Section X - Drugs Used for Immunomodulation

Chia sẻ: Tran Anh Van | Ngày: | Loại File: DOC | Số trang:26

Thêm vào BST

Báo xấu

70
lượt xem 9
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

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....

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Section X - Drugs Used for Immunomodulation

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; PIP2, 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 Ca2+-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 G1 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). Disposition and Pharmacokinetics Following oral administration, sirolimus is absorbed rapidly and reaches a peak blood concentration within about 1 hour after a single dose in healthy subjects and within about 2 hours after multiple oral doses in renal transplant patients (Napoli and Kahan, 1996; Zimmerman and Kahan, 1997). Systemic availability is approximately 15%, and blood concentrations are proportional to dose between 3 and 12 mg/m2. A high-fat meal decreases peak blood concentration by 34%; sirolimus therefore should be taken consistently either with or without food, and blood levels should be monitored closely. About 40% of sirolimus in plasma is bound to protein, especially albumin. The drug partitions into formed elements of blood, with a blood-to-plasma ratio of 38 in renal transplant patients. Sirolimus is extensively metabolized by CYP3A4 and is transported by P-glycoprotein. Seven major metabolites have been identified in whole blood (Salm et al., 1999). Metabolites also are detectable in feces and urine, with the bulk of total excretion being in feces. Although some of its metabolites are active, sirolimus per se is the major component in whole blood and contributes greater than 90% of the immunosuppressive effect. The blood half-life after multiple dosing in stable renal transplant patients is 62 hours (Napoli and Kahan, 1996; Zimmerman and Kahan, 1997). A loading dose of three times the maintenance dose will provide nearly steady-state concentrations within one day in most patients. Therapeutic Uses Sirolimus is indicated for prophylaxis of organ transplant rejection in combination therapy with a calcineurin inhibitor and glucocorticoids (Kahan et al., 1999a). In patients experiencing or at high risk for calcineurin inhibitor–associated nephrotoxicity, sirolimus has been used with glucocorticoids and mycophenolate mofetil to avoid permanent renal damage. The initial dosage in
patients 13 years or older who weigh less than 40 kg should be adjusted based on body surface area (1 mg/m2 per day) with a loading dose of 3 mg/m2. Data regarding doses for pediatric and geriatric patients are lacking at this time (Kahan, 1999). It is recommended that the maintenance dose be reduced by approximately one-third in patients with hepatic impairment (Watson et al., 1999). Toxicity The use of sirolimus in renal transplant patients is associated with a dose-dependent increase in serum cholesterol and triglycerides that may require treatment (Murgia et al., 1996). While immunotherapy with sirolimus per se is not nephrotoxic, patients treated with cyclosporine plus sirolimus have impaired renal function compared to patients treated with cyclosporine and either azathioprine or placebo. Renal function therefore must be monitored closely in such patients. Lymphocoele, a known surgical complication associated with renal transplantation, has occurred significantly more often in a dose-dependent fashion in sirolimus-treated patients, requiring close postoperative follow-up. Other adverse effects include anemia, leukopenia, thrombocytopenia (Hong and Kahan, 2000b), hypokalemia or hyperkalemia, fever, and gastrointestinal effects. As with other immunosuppressive agents, there is an increased risk of neoplasms, especially lymphomas, and infections. Prophylaxis for Pneumocystis carinii pneumonia and cytomegalovirus is recommended (Groth et al., 1999). Drug Interactions Since sirolimus is a substrate for cytochrome CYP3A4 and is transported by P-glycoprotein, close attention to interactions with other drugs that are metabolized or transported by these proteins is required (Yoshimura et al., 1999). As noted above, cyclosporine and sirolimus interact, and their administration should be separated by time. Dose adjustment may be required with coadministration of sirolimus with cyclosporine, diltiazem, or rifampin. No dosage adjustment appears to be required when sirolimus is coadministered with acyclovir, digoxin, glyburide, nifedipine, norgestrel/ethinyl estadiol, prednisolone, or sulfamethoxazole/trimethoprim. This list is incomplete, and blood levels and potential drug interactions must be monitored closely. Azathioprine Azathioprine (IMURAN) is a purine antimetabolite (Elion, 1993). It is an imidazolyl derivative of 6- mercaptopurine (Figure 53–1). Mechanism of Action Following exposure to nucleophiles, such as glutathione, azathioprine is cleaved to 6- mercaptopurine, which, in turn, is converted to additional metabolites that inhibit de novo purine synthesis (Bertino, 1973). 6-Thio-IMP, a fraudulent nucleotide, is converted to 6-thio-GMP and finally to 6-thio-GTP, which is incorporated into DNA and gene translation is inhibited (Chan et al., 1987). Cell proliferation is prevented, inhibiting a variety of lymphocyte functions. Azathioprine appears to be a more potent immunosuppressive agent than does 6-mercaptopurine itself, which may reflect differences in drug uptake or pharmacokinetic differences in the resulting metabolites. Disposition and Pharmacokinetics Azathioprine is well absorbed orally and reaches maximum blood levels within 1 to 2 hours after administration. The half-life of azathioprine itself is about 10 minutes, and that of mercaptopurine is
about an hour. Other metabolites have half-lives of up to 5 hours. Blood levels have little predictive value because of extensive metabolism, significant activity of many different metabolites, and high tissue levels attained. Azathioprine and mercaptopurine are moderately bound to plasma proteins and are partially dialyzable. Both azathioprine and mercaptopurine are rapidly removed from the blood by oxidation or methylation in the liver and/or erythrocytes. Renal clearance is of little impact in biological effectiveness or toxicity, but dose reduction is practiced in patients with renal failure. Therapeutic Uses Azathioprine was first introduced as an immunosuppressive agent in 1961, helping to make allogeneic kidney transplantation possible (Murray et al., 1963). It is indicated as an adjunct for prevention of organ transplant rejection and in severe rheumatoid arthritis (Hong and Kahan, 2000a; Gaffney and Scott, 1998). Although the dose of azathioprine required to prevent organ rejection and minimize toxicity varies among patients, 3 to 5 mg/kg per day is the usual starting dose. Lower initial doses (1 mg/kg per day) are used in treating rheumatoid arthritis. Complete blood count and liver function tests should be monitored. Toxicity The major side effect of azathioprine is bone marrow suppression with leukopenia (common), thrombocytopenia (less common), and/or anemia (uncommon). Other important adverse effects include increased susceptibility to infections (especially varicella and herpes simplex viruses), hepatotoxicity, alopecia, gastrointestinal toxicity, pancreatitis, and increased risk of neoplasia. Drug Interactions Xanthine oxidase, an enzyme of major importance in the catabolism of metabolites of azathioprine, is blocked by allopurinol (Venkat Raman, et al., 1990). If azathioprine and allopurinol are used in the same patient, the azathioprine dose must be decreased to 25% to 33% of the usual dose, but it is best not to use these two drugs together. Adverse effects resulting from coadministration of azathioprine with other myelosuppressive agents or angiotensin converting enzyme inhibitors include leukopenia, thrombocytopenia, and/or anemia as a result of myelosuppression. Mycophenolate Mofetil Mycophenolate mofetil (CELLCEPT) is the 2-morpholinoethyl ester of mycophenolic acid (MPA) (Allison and Eugui, 1993). Its structure is shown in Figure 53–1. Mechanism of Action Mycophenolate mofetil is a prodrug that is rapidly hydrolyzed to the active drug, mycophenolic acid (MPA), a selective, uncompetitive and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH) (Natsumeda and Carr, 1993), an important enzyme in the de novo pathway of guanine nucleotide synthesis. B and T lymphocytes are highly dependent on this pathway for cell proliferation, while other cell types can use salvage pathways; MPA therefore selectively inhibits lymphocyte proliferation and functions, including antibody formation, cellular adhesion, and migration. The effects of MPA on lymphocytes can be reversed by adding guanosine or deoxyguanosine to the cells. Disposition and Pharmacokinetics
Mycophenolate mofetil undergoes rapid and complete metabolism to MPA after oral or intravenous administration. MPA, in turn, is metabolized to the inactive phenolic glucuronide, MPAG. The parent drug is cleared from the blood within a few minutes. The half-life of MPA is about 16 hours. Negligible amounts (
Methotrexate is used for treatment of graft-versus-host disease, rheumatoid arthritis, and psoriasis as well as in anticancer thereapy (see Chapter 52: Antineoplastic Agents) (Grosflam and Weinblatt, 1991). Cyclophosphamide and chlorambucil are used in treating childhood nephrotic syndrome (Neuhaus et al., 1994) as well as in treating of a variety of malignancies (see Chapter 52: Antineoplastic Agents). Cyclophosphamide also is widely used for treatment of severe systemic lupus erythematosus (Valeri et al., 1994). Leflunomide (ARAVA) is a pyrimidine-synthesis inhibitor indicated for the treatment of adults with rheumatoid arthritis (Prakash and Jarvis, 1999). The drug inhibits dihydroorotate dehydrogenase in the de novo pathway of pyrimidine synthesis. It is hepatotoxic and can cause fetal injury when administered to pregnant women. Antibodies Both polyclonal and monoclonal antibodies against lymphocyte cell-surface antigens are widely used for prevention and treatment of organ transplant rejection. Polyclonal antisera are generated by repeated injections of human thymocytes (antithymocyte globulin, ATG) or lymphocytes (antilymphocyte globulin, ALS) into animals such as horses, rabbits, sheep, or goats and then purifying the serum immunoglobulin fraction (Mannick et al., 1971). Although highly effective immunosuppressive agents, these preparations vary in efficacy and toxicity from batch to batch. The advent of hybridoma technology to produce monoclonal antibodies was a major advance in immunology (Kohler and Milstein, 1975). It is now possible to make essentially unlimited amounts of a single antibody of a defined specificity (Figure 53–3). These monoclonal reagents have overcome the problems of variability in efficacy and toxicity seen with the polyclonal products, but they are more limited in their target specificity. Thus, both polyclonal and monoclonal products have a place in immunosuppressive therapy. Figure 53–3. Generation of Monoclonal Antibodies. Mice are immunized with the selected antigen, and spleen or lymph node is harvested and B cells separated. These B cells are fused to a suitable B-cell myeloma that has been selected for its inability to grow in medium supplemented with hypoxanthine, aminopterin, and thymidine (HAT). Only myelomas that fuse with B cells can survive in HAT- supplemented medium. The hybridomas expand in culture. Those of interest based upon a specific screening technique are then selected and cloned by limiting dilution. Monoclonal antibodies can be used directly as supernatants or ascites fluid experimentally but are purified for clinical use. HPRT, hypoxanthine-guanine phosphoribosyl transferase. (From Krensky, A.M., 1999, with permission.)
Antithymocyte Globulin Antithymocyte globulin (THYMOGLOBULIN) is a purified gamma globulin from the serum of rabbits immunized with human thymocytes (Regan et al., 1999). It is provided as a sterile, freeze-dried product for intravenous administration after reconstitution with sterile water. Mechanism of Action Antithymocyte globulin contains cytotoxic antibodies that bind to CD2, CD3, CD4, CD8, CD11a, CD18, CD25, CD44, CD45, and HLA class I and II molecules on the surface of human T lymphocytes (Bourdage and Hamlin, 1995). The antibodies deplete circulating lymphocytes by direct cytotoxicity (both complement and cell-mediated) and block lymphocyte function by binding to cell surface molecules involved in the regulation of cell function. Therapeutic Uses Antithymocyte globulin is indicated for induction immunosuppression and the treatment of acute renal transplant rejection in combination with other immunosuppressive agents (Mariat et al., 1998). Because it is a highly effective immunosuppressant, a course of 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 rejection of other types of organ transplants and for prophylaxis of rejection (Wall, 1999). Studies to examine its use as induction therapy at the time of transplantation are in progress (Szczech and Feldman, 1999). Toxicity
The major side effects are fever and chills with the potential for hypotension. Premedication with corticosteroids, acetaminophen, and/or an antihistamine and administration of the antiserum by slow infusion (over 4 to 6 hours) into a large-diameter vessel minimize such reactions. Outright serum sickness and glomerulonephritis can occur; anaphylaxis is a rare event. Hematologic complications include leukopenia and thrombocytopenia. As with other immunosuppressive agents, there is an increased risk of infection and malignancy, especially when multiple immunosuppressive agents are used in combination. No drug interactions have been described, but anti-antibodies develop, limiting repeated use of this or any other rabbit antibody preparations. As an example, in one trial, 68% of patients developed antirabbit antibodies. Monoclonal Antibodies Anti-CD3 Monoclonal Antibodies Antibodies directed at the CD3 antigen on the surface of human T lymphocytes have been used since the early 1980s in human transplantation and have proven to be extremely effective immunosuppressive agents. The original mouse IgG2a antihuman CD3 monoclonal 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, thereby preventing subsequent recognition of antigen. Administration of the antibody is followed rapidly by depletion and extravasation of a majority of T cells from the bloodstream and the peripheral lymphoid organs such as lymph nodes and spleen. This absence of detectable T cells from the usual lymphoid regions is secondary both to cell death following complement activation and activation-induced cell death and to margination of T cells onto vascular endothelial walls and redistribution of T cells to nonlymphoid organs such as the lungs. Muromonab-CD3 also induces a reduction in function of the remaining T cells, as defined by lack of IL-2 production and great reduction in the production of multiple cytokines, perhaps with the exception of IL-4 and IL-10. Therapeutic Uses Muromonab-CD3 is indicated for treatment of acute organ transplant rejection (Ortho Multicenter Transplant Group, 1985; Woodle et al., 1999; Rostaing et al., 1999). Muromonab-CD3 is provided as a sterile solution containing 5 mg per ampule. The recommended dose is 5 mg/day (in adults; less for children) in a single intravenous bolus (less than one minute) for 10 to 14 days. Antibody levels increase over the first three days and then level off. Circulating T cells disappear from the blood within minutes of administration and return within approximately one week after termination of therapy. Repeated use of muromonab-CD3 results in the immunization of the patient against the mouse determinants of the antibody, which can neutralize and prevent its immunosuppressive efficacy (Jaffers et al., 1983). Thus, repeated treatment with the muromonab-CD3 or other mouse monoclonal antibodies is contraindicated in many patients. Toxicity The major side effect of anti-CD3 therapy is the "cytokine release syndrome" (Wilde and Goa,
1996; Ortho Multicenter Transplant Study Group, 1985). Administration of glucocorticoids prior to the injection of muromonab-CD3 prevents the release of cytokines and reduces first-dose reactions considerably and is now a standard procedure. Antibody binding to the T-cell receptor complex combined with Fc receptor (FcR)–mediated crosslinking is the basis for the initial activating properties of this agent. The syndrome typically begins 30 minutes after infusion of the antibody (but can occur later) and may persist for hours. The symptomatology usually is worst with the first dose; both the frequency and severity decrease with subsequent doses. Common clinical manifestations include high fever, chills/rigor, headache, tremor, nausea/vomiting, diarrhea, abdominal pain, malaise, muscle/joint aches and pains, and generalized weakness. Less common complaints include skin reactions and cardiorespiratory and central nervous system (CNS) disorders, including aseptic meningitis. Potentially fatal, severe pulmonary edema, adult respiratory distress syndrome, cardiovascular collapse, cardiac arrest, and arrhythmias have been described. The syndrome is associated with and attributed to increased serum levels of cytokines [including tumor necrosis factor (TNF)- , IL-2, IL-6, and interferon gamma], which are released by activated T cells and/or monocytes. In several studies, the production of the TNF- cytokine has been shown to be the major cause of the toxicity (Herbelin et al., 1995). Fluid status of patients must be monitored carefully before therapy; steroids and other premedications should be given, and a fully competent resuscitation facility must be immediately available 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 monoclonal antibodies have been developed that are "humanized" to minimize the occurrence of anti-antibody responses and mutated to prevent binding to FcRs (Friend et al., 1999). The rationale for developing this new generation of anti-CD3 monoclonal antibodies is that they could induce selective immunomodulation in the absence of toxicity associated with conventional anti-CD3 monoclonal antibody therapy. In initial clinical trials, a humanized anti-CD3 monoclonal antibody that does not bind to FcRs reversed acute renal allograft 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, have been produced by recombinant DNA technology (Wiseman and Faulds, 1999). The composite daclizumab antibody consists of human (90%) constant domains of IgG 1 and variable framework regions of the Eu myeloma antibody and murine (10%) CDR of the 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 of acute organ rejection in adult patients as part of combination therapy (with glucocorticoids, a calcineurin inhibitor, with or without azathioprine or mycophenolate mofetil) (Kovarik et al., 1999; Hong and Kahan, 1999; Kahan et al., 1999b; Hirose et al., 2000). Daclizumab 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 two doses of 20 mg each, the first two hours before surgery and the second four days after. Toxicity No cytokine-release syndrome has been observed with these antibodies. Anaphylactic reactions can occur. Although lymphoproliferative disorders and opportunistic infections may occur, as with other immunosuppressive agents, the incidence ascribed to anti-CD25 treatment appears remarkably low. No significant drug interactions with anti–IL-2-receptor antibodies have been described ( Hong and Kahan, 1999). Infliximab Infliximab (REMICADE) is a chimeric anti–TNF- monoclonal antibody containing a human constant region and a murine variable region. It binds with high affinity to TNF- and prevents the cytokine from binding to its receptors. Patients with rheumatoid arthritis have elevated levels of TNF- in their joints, and patients with Crohn's disease have elevated levels of TNF- in their stools. A clinical trial has revealed that patients treated with infliximab plus methotrexate have fewer signs and symptoms of rheumatoid arthritis than do patients treated with methotrexate alone. Patients with active Crohn's disease who had not responded to other immunosuppressive therapies have shown improvement when treated with infliximab, and patients with fistulizing Crohn's disease have had fewer draining fistulas after treatment with the antibody. Infliximab is approved in the United States for treating the symptoms of rheumatoid arthritis, in combination with methotrexate, in patients who do not respond to methotrexate alone. Infliximab also is approved for use in the treatment of symptoms of moderately to severely active Crohn's disease in patients who have failed to respond to conventional therapy and in treatment to reduce the number of draining fistulas in Crohn's disease patients (see Chapter 39: Agents Used for Diarrhea, Constipation, and Inflammatory Bowel Disease; Agents Used for Biliary and Pancreatic Disease). About 1 of 6 patients receiving infliximab has experienced an infusion reaction within 1 to 2 hours after administration of the antibody. The reaction has included fever, urticaria, hypotension, and dyspnea. Serious infections also have occurred in infliximab- treated patients, most frequently in the upper respiratory and urinary tracts. The development of antinuclear antibodies and, rarely, a lupus-like syndrome have been reported to occur after treatment with infliximab. A therapeutic agent related to infliximab in its mechanism of action, although not a monoclonal antibody, is etanercept (ENBREL), which is a fusion protein containing the ligand-binding portion of a human TNF- receptor linked to the Fc portion of human IgG1. Like infliximab, etanercept binds to TNF- and prevents it from interacting with its receptors. It is approved in the United States for treatment of the symptoms of rheumatoid arthritis in patients who have not responded to other treatments. Etanercept can be used in combination with methotrexate in patients who have not responded adequately to methotrexate alone. As with infliximab, serious infections have occurred
after treatment with etanercept. Injection-site reactions (erythema, itching, pain, or swelling) have occurred in more than one-third of etanercept-treated patients. Tolerance Immunosuppression has concomitant risks of opportunistic infections and secondary tumors. Therefore, the ultimate goal of research on organ transplantation and autoimmune diseases is the induction and maintenance of immunologic tolerance, the active state of antigen-specific nonresponsiveness (Krensky and Clayberger, 1994; Hackett and Dickler, 1999). If tolerance can be attained, it would represent a true cure for conditions discussed above without the side effects of the various immunosuppressive therapies discussed. The calcineurin inhibitors prevent tolerance induction in some, but not all, preclinical models (Wood, 1991; Van Parijs and Abbas, 1998). In contrast, in these same model systems, sirolimus does not prevent tolerance and, in fact, in some cases promotes tolerance induction (Li et al., 1998). Several other approaches have exciting promise as well and are being evaluated in clinical trials. Because these approaches are still experimental, they are only briefly discussed here. Costimulatory Blockade Induction of specific immune responses by T lymphocytes requires two signals: an antigen-specific signal via the T-cell receptor and a costimulatory signal provided by molecules such as CD28 on the T cell interacting with CD80 and CD86 on the antigen-presenting cell (APC) (Figure 53–4) (Khoury et al., 1999). Preclinical studies have shown that inhibition of the costimulatory signal can induce tolerance (Larsen et al., 1996; Kirk et al., 1997). Experimental approaches to inhibition of costimulation include a recombinant fusion protein molecule, CTLA4Ig, and anti-CD80 and/or anti- CD86 monoclonal antibodies. CTLA4Ig contains the binding region of CTLA4, which is a CD28 homolog, and the constant region of the human IgG1. CTLA4Ig is a competitive inhibitor of CD28. Both CTLA4Ig and a lytic anti-CD80 monoclonal antibody are in clinical trials. A second costimulatory pathway undergoing clinical evaluation involves the interaction of CD40 on activated T cells with CD40 ligand (CD154) on B cells, endothelium, and/or APCs (Figure 53–4). Among the purported activities of anti-CD154 antibody treatment is its blockade of the induced B7 expression following immune activation. At least two anti-CD154 monoclonal antibodies are under clinical evaluation in organ transplantation and autoimmunity. Other 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. Signal 1 in the absence of signal 2 results in an inactivated T cell. B. One important costimulatory pathway involves CD28 on the T cell and B7-1 (CD80) and B7-2 (CD86) on the antigen-presenting cell (APC). After a T cell is activated, it expresses additional costimulatory molecules. CD152 is CD40 ligand, which interacts with CD40 as a costimulatory pair. CD154 (CTLA4) interacts with CD80 and CD86 to dampen or down-regulate an immune response. Antibodies against CD80, CD86, and CD152 are being evaluated as potential therapeutic agents. CTLA4-Ig, a chimeric protein consisting of part of an immunoglobulin molecule and part of CD154, also has been tested as a therapeutic agent. (From Clayberger et al., 2001, with permission.)