The calcineurin inhibitors ciclosporin (cyclosporine) and tacrolimus are immunosuppressant drugs used for the prevention of organ rejection following transplantation Both agents are metabolic substrates for cytochrome P450 (CYP) 3A enzymes — in particular, CYP3A4 and CYP3A5 — and are transported out of cells via P-glycoprotein (ABCB1) Several single nucleotide polymorphisms (SNPs) have been identified in the genes encoding for CYP3A4, CYP3A5 and P-glycoprotein, including CYP3A4 —392A>G (rs2740574), CYP3A5 6986A>G (rs776746), ABCB1 3435C>T (rs1045642), ABCB1 1236C>T (rs1 128503) and ABCB1 2677G>T/A (rs2032582) The aim of this review is to provide the clinician with an extensive overview of the recent literature on the known effects of these SNPs on the pharmacodynamics of ciclosporin and tacrolimus in solid-organ transplant recipients Literature searches were performed and all relevant primary research articles were critiqued and summarized There is no evidence that the CYP3A4 —392A>G SNP has an effect on the pharmacodynamics of either ciclosporin or tacrolimus; however, studies have been limited For patients prescribed ciclosporin, the CYP3A5 6986A>G SNP may influence long-term survival, possibly because of a different metabolite pattern over time This SNP has no clear association with acute rejection during ciclosporin therapy Despite a strong association between the CYP3A5 6986A>G SNP and tacrolimus pharmacokinetics, there is no consistent evidence of organ rejection as a result of genotype-related under-immunosuppression This is likely to be explained by the practice of performing tacrolimus dose adjustments in the early phase after transplantation The effect of the CYP3A5 6986A>G SNP on ciclosporin-and tacrolimus-related nephrotoxicity and development of hypertension is unclear Similarly, the ABCB1 SNPs exert no clear influence on either ciclosporin or tacrolimus pharmacodynamics, with studies showing conflicting results in regard to the main parameters of acute rejection and nephrotoxicity In kidney transplant patients, consideration of the donor kidney genotype rather than the recipient genotype may be more important when assessing development of nephrotoxicity Studies with low patient numbers may account for many inconsistent results to date The majority of studies have only evaluated the effects of individual SNPs; however, multiple polymorphisms may interact to produce a combined effect Further haplotype analyses are likely to be useful, particularly ones that consider both donor and recipient genotype The effects of polymorphisms associated with the pregnane X receptor, organic anion transporting polypeptides, calcineurin inhibitor target sites and immune response pathways need to be further investigated A large standardized clinical trial is now required to evaluate the relationship between the pharmacokinetics and pharmacodynamics of CYP3A5-mediated tacrolimus metabolism, particularly in regard to the outcomes of acute rejection and nephrotoxicity It is not yet clear whether pharmacogenetic profiling of calcineurin inhibitors will be a useful clinical tool for personalizing immunosuppressant therapy

Effect of CYP3A and ABCB1 Single Nucleotide Polymorphisms on the Pharmacokinetics and Pharmacodynamics of Calcineurin Inhibitors: Part II

The calcineurin inhibitor tacrolimus is the backbone of immunosuppressive drug therapy after solid organ transplantation. Tacrolimus is effective in preventing acute rejection but has considerable toxicity and displays marked inter-individual variability in its pharmacokinetics and pharmacodynamics. The genetic basis of these phenomena is reviewed here. With regard to its pharmacokinetic variability, a single nucleotide polymorphism (SNP) in cytochrome P450 (CYP) 3A5 (6986A>G) has been consistently associated with tacrolimus dose requirement. Patients expressing CYP3A5 (those carrying the A nucleotide, defined as the *1 allele) have a dose requirement that is around 50 % higher than non-expressers (those homozygous for the G nucleotide, defined as the *3 allele). A randomised controlled study in kidney transplant recipients has demonstrated that a CYP3A5 genotype-based approach to tacrolimus dosing leads to more patients reaching the target concentration early after transplantation. However, no improvement of clinical outcomes (rejection incidence, toxicity) was observed, which may have been the result of the design of this particular study. In addition to CYP3A5 genotype, other genetic variants may also contribute to the variability in tacrolimus pharmacokinetics. Among these, the CYP3A4*22 and POR*28 SNPs are the most promising. Individuals carrying the CYP3A4*22 T-variant allele have a lower tacrolimus dose requirement than individuals with the CYP3A4*22 CC genotype and this effect appears to be independent of CYP3A5 genotype status. Individuals carrying the POR*28 T-variant allele have a higher tacrolimus dose requirement than POR*28 CC homozygotes but this association was only found in CYP3A5-expressing individuals. Other, less well-defined SNPs have been inconsistently associated with tacrolimus dose requirement. It is envisaged that in the future, algorithms incorporating clinical, demographic and genetic variables will be developed that will aid clinicians with the determination of the tacrolimus starting dose for an individual transplant recipient. Such an approach may limit early tacrolimus under-exposure and toxicity. With regard to tacrolimus pharmacodynamics, no strong genotype–phenotype relationships have been identified. Certain SNPs associate with rejection risk but these observations await replication. Likewise, the genetic basis of tacrolimus-induced toxicity remains unclarified. SNPs in the genes encoding for the drug transporter ABCB1 and the CYP3A enzymes may relate to chronic nephrotoxicity but findings have been inconsistent. No genetic markers reliably predict new-onset diabetes mellitus after transplantation, hypertension or neurotoxicity. The CYP3A5*1 SNP is currently the most promising biomarker for tailoring tacrolimus treatment. However, before CYP3A5 genotyping is incorporated into the routine clinical care of transplant recipients, prospective clinical trials are needed to determine whether such a strategy improves patient outcomes. The role of pharmacogenetics in tacrolimus pharmacodynamics should be explored further by the study of intra-lymphocyte and tissue tacrolimus concentrations.

The Role of Pharmacogenetics in the Disposition of and Response to Tacrolimus in Solid Organ Transplantation

Tacrolimus (FK506) and cyclosporine (cyclosporin A, CsA) are cornerstone immunosuppressive agents administered to solid organ transplant recipients to prevent and treat allograft rejection. The discovery of cyclosporine in the 1970s, and its entry into the collection of immunosuppressants in the early 1980s, was a major breakthrough in medicine. Cyclosporine was the most successful antirejection drug to date, and it radically improved the chance of survival for transplant recipients. In 1994, the Food and Drug Administration (FDA) approved tacrolimus, an effective alternative to cyclosporine [1]. Since then, tacrolimus and cyclosporine have become the principal immunosuppressive drugs for solid organ transplantation. The United States Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients showed that in 2011, 86% of the 16 055 patients who received a kidney transplant were prescribed tacrolimus upon discharge, and 2.4% were prescribed cyclosporine. One year after transplant, 84 and 4% of patients received tacrolimus and cyclosporine therapy, respectively [2]. Global differences exist in the usage of tacrolimus and cyclosporine: 2008 figures from the Australia and New Zealand Dialysis and Transplant Registry show that 61% of the 391 Australian patients who received a deceased kidney donor graft were prescribed tacrolimus, and 35% were prescribed cyclosporine. At 1-year post-transplant, these numbers changed to 55 and 33% for tacrolimus and cyclosporine, respectively [3]. Both drugs are also prescribed for liver, intestinal, lung, and heart transplant recipients [2], and can be used to manage severe autoimmune conditions, such as atopic dermatitis [4,5] and rheumatoid arthritis [6,7].

Tacrolimus and cyclosporine differ in their chemical structure: cyclosporine is a cyclic endecapeptide [8], whereas tacrolimus is a macrocyclic lactone [9]. However, they act in a similar manner. Both are calcineurin inhibitors; their main mechanism of action involves inhibition of this important phosphatase [1]. Tacrolimus exhibits similar effects to cyclosporine, but at concentrations 100 times lower [10]. Despite these differences in potency, tacrolimus and cyclosporine both show excellent survival rates for grafts across many comparative studies (summarized in Maes and Vanrenterghem [11]). However, several studies have shown that use of tacrolimus is associated with a lower allograft rejection rate compared with cyclosporine [12-14].

The principal adverse effects associated with tacrolimus and cyclosporine treatment are neurotoxicity, nephrotoxicity, hypertension, hyperglycemia, gastrointestinal disturbances, infections, and malignancy [15]. Although the two drugs have similar side-effect profiles, they may differ in the frequency of effects. For example, tacrolimus is more likely to cause alopecia [16], tremors [17], and new-onset diabetes mellitus [12], whereas cyclosporine is associated with hyperlipidemia [18], hypertrichosis, and gingival hyperplasia [19]. The idea that tacrolimus is less nephrotoxic than cyclosporine remains controversial [20], particularly as most studies of renal injury are based on evaluations in renal transplant patients, making it difficult to discriminate between drug-induced organ damage and other causes of organ dysfunction [21]. A recent study in pancreatic transplant recipients examined baseline kidney biopsies and 5-year post-transplant biopsies, and reported that the chronic nephrotoxic effects of tacrolimus and cyclosporine were similar [20].

Despite the success of both drugs, treatment is complicated by narrow therapeutic indices and large intrapatient and interpatient pharmacokinetic variability [22,23]. Although adequate exposure is essential to prevent rejection, overexposure can lead to toxicities that reduce tolerability and affect long-term allograft and patient survival [24]. Therapeutic drug monitoring (TDM), therefore, is mandatory for both drugs. However, because individual transplant recipients respond differently to similar immunosuppressant concentrations, achieving the recommended therapeutic target range does not guarantee absence of drug toxicity or complete immunosuppressant efficacy. A mechanistic understanding of the underlying factors affecting the pharmacokinetics and pharmacodynamics of calcinuerin inhibitors may prove useful in being able to further personalize these therapies.

This review aims to provide a broad overview of recently published literature on the pharmacokinetics, pharmacodynamics, and pharmacogenetics of tacrolimus and cyclosporine in transplant patients, with the goals of clarifying current understanding and identifying areas of future research. In doing so, this review builds on the work of others in this field [1,8,24-27]. A particular emphasis is given to pharmacogenetics, as developments in this area may provide a way to optimize treatment with these drugs, potentially avoiding negative side effects while still maintaining efficacy.

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PharmGKB summary: cyclosporine and tacrolimus pathways.

This review aims to provide an update of the literature on the pharmacology and toxicology of mycophenolate in solid organ transplant recipients. Mycophenolate is now the antimetabolite of choice in immunosuppressant regimens in transplant recipients. The active drug moiety mycophenolic acid (MPA) is available as an ester pro-drug and an enteric-coated sodium salt. MPA is a competitive, selective and reversible inhibitor of inosine-5'-monophosphate dehydrogenase (IMPDH), an important rate-limiting enzyme in purine synthesis. MPA suppresses T and B lymphocyte proliferation; it also decreases expression of glycoproteins and adhesion molecules responsible for recruiting monocytes and lymphocytes to sites of inflammation and graft rejection; and may destroy activated lymphocytes by induction of a necrotic signal. Improved long-term allograft survival has been demonstrated for MPA and may be due to inhibition of monocyte chemoattractant protein 1 or fibroblast proliferation. Recent research also suggested a differential effect of mycophenolate on the regulatory T cell/helper T cell balance which could potentially encourage immune tolerance. Lower exposure to calcineurin inhibitors (renal sparing) appears to be possible with concomitant use of MPA in renal transplant recipients without undue risk of rejection. MPA displays large between- and within-subject pharmacokinetic variability. At least three studies have now reported that MPA exhibits nonlinear pharmacokinetics, with bioavailability decreasing significantly with increasing doses, perhaps due to saturable absorption processes or saturable enterohepatic recirculation. The role of therapeutic drug monitoring (TDM) is still controversial and the ability of routine MPA TDM to improve long-term graft survival and patient outcomes is largely unknown. MPA monitoring may be more important in high-immunological recipients, those on calcineurin-inhibitor-sparing regimens and in whom unexpected rejection or infections have occurred. The majority of pharmacodynamic data on MPA has been obtained in patients receiving MMF therapy in the first year after kidney transplantation. Low MPA area under the concentration time from 0 to 12 h post-dose (AUC0-12) is associated with increased incidence of biopsy-proven acute rejection although AUC0-12 optimal cut-off values vary across study populations. IMPDH monitoring to identify individuals at increased risk of rejection shows some promise but is still in the experimental stage. A relationship between MPA exposure and adverse events was identified in some but not all studies. Genetic variants within genes involved in MPA metabolism (UGT1A9, UGT1A8, UGT2B7), cellular transportation (SLCOB1, SLCO1B3, ABCC2) and targets (IMPDH) have been reported to effect MPA pharmacokinetics and/or response in some studies; however, larger studies across different ethnic groups that take into account genetic linkage and drug interactions that can alter a patient's phenotype are needed before any clinical recommendations based on patient genotype can be formulated. There is little data on the pharmacology and toxicology of MPA in older and paediatric transplant recipients.

Pharmacology and toxicology of mycophenolate in organ transplant recipients: an update.

Although therapeutic drug monitoring (TDM) of immunosuppressive drugs has been an integral part of routine clinical practice in solid organ transplantation for many years, ongoing research in the field of immunosuppressive drug metabolism, pharmacokinetics, pharmacogenetics, pharmacodynamics, and clinical TDM keeps yielding new insights that might have future clinical implications. In this review, the authors will highlight some of these new insights for the calcineurin inhibitors (CNIs) cyclosporine and tacrolimus and the antimetabolite mycophenolic acid (MPA) and will discuss the possible consequences. For CNIs, important relevant lessons for TDM can be learned from the results of 2 recently published large CNI minimization trials. Furthermore, because acute rejection and drug-related adverse events do occur despite routine application of CNI TDM, alternative approaches to better predict the dose-concentration-response relationship in the individual patient are being explored. Monitoring of CNI concentrations in lymphocytes and other tissues, determination of CNI metabolites, and CNI pharmacogenetics and pharmacodynamics are in their infancy but have the potential to become useful additions to conventional CNI TDM. Although MPA is usually administered at a fixed dose, there is a rationale for MPA TDM, and this is substantiated by the increasing knowledge of the many nongenetic and genetic factors contributing to the interindividual and intraindividual variability in MPA pharmacokinetics. However, recent, large, randomized clinical trials investigating the clinical utility of MPA TDM have reported conflicting data. Therefore, alternative pharmacokinetic (ie, MPA free fraction and metabolites) and pharmacodynamic approaches to better predict drug efficacy and toxicity are being explored. Finally, for MPA and tacrolimus, novel formulations have become available. For MPA, the differences in pharmacokinetic behavior between the old and the novel formulation will have implications for TDM, whereas for tacrolimus, this probably will not to be the case.

New insights into the pharmacokinetics and pharmacodynamics of the calcineurin inhibitors and mycophenolic acid: possible consequences for therapeutic drug monitoring in solid organ transplantation

Renal transplant outcomes exhibit large inter-individual variability, possibly on account of genetic variation in immune-response mediators and genes influencing the pharmacodynamics/pharmacokinetics of immunosuppressants. We examined 21 polymorphisms from 10 genes in 237 de novo renal transplant recipients participating in an open-label, multicenter study [Cyclosporine Avoidance Eliminates Serious Adverse Renal-toxicity (CAESAR)] investigating renal function and biopsy-proven acute rejection (BPAR) with different cyclosporine A regimens and mycophenolate mofetil. Genes were selected for their immune response and pharmacodynamic/pharmacokinetic relevance and were tested for association with BPAR. Four polymorphisms were significantly associated with BPAR. The ABCB1 2677T allele tripled the odds of developing BPAR (OR: 3.16, 95% CI [1.50-6.67]; P = 0.003), as did the presence of at least one IMPDH2 3757C allele (OR: 3.39, 95% CI [1.42-8.09]; P = 0.006). BPAR was almost fivefold more likely in patients homozygous for IL-10 -592A (OR: 4.71, 95% CI [1.52-14.55]; P = 0.007) and twice as likely in patients with at least one A allele of TNF-alpha G-308A (OR: 2.18, 95% CI [1.08-4.41]; P = 0.029). There were no statistically significant interactions between polymorphisms, or the different treatment regimens. Variation in genes of immune response and pharmacodynamic/pharmacokinetic relevance may be important in understanding acute rejection after renal transplant. (Less)

Michelle Rashford

Papers

Association of four DNA polymorphisms with acute rejection after kidney transplantation