Section X - Drugs Used for Immunomodulation doc

Prednisone, prednisolone, and other glucocorticoids are used alone and in combination with other immunosuppressive agents for treatment of transplant rejection and autoimmune disorders..

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 themechanism 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 drugtoxicities 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 theinnate 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 diseasesinclude 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 tomodulate 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)

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 orfor 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

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 transductionare 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.)

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 ofNFAT 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 alsoincreases 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 vehiclewhich 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), themost 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

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 cardiactransplant 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 ofcyclosporine (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 toachieve 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 cell growth factor receptors (Figure 53–2) (Kuo et al , 1992) Sirolimus, like cyclosporine and

T-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 concentrationwithin 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).

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 requiredwhen 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 isabout 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 andminimize 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

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 (<1%) of MPA are excreted in the urine (Bardsley-Elliot et al , 1999) Most

(87%) is excreted in the urine as MPAG Plasma concentrations of both MPA and MPAG are increased in patients with renal insufficiency In early renal transplant patients (<40 days

posttransplant), plasma concentrations of MPA after a single dose of mycophenolate mofetil are about half of those found in healthy volunteers or stable renal transplant patients Studies in the pediatric population are limited; safety and effectiveness in this population have not been

established (Butani et al , 1999).