CLINICAL PHARMACY IN ALLERGOLOGY. SYMPTOMS AND SYNDROMES OF ALLERGIES. CLINICAL PHARMACOLOGY OF DRUGS USED IN ALLERGIES
The effects of histamine released in the body can be reduced in several ways. Physiologic antagonists, especially epinephrine, have smooth muscle actions opposite to those of histamine, but they act at different receptors. This is important clinically because injection of epinephrine can be lifesaving in systemic anaphylaxis and in other conditions in which massive release of histamine—and other mediators—occurs.
Release inhibitors reduce the degranulation of mast cells that results from immunologic triggering by antigen-IgE interaction. Cromolyn and nedocromil appear to have this effect and are used in the treatment of asthma, although the molecular mechanism underlying their action is not fully understood. Beta2-adrenoceptor agonists also appear capable of reducing histamine release.
Histamine receptor antagonists represent a third approach to the reduction of histamine-mediated responses. For over 60 years, compounds have been available that competitively antagonize many of the actions of histamine on smooth muscle. However, not until the H2-receptor antagonist burimamide was described in 1972 was it possible to antagonize the gastric acid-stimulating activity of histamine. The development of selective H2-receptor antagonists has led to more effective therapy for peptic disease. Selective H3 and H4 antagonists are not yet available for clinical use. However, potent and partially selective experimental H3-receptor antagonists, thioperamide and clobenpropit, have been developed.
H1-Receptor Antagonists
Compounds that competitively block histamine at H1 receptors have been used in the treatment of allergic conditions for many years, and many H1 antagonists are currently marketed in the
Basic Pharmacology of H1-Receptor Antagonists
The H1 antagonists are conveniently divided into first-generation and second-generation agents. These groups are distinguished by the relatively strong sedative effects of most of the first-generation drugs. The first-generation agents are also more likely to block autonomic receptors. Second-generation H1 blockers are less sedating, owing in part to their less complete distribution into the central nervous system.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
nd, no data found. |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
These agents are rapidly absorbed after oral administration, with peak blood concentrations occurring in 1–2 hours. They are widely distributed throughout the body, and the first-generation drugs enter the central nervous system readily. Some of them are extensively metabolized, primarily by microsomal systems in the liver. Several of the second-generation agents are metabolized by the CYP3A4 system and thus are subject to important interactions when other drugs (such as ketoconazole) inhibit this subtype of P450 enzymes. Most of the drugs have an effective duration of action of 4–6 hours following a single dose, but meclizine and several second-generation agents are longer-acting, with a duration of action of 12–24 hours. The newer agents are considerably less lipid-soluble than the first-generation drugs and are substrates of the P-glycoprotein transporter in the blood-brain barrier; as a result they enter the central nervous system with difficulty or not at all. Many H1 antagonists have active metabolites. The active metabolites of hydroxyzine, terfenadine, and loratadine are available as drugs (cetirizine, fexofenadine, and desloratadine, respectively).
Pharmacodynamics
Both neutral H1 antagonists and inverse H1 agonists reduce or block the actions of histamine by reversible competitive binding to the H1 receptor. Several have been clearly shown to be inverse agonists, and it is possible that all act by this mechanism. They have negligible potency at the H2 receptor and little at the H3 receptor. For example, histamine-induced contraction of bronchiolar or gastrointestinal smooth muscle can be completely blocked by these agents, but the effects on gastric acid secretion and the heart are unmodified.
The first-generation H1-receptor antagonists have many actions in addition to blockade of the actions of histamine. The large number of these actions probably results from the similarity of the general structure to the structure of drugs that have effects at muscarinic cholinoceptor, α-adrenoceptor, serotonin, and local anesthetic receptor sites. Some of these actions are of therapeutic value and some are undesirable.
A common effect of first-generation H1 antagonists is sedation, but the intensity of this effect varies among chemical subgroups and among patients as well. The effect is sufficiently prominent with some agents to make them useful as “sleep aids” and unsuitable for daytime use. The effect resembles that of some antimuscarinic drugs and is considered very unlike the disinhibited sedation produced by sedative-hypnotic drugs. Compulsive use has not been reported. At ordinary dosages, children occasionally (and adults rarely) manifest excitation rather than sedation. At very high toxic dose levels, marked stimulation, agitation, and even convulsions may precede coma. Second-generation H1 antagonists have little or no sedative or stimulant actions. These drugs (or their active metabolites) also have far fewer autonomic effects than the first-generation antihistamines.
Antinausea and Antiemetic Actions
Several first-generation H1 antagonists have significant activity in preventing motion sickness. They are less effective against an episode of motion sickness already present. Certain H1 antagonists, notably doxylamine (in Bendectin), were used widely in the past in the treatment of nausea and vomiting of pregnancy (see below).
Some of the H1 antagonists, especially diphenhydramine, have significant acute suppressant effects on the extrapyramidal symptoms associated with certain antipsychotic drugs. This drug is given parenterally for acute dystonic reactions to antipsychotics.
Many first-generation agents, especially those of the ethanolamine and ethylenediamine subgroups, have significant atropine-like effects on peripheral muscarinic receptors. This action may be responsible for some of the (uncertain) benefits reported for nonallergic rhinorrhea but may also cause urinary retention and blurred vision.
Alpha-receptor blocking effects can be demonstrated for many H1 antagonists, especially those in the phenothiazine subgroup, eg, promethazine. This action may cause orthostatic hypotension in susceptible individuals. Beta-receptor blockade is not observed.
Strong blocking effects at serotonin receptors have been demonstrated for some first-generation H1 antagonists, notably cyproheptadine. This drug is promoted as an antiserotonin agent and is discussed with that drug group. Nevertheless, its structure resembles that of the phenothiazine antihistamines, and it is a potent H1-blocking agent.
Several first-generation H1 antagonists are potent local anesthetics. They block sodium channels in excitable membranes in the same fashion as procaine and lidocaine. Diphenhydramine and promethazine are actually more potent than procaine as local anesthetics. They are occasionally used to produce local anesthesia in patients allergic to conventional local anesthetic drugs. A small number of these agents also block potassium channels; this action is discussed below (see Toxicity).
Certain H1 antagonists, eg, cetirizine, inhibit mast cell release of histamine and some other mediators of inflammation. This action is not due to H1-receptor blockade and may reflect an H4-receptor effect (see below). The mechanism is not fully understood but could play a role in the beneficial effects of these drugs in the treatment of allergies such as rhinitis. A few H1 antagonists (eg, terfenadine, acrivastine) have been shown to inhibit the P-glycoprotein transporter found in cancer cells, the epithelium of the gut, and the capillaries of the brain. The significance of this effect is not known.
Clinical Pharmacology of H1-Receptor Antagonists
First-generation H1-receptor blockers are among the most extensively promoted and used over-the-counter drugs. The prevalence of allergic conditions and the relative safety of the drugs contribute to this heavy use. The fact that they do cause sedation contributes to heavy prescribing of second-generation antihistamines.
The H1 antihistaminic agents are often the first drugs used to prevent or treat the symptoms of allergic reactions. In allergic rhinitis (hay fever) and urticaria, in which histamine is the primary mediator, the H1 antagonists are the drugs of choice and are often quite effective if given before exposure. However, in bronchial asthma, which involves several mediators, the H1 antagonists are largely ineffective.
Angioedema may be precipitated by histamine release but appears to be maintained by peptide kinins that are not affected by antihistaminic agents. For atopic dermatitis, antihistaminic drugs such as diphenhydramine are used mostly for their sedative side effect, which reduces awareness of itching.
The H1 antihistamines used for treating allergic conditions such as hay fever are usually selected with the goal of minimizing sedative effects; in the
The second-generation H1 antagonists are used mainly for the treatment of allergic rhinitis and chronic urticaria. Several double-blind comparisons with older agents (eg, chlorpheniramine) indicated about equal therapeutic efficacy. However, sedation and interference with safe operation of machinery, which occur in about 50% of subjects taking first-generation antihistamines, occurred in only about 7% of subjects taking second-generation agents. The newer drugs are much more expensive, even in over-the-counter formulations.
Motion Sickness and Vestibular Disturbances
Scopolamine and certain first-generation H1 antagonists are the most effective agents available for the prevention of motion sickness. The antihistaminic drugs with the greatest effectiveness in this application are diphenhydramine and promethazine. Dimenhydrinate, which is promoted almost exclusively for the treatment of motion sickness, is a salt of diphenhydramine. The piperazines (cyclizine and meclizine) also have significant activity in preventing motion sickness and are less sedating than diphenhydramine in most patients. Dosage is the same as that recommended for allergic disorders. Both scopolamine and the H1 antagonists are more effective in preventing motion sickness when combined with ephedrine or amphetamine.
It has been claimed that the antihistaminic agents effective in prophylaxis of motion sickness are also useful in Ménière’s syndrome, but efficacy in the latter application is not established.
Nausea and Vomiting of Pregnancy
Several H1-antagonist drugs have been studied for possible use in treating “morning sickness.” The piperazine derivatives were withdrawn from such use when it was demonstrated that they have teratogenic effects in rodents. Doxylamine, an ethanolamine H1 antagonist, was promoted for this application as a component of Bendectin, a prescription medication that also contained pyridoxine. Possible teratogenic effects of doxylamine were widely publicized in the lay press after 1978 as a result of a few case reports of fetal malformation associated with maternal ingestion of Bendectin. However, several large prospective studies involving over 60,000 pregnancies, of which more than 3000 involved maternal Bendectin ingestion, disclosed no increase in the incidence of birth defects. However, because of the continuing controversy, adverse publicity, and lawsuits, the manufacturer of Bendectin withdrew the product from the market.
The wide spectrum of nonantihistaminic effects of the H1 antihistamines is described above. Several of these effects (sedation, antimuscarinic action) have been used for therapeutic purposes, especially in over-the-counter remedies. Nevertheless, these two effects constitute the most common undesirable actions when these drugs are used to block histamine receptors.
Less common toxic effects of systemic use include excitation and convulsions in children, postural hypotension, and allergic responses. Drug allergy is relatively common after topical use of H1 antagonists. The effects of severe systemic overdosage of the older agents resemble those of atropine overdosage and are treated in the same way (see Chapters 8 and 58). Overdosage of astemizole or terfenadine may induce cardiac arrhythmias, but these drugs are no longer marketed in the
Lethal ventricular arrhythmias occurred in several patients taking either of the early second-generation agents, terfenadine or astemizole, in combination with ketoconazole, itraconazole, or macrolide antibiotics such as erythromycin. These antimicrobial drugs inhibit the metabolism of many drugs by CYP3A4 and cause significant increases in blood concentrations of the antihistamines. The mechanism of this toxicity involves blockade of the HERG (IKr) potassium channels in the heart that are responsible for repolarization of the action potential (see Chapter 14). The result is prolongation of the action potential, and excessive prolongation leads to arrhythmias. Both terfenadine and astemizole were withdrawn from the
For those H1 antagonists that cause significant sedation, concurrent use of other drugs that cause central nervous system depression produces additive effects and is contraindicated while driving or operating machinery. Similarly, the autonomic blocking effects of older antihistamines are additive with those of muscarinic and α-blocking drugs.
IMMUNOLOGIC REACTIONS TO DRUGS & DRUG ALLERGY
The basic immune mechanism and the ways in which it can be suppressed or stimulated by drugs have been discussed in previous sections of this chapter. Drugs also activate the immune system in undesirable ways that are manifested as adverse drug reactions. These reactions are generally grouped in a broad classification as “drug allergy.” Indeed, many drug reactions such as those to penicillin, iodides, phenytoin, and sulfonamides are allergic in nature. These drug reactions are manifested as skin eruptions, edema, anaphylactoid reactions, glomerulonephritis, fever, and eosinophilia.
Drug reactions mediated by immune responses can have several different mechanisms. Thus, any of the four major types of hypersensitivity discussed earlier in this chapter (page 912) can be associated with allergic drug reactions:
· Type I: IgE-mediated acute allergic reactions to stings, pollens, and drugs, including anaphylaxis, urticaria, and angioedema. IgE is fixed to tissue mast cells and blood basophils, and after interaction with antigen the cells release potent mediators.
· Type II: Drugs often modify host proteins, thereby eliciting antibody responses to the modified protein. These allergic responses involve IgG or IgM in which the antibody becomes fixed to a host cell, which is then subject to complement-dependent lysis or to antibody-dependent cellular cytotoxicity.
· Type III: Drugs may cause serum sickness, which involves immune complexes containing IgG and is a multisystem complement-dependent vasculitis that may also result in urticaria.
· Type IV: Cell-mediated allergy is the mechanism involved in allergic contact dermatitis from topically applied drugs or induration of the skin at the site of an antigen injected intradermally.
In some drug reactions, several of these hypersensitivity responses may present simultaneously. Some adverse reactions to drugs may be mistakenly classified as allergic or immune when they are actually genetic deficiency states or are idiosyncratic and not mediated by immune mechanisms (eg, hemolysis due to primaquine in glucose-6-phosphate dehydrogenase deficiency, or aplastic anemia caused by chloramphenicol).
IMMEDIATE (TYPE I) DRUG ALLERGY
Type I (immediate) sensitivity allergy to certain drugs occurs when the drug, not capable of inducing an immune response by itself, covalently links to a host carrier protein (hapten). When this happens, the immune system detects the drug-hapten conjugate as “modified self” and responds by generating IgE antibodies specific for the drug-hapten. It is not known why some people mount an IgE response to a drug, while others mount IgG responses. Under the influence of IL-4, IL-5, and IL-13 secreted by TH2 cells, B cells specific for the drug secrete IgE antibody.
Fixation of the IgE antibody to high-affinity Fc receptors (FceRs) on blood basophils or their tissue equivalent (mast cells) sets the stage for an acute allergic reaction. The most important sites for mast cell distribution are skin, nasal epithelium, lung, and gastrointestinal tract. When the offending drug is reintroduced into the body, it binds and cross-links basophil and mast cell-surface IgE to signal release of the mediators (eg, histamine, leukotrienes) from granules. Mediator release is associated with calcium influx and a fall in intracellular cAMP within the mast cell. Many of the drugs that block mediator release appear to act through the cAMP mechanism (eg, catecholamines, glucocorticoids, theophylline), others block histamine release, and still others block histamine receptors. Other vasoactive substances such as kinins may also be generated during histamine release. These mediators initiate immediate vascular smooth muscle relaxation, increased vascular permeability, hypotension, edema, and bronchoconstriction.
Drug Treatment of Immediate Allergy
One can test an individual for possible sensitivity to a drug by a simple scratch test, ie, by applying an extremely dilute solution of the drug to the skin and making a scratch with the tip of a needle. If allergy is present, an immediate (within 10-15 minutes) wheal (edema) and flare (increased blood flow) will occur. However, skin tests may be negative in spite of IgE hypersensitivity to a hapten or to a metabolic product of the drug, especially if the patient is taking steroids or antihistamines.
Drugs that modify allergic responses act at several links in this chain of events. Prednisone, often used in severe allergic reactions, is immunosuppressive; it blocks proliferation of the IgE-producing clones and inhibits IL-4 production by T helper cells in the IgE response, since glucocorticoids are generally toxic to lymphocytes. In the efferent limb of the allergic response, isoproterenol, epinephrine, and theophylline reduce the release of mediators from mast cells and basophils and produce bronchodilation. Epinephrine opposes histamine; it relaxes bronchiolar smooth muscle and contracts vascular muscle, relieving both bronchospasm and hypotension. The antihistamines competitively inhibit histamine, which would otherwise produce bronchoconstriction and increased capillary permeability in the end organ. Glucocorticoids may also act to reduce tissue injury and edema in the inflamed tissue, as well as facilitating the actions of catecholamines in cells that may have become refractory to epinephrine or isoproterenol. Several agents directed toward the inhibition of leukotriene synthesis may be useful in acute allergic and inflammatory disorders.
When reasonable alternatives are not available, certain drugs (eg, penicillin, insulin) must be used for life-threatening illnesses even in the presence of known allergic sensitivity. In such cases, desensitization can sometimes be accomplished by starting with very small doses of the drug and gradually increasing the dose over a period of hours to the full therapeutic range. This practice is hazardous and must be performed under direct medical supervision, as anaphylaxis may occur before desensitization has been achieved. It is thought that slow and progressive administration of the drug gradually binds all available IgE on mast cells, triggering a gradual release of granules. Once all of the IgE on the mast cell surfaces has been bound and the cells have been degranulated, therapeutic doses of the offending drug may be given with minimal further immune reaction. Therefore, a patient is only desensitized during administration of the drug.
AUTOIMMUNE (TYPE II) REACTIONS TO DRUGS
Certain autoimmune syndromes can be induced by drugs. Examples include systemic lupus erythematosus following hydralazine or procainamide therapy, “lupoid hepatitis” due to cathartic sensitivity, autoimmune hemolytic anemia resulting from methyldopa administration, thrombocytopenic purpura due to quinidine, and agranulocytosis due to a variety of drugs. As indicated in other chapters of this book, a number of drugs are associated with type I and type II reactions. In these drug-induced autoimmune states, IgG antibodies bind to drug-modified tissue and are destroyed by the complement system or by phagocytic cells with Fc receptors. Fortunately, autoimmune reactions to drugs usually subside within several months after the offending drug is withdrawn. Immunosuppressive therapy is warranted only when the autoimmune response is unusually severe.
SERUM SICKNESS & VASCULITIC (TYPE III) REACTIONS
Immunologic reactions to drugs resulting in serum sickness are more common than immediate anaphylactic responses, but type II and type III hypersensitivities often overlap. The clinical features of serum sickness include urticarial and erythematous skin eruptions, arthralgia or arthritis, lymphadenopathy, glomerulonephritis, peripheral edema, and fever. The reactions generally last 6-12 days and usually subside once the offending drug is eliminated. Antibodies of the IgM or IgG class are usually involved. The mechanism of tissue injury is immune complex formation and deposition on basement membranes (eg, lung, kidney), followed by complement activation and infiltration of leukocytes, causing tissue destruction. Glucocorticoids are useful in attenuating severe serum sickness reactions to drugs. In severe cases plasmapheresis can be used to remove the offending drug and immune complexes from circulation.
Immune vasculitis can also be induced by drugs. The sulfonamides, penicillin, thiouracil, anticonvulsants, and iodides have all been implicated in the initiation of hypersensitivity angiitis. Erythema multiforme is a relatively mild vasculitic skin disorder that may be secondary to drug hypersensitivity. Stevens-Johnson syndrome is probably a more severe form of this hypersensitivity reaction and consists of erythema multiforme, arthritis, nephritis, central nervous system abnormalities, and myocarditis. It has frequently been associated with sulfonamide therapy. Administration of nonhuman monoclonal or polyclonal antibodies such as rattlesnake antivenin may cause serum sickness.
NORMAL IMMUNE RESPONSES
The immune system has evolved to protect the host from invading pathogens and to eliminate disease. At its functioning best, the immune system is exquisitely responsive to invading pathogens while retaining the capacity to recognize self antigens to which it is tolerant. Protection from infection and disease is provided by the collaborative efforts of the innate and adaptive immune systems.
The innate immune system is the first line of defense against an invading pathogen (antigen) and includes physical (eg, skin), biochemical (eg, complement, lysozyme, interferons), and cellular components (neutrophils, monocytes, macrophages, natural killer [NK], and natural killer-T [NKT] cells). An intact skin or mucosa is the first barrier to infection. When this barrier is breached, destruction of the pathogen (eg, bacteria, fungi, parasites) is accomplished by biochemical components such as lysozyme (which breaks down the protective peptidoglycan cell wall) and the split products arising from complement activation. Complement components enhance macrophage and neutrophil phagocytosis by acting as opsonins (C3b) and chemoattractants (C3a, C5a) that recruit immune cells to inflammatory sites. The activation of complement eventually leads to pathogen lysis via the generation of a membrane attack complex that creates holes in the membrane and results in leakage of cellular components.
During the inflammatory response triggered by infection, neutrophils and monocytes enter the tissue sites from the peripheral circulation. This cellular influx is mediated by the release and action of chemoattractant cytokines (eg, IL-8 [CXCL8], macrophage chemotactic protein-1 [MCP-1; CCL2], and macrophage inflammatory protein-1 [MIP-1a; CCL3]) from activated endothelial cells and immune cells (mostly tissue macrophages) at the inflammatory site. It is triggered by the adhesion of cell surface receptors on the immune cells to ligands on the activated endothelial cell surface. If these events occur successfully, the invading pathogen is ingested, degraded, and eliminated, and disease is either prevented or is of short duration.
Natural killer (NK) and natural killer-T (NKT) cells recruited to the inflammatory site also contribute to the innate response by secreting interferon-gamma (IFN-g), which activates resident tissue macrophages and dendritic cells. NK cells are so called because they are able to recognize and destroy virus-infected normal cells as well as tumor cells without prior stimulation. This activity is regulated by so-called “killer cell immunoglobulin-like receptors” (KIRs) on the NK cell surface that are specific for major histocompatibility complex (MHC) class I molecules. When NK cells bind self MHC class I proteins (expressed on all nucleated cells), these receptors deliver inhibitory signals, preventing them from killing normal host cells. Tumor cells or virus-infected cells that have down-regulated MHC class I expression do not engage these KIRs, resulting in activation of NK cells and subsequent destruction of the target cell. NK cells kill target cells by releasing cytotoxic granules that induce programmed cell death.
NKT cells express T-cell receptors as well as receptors commonly found on NK cells. NKT cells recognize microbial lipid antigens presented by a unique class of MHC-like molecules known as CD1 and have been implicated in host defense against microbial agents, autoimmune diseases, and tumors.
The adaptive immune system is mobilized by cues from the innate response when the innate processes are incapable of coping with an infection. The adaptive immune system has a number of characteristics that contribute to its success in eliminating pathogens. These include the ability to (1) respond to a variety of antigens, each in a specific manner; (2) discriminate between foreign (“non-self”) antigens (pathogens) and self antigens of the host; and (3) respond to a previously encountered antigen in a learned way by initiating a vigorous memory response. This adaptive response culminates in the production of antibodies, which are the effectors of humoral immunity; and the activation of T lymphocytes, which are the effectors of cell-mediated immunity.
The induction of specific adaptive immunity requires the participation of professional antigen-presenting cells (APCs), which include dendritic cells (DC), macrophages, and B lymphocytes. These cells play pivotal roles in the induction of an adaptive immune response because of their capacity to phagocytize or endocytose protein antigens, and enzymatically digest them to generate peptides, which are then loaded onto class I or class II MHC proteins and “presented” to the cell surface T-cell receptor (TCR). CD8 T cells recognize class I-MHC peptide complexes while CD4 T cells recognize class II-MHC peptide complexes. At least two signals are necessary for the activation of T cells. The first signal is delivered following engagement of the TCR with peptide-bound MHC molecules. In the absence of a second signal, the T cells become unresponsive (anergic) or undergo apoptosis. The second signal involves ligation of costimulatory molecules (CD40, CD80 [also known as B7-1], and CD86 [also known as B7-2]) on the antigen-presenting cell to their respective ligands (CD40L for CD40, CD28 for CD80 or CD86). Activation of T cells is regulated via a negative feedback loop involving another molecule known as T-lymphocyte-associated antigen 4 (CTLA-4). Following engagement of CD28 with CD80 or CD86, CTLA-
Studies using murine T-cell clones have demonstrated the presence of two subsets of T helper lymphocytes (TH1 and TH2) based on the cytokines they secrete after activation. This demarcation is not so clear-cut in humans. The TH1 subset characteristically produces interferon-g (IFN-g), interleukin-2 (IL-2), and tumor necrosis factor-b (TNF-b) and induces cell-mediated immunity by activation of macrophages, cytotoxic T cells (CTLs), and NK cells. The TH2 subset produces IL-4, IL-5, IL-6, and IL-10 (and sometimes IL-13), which induce B-cell proliferation and differentiation into antibody-secreting plasma cells. IL-10 produced by TH2 cells inhibits cytokine production by TH1 cells via the down-regulation of MHC expression by APCs. Conversely, IFN-g produced by TH1 cells inhibits the proliferation of TH2 cells. Although these subsets have been well described in vitro, the nature of the antigenic challenge that elicits a TH1 or TH2 phenotype is less clear. Extracellular bacteria typically cause the elaboration of TH2 cytokines, culminating in the production of neutralizing or opsonic antibodies. In contrast, intracellular organisms (eg, mycobacteria) elicit the production of TH1 cytokines, which lead to the activation of effector cells such as macrophages. A less well defined T-cell subset (TH3) has been described that produces transforming growth factor-b (TGF-b), whose numerous functions include down-regulation of proliferation and differentiation of T lymphocytes.
CD8 T lymphocytes recognize endogenously processed peptides presented by virus-infected cells or tumor cells. These peptides are usually nine-amino-acid fragments derived from virus or protein tumor antigens in the cytoplasm and are loaded onto MHC class I molecules in the endoplasmic reticulum. In contrast, class II MHC molecules present peptides (usually 11-22 amino acids) derived from extracellular (exogenous) pathogens to CD4 T helper cells. In some instances, exogenous antigens, upon ingestion by APCs, can be presented on class I MHC molecules to CD8 T cells. This phenomenon is referred to as “cross-presentation” and is thought to be useful in generating effective immune responses against infected host cells that are incapable of priming T lymphocytes. Upon activation, CD8 T cells induce target cell death via lytic granule enzymes (“granzymes”), perforin, and the Fas-Fas ligand (Fas-FasL) apoptosis pathways.
B lymphocytes undergo selection in the bone marrow, during which self-reactive B lymphocytes are clonally deleted while B-cell clones specific for foreign antigens are retained and expanded. The repertoire of antigen specificities by T cells is genetically determined and arises from T-cell receptor gene rearrangement while the specificities of B cells arise from immunoglobulin gene rearrangement; for both types of cells, these determinations occur prior to encounters with antigen. Upon an encounter with antigen, a mature B cell binds the antigen, internalizes and processes it, and presents its peptide bound to class II MHC to CD4 helper cells, which in turn secrete IL-4 and IL-5. These interleukins stimulate B-cell proliferation and differentiation into memory B cells and antibody-secreting plasma cells. The primary antibody response consists mostly of IgM-class immunoglobulins. Subsequent antigenic stimulation results in a vigorous “booster” response accompanied by class (isotype) switching to produce IgG, IgA, and IgE antibodies with diverse effector functions. These antibodies also undergo affinity maturation, which allows them to bind more efficiently to the antigen. With the passage of time, this results in accelerated elimination of microorganisms in subsequent infections. Antibodies mediate their functions by acting as opsonins to enhance phagocytosis and cellular cytotoxicity and by activating complement to elicit an inflammatory response and induce bacterial lysis.
ABNORMAL IMMUNE RESPONSES
Whereas the normally functioning immune response can successfully neutralize toxins, inactivate viruses, destroy transformed cells, and eliminate pathogens, inappropriate responses can lead to extensive tissue damage (hypersensitivity) or reactivity against self antigens (autoimmunity); conversely, impaired reactivity to appropriate targets (immunodeficiency) may occur and abrogate essential defense mechanisms.
Hypersensitivity
Hypersensitivity can be classified as antibody-mediated or cell-mediated. Three types of hypersensitivity are antibody-mediated (types I-III), while the fourth is cell-mediated (type IV). Hypersensitivity occurs in two phases: the sensitization phase and the effector phase. Sensitization occurs upon initial encounter with an antigen; the effector phase involves immunologic memory and results in tissue pathology upon a subsequent encounter with that antigen.
A. IMMEDIATE HYPERSENSITIVITY
Immediate, or type I, hypersensitivity is IgE-mediated, with symptoms usually occurring within minutes following the patient’s encounter with antigen.
1. Type I¾ Type I hypersensitivity results from cross-linking of membrane-bound IgE on blood basophils or tissue mast cells by antigen. This cross-linking causes cells to degranulate, releasing substances such as histamine, leukotrienes, and eosinophil chemotactic factor, which induce anaphylaxis, asthma, hay fever, or urticaria (hives) in affected individuals. A severe type I hypersensitivity reaction such as systemic anaphylaxis (eg, from insect envenomation, ingestion of certain foods, or drug hypersensitivity) requires immediate medical intervention.
2. Type II¾ Type II hypersensitivity results from the formation of antigen-antibody complexes between foreign antigen and IgM or IgG immunoglobulins. One example of this type of hypersensitivity is a blood transfusion reaction that can occur if blood is not cross-matched properly. Preformed antibodies bind to red blood cell membrane antigens that activate the complement cascade, generating a membrane attack complex that lyses the transfused red blood cells. In hemolytic disease of the newborn, anti-Rh IgG antibodies produced by an Rh-negative mother cross the placenta, bind to red blood cells of an Rh-positive fetus, and damage them. The disease is prevented in subsequent pregnancies by the administration of anti-Rh antibodies to the mother 24-48 hours after delivery (see Antibodies as Immunosuppressive Agents, below). Type II hypersensitivity can also be drug-induced and may occur during the administration of penicillin to allergic patients. In these patients, penicillin binds to red blood cells or other host tissue to form a neoantigen that evokes production of antibodies capable of inducing complement-mediated red cell lysis. In some circumstances, subsequent administration of the drug can lead to systemic anaphylaxis (type I hypersensitivity).
3. Type III¾ Type III hypersensitivity is due to the presence of elevated levels of antigen-antibody complexes that deposit on basement membranes in tissues and vessels. Immune complex deposition activates complement to produce components with anaphylatoxic and chemotactic activities (C5a, C3a, C4a) that increase vascular permeability and recruit neutrophils to the site of complex deposition. Complex deposition and the action of lytic enzymes released by neutrophils can cause skin rashes, glomerulonephritis, and arthritis in these individuals. If patients have type III hypersensitivity against a particular antigen, clinical symptoms usually occur 3 to 4 days after exposure to the antigen.
B. TYPE IV: DELAYED-TYPE HYPERSENSITIVITY
Unlike type I, II, and III hypersensitivities, delayed-type hypersensitivity (DTH) is cell-mediated, and responses occur 2-3 days after exposure to the sensitizing antigen. DTH is caused by antigen-specific DTH TH1 cells and induces a local inflammatory response that causes tissue damage characterized by the influx of antigen-nonspecific inflammatory cells, especially macrophages. These cells are recruited under the influence of TH1-produced cytokines, which chemoattract circulating monocytes and neutrophils, induce myelopoiesis, and activate macrophages. The activated macrophages are primarily responsible for the tissue damage associated with DTH. Although widely considered to be deleterious, DTH responses are very effective in eliminating infections caused by intracellular pathogens such as Mycobacterium tuberculosis and Leishmania species. Clinical manifestations of DTH include tuberculin and contact hypersensitivities. Tuberculosis exposure is determined using a DTH skin test. Positive responses show erythema and induration caused by accumulation of macrophages and DTH T cells at the site of the tuberculin injection. Poison ivy is the most common cause of contact hypersensitivity, in which pentadecacatechol, the lipophilic chemical in poison ivy, modifies cellular tissue and results in a DTH T-cell response.
Autoimmune disease arises when the body mounts an immune response against itself due to failure to distinguish self tissues and cells from foreign (nonself) antigens. This phenomenon derives from the activation of self-reactive T and B lymphocytes that generate cell-mediated or humoral immune responses directed against self antigens. The pathologic consequences of this reactivity constitute several types of autoimmune diseases. Autoimmune diseases are highly complex due to MHC genetics, environmental conditions, infectious entities, and dysfunctional immune regulation. Examples of such diseases include rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, and insulin-dependent diabetes mellitus (type 1 diabetes). In rheumatoid arthritis, IgM antibodies (rheumatoid factors) are produced that react with the Fc portion of IgG and may form immune complexes that activate the complement cascade, causing chronic inflammation of the joints and kidneys. In systemic lupus erythematosus, antibodies are made against DNA, histones, red blood cells, platelets, and other cellular components. In multiple sclerosis and type 1 diabetes, cell-mediated autoimmune attack destroys myelin surrounding nerve cells and insulin-producing islet B (beta) cells of the pancreas, respectively. In type 1 diabetes, activated CD4 TDTH cells that infiltrate the islets of Langerhans and recognize self islet B cell peptides are thought to produce cytokines that stimulate macrophages to produce lytic enzymes, which destroy islet B cells. Autoantibodies directed against the islet B cell antigens are produced, but do not contribute significantly to disease.
A number of mechanisms have been proposed to explain autoimmunity:
(1) Exposure of self-reactive T lymphocytes to antigens previously sequestered from the immune system (eg, lens protein, myelin basic protein).
(2) Molecular mimicry by invading pathogens, in which immune responses are directed at antigenic determinants on pathogens that share identical or similar epitopes with normal host tissue. This phenomenon occurs in rheumatic fever following Streptococcus pyogenes infection, in which heart damage is thought to arise from an immune response directed against streptococcal antigens shared with heart muscle. The suggested viral etiology of autoimmune diseases has been ascribed to immune responses (both cell-mediated and humoral) directed against virus epitopes that mimic sequestered self antigens.
(3) Inappropriate expression of class II MHC molecules on the membranes of cells that normally do not express class II MHC (eg, islet b cells). Increased expression of MHC II may increase presentation of self peptides to T helper cells, which in turn induce CTL, TDTH, and B-lymphocyte cells that react against self antigens.
Immunodeficiency diseases result from inadequate function in the immune system; the consequences include increased susceptibility to infections and prolonged duration and severity of disease. Immunodeficiency diseases are either congenitally acquired or arise from extrinsic factors such as bacterial or viral infections or drug treatment. Affected individuals frequently succumb to infections caused by opportunistic organisms of low pathogenicity for the immunocompetent host. Examples of congenitally acquired immunodeficiency disease include X-linked agammaglobulinemia, DiGeorge’s syndrome, and severe combined immunodeficiency disease (SCID) due to adenosine deaminase (ADA) deficiency.
X-linked agammaglobulinemia is a disease affecting males that is characterized by a failure of immature B-lymphocytes to mature into antibody-producing plasma cells. These individuals are susceptible to recurrent bacterial infections, although the cell-mediated responses directed against viruses and fungi are preserved. DiGeorge’s syndrome is due to failure of the thymus to develop, resulting in diminished T-cell responses (TDTH, CTL), while the humoral response is unaffected.
The ADA enzyme normally prevents the accumulation of toxic deoxy-ATP in cells. Deoxy-ATP is particularly toxic to lymphocytes, and leads to death of T and B cells. Absence of the enzyme therefore results in SCID. Infusion of the purified enzyme (pegademase, from bovine sources) and transfer of ADA gene-modified lymphocytes have both been used successfully to treat this disease.
AIDS represents the classic example of immunodeficiency disease caused by extrinsic factors, in this instance the human immunodeficiency virus (HIV). This virus exhibits a strong tropism for CD4 T helper cells; these become depleted, giving rise to increased frequency of opportunistic infections and malignancies in infected individuals. AIDS is also characterized by an imbalance in TH1 and TH2 cells, and the ratios of cells and their functions are skewed toward TH2. This results in hypergammaglobulinemia, loss of cytotoxic lymphocyte activity, and delayed hypersensitivity.
Immunosuppressive agents have proved very useful in minimizing the occurrence or impact of deleterious effects of exaggerated or inappropriate immune responses. Unfortunately, these agents also have the potential to cause disease and to increase the risk of infection and malignancies.
Glucocorticoids (corticosteroids) were the first hormonal agents recognized as having lympholytic properties. Administration of any glucocorticoid reduces the size and lymphoid content of the lymph nodes and spleen, although it has no toxic effect on proliferating myeloid or erythroid stem cells in the bone marrow.
Glucocorticoids are thought to interfere with the cell cycle of activated lymphoid cells. Glucocorticoids are quite cytotoxic to certain subsets of T cells, but their immunologic effects are probably due to their ability to modify cellular functions rather than to direct cytotoxicity. Although cellular immunity is more affected than humoral immunity, the primary antibody response can be diminished, and with continued use, previously established antibody responses are also decreased. Additionally, continuous administration of corticosteroid increases the fractional catabolic rate of IgG, the major class of antibody immunoglobulins, thus lowering the effective concentration of specific antibodies. Contact hypersensitivity mediated by DTH T cells, for example, is usually abrogated by glucocorticoid therapy.
Glucocorticoids are used in a wide variety of conditions. It is thought that the immunosuppressive and anti-inflammatory properties of corticosteroids account for their beneficial effects in diseases like idiopathic thrombocytopenic purpura and rheumatoid arthritis. Glucocorticoids modulate allergic reactions and are useful in the treatment of diseases like asthma or as premedication for other agents (eg, blood products, chemotherapy) that might cause undesirable immune responses. Glucocorticoids are first-line immunosuppressive therapy for both solid organ and hematopoietic stem cell transplant recipients, with variable results. The toxicities of long-term glucocorticoid therapy can be severe.
1. Cyclosporine
Cyclosporine (cyclosporin A, CSA) is an immunosuppressive agent with efficacy in human organ transplantation, in the treatment of graft-versus-host disease after hematopoietic stem cell transplantation, and in the treatment of selected autoimmune disorders. Cyclosporine is a peptide antibiotic that appears to act at an early stage in the antigen receptor-induced differentiation of T cells and blocks their activation. Cyclosporine binds to cyclophilin, a member of a class of intracellular proteins called immunophilins. Cyclosporine and cyclophilin form a complex that inhibits the cytoplasmic phosphatase, calcineurin, which is necessary for the activation of a T-cell-specific transcription factor. This transcription factor, NF-AT, is involved in the synthesis of interleukins (eg, IL-2) by activated T cells. In vitro studies have indicated that cyclosporine inhibits the gene transcription of IL-2, IL-3, IFN-g, and other factors produced by antigen-stimulated T cells, but it does not block the effect of such factors on primed T cells nor does it block interaction with antigen.
Cyclosporine may be given intravenously or orally, though it is slowly and incompletely absorbed (20-50%). The absorbed drug is primarily metabolized by the P450 3A enzyme system in the liver with resultant multiple drug interactions. This propensity for drug interaction contributes to significant interpatient variability in bioavailability, such that cyclosporine requires individual patient dosage adjustments based on steady-state blood levels and the desired therapeutic ranges for the drug. Cyclosporine ophthalmic solution is now available for severe dry eye syndrome, as well as ocular graft-versus-host disease. Inhaled cyclosporine is being investigated for use in lung transplantation.
Toxicities are numerous and include nephrotoxicity, hypertension, hyperglycemia, liver dysfunction, hyperkalemia, altered mental status, seizures, and hirsutism. Cyclosporine causes very little bone marrow toxicity. While an increased incidence of lymphoma and other cancers (Kaposi’s sarcoma, skin cancer) have been observed in transplant recipients receiving cyclosporine, other immunosuppressive agents may also predispose recipients to cancer. Some evidence suggests that tumors may arise after cyclosporine treatment because the drug induces TGF-b, which promotes tumor invasion and metastasis.
Cyclosporine may be used alone or in combination with other immunosuppressants, particularly glucocorticoids. It has been used successfully as the sole immunosuppressant for cadaveric transplants of the kidney, pancreas, and liver, and it has proved extremely useful in cardiac transplants as well. In combination with methotrexate, cyclosporine is a standard prophylactic regimen to prevent graft-versus-host disease after allogeneic stem cell transplants. Cyclosporine has also proved useful in a variety of autoimmune disorders, including uveitis, rheumatoid arthritis, psoriasis, and asthma. Its combination with newer agents is showing considerable efficacy in clinical and experimental settings where effective and less toxic immunosuppression is needed. Newer formulations of cyclosporine have been developed that are improving patient compliance (smaller, better tasting pills), and increasing bioavailability.
2. Tacrolimus
Tacrolimus (FK 506) is an immunosuppressant macrolide antibiotic produced by Streptomyces tsukubaensis. It is not chemically related to cyclosporine, but their mechanisms of action are similar. Both drugs bind to cytoplasmic peptidyl-prolyl isomerases that are abundant in all tissues. While cyclosporine binds to cyclophilin, tacrolimus binds to the immunophilin FK-binding protein (FKBP). Both complexes inhibit calcineurin, which is necessary for the activation of the T-cell-specific transcription factor NF-AT.
On a weight basis, tacrolimus is 10-100 times more potent than cyclosporine in inhibiting immune responses. Tacrolimus is utilized for the same indications as cyclosporine, particularly in organ and stem cell transplantation. Multicenter studies in the USA and in Europe indicate that both graft and patient survival are similar for the two drugs. Tacrolimus has been proven to be effective therapy for preventing rejection in solid-organ transplant patients even after failure of standard rejection therapy, including anti-T-cell antibodies. It is now considered a standard prophylactic agent (usually in combination with methotrexate or mycophenolate mofetil) for graft-versus-host disease.
Tacrolimus can be administered orally or intravenously. The half-life of the intravenous form is approximately 9-12 hours. Like cyclosporine, tacrolimus is metabolized primarily by P450 enzymes in the liver, and there is potential for drug interactions. The dosage is determined by trough blood level at steady state. Its toxic effects are similar to those of cyclosporine and include nephrotoxicity, neurotoxicity, hyperglycemia, hypertension, hyperkalemia, and gastrointestinal complaints.
Because of the effectiveness of systemic tacrolimus in some dermatologic diseases, a topical preparation is now available. Tacrolimus ointment is currently used in the therapy of atopic dermatitis and psoriasis.
3. Sirolimus
Sirolimus (rapamycin) is derived from Streptomyces hygroscopicus and binds immunophilins and inhibits calcineurin, as do cyclosporine and tacrolimus. However, it does not block interleukin production by activated T cells but instead blocks the response of T cells to cytokines. In vitro, it antagonizes tacrolimus-induced T-cell responses but seems to be synergistic with cyclosporine. Furthermore, it is a potent inhibitor of B-cell proliferation and immunoglobulin production. Sirolimus also inhibits the mononuclear cell proliferative response to colony-stimulating factors and suppresses hematopoietic recovery after myelotoxic treatment in mice.
Sirolimus is available only as an oral drug. It is rapidly absorbed and its elimination is similar to that of cyclosporine and tacrolimus, being a substrate for both cytochrome P450 3A and P-glycoprotein. Significant drug interactions can occur, and the drug level in the blood may need to be monitored.
Sirolimus has been used effectively alone and in combination with other immunosuppressants (corticosteroids, cyclosporine, tacrolimus, and mycophenolate mofetil) to prevent rejection of solid organ allografts. Sirolimus is being investigated as therapy for steroid-refractory acute and chronic graft-versus-host disease in hematopoietic stem cell transplant recipients. Topical sirolimus is also used in some dermatologic disorders and, in combination with cyclosporine, in the management of uveoretinitis. Recently, sirolimus-eluting coronary stents have been shown to reduce restenosis and additional adverse cardiac events in patients with severe coronary artery disease, due to its antiproliferative effects. A derivative of sirolimus, everolimus, is a proliferation-signal inhibitor that may be of benefit in decreasing rejection in cardiac transplantation.
Toxicities of sirolimus can include profound myelosuppression (especially thrombocytopenia), hepatotoxicity, diarrhea, hypertriglyceridemia, and headache.
Mycophenolate mofetil (MMF) is a semisynthetic derivative of mycophenolic acid, isolated from the mold Penicillium glaucum. In vitro, it inhibits T- and B-lymphocyte responses, including mitogen and mixed lymphocyte responses, probably by inhibition of de novo synthesis of purines. Mycophenolate mofetil is hydrolyzed to mycophenolic acid, the active immunosuppressive moiety; it is synthesized and administered as MMF to enhance bioavailability. Mycophenolate mofetil is used in solid organ transplant patients for refractory rejection and, in combination with prednisone, as an alternative to cyclosporine or tacrolimus in patients who do not tolerate those drugs. Mycophenolate mofetil is used to treat steroid-refractory graft-versus-host disease in hematopoietic stem cell transplant patients. It is also used in combination with tacrolimus or other immunosuppressants as prophylaxis to prevent graft-versus-host disease. Newer immunosuppressant applications for MMF include lupus nephritis, rheumatoid arthritis, and some dermatologic disorders.
Mycophenolate mofetil is available in both oral and intravenous forms. The oral form is rapidly metabolized to mycophenolic acid but not by the cytochrome P450 3A system, though some drug interactions still occur.
Toxicities include gastrointestinal disturbances (nausea and vomiting, diarrhea, abdominal pain) headache, hypertension and reversible myelosuppression (primarily neutropenia).
Thalidomide is a sedative drug that was withdrawn from the market in the 1960s because of its disastrous teratogenic effects when used during pregnancy. Nevertheless, it has significant immunomodulatory actions and is currently in active use or in clinical trials for over 40 different illnesses. Thalidomide inhibits angiogenesis and has anti-inflammatory and immunomodulatory effects. It inhibits TNF-a, reduces phagocytosis by neutrophils, increases production of IL-10, alters adhesion molecule expression, and enhances cell-mediated immunity via interactions with T cells. The complex actions of thalidomide continue to be studied as its clinical use evolves.
Thalidomide is currently used in the treatment of multiple myeloma at initial diagnosis and for relapsed-refractory disease. Patients generally show signs of response within 2-3 months of starting the drug, with response rates from 20 to 70%. When combined with dexamethasone, the response rates in myeloma are 90% or more in some studies. Many patients have durable responses¾up to 12-18 months in refractory disease and even longer in some patients treated at diagnosis. The success of thalidomide in myeloma has lead to numerous clinical trials in other diseases such as myelodysplastic syndrome, acute myelogenous leukemia, and graft-versus-host disease, as well as in solid tumors like colon cancer, renal cell carcinoma, melanoma, and prostate cancer, with variable results to date. Thalidomide has been used for many years in the treatment of some manifestations of leprosy and has been reintroduced in the USA for erythema nodosum leprosum; it is also useful in management of the skin manifestations of lupus erythematosus.
The adverse effect profile of thalidomide is extensive. The most important toxicity is teratogenesis. Because of this effect, thalidomide prescription and use is closely regulated by the manufacturer. Other adverse effects of thalidomide include peripheral neuropathy, constipation, rash, fatigue, hypothyroidism, and increased risk of deep vein thrombosis. Thrombosis is sufficiently frequent, particularly in the myeloma population, that most patients are placed on warfarin when thalidomide treatment is initiated.
Owing to thalidomide’s serious toxicity profile, considerable effort has been expended in the development of analogs. Immunomodulatory derivatives of thalidomide are termed IMiDs. Some IMiDs are much more potent than thalidomide in regulating cytokines and affecting T-cell proliferation. Lenalidomide is an IMiD that in animal and in vitro studies has been shown to be similar to thalidomide in action, but with less toxicity, especially teratogenicity. Lenalidomide was approved by the Food and Drug Administration in late 2005 as a consequence of trials that showed its effectiveness in the treatment of the myelodysplastic syndrome with the chromosome 5q31 deletion. Several clinical trials using lenalidomide to treat relapsed or refractory myeloma are showing benefits and it is likely to be approved by the FDA for that indication as well.
CC-4047 (Actimid) is another IMiD that is being investigated for the treatment of myelodysplastic syndrome, myeloma, and prostate cancer.
Another group of thalidomide analogs, selective cytokine inhibitory drugs (SelCIDs), are phosphodiesterase type 4 inhibitors with potent anti-TNF-a activity but no T-cell co-stimulatory activity. Several SelCIDs are currently under investigation for clinical use.
1. Azathioprine
Azathioprine is a prodrug of mercaptopurine and, like mercaptopurine, functions as an antimetabolite. Although its action is presumably mediated by conversion to mercaptopurine and further metabolites, it has been more widely used than mercaptopurine for immunosuppression in humans. These agents represent prototypes of the antimetabolite group of cytotoxic immunosuppressive drugs, and many other agents that kill proliferative cells appear to work at a similar level in the immune response.
Azathioprine is well absorbed from the gastrointestinal tract and is metabolized primarily to mercaptopurine. Xanthine oxidase splits much of the active material to 6-thiouric acid prior to excretion in the urine. After administration of azathioprine, small amounts of unchanged drug and mercaptopurine are also excreted by the kidney, and as much as a twofold increase in toxicity may occur in anephric or anuric patients. Since much of the drug’s inactivation depends on xanthine oxidase, patients who are also receiving allopurinol for control of hyperuricemia should have the dose of azathioprine reduced to one-fourth to one-third the usual amount to prevent excessive toxicity.
Azathioprine and mercaptopurine appear to produce immunosuppression by interfering with purine nucleic acid metabolism at steps that are required for the wave of lymphoid cell proliferation that follows antigenic stimulation. The purine analogs are thus cytotoxic agents that destroy stimulated lymphoid cells. Although continued messenger RNA synthesis is necessary for sustained antibody synthesis by plasma cells, these analogs appear to have less effect on this process than oucleic acid synthesis in proliferating cells. Cellular immunity as well as primary and secondary serum antibody responses can be blocked by these cytotoxic agents.
Azathioprine and mercaptopurine appear to be of definite benefit in maintaining renal allografts and may be of value in transplantation of other tissues. These antimetabolites have been used with some success in the management of acute glomerulonephritis and in the renal component of systemic lupus erythematosus. They have also proved useful in some cases of rheumatoid arthritis, Crohn’s disease, and multiple sclerosis. The drugs have been of occasional use in prednisone-resistant antibody-mediated idiopathic thrombocytopenic purpura and autoimmune hemolytic anemias.
The chief toxic effect of azathioprine and mercaptopurine is bone marrow suppression, usually manifested as leukopenia, although anemia and thrombocytopenia may occur. Skin rashes, fever, nausea and vomiting, and sometimes diarrhea occur, with the gastrointestinal symptoms seen mainly at higher dosages. Hepatic dysfunction, manifested by very high serum alkaline phosphatase levels and mild jaundice, occurs occasionally, particularly in patients with preexisting hepatic dysfunction.
The alkylating agent cyclophosphamide is one of the most efficacious immunosuppressive drugs available. Cyclophosphamide destroys proliferating lymphoid cells but also appears to alkylate some resting cells. It has been observed that very large doses (eg, > 120 mg/kg intravenously over several days) may induce an apparent specific tolerance to a new antigen if the drug is administered simultaneously with, or shortly after, the antigen. In smaller doses, it has been effective against autoimmune disorders (including systemic lupus erythematosus) and in patients with acquired factor XIII antibodies and bleeding syndromes, autoimmune hemolytic anemia, antibody-induced pure red cell aplasia, and Wegener’s granulomatosis.
Treatment with large doses of cyclophosphamide carries considerable risk of pancytopenia and hemorrhagic cystitis and therefore is generally combined with stem cell rescue (transplant) procedures. Although cyclophosphamide appears to induce tolerance for marrow or immune cell grafting, its use does not prevent the subsequent graft-versus-host disease syndrome, which may be serious or lethal if the donor is a poor histocompatibility match (despite the severe immunosuppression induced by high doses of cyclophosphamide). Other adverse effects of cyclophosphamide include nausea, vomiting, cardiac toxicity, and electrolyte disturbances.
3. Leflunomide
Leflunomide is a prodrug of an inhibitor of pyrimidine synthesis (rather than purine synthesis). It is orally active, and the active metabolite has a long half-life of several weeks. Thus, the drug should be started with a loading dose, but it can be taken once daily after reaching steady state. It is approved only for rheumatoid arthritis at present, though studies are underway combining leflunomide with mycophenolate mofetil for a variety of autoimmune and inflammatory skin disorders, as well as preservation of allografts in solid organ transplantation. Leflunomide also appears (from murine data) to have antiviral activity.
Toxicities include elevation of liver enzymes with some risk of liver damage, renal impairment, and teratogenic effects. A low frequency of cardiovascular effects (angina, tachycardia) was reported in clinical trials of leflunomide.
4. Hydroxychloroquine
Hydroxychloroquine is an antimalarial agent with immunosuppressant properties. It is thought to suppress intracellular antigen processing and loading of peptides onto MHC class II molecules by increasing the pH of lysosomal and endosomal compartments, thereby decreasing T-cell activation.
Because of these immunosuppressant activities, hydroxychloroquine is used to treat some autoimmune disorders, eg, rheumatoid arthritis and systemic lupus erythematosus. It has also been used to both treat and prevent graft-versus-host disease after allogeneic stem cell transplantation.
Other cytotoxic agents, including vincristine, methotrexate, and cytarabine, also have immunosuppressive properties. Methotrexate has been used extensively in rheumatoid arthritis and in the treatment of graft-versus-host disease. Although the other agents can be used for immunosuppression, their use has not been as widespread as the purine antagonists, and their indications for immunosuppression are less certain. The use of methotrexate (which can be given orally) appears reasonable in patients with idiosyncratic reactions to purine antagonists. The antibiotic dactinomycin has also been used with some success at the time of impending renal transplant rejection. Vincristine appears to be quite useful in idiopathic thrombocytopenic purpura refractory to prednisone. The related vinca alkaloid vinblastine has been shown to prevent mast cell degranulation in vitro by binding to microtubule units within the cell and to prevent release of histamine and other vasoactive compounds. Pentostatin is an adenosine deaminase inhibitor primarily used as an antineoplastic agent for lymphoid malignancies, and produces a profound lymphopenia. It is now frequently used for steroid-resistant graft-versus-host disease after allogeneic stem cell transplantation, as well as in preparative regimens prior to those transplants to provide severe immunosuppression to prevent allograft rejection.
IMMUNOSUPPRESSIVE ANTIBODIES
The development of hybridoma technology by Milstein and Kohler in 1975 revolutionized the antibody field and radically increased the purity and specificity of antibodies used in the clinic and for diagnostic tests in the laboratory. Hybridomas consist of antibody-forming cells fused to immortal plasmacytoma cells. Hybrid cells that are stable and produce the required antibody can be subcloned for mass culture for antibody production. Large-scale fermentation facilities are now used for this purpose in the pharmaceutical industry.
More recently, molecular biology has been used to develop monoclonal antibodies. Combinatorial libraries of cDNAs encoding immunoglobulin heavy and light chains expressed on bacteriophage surfaces are screened against purified antigens. The result is an antibody fragment with specificity and high affinity for the antigen of interest. This technique has been used to develop antibodies specific for viruses (eg, HIV), bacterial proteins, tumor antigens, and even cytokines. Several antibodies developed in this manner are in clinical trials.
Other genetic engineering techniques involve production of chimeric and humanized versions of murine monoclonal antibodies in order to reduce their antigenicity and increase the half-life of the antibody in the patient. Murine antibodies administered as such to human patients evoke production of human antimouse antibodies (HAMA), which clear the original murine proteins very rapidly. Humanization involves replacing most of the murine antibody with equivalent human regions while keeping only the variable, antigen-specific regions intact. Chimeric mouse-human antibodies have similar properties with less complete replacement of the murine components. The current naming convention for these engineered substances uses the suffix “umab” or “zumab” for humanized antibodies, and “imab” or “ximab” for chimeric products. These procedures have been successful in reducing or preventing HAMA production for many of the antibodies discussed below.
1. Antilymphocyte & Antithymocyte Antibodies
Antisera directed against lymphocytes have been prepared sporadically for over 100 years. With the advent of human organ transplantation as a therapeutic option, heterologous antilymphocyte globulin (ALG) took oew importance. ALG and antithymocyte globulin (ATG) are now in clinical use in many medical centers, especially in transplantation programs. The antiserum is usually obtained by immunization of large animals such as horses or sheep with human lymphoid cells.
Antilymphocyte antibody acts primarily on the small, long-lived peripheral lymphocytes that circulate between the blood and lymph. With continued administration, “thymus-dependent” lymphocytes from lymphoid follicles are also depleted, as they normally participate in the recirculating pool. As a result of the destruction or inactivation of T cells, an impairment of delayed hypersensitivity and cellular immunity occurs while humoral antibody formation remains relatively intact. ALG and ATG are useful for suppressing certain major compartments (ie, T cells) of the immune system and play a definite role in the management of solid organ and bone marrow transplantation.
Monoclonal antibodies directed against specific antigens such as CD3, CD4, CD25, CD40, IL-2 receptor, and TNF (discussed below) much more selectively influence T-cell subset function. The high specificity of these antibodies improves selectivity and reduces toxicity of the therapy and alters the disease course in several different autoimmune disorders.
In the management of transplants, ALG and monoclonal antibodies can be used in the induction of immunosuppression, in the treatment of initial rejection, and in the treatment of steroid-resistant rejection. There has been some success in the use of ALG and ATG plus cyclosporine to prepare recipients for bone marrow transplantation. In this procedure, the recipient is treated with ALG or ATG in large doses for 7-10 days prior to transplantation of bone marrow cells from the donor. Residual ALG appears to destroy the T cells in the donor marrow graft, and the probability of severe graft-versus-host syndrome is reduced.
The adverse effects of ALG are mostly those associated with injection of a foreign protein obtained from heterologous serum. Local pain and erythema often occur at the injection site (type III hypersensitivity). Since the humoral antibody mechanism remains active, skin-reactive and precipitating antibodies may be formed against the foreign IgG. Similar reactions occur with monoclonal antibodies of murine origin, and reactions thought to be caused by the release of cytokines by T cells and monocytes have also been described.
Anaphylactic and serum sickness reactions to ALG and murine monoclonal antibodies have been observed and usually require cessation of therapy. Complexes of host antibodies with horse ALG may precipitate and localize in the glomeruli of the kidneys. Even more disturbing has been the development of histiocytic lymphomas in the buttock at the site of ALG injection. The incidence of lymphoma as well as other forms of cancer is increased in kidney transplant patients. It appears likely that part of the increased risk of cancer is related to the suppression of a normally potent defense system against oncogenic viruses or transformed cells. The preponderance of lymphoma in these cancer cases is thought to be related to the concurrence of chronic immune suppression with chronic low-level lymphocyte proliferation.
2. Muromonab-CD3
Monoclonal antibodies against T-cell surface proteins are increasingly being used in the clinic for autoimmune disorders and in transplantation settings. Clinical studies have shown that the murine monoclonal antibody muromonab-CD3 (OKT3) directed against the CD3 molecule on the surface of human thymocytes and mature T cells can also be useful in the treatment of renal transplant rejection. In vitro, muromonab-CD3 blocks killing by cytotoxic human T cells and several other T-cell functions. In a prospective randomized multicenter trial with cadaveric renal transplants, use of muromonab-CD3 (along with lower doses of steroids or other immunosuppressive drugs) proved more effective at reversing acute rejection than did conventional steroid treatment. Muromonab-CD3 is approved for the treatment of renal allograft rejection crises. Several other monoclonal antibodies directed against surface markers on lymphocytes are approved for certain indications (see monoclonal antibody section below), while others are in various stages of development and clinical trials.
3. Immune Globulin Intravenous (IGIV)
A quite different approach to immunomodulation is the intravenous use of polyclonal human immunoglobulin. This immunoglobulin preparation (usually IgG) is prepared from pools of thousands of healthy donors, and no specific antigen is the target of the “therapeutic antibody.” Rather, one expects that the pool of different antibodies will have a normalizing effect upon the patient’s immune networks.
IGIV in high doses (2 g/kg) has proved effective in a variety of different conditions ranging from immunoglobulin deficiencies to autoimmune disorders to HIV disease to bone marrow transplants. In patients with Kawasaki’s disease, it has been shown to be safe and effective, reducing systemic inflammation and preventing coronary artery aneurysms. It has also brought about good clinical responses in systemic lupus erythematosus and refractory idiopathic thrombocytopenic purpura. Possible mechanisms of action of IGIV include a reduction of T helper cells, increase of suppressor T cells, decreased spontaneous immunoglobulin production, Fc receptor blockade, increased antibody catabolism, and idiotypic-anti-idiotypic interactions with “pathologic antibodies.” Although its precise mechanism of action is still controversial, IGIV brings undeniable clinical benefit to many patients with a variety of immune syndromes.
4. Rho(D) Immune Globulin Micro-Dose
One of the earliest major advances in immunopharmacology was the development of a technique for preventing Rh hemolytic disease of the newborn. The technique is based on the observation that a primary antibody response to a foreign antigen can be blocked if specific antibody to that antigen is administered passively at the time of exposure to antigen. Rho(D) immune globulin is a concentrated (15%) solution of human IgG containing a higher titer of antibodies against the Rho(D) antigen of the red cell.
Sensitization of Rh-negative mothers to the D antigen occurs usually at the time of birth of an Rho(D)-positive or Du-positive infant, when fetal red cells may leak into the mother’s bloodstream. Sensitization might also occur occasionally with miscarriages or ectopic pregnancies. In subsequent pregnancies, maternal antibody against Rh-positive cells is transferred to the fetus during the third trimester, leading to the development of erythroblastosis fetalis (hemolytic disease of the newborn).
If an injection of Rho(D) antibody is administered to the mother within 24-72 hours after the birth of an Rh-positive infant, the mother’s own antibody response to the foreign Rho(D)-positive cells is suppressed because the infant’s red cells are cleared from circulation before the mother can generate a B-cell response against Rho(D). Therefore she has no memory B cells that can activate upon subsequent pregnancies with an Rho(D)-positive fetus.
When the mother has been treated in this fashion, Rh hemolytic disease of the newborn has not been observed in subsequent pregnancies. For this prophylactic treatment to be successful, the mother must be Rho(D)-negative and Du-negative and must not already be immunized to the Rho(D) factor. Treatment is also often advised for Rh-negative mothers who have had miscarriages, ectopic pregnancies, or abortions, when the blood type of the fetus is unknown. Note: Rho(D) immune globulin is administered to the mother and must not be given to the infant.
The usual dose of Rho(D) immune globulin is 2 mL intramuscularly, containing approximately 300 mcg anti-Rho(D) IgG. Adverse reactions are infrequent and consist of local discomfort at the injection site or, rarely, a slight temperature elevation.
5. Hyperimmune Immunoglobulins
Hyperimmune immunoglobulin preparations are IGIV preparations made from pools of selected human or animal donors with high titers of antibodies against particular agents of interest such as viruses or toxins. Various hyperimmune IGIVs are available for treatment of respiratory syncytial virus, cytomegalovirus, varicella zoster, human herpesvirus 3, hepatitis B virus, rabies, tetanus, and digoxin overdose. Intravenous administration of the hyperimmune globulins is a passive transfer of high titer antibodies that either reduces risk or reduces the severity of infection. Rabies hyperimmune globulin is injected around the wound and given intravenously. Tetanus hyperimmune globulin is administered intravenously when indicated for prophylaxis. Rattlesnake and coral snake hyperimmune globulins (antivenins) are of equine origin and are effective for North and South American rattlesnakes and some coral snakes (but not Arizona coral snake). Equine and ovine antivenins are available for rattlesnake envenomations, but only equine antivenin is available for coral snake bite. The ovine antivenin is a Fab preparation and is less immunogenic than whole equine IgG antivenins, but retains the ability to neutralize the rattlesnake venom.
MONOCLONAL ANTIBODIES (MABS)
Recent advances in the ability to manipulate the genes of immunoglobulins have resulted in development of a wide array of humanized and chimeric monoclonal antibodies directed against therapeutic targets. The only murine elements of humanized monoclonal antibodies are the complementarity-determining regions in the variable domains of immunoglobulin heavy and light chains. Complementarity-determining regions are primarily responsible for the antigen-binding capacity of antibodies. Chimeric antibodies typically contain antigen-binding murine variable regions and human constant regions. The following are brief descriptions of the engineered antibodies that have been approved by the FDA.
Alemtuzumab is a humanized IgG1 with a kappa chain that binds to CD52 found oormal and malignant B and T lymphocytes, NK cells, monocytes, macrophages, and a small population of granulocytes. Currently, alemtuzumab is approved for the treatment of B-cell chronic lymphocytic leukemia in patients who have been treated with alkylating agents and have failed fludarabine therapy. Alemtuzumab appears to deplete leukemic and normal cells by direct antibody-dependent lysis. Patients receiving this antibody become lymphopenic and may also become neutropenic, anemic, and thrombocytopenic. As a result patients should be closely monitored for opportunistic infections and hematologic toxicity.
Bevacizumab is a humanized IgG1 monoclonal antibody that binds to vascular endothelial growth factor (VEGF) and inhibits VEGF from binding to its receptor, especially on endothelial cells. It is an antiangiogenic drug that has been shown to inhibit growth of blood vessels (angiogenesis) in tumors. It is approved for first-line treatment of patients with metastatic colorectal cancer alone or in combination with 5-FU-based chemotherapy. Since bevacizumab is antiangiogenic, it should not be administered until patients heal from surgery. Patients taking the drug should be watched for hemorrhage, gastrointestinal perforations, and wound healing problems.
Cetuximab is a human-mouse chimeric monoclonal antibody that targets epidermal growth factor receptor (EGFR). Binding of cetuximab to EGFR inhibits tumor cell growth by a variety of mechanisms, including decreases in kinase activity, matrix metalloproteinase activity, and growth factor production, and increased apoptosis. It is indicated for use in patients with metastatic colorectal cancer whose tumors overexpress EGFR. Cetuximab may be administered in combination with irinotecan or alone in patients who cannot tolerate irinotecan.
Gemtuzumab is a humanized IgG4 monoclonal antibody with a kappa light chain specific for CD33, a sialoadhesion protein found on leukemic blast cells in 80-90% of patients with acute myelogenous leukemia (AML). Gemtuzumab alone has some antiblast activity. In the clinical formulation, gemtuzumab is coupled to the cytotoxic agent, ozogamicin, which is a semisynthetic derivative of calicheamicin, an antibiotic with antitumor activity. Internalization of gemtuzumab-ozogamicin by the tumor cell results in release of the cytotoxin from the antibody in the lysosome. Ozogamicin then binds to the minor groove in DNA, causing double-strand breaks and cell death.
Gemtuzumab is approved for the treatment of patients 60 years and older in first relapse with CD33 acute myelogenous leukemia who are not considered candidates for other types of cytotoxic chemotherapy. Adverse events due to the administration of gemtuzumab-ozogamicin include severe myelosuppression, especially neutropenia, requiring careful hematologic monitoring. Other adverse events associated with gemtuzumab are significant hepatotoxicity and various hypersensitivity reactions.
Rituximab is a chimeric murine-human monoclonal IgG1 (human Fc) that binds to the CD20 molecule oormal and malignant B lymphocytes and is approved for the therapy of patients with relapsed or refractory low-grade or follicular, B-cell non-Hodgkin’s lymphoma. The mechanism of action includes complement-mediated lysis, antibody-dependent cellular cytotoxicity, and induction of apoptosis in the malignant lymphoma cells. This drug appears to be synergistic with chemotherapy (eg, fludarabine, CHOP) for lymphoma.
Trastuzumab is a recombinant DNA-derived, humanized monoclonal antibody that binds to the extracellular domain of the human epidermal growth factor receptor HER-2/neu. This antibody blocks the natural ligand from binding and down-regulates the receptor. Trastuzumab is approved for the treatment of metastatic breast cancer in patients whose tumors overexpress HER-2/neu. As a single agent it induces remission in about 15-20% of patients; in combination with chemotherapy, it increases response rate and duration as well as 1-year survival. Trastuzumab is under investigation for other tumors that express HER-2.
MABs Used to Deliver Isotopes to Tumors
Arcitumomab is a murine F(ab¢)2 fragment from an anti-carcinoembryonic antigen (CEA) antibody labeled with technetium 99m (99mTc) that is used for imaging patients with metastatic colorectal carcinoma (immunoscintigraphy) to determine extent of disease. CEA is often upregulated on tumor in patients with gastrointestinal carcinomas. The use of the F(ab¢)2 fragment decreases the immunogenicity of the agent so that it can be given more than once, unlike other intact murine monoclonal antibodies.
Capromab pendetide is a murine monoclonal antibody specific for prostate specific membrane antigen. It is coupled to isotopic indium (111In) and is used in immunoscintigraphy for patients with biopsy-confirmed prostate cancer and post-prostatectomy in patients with rising prostate specific antibody level to determine extent of disease.
Ibritumomab tiuxetan is an anti-CD20 murine monoclonal antibody labeled with isotopic yttrium (90Y) or 111In. The radiation of the isotope provides the major antitumor activity. Ibritumomab is approved for use in patients with relapsed or refractory low-grade, follicular, or B-cell non-Hodgkin’s lymphoma, including patients with rituximab-refractory follicular disease. It is used in conjunction with rituximab in a two-step therapeutic regimen.
Nofetumomab is a mouse monoclonal antibody coupled to 99mTc that is used for diagnostic purposes to determine extent of disease and to stage patients with small cell lung cancer. It binds a 40 kD antigen found on many tumor cell types, but also on some normal cells. It is an accurate indicator of extent of disease in biopsy-confirmed small cell lung cancer except in those patients with brain or adrenal metastases.
Tositumomab is another anti-CD20 monoclonal antibody and is complexed with iodine 131 (131I). Tositumomab is used in two-step therapy in patients with CD20-positive, follicular non-Hodgkin’s lymphoma whose disease is refractory to rituximab and standard chemotherapy. Toxicities are similar to those for ibritumomab and include severe cytopenias such as thrombocytopenia and neutropenia. Tositumomab should not be administered to patients with greater than 25% bone marrow involvement.
MABs Used as Immunosuppressants and Anti-Inflammatory Agents
A. ANTI-TNF-ALPHA MABS
Adalimumab, etanercept, and infliximab are antibodies that bind TNF-a, a proinflammatory cytokine. Blocking TNF-a from binding to TNF receptors on inflammatory cell surfaces results in suppression of downstream inflammatory cytokines such as IL-1 and IL-6 and adhesion molecules involved in leukocyte activation and migration. An increased risk of lymphoma is common to each of these agents.
Adalimumab is a completely human IgG1 approved for use in rheumatoid arthritis. Like the other anti-TNF-a biologicals, adalimumab blocks the interaction of TNF-a with TNF receptors on cell surfaces; it does not bind TNF-b. Pharmacodynamic studies showed that administration of adalimumab reduced levels of Creactive protein, erythrocyte sedimentation rate, serum IL-6 and matrix metalloproteinases MMP-1 and MMP-
Etanercept is a dimeric fusion protein composed of human IgG1 constant regions (CH2, CH3, and hinge, but not CH1) fused to the TNF receptor. Etanercept binds to both TNF-a and TNF-b and appears to have effects similar to that of infliximab, ie, inhibition of TNF-a-mediated inflammation, but its half-life is shorter due to its physical form (fusion protein) and the route of injection (subcutaneously, twice weekly). Etanercept is approved for adult RA, polyarticular-course juvenile RA, and psoriatic arthritis. It may be used in combination with methotrexate.
Infliximab is a human-mouse chimeric IgG1 monoclonal antibody possessing human constant (Fc) regions and murine variable regions. Infliximab is currently approved for use in Crohn’s disease, ulcerative colitis, rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis.
B. ABATACEPT
Abatacept is a recombinant fusion protein composed of the extracellular domain of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) fused to human IgG Fc. CTLA-4 is a costimulatory molecule found on T cells that binds to CD80 and CD86 on antigen presenting cells. This fusion protein blocks activation of T cells by binding to CD80 or 86 so that CD28 on T cells cannot bind and stimulate the T cell and lead to cytokine release. Abatacept is approved for patients with severe rheumatoid arthritis who have failed other DMARDS. Patients should not take other anti-TNF drugs or anakinra while taking abatacept.
C. ALEFACEPT
Alefacept is an engineered protein consisting of the CD2-binding portion of leukocyte-function-associated antigen-3 (LFA-3) fused to a human IgG1 Fc region (hinge, CH1, and CH2), approved for the treatment of plaque psoriasis. It inhibits activation of T cells by binding to cell surface CD2, inhibiting the normal CD2/LFA-3 interaction. Treatment of patients with alefacept also results in a dose-dependent reduction of the total number of circulating T cells, including those that predominate in psoriatic plaques. Therefore, T-cell counts of patients receiving alefacept must be monitored, and the drug discontinued if CD4 lymphocyte levels fall below 250 cells/uL.
D. BASILIXIMAB
Basiliximab is a chimeric mouse-human IgG1 that binds to CD25, the IL-2 receptor alpha chain on activated lymphocytes. It functions as an IL-2 antagonist, blocking IL-2 from binding to activated lymphocytes, and is therefore immunosuppressive. It is indicated for prophylaxis of acute organ rejection in renal transplant patients and is usually used as part of an immunosuppressive regimen that also includes glucocorticoids and cyclosporine A.
E. DACLIZUMAB
Daclizumab is a humanized IgG1 that binds to the alpha subunit of the IL-2 receptor. Its indications are identical to that of basiliximab, but the mode of administration differs.
F. EFALIZUMAB
Efalizumab is a recombinant humanized anti-CD11a monoclonal antibody approved for the treatment of adult patients with severe psoriasis. Binding of efalizumab to CD11a (the alpha subunit of LFA-1) inhibits the interaction of LFA-1 on all lymphocytes with intercellular adhesion molecule-1 (ICAM-1), thereby inhibiting the adhesion, activation, and migration of lymphocytes into skin. Efalizumab is administered by subcutaneous injection.
G. OMALIZUMAB
Omalizumab is an anti-IgE recombinant humanized monoclonal antibody that is approved for the treatment of allergic asthma in adult and adolescent patients whose symptoms are refractory to inhaled corticosteroids. The antibody blocks the binding of IgE to the high-affinity Fce receptor on basophils and mast cells, which suppresses IgE-mediated release of type I allergy mediators such as histamine and leukotrienes. Total serum IgE levels may remain elevated in patients for up to 1 year after administration of this antibody.
Abciximab is a Fab fragment of a murine-human monoclonal antibody that binds to the integrin GPIIb/IIIa receptor on activated platelets and inhibits fibrinogen, von Willebrand factor, and other adhesion molecules from binding to activated platelets, thus preventing their aggregation.
Palivizumab is a monoclonal antibody that binds to the fusion protein of respiratory syncytial virus, preventing infection in susceptible cells in the airways. It is used ieonates at risk for this viral infection and reduces the frequency of infection and hospitalization by about 50%.
III. CLINICAL USES OF IMMUNOSUPPRESSIVE DRUGS
Immunosuppressive agents are commonly used in two clinical circumstances: transplantation and autoimmune disorders. The agents used differ somewhat for the specific disorders treated, as do administration schedules. Because autoimmune disorders are very complex, optimal treatment schedules have yet to be established in many clinical situations.
SOLID ORGAN AND BONE MARROW TRANSPLANTATION
In organ transplantation, tissue typing¾based on donor and recipient histocompatibility matching with the human leukocyte antigen (HLA) haplotype system¾is required. Close histocompatibility matching reduces the likelihood of graft rejection and may also reduce the requirements for intensive immunosuppressive therapy. Prior to transplant, patients may receive an immunosuppressive regimen, including antithymocyte globulin, muromonab-CD3, daclizumab, or basiliximab. Four types of rejection can occur in a solid organ transplant recipient: hyperacute, accelerated, acute, and chronic. Hyperacute rejection is due to preformed antibodies against the donor organ, such as anti-blood group antibodies. Hyperacute rejection occurs within hours of the transplant and cannot be stopped with immunosuppressive drugs. It results in rapid necrosis and failure of the transplanted organ. Accelerated rejection is mediated by both antibodies and T cells, but it also cannot be stopped by immunosuppressive drugs. Acute rejection of an organ occurs within days to months and involves mainly cellular immunity. Reversal of acute rejection is usually possible with general immunosuppressive drugs such as azathioprine, mycophenolate mofetil, cyclosporine, tacrolimus, glucocorticoids, cyclophosphamide, methotrexate, and sirolimus. Recently, biologic agents such as anti-CD3 monoclonal antibody have been used to stem acute rejection. Chronic rejection usually occurs months or even years after transplantation. It is characterized by thickening and fibrosis of the vasculature of the transplanted organ, involving both cellular and humoral immunity. Chronic rejection is treated with the same drugs as those used for acute rejection.
Allogeneic hematopoietic stem cell transplantation is a well established treatment for many malignant and nonmalignant diseases. An HLA-matched donor, usually a family member, is located, patients are conditioned with high-dose chemotherapy or radiation therapy, and then donor stem cells are infused. The conditioning regimen is used not only to kill cancer cells in the case of malignant disease, but also to totally suppress the immune system so that the patient does not reject the donor stem cells. As patients’ blood counts recover (after reduction by the conditioning regimen) they develop a new immune system that is created from the donor stem cells. Rejection of donor stem cells is uncommon, and can only be treated by infusion of more stem cells from the donor.
Graft-versus-host disease (GVHD), however, is very common, occurring in the majority of patients who receive an allogeneic transplant. Graft-versus-host disease occurs as donor T cells fail to recognize the patient’s skin, liver, and gut (usually) as self and attack those tissues. Although patients are given immunosuppressive therapy (cyclosporine, methotrexate, and others) early in the transplant course to help prevent this development, it usually occurs despite these medications. Acute graft-versus-host disease occurs within the first 100 days, and is usually manifested as a skin rash, severe diarrhea, or hepatotoxicity. Additional medications are added, invariably starting with high-dose corticosteroids, and adding drugs such as mycophenolate mofetil, sirolimus, tacrolimus, daclizumab, and others, with variable success rates. Patients generally progress to chronic graft-versus-host disease (after 100 days) and require therapy for variable periods thereafter. Unlike solid organ transplantation, however, stem cell transplant patients generally are able to discontinue immunosuppressive drugs as graft-versus-host disease resolves (generally 1-2 years after their transplant).
The effectiveness of immunosuppressive drugs in autoimmune disorders varies widely. Nonetheless, with immunosuppressive therapy, remissions can be obtained in many instances of autoimmune hemolytic anemia, idiopathic thrombocytopenic purpura, type 1 diabetes, Hashimoto’s thyroiditis, and temporal arteritis. Improvement is also often seen in patients with systemic lupus erythematosus, acute glomerulonephritis, acquired factor VIII inhibitors (antibodies), rheumatoid arthritis, inflammatory myopathy, scleroderma, and certain other autoimmune states.
Immunosuppressive therapy is utilized in chronic severe asthma, where cyclosporine is often effective and sirolimus is another alternative. Omalizumab (anti-IgE antibody) has recently been approved for the treatment of severe asthma (see previous section). Tacrolimus is currently under clinical investigation for the management of autoimmune chronic active hepatitis and of multiple sclerosis, where IFN-b has a definitive role.
The development of agents that modulate the immune response rather than suppress it has become an important area of pharmacology. The rationale underlying this approach is that such drugs may increase the immune responsiveness of patients who have either selective or generalized immunodeficiency. The major potential uses are in immunodeficiency disorders, chronic infectious diseases, and cancer. The AIDS epidemic has greatly increased interest in developing more effective immunomodulating drugs.
The cytokines are a large and heterogeneous group of proteins with diverse functions. Some are immunoregulatory proteins synthesized within lymphoreticular cells and play numerous interacting roles in the function of the immune system and in the control of hematopoiesis. In most instances, cytokines mediate their effects through receptors on relevant target cells and appear to act in a manner similar to the mechanism of action of hormones. In other instances, cytokines may have antiproliferative, antimicrobial, and antitumor effects.
The first group of cytokines discovered, the interferons (IFNs), were followed by the colony-stimulating factors (CSFs). The latter regulate the proliferation and differentiation of bone marrow progenitor cells. Most of the more recently discovered cytokines have been classified as interleukins (ILs) and numbered in the order of their discovery. Cytokines are produced using gene cloning techniques.
Most cytokines (including TNF-a, IFN-g, IL-2, granulocyte colony-stimulating factor [G-CSF], and granulocyte-macrophage colony-stimulating factor [GM-CSF]) have very short serum half-lives (minutes). The usual subcutaneous route of administration provides slower release into the circulation and a longer duration of action. Each cytokine has its own unique toxicity, but some toxicities are shared. For example, IFN-a, IFN-b, IFN-g, IL-2, and TNF-a all induce fever, flulike symptoms, anorexia, fatigue, and malaise.
Interferons are proteins that are currently grouped into three families: IFN-a, IFN-b, and IFN-g. The IFN-a and IFN-b families comprise type I IFNs, ie, acid-stable proteins that act on the same receptor on target cells. IFN-g, a type II IFN, is acid-labile and acts on a separate receptor on target cells. Type I IFNs are usually induced by virus infections, with leukocytes producing IFN-a. Fibroblasts and epithelial cells produce IFN-b. IFN-g is usually the product of activated T lymphocytes.
IFNs interact with cell receptors to produce a wide variety of effects that depend on the cell and IFN types. IFNs, particularly IFN-g, display immune-enhancing properties, which include increased antigen presentation and macrophage, NK cell, and cytotoxic T-lymphocyte activation. IFNs also inhibit cell proliferation. In this respect, IFN-a and IFN-b are more potent than IFN-g. Another striking IFN action is increased expression of MHC molecules on cell surfaces. While all three types of IFN induce MHC class I molecules, only IFN-g induces class II expression. In glial cells, IFN-b antagonizes this effect and may, in fact, decrease antigen presentation within the nervous system.
IFN-a is approved for the treatment of several neoplasms, including hairy cell leukemia, chronic myelogenous leukemia, malignant melanoma, and Kaposi’s sarcoma, and for use in hepatitis B and C infections. It has also shown activity as an anticancer agent in renal cell carcinoma, carcinoid syndrome, and T cell leukemia. IFN-b is approved for use in relapsing-type multiple sclerosis. IFN-g is approved for the treatment of chronic granulomatous disease and IL-2, for metastatic renal cell carcinoma and malignant melanoma. Numerous clinical investigations of the other cytokines, including IL-1, -3, -4, -6, -11, and -12, are still in progress. Toxicities of IFNs, which include fever, chills, malaise, myalgias, myelosuppression, headache, and depression, can severely restrict their clinical use.
TNF-a has been extensively tested in the therapy of various malignancies, but results have been disappointing due to dose-limiting toxicities. One exception is the use of intra-arterial high-dose TNF-a for malignant melanoma and soft tissue sarcoma of the extremities. In these settings, response rates greater than 80% have beeoted.
Cytokines have been under clinical investigation as adjuvants to vaccines, and IFNs and IL-2 have shown some positive effects in the response of human subjects to hepatitis B vaccine. IL-12 and GM-CSF have also shown adjuvant effects with vaccines. GM-CSF is of particular interest because it promotes recruitment of professional antigen-presenting cells such as the dendritic cells required for priming naive antigen-specific T-lymphocyte responses. There are some claims that GM-CSF can itself stimulate an antitumor immune response, resulting in tumor regression in melanoma and prostate cancer.
It is important to emphasize that cytokine interactions with target cells often result in the release of a cascade of different endogenous cytokines, which exert their effects sequentially or simultaneously. For example, IFN-g exposure increases the number of cell surface receptors on target cells for TNF-a. Therapy with IL-2 induces the production of TNF-a, while therapy with IL-12 induces the production of IFN-g.
A more recent application of immunomodulation therapy involves the use of cytokine inhibitors for inflammatory diseases and septic shock, conditions in which cytokines such as IL-1 and TNF-a are involved in the pathogenesis. Drugs now under investigation include anticytokine monoclonal antibodies, soluble cytokine receptors (soluble forms of IL-1 and TNF-a receptors occur naturally in humans), and the IL-1 receptor antagonist (IL-1Ra), anakinra. Anakinra is a recombinant form of the naturally occurring IL-1 receptor antagonist that prevents IL-1 from binding to its receptor, stemming the cascade of cytokines released if IL-1 were to bind to the IL-1R. Anakinra is approved for use in adult rheumatoid arthritis patients who have failed treatment with one or more disease-modifying antirheumatic drugs. Patients must be carefully monitored if they are also taking an anti-TNF-a drug, have chronic infections, or are otherwise immunosuppressed.
ADA Adenosine deaminase ALG Antilymphocyte globulin APC Antigen-presenting cell ATG Antithymocyte globulin CD Cluster of differentiation CSF Colony-stimulating factor CTL Cytotoxic T lymphocyte DC Dendritic cell DTH Delayed-type hypersensitivity FKBP FK-binding protein HAMA Human antimouse antibody HLA Human leukocyte antigen IFN Interferon IGIV Immune globulin intravenous IL Interleukin LFA Leukocyte function-associated antigen MAB Monoclonal antibody MHC Major histocompatibility complex NK cell Natural killer cell SCID Severe combined immunodeficiency disease TCR T cell receptor TGF-b Transforming growth factor-b TH1, TH2 T helper cell types 1 and 2 TNF Tumor necrosis factor
References:
1. Sally Roach (2007). Introductory Clinical Pharmacology, 7th ed., pp.215-224
2. Matthews, H. W. & Johnson, J. (2000). Racial, ethnic, and gender differences in response to drugs. In E. T. Herfindal & D. R. Gourley (Eds.), Textbook of therapeutics: Drug and disease management, 7th ed., pp. 93–103.
3.
