PREPARATIONS INFLUENCE ON
THE BLOOD SYSTEM – 1: (Ferri lactas, Ferri sulfas, Ferrum Lek,
Fercoven, Hemostimulinum, Cyanocobalaminum, Acidum folicum, Pentoxilum, Natrii
nucleas, Methyluracilum, Leukogenum, Molgramostinum (leikomax), Natrii phosphas
Ð32)
PREPARATIONS INFLUENCE ON
THE BLOOD SYSTEM –2: (Vicasolum, Fibrinogenum, Etamzilatum,
Hemostatic sponge, Thrombinum, Calcii chloridum, Heparinum, Fraxiparine,
Protamini sulfas, Neodicumarinum, Phenilinum, Acidum acetylsalicilicum,
Dipyridamolum, Ticlopidin (Ticlid), Pentoxiphillinum)
Agents used in Anemias,
Hematopoietic Growth Factors
Hematopoiesis, the production from
undifferentiated stem cells of circulating erythrocytes, platelets, and
leukocytes, is a remarkable process that produces over 200 billion new cells
per day in the normal person and even greater numbers of blood cells in people
with conditions that cause loss or destruction of blood cells. The
hematopoietic machinery resides primarily in the bone marrow in adults and
requires a constant supply of three essential nutrients—iron, vitamin B12, and folic acid—as
well as the presence of hematopoietic
growth factors, proteins that regulate the proliferation and
differentiation of hematopoietic cells. Inadequate supplies of either the
essential nutrients or the growth factors result in deficiency of functional
blood cells.
Erythropoiesis in bone marrow
Anemia, a deficiency in oxygen-carrying erythrocytes,
is the most common and easily treated of these conditions,
but thrombocytopenia and neutropenia are not rare and in some forms are amenable to drug therapy. In this chapter, we
first consider treatment of anemia due to deficiency of iron, vitamin B12, or
folic acid and then turn to the medical use of hematopoietic growth factors to
combat anemia, thrombocytopenia, and neutropenia.
Agents Used in
Anemias
Iron
Basic Pharmacology
Iron deficiency is the most common cause of chronic anemia—anemia that
develops over time. Like other forms of chronic anemia, iron deficiency anemia
leads to pallor, fatigue, dizziness, exertional dyspnea, and other generalized
symptoms of tissue ischemia. The cardiovascular adaptations to chronic
anemia—tachycardia, increased cardiac output, vasodilation—can worsen the
condition of patients with underlying cardiovascular disease. Iron forms the
nucleus of the iron-porphyrin heme ring, which together with globin chains
forms hemoglobin.
Hemoglobin
reversibly binds oxygen and provides the critical mechanism for oxygen delivery
from the lungs to other tissues. In the absence of adequate iron, small
erythrocytes with insufficient hemoglobin are formed, giving rise to microcytic
hypochromic anemia. Pharmacokinetics The body has an elaborate system for
maintaining the supply of the iron required for hematopoiesis.
Absorption
Iron
is normally absorbed in the duodenum and proximal jejunum, though the more
distal small intestine can absorb iron if necessary. The average diet in the
A normal individual without iron deficiency absorbs
5–10% of this iron, or about 0.5–1 mg daily.
Iron absorption increases in response to low iron stores or increased iron
requirements. Total iron absorption increases
to 1–2 mg/d in normal menstruating women and may be as high as 3–4 mg/d in
pregnant women.
Infants
and adolescents also have increased iron requirements during rapid growth
periods. Iron is available in a wide variety of foods but is especially
abundant in meat. The iron in meat protein can be efficiently absorbed, since
heme iron in meat hemoglobin and myoglobin can be absorbed intact without first
having to be broken down into elemental iron. Iron in other foods, especially
vegetables and grains, is often tightly bound to phytates or other complexing
agents and may be much less available for absorption. Nonheme iron in foods and
iron in inorganic iron salts and complexes must be reduced to ferrous (Fe2+)
iron before it can be absorbed by the intestinal mucosal cells. Such absorption
is decreased by the presence of chelators or complexing agents in the
intestinal lumen and is increased in the presence of hydrochloric acid and
vitamin C. Iron crosses the intestinal mucosal cell by active transport. The
rate of iron uptake is regulated by mucosal cell iron stores such that more
iron is transported when stores are low. Together with iron split from absorbed
heme, the newly absorbed iron can be made available for immediate transport
from the mucosal cell to the plasma via transferrin or can be stored in the
mucosal cell as ferritin, a water-soluble complex consisting of a core crystal
of ferric hydroxide covered by a shell of a specialized storage protein called
apoferritin. In general, when total body iron stores are high and iron
requirements by the body are low, newly absorbed iron is diverted into ferritin
in the intestinal mucosal cells rather than being transported to other sites.
When iron stores are low or iron requirements are high, however, newly absorbed
iron is immediately transported from the mucosal cells to the bone marrow for
the production of hemoglobin.
Iron:
possible routes of administration and fate in the organism
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Transport
Iron is transported in the plasma
bound to transferrin, a
-globulin that specifically binds ferric iron. The transferrin-ferric iron
complex enters maturing erythroid cells by a specific receptor mechanism.
Transferrin receptors—integral membrane glycoproteins present in large numbers
on proliferating erythroid cells—bind the transferrin-iron complex and
internalize the iron, releasing it within the cell. The transferrin and
transferrin receptor are then recycled, providing an efficient mechanism for
incorporating iron into hemoglobin in developing red blood cells. Increased
erythropoiesis is associated with an increase in the number of transferrin
receptors on developing erythroid cells. Iron store depletion and iron
deficiency anemia are associated with an increased concentration of serum
transferrin.
Storage
Iron binds avidly to a protein, apoferritin, and
forms the complex ferritin. Iron is stored, primarily as ferritin, in
intestinal mucosal cells and in macrophages in the liver, spleen, and bone.
Apoferritin synthesis is regulated by the levels of free iron.
When
these levels are low, apoferritin synthesis is inhibited and the balance of
iron binding shifts toward transferrin. When free iron levels are high, more
apoferritin is produced in an effort to safely sequester more iron and protect
organs from the toxic effects of excess free iron. Ferritin is also detectable
in plasma. Since the ferritin present in plasma is in equilibrium with storage
ferritin in reticuloendothelial tissues, the plasma (or serum) ferritin level
can be used to estimate total body iron stores.
Elimination
There is no mechanism for excretion of
iron. Small amounts are lost by exfoliation of intestinal mucosal cells into
the stool, and trace amounts are excreted in bile, urine, and sweat. These
losses account for no more than 1 mg of iron per day. Because the body's
ability to increase excretion of iron is so limited, regulation of iron balance
must be achieved by changing intestinal absorption and storage of iron, in
response to the body's needs.
Clinical Pharmacology
Indications for the Use of Iron
The only clinical indication for the use of iron
preparations is the treatment or prevention of iron deficiency anemia.
Iron
deficiency is commonly seen in populations with increased iron requirements.
These include infants, especially premature infants; children during rapid
growth periods; and pregnant and lactating women. Iron deficiency also occurs
frequently after gastrectomy and in patients with severe small bowel disease
that results in generalized malabsorption. Iron deficiency in these
gastrointestinal conditions is due to inadequate iron absorption. The most
common cause of iron deficiency in adults is blood loss. Menstruating women
lose about 30 mg of iron with each menstrual period; women with heavy menstrual
bleeding may lose much more. Thus, many premenopausal women have low iron
stores or even iron deficiency. In men and postmenopausal women, the most
common site of blood loss is the gastrointestinal tract. Patients with
unexplained iron deficiency anemia should be evaluated for occult
gastrointestinal bleeding. As iron deficiency develops, storage iron decreases
and then disappears; next, serum ferritin decreases; and then serum iron
decreases and iron-binding capacity increases, resulting in a decrease in
iron-binding (transferrin) saturation.
Thereafter, anemia begins to develop.
Red cell indices (mean corpuscular volume [MCV]: normal = 80–100 fL; mean
corpuscular hemoglobin concentration [MCHC]: normal = 32–36 g/dL) are usually
low normal when iron deficiency anemia is mild, but cells become
progressively more microcytic (low MCV) and hypochromic (low MCHC) as anemia
becomes more severe. By the time iron deficiency is diagnosed, serum iron is
usually less than 40 g/dL; total iron-binding capacity (TIBC) is greater than
400 g/dL; ironbinding saturation is less than 10%; and serum ferritin is less
than 10 g/L. These laboratory measurements can be used to confirm a diagnosis
of iron deficiency anemia in patients who present with signs and symptoms of
microcytic anemia.
Treatment
The
treatment of iron deficiency anemia consists of administration of oral or
parenteral iron preparations. Oral iron corrects the anemia just as rapidly and
completely as parenteral iron in most cases if iron absorption from the
gastrointestinal tract is normal.
Oral Iron Therapy
A wide variety of oral iron preparations
are available. Since ferrous iron is most efficiently absorbed, only ferrous
salts should be used. Ferrous sulfate, ferrous gluconate, and ferrous fumarate
are all effective and inexpensive and are recommended for the treatment of most
patients. Different iron salts provide different amounts of elemental iron, as
shown in Table 33–2. In an irondeficient individual, about 50–100 mg of iron
can be incorporated into hemoglobin daily, and about 25% of oral iron given as
ferrous salt can be absorbed. Therefore, 200–400 mg of elemental iron should be
given daily to correct iron deficiency most rapidly. Patients unable to
tolerate such large doses of iron can be given lower daily doses of iron, which
results in slower but still complete correction of iron deficiency. Treatment
with oral iron should be continued for 3–6 months. This will correct the anemia
and replenish iron stores.
Common adverse effects of oral iron
therapy include nausea, epigastric discomfort, abdominal cramps, constipation,
and diarrhea. These effects are usually dose-related and can often be overcome
by lowering the daily dose of iron or by taking the tablets immediately after
or with meals. Some patients have less severe gastrointestinal adverse effects
with one iron salt than another and benefit from changing preparations.
Patients taking oral iron develop black stools; this itself has no clinical
significance but may obscure the diagnosis of continued gastrointestinal blood
loss.
Parenteral Iron Therapy
Parenteral therapy should be reserved
for patients with documented iron deficiency unable to tolerate or absorb oral
iron and patients with extensive chronic blood loss who cannot be maintained
with oral iron alone. This includes patients with various postgastrectomy
conditions and previous small bowel resection, inflammatory bowel disease
involving the proximal small bowel, and malabsorption syndromes.
Iron dextran is a stable complex of ferric hydroxide and
low-molecular-weight dextran containing 50 mg of elemental iron per milliliter of
solution. Iron-sucrose complex and iron sodium gluconate complex are
newer, alternative preparations.
These agents can be given either by
deep intramuscular injection or by intravenous infusion. Adverse effects of parenteral
iron therapy include local pain and tissue staining (brown discoloration of the
tissues overlying the injection site), headache, light-headedness, fever,
arthralgias, nausea and vomiting, back pain, flushing, urticaria, bronchospasm,
and, rarely, anaphylaxis and death. Most adults with iron deficiency anemia
require 1–2 g of replacement iron, or 20–40 mL of iron dextran. Most physicians
prefer to give the entire dose in a single intravenous infusion in several
hundred milliliters of normal saline over 1–2 hours. Intravenous administration
eliminates the local pain and tissue staining that often occur with the
intramuscular route and allows delivery of the entire dose of iron necessary to
correct the iron deficiency at one time. There is no clear evidence that any of
the adverse effects, including anaphylaxis, are more likely to occur with
intravenous than with intramuscular administration. Owing to the risk of a
hypersensitivity reaction, a small test dose of iron dextran should always be
given before full intramuscular or intravenous doses are given. Patients with a
strong history of allergy and patients who have previously received parenteral
iron are more likely to have hypersensitivity reactions following treatment
with parenteral iron dextran.
Clinical Toxicity
Acute Iron Toxicity
Acute iron toxicity is seen almost
exclusively in young children who have ingested a number of iron tablets.
Although adults are able to tolerate large doses of oral iron without serious
consequences, as few as ten tablets of any of the commonly available oral iron
preparations can be lethal in young children. Patients taking oral iron
preparations should be instructed to store tablets in child-proof containers
out of the reach of children. Large amounts of oral iron cause necrotizing
gastroenteritis, with vomiting, abdominal pain, and bloody diarrhea followed by
shock, lethargy, and dyspnea. Subsequently, improvement is often noted, but
this may be followed by severe metabolic acidosis, coma, and death. Urgent
treatment of acute iron toxicity is necessary, especially in young children.
Activated charcoal, a highly effective adsorbent for most toxins, does not bind iron and thus is
ineffective. Whole bowel irrigation (see Chapter 59: Management of the Poisoned
Patient) should be performed to flush out unabsorbed pills. Deferoxamine, a
potent iron-chelating compound, can be given systemically to bind iron that has
already been absorbed and to promote its excretion in urine and feces.
Appropriate supportive therapy for gastrointestinal bleeding, metabolic
acidosis, and shock must also be provided.
Chronic Iron Toxicity
Chronic iron toxicity (iron overload), also known as hemochromatosis,
results when excess iron is deposited in the heart, liver, pancreas, and
other organs.
It can lead to organ failure and
death. It most commonly occurs in patients with inherited hemochromatosis, a
disorder characterized by excessive iron absorption, and in patients who
receive many red cell transfusions over a long period of time. Chronic iron
overload in the absence of anemia is most efficiently treated by intermittent
phlebotomy. One unit of blood can be removed every week or so until all of the
excess iron is removed. Iron chelation therapy using parenteral deferoxamine is
much less efficient as well as more complicated, expensive, and hazardous, but
it can be useful for severe iron overload that cannot be managed by phlebotomy.
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
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
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 (Figure 33–1). In
one, methylcobalamin serves as an intermediate in the transfer of a methyl
group from N_5- methyltetrahydrofolate
to methionine (Figure 33–1 A; Figure 33–2, reaction 1).
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 as shown in Figure 33–3, reaction 2.
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 (Figure 33–2, reaction 3)
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
(Figure 33–1 B). 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.
Figure 33–1.
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.)
Figure 33–2.
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
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 (Figure 33–3). 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 (Figure 33–2, reaction 3). 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 (Figure 33–2). 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
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 (Figure 33–2, reaction 1).
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 (Figure 33–2, reaction 2).
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 (Figure 33–1 A; Figure 33–2, reaction
1). 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
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
Hematopoietic Growth
Factors
The
hematopoietic growth factors are glycoprotein hormones that regulate the
proliferation and differentiation of hematopoietic progenitor cells in the bone
marrow. The first growth factors to be identified were called
colony-stimulating factors because they could stimulate the growth of colonies
of various bone marrow progenitor cells in vitro. In the past decade, many of
these growth factors have been purified and cloned, and their effects on
hematopoiesis have been extensively studied. Quantities of these growth factors
sufficient for clinical use are produced by recombinant DNA technology.
Of the known hematopoietic growth factors, erythropoietin
(epoetin alfa), granulocyte colonystimulating factor (G-CSF),
granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin
11 are currently in clinical use.
Thrombopoietin
is undergoing clinical trials and will probably become available soon. Other
potentially useful hematopoietic factors are still indevelopment.
The hematopoietic
growth factors have complex effects on the function of a wide variety of cell
types, including nonhematologic cells. Their utility in other areas of
medicine, particularly as potential anticancer and anti-inflammatory drugs, is
being investigated.
Erythropoietin
Chemistry & Pharmacokinetics
Erythropoietin,
a 34-39 kDA glycoprotein, was the first human hematopoietic growth factor to be
isolated. It was originally purified from the urine of patients with severe
anemia. Recombinant human erythropoietin (rHuEpo, epoetin alfa) is produced in
a mammalian cell expression system using recombinant DNA technology. After
intravenous administration, erythropoietin has a serum half-life of 4–13 hours
in patients with chronic renal failure. It is not cleared by dialysis. It is
measured in international units (IU). Darbopoetin alfa is a glycosylated form
of erythropoietin and differs from it functionally only in having a twofold to
threefold longer half-life.
Pharmacodynamics
Erythropoietin
stimulates erythroid proliferation and differentiation by interacting with
specific erythropoietin receptors on red cell progenitors. It also induces
release of reticulocytes from the bone marrow. Endogenous erythropoietin is
produced by the kidney in response to tissue hypoxia. When anemia occurs, more
erythropoietin is produced by the kidney, signaling the bone marrow to produce
more red blood cells. This results in correction of the anemia provided that
bone marrow response is not impaired by red cell nutritional deficiency
(especially iron deficiency), primary bone marrow disorders (see below), or
bone marrow suppression from drugs or chronic diseases.
Normally
there is an inverse relationship between the hematocrit or hemoglobin level and
the serum erythropoietin level. Nonanemic individuals have serum erythropoietin
levels of less than 20 IU/L. As the hematocrit and hemoglobin levels fall and
anemia becomes more severe, the serum erythropoietin level rises exponentially.
Patients with moderately severe anemias usually have erythropoietin levels in
the 100–500 IU/L range, and patients with severe anemias may have levels of
thousands of IU/L. The most important exception to this inverse relationship is
in the anemia of chronic renal failure. In patients with renal disease, erythropoietin
levels are usually low because the kidneys cannot produce the growth factor.
These patients are the most likely to respond to treatment with exogenous
erythropoietin. In most primary bone marrow disorders (aplastic anemia,
leukemias, myeloproliferative and myelodysplastic disorders, etc) and most
nutritional and secondary anemias, endogenous erythropoietin levels are high,
so there is less likelihood of a response to exogenous erythropoietin (but see
below).
Clinical Pharmacology
The
availability of erythropoietin has had a significant positive impact for
patients with chronic renal failure. Erythropoietin consistently improves the
hematocrit and hemoglobin level and usually eliminates the need for
transfusions in these patients. An increase in reticulocyte count is usually
observed in about 10 days and an increase in hematocrit and hemoglobin levels
in 2–6 weeks. Most patients can maintain a hematocrit of about 35% with
erythropoietin doses of 50–150 IU/kg intravenously or subcutaneously three
times a week. Failure to respond to erythropoietin is most commonly due to
concurrent iron deficiency, which can be corrected by giving oral iron. Folate
supplementation may also be necessary in some patients.
In
selected patients, erythropoietin may also be useful for the treatment of
anemia due to primary bone marrow disorders and secondary anemias. This
includes patients with aplastic anemia and other bone marrow failure states,
myeloproliferative and myelodysplastic disorders, multiple myeloma and perhaps
other chronic bone marrow malignancies, and the anemias associated with chronic
inflammation, AIDS, and cancer. Patients with these disorders who have
disproportionately low serum erythropoietin levels for their degree of anemia
are most likely to respond to treatment with this growth factor. Patients with
endogenous erythropoietin levels of less than 100 IU/L have the best chance of
response, though patients with erythropoietin levels between 100 and 500 IU/L
respond occasionally. These patients generally require higher erythropoietin
doses (150–300 IU/kg three times a week) to achieve a response, and responses
are often incomplete.
Erythropoietin
has been used successfully to offset the anemia produced by zidovudine
treatment in patients with HIV infection and in the treatment of the anemia of
prematurity. It can also be used to accelerate erythropoiesis after
phlebotomies, when blood is being collected for autologous transfusion for
elective surgery, or for treatment of iron overload (hemochromatosis).
Erythropoietin
is one of the drugs banned by the International Olympic Committee. The use of
erythropoietin by athletes is based on their hope that increased red blood cell
concentration will increase oxygen delivery and improve performance.
Toxicity
The most common adverse effects of
erythropoietin are associated with a rapid increase in hematocrit and
hemoglobin and include hypertension and thrombotic complications. These
difficulties can be minimized by raising the hematocrit and hemoglobin slowly and
by adequately monitoring and treating hypertension. Allergic reactions have
been infrequent and mild.
Myeloid Growth Factors
Chemistry & Pharmacokinetics
G-CSF
and GM-CSF, the two myeloid growth factors
currently available for clinical use, were originally purified from cultured
human cell lines. Recombinant human G-CSF (rHuG-CSF; filgrastim) is
produced in a bacterial expression system using recombinant DNA technology. It
is a nonglycosylated peptide of 175 amino acids, with a molecular weight of 18
kDa. Recombinant human GM-CSF (rHuGM-CSF; sargramostim) is produced in a
yeast expression system using recombinant DNA technology. It is a partially
glycosylated peptide of 127 amino acids, with three molecular species with
molecular weights of 15,500, 15,800, and 19,500. These preparations have serum
half-lives of 2–7 hours after intravenous or subcutaneous administration. Pegfilgrastim,
a covalent conjugation product of filgrastim and a form of polyethylene
glycol, has a much longer serum half-life than recombinant G-CSF, and so it can
be injected once per myelosuppressive chemotherapy cycle instead of daily for
several days.
Pharmacodynamics
The
myeloid growth factors stimulate proliferation and differentiation by
interacting with specific receptors found on various myeloid progenitor cells.
These receptors are members of the superfamily of receptors that transduce
signals by association with cytoplasmic tyrosine kinases in the JAK/STAT
pathway. G-CSF stimulates proliferation and differentiation of progenitors
already committed to the neutrophil lineage. It also activates the phagocytic
activity of mature neutrophils and prolongs their survival in the circulation.
G-CSF also has a remarkable ability to mobilize hematopoietic stem cells, ie,
to increase their concentration in peripheral blood. This biologic effect
underlies a major advance in transplantation—the use of peripheral blood stem
cells (PBSCs) instead of bone marrow stem cells for autologous and allogeneic
hematopoietic stem cell transplantation (see below).
GM-CSF has broader biologic actions
than G-CSF. It is a multipotential hematopoietic growth factor that stimulates
proliferation and differentiation of early and late granulocytic progenitor
cells as well as erythroid and megakaryocyte progenitors. Like G-CSF, GM-CSF
also stimulates the function of mature neutrophils. GM-CSF acts together with
interleukin-2 to stimulate T cell proliferation and appears to be a locally
active factor at the site of inflammation. GM-CSF mobilizes peripheral blood stem
cells, but it is significantly less efficacious than G-CSF in this regard.
Clinical Pharmacology
Neutropenia,
a common adverse effect of the cytotoxic drugs used to treat cancer, puts
patients at high risk of serious infection. Unlike the treatment of anemia and
thrombocytopenia, transfusion of neutropenic patients with granulocytes
collected from donors is performed rarely and with limited success. The
introduction of G-CSF in 1991 represented a milestone in the treatment of
chemotherapy-induced neutropenia. This growth factor dramatically accelerates
the rate of neutrophil recovery after dose-intensive myelosuppressive
chemotherapy (Figure 33–4). It reduces the duration of neutropenia and usually
raises the nadir, the lowest neutrophil count seen following a cycle of
chemotherapy.
While
the ability of G-CSF to increase neutrophil counts after myelosuppressive
chemotherapy is nearly universal, its impact upon clinical outcomes is more
variable. Some clinical trials have shown that G-CSF reduces episodes of
febrile neutropenia, requirements for broad-spectrum antibiotics, and days of
hospitalization; however, other trials failed to find these favorable outcomes.
To date, no clinical trial has shown improved survival in cancer patients
treated with GCSF. Clinical guidelines for the use of G-CSF after cytotoxic
chemotherapy have been published (Ozer et al, 2001). These guidelines recommend
reserving G-CSF for patients with a prior episode of febrile neutropenia after
cytotoxic chemotherapy, patients receiving dose-intensive chemotherapy,
patients at high risk of febrile neutropenia, and patients who are unlikely to
survive an episode of febrile neutropenia. Pegfilgrastim is an alternative to
G-CSF for prevention of chemotherapy-induced febrile neutropenia. Pegfilgrastim
can be administered less frequently, and it may shorten the period of severe
neutropenia slightly more than G-CSF. Like G-CSF and pegfilgrastim, GM-CSF also
reduces the duration of neutropenia after cytotoxic chemotherapy. It has been
more difficult to show that GM-CSF reduces the incidence of febrile
neutropenia, probably because GM-CSF itself can induce fever. In the treatment
of chemotherapyinduced neutropenia, G-CSF, 5 g/kg/d, or GM-CSF, 250 g/m2/d, is
usually started within 24–72 hours after completing chemotherapy and is
continued until the absolute neutrophil count is >10,000 cells/ L.
Pegfilgrastim is given as a single dose instead of as daily injections.
The
utility and safety of the myeloid growth factors in the postchemotherapy
supportive care of patients with acute myeloid leukemia (AML) has been the
subject of a number of clinical trials. Since leukemic cells arise from
progenitors whose proliferation and differentiation are normally regulated by
hematopoietic growth factors, including GM-CSF and G-CSF, there was concern
that myeloid growth factors could stimulate leukemic cell growth and increase
the rate of relapse. The results of randomized clinical trials suggest that
both G-CSF and GM-CSF are safe following induction and consolidation treatment
of myeloid and lymphoblastic leukemia.
There
has been no evidence that these growth factors reduce the rate of remission or
increase relapse rate. On the contrary, the growth factors accelerate
neutrophil recovery and reduce infection rates and days of hospitalization.
Both G-CSF and GM-CSF have FDA approval for treatment of patients with AML.
G-CSF
and GM-CSF have also been shown to be effective in treating the neutropenia
associated with congenital neutropenia, cyclic neutropenia, myelodysplasia, and
aplastic anemia. Many patients with these disorders respond with a prompt and
sometimes dramatic increase in neutrophil count. In some cases this results in
a decrease in the frequency of infections. Since G-CSF and GMCSF do not
stimulate the formation of erythrocytes or platelets, they are sometimes used
in combination with other growth factors for treatment of pancytopenia.
The
myeloid growth factors play an important role in autologous stem cell
transplantation for patients undergoing high-dose chemotherapy. High-dose
chemotherapy with autologous stem cell support is increasingly being used to
treat patients with tumors that are resistant to standard doses of
chemotherapeutic drugs. The high-dose regimens produce extreme
myelosuppression; the myelosuppression is then counteracted by reinfusion of
the patient's own hematopoietic stem cells (which are collected prior to
chemotherapy). The administration of G-CSF or GM-CSF early after autologous
stem cell transplantation has been shown to reduce the time to engraftment and
to recovery from neutropenia in patients receiving stem cells obtained either
from bone marrow or from peripheral blood. These effects are seen in patients
being treated for lymphoma or for solid tumors. G-CSF and GM-CSF are also used
to support patients who have received allogeneic bone marrow transplantation
for treatment of hematologic malignancies or bone marrow failure states. In
this setting, the growth factors speed the recovery from neutropenia without
increasing the incidence of acute graft-versus-host disease.
Probably
the most important role of the myeloid growth factors in transplantation is for
mobilization of peripheral blood stem cells (PBSCs). Stem cells collected from peripheral
blood have nearly replaced bone marrow as the hematopoietic preparation used
for autologous transplantation. The cells can be collected in an outpatient
setting with a procedure that avoids much of the risk and discomfort of bone
marrow collection, including the need for general anesthesia. In addition,
there is evidence that PBSC transplantation results in more rapid engraftment
of all hematopoietic cell lineages and in reduced rates of graft failure or
delayed platelet recovery. The use of PBSCs for allogeneic transplantation is
also being investigated. In allogeneic transplantation, donors are treated with
G-CSF in order to mobilize their PBSCs prior to
leukapheresis, the procedure that
separates the fraction containing stem cells from the other components in
blood.
G-CSF
is the cytokine most commonly used for PBSC mobilization because of its
increased efficacy and reduced toxicity compared with GM-CSF. To mobilize stem
cells, patients or donors are given 5–10 g/kg/d subcutaneously for 4 days. On
the fifth day, they undergo leukapheresis.
The
success of PBSC transplantation depends upon transfusion of adequate numbers of
stem cells. CD34, an antigen present on early progenitor cells and absent from
later, committed, cells, is used as a marker for the requisite stem cells. The
goal is to reinfuse at least 5 x 106 CD34 cells/kg; this number of CD34 cells
usually results in prompt and durable engraftment of all cell lineages. It can
take several separate leukaphereses to collect enough CD34 cells, especially
from older patients and patients who have been exposed to radiotherapy or
chemotherapy.
Toxicity
Although the two growth factors have
similar effects on neutrophil counts, G-CSF is used more frequently because it
is better tolerated. G-CSF can cause bone pain, which clears when the drug is
discontinued. GM-CSF can cause more severe side effects, particularly at higher
doses. These include fevers, malaise, arthralgias, myalgias, and a capillary
leak syndrome characterized by peripheral edema and pleural or pericardial
effusions. Allergic reactions may occur but are infrequent. Splenic rupture is
a rare but serious complication of the use of G-CSF for PBSC.
Megakaryocyte
Growth Factors
Chemistry & Pharmacokinetics
Interleukin-11
(IL-11) is a 65–85 kDa protein produced by
fibroblasts and stromal cells in the bone marrow. Oprelvekin, the
recombinant form of interleukin-11 approved for clinical use, is produced by
expression in E coli. The half-life of IL-11 is 7–8 hours when the drug
is injected subcutaneously.
Thrombopoietin,
a 65–85 kDa glycosylated protein, is constitutively
expressed by a variety of organs and cell types. Hepatocytes appear to be the
major source of human thrombopoietin, and patients with cirrhosis and
thrombocytopenia have low serum thrombopoietin levels. Recombinant
thrombopoietin is produced by expression in human cells; the recombinant
product contains two intramolecular disulfide bonds and a number of
carbohydrate side chains.
Pharmacodynamics
Interleukin-11
acts through a specific cell surface cytokine receptor to stimulate the growth
of multiple lymphoid and myeloid cells. It acts synergistically with other
growth factors to stimulate the growth of primitive megakaryocytic progenitors
and, most importantly, increases the number of peripheral platelets and
neutrophils.
Acting
through its own cytokine receptor, thrombopoietin also independently stimulates
the growth of primitive megakaryocytic progenitors. In addition, it stimulates
mature megakaryocytes and even activates mature platelets to respond to
aggregation-inducing stimuli. The critical in vivo role of thrombopoietin has
been demonstrated in genetically engineered knockout mice who lack either
thrombopoietin or its receptor. These mice have marked thrombocytopenia but do not
display anemia or leukopenia.
Clinical Pharmacology
Patients with thrombocytopenia have a
high risk of hemorrhage. While platelet transfusion is commonly used to treat
thrombocytopenia, this procedure can cause adverse reactions in the recipient;
furthermore, a significant number of patients fail to exhibit the expected
increase in platelet count.
Interleukin-11
is the first growth factor to gain FDA approval for treatment of
thrombocytopenia. It is approved for the secondary prevention of
thrombocytopenia in patients receiving cytotoxic chemotherapy for treatment of
nonmyeloid cancers. Clinical trials show that it reduces the number of platelet
transfusions required by patients who experienced severe thrombocytopenia after
a previous cycle of chemotherapy. Although IL-11 has broad stimulatory effects
on hematopoietic cell lineages in vitro, it does not appear to have significant
effects on the leukopenia or neutropenia caused by myelosuppressive
chemotherapy. Interleukin-11 is given by subcutaneous injection at a
dose of 50 g/kg/d. It is started 6–24
hours after completion of chemotherapy and continued for 14–21 days or until
the platelet count passes the nadir and rises to > 50,000 cells/ L.
Recombinant
thrombopoietin is still an investigational agent. The primary focus of current
clinical trials is for the treatment of chemotherapy-induced thrombocytopenia
and thrombocytopenia accompanying hematologic stem cell transplantation. Other
trials are looking into the possibility of administering thrombopoietin to
normal donors in order to increase the number of cells recovered by platelet
apheresis. Approval of the latter application will require that thrombopoietin
be shown to have an excellent short- and long-term safety profile.
Toxicity
The
most common side effects of interleukin-11 are fatigue, headache, dizziness,
and
cardiovascular effects. The
cardiovascular effects include anemia (due to hemodilution), dyspnea (due to
fluid accumulation in the lungs), and transient atrial arrhythmias. Hypokalemia
has also been seen in some patients. All of these adverse effects appear to be
reversible. In the limited clinical trial data available thus far, recombinant
thrombopoietin appears to be well tolerated.
Drugs Used in
Disorsers of Coagulation
Excessive
bleeding and thrombosis may represent altered states of hemostasis. Impaired
hemostasis results in spontaneous bleeding; stimulated hemostasis results in
thrombus formation. The drugs used to arrest abnormal bleeding and to inhibit
thrombosis are the subjects of this chapter.
Mechanisms of Blood Coagulation
Thrombogenesis
Hemostasis
is the spontaneous arrest of bleeding from a damaged blood vessel. The normal
vascular endothelial cell is not thrombogenic, and circulating blood platelets
and clotting factors do not normally adhere to it to an appreciable extent.
The
immediate hemostatic response of a damaged vessel is vasospasm. Within
seconds, platelets stick to the exposed collagen of the damaged endothelium (platelet
adhesion) and to each other (platelet aggregation). Platelets then
lose their individual membranes and form a gelatinous mass during viscous
metamorphosis. This platelet plug quickly arrests bleeding but must
be reinforced by fibrin for long-term effectiveness. Fibrin reinforcement
results from local stimuli to blood coagulation: the exposed collagen of
damaged vessels and the membranes and released contents of platelets (Figure
34–1). The local production of thrombin not only releases platelet adenosine
diphosphate (ADP), a powerful inducer of platelet aggregation, but also
stimulates the synthesis of prostaglandins from the arachidonic acid of
platelet membranes. These powerful substances are composed of two groups of
eicosanoids that have opposite effects on thrombogenesis.
Thromboxane
A2 (TXA2) is synthesized within platelets and induces
thrombogenesis and vasoconstriction. Prostacyclin (PGI2) is synthesized
within vessel
walls and inhibits thrombogenesis. Serotonin
(5-HT) is also released from the platelets, stimulating further aggregation
and vasoconstriction.
The
platelet is central to normal hemostasis and to all thromboembolic disease. A white
thrombus forms initially in high-pressure arteries by adherence of
circulating platelets to areas of abnormal endothelium as described above. The
growing thrombus of aggregated platelets reduces arterial flow. This localized
stasis triggers fibrin formation, and a red thrombus forms around the nidal
white thrombus.
Thrombogenesis
A red
thrombus can form around a white thrombus as mentioned above or de novo in
low-pressure veins, initially by adherence of platelets (as in arteries) but
followed promptly by the process of blood coagulation so that the bulk of the
thrombus forms a long tail consisting of a fibrin network in which red cells
are enmeshed. These tails become detached easily and travel as emboli to the
pulmonary arteries. Such emboli often arise from a deep venous thrombosis
(DVT)—a thrombus in the veins of the legs or pelvis. Although all thrombi are
mixed, the platelet nidus dominates the arterial thrombus and the fibrin tail
the venous thrombus. Arterial thrombi cause serious disease by producing local
occlusive ischemia; venous thrombi, by giving rise to distant embolization.
Blood Coagulation
Blood
coagulates by the transformation of soluble fibrinogen into insoluble fibrin.
Several circulating proteins interact in a cascading series of limited
proteolytic reactions. At each step, a clotting factor zymogen (eg, factor VII)
undergoes limited proteolysis and becomes an active protease (eg, factor VIIa).
Thus, each protease factor activates the next clotting factor until finally a
solid fibrin clot is formed. Fibrinogen (factor I), the soluble precursor of
fibrin, is the substrate for the enzyme thrombin (factor IIa). This protease is
formed during coagulation by activation of its zymogen, prothrombin (factor
II). Prothrombin is bound by calcium to a platelet phospholipid (PL) surface,
where activated factor X (Xa), in the presence of factor Va, converts it into
circulating thrombin. Several of the blood clotting factors are targets for
drug therapy (Table 34–1).
The
main initiator of blood coagulation is the tissue factor (TF)/factor VIIa
pathway. The exposure of TF on damaged endothelium binds and activates
circulating factor VII (Figure 34–2). This complex, in turn, activates factors
X and IX, with the eventual generation of thrombin. Thrombin, in turn,
activates upstream proteins, primarily factors V, VIII, and XI, resulting in
further thrombin generation. Additionally, thrombin is a potent activator of
platelets, converts fibrinogen to fibrin, and activates factor XIII, resulting
in an insoluble, cross-linked fibrin molecule.
The
TF/factor VII/factor X process is inhibited and regulated by tissue factor
pathway inhibitor (TFPI). Oral anticoagulant drugs inhibit the hepatic
synthesis of several clotting factors. Heparin inhibits the activity of several
of these activated clotting factors by enhancing the anticoagulant activity of antithrombin,
which inactivates the serine proteases IIa, IXa, Xa, XIa, and XIIa. The
endogenous anticoagulants protein C and protein S diminish amplification in the
blood clotting
cascade by proteolysis of factors Va
and VIIIa.
Regulation of Coagulation & Fibrinolysis
Blood
coagulation and thrombus formation must be confined to the smallest possible
area to achieve local hemostasis in response to bleeding from trauma or surgery
without causing disseminated coagulation or impaired blood flow.
Two major systems regulate and delineate these processes: fibrin
inhibition and fibrinolysis.
Plasma contains protease inhibitors that rapidly inactivate the
coagulation proteins as they escape from the site of vessel injury. The most
important proteins of this system are 1-antiprotease, 2- macroglobulin,
2-antiplasmin, and antithrombin. If this system is overwhelmed, generalized
intravascular clotting may occur. This process is called disseminated
intravascular coagulation (DIC) and may follow massive tissue injury, cell
lysis in malignant neoplastic disease, obstetric emergencies such as abruptio
placentae, or bacterial sepsis.
The central process of fibrinolysis is conversion of inactive
plasminogen to the proteolytic enzyme plasmin. Injured cells release
activators of plasminogen. Plasmin remodels the thrombus and limits the
extension of thrombosis by proteolytic digestion of fibrin.
Regulation of the fibrinolytic system is useful in therapeutics.
Increased fibrinolysis is effective therapy for thrombotic disease. Tissue
plasminogen activator (t-PA), urokinase, and streptokinase all
activate the fibrinolytic system (Figure 34–3). Conversely, decreased
fibrinolysis protects clots from lysis and reduces the bleeding of hemostatic
failure. Aminocaproic acid is a clinically useful inhibitor of
fibrinolysis. Heparin and the oral anticoagulant drugs do not affect the
fibrin-olytic mechanism.
Basic Pharmacology of the Anticoagulant Drugs
Indirect Thrombin Inhibitors
The indirect thrombin inhibitors are so named because their
antithrombotic effect is exerted by their interaction with antithrombin. Unfractionated
heparin (UFH), low-molecular-weight heparin (LMWH), and the synthetic
pentasaccharide fondaparinux bind to antithrombin and enhance its
inactivation of factor Xa. UFH and to a lesser extent LMWH also enhance
antithrombin's inactivation of thrombin (IIa).
Heparin
Chemistry & Mechanism of Action
Heparin
is a heterogeneous mixture of sulfated mucopolysaccharides. It binds to
endothelial cell surfaces and a variety of plasma proteins. As noted above, its
biologic activity is dependent upon the plasma protease inhibitor antithrombin.
Antithrombin inhibits clotting factor proteases, especially thrombin (IIa),
IXa, and Xa, by forming equimolar stable complexes with them. In the absence of
heparin, these reactions are slow; in the presence of heparin, they are
accelerated 1000- fold. Only about a third of the molecules in commercial
heparin preparations have an accelerating effect because the remainder lack the
unique pentasaccharide sequence needed for high-affinity binding to
antithrombin (Figure 34–4). The active heparin molecules bind tightly to
antithrombin and cause a conformational change in this inhibitor. The conformational
change of antithrombin exposes its active site for more rapid interaction with
the proteases (the activated clotting factors).
Heparin catalyzes the
antithrombin-protease reaction without being consumed. Once the
antithrombin-protease complex is formed, heparin is released intact for renewed
binding to more antithrombin.
The
antithrombin binding region of commercial unfractionated heparin consists of
repeating sulfated disaccharide units composed of D-glucosamine-L-iduronic acid
and D-glucosamine-Dglucuronic acid (Figure 34–4). High-molecular-weight (HMW)
fractions of heparin with high affinity for antithrombin markedly inhibit blood
coagulation by inhibiting all three factors, especially thrombin and factor Xa.
The
antithrombin binding region of commercial unfractionated heparin consists of
repeating sulfated disaccharide units composed of D-glucosamine-L-iduronic acid
and D-glucosamine-Dglucuronic acid (Figure 34–4). High-molecular-weight (HMW)
fractions of heparin with high affinity for antithrombin markedly inhibit blood
Figure 34–3.
coagulation
by inhibiting all three factors, especially thrombin and factor Xa.
Unfractionated heparin has a MW range of 5000–30,000. In contrast, the
shorter-chain low-molecular-weight (LMW) fractions of heparin inhibit activated
factor X but have less effect on thrombin (and on coagulation in general) than
the HMW species. Nevertheless, numerous studies have demonstrated that LMW heparins such as enoxaparin, dalteparin, and
tinzaparin are effective in several thromboembolic conditions. In
fact, these LMW heparins—in comparison with UFH—have equal efficacy, increased
bioavailability from the subcutaneous site of injection, and less frequent
dosing requirements (once or twice daily is sufficient).
Because
commercial heparin consists of a family of molecules of different molecular
weights, the correlation between the concentration of a given heparin
preparation and its effect on coagulation often is poor. Therefore, UFH is
standardized by bioassay. Heparin sodium USP must contain at least 120 USP
units per milligram. Heparin is generally used as the sodium salt, but calcium
heparin is equally effective. Lithium heparin is used in vitro as an
anticoagulant for blood samples. Commercial heparin is
Heparin: origin, structure, and mechanism of action
extracted
from porcine intestinal mucosa and bovine lung. Enoxaparin is obtained from the
same sources as regular heparin, but doses are specified in milligrams.
Dalteparin, tinzaparin and danaparoid (an LMW heparanoid containing heparan
sulfate, dermatan sulfate, and chondroitin sulfate that is no longer available
in the United States), on the other hand, are specified in anti-factor Xa
units.
Toxicity
The
major adverse effect of heparin is bleeding. This risk can be decreased by
scrupulous patient selection, careful control of dosage, and close monitoring
of the activated partial thromboplastin time (aPTT) in those patients receiving
unfractionated heparin. Levels for UFH may also be determined by protamine
titration (therapeutic levels 0.2–0.4 unit/mL) or anti-Xa units (therapeutic
levels 0.3–0.7 unit/mL). Weight-
based
dosing of the LMW heparins results in predictable pharmacokinetics and plasma
levels in patients with normal renal function. Therefore, LMW heparin levels
are not generally measured except in the setting of renal insufficiency,
obesity, and pregnancy. LMW heparin levels are determined by anti-Xa units.
Peak therapeutic levels are 0.5–1 unit/mL for twice daily dosing, determined 4
hours after administration, and approximately 1.5 units/mL for once daily
dosing. Elderly women and patients with renal failure are more prone to
hemorrhage. Heparin is of animal origin and should be used cautiously in
patients with allergy. Increased loss of hair and reversible alopecia have been
reported. Long-term heparin therapy is associated with osteoporosis and
spontaneous fractures. Heparin accelerates the clearing of postprandial lipemia
by causing the release of lipoprotein lipase from tissues, and long-term use is
associated with mineralocorticoid deficiency.
Heparin
causes transient thrombocytopenia in 25% or more of patients and severe
thrombocytopenia in 5%. Mild platelet
reduction within the first 5 days of therapy may result from heparin-induced
aggregation that is postulated to be benign and transient in character. A
smaller subset of patients may develop an antibody-mediated thrombocytopenia
that is associated with paradoxical thrombosis. In these instances, the
heparin-induced antibody is directed against the heparin-platelet factor 4
complex. These antigen-antibody complexes bind to Fc receptors on adjacent
platelets, causing aggregation and thromboembolism. The following points should
be considered in all patients receiving heparin: Platelet counts should be
performed frequently; thrombocytopenia should be considered to be
heparin-induced; any new thrombus can be the result of heparin; and
thromboembolic disease thought to be heparin-induced should be treated by
discontinuance of heparin and administration of an alternative drug, such as a
direct thrombin inhibitor (see below). Administration of warfarin alone is
contraindicated since it may exacerbatethe prothrombotic state associated with heparin-induced
thrombocytopenia.
Contraindications
Heparin
is contraindicated in patients who are hypersensitive to the drug, are actively
bleeding, or have hemophilia, significant thrombocytopenia, purpura, severe
hypertension, intracranial hemorrhage, infective endocarditis, active
tuberculosis, ulcerative lesions of the gastrointestinal tract, threatened
abortion, visceral carcinoma, or advanced hepatic or renal disease. Heparin
should be avoided in those patients who have recently had surgery of the brain,
spinal cord, or eye and in patients who are undergoing lumbar puncture or
regional anesthetic block. Despite the apparent lack of placental transfer,
heparin should be used in pregnant women only when clearly indicated.
Administration & Dosage
The
indications for the use of heparin are described in the section on clinical
pharmacology. A plasma concentration of heparin of 0.2–0.4 unit/mL (by
protamine titration) or 0.3–0.7 unit/mL (anti-Xa units) usually prevents
pulmonary emboli in patients with established venous thrombosis. This
concentration of heparin will prolong the activated partial thromboplastin time
(aPTT) to 2–2.5 times that of the control value. This degree of anticoagulant
effect should be maintained throughout the course of continuous intravenous
heparin therapy. When intermittent heparin administration is used, the
aPTT should be measured 6 hours after the administered dose to maintain
prolongation of the aPTT to 2–2.5 times that of the control value.
Hirudin
For a
number of years, surgeons have used medicinal leeches (Hirudo medicinalis) to
prevent thrombosis in the fine vessels of reattached digits. Hirudin is a
specific, irreversible thrombin inhibitor from the leech that is now available
in recombinant form as lepirudin. Its action is independent of
antithrombin, which means it can reach and inactivate fibrin-bound thrombin in
thrombi. Lepirudin has little effect on platelets or the bleeding time. Like
heparin, it must be administered parenterally and is monitored by the aPTT.
Lepirudin is FDA-approved for use in patients with thrombosis related to
heparin-induced thrombocytopenia. This drug has a short halflife, but it
accumulates in renal insufficiency and no antidote exists. Up to 40% of
patients on longterm infusions develop an antibody directed against the
thrombin-lepirudin complex. These antigenantibody complexes are not cleared by
the kidney and may result in an enhanced anticoagulant effect.
Bivalirudin,
another bivalent inhibitor of thrombin, is administered
intravenously, with a rapid onset and offset of action. The drug has a short
half-life with clearance that is 20% renal and the remainder metabolic.
Bivalirudin inhibits platelet activation and been FDA-approved for use in
percutaneous coronary angioplasty.
Argatroban
is a small molecule thrombin inhibitor that is FDA
approved for use in patients with heparin-induced thrombocytopenia (HIT) with
or without thrombosis and coronary angioplasty in patients with HIT. It, too,
has a short half-life, is given by continuous intravenous infusion, and
monitoring is done by aPTT. Its clearance is not affected by renal disease but
is dependent on liver function. The drug requires dose reduction in patients
with liver disease. Patients on argatroban will demonstrate elevated INRs
because of test interference, rendering the transition to warfarin difficult.
Melagatran
is the fourth parenteral drug in this class. It and its oral prodrug,
ximelagatran, are under intensive study. Attractive features of ximelagatran
include predictable pharmacokinetics and bioavailability—allowing for fixed
dosing and predictable anticoagulant response; no need for routine coagulation
monitoring; lack of interaction with P450-interacting drugs; rapid onset and
offset of action—allowing for immediate anticoagulation and thus no need for
overlap with additional anticoagulant drugs. A published phase 3 trial in
patients status post major orthopedic surgery found ximelagatran equivalent to
warfarin in preventing postoperative DVT. Clinical trials in patients with
acute DVT and chronic atrial fibrillation are on-going.
Coumarin Anticoagulants
Chemistry & Pharmacokinetics
The
clinical use of the coumarin anticoagulants can be traced to the discovery of
an anticoagulant substance formed in spoiled sweet clover silage. It produced a
deficiency of plasma prothrombin and consequent hemorrhagic disease in cattle.
The toxic agent was identified as bishydroxycoumarin and synthesized as
dicumarol. This drug and its congeners, most notably warfarin (Figure 34–5),
are widely used as rodenticides in addition to their application as
antithrombotic agents in humans. Warfarin is the most reliable member of this
group, and the other coumarin anticoagulants are almost never used in the
Figure 34–5.
Warfarin
is generally administered as the sodium salt and has 100% bioavailability. Over
99% of racemic warfarin is bound to plasma albumin, which may contribute to its
small volume of distribution (the albumin space), its long half-life in plasma
(36 hours), and the lack of urinary excretion of unchanged drug. Warfarin used
clinically is a racemic mixture composed of equal amounts of two enantiomorphs.
The levorotatory S-warfarin is four times more potent than the
dextrorotatory R-warfarin. This observation is useful in understanding
the stereoselective nature of several drug interactions involving warfarin.
Mechanism of Action
Coumarin
anticoagulants block the -carboxylation of several glutamate residues in
prothrombinand factors VII, IX, and X as well as the endogenous anticoagulant
proteins C and S (Figure 34–2).
The
blockade results in incomplete molecules that are biologically inactive in
coagulation. 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 (Figure 34–6). 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.
Figure 34–6.
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. These drugs not only
augment the hypoprothrombinemia but also inhibit platelet function and may
induce peptic ulcer disease (see Chapter 36: Nonsteroidal Anti-Inflammatory
Drugs, Disease-Modifying Antirheumatic Drugs, Nonopioid Analgesics, & Drugs
Used in Gout).
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
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).
Drugs
with no significant effect on anticoagulant therapy include ethanol,
phenothiazines, benzodiazepines, acetaminophen, opioids, indomethacin, and most
antibiotics.
Reversal of Action
Excessive
anticoagulant effect and bleeding from warfarin can be reversed by stopping the
drug and administering vitamin K1 (phytonadione), fresh-frozen plasma,
prothrombin complex concentrates (PCC) such as Bebulin and Proplex T, and
recombinant factor VIIa (rFVIIa). The disappearance of excessive effect is not
correlated with plasma warfarin concentrations but rather with reestablishment
of normal activity of the clotting factors. A modest excess of anticoagulant
effect without bleeding may require no more than cessation of the drug. The
warfarin effect can be rapidly reversed in the setting of severe bleeding with
the administration of a prothrombin complex or recombinant factor VIIa coupled
with intravenous vitamin K.
Analogs & Variants
Vitamin K antagonists other than
warfarin are seldom used, because they have less favorable pharmacologic
properties or greater toxicity. Dicumarol is incompletely absorbed and
frequently causes gastrointestinal symptoms. Phenprocoumon has a long
half-life of 6 days—a disadvantage should toxicity occur. The indanedione
group, which includes phenindione and diphenadione, has potentially serious
adverse effects in the kidney and liver and is of little clinical use.
Basic Pharmacology of the Fibrinolytic Drugs
Fibrinolytic drugs rapidly lyse thrombi by catalyzing the formation of
the serine protease plasmin from its precursor zymogen, plasminogen
(Figure 34–3). These drugs (streptokinase, alteplase, anistreplase, tissue
plasminogen activator, reteplase, tenecteplase, and urokinase) create
a generalized lytic state when administered intravenously. Thus, both
protective hemostatic thrombi and target thromboemboli are broken down. The
section Thrombolytic Drugs for Acute Myocardial Infarction describes the use of
these drugs in one major application.
Pharmacology
Streptokinase
is a protein (but not an enzyme in itself) synthesized by streptococci that
combines with the proactivator plasminogen. This enzymatic complex catalyzes
the conversion of inactive plasminogen to active plasmin. Urokinase is a
human enzyme synthesized by the kidney that directly converts plasminogen to
active plasmin. Plasmin itself cannot be used because naturally occurring
inhibitors in plasma prevent its effects. However, the absence of inhibitors
for urokinase and the streptokinase-proactivator complex permit their use
clinically. Plasmin formed inside a thrombus by these activators is protected
from plasma antiplasmins, which allows it to lyse the thrombus from within.
Anistreplase
(anisoylated plasminogen streptokinase activator
complex; APSAC) consists of a complex of purified human plasminogen and
bacterial streptokinase that has been acylated to protect the enzyme's active
site. When administered, the acyl group spontaneously hydrolyzes, freeing the
activated streptokinase-proactivator complex. This product (recently
discontinued in the
Plasminogen
can also be activated endogenously by tissue plasminogen activators (t-PA). These
activators preferentially activate plasminogen that is bound to fibrin, which
(in theory) confines fibrinolysis to the formed thrombus and avoids systemic
activation. Human t-PA is manufactured as alteplase by means of recombinant DNA
technology.
Reteplase
is another recombinant human t-PA from which several
amino acid sequences have been deleted. Reteplase is less expensive to produce
than t-PA. Because it lacks the major fibrin-binding domain, reteplase is less
fibrin-specific than t-PA. Tenecteplase is a mutant form of t-PA that
has a longer half-life, and it can be given as an intravenous bolus.
Tenecteplase is slightly more fibrinspecific than t-PA.
Indications & Dosage
Use of fibrinolytic drugs by the
intravenous route is indicated in cases of multiple pulmonary emboli that
are not massive enough to require surgical management. Intravenous fibrinolytic
drugs are also indicated in cases of central deep venous thrombosis such
as the superior vena caval syndrome and ascending thrombophlebitis of the
iliofemoral vein. They have also been used intraarterially, especially for
peripheral vascular disease.
Thrombolytic therapy in the management of acute myocardial infarction
requires careful patient selection, the use of a specific thrombolytic
agent, and the benefit of adjuvant therapy. Considerable controversy surrounds
the question of greater safety or efficacy of t-PA compared with the other
thrombolytic agents .
Streptokinase is administered by intravenous infusion of a loading dose
of 250,000 units, followed by 100,000 units/h for 24–72 hours.
Patients
with antistreptococcal antibodies can develop fever, allergic reactions, and
therapeutic resistance. Urokinase requires a loading dose of 300,000 units
given over 10 minutes and a maintenance dose of 300,000 units/h for 12 hours.
Alteplase (t-PA) is given by intravenous infusion of 60 mg over the first hour
and then 40 mg at a rate of 20 mg/h. Reteplase is given as two intravenous
bolus injections of 10 units each, separated by 30 minutes. Tenecteplase is
given as a single intravenous bolus of 0.5 mg/kg. Anistreplase is given as a
single intravenous injection of 30 units over 3–5 minutes. A single course of
fibrinolytic drugs is expensive: hundreds of dollars for streptokinase and
thousands for urokinase and t-PA.
Recombinant
tissue plasminogen activator has increasingly been used for patients presenting
with acute stroke symptoms. A recent outcomes study demonstrated an advantage
with respect to neurologic disability at 1 year in those patients with acute
ischemic stroke who received intravenous t-PA within 3 hours after onset of
symptoms.
Thrombolytic Drugs for Acute Myocardial Infarction
Background:
The paradigm shift in 1980 on the causation of acute
myocardial infarction to acute coronary occlusion by a thrombus created the
rationale for thrombolytic therapy of this common lethal disease. At that
time—and for the first time—intravenous thrombolytic therapy for acute
myocardial infarction in the European Cooperative Study Group trial was found
to reduce mortality significantly. Later studies, with thousands of patients in
each trial, provided enough statistical power for the 20% reduction in
mortality to be considered statistically significant. Although the standard of
care in areas with adequate facilities and experience in percutaneous coronary
intervention (PCI) now favors catheterization and placement of a stent,
thrombolytic therapy is still very important where PCI is not readily available.
Patients:
The selection of patients for thrombolytic therapy is
critical. The diagnosis of acute myocardial infarction is made clinically and
is confirmed by electrocardiography. Patients with ST segment elevation and
bundle branch block on electrocardiography do best; those with ST segment
depression or a normal ECG do less well; and those with non-Q-wave acute
myocardial infarction may even be harmed. All trials to date show the greatest
benefit for thrombolytic therapy when it is given early, within 6 hours after
symptomatic onset of acute myocardial infarction.
Clinical
trials: One of the trials (ISIS-3) showed
that streptokinase plus aspirin performed as well as recombinant tissue-type
plasminogen activator (rt-PA) or complex formulations of streptokinase such as
anistreplase (APSAC). The GUSTO trial showed a small advantage for the much
more expensive t-PA over streptokinase, but with a significantly higher risk of
hemorrhagic stroke. Nine clinical trials—each containing over 1000 patients
with suspected acute myocardial infarction— reported 11.6% mortality at 35 days
in the control group and 9.5% in the treatment group, an 18% reduction in
mortality.
Adjunctive
drugs: The best results occur with thrombolytic drugs
supplemented by other drugs. - Blocker drugs reduced myocardial ischemia and
infarct size, prevented arrhythmias, decreased reinfarction, and improved
survival in ISIS-1 and GISSI-1. Aspirin alone and with streptokinase reduced
mortality in ISIS-2. Nitroglycerin, given early in acute myocardial infarction
for pain relief, had no beneficial effect on mortality in GISSI-3 and ISIS-4.
Oral anticoagulants are best used in patients with depressed left ventricular
function or systemic embolization. Heparin was a necessary adjunct for t-PA in GUSTO
but was less useful with streptokinase plus aspirin because of the increased
risk of bleeding. ACE inhibitors reduced infarct expansion and arrhythmias and
improved survival in GISSI-3. Direct thrombin inhibitors like hirudin and
bivalirudin are undergoing clinical trials to better determine their efficacy
in conjunction with thrombolytic therapy (TIMI-9B, GUSTO-2A, OASIS, HERO).
Summary:
Thrombolytic drugs reduce the mortality of acute
myocardial infarction. The early and appropriate use of any thrombolytic drug
probably transcends individual advantages. Adjunctive drugs like aspirin,
-blockers, and ACE inhibitors reduce mortality even further. The principles of
management are outlined in part 7 of the American Heart Association Guidelines,
2000.
Basic Pharmacology of Antiplatelet Agents
Platelet
function is regulated by three categories of substances. The first group
consists of agents generated outside the platelet that interact with platelet
membrane receptors, eg, catecholamines, collagen, thrombin, and prostacyclin.
The second category contains agents generated within the platelet that interact
with membrane receptors, eg, ADP, prostaglandin D2, prostaglandin E2, and
serotonin. The third group comprises agents generated within the platelet that
act within the platelet, eg, prostaglandin endoperoxides and thromboxane A2,
the cyclic nucleotides cAMP and cGMP, and calcium ion. From this list of
agents, several targets for platelet inhibitory drugs have been identified
(Figure 34–1): inhibition of prostaglandin metabolism (aspirin), inhibition of
ADPinduced platelet aggregation (clopidogrel, ticlopidine), and blockade of GP
IIb/IIIa receptors on platelets (abciximab, tirofiban, and eptifibatide).
Dipyridamole and cilostazol are additional antiplatelet drugs.
Aspirin
The
prostaglandin thromboxane A2 is an arachidonate product that causes
platelets to change shape, to release their granules, and to aggregate (see
Chapter 18: The Eicosanoids: Prostaglandins, Thromboxanes, Leukotrienes, &
Related Compounds). Drugs that antagonize this pathway interfere with platelet
aggregation in vitro and prolong the bleeding time in vivo. Aspirin is
the prototype of this class of drugs. Drugs that modulate the intraplatelet
concentration of cAMP do not prolong the bleeding time. However, dietary
therapy can be useful in the prophylaxis of thrombosis. Ingestion of the
unsaturated fatty acid eicosapentaenoic acid, which is high in cold
water fish, generates prostaglandin I3, an effective antiaggregating substance
like prostacyclin, and thromboxane A3, which is much less active than TXA2.
Related
Compounds, aspirin inhibits the synthesis of thromboxane A2 by irreversible
acetylation of the enzyme cyclooxygenase. Other salicylates and nonsteroidal
anti-inflammatory drugs also inhibit cyclooxygenase but have a shorter duration
of inhibitory action because they cannot acetylate cyclooxygenase, ie, their
action is reversible.
The
FDA has approved the use of 325 mg/d for primary prophylaxis of
myocardial infarction but urges caution in this use of aspirin by the general
population except when prescribed as an adjunct to risk factor management by
smoking cessation and lowering of blood cholesterol and blood pressure.
Meta-analysis of many published trials of aspirin and other antiplatelet agents
confirms the value of this intervention in the secondary prevention of vascular events among patients with a
history of vascular events.
Inhibitors of platelet aggregation
Clopidogrel & Ticlopidine
Clopidogrel and ticlopidine reduce
platelet aggregation by inhibiting the ADP pathway of platelets. These drugs
are thienopyridine derivatives that achieve their antiplatelet effects by
irreversibly blocking the ADP receptor on platelets. Unlike aspirin, these
drugs have no effect on prostaglandin metabolism. Randomized clinical trials
with both drugs report efficacy in the prevention of vascular events among
patients with transient ischemic attacks, completed strokes, and unstable
angina pectoris. Use of clopidogrel or ticlopidine to prevent thrombosis is now
considered standard practice in patients undergoing placement of a coronary
stent.
Presystemic inactivation of platelet cyclooxygenase by acetylsalicylic
acid
Adverse
effects of ticlopidine include nausea, dyspepsia, and diarrhea in up to 20% of
patients, hemorrhage in 5%, and, most seriously, leukopenia in 1%. The
leukopenia is detected by regular monitoring of the white blood cell count
during the first 3 months of treatment. Development of thrombotic
thrombocytopenic purpura (TTP) has also been associated with the ingestion of
ticlopidine. The dosage of ticlopidine is 250 mg twice daily. It is
particularly useful in patients who cannot tolerate aspirin. Doses of
ticlopidine less than 500 mg/d may be efficacious with fewer adverse effects.
Clopidogrel
has fewer adverse effects than ticlopidine and is rarely associated with
neutropenia. Thrombotic thrombocytopenic purpura associated with clopidogrel
has recently been reported. Because of its superior side effect profile and dosing
requirements, clopidogrel is preferred over ticlopidine. The antithrombotic
effects of clopidogrel are dose-dependent; within 5 hours after an oral loading
dose of 300 mg, 80% of platelet activity will be inhibited. The maintenance
dose of clopidogrel is 75 mg/d, which achieves maximum platelet inhibition. The
duration of the antiplatelet effect is 7–10 days.
Blockade of Platelet Gp IIb/IIIa Receptors
The glycoprotein IIb/IIIa inhibitors
are used in patients with acute coronary syndromes. These drugs target the
platelet IIb/IIIa receptor complex (Figure 34–1). The IIb/IIIa complex
functions as a receptor mainly for fibrinogen and vitronectin but also for
fibronectin and von Willebrand factor. Activation of this receptor complex is
the "final common pathway" for platelet aggregation. There are
approximately 50,000 copies of this complex on the platelet surface. Persons
lacking this receptor have a bleeding disorder called Glanzmann's
thrombasthenia.
Abciximab, a humanized monoclonal antibody directed against the IIb/IIIa complex
including the vibronectin receptor, was the first agent approved in this class
of drugs. It has been approved for use in percutaneous coronary intervention
and in acute coronary syndromes. Eptifibatide is an analog of the
sequence at the extreme carboxyl terminal of the delta chain of fi-brinogen,
which mediates the binding of fibrinogen to the receptor. Tirofiban is a
smaller molecule with similar properties. Eptifibatide and tirofiban inhibit
ligand binding to the IIb/IIIa receptor by their occupancy of the receptor but
do not block the vibronectin receptor. The three agents described above are
administered parenterally. Oral formulations of IIb/IIIa antagonists have been
developed and are in various stages of development. Thus far, however, lack of
efficacy and significant thrombocytopenia have prevented progress with the oral
analogs.
Additional Antiplatelet-Directed Drugs
Dipyridamole
is a vasodilator that inhibits platelet function by
inhibiting adenosine uptake and cyclic GMP phosphodiesterase activity.
Dipyridamole by itself has little or no beneficial effect. Therefore,
therapeutic use of this agent is primarily in combination with aspirin to
prevent cerebrovascular ischemia. It may also be used in combination with
warfarin for primary prophylaxis of thromboemboli in patients with prosthetic
heart valves. A combination of dipyridamole complexed with 25 mg of aspirin is
now available for secondary prophylaxis of cerebrovascular disease.
Cilostazol
is a newer phosphodiesterase inhibitor that promotes
vasodilation and inhibition of platelet aggregation. Cilostazol is used
primarily to treat intermittent claudication.
Clinical Pharmacology of Drugs Used to Prevent
Clotting
Venous Thrombosis
Risk Factors
Inherited Disorders
The inherited disorders characterized
by an tendency to form thrombi (thrombophilia) derive from either quantitative
or qualitative abnormalities of the natural anticoagulant system. Deficiencies
in the natural anticoagulants antithrombin, protein C, and protein S account
for approximately 15% of selected patients with juvenile or recurrent
thrombosis and 5–10% of unselected cases of acute venous thrombosis. Additional
causes of thrombophilia include the factor V Leiden mutation,
hyperhomocystinemia, and the prothrombin 20210 mutation that together account
for the greater number of hypercoagulable patients.
Acquired Disease
The increased risk of thromboembolism
associated with arrhythmia, primarily atrial fibrillation, and the placement of
mechanical heart valves has long been recognized. Similarly, prolonged bed
rest, high-risk surgical procedures, and the presence of cancer are clearly
associated with an increased incidence of deep venous thrombosis and embolism.
Antithrombotic Management
Prevention
Primary
prevention of venous thrombosis reduces the incidence of and mortality rate
from pulmonary emboli. Heparin and warfarin may be used to prevent venous
thrombosis. Subcutaneous administration of low-dose unfractionated heparin,
low-molecular-weight heparin, or fondaparinux provides effective prophylaxis.
Warfarin is also effective but requires laboratory monitoring of the
prothrombin time.
Treatment of Established Disease
Treatment
for established venous thrombosis is initiated with unfractionated or
low-molecularweight heparin for the first 5–7 days, with an overlap with
warfarin. Once therapeutic effects of warfarin have been established, therapy
with warfarin is continued for a minimum of 3–6 months. Patients with recurrent
disease or identifiable, nonreversible risk factors may be treated
indefinitely. Small thrombi confined to the calf veins may be managed without
anticoagulants if there is documentation over time that the thrombus is not
extending. Warfarin readily crosses the placenta. It can cause hemorrhage at
any time during pregnancy as well as developmental defects when administered
during the first trimester. Therefore, venous thromboembolic disease in
pregnant women is generally treated with heparin, best administered by
subcutaneous injection.
Arterial
Thrombosis
Activation
of platelets is considered an essential process for arterial thrombosis. Thus,
treatment with platelet-inhibiting drugs such as aspirin and ticlopidine or
clopidogrel is indicated in patients with transient ischemic attacks and strokes
or unstable angina and acute myocardial infarction. In angina and infarction,
these drugs are often used in conjunction with -blockers, calcium channel
blockers, and fibrinolytic drugs.
Drugs Used in
Bleeding Disorders
VItamin K
Vitamin K confers biologic activity
upon prothrombin and factors VII, IX, and X by participating in their
postribosomal modification. Vitamin K is a fat-soluble substance found
primarily in leafy green vegetables. The dietary requirement is low, because the
vitamin is additionally synthesized by bacteria that colonize the human
intestine. Two natural forms exist: vitamins K1 and K2. Vitamin K1
(phytonadione; Figure 34–5) is found in food. Vitamin K2 (menaquinone) is found
in human tissues and is synthesized by intestinal bacteria. Vitamins K1 and K2 require bile salts for
absorption from the intestinal tract. Vitamin K1 is available clinically in 5
mg tablets and 50 mg ampules. Onset of effect is delayed for 6 hours but the
effect is complete by 24 hours when treating depression of prothrombin activity
by excess warfarin or vitamin K deficiency. Intravenous administration of
vitamin K1 should be slow, because rapid infusion can produce dyspnea, chest
and back pain, and even death. Vitamin K repletion is best achieved with
intravenous or oral administration, because its bioavailability after
subcutaneous administration is erratic.
Vitamin
K1 is currently administered to all newborns to prevent the hemorrhagic disease
of vitamin K deficiency, which is especially common in premature infants. The
water-soluble salt of vitamin K3 (menadione) should never be used in
therapeutics. It is particularly ineffective in the treatment of warfarin
overdosage.
Vitamin
K deficiency frequently occurs in hospitalized patients in intensive care units
because of poor diet, parenteral nutrition, recent surgery, multiple antibiotic
therapy, and uremia. Severe hepatic failure results in diminished protein
synthesis and a hemorrhagic diathesis that is unresponsive to vitamin K.
Plasma Fractions
Sources & Preparations
Deficiencies
in plasma coagulation factors can cause bleeding (Table 34–3). Spontaneous
bleeding occurs when factor activity is less than 5–10% of normal. Factor VIII
deficiency (classic hemophilia, or hemophilia A) and factor IX deficiency
(Christmas disease, or hemophilia B) account for most of the heritable
coagulation defects. Concentrated plasma fractions are available for the
treatment of these deficiencies. Administration of plasma-derived, heat- or
detergent-treated factor concentrates and recombinant factor concentrates are
the standard treatments for bleeding associated with hemophilia. Lyophilized
factor VIII concentrates are prepared from large pools of plasma. Transmission
of viral diseases such as hepatitis B and C and AIDS is reduced or eliminated
by pasteurization and by extraction of plasma with solvents and detergents. The
best use of these therapeutic materials requires diagnostic specificity of the
deficient factor and quantitation of its activity in plasma. Intermediate
purity
factor VIII concentrates (as opposed to recombinant or highpurity concentrates)
contain significant amounts of von Willebrand factor. Humate-P is a factor VIII
concentrate that is approved by the FDA for the treatment of bleeding associated
with von Willebrand disease.
Vitamin
K-antagonists of the coumarin type and vitamin K
Clinical Uses
An
uncomplicated hemorrhage into a joint should be treated with sufficient factor
VIII or factor IX replacement to maintain a level of at least 30–50% of the
normal concentration for 24 hours. Soft tissue hematomas require a minimum of
100% activity for 7 days. Hematuria requires at least 10% activity for 3 days.
Surgery and major trauma require a minimum of 100% activity for 10 days. The
initial loading dose for factor VIII is 50 units/kg of body weight to achieve
100% activity of factor VIII from a baseline of 1%, assuming a normal
hemoglobin. Each unit of factor VIII per kilogram of body weight raises its
activity in plasma 2%. Replacement should be administered every 12 hours.
Factor IX therapy requires twice the dose of factor VIII, but with an
administration of about every 24 hours because of its longer half-life.
Recombinant factor IX has only 80% recovery compared to plasma-derived factor
IX products. Therefore, dosing with recombinant factor IX requires 120% of the
dose used with the plasma-derived product.
Desmopressin
acetate (arginine vasopressin) increases the factor VIII
activity of patients with mild hemophilia A or von Willebrand disease. It can
be used in preparation for minor surgery such as tooth extraction without any
requirement for infusion of clotting factors if the patient has a documented
adequate response. High-dose intranasal desmopressin is available and has been
shown to be efficacious and well tolerated by patients.
Freeze-dried
concentrates of plasma containing prothrombin, factors IX and X, and varied
amounts of factor VII (Proplex, etc) are commercially available for treating
deficiencies of these factors (Table 34–3). Each unit of factor IX per kilogram
of body weight raises its activity in plasma 1.5%. Heparin is often added to
inhibit coagulation factors activated by the manufacturing process. However,
addition of heparin does not eliminate all thromboembolic events.
Some preparations of factor IX
concentrate contain activated clotting factors, which has led to their
use in treating patients with inhibitors or antibodies to factor VIII or factor
IX. Two products are available expressly for this purpose: Autoplex (with
factor VIII correctional activity) and Feiba (with factor VIII inhibitor
bypassing activity). These products are not uniformly successful in arresting
hemorrhage, and the factor IX inhibitor titers often rise after treatment with
them. Acquired inhibitors of coagulation factors may also be treated with
porcine factor VIII (for factor VIII inhibitors) and recombinant activated
factor VII. Recombinant activated factor VII (NovoSeven) is being
increasingly used to treat coagulopathy associated with liver disease and major
blood loss in trauma and surgery. These recombinant and plasma-derived factor
concentrates are very expensive, and the indications for them are very precise.
Therefore, close consultation with a hematologist knowledgeable in this area is
essential.
Cryoprecipitate
is a plasma protein fraction obtainable from whole
blood. It is used to treat deficiencies or qualitative abnormalities of
fibrinogen, such as that which occurs with disseminated intravascular
coagulation and liver disease. A single unit of cryoprecipitate contains 300 mg
of fibrinogen.
Cryoprecipitate
may also be used for patients with factor VIII deficiency and von Willebrand
disease if desmopressin is not indicated and a pathogen-inactivated recombinant
or plasma-derived product is not available. The concentration of factor VIII
and von Willebrand factor in cryoprecipitate is not as great as that found in
the concentrated plasma fractions. Moreover, cryoprecipitate is not treated in
any manner to decrease the risk of viral exposure. For infusion, the frozen
cryoprecipitate unit is thawed and dissolved in a small volume of sterile
citrate-saline solution and pooled with other units. Rh-negative women with
potential for childbearing should receive only Rh-negative cryoprecipitate because
of possible contamination of the product with Rhpositive blood cells.
Fibrinolytic Inhibitors: Aminocaproic Acid
Aminocaproic
acid (EACA), which is chemically similar to the amino acid lysine, is a
synthetic inhibitor of fibrinolysis. It competitively inhibits plasminogen
activation (Figure 34–3). It is rapidly absorbed orally and is cleared from the
body by the kidney. The usual oral dosage of EACA is
Clinical
uses of aminocaproic acid are as adjunctive therapy in hemophilia, as therapy
for bleeding from fibrinolytic therapy, and as prophylaxis for rebleeding from
intracranial aneurysms. Treatment success has also been reported in patients
with postsurgical gastrointestinal bleeding and postprostatectomy bleeding and
bladder hemorrhage secondary to radiation- and drug-induced cystitis. Adverse
effects of the drug include intravascular thrombosis from inhibition of
plasminogen activator, hypotension, myopathy, abdominal discomfort, diarrhea,
and nasal stuffiness. The drug should not be used in patients with disseminated
intravascular coagulation or genitourinary bleeding of the upper tract, eg,
kidney and ureters, because of the potential for excessive clotting.
Serine Protease Inhibitors: Aprotinin
Aprotinin
is a serine protease inhibitor ("serpin") that inhibits fibrinolysis
by free plasmin and may have other antihemorrhagic effects as well. It also
inhibits the plasmin-streptokinase complex in patients who have received that
thrombolytic agent. Aprotinin will reduce bleeding—by as much as 50%—from many
types of surgery, especially that involving extracorporeal circulation for open
heart procedures and liver transplantation. It is currently approved for use in
patients undergoing coronary artery bypass grafting who are at high risk of
excessive blood loss. In placebo-controlled trials, adverse effects of
aprotinin were little different from those reported in patients in the placebo
group. In larger studies, a possible association with anaphylaxis has been
reported in < 0.5% of cases. Therefore, a small test dose is recommended
before the full therapeutic dose is given.
ANTICOAGULANT
THERAPY
Thrombosis:
· The formation of clots at the wrong time in the wrong
place
· Can be arterial or venous
Risk factors are different:
VENOUS ARTERIAL
immobility smoking
obesity male
sex
trauma/surgery hypertension
cancer hyperlipidaemia
pregnancy diabetes
oral
contraceptive raised
fibrinogen
thrombophilia inherited
factors
increasing
age increasing
age
Complications include:
Venous:
· Deep vein thrombosis (DVT)
· Pulmonary embolism (PE)
· Postphlebitic syndrome
Arterial:
· Myocardial infarction (coronary thrombosis)
· Stroke (cerebral thrombosis)
· Peripheral arterial occlusion (infarction, gangrene)
Krysko
A.A., Malovichko O.L., Kabanova T.A. and Mazepa A.V. Russian Journal of
Bioorganic Chemistry, 2004, 30, 534.
ANTICOAGULANT
THERAPY
Two main indications:
·
Prevention of
thrombosis (prophylaxis)
·
Treatment of thrombosis
3 main
drugs are used:
·
Aspirin
·
Heparins
·
Warfarin
1.
ASPIRIN
q Mainly used for the prevention of arterial thrombosis
q Is an “anti-platelet” drug, ie inhibits platelet
function
q Works by irreversibly inactivating cyclo-oxygenase and reducing the
formation of thromboxane (activates
and aggregates platelets)
q Usual dose 75mg (“mini-dose aspirin”)
q Prevention can be:
· “PRIMARY”
in patients known to be at
high risk but
have been OK so far
· or “SECONDARY”
to prevent further problems in
a patient who has already had a
thrombotic event such as MI or stroke
2.
HEPARINS
q These are naturally occurring glycosaminoglycans
produced by cells such as vascular endothelium
q Work by binding to the natural anticoagulant “Antithrombin” and increasing its ability
to inhibit several points in the clotting cascade, especially Xa and
Thrombin
q Not active orally and has to be given by Intravenous
or Subcutaneous injection
q Main uses:
·
Initial treatment of acute venous or
arterial thrombosis
·
Prophylaxis during major surgery
·
Prophylaxis in pregnancy (where
warfarin is contraindicated)
q 2 main types of heparin:
·
Unfractionated heparin (UFH)
-
usually given by
IV infusion
-
narrow
“therapeutic window” between anticoagulant effect and risk of bleeding
-
requires
laboratory monitoring with aim of
keeping APTT between 1.5 and 2.5 x normal
·
Low molecular weight heparins
-
made by enzymatic
digestion of UFH
-
main therapeutic
effect is against Factor Xa
-
bigger therapeutic
window, lower risk of bleeding
-
do not usually
need laboratory monitoring (prescribed on a dose per weight basis as have a
much more predictable effect than UFH)
-
usually given by
subcutaneous injection and patients can be taught to self-inject
3.
WARFARIN
- Works by inhibiting the formation of those clotting
factors which require Vitamin K for full activation:
-
Factors II, VII,
IX and X
- Monitored by measuring the Prothrombin Time
(standardised throughout the world as the:
International Normalised Ratio
(INR))
- Main side-effect is bleeding; depends on degree of
anticoagulation, duration of treatment, quality of control (patients usually
attend a special Anticoagulant Service), drug interactions (common in elderly)
- Orally administered
- Takes several days to achieve a full anticoagulant
effect therefore, for treatment of thrombosis, usually start both heparin
(instantaneous effect) and warfarin together and stop the heparin when reach
therapeutic INR
INDICATIONS RECOMMENDED INR
Treatment of DVT
and/or PE 2.0-3.0
Atrial
Fibrillation 2.0-3.0
Recurrent DVT
and/or PE 3.0-4.5
Metal heart valves 3.0-4.5
Duration of treatment:
· Limited DVT with reversible 6 weeks
risk factor (eg surgery)
· More extensive DVT or 3 to 6 months
Pulmonary embolism
· Long term risk factors
eg Atrial Fibrillation, metal Lifelong
heart valve, recurrent
thrombosis
Control of warfarin treatment:
· Patients usually attend an Anticoagulant Clinic
(now usually “community
based”, eg at Health Centres)
· Initially need frequent checks on INR
· Once stabilised can go for up to 12 weeks between
checks
· Many clinics now use computer-assisted dosing (more
standardised than dosing by doctors or nurses, but needs watching
intelligently!)
· Some patients buy home coagulation monitors and can
download results to central clinic for computer-dosing via e-mail
· Biggest problems occur in the elderly, those on
multiple drugs and the poorly compliant patient
Treatment of Warfarin Overdose
Major bleeding:
·
Stop warfarin
·
Give “Prothrombin
Complex”
(Factor II, VII, IX, X concentrate)
OR Fresh Frozen Plasma (FFP)
·
Give Vitamin K
(5mg oral or IV)
Patients with a high INR but no bleeding can usually
be managed by temporarily stopping the warfarin
If regarded as high
risk of bleeding, can give a small dose of Vitamin K (0.25-2.5mg) orally
1. http://www.youtube.com/watch?v=8W0UzjmSPWc&feature=related
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8. http://www.youtube.com/watch?v=6zEiH7X1Jz4&NR=1
9. http://www.youtube.com/watch?v=kUiEEski4Aw&feature=related