Section X. Drugs Used for Immunomodulation Chapter 53. Immunomodulators: Immunosuppressive Agents, Tolerogens, and Immunostimulants Overview This chapter provides a brief overview of the immune response as background for understanding the mechanism of action of immunomodulatory agents. The general principles of pharmacological immunosuppression are discussed in the context of potential targets, major indications, and unwanted side effects. Four major classes of immunosuppressive drugs are discussed: glucocorticoids (see also Chapter 60: Adrenocorticotropic Hormone; Adrenocortical Steroids and Their Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical Hormones), calcineurin inhibitors, antiproliferative and antimetabolic agents (see also Chapter 52: Antineoplastic Agents), and antibodies. The "holy grail" of immunomodulation is the induction and maintenance of immune tolerance, the active state of antigen-specific nonresponsiveness. Approaches expected to overcome the risks of infections and tumors with immunosuppression are reviewed. These include costimulatory blockade, donor-cell chimerism, soluble human leukocyte antigens (HLA), and antigen-based therapies. Lastly, a general discussion of the limited number of immunostimulant agents is presented, concluding with an overview of active and passive immunization. New immunotherapeutic approaches will address not only the issues of specific drug toxicities and efficacy but also long-term economic, metabolic, and quality-of-life outcomes. The Immune Response The immune system evolved to discriminate self from nonself. Multicellular organisms were faced with the problem of destroying infectious invaders (microbes) or dysregulated self (tumors) while leaving normal cells intact. These organisms responded by developing a robust array of receptor- mediated sensing and effector mechanisms broadly described as innate and adaptive. Innate, or natural, immunity is primitive, does not require priming, is of relatively low affinity, but is broadly reactive. Adaptive, or learned, immunity is antigen-specific, depends upon antigen exposure or priming, and can be of very high affinity. The two arms of immunity work closely together, with the innate immune system being most active early in an immune response and adaptive immunity becoming progressively dominant over time. The major effectors of innate immunity are complement, granulocytes, monocytes/macrophages, natural killer cells, mast cells, and basophils. The major effectors of adaptive immunity are B and T cells. B cells make antibodies; T cells function as helper, cytolytic, and regulatory (suppressor) cells. These cells are important in the normal immune response to infection and tumors but also mediate transplant rejection and autoimmunity (Janeway et al. , 1999; Paul, 1999). Immunoglobulins (antibodies) on the B-cell surface are receptors for a large variety of specific structural conformations. In contrast, T cells recognize antigens as peptide fragments in the context of self major histocompatibility complex (MHC) antigens (called HLA in human beings) on the surface of antigen-presenting cells (APCs), such as dendritic cells, macrophages, and other cell types expressing MHC class I (HLA-A, B, and C) and class II antigens (HLA-DR, DP, and DQ) in human beings. Once activated by specific antigen recognition via their respective clonally restricted cell-surface receptors, both B and T cells are triggered to differentiate and divide, leading to release of soluble mediators (cytokines, lymphokines) that perform as effectors and regulators of the immune response. The impact of the immune system in human disease is enormous. Developing vaccines against emerging infectious agents from human immunodeficiency virus (HIV) to Ebola virus is among the most critical challenges facing the research community. Immune system-mediated diseases are significant health-care problems. Immunological diseases are growing at epidemic proportions that require aggressive and innovative approaches to the development of new treatments. These diseases include a broad spectrum of autoimmune diseases such as rheumatoid arthritis, diabetes mellitus, systemic lupus erythematosus, and multiple sclerosis; solid tumors and hematologic malignancies; infectious diseases; asthma; and various allergic conditions. Furthermore, one of the great therapeutic opportunities for the treatment of many disorders is organ transplantation. However, immune system–mediated graft rejection remains the single greatest barrier to widespread use of this technology. An improved understanding of the immune system has led to the development of new therapies to treat immune system–mediated diseases. This chapter briefly reviews drugs used to modulate the immune response in three ways: immunosuppression, tolerance, and immunostimulation. Immunosuppression Immunosuppressive drugs are used to dampen the immune response in organ transplantation and autoimmune disease. In transplantation, the major classes of drugs used today are: (1) glucocorticoids, (2) calcineurin inhibitors, and (3) antiproliferative/antimetabolic agents. These drugs have met with a high degree of clinical success in treating conditions such as acute immune rejection of organ transplants and severe autoimmune diseases. However, such therapies require lifelong use and nonspecifically suppress the entire immune system, exposing patients to considerably higher risks of infection and cancer. The calcineurin inhibitors and steroids, in particular, are nephrotoxic and diabetogenic, thus limiting their usefulness in a variety of clinical settings. Monoclonal and polyclonal antibody preparations directed at reactive T cells are important adjunct therapies and provide a unique opportunity to selectively target specific immune-reactive cells and thus promote more specific treatments. Finally, new agents recently have expanded the arsenal of immunosuppressive agents. In particular, sirolimus and anti–CD25 [interleukin (IL)-2 receptor] antibodies (basiliximab, daclizumab) are being used to target growth factor pathways, substantially limiting clonal expansion and thus promoting tolerance. The most commonly used immunosuppressive drugs are described below. Nevertheless, many new, more selective, therapeutic agents are on the horizon and are expected to revolutionize immunotherapy in the next decade. General Approach to Organ Transplantation Therapy Organ transplant therapy is organized around five general principles. The first principle is careful patient preparation and selection of the best available ABO-compatible HLA match for organ donation (Legendre and Guttman, 1989). Second, a multitiered approach to immunosuppressive drug therapy, similar to that in cancer chemotherapy, is employed. Several agents are used simultaneously, each of which is directed at a different molecular target within the allograft response (Table 53–1; Krensky, et al. , 1990; Hong and Kahan, 2000a). Synergistic effects are obtained through application of the various agents at relatively low doses, thereby limiting specific toxicities while maximizing the immunosuppressive effect. The third principle is that greater immunosuppression is required to gain early engraftment and/or to treat established rejection than to maintain immunosuppression in the long term. Therefore, intensive induction and lower-dose maintenance drug protocols are employed. Fourth, careful investigation of each episode of transplant dysfunction is required, including evaluation for rejection, drug toxicity, and infection, keeping in mind that these various problems can and often do coexist. The fifth principle involves reduction or withdrawal of a therapeutic agent when its toxicity exceeds its benefit. Sequential Immunotherapy In many organ transplant centers, muromonab-CD3, anti-CD25 monoclonal antibodies, or polyclonal antilymphocyte antibodies are used as induction therapy in the immediate posttransplantation period (Wilde and Goa, 1996; Brennan et al. , 1999). This treatment enables initial engraftment without the use of high doses of nephrotoxic calcineurin inhibitors. Such protocols reduce the incidence of early rejection and appear to be particularly beneficial for patients at high risk for graft rejection (broadly presensitized or retransplant patients, pediatric recipients, or African Americans). Maintenance Immunotherapy The basic immunosuppressive protocol used in most transplant centers involves the use of multiple drugs simultaneously. Therapy typically involves a calcineurin inhibitor, steroids, and mycophenolate mofetil (a purine metabolism inhibitor), each directed at a discrete site in T-cell activation (Suthanthiran et al. , 1996; Perico and Remuzzi, 1997). Glucocorticoids, azathioprine, cyclosporine, tacrolimus, mycophenolate mofetil, sirolimus, and various monoclonal and polyclonal antibodies currently are approved by the United States Food and Drug Administration (FDA) for use in transplantation. Therapy for Established Rejection Although low doses of prednisone, calcineurin inhibitors, purine-metabolism inhibitors, or sirolimus are effective in preventing acute cellular rejection, they are not as effective in blocking T cells that already are activated, and they are not very effective against established, acute rejection or for the total prevention of chronic rejection (Monaco et al. , 1999). Therefore, treatment of established rejection requires the use of agents directed against activated T cells. These include glucocorticoids in high doses (pulse therapy), polyclonal antilymphocyte antibodies, or muromonab-CD3 monoclonal antibody. Adrenocortical Steroids The introduction of glucocorticoids as immunosuppressive drugs in the 1960s played a key role in making organ transplantation possible. The chemistry, pharmacokinetics, and drug interactions of adrenocortical steroids are described in Chapter 60: Adrenocorticotropic Hormone; Adrenocortical Steroids and Their Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical Hormones. Prednisone, prednisolone, and other glucocorticoids are used alone and in combination with other immunosuppressive agents for treatment of transplant rejection and autoimmune disorders. Mechanism of Action The immunosuppressive effects of glucocorticoids long have been known, but the specific mechanism(s) of their immunosuppressive action remains somewhat elusive (Rugstad, 1988; Beato, 1989). Steroids lyse and possibly induce the redistribution of lymphocytes, causing a rapid, transient decrease in peripheral blood lymphocyte counts. To effect longer-term responses, steroids bind to receptors inside cells, and either these receptors or glucocorticoid-induced proteins bind to DNA in the vicinity of response elements that regulate the transcription of numerous other genes (see Chapter 60: Adrenocorticotropic Hormone; Adrenocortical Steroids and Their Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical Hormones). Additionally, glucocorticoid-receptor complexes increase I B expression, thereby curtailing activation of NF B, which results in increased apoptosis of activated cells (Auphan et al. , 1995). Of central importance in this regard is the downregulation of important proinflammatory cytokines, such as IL-1 and IL-6. T cells are inhibited from making IL-2 and proliferating. The activation of cytotoxic T lymphocytes is inhibited. Neutrophils and monocytes display poor chemotaxis and decreased lysosomal enzyme release. Therefore, glucocorticoids have broad antiinflammatory effects on cellular immunity. In contrast, they have relatively little effect on humoral immunity. Therapeutic Uses Glucocorticoids commonly are used in combination with other immunosuppressive agents to both prevent and treat transplant rejection. High doses of intravenous methylprednisolone sodium succinate (SOLU-MEDROL, A-METHAPRED) (pulses) are used to reverse acute transplant rejection and acute exacerbations of selected autoimmune disorders (Shinn et al. , 1999; Laan et al. , 1999). There are numerous indications for glucocorticoids (Zoorob and Cender, 1998). They are efficacious for treatment of graft-versus-host disease in bone-marrow transplantation. Among autoimmune disorders, glucocorticoids are used routinely to treat rheumatoid and other arthritides, systemic lupus erythematosus, systemic dermatomyositis, psoriasis and other skin conditions, asthma and other allergic disorders, inflammatory bowel disease, inflammatory ophthalmic diseases, autoimmune hematologic disorders, and acute exacerbations of multiple sclerosis. In addition, glucocorticoids limit allergic reactions that occur with other immunosuppressive agents and are used in transplant recipients to block first-dose cytokine storm caused by treatment with muromonad-CD3 (see below). Toxicity Unfortunately, because there are numerous steroid-responsive tissues and genes, the extensive use of steroids has resulted in disabling and life-threatening adverse effects in many patients. These effects include growth retardation, avascular necrosis of bone, osteopenia, increased risk of infection, poor wound healing, cataracts, hyperglycemia, and hypertension (see Chapter 60: Adrenocorticotropic Hormone; Adrenocortical Steroids and Their Synthetic Analogs; Inhibitors of the Synthesis and Actions of Adrenocortical Hormones). The advent of concomitant glucocorticoid/cyclosporine regimens has allowed a reduction in the dosages of steroids administered, yet steroid-induced morbidity is still a major problem in many transplant patients. Calcineurin Inhibitors Perhaps the most effective immunosuppressive drugs in routine clinical use are calcineurin inhibitors, cyclosporine and tacrolimus, drugs that target intracellular signaling pathways induced as a consequence of T-cell-receptor activation (Schreiber and Crabtree, 1992). Although they are structurally unrelated (Figure 53–1) and bind to different (but related) molecular targets, the mechanisms of action of cyclosporine and tacrolimus in inhibiting normal T-cell signal transduction are the same (Figure 53–2). Cyclosporine and tacrolimus do not act per se as immunosuppressive agents. Instead, these drugs "gain function" after binding to cyclophilin or FKBP-12, resulting in subsequent interaction with calcineurin to block the activity of this phosphatase. Calcineurin- catalyzed dephosphorylation is required for movement of a component of the nuclear factor of activated T lymphocytes (NFAT) into the nucleus (Figure 53–2). NFAT, in turn, is required for induction of a number of cytokine genes, including that for interleukin-2 (IL-2), a prototypic T-cell growth and differentiation factor. Figure 53–1. Chemical Structures of Immunosuppressive Drugs: Azathioprine, Mycophenolate Mofetil, Cyclosporine, Tacrolimus, and Sirolimus. Figure 53–2. Mechanisms of Action of Cyclosporine, Tacrolimus, and Sirolimus. Both cyclosporine and tacrolimus bind to immunophilins [cyclophilin and FK506-binding protein (FKBP), respectively], forming a complex that binds the phosphatase calcineurin and inhibits the calcineurin-catalyzed dephosphorylation essential to permit movement of the nuclear factor of activated T cells (NFAT) into the nucleus. NFAT is required for transcription of interleukin-2 (IL-2) and other growth and differentiation–associated cytokines (lymphokines). Sirolimus (rapamycin) works at a later stage in T-cell activation, downstream of the IL-2 receptor. Sirolimus also binds FKBP, but the FKBP- sirolimus complex binds to and inhibits the mammalian target of rapamycin (mTOR), a kinase involved in cell-cycle progression (proliferation). DG, diacylglycerol; PIP 2 , phosphatidylinositol bisphosphate; PLC, phospholipase C; PKC, protein kinase C; TCR, T-cell receptor. (From Pattison et al. , 1997, with permission.) Cyclosporine Chemistry Cyclosporine (cyclosporin A) is a cyclic polypeptide consisting of 11 amino acids, produced as a metabolite of the fungus species Beauveria nivea (Borel et al. , 1976). Of note, all amide nitrogens are either hydrogen bonded or methylated, the single D-amino acid is at position 8, the methyl amide between residues 9 and 10 is in the cis configuration, and all other methyl amide moieties are in the trans form (Figure 53–1). Since cyclosporine is lipophilic and highly hydrophobic, it must be solubilized for clinical administration. Mechanism of Action Cyclosporine suppresses some humoral immunity but is more effective against T cell–dependent immune mechanisms such as those underlying transplant rejection and some forms of autoimmunity (Kahan, 1989). It preferentially inhibits antigen-triggered signal transduction in T lymphocytes, blunting expression of many lymphokines, including IL-2, as well as expression of antiapoptotic proteins. Cyclosporine forms a complex with cyclophilin, a cytoplasmic receptor protein present in target cells. This complex binds to calcineurin, inhibiting Ca 2+ -stimulated dephosphorylation of the cytosolic component of NFAT (Schreiber and Crabtree, 1992). When the cytoplasmic component of NFAT is dephosphorylated, it translocates to the nucleus, where it complexes with nuclear components required for complete T-cell activation, including transactivation of IL-2 and other lymphokine genes. Calcineurin enzymatic activity is inhibited following physical interaction with the cyclosporine/cyclophilin complex. This results in the blockade of NFAT dephosphorylation; thus, the cytoplasmic component of NFAT does not enter the nucleus, gene transcription is not activated, and the T lymphocyte fails to respond to specific antigenic stimulation. Cyclosporine also increases expression of transforming growth factor (TGF- ), a potent inhibitor of IL-2-stimulated T-cell proliferation and generation of cytotoxic T lymphocytes (CTL) (Khanna et al. , 1994). Disposition and Pharmacokinetics Cyclosporine can be administered intravenously or orally. The intravenous preparation (SANDIMMUNE Injection) is provided as a solution in an ethanol-polyoxyethylated castor oil vehicle which must be further diluted in 0.9%sodium chloride solution or 5%dextrose solution before injection. The oral dosage forms include soft gelatin capsules and oral solutions. Cyclosporine supplied in the original soft gelatin capsule is absorbed slowly with 20% to 50% bioavailability. A modified microemulsion formulation (NEORAL) was developed to improve absorption and was approved by the FDA for use in the United States in 1995 (Noble and Markham, 1995). It has more uniform and slightly increased bioavailability compared to SANDIMMUNE and is provided as 25-mg and 100-mg soft gelatin capsules and a 100-mg/ml oral solution. Since SANDIMMUNE and NEORAL are not bioequivalent, they cannot be used interchangeably without supervision by a physician and monitoring of drug concentration in plasma. Comparison of blood concentrations in published literature and in clinical practice must be performed with a detailed knowledge of the assay system employed. Although generic cyclosporine formulations have become available (Halloran, 1997), the most carefully studied generic product recently was withdrawn from the United States market by the FDA because of questions raised about bioequivalence. As described above, absorption of cyclosporine is incomplete following oral administration. The extent of absorption depends upon several variables, including the individual patient and formulation used. The elimination of cyclosporine from the blood is generally biphasic, with a terminal half-life of 5 to 18 hours (Faulds et al. , 1993; Noble and Markham, 1995). After intravenous infusion, clearance is approximately 5 to 7 ml/min per kg in adult recipients of renal transplants, but results differ by age and patient populations. For example, clearance is slower in cardiac transplant patients and more rapid in children. The relationship between administered dose and the area under the plasma concentration–versus-time curve (AUC; see Chapter 1: Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, and Elimination) is linear within the therapeutic range, but the intersubject variability is so large that individual monitoring is required (Faulds et al. , 1993; Noble and Markham, 1995). Following oral administration of cyclosporine (as NEORAL), the time to peak blood concentrations is 1.5 to 2.0 hours (Faulds et al. , 1993; Noble and Markham, 1995). Administration with food both delays and decreases absorption. High- and low-fat meals consumed within 30 minutes of administration decrease the AUC by approximately 13% and the maximum concentration by 33%. This makes it imperative to individualize dosage regimens for outpatients. Cyclosporine is distributed extensively outside the vascular compartment. After intravenous dosing, the steady-state volume of distribution has been reported to be as high as 3 to 5 liters/kg in solid- organ transplant recipients. Only 0.1% of cyclosporine is excreted unchanged in urine (Faulds et al. , 1993). Cyclosporine is extensively metabolized in the liver by the cytochrome-P450 3A (CYP3A) enzyme system and to a lesser degree by the gastrointestinal tract and kidneys (Fahr, 1993). At least 25 metabolites have been identified in human bile, feces, blood, and urine (Christians and Sewing, 1993). Although the cyclic peptide structure of cyclosporine is relatively resistant to metabolism, the side chains are extensively metabolized. All of the metabolites have both reduced biological activity and toxicity compared to the parent drug. Cyclosporine and its metabolites are excreted principally through the bile into the feces, with only approximately 6% being excreted in the urine. Cyclosporine also is excreted in human milk. In the presence of hepatic dysfunction, dosage adjustments are required. No adjustments generally are necessary for dialysis or renal failure patients. Therapeutic Uses Clinical indications for cyclosporine are kidney, liver, heart, and other organ transplantation; rheumatoid arthritis; and psoriasis (Faulds et al. , 1993). Its use in dermatology is discussed in Chapter 65: Dermatological Pharmacology. Cyclosporine generally is recognized as the agent that ushered in the modern era of organ transplantation, increasing the rates of early engraftment, extending graft survival for kidneys, and making cardiac and liver transplantation possible. Cyclosporine usually is used in combination with other agents, especially glucocorticoids and either azathioprine or mycophenolate mofetil and, most recently, sirolimus. The dosage of cyclosporine used is quite variable, depending upon the organ transplanted and the other drugs used in the specific treatment protocol(s). The initial dose generally is not given pretransplant because of the concern about neurotoxicity. Especially for renal transplant patients, therapeutic algorithms have been developed to delay cyclosporine introduction until a threshold renal function has been attained. The amount of the initial dose and reduction to maintenance dosing is sufficiently variable that no specific recommendation is provided here. Dosage is guided by signs of rejection (too low a dose), renal or other toxicity (too high a dose), and close monitoring of blood levels. Great care must be taken to differentiate renal toxicity from rejection in kidney transplant patients. Because adverse reactions have been ascribed frequently to the intravenous formulation, this route of administration is discontinued as soon as the patient is able to take an oral form of the drug. In rheumatoid arthritis, cyclosporine is used in cases of severe disease that have not responded to methotrexate. Cyclosporine can be used in combination with methotrexate, but the levels of both drugs must be monitored closely (Baraldo et al. , 1999). In psoriasis, cyclosporine is indicated for treatment of adult nonimmunocompromised patients with severe and disabling disease who have failed other systemic therapies (Linden and Weinstein, 1999). Because of its mechanism of action, there is a theoretical basis for the use of cyclosporine in a variety of other T cell–mediated diseases (Faulds et al. , 1993). Cyclosporine has been reported to be effective in Behçet's acute ocular syndrome, endogenous uveitis, atopic dermatitis, inflammatory bowel disease, and nephrotic syndrome when standard therapies have failed. Toxicity The principal adverse reactions to cyclosporine therapy are renal dysfunction, tremor, hirsutism, hypertension, hyperlipidemia, and gum hyperplasia (Burke et al. , 1994). Nephrotoxicity is limiting and occurs in the majority of patients treated. It is the major indication for cessation or modification of therapy. Hypertension may occur in approximately 50% of renal transplant and almost all cardiac transplant patients. Combined use of calcineurin inhibitors and glucocorticoids is particularly diabetogenic, with diabetes being more frequent in patients treated with tacrolimus than in those receiving cyclosporine. Drug Interactions Cyclosporine interacts with a wide variety of commonly used drugs, and close attention must be paid to drug interactions. Any drug that affects microsomal enzymes, especially the CYP3A system, may affect cyclosporine blood concentrations (Faulds et al. , 1993). Substances that inhibit this enzyme can decrease cyclosporine metabolism and increase blood concentrations. These include calcium channel blockers (e.g., verapamil, nicardipine), antifungal agents (e.g., fluconazole, ketoconazole), antibiotics (e.g., erythromycin), glucocorticoids (e.g., methylprednisolone), HIV- protease inhibitors (e.g., indinavir), and other drugs (e.g., allopurinol and metoclopramide). In addition, grapefruit and grapefruit juice block the CYP3A system and increase cyclosporine blood concentrations and thus should be avoided by patients receiving the drug. In contrast, drugs that induce CYP3A activity can increase cyclosporine metabolism and decrease blood concentrations. Drugs that can decrease cyclosporine concentrations in this manner include antibiotics (e.g., nafcillin and rifampin), anticonvulsants (e.g., phenobarbital, phenytoin), and other drugs (e.g., octreotide, ticlopidine). In general, close monitoring of cyclosporine blood levels and the levels of other drugs is required when such combinations are used. Interactions between cyclosporine and sirolimus have led to the recommendation that administration of the two drugs be separated by time. Sirolimus aggravates cyclosporine-induced renal dysfunction, while cyclosporine increases sirolimus-induced hyperlipemia and myelosuppression. Other cyclosporine–drug interactions of concern include additive nephrotoxicity when coadministered with nonsteroidal antiinflammatory drugs and other drugs that cause renal dysfunction; elevation in methotrexate levels when the two drugs are coadministered; and reduced clearance of other drugs, including prednisolone, digoxin, and lovastatin. Tacrolimus Tacrolimus (PROGRAF, FK506) is a macrolide antibiotic produced by Streptomyces tsukubaensis (Goto et al. , 1987). Its formula is shown in Figure 53–1. Mechanism of Action Like cyclosporine, tacrolimus inhibits T-cell activation by inhibiting calcineurin (Schreiber and Crabtree, 1992). Tacrolimus binds to an intracellular protein, FK506-binding protein–12 (FKBP- 12), an immunophilin structurally related to cyclophilin. A complex of tacrolimus-FKBP-12, calcium, calmodulin, and calcineurin then forms, and calcineurin phosphatase activity is inhibited. As described for cyclosporine and depicted in Figure 53–2, the inhibition of phosphatase activity prevents dephosphorylation and nuclear translocation of NFAT and leads to inhibition of T-cell activation. Thus, although the intracellular receptors differ, cyclosporine and tacrolimus appear to share a single common pathway for immunosuppression (Plosker and Foster, 2000). Disposition and Pharmacokinetics Tacrolimus is available for oral administration as capsules (0.5, 1, and 5 mg) and a sterile solution for injection (5 mg/ml). Immunosuppressive activity resides primarily in the parent drug. Because of intersubject variability in pharmacokinetics, individualization of dosing is required for optimal therapy (Fung and Starzl, 1995). Whole blood, rather than plasma, is the most appropriate sampling compartment to describe tacrolimus pharmacokinetics. Gastrointestinal absorption is incomplete and variable. Food decreases both the rate and extent of absorption. Plasma protein binding of tacrolimus is 75% to 99%, involving primarily albumin and 1 -acid glycoprotein. Its half-life is about 12 hours. Tacrolimus is extensively metabolized in the liver by CYP3A, and at least some of the metabolites are active. The bulk of excretion of parent drug and metabolites is in the feces. Less than 1% of administered tacrolimus is excreted unchanged in the urine. Therapeutic Uses Tacrolimus is indicated for the prophylaxis of solid-organ allograft rejection in a manner similar to cyclosporine and as rescue therapy in patients with rejection episodes despite "therapeutic" levels of cyclosporine (Mayer et al. , 1997; The U.S. Multicenter FK506 Liver Study Group, 1994). The recommended starting dose for tacrolimus injection is 0.03 to 0.05 mg/kg per day as a continuous infusion. Recommended initial oral doses are 0.2 mg/kg per day for adult kidney transplant patients, 0.1 to 0.15 mg/kg per day for adult liver transplant patients, and 0.15 to 0.2 mg/kg per day for pediatric liver transplant patients in two divided doses 12 hours apart. These dosages are intended to achieve typical blood trough levels in the 5- to 20-ng/ml range. Pediatric patients generally require higher doses than do adults (Shapiro, 1998). Toxicity Nephrotoxicity, neurotoxicity (tremor, headache, motor disturbances, seizures), gastrointestinal complaints, hypertension, hyperkalemia, hyperglycemia, and diabetes are associated with tacrolimus use (Plosker and Foster, 2000). As with cyclosporine, nephrotoxicity is limiting (Mihatsch et al. , 1998; Henry, 1999). Tacrolimus has a negative effect on the pancreatic islet beta cell, and both glucose intolerance and diabetes mellitus are well- recognized complications of tacrolimus-based immunosuppression among adult solid-organ transplant recipients. As with other immunosuppressive agents, there is an increased risk of secondary tumors and opportunistic infections. Drug Interactions Because of its potential for nephrotoxicity, blood levels of tacrolimus and renal function should be monitored closely, especially when tacrolimus is used with other potentially nephrotoxic drugs. Coadministration with cyclosporine results in additive or synergistic nephrotoxicity; therefore, a delay of at least 24 hours is required when switching a patient from cyclosporine to tacrolimus. Since tacrolimus is metabolized mainly by CYP3A, the potential interactions described for cyclosporine (above) apply for tacrolimus as well (Venkataramanan et al. , 1995; Yoshimura et al. , 1999). Antiproliferative and Antimetabolic Drugs Sirolimus Sirolimus (rapamycin; RAPAMUNE) is a macrocyclic lactone produced by Streptomyces hygroscopicus (Vezina, et al. , 1975). Its structure is shown in Figure 53–1. Mechanism of Action Sirolimus inhibits T-lymphocyte activation and proliferation downstream of the IL-2 and other T- cell growth factor receptors (Figure 53–2) (Kuo et al. , 1992). Sirolimus, like cyclosporine and tacrolimus, is a drug whose therapeutic action requires formation of a complex with the immunophilin, FKBP-12. However, the sirolimus-FKBP-12 complex does not affect calcineurin activity, but binds to and inhibits the mammalian kinase, target of rapamycin (mTOR), which is a key enzyme in cell-cycle progression (Brown et al. , 1994). Inhibition of this kinase blocks cell cycle progression at the G 1 S phase transition. In animal models, sirolimus not only inhibits transplant rejection, graft-versus-host disease, and a variety of autoimmune diseases, but its effect also lasts several months after discontinuing therapy, suggesting a tolerizing effect (see"Tolerance," below) (Groth et al. , 1999). [...]... antagonists of T-cell costimulation, including anti-CD2, anti-ICAM-1 (CD54) and anti-LFA-1 monoclonal antibodies, have shown promise in preclinical models of tolerance (Salmela et al., 1999) Figure 53–4 Costimulation A Two signals are required for T-cell activation Signal 1 is via the T-cell receptor (TCR) and signal 2 is via a costimulatory receptor-ligand pair Both signals are required for T-cell activation... Cytotoxic Agents Many of the cytotoxic and antimetabolic agents used in cancer chemotherapy (see Chapter 52: Antineoplastic Agents) are immunosuppressive due to their action on lymphocytes and other cells of the immune system Other cytotoxic drugs that have been used as immunosuppressive agents include methotrexate, cyclophosphamide (CYTOXAN), thalidomide, and chlorambucil (LEUKERAN) Methotrexate is used. .. anti-Tac antibody Mechanism of Action The antibodies bind with high affinity to the alpha subunit of the IL-2 receptor (p55 alpha, CD25) present on the surface of activated, but not resting, T lymphocytes and block IL-2–mediated T-cell activation events Daclizumab has a somewhat lower affinity than does basiliximab Therapeutic Uses Anti–IL-2-receptor monoclonal antibodies are recommended for prophylaxis... rejection in the absence of the first-dose cytokine-release syndrome (Woodle et al., 1999) Clinical efficacy of these agents in autoimmune diseases is being evaluated Anti-IL-2 Receptor (Anti-CD25) Antibodies Daclizumab (ZENAPAX), a humanized murine complementarity- determining region (CDR)/human IgG1 chimeric monoclonal antibody, and basiliximab (SIMULECT), a murine-human chimeric monoclonal antibody,... for patients receiving their first several doses of this therapy Other toxicities associated with anti-CD3 therapy include anaphylaxis and the usual infections and neoplasms associated with immunosuppressive therapy A high rate of "rebound" rejection has been observed when muromonab-CD3 treatment is stopped (Wilde and Goa, 1996) New-Generation Anti-CD3 Antibodies Recently, genetically altered anti-CD3... gastrointestinal distress, anorexia, weight loss, myalgia, and depression Interferon beta-1a (AVONEX), a 166–amino acid recombinant glycoprotein, and interferon beta-1b (BETASERON), a 165–amino acid recombinant protein, have antiviral and immunomodulatory properties They are FDA-approved for the treatment of relapsing and relapsing-remitting multiple sclerosis to reduce the frequency of clinical exacerbations The... unclear Flu-like symptoms (fever, chills, myalgia) and injection-site reactions have been common adverse effects Further discussion of the use of these and other interferons in the treatment of viral diseases can be found in Chapter 50: Antimicrobial Agents: Antiviral Agents (Nonretroviral) Interleukin-2 Human recombinant interleukin-2 (aldesleukin, PROLEUKIN; des-alanyl-1, serine-125 human IL-2) is produced... antithymocyte-globulin treatment often is given to renal transplant patients with delayed graft function to allow withdrawal of nephrotoxic calcineurin inhibitors and thereby aid in recovery from ischemic reperfusion injury The recommended dose for acute rejection of renal grafts is 1.5 mg/kg per day (over 4 to 6 hours) for 7 to 14 days Mean T-cell counts fall by day 2 of therapy It also is used for acute... antibody, muromonab-CD3 (OKT3, ORTHOCLONE OKT3), is still used to reverse corticosteroid-resistant rejection episodes (Cosimi, et al., 1981) Mechanism of Action Muromonab-CD3 binds to CD3, a monomorphic component of the T-cell receptor complex involved in antigen recognition, cell signaling, and proliferation (Hooks et al., 1991) Antibody treatment induces rapid internalization of the T-cell receptor,... and basiliximab are supplied as sterile concentrates that are diluted before intravenous administration Renal transplant patients receiving 1 mg/kg of daclizumab intravenously every 14 days for 5 doses have saturating blockade of the IL-2 receptor for 120 days posttransplant (Vincenti et al., 1998) No significant change in circulating lymphocyte markers has been observed Basiliximab is given for only . Other cytotoxic drugs that have been used as immunosuppressive agents include methotrexate, cyclophosphamide (CYTOXAN), thalidomide, and chlorambucil (LEUKERAN). Methotrexate is used for treatment. Cyclosporine also increases expression of transforming growth factor (TGF- ), a potent inhibitor of IL-2-stimulated T-cell proliferation and generation of cytotoxic T lymphocytes (CTL) (Khanna. block first-dose cytokine storm caused by treatment with muromonad-CD3 (see below). Toxicity Unfortunately, because there are numerous steroid-responsive tissues and genes, the extensive use