This chapter reviews the components of the immune response and drugs that modulate immunity in 3 ways: immunosuppression, tolerance, and immunostimulation. Four major classes of immunosuppressive drugs are discussed: glucocorticoids (see Chapter 42), calcineurin inhibitors, antiproliferative and antimetabolic agents (see Chapter 61), and antibodies. The chapter ends with a brief case study of immunotherapy for MS.
THE IMMUNE RESPONSE
The immune system evolved to discriminate self from nonself. Innate immunity (natural immunity) is primitive, does not require priming, and is of relatively low affinity, but is broadly reactive. Adaptive immunity (learned immunity) is antigen specific, depends on antigen exposure or priming, and can be of very high affinity. The 2 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 lymphocytes. B lymphocytes make antibodies; T lymphocytes 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.
Immunoglobulins (antibodies) on the B-lymphocyte surface are receptors for a large variety of specific structural conformations. In contrast, T lymphocytes recognize antigens as peptide fragments in the context of self major histocompatibility complex (MHC) antigens (called human leukocyte antigens [HLAs] in humans) on the surface of antigen-presenting cells, such as dendritic cells, macrophages, and other cell types expressing MHC class I and class II antigens. Once activated by specific antigen recognition, both B and T lymphocytes are triggered to differentiate and divide, leading to release of soluble mediators (cytokines, lymphokines) that perform as effectors and regulators of the immune response.
Immunosuppressive drugs are used to dampen the immune response in organ transplantation and autoimmune disease. In transplantation, the major classes of immunosuppressive drugs used today are:
• Calcineurin inhibitors
• Antiproliferative/antimetabolic agents
• Biologicals (antibodies)
Table 35–1 summarizes the sites of action of representative immunosuppressants on T-cell activation.
Sites of Action of Selected Immunosuppressive Agents on T-Cell Activation
These drugs are used in treating conditions such as acute immune rejection of organ transplants and severe auto-immune 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 glucocorticoids, in particular, are nephrotoxic and diabetogenic, respectively, thus restricting 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 target specifically immune-reactive cells. Finally, newer small molecules and antibodies have expanded the arsenal of immunosuppressives. In particular, mammalian target of rapamycin (mTOR) inhibitors (sirolimus, everolimus) and anti-CD25 (interleukin-2 receptor [IL-2R]) antibodies (basiliximab, daclizumab) target growth-factor pathways, substantially limiting clonal expansion and thus potentially promoting tolerance.
GENERAL APPROACH TO ORGAN TRANSPLANTATION THERAPY. Organ transplantation therapy is organized around 5 general principles.
• Carefully prepare patient and select the best available ABO blood type–compatible HLA match for organ donation.
• Employ multitiered immunosuppressive therapy; simultaneously use several agents, each of which is directed at a different molecular target within the allograft response. Synergistic effects permit use of the various agents at relatively low doses, thereby limiting specific toxicities while maximizing the immunosuppressive effect.
• Employ intensive induction and lower-dose maintenance drug protocols; greater immunosuppression is required to gain early engraftment and/or to treat established rejection than to maintain long-term immunosuppression.
• Investigation of each episode of transplant dysfunction is required, including evaluation for rejection, drug toxicity, and infection.
• Reduce dosage or withdraw a drug if its toxicity exceeds its benefit.
BIOLOGICAL INDUCTION THERAPY. Induction therapy with polyclonal and monoclonal antibodies (mAbs) has been an important component of immunosuppression since the 1960s, when Starzl and colleagues demonstrated the beneficial effect of antilymphocyte globulin (ALG) in the prophylaxis of rejection. Two preparations are FDA-approved for use in transplantation: lymphocyte immune globulin (ATGAM) and antithymocyte globulin (ATG; THYMOGLOBULIN). ATG is the most frequently used depleting agent. Alemtuzumab, a humanized anti-CD52 monoclonal antibody that produces prolonged lymphocyte depletion, is approved for use in chronic lymphocytic leukemia but is increasingly used off label as induction therapy in transplantation.
In many transplant centers, induction therapy with biological agents is used to delay the use of the nephrotoxic calcineurin inhibitors or to intensify the initial immunosuppressive therapy in patients at high risk of rejection (i.e., repeat transplants, broadly presensitized patients, African American patients, or pediatric patients). Most of the limitations of murine-based mAbs generally were overcome by the introduction of chimeric or humanized mAbs that lack antigenicity and have a prolonged serum t1/2. Antibodies derived from transgenic mice carrying human antibody genes are labeled “humanized” (90-95% human) or “fully human” (100% human); antibodies derived from human cells are labeled “human.” However, all 3 types of antibodies probably are of equal efficacy and safety. Chimeric antibodies generally contain ~33% mouse protein and 67% human protein and can still produce an antibody response, resulting in reduced efficacy and shorter t1/2 compared to humanized antibodies.
Biological agents for induction therapy in the prophylaxis of rejection currently are used in ~70% of de novo transplant patients. Biological agents for induction can be divided into 2 groups: the depleting agents and the immune modulators. The depleting agents consist of lymphocyte immune globulin, ATG, and muromonab-CD3 mAb; their efficacy derives from their ability to deplete the recipient’s CD3-positive cells at the time of transplantation and antigen presentation. The second group of biological agents, the anti–IL-2R mAbs, do not deplete T lymphocytes, with the possible exception of T regulatory cells, but rather block IL-2–mediated T-cell activation by binding to the α chain of IL-2R. For patients with high levels of anti-HLA antibodies and humoral rejection, more aggressive therapies include plasmapheresis, intravenous immunoglobulin, and rituximab, a chimeric anti-CD20 monoclonal antibody.
MAINTENANCE IMMUNOTHERAPY. Basic immunosuppressive therapy uses multiple drugs simultaneously, typically a calcineurin inhibitor, glucocorticoids, and mycophenolate (a purine metabolism inhibitor), each directed at a discrete site in T-cell activation. Glucocorticoids, azathioprine, cyclosporine, tacrolimus, mycophenolate, sirolimus, and various monoclonal and polyclonal antibodies all are approved for use in transplantation.
THERAPY FOR ESTABLISHED REJECTION. Low doses of prednisone, calcineurin inhibitors, purine metabolism inhibitors, or sirolimus are effective in preventing acute cellular rejection; they are less effective in blocking activated T lymphocytes and thus are not very effective against established, acute rejection or for the total prevention of chronic rejection. 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.
The glucocorticoids are described in Chapter 42. 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. Glucocorticoids have broad anti-inflammatory effects on multiple components of cellular immunity, but relatively little effect on humoral immunity. Glucocorticoids bind to receptors inside cells and regulate the transcription of numerous other genes (see Chapter 42). Glucocorticoids also curtail activation of NF-κB, suppress formation of pro-inflammatory cytokines such as IL-1 and IL-6, inhibit T cells from making IL-2 and proliferating, and inhibit the activation of cytotoxic T lymphocytes. In addition, neutrophils and monocytes display poor chemotaxis and decreased lysosomal enzyme release.
Therapeutic Uses. Glucocorticoids commonly are combined with other immunosuppressive agents to prevent and treat transplant rejection. Glucocorticoids also are efficacious for treatment of graft-versus-host disease in bone-marrow transplantation. Glucocorticoids are routinely used to treat autoimmune disorders such as 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, auto-immune hematological disorders, and acute exacerbations of MS (see “Multiple Sclerosis,” below). 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 muromonab-CD3 and to a lesser extent ATG (see “Antithymocyte Globulin”).
Toxicity. Extensive steroid use often results in disabling and life-threatening adverse effects described in Chapter 42. The advent of combined glucocorticoid/calcineurin inhibitor regimens has allowed reduced doses or rapid withdrawal of steroids, resulting in lower steroid-induced morbidities.
The most effective immunosuppressive drugs in routine use are the calcineurin inhibitors, cyclosporine and tacrolimus, which target intracellular signaling pathways induced as a consequence of T-cell–receptor activation (Figure 35–1). Cyclosporine and tacrolimus bind to an immunophilin (cyclophilin for cyclosporine, or FKBP-12 for tacrolimus), resulting in subsequent interaction with calcineurin to block its phosphatase activity. Calcineurin-catalyzed dephosphorylation is required for movement of a component of the nuclear factor of activated T lymphocytes (NFAT) into the nucleus. NFAT, in turn, is required to induce a number of cytokine genes, including that for IL-2, a prototypic T-cell growth and differentiation factor.
Figure 35–1 Mechanisms of action of cyclosporine, tacrolimus, and sirolimus on T cells. Cyclosporine and tacrolimus bind to immunophilins (cyclophilin and FK506-binding protein [FKBP], respectively), forming a complex that inhibits the phosphatase calcineurin and the calcineurin-catalyzed dephosphorylation that permits translocation of 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 downstream of the IL-2 receptor, binding to FKBP; the FKBP-sirolimus complex binds to and inhibits the mammalian target of rapamycin (mTOR), a kinase involved in cell-cycle progression (proliferation). TCR, the T-cell receptor that recognizes antigens bound to the major histocompatibility complex. (Reproduced with permission from Clayberger C, Krensky AM. Mechanisms of allograft rejection. In Neilson EG, Couser WG, eds, Immunologic Renal Diseases, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001, pp 321–346. http://lww.com.)
TACROLIMUS. Tacrolimus (PROGRAF, FK506) is a macrolide antibiotic produced by Streptomyces tsukubaensis. Because of perceived slightly greater efficacy and ease of blood level monitoring, tacrolimus has become the preferred calcineurin inhibitor in most transplant centers.
Mechanism of Action. Like cyclosporine, tacrolimus inhibits T-cell activation by inhibiting calcineurin. Tacrolimus binds to an intracellular protein, FK506-binding protein–12 (FKBP-12), an immunophilin structurally related to cyclophilin. A complex of tacrolimus-FKBP-12, Ca2+, calmodulin, and calcineurin then forms, and calcineurin phosphatase activity is inhibited (see Figure 35–1). Inhibition of phosphatase activity prevents dephosphorylation and nuclear translocation of NFAT and inhibits T-cell activation. Thus, although the intracellular receptors differ, cyclosporine and tacrolimus target the same pathway for immunosuppression.
ADME. Tacrolimus is available for oral administration as capsules (0.5, 1, and 5 mg) and as a solution for injection (5 mg/mL). Because of intersubject variability in pharmacokinetics, individualized dosing is required for optimal therapy. For tacrolimus, whole blood seems to be the best sampling compartment; the trough drug level in whole blood seems to correlate better with clinical events for tacrolimus than for cyclosporine. Target concentrations are 10-15 ng/mL in the early preoperative period and 100-200 ng/mL 3 months after transplantation. GI absorption is incomplete and variable. Food decreases the rate and extent of absorption. Plasma protein binding of tacrolimus is 75-99%, involving primarily albumin and α1-acid glycoprotein. The t1/2 of tacrolimus is ~12 h. Tacrolimus is extensively metabolized in the liver by CYP3A; some of the metabolites are active. The bulk of excretion of the parent drug and metabolites is in the feces.
Therapeutic Uses. Tacrolimus is indicated for the prophylaxis of solid-organ allograft rejection in a manner similar to cyclosporine (see “Cyclosporine”) and is used off label as rescue therapy in patients with rejection episodes despite “therapeutic” levels of cyclosporine. Recommended initial oral doses are 0.2 mg/kg/day for adult kidney transplant patients, 0.1-0.15 mg/kg/day for adult liver transplant patients, 0.075 mg/kg/day for adult heart transplant patients, and 0.15-0.2 mg/kg/day for pediatric liver transplant patients in 2 divided doses 12 h apart. These dosages are intended to achieve typical blood trough levels in the 5- to 20-ng/mL range.
Toxicity. Nephrotoxicity, neurotoxicity (e.g., tremor, headache, motor disturbances, seizures), GI complaints, hypertension, hyperkalemia, hyperglycemia, and diabetes all are associated with tacrolimus use. Tacrolimus has a negative effect on pancreatic islet β cells, and glucose intolerance and diabetes mellitus are well-recognized complications of tacrolimus-based immunosuppression. As with other immunosuppressive agents, there is an increased risk of secondary tumors and opportunistic infections. Notably, tacrolimus does not adversely affect uric acid or LDL cholesterol. Diarrhea and alopecia are common in patients on concomitant mycophenolate therapy.
Drug Interactions. Because of its potential for nephrotoxicity, tacrolimus blood levels and renal function should be monitored closely. Coadministration with cyclosporine results in additive or synergistic nephrotoxicity; therefore, a delay of at least 24 h is required when switching a patient from cyclosporine to tacrolimus. Because tacrolimus is metabolized mainly by CYP3A, the potential interactions described in the following section for cyclosporine also apply for tacrolimus.
CYCLOSPORINE. Cyclosporine (cyclosporin A), a cyclic polypeptide of 11 amino acids, is produced by the fungus Beauveria nivea. Figure 35–1 depicts the molecular action of cyclosporine to inhibit calcineurin activity. At the level of immune system function, 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. It preferentially inhibits antigen-triggered signal transduction in T lymphocytes, blunting expression of many lymphokines, including IL-2, and the expression of anti-apoptotic proteins. Cyclosporine also increases expression of transforming growth factor β (TGF-β), a potent inhibitor of IL-2–stimulated T-cell proliferation and generation of cytotoxic T lymphocytes (CTLs).
ADME. Because cyclosporine is lipophilic and highly hydrophobic, it is formulated for clinical administration using castor oil or other strategies to ensure solubilization. Cyclosporine can be administered intravenously or orally. The intravenous preparation (SANDIMMUNE, others) is provided as a solution in an ethanol-polyoxyethylated castor oil vehicle that 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 20-50% bioavailability. A modified microemulsion formulation (NEORAL) has become the most widely used preparation. It has more uniform and slightly increased bioavailability compared to the original formulation. It is provided as 25-mg and 100-mg soft gelatin capsules and a 100-mg/mL oral solution. The original and microemulsion formulations are not bioequivalent and cannot be used interchangeably without supervision by a physician and monitoring of drug concentrations in plasma. Generic preparations of both NEORAL and SANDIMMUNE are bioequivalent by FDA criteria. Transplant units need to educate patients that SANDIMMUNE and its generics are not the same as NEORAL and its generics, such that one preparation cannot be substituted for another without risk of inadequate immunosuppression or increased toxicity.
Blood levels taken 2 h after a dose administration (so-called C2 levels) may correlate better with the AUC than other single points, but no single time point can simulate the exposure better than more frequent drug sampling. In practice, if a patient has clinical signs or symptoms of toxicity, or if there is unexplained rejection or renal dysfunction, a pharmacokinetic profile can be used to estimate that person’s exposure to the drug.
Cyclosporine absorption is incomplete following oral administration and varies with the individual patient and the formulation used. The elimination of cyclosporine from the blood generally is biphasic, with a terminal t1/2 of 5-18 h. After intravenous infusion, clearance is ~5-7 mL/min/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. Thus, the intersubject variability is so large that individual monitoring is required. After oral administration of cyclosporine (as NEORAL), the time to peak blood concentrations is 1.5-2 h. Administration with food delays and decreases absorption. High- and low-fat meals consumed within 30 min of administration decrease the AUC by ~13% and the maximum concentration by 33%. This makes it imperative to individualize dosage regimens for outpatients. Cyclosporine is extensively metabolized by hepatic CYP3A and to a lesser degree by the GI tract and kidneys. Cyclosporine and its metabolites are excreted principally through the bile into the feces, with ~6% 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. Its use in dermatology is discussed in Chapter 65. Cyclosporine usually is combined with other agents, especially glucocorticoids and either azathioprine or mycophenolate and, most recently, sirolimus. The dose of cyclosporine varies, depending on the organ transplanted and the other drugs used in the specific treatment protocol(s). The initial dose generally is not given before the transplant because of the concern about nephrotoxicity. Dosing 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. Ultrasound-guided allograft biopsy is the best way to assess the reason for renal dysfunction. Because adverse reactions have been ascribed more frequently to the intravenous formulation, this route of administration is discontinued as soon as the patient can take the drug orally.
In rheumatoid arthritis, cyclosporine is used in severe cases that have not responded to methotrexate. Cyclosporine can be combined with methotrexate, but the levels of both drugs must be monitored closely. In psoriasis, cyclosporine is indicated for treatment of adult immunocompetent patients with severe and disabling disease for whom other systemic therapies have failed. 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. Cyclosporine reportedly is effective in Behçet’s acute ocular syndrome, endogenous uveitis, atopic dermatitis, inflammatory bowel disease, and nephrotic syndrome, even when standard therapies have failed.
Toxicity. The principal adverse reactions to cyclosporine therapy are renal dysfunction and hypertension; tremor, hirsutism, hyperlipidemia, and gum hyperplasia also are frequently encountered. Hypertension occurs in ~50% of renal transplant and almost all cardiac transplant patients. Hyperuricemia may lead to worsening of gout, increased P-glycoprotein activity, and hypercholesterolemia. Nephrotoxicity occurs in the majority of patients and is the major reason for cessation or modification of therapy. Combined use of calcineurin inhibitors and glucocorticoids is particularly diabetogenic. Especially at risk are obese patients, African American or Hispanic transplant recipients, or those with a family history of type II diabetes or obesity. Cyclosporine, as opposed to tacrolimus, is more likely to produce elevations in LDL cholesterol.
Drug Interactions. Any drug that affects CYPs, especially CYP3A, may impact cyclosporine blood concentrations. Substances that inhibit this enzyme can decrease cyclosporine metabolism and increase blood concentrations. These include Ca2+ 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, metoclopramide). Grapefruit juice inhibits CYP3A and the P-glycoprotein multidrug efflux pump and thereby can increase cyclosporine blood concentrations. In contrast, drugs that induce CYP3A activity can increase cyclosporine metabolism and decrease blood concentrations. Such drugs include antibiotics (e.g., nafcillin, rifampin), anticonvulsants (e.g., phenobarbital, phenytoin), and others (e.g., octreotide, ticlopidine).
Interactions between cyclosporine and sirolimus require that administration of the 2 drugs be separated by time. Sirolimus aggravates cyclosporine-induced renal dysfunction, while cyclosporine increases sirolimus-induced hyperlipidemia and myelosuppression. Additive nephrotoxicity may occur when cyclosporine is coadministered with NSAIDs and other drugs that cause renal dysfunction; elevation of methotrexate levels when the 2 drugs are coadministered; and reduced clearance of other drugs, including prednisolone, digoxin, and statins.
ANTIPROLIFERATIVE AND ANTIMETABOLIC DRUGS
Sirolimus (rapamycin; RAPAMUNE) is a macrocyclic lactone produced by Streptomyces hygroscopicus. Sirolimus inhibits T-lymphocyte activation and proliferation downstream of the IL-2 and other T-cell growth factor receptors (see Figure 35–1). Like cyclosporine and tacrolimus, therapeutic action of sirolimus requires formation of a complex with an immunophilin, in this case FKBP-12. The sirolimus–FKBP-12 complex does not affect calcineurin activity; rather, it binds to and inhibits the protein kinase mTOR, which is a key enzyme in cell-cycle progression. Inhibition of mTOR blocks cell-cycle progression at the G1 → S phase transition.
ADME. After oral administration, sirolimus is absorbed rapidly and reaches a peak blood concentration within ~1 h after a single dose in healthy subjects and within ~2 h after multiple oral doses in renal transplant patients. Systemic availability is ~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 protein bound, especially to albumin. The drug partitions into formed elements of blood (blood-to-plasma ratio = 38 in renal transplant patients). Sirolimus is extensively metabolized by CYP3A4 and is transported by P-glycoprotein. Although some of its metabolites are active, sirolimus itself is the major active component in whole blood and contributes > 90% of the immunosuppressive effect. The blood t1/2 after multiple doses in stable renal transplant patients is 62 h. A loading dose of 3 times the maintenance dose will provide nearly steady-state concentrations within 1 day in most patients.
Therapeutic Uses. Sirolimus is indicated for prophylaxis of organ transplant rejection usually in combination with a reduced dose of calcineurin inhibitor and glucocorticoids. Sirolimus has been used with glucocorticoids and mycophenolate to avoid permanent renal damage. Sirolimus dosing regimens are relatively complex with blood levels generally targeted between 5 and 15 ng/mL. It is recommended that the daily maintenance dose be reduced by approximately one-third in patients with hepatic impairment. Sirolimus also has been incorporated into stents to inhibit local cell proliferation and blood vessel occlusion.
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. Although immunotherapy with sirolimus per se is not nephrotoxic, patients treated with cyclosporine plus sirolimus have impaired renal function compared to patients treated with cyclosporine alone. Sirolimus also may prolong delayed graft function in deceased-donor kidney transplants, presumably because of its antiproliferative action. Renal function therefore must be monitored closely in such patients. Lymphocele, a known surgical complication associated with renal transplantation, is increased in a dose-dependent fashion by sirolimus, requiring close postoperative follow-up. Other adverse effects include anemia, leukopenia, thrombocytopenia, mouth ulcer, hypokalemia, proteinuria, and GI effects. Delayed wound healing may occur with sirolimus use. As with other immunosuppressive agents, there is an increased risk of neoplasms, especially lymphomas, and infections.
Drug Interactions. Because sirolimus is a substrate for CYP3A4 and is transported by P-glycoprotein, close attention to interactions with other drugs that are metabolized or transported by these proteins is required. Dose adjustment may be required when sirolimus is coadministered with diltiazem or rifampin.
Everolimus [40-O-(2-hydroxyethyl)-rapamycin] is closely related to sirolimus but has distinct pharmacokinetics. The main difference is a shorter t1/2 and thus a shorter time to achieve steady-state concentrations of the drug. Dosage on a milligram per kilogram basis is similar to that of sirolimus. As with sirolimus, the combination of a calcineurin inhibitor and an mTOR inhibitor produces worse renal function at 1 year than does calcineurin inhibitor therapy alone. The toxicity of everolimus and the drug interactions seem to be the same as with sirolimus.
Azathioprine (IMURAN, others) is a purine antimetabolite. It is an imidazolyl derivative of 6-mercaptopurine, metabolites of which can inhibit purine synthesis.
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 (see Chapter 61). A fraudulent nucleotide, 6-thio-IMP, is converted to 6-thio-GMP and finally to 6-thio-GTP, which is incorporated into DNA. Cell proliferation thereby is inhibited, impairing a variety of lymphocyte functions. Azathioprine appears to be a more potent immunosuppressive agent than 6-mercaptopurine.
Disposition and Pharmacokinetics. Azathioprine is well absorbed orally and reaches maximum blood levels within 1-2 h after administration. The t1/2 of azathioprine is ~10 min, and t1/2 of 6-mercaptopurine is ~1 h. Blood levels have limited 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 are rapidly removed from the blood by oxidation or methylation in the liver and/or erythrocytes.
Therapeutic Uses. Azathioprine is indicated as an adjunct for prevention of organ transplant rejection and in severe rheumatoid arthritis. The usual starting dose of azathioprine is 3-5 mg/kg/day. Lower initial doses (1 mg/kg/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, including 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, GI toxicity, pancreatitis, and increased risk of neoplasia.
Drug Interactions. Xanthine oxidase, an enzyme of major importance in the catabolism of azathioprine metabolites, is blocked by allopurinol. Adverse effects resulting from coadministration of azathioprine with other myelosuppressive agents or angiotensin-converting enzyme inhibitors include leukopenia, thrombocytopenia, and anemia as a result of myelosuppression.
Mycophenolate mofetil (MMF; CELLCEPT) is the 2-morpholinoethyl ester of mycophenolic acid (MPA). MMF is a prodrug that is rapidly hydrolyzed to the active drug, MPA, a selective, noncompetitive, reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), 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; MPA thus selectively inhibits lymphocyte proliferation and functions, including antibody formation, cellular adhesion, and migration.
Disposition and Pharmacokinetics. MMF undergoes rapid and complete metabolism to MPA after oral or intravenous administration. MPA is then metabolized to the inactive glucuronide MPAG. The t1/2 of MPA is ~16 h. Most (87%) is excreted in the urine as MPAG. Plasma concentrations of MPA and MPAG are increased in patients with renal insufficiency.
Therapeutic Uses. MMF is indicated for prophylaxis of transplant rejection, and it typically is used in combination with glucocorticoids and a calcineurin inhibitor but not with azathioprine. Combined treatment with sirolimus is possible, although potential drug interactions necessitate careful monitoring of drug levels. For renal transplants, 1 g is administered orally or intravenously (over 2 h) twice daily (2 g/day). A higher dose, 1.5 g twice daily (3 g/day), may be recommended for African American renal transplant patients and all liver and cardiac transplant patients. MMF is increasingly used off label in systemic lupus. A delayed-release formulation of MPA (MYFORTIC) is available. It does not release MPA under acidic conditions (pH < 5), as in the stomach, but is soluble in neutral pH, as in the intestine. The enteric coating results in a delay in the time to reach maximum MPA concentrations.
Toxicity. The principal toxicities of MMF are GI and hematologic: leukopenia, pure red cell aplasia, diarrhea, and vomiting. There also is an increased incidence of some infections, especially sepsis associated with cytomegalovirus. Tacrolimus in combination with MMF has been associated with activation of polyoma viruses such as BK virus, which can cause interstitial nephritis. The use of mycophenolate in pregnancy is associated with congenital anomalies and increased risk of pregnancy loss.
Drug Interactions. Tacrolimus delays elimination of MMF by impairing the conversion of MPA to MPAG. This may enhance GI toxicity. Coadministration with antacids containing aluminum or magnesium hydroxide leads to decreased absorption of MMF; thus, these drugs should not be administered simultaneously. MMF should not be administered with cholestyramine or other drugs that affect enterohepatic circulation. Such agents decrease plasma MPA concentrations, probably by binding free MPA in the intestines. Acyclovir and ganciclovir may compete with MPAG for tubular secretion, possibly resulting in increased concentrations of both MPAG and the antiviral agents in the blood, an effect that may be compounded in patients with renal insufficiency.
OTHER ANTIPROLIFERATIVE AND CYTOTOXIC AGENTS
Many of the cytotoxic and antimetabolic agents used in cancer chemotherapy (see Chapter 61) are immunosuppressive due to their action on lymphocytes and other cells of the immune system. Other cytotoxic drugs that have been used off label as immunosuppressive agents include methotrexate, cyclophosphamide, thalidomide (THALOMID), and chlorambucil (LEUKERAN). Methotrexate is used for treatment of graft-versus-host disease, rheumatoid arthritis, psoriasis, and some cancers. Cyclophosphamide and chlorambucil are used in leukemia and lymphomas and a variety of other malignancies. Cyclophosphamide also is FDA-approved for childhood nephrotic syndrome and is used widely for treatment of severe systemic lupus erythematosus and other vasculitides such as Wegener granulomatosis. Leflunomide (ARAVA, others) is a pyrimidine-synthesis inhibitor indicated for the treatment of adults with rheumatoid arthritis. This drug has found increasing empirical use in the treatment of polyomavirus nephropathy seen in immunosuppressed renal transplant recipients. There are no controlled studies showing efficacy compared with control patients treated with only withdrawal or reduction of immunosuppression alone in BK virus nephropathy. 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.
FINGOLIMOD (FTY720). This is the first agent in a new class of small molecules, sphingosine-1-phosphate receptor (S1P-R) agonists. This S1P receptor prodrug reduces recirculation of lymphocytes from the lymphatic system to the blood and peripheral tissues, thereby shunting lymphocytes away from inflammatory lesions and organ grafts.
Mechanism of Action. FTY720 specifically and reversibly causes sequestration of host lymphocytes into the lymph nodes and Peyer patches and thus away from the circulation, thereby protecting lesions and grafts from T cell–mediated attack. FTY720 does not impair T- and B-cell functions. Sphingosine kinase-2 phosphorylates FTY720; the FTY720-phosphate product is a potent agonist of S1P receptors. Altered lymphocyte traffic induced by FTY720 clearly results from its effect on S1P receptors.
Therapeutic Uses. The drug has not been as effective as standard regimens in phase III trials, and further drug development has been limited.
Toxicity. Lymphopenia, the predictable and most common side effect of FTY720, reverses upon discontinuation of the drug. Of greater concern is the negative chronotropic effect of FTY720 on the heart, which has been observed with the first dose in up to 30% of patients.
IMMUNOSUPPRESSION ANTIBODIES AND FUSION RECEPTOR PROTEIN
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 (ATG) or lymphocytes (ALG) into animals and then purifying the serum immunoglobulin fraction. These preparations vary in efficacy and toxicity from batch to batch. The capacity to produce monoclonal antibodies has overcome the problems of variability in efficacy and toxicity seen with the polyclonal products, but monoclonal Abs are more limited in their target specificity.
Another class of biological agents being developed for both autoimmunity and transplantation are fusion receptor proteins. These agents consist of the ligand-binding domains of receptors bound to the Fc region of an immunoglobulin (usually IgG1) to provide a longer t1/2.
ATG is a purified gamma globulin from the serum of rabbits immunized with human thymocytes.
MECHANISM OF ACTION. ATG 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. 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. ATG is used for induction immunosuppression, although the only approved indication is in the treatment of acute renal transplant rejection in combination with other immunosuppressive agents. Antilymphocyte-depleting agents (THYMOGLOBULIN, ATGAM, and OKT3) are not registered for use as induction immunosuppression. A course of antithymocyte-globulin often is given to renal transplant patients with delayed graft function to avoid early treatment with the nephrotoxic calcineurin inhibitors, thereby aiding in recovery from ischemic reperfusion injury. The recommended dose for acute rejection of renal grafts is 1.5 mg/kg/day (over 4-6 h) for 7-14 days. Mean T-cell counts fall by day 2 of therapy. ATG also is used for acute rejection of other types of organ transplants and for prophylaxis of rejection.
Toxicity. Polyclonal antibodies are xenogeneic proteins that can elicit major side effects, including 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 4-6 h) into a large-diameter vessel minimize such reactions. Serum sickness and glomerulonephritis can occur; anaphylaxis is rare. 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 combined.
ANTI-CD3 MONOCLONAL ANTIBODIES. Antibodies directed at the A chain of CD3, a trimeric molecule adjacent to the T-cell receptor on the surface of human T lymphocytes, have been used with considerable efficacy in human transplantation. The original mouse IgG2a antihuman CD3 monoclonal antibody, muromonab-CD3 (OKT3, ORTHOCLONEOKT3), still is used to reverse glucocorticoid-resistant rejection episodes.
Mechanism of Action. Muromonab-CD3 binds to the a chain of CD3, a monomorphic component of the T-cell receptor complex involved in antigen recognition, cell signaling, and proliferation. Antibody treatment induces rapid internalization of the T-cell receptor, thereby preventing subsequent antigen recognition. Administration of the antibody is followed rapidly by depletion and of a majority of T cells from the bloodstream and peripheral lymphoid organs such as lymph nodes and spleen. This absence of detectable T cells from the usual lymphoid regions is secondary both to complement 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 reduces 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. The recommended dose is 5 mg/day (in adults; less for children) in a single intravenous bolus (< 1 min) for 10-14 days. Circulating T cells disappear from the blood within minutes of administration and return within ~1 week after termination of therapy. Repeated use of muromonab-CD3 results in the immunization of the patient against the mouse determinants of the antibody and generally is contraindicated. Administration of glucocorticoids before the injection of muromonab-CD3 is standard; it prevents the release of cytokines and reduces first-dose reactions considerably, and now is a standard procedure. Volume status of patients also must be monitored carefully before therapy; a fully competent resuscitation facility must be immediately available for patients receiving their first several doses of this therapy.
Toxicity. The major side effect of anti-CD3 therapy is the “cytokine release syndrome.” The syndrome typically begins 30 min after infusion of the antibody (but can occur later) and may persist for h. The syndrome is associated with increased serum levels of cytokines (including tumor necrosis factor-α [TNF-α], IL-2, IL-6, and interferon-γ [IFN-γ]) released by activated T cells and/or monocytes. Clinical manifestations include high fever, chills/rigor, headache, tremor, nausea, vomiting, diarrhea, abdominal pain, malaise, myalgias, arthralgias, and generalized weakness. Less common complaints include skin reactions and cardiorespiratory and CNS disorders. Potentially fatal pulmonary edema, acute respiratory distress syndrome, cardiovascular collapse, cardiac arrest, and arrhythmias have been described. Other toxicities associated with anti-CD3 therapy include anaphylaxis and the usual infections and neoplasms associated with immunosuppressive therapy. “Rebound” rejection has been observed when muromonab-CD3 treatment is stopped. Anti-CD3 therapies may be limited by anti-idiotypic or antimurine antibodies in the recipient. Muromonab-CD3 rarely is used in transplantation. It has been replaced by ATG and alemtuzumab.
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 Fc receptors. In initial clinical trials, a humanized anti-CD3 monoclonal antibody that does not bind to Fc receptors reversed acute renal allograft rejection without causing the first-dose cytokine-release syndrome.
ANTI-IL-2 RECEPTOR (ANTI-CD25) ANTIBODIES. Daclizumab (ZENAPAX) is a humanized murine complementarity-determining region (CDR)/human IgG1 chimeric monoclonal antibody. Basiliximab (SIMULECT) is a murine-human chimeric monoclonal antibody.
Mechanism of Action. The anti-CD25 mAbs bind to the IL-2 receptor on the surface of activated T cells. Significant depletion of T cells does not appear to play a major role in the mechanism of action of these mAbs. Therapy with the anti IL-2R mAbs results in a relative decrease of the expression of the α chain epitope of the IL-2R on activated lymphocytes. The β chain may be downregulated by the anti-CD25 antibody. Daclizumab has a somewhat lower affinity but a longer t1/2 (20 days) than basiliximab.
Therapeutic Uses. Anti–IL-2-receptor monoclonal antibodies are used for prophylaxis of acute organ rejection in adult patients.
Clinical trials indicate that the t1/2 of daclizumab is 20 days, resulting in saturation of the IL-2Rα on circulating lymphocytes for up to 120 days after transplantation. Daclizumab was administered in 5 doses (1 mg/kg given intravenously over 15 min in 50-100 mL of normal saline) starting immediately preoperatively, and subsequently at biweekly intervals. Daclizumab was used with maintenance immunosuppressive regimens (cyclosporine, azathioprine, and steroids; cyclosporine and steroids).
The t1/2 of basiliximab is 7 days. In trials, basiliximab was administered in a fixed dose of 20 mg preoperatively and on days 0 and 4 after transplantation. This regimen of basiliximab saturated IL-2R on circulating lymphocytes for 25-35 days after transplantation. Basiliximab was used with a maintenance regimen consisting of cyclosporine and prednisone and was found to be safe and effective when used in a maintenance regimen consisting of cyclosporine, MMF, and prednisone.
Toxicity. No cytokine-release syndrome has been noted, but anaphylactic reactions and rare lymphoproliferative disorders and opportunistic infections may occur. No drug interactions have been described.
Alemtuzumab. Alemtuzumab (CAMPATH) is a humanized mAb approved for use in chronic lymphocytic leukemia. The antibody targets CD52, a glycoprotein expressed on lymphocytes, monocytes, macrophages, and natural killer cells; the drug causes lympholysis by inducing apoptosis of targeted cells. It has achieved some use in renal transplantation because it produces prolonged T- and B-cell depletion and allows drug minimization.
ANTI-TNF REAGENTS. TNF-T is a proinflammatory cytokine that has been implicated in the pathogenesis of several immune-mediated intestinal, skin, and joint diseases. Several diseases (rheumatoid arthritis, Crohn disease) are associated with elevated levels of TNF-α. As a result, a number of anti-TNF agents have been developed as treatments.
Infliximab (REMICADE) is a chimeric IgG1 monoclonal antibody containing a human constant (Fc) region and a murine variable region. It binds with high affinity to TNF-α and prevents the cytokine from binding to its receptors. Infliximab is approved in the U.S. for treating the symptoms of rheumatoid arthritis and is typically used in combination with methotrexate in patients who do not respond to methotrexate alone. Infliximab also is approved for treatment of symptoms of Crohn disease, ankylosing spondylitis, plaque psoriasis, psoriatic arthritis, and ulcerative colitis. About 1 of 6 patients receiving infliximab experiences an infusion reaction characterized by fever, urticaria, hypotension, and dyspnea within 1-2 h after antibody administration. The development of antinuclear antibodies, and rarely a lupus-like syndrome, has been reported after treatment with infliximab.
Etanercept (ENBREL) is a fusion protein that targets TNF-α. Etanercept contains the ligand-binding portion of a human TNF-α. receptor fused to the Fc portion of human IgG1, and binds to TNF-α, and prevents it from interacting with its receptors. It is approved for treatment of the symptoms of rheumatoid arthritis, ankylosing spondylitis, plaque psoriasis, polyarticular juvenile idiopathic arthritis, and psoriatic arthritis. Etanercept can be used in combination with methotrexate in patients who have not responded adequately to methotrexate alone. Injection-site reactions (i.e., erythema, itching, pain, or swelling) have occurred.
Adalimumab (HUMIRA) is another anti-TNF product for intravenous use. This recombinant human IgG1 monoclonal antibody is approved for use in rheumatoid arthritis, ankylosing spondylitis, Crohn disease, juvenile idiopathic arthritis, plaque psoriasis, psoriatic arthritis, and ulcerative colitis.
Toxicity. All anti-TNF agents (i.e., infliximab, etanercept, adalimumab) increase the risk for serious infections, lymphomas, and other malignancies. For example, fatal hepatosplenic T-cell lymphomas have been reported in adolescent and young adult patients with Crohn disease treated with infliximab in conjunction with azathioprine or 6-mercaptopurine.
Plasma IL-1 levels are increased in patients with active inflammation (see Chapter 34). In addition to the naturally occurring IL-1 receptor antagonist (IL-1RA), several IL-1 receptor antagonists are in development and a few have been approved for clinical use. Anakinra is an FDA-approved recombinant, nonglycosylated form of human IL-1RA for the management of joint disease in rheumatoid arthritis. It can be used alone or in combination with anti-TNF agents such as etanercept, infliximab, or adalimumab. Canakinumab (ILARIS) is an IL-1 β monoclonal antibody that is FDA-approved for Cryopyrin-associated periodic syndromes (CAPS), a group of rare inherited inflammatory diseases associated with overproduction of IL-1 that includes familial cold autoinflammatory and Muckle-Wells syndromes. Canakinumab is also being evaluated for use in chronic obstructive pulmonary disease. Rilonacept (IL-1 TRAP) is another IL-1 blocker (a fusion protein that binds IL-1) that is being evaluated for gout; IL-1 is an inflammatory mediator of joint pain associated with elevated uric acid crystals.
LYMPHOCYTE FUNCTION–ASSOCIATED ANTIGEN-1 (LFA-1) INHIBITION
Efalizumab (RAPTIVA) is a humanized IgG1 mAb targeting the CD11a chain of lymphocyte function–associated antigen-1 (LFA-1). Efalizumab binds to LFA-1 on lymphocytes and prevents the LFA-1 interaction with intercellular adhesion molecule (ICAM) thereby inhibiting T-cell adhesion, trafficking, and activation. Efalizumab is approved for use in patients with psoriasis.
Alefacept (AMEVIVE) is a human LFA-3-IgG1 fusion protein. The LFA-3 portion of alefacept binds to CD2 on T lymphocytes, blocking the interaction between LFA-3 and CD2 and interfering with T-cell activation. Alefacept is approved for use in psoriasis. Treatment with alefacept has been shown to produce a dose-dependent reduction in T-effector memory cells (CD45, RO+) but not in naïve cells (CD45, RA+).
TARGETING B CELLS
Most of the advances in transplantation can be attributed to drugs designed to inhibit T-cell responses. As a result, T cell–mediated acute rejection has been become much less of a problem, while B cell–mediated responses such as antibody-mediated rejection and other effects of donor-specific antibodies have become more evident. Both biologicals and small molecules with B-cell specific effects now are in development for transplantation, including humanized monoclonal antibodies to CD20 and inhibitors of the 2 B cell–activation factors BLYS and APRIL and their respective receptors. Belimumab (BENLYSTA), a monoclonal antibody that targets BLYS, was recently approved for use in patients with systemic lupus erythromatosus.
Immunosuppression has concomitant risks of opportunistic infections and secondary tumors. Therefore, the ultimate goal of research on organ transplantation and autoimmune diseases is to induce and maintain immunological tolerance, the active state of antigen-specific nonresponsiveness. Tolerance, if attainable, would represent a true cure for conditions discussed earlier in this section without the side effects of the various immunosuppressive therapies. The calcineurin inhibitors prevent tolerance induction in some, but not all, preclinical models. In these same model systems, sirolimus does not prevent tolerance and may even promote tolerance.
CO-STIMULATORY BLOCKADE. Induction of specific immune responses by T lymphocytes requires 2 signals: an antigen-specific signal via the T-cell receptor and a co-stimulatory signal provided by the interaction of molecules such as CD28 on the T lymphocyte and CD80 and CD86 on the antigen-presenting cell (Figure 35–2). Inhibition of the co-stimulatory signal has been shown to induce tolerance.
Figure 35–2 Co-stimulation. A. Two signals are required for T-cell activation. Signal 1 is via the T-cell receptor (TCR) and signal 2 is via a co-stimulatory receptor-ligand pair. Signal 1 in the absence of signal 2 results in an inactivated T cell. B. One important co-stimulatory 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 co-stimulatory molecules. CD152 is CD40 ligand, which interacts with CD40 as a co-stimulatory pair. CD154 (CTLA4) interacts with CD80 and CD86 to dampen or downregulate 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. (Adapted with permission from Clayberger C, Krensky AM. Mechanisms of allograft rejection. In Neilson EG, Couser WG, eds,Immunologic Renal Diseases, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001, pp 321–346. http://lww.com.)
Abatacept (CTLA4-Ig) is a fusion protein that contains the binding region of cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), which is a CD28 homolog, and the Fc region of the human IgG1. CTLA4-Ig competitively inhibits CD28 binding to CD80 and CD86 and thus activation of T-cells. CTLA4-Ig is effective in the treatment of rheumatoid arthritis in patients resistant to other drugs.
Belatacept (NULOJIX, LEA29Y) is a second-generation CTLA4-Ig with 2 amino acid substitutions. Belatacept has higher affinity for CD80 (2-fold) and CD86 (4-fold), yielding a 10-fold increase in potency in vitro as compared to CTLA4-Ig. Preclinical renal transplant studies in nonhuman primates showed that belatacept did not induce tolerance but did prolong graft survival. Because of the risk of post-transplant lymphoproliferative disease (PTLD), EBV negative patients should not be treated with belatacept. Belatacept is approved as an immunosuppressant to prevent organ rejection in renal transplantation.
A second co-stimulatory pathway involves the interaction of CD40 on activated T cells with CD40 ligand (CD154) on B cells, endothelium, and/or antigen-presenting cells (see Figure 35–2). Among the purported activities of anti-CD154 antibody treatment is the blockade of B7 expression induced by immune activation. Two humanized anti-CD154 monoclonal antibodies have been used in clinical trials in renal transplantation and autoimmune diseases. The development of these antibodies, however, is on hold because of associated thromboembolic events. An alternative approach to block the CD154-CD40 pathway is to target CD40 with monoclonal antibodies. These antibodies are undergoing trials in non-Hodgkin lymphoma but are also likely to be developed for autoimmunity and transplantation.
DONOR CELL CHIMERISM. A promising approach is induction of chimerism (coexistence of cells from 2 genetic lineages in a single individual) by first dampening or eliminating immune function in the recipient with ionizing radiation, drugs such as cyclophosphamide, and/or antibody treatment, and then providing a new source of immune function by adoptive transfer (transfusion) of bone marrow or hematopoietic stem cells. Upon reconstitution of immune function, the recipient no longer recognizes new antigens provided during a critical period as “nonself.” Such tolerance is long lived and less likely to be complicated by the use of calcineurin inhibitors.
ANTIGENS. Specific antigens induce immunological tolerance in preclinical models of diabetes mellitus, arthritis, and MS. In vitro and preclinical in vivo studies demonstrate that one can selectively inhibit immune responses to specific antigens without the associated toxicity of immunosuppressive therapies. With these insights comes the promise of specific immune therapies to treat an array of immune disorders from auto-immunity to transplant rejection.
In contrast to immunosuppressive agents that inhibit the immune response in transplant rejection and autoimmunity, a few immunostimulatory drugs have been developed with applicability to infection, immunodeficiency, and cancer.
THALIDOMIDE. Thalidomide (THALOMID) is best known for the severe, life-threatening birth defects it caused when administered to pregnant women. Thalidomide should never be taken by women who are pregnant or who could become pregnant while taking the drug. It is indicated for the treatment of patients with erythema nodosum leprosum (see Chapter 56) and multiple myeloma. In addition, it has orphan drug status for mycobacterial infections, Crohn disease, HIV-associated wasting, Kaposi sarcoma, lupus, myelofibrosis, brain malignancies, leprosy, graft-versus-host disease, and aphthous ulcers. Its mechanism of action is unclear.
LENALIDOMIDE. Lenalidomide (REVLIMID) is a thalidomide analog with immunomodulatory and anti-angiogenic properties. The drug is FDA-approved for the treatment of transfusion-dependent anemia. Lenalidomide causes significant neutropenia and thrombocytopenia, is associated with a significant risk for deep vein thrombosis, and carries the same risk of teratogenicity as thalidomide (pregnancy must be avoided).
BACILLUS CALMETTE-GUéRIN (BCG). Live BCG (T ICEBCG, THERACYS) is an attenuated, live culture of the bacillus of Calmette and Guérin strain of Mycobacterium bovis that induces a granulomatous reaction at the site of administration. By unclear mechanisms, this preparation is active against tumors and is indicated for the treatment and prophylaxis of carcinoma in situ of the urinary bladder and for prophylaxis of primary and recurrent stage Ta and/or T1 papillary tumors after transurethral resection. Adverse effects include hypersensitivity, shock, chills, fever, malaise, and immune complex disease.
LEVAMISOLE. Levamisole (ERGAMISOL) was synthesized originally as an anthelmintic but appears to “restore” depressed immune function of B lymphocytes, T lymphocytes, monocytes, and macrophages. Its only clinical indication was as adjuvant therapy with 5-fluorouracil after surgical resection in patients with Dukes’ stage C colon cancer. Levamisole is associated with risk for fatal agranulocytosis and was withdrawn from the U.S. market.
INTERFERONS. Although interferons (α, β, and γ,) initially were identified by their antiviral activity, these agents also have important immunomodulatory activities. The interferons bind to specific cell-surface receptors that initiate a series of intracellular events: induction of certain enzymes, inhibition of cell proliferation, and enhancement of immune activities, including increased phagocytosis by macrophages and augmentation of specific cytotoxicity by T lymphocytes.
Recombinant IFN-a-2b (INTRON A) is obtained from Escherichia coli by recombinant expression. It is a member of a family of naturally occurring small proteins (15-27 KDa), produced and secreted by cells in response to viral infections and other inducers. IFN-c-2b is indicated in the treatment of myriad tumors (e.g., hairy cell leukemia, malignant melanoma, follicular lymphoma, and AIDS-related Kaposi sarcoma) and for infectious diseases, chronic hepatitis B, and condylomata acuminata. IFN-d-2b is supplied in combination with ribavirin (REBETRON) for treatment of chronic hepatitis C in patients with compensated liver function not treated previously with IFN-α-2b or who have relapsed after IFN-α-2b therapy. Flu-like symptoms are the most common adverse effects after IFN-α-2b administration. Adverse cardiovascular effects (e.g., hypotension, arrhythmias, cardiomyopathy, and myocardial infarction) and CNS effects (e.g., depression, confusion) are less frequent. All α interferons carry a black-box warning regarding development of pulmonary hypertension.
IFN-γ-1b (ACTIMMUNE) is a recombinant polypeptide that activates phagocytes and induces their generation of oxygen metabolites that are toxic to a number of microorganisms. It is indicated to reduce the frequency and severity of serious infections associated with chronic granulomatous disease and to delay the time to progression in severe malignant osteopetrosis. Adverse reactions include fever, headache, rash, fatigue, GI distress, anorexia, weight loss, myalgia, and depression.
IFN-β-1a (AVONEX,REBIF), a 166–amino acid recombinant glycoprotein, and IFN-β-1b (BETASERON), a 165–amino acid recombinant protein, have antiviral and immunomodulatory properties. They are FDA-approved for the treatment of relapsing MS to reduce the frequency of clinical exacerbations (see below). The mechanism of their action in MS is unclear. Flu-like symptoms (e.g., fever, chills, myalgia) and injection-site reactions are common adverse effects.
INTERLEUKIN-2. Human recombinant IL-2 (aldesleukin, PROLEUKIN; des-alanyl-1, serine-125 human IL-2) differs from native IL-2 in that it is not glycosylated, has no amino-terminal alanine, and has a serine substituted for the cysteine at amino acid 125. Aldesleukin activates cellular immunity, with lymphocytosis, eosinophilia, thrombocytopenia, and release of multiple cytokines (e.g., TNF, IL-1, IFN-γ). Aldesleukin is indicated for the treatment of adults with metastatic renal cell carcinoma and melanoma.
The potency of the preparation is represented in International Units in a lymphocyte proliferation assay such that 1.1 mg of recombinant IL-2 protein equals 18 million IU. Administration of aldesleukin has been associated with serious cardiovascular toxicity resulting from capillary leak syndrome, which involves loss of vascular tone and leak of plasma proteins and fluid into the extravascular space. Hypotension, reduced organ perfusion, and death may occur. An increased risk of disseminated infection due to impaired neutrophil function also has been associated with aldesleukin treatment.
Active immunization involves stimulation with an antigen to develop immunological defenses against a future exposure. Passive immunization involves administration of pre-formed antibodies to an individual who is already exposed or is about to be exposed to an antigen.
VACCINES. Active immunization, vaccination, involves administration of an antigen as a whole, killed (inactivated) organism; attenuated (live) organism; or a specific protein or peptide constituent of an organism. Booster doses often are required, especially when killed organisms are used as the immunogen. In the U.S., vaccination has sharply curtailed or practically eliminated a variety of major infections, including diphtheria, measles, mumps, pertussis, rubella, tetanus, Haemophilus influenzae type b, and pneumococcus.
Although most vaccines have targeted infectious diseases, a new generation of vaccines may provide complete or limited protection from specific cancers or autoimmune diseases. Because T cells optimally are activated by peptides and co-stimulatory ligands that are present on antigen-presenting cells (APCs), one approach for vaccination has consisted of immunizing patients with APCs expressing a tumor antigen. Multiple studies have demonstrated the efficacy of DNA vaccines in small- and large-animal models of infectious diseases and cancer. Compared to peptide immunization, DNA immunization has the advantage of permitting generation of entire proteins, enabling determinant selection to occur in the host without having to restrict immunization to patients bearing specific HLA alleles. However, a safety concern about this technique is the potential for integration of the plasmid DNA into the host genome, possibly disrupting important genes and thereby leading to phenotypic mutations or carcinogenicity. A final approach to generate or enhance immune responses against specific antigens consists of infecting cells with recombinant viruses that encode the protein antigen of interest.
IMMUNE GLOBULIN. Passive immunization is indicated when an individual is deficient in antibodies because of a congenital or acquired immunodeficiency, when an individual with a high degree of risk is exposed to an agent and there is inadequate time for active immunization (e.g., measles, rabies, hepatitis B), or when a disease is already present but can be ameliorated by passive antibodies (e.g., botulism, diphtheria, tetanus). Passive immunization may be provided by several different products (Table 35–2).
Selected Immune Globulin Preparations
Nonspecific immunoglobulins or highly specific immunoglobulins may be provided based on the indication. The protection provided usually lasts 1-3 months. Immune globulin is derived from pooled plasma of adults by an alcohol-fractionation procedure. It contains largely IgG (95%) and is indicated for antibody-deficiency disorders, exposure to infections such as hepatitis A and measles, and specific immunological diseases such as immune thrombocytopenic purpura and Guillain-Barré syndrome. In contrast, specific immune globulins (“hyperimmune”) differ from other immune globulin preparations in that donors are selected for high titers of the desired antibodies. Specific immune globulin preparations are available for hepatitis B, rabies, tetanus, varicella-zoster, cytomegalovirus, botulism, and respiratory syncytial virus. Rho(D) immune globulin is a specific hyperimmune globulin for prophylaxis against hemolytic disease of the newborn due to Rh incompatibility between mother and fetus. All such plasma-derived products carry the theoretical risk of transmission of infectious disease.
RHO(D) IMMUNE GLOBULIN. The commercial forms of Rho(D) immune globulin (see Table 35–2) consist of IgG containing a high titer of antibodies against the Rh(D) antigen on the surface of red blood cells. All donors are carefully screened to reduce the risk of transmitting infectious diseases. Fractionation of the plasma is performed by precipitation with cold alcohol followed by passage through a viral clearance system. Rho(D) immune globulin binds Rho antigens, preventing sensitization. Rh-negative women may be sensitized to the “foreign” Rh antigen on red blood cells via the fetus at the time of birth, miscarriage, ectopic pregnancy, or any transplacental hemorrhage. If the women go on to have a primary immune response, they will make antibodies to Rh antigen that can cross the placenta and damage subsequent fetuses by lysing red blood cells. This syndrome, called hemolytic disease of the newborn, is life threatening but is largely preventable by Rho(D) immune globulin. Rho(D) immune globulin is indicated whenever fetal red blood cells are suspected to have entered the circulation of an Rh-negative mother unless the fetus is known to be Rh negative. The drug is given intramuscularly. The t1/2 of circulating immunoglobulin is ~21-29 days. Systemic reactions are extremely rare; myalgia, lethargy, and anaphylactic shock have been reported.
INTRAVENOUS IMMUNOGLOBULIN (IVIG). Indications for the use of IVIG have expanded beyond replacement therapy for agammaglobulinemia and other immunodeficiencies to include a variety of bacterial and viral infections, and an array of autoimmune and inflammatory diseases as diverse as thrombocytopenic purpura, Kawasaki disease, and autoimmune skin, neuromuscular, and neurological diseases. The mechanism of action of IVIG in immune modulation remains largely unknown.
IMMUNOTHERAPY FOR MULTIPLE SCLEROSIS
CLINICAL FEATURES AND PATHOLOGY. MS is a demyelinating inflammatory disease of the CNS white matter that displays a triad of pathogenic symptoms: mononuclear cell infiltration, demyelination, and scarring (gliosis). The peripheral nervous system is uninvolved. MS may be episodic or progressive, and occurs with prevalence increasing from late adolescence to 35 years of age and then declining. MS is roughly 3-fold more common in females than in males and occurs mainly in higher latitudes of the temperate climates. Epidemiologic studies suggest a role for environmental factors in the pathogenesis of MS; despite many suggestions, associations with infectious agents have proven inconclusive. A stronger linkage is the genetic one: people of northern European ancestry have a higher susceptibility to MS, and studies in twins and siblings suggest a strong genetic component.
MS is a complex genetic disease in which multiple allelic variants lead to disease susceptibility. HLA-DR2 clearly is associated with risk of developing MS. There also is substantial evidence of an autoimmune component to MS: In MS patients, there are activated T cells that are reactive to different myelin antigens, including myelin basic protein (MBP). In addition, there is evidence for the presence of auto-antibodies to myelin oligodendrocyte glycoprotein (MOG) and to MBP that can be eluted from the CNS plaque tissue. These antibodies may act with pathogenic T cells to produce some of the cellular pathology of MS. The neurophysiological result is altered conduction (both positive and negative) in myelinated fibers within the CNS (cerebral white matter, brain stem, cerebellar tracts, optic nerves, spinal cord); some alterations appear to result from exposure of voltage-dependent K+ channels that normally are covered by myelin.
Attacks are classified by type and severity and likely correspond to specific degrees of CNS damage and pathological processes. Thus, physicians refer to relapsing-remitting MS (the form in 85% of younger patients), secondary progressive MS (progressive neurological deterioration following a long period of relapsing-remitting disease), and primary progressive MS (~15% of patients, wherein deterioration with relatively little inflammation is apparent at onset).
PHARMACOTHERAPY FOR MS. Table 35–3 summarizes current immunomodulatory therapies for MS. Specific therapies are aimed at resolving acute attacks, reducing recurrences and exacerbations, and slowing the progression of disability. Nonspecific therapies focus on maintaining function and quality of life. For acute attacks, pulse glucocorticoids often are employed (typically, 1 g/day of methylprednisolone administered intravenously for 3-5 days). For relapsing-remitting attacks, immunomodulatory therapies are approved: β-1 interferons [IFN-β-1a, IFN-β-1b], and glatiramer acetate (COPAXONE).
Pharmacotherapy of Multiple Sclerosis
Random polymers that contain amino acids commonly used as MHC anchors and T cell–receptor contact residues have been proposed as possible “universal APLs (altered peptide ligands).” Glatiramer acetate (GA), a random-sequence polypeptide consisting of 4 amino acids [alanine (A), lysine (K), glutamate (E), and tyrosine (Y)] with an average length of 40-100 amino acids, binds efficiently to MHC class II DR molecules, but does not bind MHC class II DQ or MHC class I molecules in vitro. In clinical trials, GA, administered subcutaneously to patients with relapsing-remitting MS, decreased the rate of exacerbations by ~30%. In vivo administration of GA induces highly cross-reactive CD4+ T cells that are immune deviated to secrete Th2 cytokines and prevents the appearance of new lesions detectable by magnetic resonance imaging. This represents one of the first successful uses of an agent that ameliorates autoimmune disease by altering signals through the T cell–receptor complex.
For relapsing-remitting attacks and for secondary progressive MS, the alkylating agent cyclophosphamide and mitoxantrone (NOVANTRONE, others) currently are used in patients refractory to other immunomodulators. These agents, primarily used for cancer chemotherapy, have significant toxicities (see Chapter 61). Mitoxantrone generally is tolerated only up to an accumulated dose of 100-140 mg/m2. However, the FDA recommends that left ventricular ejection fraction (LVEF) be evaluated before initiating therapy, prior to each dose, and annually after patients have finished treatment to detect late-occurring cardiac toxicity.
The monoclonal antibody, natalizumab (TYSABRI), directed against the adhesion molecule α4 integrin, antagonizes interactions with integrin heterodimers containing α4 integrin, such as α4 β1 integrin that is expressed on the surface of activated lymphocytes and monocytes. An interaction of α4 β1 integrin with vascular-cellular adhesion molecule (VCAM)-1 seems critical for T-cell trafficking from the periphery into the CNS; blocking this interaction would hypothetically inhibit disease exacerbations. Use of natalizumab has been associated with the development of progressive multifocal leukoencephalopathy, and availability is limited to a special distribution program (TOUCH) administered by the manufacturer. Monoclonal antibodies directed against the IL-2 receptor and against CD52 (alemtuzumab; CAMPATH) are in phase III clinical trials.
Each of the agents mentioned has side effects and contraindications that may be limiting: infections (for glucocorticoids), hypersensitivity and pregnancy (for immunomodulators), and prior anthracycline/anthracenedione use, mediastinal irradiation, or cardiac disease (mitoxantrone). With all of these agents, the earlier they are used, the more effective they are in preventing disease relapses. What is not clear is whether any of these agents will prevent or diminish the later onset of secondary progressive disease, which causes the more severe disability.