VITAMINS. ENZYMES DRUGS. ANGIOPROTECTORS. (Thiamimi bromidum, Riboflavinum, Calcii pangamanas, Acidum folicum, Acidum nicotinicum, Piridoxinum, Cyanocobalaminum, Calcii pantotenas, Acidum ascorbinicum, Rutinum, Cvercitinum, Retinoli acetas, Ergocalcipherolum, Tocopheroli acetas, Vicasolium)

ACIDS, ALKALIS, GLUCOSE. PLASMA SUBSTITUTES. SOLUTIONS FOR PARENTERAL NUTRITION. (Acidum salicilicum, Acidum benzoicum, acidum dehydrochloricum delutum, Natrii hydrocarbonas, Magnesii oxydum, Solutio Ammonii caustici, Aluminii hydroxidum, Kalii chloridum, Asparcam (Pananginum), Magnesii sulfas, Calcii chloridum, Calcii gluconas, Natrii chloridum, Solutio Ringer-Lokka, Trisol, Lipofundinum, Glucosa, Albuminum, Natrii carbonas, Trisaminum, Reopolyglukin, Gelatynolum, Neohemodes)

ANTI-INFLAMMATORY AGENTS. ANTIALLERGIC AGENTS. IMMUNOMODULATORS. (Acidum acetylsalicilicum, Acidum mephenamicum, Butadionum, Indometacinum (Metindol), Dyclofenac-natrium, Ibuprophen, Naproxen, Pyroxicam, Meloxicam, Celecoxib, Nimesulid, Dimedrolum, Tavegilum, Fencarolum, Suprastinum, Diazolinum, Loratinum, Diprasinum, Fexofenadinum, Ranitidinum, Famotidinum, Levamisolum, Timalinum, Т-activinum, Prodigiosanum, Natrii Nuclrinas, Methyluracilum, Pentoxilum)

Vitamins

History: Approximately 40 vitamin and mineral nutrients are required by man. Vitamins can be defined as organic substances that must be provided in not more than small amounts from the environment to sustain healthy life.

Vitamins are either not synthesized at all by the body or are synthesized in quantities too small to fulfill daily nutritional needs. For centuries, some diseases have been known to be related to deficient intake of a specific vitamin including night blindness (vitamin A deficiency), beriberi (thiamine deficiency), pellagra (niacin deficiency), scurvy (ascorbic acid deficiency), and rickets (vitamin D deficiency). Folic acid deficiency during gestation has recently been associated with neural tube defects in the fetus.

Approximately 40 vitamin and mineral nutrients are required by man. Vitamins can be defined as organic substances that must be provided in not more than small amounts from the environment to sustain healthy life. Vitamins are either not synthesized at all by the body or are synthesized in quantities too small to fulfill daily nutritional needs. For centuries, some diseases have been known to be related to deficient intake of a specific vitamin including night blindness (vitamin A deficiency), beriberi (thiamine deficiency), pellagra (niacin deficiency), scurvy (ascorbic acid deficiency), and rickets (vitamin D deficiency). Folic acid deficiency during gestation has recently been associated with neural tube defects in the fetus.
Vitamins were originally identified through animal experiments. Animal were fed a diet that was thought to cause a particular disease in man and then treated with the nutrient that was deficient in the diet producing the disease. Funk, in 1911, was able to identify an extract that prevented beriberi and coined the term "vitamine" because he believed the substance to be an amine that was vital to life. It was later confirmed by McCollum and Davis that a number of factors were present in fats (fat-soluble A) which were different from water-soluble factors they called a "water-soluble B" fraction. The B vitamins were found in an extract from rice husks and continue to be classified together even though they have different chemical structures and biologic functions.
Today, the fat-soluble vitamins are known as vitamins A, D, E, and K. Water-soluble vitamins include thiamine, riboflavin, nicotinic acid (niacin), pyridoxine, pantothenic acid, biotin, folic acid, and cyanocobalamin.
The daily requirements for vitamins are estimated in the United States by the Food and Nutrition Board of the National Academy of Sciences. Recommended dietary allowances (RDA) are published for males and females of different ages and have been periodically revised since 1941. The RDA document also discusses substances that have not been proven essential by man. These substances are grouped into four categories: (1) those known to be essential for some animals but not shown to be needed by man (e.g., nickel, vanadium, and silicon); (2) substances that act as growth factors for lower forms of life (e.g., para-aminobenzoic acid, carnitine, and pimelic acid); (3) substances that are in foods but whose actions are probably pharmacologic or non-existent and; (4) substances for which scientific proof of a nutrient action has not been established (e.g., pangamic acid, laetrile). This latter category includes substances often promoted by the health-food industry.
Vitamin products are regulated by the US FDA primarily as foods and not as drugs. Therefore, most vitamin products are not subject to the same requirements to establish safety and efficacy as are OTC and prescription drugs. The distinction as to whether a vitamin is a drug or a food supplement is determined by its intended use. If the vitamin is intended to treat or prevent disease, it is considered a drug. If, however, its use is simply as a nutritional supplement, then the vitamin is considered a food supplement and is not subject to the strict guidelines as mandated in the Food, Drug, and Cosmetic Act. Vitamin products must contain ingredients as labeled but there is no requirement to establish that the ingredients in the product are able to be absorbed from the product or are active after oral administration. In part to remedy this situation, the USP has published voluntary standards governing in vitro dosage form disintegration and dissolution. Vitamin and mineral supplement manufacturers may choose to test their products against these standards and indicate that they passed the tests on their product labels.
Mechanism of Action: Since vitamins represent a diverse collection of biologically active compounds, they exert their effects through a wide range of mechanisms. Their classification as vitamins is not because they have similar biochemical effects but because all are needed for continued good health. In general, most vitamins exert their effect by binding to a specific cofactor. Because it is thought that binding to the cofactor can be saturated at some vitamin concentration, increasing the dose of the vitamin, does not produce proportionately greater physiologic effects. Rather, pharmacologic or toxic effects of the vitamin may occur. An example of an effect of a pharmacologic effect for a vitamin is the cholesterol lowering action of niacin (vitamin B₃) when used in doses at least 40 times the RDA. Nevertheless, many individuals attribute near magical qualities to vitamins despite the fact that they merely represent dietary nutrients in tablet or capsule form.
Recently, however, great interest in the antioxidant properties of vitamins C and E and beta-carotene has arisen. These data have been reviewed by Jha et al. It is thought that the body, particularly in smokers, generates highly reactive oxidative molecules which can be damaging to tissues unless neutralized. Adequate concentrations of the "antioxidant" vitamins affords the protection from these molecules the body needs. Oxidation of LDL cholesterol is an important step in the pathogenesis of atherosclerotic lesions. Vitamins with antioxidant properties include vitamin E (alpha-tocopherol), beta-carotene, and vitamin C.
Other pharmacologic actions of vitamins are discussed in detail on the respective monographs for each vitamin.
Vitamins were originally identified through animal experiments. Animal were fed a diet that was thought to cause a particular disease in man and then treated with the nutrient that was deficient in the diet producing the disease. Funk, in 1911, was able to identify an extract that prevented beriberi and coined the term "vitamine" because he believed the substance to be an amine that was vital to life. It was later confirmed by McCollum and Davis that a number of factors were present in fats (fat-soluble A) which were different from water-soluble factors they called a "water-soluble B" fraction. The B vitamins were found in an extract from rice husks and continue to be classified together even though they have different chemical structures and biologic functions.

Today, the fat-soluble vitamins are known as vitamins A, D, E, and K. Water-soluble vitamins include thiamine, riboflavin, nicotinic acid (niacin), pyridoxine, pantothenic acid, biotin, folic acid, and cyanocobalamin.

The daily requirements for vitamins are estimated in the United States by the Food and Nutrition Board of the National Academy of Sciences. Recommended dietary allowances (RDA) are published for males and females of different ages and have been periodically revised since 1941.

The RDA document also discusses substances that have not been proven essential by man. These substances are grouped into four categories: (1) those known to be essential for some animals but not shown to be needed by man (e.g., nickel, vanadium, and silicon); (2) substances that act as growth factors for lower forms of life (e.g., para-aminobenzoic acid, carnitine, and pimelic acid); (3) substances that are in foods but whose actions are probably pharmacologic or non-existent and; (4) substances for which scientific proof of a nutrient action has not been established (e.g., pangamic acid, laetrile). This latter category includes substances often promoted by the health-food industry.

Vitamin products are regulated by the US FDA primarily as foods and not as drugs. Therefore, most vitamin products are not subject to the same requirements to establish safety and efficacy as are OTC and prescription drugs. The distinction as to whether a vitamin is a drug or a food supplement is determined by its intended use. If the vitamin is intended to treat or prevent disease, it is considered a drug.

If, however, its use is simply as a nutritional supplement, then the vitamin is considered a food supplement and is not subject to the strict guidelines as mandated in the Food, Drug, and Cosmetic Act. Vitamin products must contain ingredients as labeled but there is no requirement to establish that the ingredients in the product are able to be absorbed from the product or are active after oral administration. In part to remedy this situation, the USP has published voluntary standards governing in vitro dosage form disintegration and dissolution. Vitamin and mineral supplement manufacturers may choose to test their products against these standards and indicate that they passed the tests on their product labels.

Mechanism of Action: Since vitamins represent a diverse collection of biologically active compounds, they exert their effects through a wide range of mechanisms. Their classification as vitamins is not because they have similar biochemical effects but because all are needed for continued good health. In general, most vitamins exert their effect by binding to a specific cofactor. Because it is thought that binding to the cofactor can be saturated at some vitamin concentration, increasing the dose of the vitamin, does not produce proportionately greater physiologic effects.

Rather, pharmacologic or toxic effects of the vitamin may occur. An example of an effect of a pharmacologic effect for a vitamin is the cholesterol lowering action of niacin (vitamin B₃) when used in doses at least 40 times the RDA. Nevertheless, many individuals attribute near magical qualities to vitamins despite the fact that they merely represent dietary nutrients in tablet or capsule form. Recently, however, great interest in the antioxidant properties of vitamins C and E and beta-carotene has arisen. These data have been reviewed by Jha et al. It is thought that the body, particularly in smokers, generates highly reactive oxidative molecules which can be damaging to tissues unless neutralized. Adequate concentrations of the "antioxidant" vitamins affords the protection from these molecules the body needs. Oxidation of LDL cholesterol is an important step in the pathogenesis of atherosclerotic lesions. Vitamins with antioxidant properties include vitamin E (alpha-tocopherol), beta-carotene, and vitamin C. Other pharmacologic actions of vitamins are discussed in detail on the respective monographs for each vitamin.

Distinguishing Features: Although dissimilar in function, fat- and water-soluble vitamins share some general characteristics. The body stores only limited amounts of water-soluble vitamins as these are easily eliminated by the kidneys. Fat-soluble vitamins are readily stored in large quantities and can accumulate to toxic concentrations. Health-food outlets and literature often promote the benefits of natural source products, however, products that are equally bioavailable are equally effective regardless of origin (natural or synthetic).

Besides satisfying the body's daily needs to prevent the development of a deficiency state, some vitamins have therapeutic uses: pyridoxine can be used in the treatment of sideroblastic anemia and to offset certain drug-induced neuropathies, niacin exerts an antilipemic action, and ascorbic acid can be used to acidify the urine. Some derivatives of vitamin A - though not nutrients in the strict definition - exert powerful effects on the skin and the hematopoetic system, underscoring how important vitamins are to general health. Although more data are needed, preliminary results indicate that routine intake of doses of vitamins with antioxidant properties in excess of the standard RDA may indeed be protective against myocardial infarction. Data is most convincing for vitamin E, however, results from randomized trials has tempered enthusiasm generated from earlier epidemiologic cohort studies. The beneficial effects of beta-carotene on risk of myocardial infarction appear to be limited to smokers. Vitamin C was found to reduce risk in only one cohort study. Clinicians should also consider that subjects using antioxidant vitamins in these studies were less likely to be smokers and have hypertension, more likely to exercise regularly, and consumed more alcohol.

Adverse Reactions: Due to their prompt elimination via the kidneys, sweat glands, and other sites of excretion, water-soluble vitamins are generally considered to be non-toxic even when taken in larger than physiologic doses. Various toxicities, however, have been associated with water-soluble vitamins. In doses greater than the RDA, niacin can be hepatotoxic, ascorbic acid has been associated with nephrolithiasis, and pyridoxine, paradoxically, in very high doses has caused peripheral neuropathy. Fat soluble vitamins, on the other hand, can readily accumulate to toxic levels when taken in doses substantially greater than the RDA. The liver is highly efficient in storing vitamin A and even modest doses of vitamin D taken in combination with calcium supplements can lead to hypercalcemia severe enough produce coma. Since there is no established benefit of taking vitamins in excess quantities, the AMA Council on Scientific Affairs has recommended that the daily intake of vitamins be limited to 150% of the RDA for any single vitamin in patients with no documented need for therapeutic doses.

VItamin B12

Vitamin B12 serves as a cofactor for several essential biochemical reactions in humans. Deficiency of vitamin B12 leads to anemia, gastrointestinal symptoms, and neurologic abnormalities.

While deficiency of vitamin B12 due to an inadequate supply in the diet is unusual, deficiency of B12 in adults—especially older adults—due to abnormal absorption of dietary vitamin B12 is a relatively common and easily treated disorder.

Chemistry

Vitamin B12 consists of a porphyrin-like ring with a central cobalt atom attached to a nucleotide.

Various organic groups may be covalently bound to the cobalt atom, forming different cobalamins. Deoxyadenosylcobalamin and methylcobalamin are the active forms of the vitamin in humans.

Vitamin B12 and folate metabolism

Cyanocobalamin and hydroxocobalamin (both available for therapeutic use) and other cobalamins found in food sources are converted to the above active forms. The ultimate source of vitamin B12 is from microbial synthesis; the vitamin is not synthesized by animals or plants. The chief dietary source of vitamin B12 is microbially derived vitamin B12 in meat (especially liver), eggs, and dairy products. Vitamin B12 is sometimes called extrinsic factor to differentiate it from intrinsic factor, a protein normally secreted by the stomach.

Pharmacokinetics

The average diet in the USA contains 5–30 g of vitamin B12 daily, 1–5 g of which is usually absorbed.

The vitamin is avidly stored, primarily in the liver, with an average adult having a total vitamin B12 storage pool of 3000–5000 g. Only trace amounts of vitamin B12 are normally lost in urine and stool. Since the normal daily requirements of vitamin B12 are only about 2 g, it would take about 5 years for all of the stored vitamin B12 to be exhausted and for megaloblastic anemia to develop if B12 absorption stopped.

Vitamin B12 in physiologic amounts is absorbed only after it complexes with intrinsic factor, a glycoprotein secreted by the parietal cells of the gastric mucosa. Intrinsic factor combines with the vitamin B12 that is liberated from dietary sources in the stomach and duodenum, and the intrinsic factor-vitamin B12 complex is subsequently absorbed in the distal ileum by a highly specific receptor-mediated transport system.

Vitamin B12 deficiency in humans most often results from malabsorption of vitamin B12, due either to lack of intrinsic factor or to loss or malfunction of the specific absorptive mechanism in the distal ileum. Nutritional deficiency is rare but may be seen in strict vegetarians after many years without meat, eggs, or dairy products. Once absorbed, vitamin B12 is transported to the various cells of the body bound to a plasma glycoprotein, transcobalamin II. Excess vitamin B12 is transported to the liver for storage. Significant amounts of vitamin B12 are excreted in the urine only when very large amounts are given parenterally, overcoming the binding capacities of the transcobalamins (50–100 g).

Pharmacodynamics

Two essential enzymatic reactions in humans require vitamin B12. In one, methylcobalamin serves as an intermediate in the transfer of a methyl group from N_5- methyltetrahydrofolate to methionine.

In the absence of vitamin B12, conversion of the major dietary and storage folate, N5-methyltetrahydrofolate, to tetrahydrofolate, the precursor of folate cofactors, cannot occur. As a result, a deficiency of folate cofactors necessary for several biochemical reactions involving the transfer of one-carbon groups develops. In particular, the depletion of tetrahydrofolate prevents synthesis of adequate supplies of the deoxythymidylate (dTMP) and purines required for DNA synthesis in rapidly dividing cells.

The accumulation of folate as N5-methyltetrahydrofolate and the associated depletion of tetrahydrofolate cofactors in vitamin B12 deficiency have been referred to as the "methylfolate trap." This is the biochemical step whereby vitamin B12 and folic acid metabolism are linked and explains why the megaloblastic anemia of vitamin B12 deficiency can be partially corrected by ingestion of relatively large amounts of folic acid. Folic acid can be reduced to dihydrofolate by the enzyme dihydrofolate reductase and thus serve as a source of the tetrahydrofolate required for synthesis of the purines and dTMP that are needed for DNA synthesis.

The other enzymatic reaction that requires vitamin B12 is isomerization of methylmalonyl-CoA to succinyl-CoA by the enzyme methylmalonyl-CoA mutase. In vitamin B12 deficiency, this conversion cannot take place, and the substrate, methylmalonyl-CoA, accumulates.

In the past, it was thought that abnormal accumulation of methylmalonyl-CoA causes the neurologic manifestations of vitamin B12 deficiency. However, newer evidence instead implicates the disruption of the methionine synthesis pathway as the cause of neurologic problems. Whatever the biochemical explanation for neurologic damage, the important point is that administration of folic acid in the setting of vitamin B12 deficiency will not prevent neurologic manifestations even though it will largely correct the anemia caused by the vitamin B12 deficiency.

Clinical Pharmacology

Vitamin B12 is used to treat or prevent deficiency. There is no evidence that vitamin B12 injections have any benefit in persons who do not have vitamin B12 deficiency. The most characteristic clinical manifestation of vitamin B12 deficiency is megaloblastic anemia.

The typical clinical findings in megaloblastic anemia are macrocytic anemia (MCV usually > 120 fL), often with associated mild or moderate leukopenia or thrombocytopenia (or both), and a characteristic hypercellular bone marrow with megaloblastic maturation of erythroid and other precursor cells.

Vitamin B12 deficiency also causes a neurologic syndrome that usually begins with paresthesias and weakness in peripheral nerves and progresses to spasticity, ataxia, and other central nervous system dysfunctions. A characteristic pathologic feature of the neurologic syndrome is degeneration of myelin sheaths followed by disruption of axons in the dorsal and lateral horns of the spinal cord and in peripheral nerves. Correction of vitamin B12 deficiency arrests the progression of neurologic disease, but it may not fully reverse neurologic symptoms that have been present for several months.

Although most patients with neurologic abnormalities caused by vitamin B12 deficiency have full-blown megaloblastic anemias when first seen, occasional patients have few if any hematologic abnormalities. Once a diagnosis of megaloblastic anemia is made, it must be determined whether vitamin B12 or folic acid deficiency is the cause. (Other causes of megaloblastic anemia are very rare.)

This can usually be accomplished by measuring serum levels of the vitamins. The Schilling test, which measures absorption and urinary excretion of radioactively labeled vitamin B12, can be used to further define the mechanism of vitamin B12 malabsorption when this is found to be the cause of the megaloblastic anemia.

The most common causes of vitamin B12 deficiency are pernicious anemia, partial or total gastrectomy, and diseases that affect the distal ileum, such as malabsorption syndromes, inflammatory bowel disease, or small bowel resection.

Pernicious anemia results from defective secretion of intrinsic factor by the gastric mucosal cells.

Patients with pernicious anemia have gastric atrophy and fail to secrete intrinsic factor (as well as hydrochloric acid). The Schilling test shows diminished absorption of radioactively labeled vitamin B12, which is corrected when hog intrinsic factor is administered with radioactive B12, since the vitamin can then be normally absorbed.

Vitamin B12 deficiency also occurs when the region of the distal ileum that absorbs the vitamin B12-intrinsic factor complex is damaged, as when the ileum is involved with inflammatory bowel disease, or when the ileum is surgically resected. In these situations, radioactively labeled vitamin B12 is not absorbed in the Schilling test, even when intrinsic factor is added. Other rare causes of vitamin B12 deficiency include bacterial overgrowth of the small bowel, chronic pancreatitis, and thyroid disease. Rare cases of vitamin B12 deficiency in children have been found to be secondary to congenital deficiency of intrinsic factor and congenital selective vitamin B12 malabsorption due to defects of the receptor sites in the distal ileum. Since almost all cases of vitamin B12 deficiency are caused by malabsorption of the vitamin, parenteral injections of vitamin B12 are required for therapy. For patients with potentially reversible diseases, the underlying disease should be treated after initial treatment with parenteral vitamin B12. Most patients, however, do not have curable deficiency syndromes and require lifelong treatment with vitamin B12 injections.

Vitamin B12 for parenteral injection is available as cyanocobalamin or hydroxocobalamin. Hydroxocobalamin is preferred because it is more highly protein-bound and therefore remains longer in the circulation. Initial therapy should consist of 100–1000 g of vitamin B12 intramuscularly daily or every other day for 1–2 weeks to replenish body stores.

Maintenance therapy consists of 100–1000 g intramuscularly once a month for life. If neurologic abnormalities are present, maintenance therapy injections should be given every 1–2 weeks for 6 months before switching to monthly injections.

Oral vitamin B12-intrinsic factor mixtures and liver extracts should not be used to treat vitamin B12 deficiency; however, oral doses of 1000 g of vitamin B12 daily are usually sufficient to treat patients with pernicious anemia who refuse or cannot tolerate the injections.

Folic Acid

Reduced forms of folic acid are required for essential biochemical reactions that provide precursors for the synthesis of amino acids, purines, and DNA. Folate deficiency is not uncommon, even though the deficiency is easily corrected by administration of folic acid. The consequences of folate deficiency go beyond the problem of anemia because folate deficiency is implicated as a cause of congenital malformations in newborns and may play a role in vascular disease (see Folic Acid Supplementation: A Public Health Dilemma).

Chemistry

Folic acid (pteroylglutamic acid) is a compound composed of a heterocycle, p-aminobenzoic acid, and glutamic acid. Various numbers of glutamic acid moieties may be attached to the pteroyl portion of the molecule, resulting in monoglutamates, triglutamates, or polyglutamates.

Folic acid can undergo reduction, catalyzed by the enzyme dihydrofolate reductase ("folate reductase"), to give dihydrofolic acid. Tetrahydrofolate can subsequently be transformed to folate cofactors possessing one-carbon units attached to the 5-nitrogen, to the 10- nitrogen, or to both positions. The folate cofactors are interconvertible by various enzymatic reactions and serve the important biochemical function of donating one-carbon units at various levels of oxidation. In most of these, tetrahydrofolate is regenerated and becomes available for reutilization.

Pharmacokinetics

The average diet in the USA contains 500–700 g of folates daily, 50–200 g of which is usually absorbed, depending on metabolic requirements (pregnant women may absorb as much as 300–400 g of folic acid daily). Various forms of folic acid are present in a wide variety of plant and animal tissues; the richest sources are yeast, liver, kidney, and green vegetables. Normally, 5–20 mg of folates are stored in the liver and other tissues.

Folates are excreted in the urine and stool and are also destroyed by catabolism, so serum levels fall within a few days when intake is diminished. Since body stores of folates are relatively low and daily requirements high, folic acid deficiency and megaloblastic anemia can develop within 1–6 months after the intake of folic acid stops, depending on the patient's nutritional status and the rate of folate utilization. Unaltered folic acid is readily and completely absorbed in the proximal jejunum. Dietary folates, however, consist primarily of polyglutamate forms of N_5-methyltetrahydrofolate.

Before absorption, all but one of the glutamyl residues of the polyglutamates must be hydrolyzed by the enzyme -1-glutamyl transferase ("conjugase") within the brush border of the intestinal mucosa The monoglutamate N_5-methyltetrahydrofolate is subsequently transported into the bloodstream by both active and passive transport and is then widely distributed throughout the body. Inside cells, N_5-methyltetrahydrofolate is converted to tetrahydrofolate by the demethylation reaction that requires vitamin B12.

Pharmacodynamics

Tetrahydrofolate cofactors participate in one-carbon transfer reactions. As described above in the section on vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthesis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N_5,N_10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP. Unlike all of the other enzymatic reactions that utilize folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, one mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, considerable amounts of tetrahydrofolate can be consumed in this reaction, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N_5,N_10-methylenetetrahydrofolate by the action of serine transhydroxy- methylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are often referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anticancer drugs; methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase. Cofactors of tetrahydrofolate participate in several other essential reactions. As described above, N_5-methy- lenetetrahydrofolate is required for the vitamin B12-dependent reaction that generates methionine from homocysteine. In addition, tetrahydrofolate cofactors donate one-carbon units during the de novo synthesis of essential purines.

In these reactions, tetrahydrofolate is regenerated and can reenter the tetrahydrofolate cofactor pool.

Clinical Pharmacology

Folate deficiency results in a megaloblastic anemia that is microscopically indistinguishable from the anemia caused by vitamin B12 deficiency (see above). However, folate deficiency does not cause the characteristic neurologic syndrome seen in vitamin B12 deficiency. In patients with megaloblastic anemia, folate status is assessed with assays for serum folate or for red blood cell folate.

Red blood cell folate levels are often of greater diagnostic value than serum levels, since serum folate levels tend to be quite labile and do not necessarily reflect tissue levels. Folic acid deficiency, unlike vitamin B12 deficiency, is often caused by inadequate dietary intake of folates. Alcoholics and patients with liver disease develop folic acid deficiency because of poor diet and diminished hepatic storage of folates. There is also evidence that alcohol and liver disease interfere with absorption and metabolism of folates. Pregnant women and patients with hemolytic anemia have increased folate requirements and may become folic acid-deficient, especially if their diets are marginal. Evidence implicates maternal folic acid deficiency in the occurrence of fetal neural tube defects, eg, spina bifida.

Patients with malabsorption syndromes also frequently develop folic acid deficiency. Folic acid deficiency is occasionally associated with cancer, leukemia, myeloproliferative disorders, certain chronic skin disorders, and other chronic debilitating diseases. Patients who require renal dialysis also develop folic acid deficiency, because folates are removed from the plasma each time the patient is dialyzed. Folic acid deficiency can be caused by drugs that interfere with folate absorption or metabolism. Phenytoin, some other anticonvulsants, oral contraceptives, and isoniazid can cause folic acid deficiency by interfering with folic acid absorption. Other drugs such as methotrexate and, to a lesser extent, trimethoprim and pyrimethamine, inhibit dihydrofolate reductase and may result in a deficiency of folate cofactors and ultimately in megaloblastic anemia.

Parenteral administration of folic acid is rarely necessary, since oral folic acid is well absorbed even in patients with malabsorption syndromes.

A dose of 1 mg of folic acid orally daily is sufficient to reverse megaloblastic anemia, restore normal serum folate levels, and replenish body stores of folates in almost all patients.

Therapy should be continued until the underlying cause of the deficiency is removed or corrected. Therapy may be required indefinitely for patients with malabsorption or dietary inadequacy.

Folic acid supplementation to prevent folic acid deficiency should be considered in high-risk patients, including pregnant women, alcoholics, and patients with hemolytic anemia, liver disease, certain skin diseases, and patients on renal dialysis.

By January 1998, all products made from enriched grains in the USA were required to be supplemented with folic acid. This FDA ruling was issued to reduce the incidence of congenital neural tube defects. Scientific studies show a strong correlation between maternal folic acid deficiency and the incidence of neural tube defects such as spinal bifida and anencephaly. The FDA requirement for folic acid supplementation is a public health measure aimed at the significant number of women in the USA who do not receive prenatal care and are not aware of the importance of adequate folic acid ingestion for preventing birth defects in their babies. Pregnant women have increased requirements for folic acid; at least 400 g/d is recommended. It is estimated that the level of folic acid fortification now required in enriched grain products provides an additional 80–100 g of folic acid per day to the diet of women of childbearing age and 70–120 g/d to the diet of middle-aged and older adults. There may be an added benefit for adults. N_5-methyltetrahydrofolate is required for the conversion of homocysteine to methionine. Impaired synthesis of N_5-methyltetrahydrofolate results in elevated serum concentrations of homocysteine. Data from several sources suggest a positive correlation between elevated serum homocysteine and occlusive vascular diseases such as ischemic heart disease and stroke.

Clinical data suggest that the folate supplementation program has improved the folate status and reduced the prevalence of hyperhomocysteinemia in a population of middle-aged and older adults who did not use vitamin supplements. It is possible, though as yet unproved, that the increased ingestion of folic acid will also reduce the risk of vascular disease in this population. While these two potential benefits of supplemental folic acid are compelling, the decision to require folic acid in grains was—and still is—controversial. As described in the text, ingestion of folic acid can partially or totally correct the anemia caused by vitamin B12 deficiency. However, folic acid supplementation will not prevent the potentially irreversible neurologic damage caused by vitamin B12 deficiency. People with pernicious anemia and other forms of vitamin B12 deficiency are usually identified because of signs and symptoms of anemia, which tend to occur before neurologic symptoms. The opponents of folic acid supplementation are concerned that increased folic acid intake in the general population will mask vitamin B12 deficiency and increase the prevalence of neurologic disease in our elderly population. To put this in perspective, approximately 4000 pregnancies, including 2500 live births, in the USA each year are affected by neural tube defects. In contrast, it is estimated that over 10% of the elderly population in the USA, or several million people, are at risk of the neuropsychiatric complications of vitamin B12 deficiency (Rothenberg, 1999). In acknowledgment of this controversy, the FDA kept its requirements for folic acid supplementation at a somewhat low level. They also recommend that all adults should keep their ingestion of folic acid below 1 mg/d.

VItamin D

Vitamin D is a secosteroid produced in the skin from 7-dehydrocholesterol under the influence of ultraviolet irradiation. Vitamin D is also found in certain foods and is used to supplement dairy products. Both the natural form (vitamin D3, cholecalciferol) and the plant-derived form (vitamin D2, ergocalciferol) are present in the diet. These forms differ in that ergocalciferol contains a double bond (C22–23) and an additional methyl group in the side chain

Vitamin D is a prohormone that serves as precursor to a number of biologically active metabolites.Vitamin D is first hydroxylated in the liver to form 25-hydroxyvitamin D (25[OH]D). This metabolite is further converted in the kidney to a number of other forms, the beststudied of which are 1,25-dihydroxyvitamin D (1,25[OH]2D) and 24,25-dihydroxyvitamin D (24,25[OH]2D). Of the natural metabolites, only vitamin D, 25(OH)D (as calcifediol), and 1,25(OH)2D (as calcitriol) are available for clinical use.

Moreover, a number of analogs of 1,25(OH)2 are being synthesized in an effort to extend the usefulness of this metabolite to a variety of nonclassic conditions. Calcipotriene (calcipotriol), for example, is currently being used to treat psoriasis, a hyperproliferative skin disorder. Doxercalciferol and paricalcitol have recently been approved for the treatment of secondary hyperparathyroidism in patients with renal failure.

Other analogs are being investigated for the treatment of various malignancies. The regulation of vitamin D metabolism is complex, involving calcium, phosphate, and a variety of hormones, the most important of which is PTH, which stimulates the production of 1,25(OH)2D by the kidney. mechanism of action of the vitamin D metabolites remains under active investigation. However, calcitriol is well established as the most potent agent with respect to stimulation of intestinal calcium and phosphate transport and bone resorption.

Calcitriol appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein) and by modulation of calcium flux across the brush border and basolateral membranes by a means does not require new protein synthesis. The molecular action of calcitriol on bone has received less attention. However, like PTH, calcitriol can induce RANK ligand in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2D are far less potent stimulators of intestinal calcium and phosphate transport or bone resorption. However, 25(OH)D appears to be more potent than 1,25(OH)2D in stimulating renal reabsorption of calcium and phosphate and may be the major metabolite regulating calcium flux and contractility in muscle. Specific receptors for 1,25(OH)2D exist in target

The issues. However, the role and even the existence of receptors for 25(OH)D and 24,25(OH)2D remain controversial.

A summary of the principal actions of PTH and vitamin D on the three main target tissues— intestine, kidney, and bone. The net effect of PTH is to raise serum calcium and reduce serum phosphate; the net effect of vitamin D is to raise both. Regulation of calcium and phosphate homeostasis is achieved through a variety of feedback loops. Calcium is the principal regulator of PTH secretion. It binds to a novel ion recognition site that is part of a Gq protein–coupled receptor and links changes in intracellular free calcium concentration to changes in

extracellular calcium. As serum calcium levels rise and bind to this receptor, intracellular calcium levels increase and inhibit PTH secretion. Phosphate regulates PTH secretion indirectly by forming complexes with calcium in the serum. Since it is the ionized concentration of calcium that is detected by the parathyroid gland, increases in serum phosphate levels reduce the ionized calcium and lead to enhanced PTH secretion. Such feedback regulation is appropriate to the net effect of PTH to raise serum calcium and reduce serum phosphate levels. Likewise, both calcium and phosphate at high levels reduce the amount of 1,25(OH)2D produced by the kidney and increase the amount of 24,25(OH)2D produced. Since 1,25(OH)2D raises serum calcium and phosphate, whereas 24,25(OH)2D has less effect, such feedback regulation is again appropriate. 1,25(OH)2D itself directly inhibits PTH secretion (independently of its effect on serum calcium) by a direct action on PTH gene transcription. This provides yet another negative feedback loop, because PTH is a major stimulus for 1,25(OH)2D production. This ability of 1,25(OH)2D to inhibit PTH secretion directly is being exploited using calcitriol analogs that have less effect on serum calcium.

Such drugs are proving useful in the management of secondary hyperparathyroidism accompanying renal failure and may be useful in selected cases of primary hyperparathyroidism.

A number of hormones modulate the actions of PTH and vitamin D in regulating bone mineral homeostasis. Compared with that of PTH and vitamin D, the physiologic impact of such secondary regulation on bone mineral homeostasis is minor. However, in pharmacologic amounts, a number of these hormones have actions on the bone mineral homeostatic mechanisms that can be exploited therapeutically.

The principal effects of calcitonin are to lower serum calcium and phosphate by actions on bone and kidney. Calcitonin inhibits osteoclastic bone resorption. Although bone formation is not impaired at first after calcitonin administration, with time both formation and resorption of bone are reduced.

Thus, the early hope that calcitonin would prove useful in restoring bone mass has not been realized. In the kidney, calcitonin reduces both calcium and phosphate reabsorption as well as reabsorption of other ions, including sodium, potassium, and magnesium. Tissues other than bone and kidney are also affected by calcitonin. Calcitonin in pharmacologic amounts decreases gastrin secretion and reduces gastric acid output while increasing secretion of sodium, potassium, chloride, and water in the gut. Pentagastrin is a potent stimulator of calcitonin secretion (as is hypercalcemia), suggesting a possible physiologic relationship between gastrin and calcitonin. In the adult human, no readily demonstrable problem develops in cases of calcitonin deficiency (thyroidectomy) or excess (medullary carcinoma of the thyroid). However, the ability of calcitonin to block bone resorption and lower serum calcium makes it a useful drug for the treatment of Paget's disease, hypercalcemia, and osteoporosis.

Clinical Pharmacology

Disorders of bone mineral homeostasis generally present with abnormalities in serum or urine calcium levels (or both), often accompanied by abnormal serum phosphate levels. These abnormal mineral concentrations may themselves cause symptoms requiring immediate treatment (eg, coma in malignant hypercalcemia, tetany in hypocalcemia). More commonly, they serve as clues to an underlying disorder in hormonal regulators (eg, primary hyperparathyroidism), target tissue response (eg, chronic renal failure), or drug misuse (eg, vitamin D intoxication). In such cases, treatment of the underlying disorder is of prime importance.

Since bone and kidney play central roles in bone mineral homeostasis, conditions that alter bone mineral homeostasis usually affect either or both of these tissues secondarily. Effects on bone can result in osteoporosis (abnormal loss of bone; remaining bone histologically normal), osteomalacia (abnormal bone formation due to inadequate mineralization), or osteitis fibrosa (excessive bone resorption with fibrotic replacement of resorption cavities). Biochemical markers of skeletal involvement include changes in serum levels of the skeletal isoenzyme of alkaline phosphatase and

osteocalcin (reflecting osteoblastic activity) and urine levels of hydroxyproline and pyridinoline cross-links (reflecting osteoclastic activity). The kidney becomes involved when the calcium-timesphosphate product in serum exceeds the point at which ectopic calcification occurs (nephrocalcinosis) or when the calcium-times-oxalate (or phosphate) product in urine exceeds saturation, leading to nephrolithiasis. Subtle early indicators of such renal involvement include polyuria, nocturia, and hyposthenuria. Radiologic evidence of nephrocalcinosis and stones is not generally observed until later. The degree of the ensuing renal failure is best followed by monitoring the decline in creatinine clearance.

Abnormal Serum Calcium & Phosphate Levels

Hypercalcemia

Hypercalcemia causes central nervous system depression, including coma, and is potentially lethal. Its major causes (other than thiazide therapy) are hyperparathyroidism and cancer with or without bone metastases. Less common causes are hypervitaminosis D, sarcoidosis, thyrotoxicosis, milkalkali syndrome, adrenal insufficiency, and immobilization. With the possible exception of hypervitaminosis D, these latter disorders seldom require emergency lowering of serum calcium. A number of approaches are used to manage the hypercalcemic crisis.

When rapidity of action is required, 1,25(OH)2D3 (calcitriol), 0.25–1 g daily, is the vitamin D metabolite of choice, since it is capable of raising serum calcium within 24–48 hours. Calcitriol also raises serum phosphate, though this action is usually not observed early in treatment. The combined effects of calcitriol and all other vitamin D metabolites and analogs on both calcium and phosphate make careful monitoring of these mineral levels especially important to avoid ectopic calcification secondary to an abnormally high serum calcium x phosphate product. Since the choice of the levels of high-energy organic

Vitamin D deficiency, once thought to be rare in this country, is being recognized more often, especially in the pediatric and geriatric populations on vegetarian diets and with reduced sunlight exposure. This problem can be avoided by daily intake of 400–800 units of vitamin D and treated by higher dosages (4000 units per day). No other metabolite is indicated. The diet should also

contain adequate amounts of calcium and phosphate.

Use of Vitamin D Preparations

The choice of vitamin D preparation to be used in the setting of chronic renal failure in the dialysis patient depends on the type and extent of bone disease and hyperparathyroidism. No consensus has been reached regarding the advisability of using any vitamin D metabolite in the predialysis patient. 1,25(OH)2D3 (calcitriol) will rapidly correct hypocalcemia and at least partially reverse the secondary hyperparathyroidism and osteitis fibrosa. Many patients with muscle weakness and bone pain gain an improved sense of well-being.

Dihydrotachysterol, an analog of 1,25(OH)2D, is also available for clinical use, though it is used much less frequently than calcitriol. Dihydrotachysterol appears to be as effective as calcitriol, differing principally in its time course of action; calcitriol increases serum calcium in 1–2 days, whereas dihydrotachysterol requires 1–2 weeks. For an equipotent dose (0.2 mg dihydrotachy-sterol versus 0.5 g calcitriol), dihydrotachysterol costs about one fourth as much as calcitriol. Adisadvantage of dihydrotachysterol is the inability to measure it in serum. Neither dihydrotachysterol nor calcitriol corrects the osteomalacic component of renal osteodystrophy in the majority of patients, and neither should be used in patients with hypercalcemia, especially if the bone disease is primarily osteomalacic.

Calcifediol (25[OH]D3) may also be used to advantage. Calcifediol is less effective than calcitriol in stimulating intestinal calcium transport, so that hypercalcemia is less of a problem with calcifediol.

Like dihydrotachysterol, calcifediol requires several weeks to restore normocalcemia in hypocalcemic individuals with chronic renal failure. Presumably because of the reduced ability of the diseased kidney to metabolize calcifediol to more active metabolites, high doses (50–100 g daily) must be given to achieve the supraphysiologic serum levels required for therapeutic effectiveness.

Vitamin D has been used in treating renal osteodystrophy. However, patients with a substantial degree of renal failure who are thus unable to convert vitamin D to its active metabolites usually are refractory to vitamin D. Its use is decreasing as more effective alternatives become available.

Two analogs of calcitriol, doxercalciferol and paricalcitol, are approved for the treatment of secondary hyperparathyroidism of chronic renal failure. Their principal advantage is that they are less likely than calcitriol to induce hypercalcemia. Their biggest impact will be in patients in whom the use of calcitriol may lead to unacceptably high serum calcium levels.

Regardless of the drug employed, careful attention to serum calcium and phosphate levels is required. Calcium supplements (dietary and in the dialysate) and phosphate restriction (dietary and with oral ingestion of phosphate binders) should be employed along with the use of vitamin D metabolites. Monitoring serum PTH and alkaline phosphatase levels is useful in determining whether therapy is correcting or preventing secondary hyperparathyroidism.

Although not generally available, percutaneous bone biopsies for quantitative histomorphometry may help in choosing appropriate therapy and following the effectiveness of such therapy. Unlike the rapid changes in serum values, changes in bone morphology require months to years. Monitoring serum levels of the vitamin D metabolites is useful to determine compliance, absorption, and metabolism.

The common features that appear to be important in this group of diseases are malabsorption of calcium and vitamin D. Liver disease may, in addition, reduce the production of 25(OH)D from vitamin D, though the importance of this in all but patients with terminal liver failure remains in dispute. The malabsorption of vitamin D is probably not limited to exogenous vitamin D. The liver secretes into bile a substantial number of vitamin D metabolites and conjugates that are reabsorbed in (presumably) the distal jejunum and ileum. Interference with this process could deplete the body of endogenous vitamin D metabolites as well as limit absorption of dietary vitamin D.

In mild forms of malabsorption, vitamin D (25,000–50,000 units three times per week) should suffice to raise serum levels of 25(OH)D into the normal range. Many patients with severe disease do not respond to vitamin D. Clinical experience with

Vitamin D is a secosteroid produced in the skin from 7-dehydrocholesterol under the influence of ultraviolet irradiation. Vitamin D is also found in certain foods and is used to supplement dairy products. Both the natural form (vitamin D3, cholecalciferol) and the plant-derived form (vitamin D2, ergocalciferol) are present in the diet. These forms differ in that ergocalciferol contains a double bond (C22–23) and an additional methyl group in the side chain.

Vitamin D is a prohormone that serves as precursor to a number of biologically active metabolites. Vitamin D is first hydroxylated in the liver to form 25-hydroxyvitamin D (25[OH]D). This metabolite is further converted in the kidney to a number of other forms, the beststudied of which are 1,25-dihydroxyvitamin D (1,25[OH]2D) and 24,25-dihydroxyvitamin D (24,25[OH]2D). Of the natural metabolites, only vitamin D, 25(OH)D (as calcifediol), and 1,25(OH)2D (as calcitriol) are available for clinical use. Moreover, a number of analogs of 1,25(OH)2 are being synthesized in an effort to extend the usefulness of this metabolite to a variety of nonclassic conditions. Calcipotriene (calcipotriol), for example, is currently being used to treat psoriasis, a hyperproliferative skin disorder. Doxercalciferol and paricalcitol have recently been approved for the treatment of secondary hyperparathyroidism in patients with renal failure. Other analogs are being investigated for the treatment of various malignancies. The regulation of vitamin D metabolism is complex, involving calcium, phosphate, and a variety of hormones, the most important of which is PTH, which stimulates the production of 1,25(OH)2D by

the kidney. mechanism of action of the vitamin D metabolites remains under active investigation. However, calcitriol is well established as the most potent agent with respect to stimulation of intestinal calcium and phosphate transport and bone resorption. Calcitriol appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein) and by modulation of calcium flux across the brush border and basolateral membranes by a means does not require new protein synthesis. The molecular action of calcitriol on bone has received less attention. However, like PTH, calcitriol can induce RANK ligand in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2D are far less potent stimulators of intestinal calcium and phosphate transport or bone resorption. However, 25(OH)D appears to be more potent than 1,25(OH)2D in stimulating renal reabsorption of calcium and phosphate and may be the major metabolite regulating calcium flux and contractility in muscle. Specific receptors for 1,25(OH)2D exist in target

However, the role and even the existence of receptors for 25(OH)D and 24,25(OH)2D remain controversial.

A summary of the principal actions of PTH and vitamin D on the three main target tissues— intestine, kidney, and bone. The net effect of PTH is to raise serum calcium and reduce serum phosphate; the net effect of vitamin D is to raise both. Regulation of calcium and phosphate homeostasis is achieved through a variety of feedback loops. Calcium is the principal regulator of PTH secretion. It binds to a novel ion recognition site that is part of a Gq protein–coupled receptor and links changes in intracellular free calcium concentration to changes in extracellular calcium. As serum calcium levels rise and bind to this receptor, intracellular calcium levels increase and inhibit PTH secretion. Phosphate regulates PTH secretion indirectly by forming complexes with calcium in the serum. Since it is the ionized concentration of calcium that is detected by the parathyroid gland, increases in serum phosphate levels reduce the ionized calcium and lead to enhanced PTH secretion. Such feedback regulation is appropriate to the net effect of PTH to raise serum calcium and reduce serum phosphate levels. Likewise, both calcium and phosphate at high levels reduce the amount of 1,25(OH)2D produced by the kidney and increase the amount of 24,25(OH)2D produced. Since 1,25(OH)2D raises serum calcium and phosphate, whereas 24,25(OH)2D has less effect, such feedback regulation is again appropriate. 1,25(OH)2D itself directly inhibits PTH secretion (independently of its effect on serum calcium) by a direct action on PTH gene transcription. This provides yet another negative feedback loop, because PTH is a major stimulus for 1,25(OH)2D production. This ability of 1,25(OH)2D to inhibit PTH secretion directly is being exploited using calcitriol analogs that have less effect on serum calcium.

Such drugs are proving useful in the management of secondary hyperparathyroidism accompanying renal failure and may be useful in selected cases of primary hyperparathyroidism.

A number of hormones modulate the actions of PTH and vitamin D in regulating bone mineral homeostasis. Compared with that of PTH and vitamin D, the physiologic impact of such secondary regulation on bone mineral homeostasis is minor. However, in pharmacologic amounts, a number of these hormones have actions on the bone mineral homeostatic mechanisms that can be exploited therapeutically.

Calcitonin

The calcitonin secreted by the parafollicular cells of the mammalian thyroid is a single-chain peptide hormone with 32 amino acids and a molecular weight of 3600. A disulfide bond between positions 1 and 7 is essential for biologic activity. Calcitonin is produced from a precursor with MW 15,000. The circulating forms of calcitonin are multiple, ranging in size from the monomer (MW 3600) to forms with an apparent molecular weight of 60,000. Whether such heterogeneity includes precursor forms or covalently linked oligomers is not known. Because of its heterogeneity, calcitonin is standardized by bioassay in rats. Activity is compared to a standard maintained by the

British Medical Research Council (MRC) and expressed as MRC units. Human calcitonin monomer has a half-life of about 10 minutes with a metabolic clearance of 8–9 mL/kg/min. Salmon calcitonin has a longer half-life and a reduced metabolic clearance (3 mL/kg/min), making it more attractive as a therapeutic agent. Much of the clearance occurs in the kidney, although little intact calcitonin appears in the urine.

The principal effects of calcitonin are to lower serum calcium and phosphate by actions on bone and kidney. Calcitonin inhibits osteoclastic bone resorption. Although bone formation is not impaired at first after calcitonin administration, with time both formation and resorption of bone are reduced.

Thus, the early hope that calcitonin would prove useful in restoring bone mass has not been realized. In the kidney, calcitonin reduces both calcium and phosphate reabsorption as well as reabsorption of other ions, including sodium, potassium, and magnesium. Tissues other than bone and kidney are also affected by calcitonin. Calcitonin in pharmacologic amounts decreases gastrin secretion and reduces gastric acid output while increasing secretion of sodium, potassium, chloride, and water in the gut. Pentagastrin is a potent stimulator of calcitonin secretion (as is hypercalcemia), suggesting a possible physiologic relationship between gastrin and calcitonin. In the adult human, no readily demonstrable problem develops in cases of calcitonin deficiency (thyroidectomy) or excess (medullary carcinoma of the thyroid). However, the ability of calcitonin to block bone resorption and lower serum calcium makes it a useful drug for the treatment of Paget's disease, hypercalcemia, and osteoporosis.

Clinical Pharmacology

Disorders of bone mineral homeostasis generally present with abnormalities in serum or urine calcium levels (or both), often accompanied by abnormal serum phosphate levels. These abnormal mineral concentrations may themselves cause symptoms requiring immediate treatment (eg, coma in malignant hypercalcemia, tetany in hypocalcemia). More commonly, they serve as clues to an underlying disorder in hormonal regulators (eg, primary hyperparathyroidism), target tissue response (eg, chronic renal failure), or drug misuse (eg, vitamin D intoxication). In such cases, treatment of the underlying disorder is of prime importance.

Since bone and kidney play central roles in bone mineral homeostasis, conditions that alter bone mineral homeostasis usually affect either or both of these tissues secondarily. Effects on bone can result in osteoporosis (abnormal loss of bone; remaining bone histologically normal), osteomalacia (abnormal bone formation due to inadequate mineralization), or osteitis fibrosa (excessive bone resorption with fibrotic replacement of resorption cavities). Biochemical markers of skeletal involvement include changes in serum levels of the skeletal isoenzyme of alkaline phosphatase and

osteocalcin (reflecting osteoblastic activity) and urine levels of hydroxyproline and pyridinoline cross-links (reflecting osteoclastic activity). The kidney becomes involved when the calcium-timesphosphate product in serum exceeds the point at which ectopic calcification occurs (nephrocalcinosis) or when the calcium-times-oxalate (or phosphate) product in urine exceeds saturation, leading to nephrolithiasis. Subtle early indicators of such renal involvement include polyuria, nocturia, and hyposthenuria. Radiologic evidence of nephrocalcinosis and stones is not generally observed until later. The degree of the ensuing renal failure is best followed by monitoring the decline in creatinine clearance.

Abnormal Serum Calcium & Phosphate Levels

Hypercalcemia

Hypercalcemia causes central nervous system depression, including coma, and is potentially lethal. Its major causes (other than thiazide therapy) are hyperparathyroidism and cancer with or without bone metastases. Less common causes are hypervitaminosis D, sarcoidosis, thyrotoxicosis, milkalkali syndrome, adrenal insufficiency, and immobilization. With the possible exception of hypervitaminosis D, these latter disorders seldom require emergency lowering of serum calcium. A number of approaches are used to manage the hypercalcemic crisis.

Calcitonin

Calcitonin has proved useful as ancillary treatment in a large number of patients. Calcitonin by itself seldom restores serum calcium to normal, and refractoriness frequently develops. However, its lack of toxicity permits frequent administration at high doses (200 MRC units or more). An effect on serum calcium is observed within 4–6 hours and lasts for 6–10 hours. Calcimar (salmon calcitonin) is available for parenteral and nasal administration.

Calcium

A number of calcium preparations are available for intravenous, intramuscular, and oral use.

Calcium gluceptate (0.9 meq calcium/mL), calcium gluconate (0.45 meq calcium/mL), and calcium chloride (0.68–1.36 meq calcium/mL) are available for intravenous therapy. Calcium gluconate is the preferred form because it is less irritating to veins. Oral preparations include calcium carbonate (40% calcium), calcium lactate (13% calcium), calcium phosphate (25% calcium), and calcium citrate (21% calcium). Calcium carbonate is often the preparation of choice because of its high percentage of calcium, ready availability (eg, Tums), low cost, and antacid properties. In achlorhydric patients, calcium carbonate should be given with meals to increase absorption or the patient switched to calcium citrate, which is somewhat better absorbed. Combinations of vitamin D and calcium are available, but treatment must be tailored to the individual patient and individual disease, a flexibility lost by fixed-dosage combinations. Treatment of severe symptomatic hypocalcemia can be accomplished with slow infusion of 5–20 mL of 10% calcium gluconate.

Rapid infusion can lead to cardiac arrhythmias. Less severe hypocalcemia is best treated with oral forms sufficient to provide approximately 400–800 mg of elemental calcium (1–2 g calcium carbonate) per day. Dosage must be adjusted to avoid hypercalcemia and hypercalciuria.

When rapidity of action is required, 1,25(OH)2D3 (calcitriol), 0.25–1 g daily, is the vitamin D metabolite of choice, since it is capable of raising serum calcium within 24–48 hours. Calcitriol also raises serum phosphate, though this action is usually not observed early in treatment. The combined effects of calcitriol and all other vitamin D metabolites and analogs on both calcium and phosphate make careful monitoring of these mineral levels especially important to avoid ectopic calcification secondary to an abnormally high serum calcium x phosphate product. Since the choice of the

levels of high-energy organic

Vitamin D deficiency, once thought to be rare in this country, is being recognized more often, especially in the pediatric and geriatric populations on vegetarian diets and with reduced sunlight exposure. This problem can be avoided by daily intake of 400–800 units of vitamin D and treated by higher dosages (4000 units per day). No other metabolite is indicated. The diet should also

contain adequate amounts of calcium and phosphate.

Use of Vitamin D Preparations

The choice of vitamin D preparation to be used in the setting of chronic renal failure in the dialysis patient depends on the type and extent of bone disease and hyperparathyroidism. No consensus has been reached regarding the advisability of using any vitamin D metabolite in the predialysis patient. 1,25(OH)2D3 (calcitriol) will rapidly correct hypocalcemia and at least partially reverse the secondary hyperparathyroidism and osteitis fibrosa. Many patients with muscle weakness and bone pain gain an improved sense of well-being.

Dihydrotachysterol, an analog of 1,25(OH)2D, is also available for clinical use, though it is used much less frequently than calcitriol. Dihydrotachysterol appears to be as effective as calcitriol, differing principally in its time course of action; calcitriol increases serum calcium in 1–2 days, whereas dihydrotachysterol requires 1–2 weeks. For an equipotent dose (0.2 mg dihydrotachy-sterol versus 0.5 g calcitriol), dihydrotachysterol costs about one fourth as much as calcitriol. Adisadvantage of dihydrotachysterol is the inability to measure it in serum. Neither dihydrotachysterol nor calcitriol corrects the osteomalacic component of renal osteodystrophy in the majority of patients, and neither should be used in patients with hypercalcemia, especially if the bone disease is primarily osteomalacic.

Calcifediol (25[OH]D3) may also be used to advantage. Calcifediol is less effective than calcitriol in stimulating intestinal calcium transport, so that hypercalcemia is less of a problem with calcifediol.

Like dihydrotachysterol, calcifediol requires several weeks to restore normocalcemia in hypocalcemic individuals with chronic renal failure. Presumably because of the reduced ability of the diseased kidney to metabolize calcifediol to more active metabolites, high doses (50–100 g daily) must be given to achieve the supraphysiologic serum levels required for therapeutic effectiveness.

Vitamin D has been used in treating renal osteodystrophy. However, patients with a substantial degree of renal failure who are thus unable to convert vitamin D to its active metabolites usually are refractory to vitamin D. Its use is decreasing as more effective alternatives become available.

Two analogs of calcitriol, doxercalciferol and paricalcitol, are approved for the treatment of secondary hyperparathyroidism of chronic renal failure. Their principal advantage is that they are less likely than calcitriol to induce hypercalcemia. Their biggest impact will be in patients in whom the use of calcitriol may lead to unacceptably high serum calcium levels.

Regardless of the drug employed, careful attention to serum calcium and phosphate levels is required. Calcium supplements (dietary and in the dialysate) and phosphate restriction (dietary and with oral ingestion of phosphate binders) should be employed along with the use of vitamin D metabolites. Monitoring serum PTH and alkaline phosphatase levels is useful in determining whether therapy is correcting or preventing secondary hyperparathyroidism.

Although not generally available, percutaneous bone biopsies for quantitative histomorphometry may help in choosing appropriate therapy and following the effectiveness of such therapy. Unlike the rapid changes in serum values, changes in bone morphology require months to years. Monitoring serum levels of the vitamin D metabolites is useful to determine compliance, absorption, and metabolism.

The common features that appear to be important in this group of diseases are malabsorption of calcium and vitamin D. Liver disease may, in addition, reduce the production of 25(OH)D from vitamin D, though the importance of this in all but patients with terminal liver failure remains in dispute. The malabsorption of vitamin D is probably not limited to exogenous vitamin D. The liver secretes into bile a substantial number of vitamin D metabolites and conjugates that are reabsorbed in (presumably) the distal jejunum and ileum. Interference with this process could deplete the body of endogenous vitamin D metabolites as well as limit absorption of dietary vitamin D.

In mild forms of malabsorption, vitamin D (25,000–50,000 units three times per week) should suffice to raise serum levels of 25(OH)D into the normal range. Many patients with severe disease do not respond to vitamin D. Clinical experience with the other metabolites is limited, but both calcitriol and calcifediol have been used successfully in doses similar to those recommended for treatment of renal osteodystrophy. Theoretically, calcifediol should be the drug of choice under these conditions, since no impairment of the renal metabolism of 25(OH)D to 1,25(OH)2D and 24,25(OH)2D exists in these patients. Both calcitriol and 24,25(OH)2D may be of importance in reversing the bone disease. As in the other diseases discussed, treatment of intestinal osteodystrophy with vitamin D and its metabolites should be accompanied by appropriate dietary calcium supplementation and monitoring of serum calcium and phosphate levels.

This protein carboxylation is physiologically coupled with the oxidative deactivation of vitamin K. The anticoagulant prevents reductive metabolism of the inactive vitamin K epoxide back to its active hydroquinone form. Mutational change of the responsible enzyme, vitamin K epoxide reductase, can give rise to genetic resistance to warfarin in humans and especially in rats.

There is an 8- to 12-hour delay in the action of warfarin. Its anticoagulant effect results from a balance between partially inhibited synthesis and unaltered degradation of the four vitamin Kdependent clotting factors. The resulting inhibition of coagulation is dependent on their degradation rate in the circulation. These half-lives are 6, 24, 40, and 60 hours for factors VII, IX, X, and II, respectively. Larger initial doses of warfarin—up to about 0.75 mg/kg—hasten the onset of the anticoagulant effect. Beyond this dosage, the speed of onset is independent of the dose size. The only effect of a larger loading dose is to prolong the time that the plasma concentration of drug remains above that required for suppression of clotting factor synthesis. The only difference among oral anticoagulants in producing and maintaining hypoprothrombinemia is the half-life of each drug.

Toxicity

Warfarin crosses the placenta readily and can cause a hemorrhagic disorder in the fetus. Furthermore, fetal proteins with -carboxyglutamate residues found in bone and blood may be affected by warfarin; the drug can cause a serious birth defect characterized by abnormal bone formation. Thus, warfarin should never be administered during pregnancy. Cutaneous necrosis with reduced activity of protein C sometimes occurs during the first weeks of therapy. Rarely, the same process causes frank infarction of breast, fatty tissues, intestine, and extremities. The pathologic lesion associated with the hemorrhagic infarction is venous thrombosis, suggesting that it is caused by warfarin-induced depression of protein C synthesis.

The most serious interactions with warfarin are those that increase the anticoagulant effect and the risk of bleeding. The most dangerous of these interactions are the pharmacokinetic interactions with the pyrazolones phenylbutazone and sulfinpyrazone.

The mechanisms for their hypoprothrombinemic interaction are a stereoselective inhibition of oxidative metabolic transformation of S-warfarin (the more potent isomer) and displacement of albumin-bound warfarin, increasing the free fraction. For this and other reasons, neither phenylbutazone nor sulfinpyrazone is in common use in the USA. Metronidazole, fluconazole, and trimethoprim-sulfamethoxazole also stereoselectively inhibit the metabolic transformation of S-warfarin, whereas amiodarone, disulfiram, and cimetidine inhibit metabolism of both enantiomorphs of warfarin. Aspirin, hepatic disease, and hyperthyroidism augment warfarin pharmacodynamically—aspirin by its effect on platelet function and the latter two by increasing the turnover rate of clotting factors. The third-generation cephalosporins eliminate the bacteria in the intestinal tract that produce vitamin K and, like warfarin, also directly inhibit vitamin K epoxide reductase. Heparin directly prolongs the prothrombin time by inhibiting the activity of several clotting factors.

Barbiturates and rifampin cause a marked decrease of the anticoagulant effect by induction of the hepatic enzymes that transform racemic warfarin. Cholestyramine binds warfarin in the intestine and reduces its absorption and bioavailability.

Pharmacodynamic reductions of anticoagulant effect occur with vitamin K (increased synthesis of clotting factors), the diuretics chlorthalidone and spironolactone (clotting factor concentration), hereditary resistance (mutation of vitamin K reactivation cycle molecules), and hypothyroidism (decreased turnover rate of clotting factors).

Enzymes drugs

The immune response occurs when immunologically competent cells are activated in response to foreign organisms or antigenic substances liberated during the acute or chronic inflammatory response. The outcome of the immune response for the host may be beneficial, as when it causes invading organisms to be phagocytosed or neutralized. On the other hand, the outcome may be deleterious if it leads to chronic inflammation without resolution of the underlying injurious process. Chronic inflammation involves the release of a number of mediators that are not prominent in the acute response. One of the most important conditions involving these mediators is rheumatoid arthritis, in which chronic inflammation results in pain and destruction of bone and cartilage that can lead to severe disability and in which systemic changes occur that can result in shortening of life.

Purine Nucleoside Kinase

The biological importance of enzymes

Exogenous adenosine is the precursor of the entire purine nucleotide pool in T vaginalis through its partial conversion to inosine and the action of purine nucleoside kinase, a unique enzyme in the organism, which converts adenosine and inosine to the corresponding nucleotides. It performs a critical role in T vaginalis purine salvage and has a unique substrate specificity suitable as a target of chemotherapy.

Background On Enzymes

The cell damage associated with inflammation acts on cell membranes to cause leukocytes to release lysosomal enzymes; arachidonic acid is then liberated from precursor compounds, and various eicosanoids are synthesized. Compounds, the cyclooxygenase pathway of arachidonate metabolism produces prostaglandins, which have a variety of effects on blood vessels, on nerve endings, and on cells involved in inflammation. The discovery of cyclooxygenase (COX) isoforms (COX-1 and COX-2) led to the concepts that the constitutive COX-1 isoform tends to be homeostatic in function, while COX-2 is induced during inflammation and tends to facilitate the inflammatory response. On this basis, highly selective COX-2 inhibitors have been developed and marketed on the assumption that such selective inhibitors would be safer than nonselective COX-1 inhibitors but without loss of efficacy. The lipoxygenase pathway of arachidonate metabolism yields leukotrienes, which have a powerful chemotactic effect on eosinophils, neutrophils, and macrophages and promote bronchoconstriction and alterations in vascular permeability. Kinins, neuropeptides, and histamine are also released at the site of tissue injury, as are complement components, cytokines, and other products of leukocytes and platelets. Stimulation of the neutrophil membranes produces oxygen-derived free radicals.

Superoxide anion is formed by the reduction of molecular oxygen, which may stimulate the production of other reactive molecules such as hydrogen peroxide and hydroxyl radicals. The interaction of these substances with arachidonic acid results in the generation of chemotactic substances, thus perpetuating the inflammatory process.

CAZy ~ Carbohydrate-Active enZymes

Reduction of inflammation with nonsteroidal anti-inflammatory drugs (NSAIDs) often results in relief of pain for significant periods. Furthermore, most of the nonopioid analgesics (aspirin, etc) also have anti-inflammatory effects, so they are appropriate for the treatment of both acute and chronic inflammatory conditions.

The enzymes are found in the ...

Another important group of agents are characterized as slow-acting antirheumatic drugs (SAARDs) or disease-modifying antirheumatic drugs (DMARDs). They may slow the bone damage associated with rheumatoid arthritis and are thought to affect more basic inflammatory mechanisms than do the NSAIDs. Unfortunately, they may also be more toxic than the nonsteroidal anti-inflammatory agents.

Nonsteroidal Anti-Inflammatory Drugs

Salicylates and other similar agents used to treat rheumatic disease share the capacity to suppress the signs and symptoms of inflammation. These drugs also exert antipyretic and analgesic effects, but it is their anti-inflammatory properties that make them most useful in the management of disorders in which pain is related to the intensity of the inflammatory process.

Although all NSAIDs are not FDA-approved for the whole range of rheumatic diseases, all are probably effective in rheumatoid arthritis, seronegative spondyloarthropathies (eg, psoriatic arthritis and arthritis associated with inflammatory bowel disease), osteoarthritis, localized musculoskeletal syndromes (eg, sprains and strains, low back pain), and gout (except tolmetin, which appears to be ineffective in gout). Since aspirin, the original NSAID, has a number of adverse effects, many other NSAIDs have been developed in attempts to improve upon aspirin's efficacy and decrease its toxicity.

Chemistry & Pharmacokinetics

This chemical diversity yields a broad range of pharmacokinetic characteristics. Although there are many differences in the kinetics of NSAIDs, they have some general properties in common. All but one of the NSAIDs are weak organic acids as given; the exception, nabumetone, is a ketone prodrug that is metabolized to the acidic active drug. Most of these drugs are well absorbed, and food does not substantially change their bioavailability. Most of the NSAIDs are highly metabolized, some by phase I followed by phase II mechanisms and others by direct glucuronidation (phase II) alone. Metabolism of most NSAIDs proceeds, in part, by way of the CYP3A or CYP2C families of P450 enzymes in the liver. While renal excretion is the most important route for final elimination, nearly all undergo varying degrees of biliary excretion and reabsorption (enterohepatic circulation). In fact, the degree of lower gastrointestinal tract irritation correlates with the amount of enterohepatic circulation. Most of the NSAIDs are highly proteinbound ( 98%), usually to albumin. Some of the NSAIDs (eg, ibuprofen) are racemic mixtures, while one, naproxen, is provided as a single enantiomer and a few have no chiral center (eg, diclofenac).

Pharmacodynamics

The anti-inflammatory activity of the NSAIDs is mediated chiefly through inhibition of biosynthesis of prostaglandins. Various NSAIDs have additional possible mechanisms of action, including inhibition of chemotaxis, down-regulation of interleukin-1 production, decreased production of free radicals and superoxide, and interference with calcium-mediated intracellular events. Aspirin irreversibly acetylates and blocks platelet cyclooxygenase, while most non-COXselective NSAIDs are reversible inhibitors. Selectivity for COX-1 versus COX-2 is variable and incomplete for the older members, but highly selective COX-2 inhibitors (celecoxib, rofecoxib, and valdecoxib) are now available and other highly selective coxibs are being developed. The highly selective COX-2 inhibitors do not affect platelet function at their usual doses. In testing using human whole blood, aspirin, indomethacin, piroxicam, and sulindac were somewhat more effective in inhibiting COX-1; ibuprofen and meclofenamate inhibited the two isozymes about equally. The

efficacy of COX-2-selective drugs equals that of the older NSAIDs, while gastrointestinal safety may be improved. On the other hand, highly selective COX-2 inhibitors may increase the incidence of edema and hypertension.

The NSAIDs decrease the sensitivity of vessels to bradykinin and histamine, affect lymphokine production from T lymphocytes, and reverse vasodilation. To varying degrees, all newer NSAIDs are analgesic, anti-inflammatory, and antipyretic, and all (except the COX-2-selective agents and the nonacetylated salicylates) inhibit platelet aggregation. NSAIDs are all gastric irritants as well, though as a group the newer agents tend to cause less gastric irritation than aspirin. Nephrotoxicity has been observed for all of the drugs for which extensive experience has been reported, and hepatotoxicity can also occur with any NSAID.

Although these drugs effectively inhibit inflammation, there is no evidence that—in contrast to drugs such as methotrexate and gold—they alter the course of an arthritic disorder.

Aspirin

Aspirin's long use and availability without prescription diminishes its glamour compared to that of the newer NSAIDs. Aspirin is now rarely used as an anti-inflammatory medication; it has been replaced by ibuprofen and naproxen, since they are effective, are also available over the counter, and have good to excellent safety records.

Pharmacokinetics

Salicylic acid is a simple organic acid with a pKa of 3.0. Aspirin (acetylsalicylic acid; ASA) has a pKa of 3.5. Sodium salicylate and aspirin are equally effective anti-inflammatory drugs, though aspirin may be more effective as an analgesic. The salicylates are rapidly absorbed from the stomach and upper small intestine, yielding a peak plasma salicylate level within 1–2 hours. Aspirin is absorbed as such and is rapidly hydrolyzed (serum half-life 15 minutes) to acetic acid and salicylate by esterases in tissue and blood. Salicylate is bound to albumin, but the binding is saturable so that the unbound fraction increases as total concentration increases. Ingested salicylate and that generated by the hydrolysis of aspirin may be excreted unchanged, but the metabolic pathways for salicylate disposition become saturated when the total body load of salicylate exceeds 600 mg. Beyond this amount, increases in salicylate dosage increase salicylate concentration disproportionately. As doses of aspirin increase, salicylate elimination halflife increases from 3–5 hours (for 600 mg/d dosage) to 12–16 hours (dosage > 3.6 g/d).

Alkalinization of the urine increases the rate of excretion of free salicylate and its water-soluble conjugates.

Mechanisms of Action

Anti-Inflammatory Effects

Aspirin is a nonselective inhibitor of both COX isoforms, but salicylate is much less effective in inhibiting either isoform. Nonacetylated salicylates may work as oxygen radical scavengers. Aspirin irreversibly inhibits COX and inhibits platelet aggregation, while nonacetylated salicylates do not.

Aspirin also interferes with the chemical mediators of the kallikrein system, thus inhibiting granulocyte adherence to damaged vasculature, stabilizing lysosomes, and inhibiting the chemotaxis of polymorphonuclear leukocytes and macrophages.

Analgesic Effects

Aspirin is most effective in reducing pain of mild to moderate intensity through its effects on inflammation and because it probably inhibits pain stimuli at a subcortical site.

Antipyretic Effects

Aspirin reduces elevated temperature, whereas normal body temperature is only slightly affected.

Aspirin's antipyretic effect is probably mediated by both COX inhibition in the central nervous system and inhibition of IL-1 (which is released from macrophages during episodes of inflammation).

Antiplatelet Effects

Single low doses of aspirin (81 mg daily) produce a slightly prolonged bleeding time, which doubles if administration is continued for a week. The change is due to irreversible inhibition of platelet COX, so that aspirin's antiplatelet effect lasts 8–10 days (the life of the platelet).

Clinical Uses

Analgesia, Antipyresis, and Anti-Inflammatory Effects

Aspirin is employed for mild to moderate pain of varied origin but is not effective for severe visceral pain. Aspirin and other NSAIDs have been combined with opioid analgesics for treatment of cancer pain, where their anti-inflammatory effects act synergistically with the opioids to enhance analgesia. High-dose salicylates are effective for treatment of rheumatic fever, rheumatoid arthritis, and other inflammatory joint conditions.

Other Effects

Aspirin decreases the incidence of transient ischemic attacks, unstable angina, coronary artery thrombosis with myocardial infarction, and thrombosis after coronary artery bypass grafting .

Epidemiologic studies suggest that long-term use of aspirin at low dosage is associated with a lower incidence of colon cancer, possibly related to its COX-inhibiting effects.

Dosage

The optimal analgesic or antipyretic dose of aspirin is less than the 0.6–0.65 g oral dose commonly used. Larger doses may prolong the effect. The usual dose may be repeated every 4 hours. The anti-inflammatory dose for children is 50–75 mg/kg/d in divided doses and the average starting anti-inflammatory dose for adults is 45 mg/kg/d in divided doses.

Adverse Effects

At the usual dosage, aspirin's main adverse effects are gastric upset (intolerance) and gastric and duodenal ulcers, while hepatotoxicity, asthma, rashes, and renal toxicity occur less frequently.

Upper gastrointestinal bleeding associated with aspirin use is usually related to erosive gastritis. A 3 mL increase in fecal blood loss is routinely associated with aspirin administration; the blood loss is greater for higher doses. On the other hand, some mucosal adaptation occurs in many patients, so that blood loss declines back to baseline over 4–6 weeks; ulcers have been shown to heal while aspirin was taken concomitantly.

With higher doses, patients may experience "salicylism"—vomiting, tinnitus, decreased hearing, and vertigo—reversible by reducing the dosage. Still larger doses of salicylates cause hyperpnea through a direct effect on the medulla. At toxic salicylate levels, respiratory alkalosis followed by metabolic acidosis (salicylate accumulation), respiratory depression, and even cardiotoxicity and glucose intolerance can occur. Two grams or less of aspirin daily usually increases serum uric acid levels, whereas doses exceeding 4 g daily decrease urate levels. Like other NSAIDs, aspirin can cause elevation of liver enzymes (a frequent but mild effect), hepatitis (rare), decreased renal function, bleeding, rashes, and asthma.

The antiplatelet action of aspirin contraindicates its use by patients with hemophilia. Although previously not recommended during pregnancy, aspirin may be valuable in treating preeclampsiaeclampsia. When overdosing occurs, gastric lavage is advised and an alkaline, high urine output state should be maintained. Hyperthermia and electrolyte abnormalities should be treated. In severe toxic reactions, ventilatory assistance may be required.

Sodium bicarbonate infusions may be employed to alkalinize the urine, which will increase the amount of salicylate excreted.

Nonacetylated Salicylates

These drugs include magnesium choline salicylate, sodium salicylate, and salicylsalicylate. All nonacetylated salicylates are effective anti-inflammatory drugs, though they may be less effective analgesics than aspirin. Because they are much less effective than aspirin as cyclooxygenase inhibitors, they may be preferable when cyclooxygenase inhibition is undesirable, such as in patients with asthma, those with bleeding tendencies, and even (under close supervision) those with renal dysfunction.

The nonacetylated salicylates are administered in the same dosage as aspirin and can be monitored using serum salicylate measurements.

COX-2 Selective Inhibitors

COX-2 selective inhibitors, or coxibs, were developed in an attempt to inhibit prostacyclin synthesis by the COX-2 isoenzyme induced at sites of inflammation without affecting the action of the constitutively active "housekeeping" COX-1 isoenzyme found in the gastrointestinal tract, kidneys, and platelets. Coxibs selectively bind to and block the active site of the COX-2 enzyme much more effectively than that of COX-1. COX-2 inhibitors have analgesic, antipyretic, and anti-inflammatory effects similar to those of nonselective NSAIDs but with fewer gastrointestinal side effects. Likewise, COX-2 inhibitors have been shown to have no impact on platelet aggregation, which is mediated by the COX-1 isoenzyme. As a result, COX-2 inhibitors do not offer the cardioprotective effects of traditional nonselective NSAIDs, which has resulted in some patients taking low-dose aspirin in addition to a coxib regimen to maintain this effect. Unfortunately, because COX-2 is constitutively active within the kidney, recommended doses of COX-2 inhibitors cause renal toxicities similar to those associated with traditional NSAIDs. They are not recommended for patients with severe renal insufficiency. Furthermore, some clinical data have suggested a higher incidence of cardiovascular thrombotic events associated with COX-2 inhibitors such as rofecoxib, but this issue has not yet been settled. Data from animal studies have also pointed to the role of the COX-2 enzyme in bone repair, resulting in a recommendation for short-term use of different drugs in postoperative patients and those undergoing bone repair. COX-2 inhibitors have been recommended mainly for treatment of osteoarthritis and rheumatoid arthritis, but other indications include primary familial adenomatous polyposis, dysmenorrhea, acute gouty arthritis, acute musculoskeletal pain, and perhaps ankylosing spondylitis.

Celecoxib

Celecoxib is a highly selective COX-2 inhibitor—about 10–20 times more selective for COX-2 thanfor COX-1.

Celecoxib is as effective as other NSAIDs in rheumatoid arthritis and osteoarthritis, and in trials it has caused fewer endoscopic ulcers than most other NSAIDs. Because it is a sulfonamide, celecoxib may cause rashes. It does not affect platelet aggregation. It interacts occasionally with warfarin—as would be expected of a drug metabolized via CYP2C9.

The coxibs continue to be investigated to determine whether their effect on prostacyclin production could lead to a prothrombotic state. The frequency of other adverse effects approximates that of other NSAIDs. Celecoxib causes no more edema or renal effects than other members of the NSAID group, but edema and hypertension have been documented.

Etoricoxib

Etoricoxib, a bipyridine derivative, is a second-generation COX-2-selective inhibitor with the highest selectivity ratio of any coxib for inhibition of COX-2 relative to COX-1. It is extensively metabolized by hepatic P450 enzymes followed by renal excretion and has an elimination half-life of 22 hours. Etoricoxib is approved in the United Kingdom for acute treatment of the signs and symptoms of osteoarthritis (60 mg once daily) and rheumatoid arthritis (90 mg once daily), for treatment of acute gouty arthritis (120 mg once daily), and for relief of acute musculoskeletal pain (60 mg once daily). Approval in the United States is pending. Clinical data have demonstrated that 90 mg of etoricoxib once daily has superior efficacy compared with 500 mg of naproxen twice daily for treatment of patients with rheumatoid arthritis over 12 weeks. Other studies have shown etoricoxib to have similar efficacy to traditional NSAIDs for treatment of osteoarthritis, acute gouty arthritis, and primary dysmenorrhea and a gastrointestinal safety profile similar to that of other coxibs. Since etoricoxib has structural similarities to diclofenac, it is appropriate to monitor hepatic function carefully in patients using this drug.

Meloxicam

Meloxicam is an enolcarboxamide related to piroxicam that has been shown to preferentially inhibit COX-2 over COX-1, particularly at its lowest therapeutic dose of 7.5 mg/d. It is not as selective as the other coxibs. The drug is popular in Europe and many other countries for most rheumatic diseases and has recently been approved for treatment of osteoarthritis in the USA. Its efficacy in this condition and rheumatoid arthritis is comparable to that of other NSAIDs. It is associated with fewer clinical gastrointestinal symptoms and complications than piroxicam, diclofenac, and naproxen. Similarly, while meloxicam is known to inhibit synthesis of thromboxane A2, it appears that even at supratherapeutic doses its blockade of thromboxane A2 does not reach levels that result Rofecoxib, a furanose derivative, is a potent, selective COX-2 inhibitor. In the USA, rofecoxib is approved for osteoarthritis and rheumatoid arthritis, and it also appears to be analgesic and antipyretic—in common with other NSAIDs. This drug does not inhibit platelet aggregation and appears to have little effect on gastric mucosal prostaglandins or lower gastrointestinal tract permeability. At high doses it is associated with occasional edema and hypertension. Other toxicities are similar to those of other coxibs.

Diclofenac

Diclofenac is a phenylacetic acid derivative that is relatively nonselective as a cyclooxygenase inhibitor.

Adverse effects occur in approximately 20% of patients and include gastrointestinal distress, occult gastrointestinal bleeding, and gastric ulceration, though ulceration may occur less frequently than with some other NSAIDs. A preparation combining diclofenac and misoprostol decreases upper gastrointestinal ulceration but may result in diarrhea. Another combination of diclofenac and omeprazole was also effective with respect to the prevention of recurrent bleeding, but renal adverse effects were common in high-risk patients. Diclofenac at a dosage of 150 mg/d appears to impair renal blood flow and glomerular filtration rate. Elevation of serum aminotransferases may occur more commonly with this drug than with other NSAIDs. A 0.1% ophthalmic preparation is recommended for prevention of postoperative ophthalmic nflammation and can be used after intraocular lens implantation and strabismus surgery. A topical gel containing 3% diclofenac is effective for solar keratoses. Diclofenac in rectal suppository form can be considered a drug of choice for preemptive analgesia and postoperative nausea.

In Europe, diclofenac is also available as an oral mouthwash and for intramuscular administration. Fenoprofen, a propionic acid derivative, is the NSAID most closely associated with the toxic effect of interstitial nephritis. This rare toxicity may be associated with a local T cell response in renal tissue.

Other adverse effects of fenoprofen include nausea, dyspepsia, peripheral edema, rash, pruritus, central nervous system and cardiovascular effects, tinnitus, and drug interactions. However, the latter effects are less common than with aspirin.

Ibuprofen

Ibuprofen is a simple derivative of phenylpropionic acid. In doses of about 2400 mg daily, ibuprofen is equivalent to 4 g of aspirin in anti-inflammatory effect.

Oral ibuprofen is often prescribed in lower doses (< 2400 mg/d), at which it has analgesic but not anti-inflammatory efficacy. It is available over the counter in low-dose forms under several trade names. A topical cream preparation appears to be absorbed into fascia and muscle; an (S)(–) formulation has been tested. Ibuprofen cream was more effective than placebo cream for the

treatment of primary knee osteoarthritis. A liquid gel preparation of ibuprofen 400 mg provided faster relief and superior overall efficacy in postsurgical dental pain. In comparison with indomethacin, ibuprofen decreases urine output less and also causes less fluid retention than indomethacin. Ibuprofen has been shown to be effective in closing patent ductus arteriosus in preterm infants, with much the same efficacy and safety as indomethacin. Oral ibuprofen is as effective as intravenous administration in this condition.

Gastrointestinal irritation and bleeding occur, though less frequently than with aspirin. The use of ibuprofen concomitantly with aspirin may decrease the total anti-inflammatory effect. The drug is relatively contraindicated in individuals with nasal polyps, angioedema, and bronchospastic reactivity to aspirin. In addition to the gastrointestinal symptoms (which can be modified by ingestion with meals), rash, pruritus, tinnitus, dizziness, headache, aseptic meningitis (particularly in patients with systemic lupus erythematosus), and fluid retention have been reported. Interaction with anticoagulants is uncommon.

The concomitant administration of ibuprofen antagonizes the irreversible platelet inhibition induced by aspirin. Thus, treatment with ibuprofen in patients with increased cardiovascular risk may limit the cardioprotective effects of aspirin. Rare hematologic effects include agranulocytosis and aplastic anemia. Effects on the kidney (as with all NSAIDs) include acute renal failure, interstitial nephritis, and nephrotic syndrome, but these occur very rarely. Finally, hepatitis has been reported.

Indomethacin

Indomethacin, introduced in 1963, is an indole derivative. It is a potent nonselective COX inhibitor and may also inhibit phospholipase A and C, reduce neutrophil migration, and decrease T cell and B cell proliferation. Probenecid prolongs indomethacin's half-life by inhibiting both renal and biliary clearance.

Clinical Uses

Indomethacin enjoys the usual indications for use in rheumatic conditions and is particularly popular for gout and ankylosing spondylitis. In addition, it has been used to treat patent ductus arteriosus. Indomethacin has been tried in numerous small or uncontrolled trials for many conditions, including Sweet's syndrome, juvenile rheumatoid arthritis, pleurisy, nephritic syndrome, diabetes insipidus, urticarial vasculitis, postepisiotomy pain, and prophylaxis of heterotopic ossification in arthroplasty, and many others. An ophthalmic preparation seems to be efficacious for conjunctival inflammation (alone and in combination with gentamicin) to reduce pain after traumatic corneal abrasion. Gingival inflammation is reduced after administration of indomethacin oral rinse. Epidural injections produce a degree of pain relief similar to that achieved with methylprednisolone in postlaminectomy syndrome.

Adverse Effects

At higher dosages, at least a third of patients have reactions to indomethacin requiring discontinuance. The gastrointestinal effects may include abdominal pain, diarrhea, gastrointestinal hemorrhage, and pancreatitis. Headache is experienced by 15–25% of patients and may be associated with dizziness, confusion, and depression. Rarely, psychosis with hallucinations has been reported. Hepatic abnormalities are rare. Serious hematologic reactions have been noted, including thrombocytopenia and aplastic anemia. Hyperkalemia has been reported and is related to inhibition of the synthesis of prostaglandins in the kidney. Renal papillary necrosis has also been observed.

Ketoprofen

Ketoprofen is a propionic acid derivative that inhibits both cyclooxygenase (nonselectively) and lipoxygenase. Concurrent administration of probenecid elevates ketoprofen levels and prolongs its plasma half-life.

The effectiveness of ketoprofen at dosages of 100–300 mg/d is equivalent to that of other NSAIDs in the treatment of rheumatoid arthritis, osteoarthritis, gout, dysmenorrhea, and other painful conditions. In spite of its dual effect on prostaglandins and leukotrienes, ketoprofen is not superior to other NSAIDs. Its major adverse effects are on the gastrointestinal tract and the central nervous system.

Ketorolac

Ketorolac is an NSAID promoted for systemic use mainly as an analgesic, not as an anti-inflammatory drug (though it has typical NSAID properties). The drug does appear to have significant analgesic efficacy and has been used successfully to replace morphine in some situations involving mild to moderate postsurgical pain. It is most often given intramuscularly or intravenously, but an oral dose formulation is available. When used with an opioid, it may decrease the opioid requirement by 25–50%. An ophthalmic preparation is available for anti-inflammatory applications. Toxicities are similar to those of other NSAIDs, although renal toxicity may be more common with chronic use.

Meclofenamate & Mefenamic Acid

Meclofenamate and mefenamic acid inhibit both COX and phospholipase A2.

Meclofenamate appears to have adverse effects similar to those of other NSAIDs, though diarrhea and abdominal pain may be more common; it has no advantages over other NSAIDs. This drug enhances the effect of oral anticoagulants. Meclofenamate is contraindicated in pregnancy; its efficacy and safety have not been established for young children.

Mefenamic acid is probably less effective than aspirin as an anti-inflammatory agent and is clearly more toxic. It should not be used for longer than 1 week and should not be given to children.

Naproxen

Naproxen is a naphthylpropionic acid derivative. It is the only NSAID presently marketed as a single enantiomer, and it is a nonselective COX inhibitor. Naproxen's free fraction is 41% higher in women than in men, though albumin binding is very high in both sexes. Naproxen is effective for the usual rheumatologic indications and is available both in a slow-release formulation and as an oral suspension. A topical preparation and an ophthalmic solution are also available.

The incidence of upper gastrointestinal bleeding in OTC use is low but still double that of OTC ibuprofen (perhaps due to a dose effect). Rare cases of allergic pneumonitis, leukocytoclastic vasculitis, and pseudoporphyria as well as the more common NSAID-associated adverse effects have been noted.

Piroxicam

Piroxicam, an oxicam, is a nonselective COX inhibitor but at high concentrations also inhibits polymorphonuclear leukocyte migration, decreases oxygen radical production, and inhibits lymphocyte function. Its long half-life permits once-daily dosing.

Piroxicam can be used for the usual rheumatic indications. Toxicity includes gastrointestinal symptoms (20% of patients), dizziness, tinnitus, headache, and rash. When piroxicam is used in dosages higher than 20 mg/d, an increased incidence of peptic ulcer and bleeding is encountered.

Clinical Pharmacology of the NSAIDs

All NSAIDs, including aspirin, are about equally efficacious with a few exceptions—tolmetin seems not to be effective for gout, and aspirin is less effective than other NSAIDs (eg, indomethacin) for ankylosing spondylitis. Thus, NSAIDs tend to be differentiated on the basis of toxicity and cost-effectiveness. For example, the gastrointestinal and renal side effects of ketorolac limit its use. Fries et al (1993), using a toxicity index, estimated that indomethacin, tolmetin, and meclofenamate were associated with the greatest toxicity, while salsalate, aspirin, and ibuprofen were least toxic. The selective COX-2 inhibitors were not included in this analysis.

For patients with renal insufficiency, nonacetylated salicylates may be best. Fenoprofen is less used because of its rare association with interstitial nephritis. Diclofenac and sulindac are associated with more liver function test abnormalities than other NSAIDs. The relatively expensive and selective COX-2 inhibitors are probably safest for patients at high risk for gastrointestinal bleeding. These drugs or a nonselective NSAID plus omeprazole or misoprostol may be appropriate in those patients at highest risk for gastrointestinal bleeding; in this subpopulation of patients, they are cost-effective despite their high acquisition costs.

The choice of an NSAID thus requires a balance of efficacy, cost-effectiveness, safety, and numerous personal factors (eg, other drugs also being used, concurrent illness, compliance, medical insurance coverage), so that there is no "best" NSAID for all patients. There may, however, be one or two best NSAIDs for a specific person.

Plasma substitutes

History: Plasma volume expanders are used for treatment of circulatory shock. They restore vascular volume thereby stabilizing circulatory hemodynamics and maintaining tissue perfusion. Two general categories of volume expanders exist: crystalloids and colloids. The crystalloids most commonly used in clinical practice are normal saline (0.9% NaCl) or lactated Ringer's (LR) solutions, although many others are available. Colloids include the naturally occuring plasma substances (albumin, plasma protein fractions) and synthetic colloids (dextran, hetastarch). Debate regarding the preferred general type of volume expander has been ongoing.

Albumin is normally present in the blood and constitutes 50-60% of the plasma proteins and 80-85% of the oncotic pressure. Plasma protein fraction consists of 88% albumin and 12% globulins. Plasma protein fraction is effective in maintaining blood volume, but it does not maintain an increased oncotic pressure. Albumin and plasma protein fraction are derived from pooled human blood, plasma, serum, or placentas. Because of the source of these products, there may be risks for hypotension (secondary to naturally occuring prekallikrein activators) and hepatitis. The purification process used in the preparation of these products reduces this risk.

Albumin has been available since 1942, but the high cost of albumin still makes its use in clinical practice somewhat prohibitive. A recombinant form is due to begin clinical trials in 1995.

Dextran and hetastarch are synthetic colloidal volume expanders. Dextran was first described by a German chemist Schleibler and was approved in 1951 as a 6% solution. Dextran 70/75 was approved by the FDA in 1953 and Dextran 40 in 1967. Hetastarch was later approved by the FDA in 1972. These products are derived by different synthetic methods. Dextran was isolated from solutions of the sugar beet, where it was formed by the action of Leuconostoc mesenteroides, a bacterium. Hetastarch, also known as hydroxyethyl starch, is prepared from amylopectin. Hydroxyethyl ether groups are introduced into amylopectin glucose residues, which retards the rate of degradation of the polymer. Compared to dextran products, hetastarch has improved oncotic effects and less antigenicity. The biggest concern with hetastarch is its effects on coagulation.

Mechanism of Action: Albumin, dextran, and hetastarch produce volume expansion by increasing the oncotic pressure within the intravascular space. Dextran 70, dextran 75 and hetastarch all exert osmotic effects similar to those of albumin. Administration of volume expander products causes water to move from interstitial spaces into the intravascular space, thereby increasing the circulating blood volume. This increased volume causes an increase in central venous pressure, cardiac output, stroke volume, blood pressure, urinary output, and capillary perfusion, and a decrease in heart rate, peripheral resistance, and blood viscosity. In dehydrated patients, albumin has little or clinical effect on circulating blood volume. Administration of a volume of 25% albumin solution causes 3.5 times the administered volume to be drawn into the circulation within 15 minutes. Following a single infusion of dextran circulating blood volume is increased maximally within a few minutes following infusion of dextran 40 and within 1 hour after dextran 70 or 75. Hetastarch produces a volume expansion that is slightly greater than the administered volume, with maximum expansion occurring within minutes. The duration of volume expansion usually lasts for approximately 24 hours for all of these products. Dextran 40, unlike the higher MW dextran products, also improves microcirculation independently of its volume-expanding effects. The exact mechanism of this activity is unknown, but it is believed to occur by minimizing erythrocyte aggregation and/or decreasing blood viscosity.

Dextran 40 is also believed to coat erythrocytes, which maintains erythrocyte electronegativity and, in turn, decreases the attraction between erythrocytes and reduces erythrocyte rigidity which aids in passage through capillaries. Dextran is used clinically in the prophylaxis of venous thrombosis and pulmonary embolism in patients undergoing surgery that carries a high risk of thromboembolic complications (e.g., hip surgery).

Distinguishing Features: Albumin is a low-molecular-weight protein derived from pooled human blood, plasma, serum, or placentas. Commercially available albumin human solutions have no blood-clotting factors, no effective Rh factor, or other antibodies. Dextran is a branched polysaccharide formed by a bacterium, Leuconostoc mesenteroides. Hetastarch is a synthetic polymer, available as a colloidal solution. Albumin is also responsible for the transport of a variety of substances including bilirubin, calcium, and many drugs. Hetastarch has no oxygen-carrying capacity. Albumin is also used in combination with loop diuretics in the treatment of nephrotic syndrome, and in combination with exchange transfusions to bind bilirubin in patients with hyperbilirubinemia and erythroblastosis fetalis. Albumin is also used to replace protein in patients with hypoproteinemia until the cause of the deficiency can be determined.

Dextran

is available in various molecular weights, and these products exhibit different osmotic and pharmacologic properties. Dextran 40 contains molecules of molecular weight (MW) 40,000 daltons while dextran 70 contains molecules of 70,000 daltons.

Both types of products contain molecules of varying molecular weights, some lower and some higher than the stated label. Hetastarch causes an increase in the erythrocyte sedimentation rate (ESR) when added to whole blood and is used to facilitate the collection of granulocytes in leukopheresis. Compared with dextran 75, hetastarch causes a greater increase in the ESR. Some clinicians have strong opinions regarding the pros and cons of using a crystalloid-type versus a colloid-type plasma expander. In a study of 26 patients with hypovolemia and septic shock, the hemodynamic and respiratory effects of NS, albumin, and hetastarch were compared. Patients were administered enough plasma volume expander to reach a target central venous pressure (CVP). Approximately 2 to 4 times greater fluid volume was needed using NS compared to albumin and hetastarch. The only hemodynamic differences included a greater increase in cardiac output and cardiac index in the albumin and hetastarch groups compared to the NS group. Colloid osmotic pressure decreased below baseline in the NS group, resulting in a significantly higher incidence of pulmonary edema in the NS group compared to the albumin and hetastarch groups. Both albumin and hetastarch groups maintained or increased the colloid osmotic pressure compared to baseline. In general there were no significant differences between the albumin and hetastarch groups.

Adverse Reactions: Anaphylactoid reactions can occur with hetastarch, albumin, or any of the dextran preparations. Dextran is formed by a bacterium, Leuconostoc mesenteroides, which contributes to its antigenicity; however, due to improved preparation techniques, the incidence of hypersensitivity reactions is reduced. Of the dextran products, dextran 40 has less potential for causing these adverse reactions. The risk of antigenicity is less with hetastarch compared to dextran. High doses or repeat administration of albumin is more likely to produce anaphylactoid reactions than low doses of albumin. Close observation during the first few minutes of administration of these products is essential. Allergic reactions include urticaria, nasal congestion, wheezing, tightness of the chest, nausea and vomiting, periorbital edema, and hypotension, which can be mild or severe. Volume expander therapy should be stopped at the first sign of allergic reactions. Because substances with a molecular weight of 50,000 or less can be filtered by the glomerulus, dextran 40 could cause renal injury if tubular flow is decreased. Dextran 40 undergoes rapid urinary excretion, increasing the viscosity and specific gravity of urine. Patients with a reduced flow of urine are especially susceptible to tubular stasis and blocking. Adequate hydration is essential during therapy with dextran 40. Dextran 70 contains molecules of roughly 70,000 daltons. Renal failure does not occur with dextran 70 or 75 because of its limited renal clearance.

Aluminum has been detected as a contaminate of albumin products. There have been reports of accumulation of the aluminum ions, with subsequent toxicity (e.g. encephalopathy, osteodystrophy). Aluminum toxicity is more likely to occur in patients with impaired renal function receiving human albumin (e.g. via plasmaphoresis procedures). Volume overload may lead to cardiovascular effects. Excessive administration of albumin, dextran or hetastarch can precipitate cardiac failure, pulmonary edema, peripheral edema of the lower extremities, hypertension, or tachycardia. Hypotension following administration of albumin and plasma protein fraction can occur. Hypotension is due to prekallikrein activators (Hageman-factor fragments) which are found in very low concentrations in albumin products. Prekallikrein activators are found in higher concentrations in plasma protein fraction, causing a higher incidence of hypotension.Bleeding is a major concern with hetastarch therapy. Hetastarch appears to affect total platelet count, and hemodilution can exacerbate this. A prolonged bleeding time, partial thromboplastin time and prothrombin time can result as a temporary adverse effect. Effects on coagulation are minor, however, at volumes of less than 1500 ml or 20 ml/kg.Adverse GI effects have been reported from use of dextran 70 or 75 and hetastarch, including abdominal pain, parotid gland enlargement, nausea, and vomiting.

Antiallergic agent

1. To understand the mechanism of the anti-inflammatory action of glucocorticoids

2. To understand the mechanism of positive and negative transcriptional regulation by the glucocorticoid receptor.

3. To understand the basic mechanism of action of commonly used non-steroidal anti-inflammatory agents

1. The Pathophysiology of Asthma

2. Physiological Actions of Glucocorticoids

3. Mechanism of Action of the Glucocorticoid Receptor

4. Pharmacology of Glucocorticoid Use in Asthma Treatment

5. Other Anti-Inflammatory Agents Used to Treat Asthma

REVIEW OF ASTHMA PATHOPHYSIOLOGY

Asthma is a chronic disorder of the airways that is characterized by reversible airflow obstruction and airway inflammation, persistent airway hyper-reactivity (AHR) and airway remodeling. The pathogenesis of asthma involves several processes. Chronic inflammation of the bronchial mucosa is prominent, with infiltration of activated T-lymphocytes and eosinophils. This results in subepithelial fibrosis and the release of chemical mediators that can damage the epithelial lining of the airway. Many of these mediators are released following activation and degranulation of mast cells in the bronchial tree. Some of these mediators act as chemotactic agents for other inflammatory cells. They also produce mucosal edema, which narrows the airway and stimulates smooth muscle contraction, leading to bronchoconstriction. Excessive production of mucus can cause further airway obstruction by plugging the bronchiolar lumen.

Inflammatory mediators in Asthma: Activation of mast cells results in secretion of several mediators that contribute to the pathogenesis of asthma. These mediators produce bronchconstriction and initiate both the acute inflammatory response and attract cells responsible for maintaining chronic inflammation. IL, interleukin; GM-CSF, granulocyte and macrophage colony-stimulating factor; PG, prostaglandin; TNF, tissue necrosis factor; IFN interferon

IMPORTANT CONCEPT 1. Asthma is an inflammatory disease and thus effective treatments for the chronic management of asthma should be directed to reduce the inflammatory response

The available agents for treating asthma can be divided into two general categories: drugs that inhibit smooth muscle contraction, i.e. bronchodilators (beta-adrenergic agonists, methylxanthines, and anticholinergics) and agents that prevent and/or reverse inflammation, i.e., the "long-term control medications" (glucocorticoids, leukotriene inhibitors and receptor antagonists, and mast cell-stabilizing agents or cromones).

GLUCOCORTICOIDS (CORTICOSTEROIDS)

Physiology

Glucocorticoids (i.e. cortisol [hydrocortisone] in humans) are synthesized in the adrenal cortex at a daily rate of 10 mg/day and exhibit a diurnal pattern of secretion (i.e. 16 µg/dL in blood @ 8 a.m. and 4 µg/dL @ 4 p.m.). These hormones have access to all tissues in the body and exert wide-ranging effects on many organ systems. Under conditions of severe stress, glucocorticoid levels can rise at least 10-fold.

The major physiologic effects of glucocorticoids are:

· Regulation of carbohydrate, protein, and lipid metabolism

· Maintenance of fluid and electrolyte balance

· Preservation of normal function of the cardiovascular system, the immune system, the kidneys, skeletal muscle, the endocrine system, and the nervous system

· Preservation of organismal homeostasis

The impact of glucocorticoids on homeostasis is illustrated by the potent anti-inflammatory and immunosuppressive actions of these hormones.

IMPORTANT CONCEPT. The anti-inflammatory and immunosuppressive actions of glucocorticoids play an important role in preventing potentially damaging effects of an unopposed inflammatory response and can be exploited therapeutically.

Given the various tissues that are affected by glucocorticoids, systemic treatment with pharmacological doses of glucocorticoids generates many adverse side effects. Since physiological glucocorticoids (i.e. cortisol) bind with reasonably high affinity to the mineralocorticoid receptor, alterations in fluid and electrolyte handling (mediated physiologically by the mineralocorticoid receptor) and ensuing hypertension are important side effects of glucocorticoid therapy.

IMPORTANT CONCEPT. The beneficial effects of glucocorticoids to limit inflammation is counter-balanced by its many adverse side effects

Mechanism of Action

The Glucocorticoid Receptor

Glucocorticoid effects in target tissues are mediated by a single receptor protein, the glucocorticoid receptor, which is a member of the nuclear receptor (NR) superfamily. The glucocorticoid receptor, like all NRs, is a transcription factor that exerts most of its physiological effects through the positive or negative regulation of specific target genes. Thus, many of the changes in cellular physiology that result from glucocorticoid exposure are not acute and require hours or even days to develop.

Glucocorticoid Regulation of Gene Expression

Each tissue and cell type contains a distinct set of target genes that are regulated by glucocorticoids. Specific sequences within genes are recognized by the hormone-bound glucocorticoid receptor. The binding of the glucocorticoid receptor to target gene sequences can either lead to increased or decreased transcription of that gene. However, many other transcription factors (directly bound to gene sequences) and transcription cofactors (recruited to gene sequences through protein-protein interactions) are required for the glucocorticoid receptor to exert its effects on transcription.

IMPORTANT CONCEPT. Tissue- and cell type-specific effects of glucocorticoids are likely to be driven by many factors that influence the gene regulatory activity of the glucocorticoid receptor.

Glucocorticoid Repression of Inflammatory Modulator Gene Expression

The transcription factors Nuclear Factor-kappa B (NF-kB) and AP-1 regulate a number of genes of the immune system and are subject to activation by many external stimuli. When bound to hormone, the glucocorticoid receptor can inhibit the action of NF-kB and AP-1 on many genes and thereby lead to repression of transcription of many genes that are activated in an immune or inflammatory response. These effects of glucocorticoids are observed in many cells of the immune system.

IMPORTANT CONCEPT. The broad anti-inflammatory actions of glucocorticoids are due primarily to transcriptional repression of many pro-inflammatory genes in multiple cell types by the glucocorticoid receptor.

Modulation of Chromatin Structure of Target Genes by Glucocorticoid Receptors

DNA within the nucleus is packaged into chromatin due to its association with basic proteins known as histones. In general, the extent of transcription from a given gene is influenced by the tightness of its binding to histones. Actively transcribed genes are generally associated with less condensed chromatin while inactive genes are associated with more condensed chromatin. When associated with its target genes, the glucocorticoid receptor also recruits large protein complexes that function to modify the chromatin structure of target genes. Thus, when activating gene transcription the glucocorticoid receptor recruits enzymes such as Histone Acetyltransferases (HATs) to the gene. Increased histone acetylation by HATs neutralizes some histone basic character and “loosens” their grip on DNA. When repressing gene transcription, the glucocorticoid receptor recruits enzymes such as Histone Deacetylases (HDAC) to the gene. Decreased histone acetylation by HDACs restores histone basic character and “tightens” their grip on DNA.

IMPORTANT CONCEPT. The glucocorticoid receptor regulates gene transcription (either positively or negatively) through the gene-selective recruitment of histone modifying enzymes.

Pharmacology

Structure/Activity Relationships

Chemical modification of cortisol can dramatically influence its half-life and efficacy. For example, prednisolone has enhanced glucocorticoid activity with reduced mineralocorticoid activity. Prednisolone is also metabolized much more slowly than cortisol. The fluorinated glucocorticoids dexamethasone and betamethasone have very long half-lives, are potent glucocorticoids, and have no detectable mineralocorticoid action. Cortisone must be enzymatically reduced by 11ß-hydroxysteroid reducatase (typically in liver) to order to be active.

IMPORTANT CONCEPT. Structural modifications of the natural glucocorticoid cortisol generate hormones with enhanced half-life and more potent and efficacious glucocorticoid activity

Glucocorticoid Withdrawal

Since glucocorticoids suppress their own synthesis through a feedback mechanism that operates at the pituitary (i.e. reduced ACTH synthesis) and the brain (reduced CRH synthesis), rapid cessation of glucocorticoid therapy leads to acute adrenal insufficiency, which can be debilitating.

IMPORTANT CONCEPT. The cessation of high dose, systemic glucocorticoid treatment must be gradual to limit acute adrenal insufficiency.

IMPORTANT CONCEPT. The aerosol delivery of glucocorticoids to the lungs limits systemic exposure to the hormone and greatly reduces side effects.

Some new analogs of potent glucocorticoids are being used (i.e. fluticasone propionate, the active component of FLONASE) that are subjected to rapid inactivation in liver. The only circulating metabolite of fluticasone propionate detected in man is its 17ß-carboxylic acid derivative, which is formed through the cytochrome P450 3A4 pathway. This inactive metabolite had less affinity (approximately 1/2,000) than the parent drug for the glucocorticoid receptor of human lung cytosol in vitro and negligible pharmacological activity in animal studies. Thus, the bioavailability of these drugs is negligible outside of the airways. The risk of systemic effects due to improper inhalation and swallowing of the drug is dramatically reduced.

Glucocorticoids can also be combined with long-acting ß-adrenergic receptor agonists in a single inhaler devise. This can lead to an enhancement of the anti-inflammatory action of glucocorticoids at lower doses.

IMPORTANT CONCEPT. New generation synthetic glucocorticoids with more rapid metabolism in the liver overcome potential side effects due to ingested hormone upon aerosol delivery.

Another strategy currently being evaluated by many researchers is the development of “dissociated” glucocorticoids. These compounds are expected to unleash the gene repression activity of the glucocorticoid receptor while having little or lessened impact on the gene activation activity of the receptor. Since the anti-inflammatory actions of glucocorticoids are mainly but not exclusively due to gene repression, these compounds should still have anti-inflammatory activity but reduced side effects. The detrimental side effects of glucocorticoids are thought to be due principally to gene activation by the glucocorticoid receptor.

IMPORTANT CONCEPT. New generation synthetic glucocorticoids that maintain gene repression but limit gene activation by the glucocorticoid receptor (i.e. dissociated glucocorticoids) may hold promise as anti-inflammatory drugs with reduced side effects.

Anti-Leukotrienes

The generation of cysteinyl leukotrienes (CysLT) (e.g. LTC4, LTD4 and LTE4) from arachadonic acid requires the action of the 5-lipoxygenase enzyme (5-LOX) and is regulated by various stimuli, cell types, genetics of the host, and cytokine stimulation. Expression, distribution, and activation of specific receptors regulate the actions of CysLTs. Their modulation of the immune response, collagen deposition, and recruitment and activation of inflammatory cells increase chronic airway obstruction and bronchial hyper-responsiveness. Several leukotriene-modifying drugs have been developed for clinical use including leukotriene receptor antagonists (Zafirlukast [ACCOLATE], montelukast [SINGULAIR]) and a 5^’-lipoxygenase enzyme inhibitor (Zileuton [ZYFLO]).

Anti-IgE Therapy

Omalizumab (XOLAIR) is a recombinant humanized monoclonal antibody against IgE that is being used for asthma treatment. When bound to Omalizumab, IgE is unable to bind to IgE receptors on mast cells, and this drug thereby blocks the inflammatory process at an early step. Omalizumab also reduces the number of IgE receptors on the surfaces of basophils further enhancing its anti-inflammatory actions.

http://www.youtube.com/watch?v=nudFAzJsTog&feature=related

1. http://www.youtube.com/watch?v=ywdk3BTjK2s&feature=related

2. http://www.youtube.com/watch?v=tbumdvae2SE&feature=related

3. http://www.youtube.com/watch?v=ywdk3BTjK2s&feature=related

4. http://www.youtube.com/watch?v=9OCQzHl960Q&feature=related

5. http://www.youtube.com/watch?v=EDrzDpsK7WY&feature=related

6. http://www.youtube.com/watch?v=NFTL51FvX4Q&feature=related

7. http://www.youtube.com/watch?v=76b7icmEoOA&feature=related

8. http://www.youtube.com/watch?v=tASI4UrS2T0&feature=related

9. http://www.youtube.com/watch?v=3iKFaXIKUrM&feature=related

10. http://www.youtube.com/watch?v=r9rYXk5pHxc&feature=channel

11. http://www.youtube.com/watch?v=R2MfczL8_mk&feature=list_related&playnext=1&list=PL7CC5BD5F10523844

12. http://www.youtube.com/watch?v=qVmu8aXDSMQ&feature=related

13. http://www.youtube.com/watch?v=1WHF0Gq5Wps&feature=related

14. http://www.youtube.com/watch?v=rpVwurEbv5c&feature=related

15. http://www.youtube.com/watch?v=KdeW9mHBTBw&feature=related

16. http://www.youtube.com/watch?v=olFD1R5Gu-A&feature=related

17. http://www.youtube.com/watch?v=z2N1LJ8A5_Q&feature=related

18. http://www.youtube.com/watch?v=G9D-Vfmbt4s&feature=related

19. http://www.youtube.com/watch?v=ejq99wLEMTw&feature=related