From Pharmacokinetics to Pharmacogenomics: A New Approach to Tailor Immunosuppressive Therapy

One of the main tasks in the management of organ transplantation is the optimization of immunosuppressive therapy, in order to provide therapeutic efficacy limiting drug-related toxicity. In the past years major efforts have been carried out to define therapeutic windows based on blood/plasma levels of each immunosuppressant relating those concentrations to drug dosing and clinical events. Although this traditional approach is able to identify environmental and nongenetic factors that can influence drug exposure during the course of treatment, it presents limitations. Therefore, complementary strategies are advocated. The advent of the genomic era gives birth to pharmacogenomics, a science that studies how the genome as a whole, including single genes as well as gene-to-gene interactions, may affect the action of a drug. This science is of particular importance for drugs characterized by a narrow therapeutic index, such as the immunosuppressants. Preliminary studies focused on polymorphisms of genes encoding for enzymes actively involved in drug metabolism, drug transport and pharmacological target. Pharmacogenomics holds promise for improvement in the ability to individualize immunosuppressive therapy based on the patient's genetic profile, and can be viewed as a support to traditional therapeutic drug monitoring. However, the clinical applicability of this approach is still to be proven.     

免疫抑制剂的药物基因组学研究进展

患者对药物反应的个体差异,一般是由药物在体内的代谢酶、转运体和靶点编码基因的多态性引起的。因为免疫抑制剂的治疗窗口较窄,患者个体差异导致的不良反应和疗效不佳,都可能引起比较严重的后果,近年人们对于利用药物基因组学进行个体化治疗给予了很高的期待。大量研究已经揭示了多种基因多态性与免疫抑制剂的药代动力学和药效动力学(PK/PD)之间的关联。本文将围绕免疫抑制剂的PK/PD,选择部分具备临床参考意义的研究,简要综述与免疫抑制剂代谢、转运和活化过程相关的药物基因组学进展。     

Clinical Impact of Polymorphisms of Transport Proteins and Enzymes Involved in the Metabolism of Immunosuppressive Drugs

Individualization of immunosuppressive therapy after solid organ transplantation is a goal that has been pursued for a long time. Nevertheless, in clinical practice, we are still stratifying patients in subgroups in which risk is assessed using demographic information and population analysis. Then, a combination of immunosuppressive drugs is chosen and doses are individualized to compensate for intra- and interindividual variabilities in drug pharmacokinetics, to obtain similar plasma/blood concentrations that are believed to be therapeutic, again based on data derived from population analysis. One step further in this strategy is to recognize, before initiation of immunotherapy, those patients at higher risk to be either under- or overexposed to currently used immunosuppressive drugs. Several studies have been undertaken to correlate single nucleotide polymorphisms in genes encoding transport proteins and metabolizing enzymes involved in the disposition of immunosuppressive drugs. Overall, the results from these studies have been mixed. The causes of these sometimes conflicting results include methodologic, genetic, or nongenetic factors. The degree of linkage disequilibrium, the measure of nonrandom associations between polymorphisms at different loci, not necessarily on the same chromosome, is perhaps the main genetic factor. The influence of the environment, physiology (such as kidney and liver functions), disease state, use of multidrug regimens, and inherent drug-to-drug interactions are present nongenetic factors. Moreover, it is also important to increase our knowledge of the genetic factors involved in the variabilities observed in drug responses of pharmacodynamics. True individualized therapy, with the ability to improve health outcomes of each transplant recipient, will depend on our knowledge of the genetic factors involved in immunological response and drug pharmacokinetics and pharmacodynamics.     

Mycophenolic acid levels in pediatric heart transplant recipients receiving mycophenolate mofetil.

BACKGROUND: Mycophenolate mofetil (MMF) is an immunosuppressive agent that has shown promise in adult patients who have undergone heart transplantation. There have been a number of studies of the pharmacokinetics of MMF in adult solid organ transplant recipients, but there is very little information in the pediatric population. The purpose of this study was to review our experience with MMF dosing and the role of mycophenolic acid (MPA) levels for therapeutic drug monitoring in a population of pediatric heart transplant recipients. METHODS: Data were obtained by review of the pediatric heart transplant database between November 1, 1997 and October 15, 1998. The data included all serum trough MPA levels, patient age, weight, height, indication for and dose of MMF, other medications, and details of all episodes of graft rejection. RESULTS: Forty-four patients (27 males) had a total of 128 serum trough MPA levels. Median age at transplant was 2.7 years (7 days to 18.4 years), and at time of review was 6.3 years (29 days to 23.5 years). MMF treatment was used for induction in 18 patients, induction and rejection in 23 patients and graft vasculopathy in 3 patients. Dosing by body surface area (mg/m(2)), age and interval from transplantation were all independently associated with MPA level. There was a trend toward requiring higher doses to achieve desired levels (>3 ng/ml) in younger patients. The average dose to achieve desired levels was higher in the immediate post-transplant period. There was a trend that MPA levels for a given dose were higher in patients on concurrent tacrolimus therapy. CONCLUSIONS: (1) There is marked individual variation in pharmacokinetics of MMF in pediatric patients; (2) dosing by body surface area may be advantageous; (3) higher MMF doses may be required at younger ages and in the early period after transplantation; (4) lower MMF doses may be required with concurrent tacrolimus therapy; and (5) serum trough MPA levels may relate to efficacy. Therefore, therapeutic drug monitoring of serum trough MPA levels may be required for individualized MMF dosing in pediatric cases.     

How pharmacokinetic and pharmacodynamic drug monitoring can improve outcome in solid organ transplant recipients

Within the field of solid organ transplantation there is an unprecedented interest in therapeutic drug monitoring of immunosuppressive drugs. Ideally therapeutic drug monitoring should cost-effectively lead to improved efficacy of the drug and to a reduction in side effects. Therapeutic drug monitoring will be most effective if there is a large interpatient variability and a small intrapatient variability. Therapeutic drug monitoring in transplantation is largely based on correlations between drug concentrations and toxicity or between drug concentrations and efficacy. Pharmacodynamic monitoring of immunosuppressive drugs has not reached the stage of widespread clinical application. In part this is caused by the fact that most of the pharmacodynamic assays are time-consuming, costly and in some cases only give a result after several days of incubation. Another reason for the limited interest in pharmacodynamic monitoring is the lack of data showing improved outcome if dose adjustment is based on pharmacodynamics rather than pharmacokinetics. On the other hand, such data are also lacking for pharmacokinetic monitoring. Prospective investigations on the contribution of therapeutic drug monitoring may result in further improvement of the safety and efficacy of our immunosuppressive regimens and more refined methods for therapeutic drug monitoring. There is no contest between pharmacokinetic and pharmacodynamic monitoring. Most likely the results of both ways of monitoring will be complementary.     

Pharmacogenetics of tacrolimus and sirolimus in renal transplant patients: from retrospective analyses to prospective studies

The promises of pharmacogenetics are to elucidate the inherited basis of differences between individual responses to drugs in order to identify the right drug and dose for each patient. The recent identification of genetic polymorphisms in drug-metabolizing enzymes and drug transporters led to the hypothesis that genetic factors may be implicated in the interindividual variability of the pharmacokinetic or pharmacodynamic characteristics of immunosuppressive drugs, major side effects, and efficacy. The purpose of this study was to provide a short overview of recent results obtained in the field of pharmacogenetics of tacrolimus and sirolimus, both substrates of the cytochrome P450 3A (CYP3A) enzymes and of the efflux pump P-glycoprotein, the product of the Multidrug Resistance-1 (MDR1) genes. A number of retrospective studies that demonstrated a link between the polymorphisms governing the CYP3A5 protein expression, with more conflicting results with the MDR1 gene polymorphisms, related to the daily dose necessary to achieve adequate blood tacrolimus levels. The CYP3A5 polymorphisms have also been associated with sirolimus pharmacokinetics. One challenge is to investigate the combined effect of a number of different polymorphisms in various genes to define genetic backgrounds with different pharmacokinetic profiles using high throughput technologies. Another challenge is to move toward prospective randomized studies to explore whether a pharmacogenetic approach, taking into account a limited number of polymorphisms prior to drug treatment, could be used on an individual basis to guide initial dosing of a given drug. The last challenge is based on "target" pharmacogenetics to investigate the role of the polymorphisms of other genes implicated in the efficacy and/or safety of the drug.     

Use of mycophenolate mofetil in liver transplantation: a literature review

Mycophenolate mofetil (MMF) is an immunosuppressive drug, exhibiting its effect through inhibition of proliferation of T and B lymphocytes. Standard primary immunosuppressive therapy after orthotopic liver transplantation (OLT) is based on a calcineurin-inhibitor (CNI): cyclosporine or tacrolimus. Renal failure with arterial hypertension, due to CNI side-effects, is a major cause of morbidity and mortality after OLT. Several studies have shown the efficacy of MMF to improve CNI-induced nephrotoxicity, blood pressure, and uric acid concentration in liver transplant patients with concomitant reduction or withdrawal of CNI. Predose plasma mycophenolic acid concentrations (MPA) are related to adverse events, drug dose, and clinical status. Blood level values outside the suggested MPA therapeutic range are associated with acute rejection episodes and side effects, which have been described in about half of the patients treated with MMF. Most authors have described gastrointestinal and hematological side-effects, whereas these appear usually dose related, responding quickly to reduction. MMF is potent and safe immunosuppressive agent, and replacement of CNI by MMF in liver transplant patients with renal dysfunction may improve not only kidney function but also other CNI-associated side-effects, such as hypertension and hyperuricemia, with a low risk of rejection.     

Immunosuppressive drug monitoring – what to use in clinical practice today to improve renal graft outcome

Therapeutic drug monitoring (TDM) of immunosuppressive therapy is becoming an increasingly complex matter as the number of compounds and their respective combinations are continuously expanding. Unfortunately, in clinical practice, monitoring predose trough blood concentrations is often not sufficient for guiding optimal long-term dosing of these drugs. The excellent short-term results obtained nowadays in renal transplantation confer a misleading feeling of safety despite the fact that long-term results have not substantially improved, definitely not to a point where longer graft survival could counteract the increasing need for transplant organs and less toxicity and side-effects could ameliorate patient survival. It is therefore a challenging task to try to tailor immunosuppressive drug therapy to the individual patient profile and this in a time-dependent manner. For the majority of currently used immunosuppressive drugs, measurement of total drug exposure by determination of the dose-interval area under the concentration curve (AUC) seems to provide more useful information for clinicians in terms of concentration-exposure and exposure-response as well as reproducibility. To simplify this laborious way of measuring drug exposure, several validated abbreviated AUC profiles, accurately predicting the dose-interval AUC, have been put forward. Together with an increasing knowledge of the time-related pharmacokinetic behaviour of immunosuppressive drug and their metabolites, studies are focusing on how to apply abbreviated AUC sampling methods in clinical transplantation, taking into account the numerous factors affecting drug pharmacokinetics. Eventually, TDM using abbreviated AUC profiles has to be prospectively tested against classic methods of drug monitoring in terms of cost-effectiveness, feasibility and clinical relevance with the ultimate goal of improving patient and graft survival.     

A pharmacogenetic strategy for immunosuppression based on the CYP3A5 genotype

Pharmacogenetics has the potential to complement therapeutic drug monitoring in achieving target blood concentrations of the immunosuppressive drugs during the critical early period after transplantation when adequate drug exposure is essential to prevent rejection. The most attractive candidate for a pharmacogenetic strategy is tacrolimus dosing based on the CYP3A5 genotype.     

Immunosuppression in nonhuman primates

Nonhuman primate organ transplant models have been and continue to be an excellent conduit for the development of immunosuppressive therapies for use in human organ transplantation. These experimental therapies often make use of immunosuppressive drugs currently used in clinical transplantation and combine them in new and exciting ways with cutting-edge reagents and strategies. This review summarizes our own research experience and the experience of other leaders in the field using a variety of immunosuppressants in nonhuman primate transplant models. Information on drug dosing and safety is included.     

Prospects for personalized immunosuppression: pharmacologic tools--a review

In the last few years, novel immunosuppressive agents and new formulations, including sirolimus, mycophenolic acid (the active metabolite of mycophenolate mofetil), tacrolimus, and microemulsion cyclosporine, have significantly improved the clinical outcome of transplant recipients. However, the majority of immunosuppressive agents need a constant monitoring of drug levels to reduce the risk of graft rejection as well as drug-induced toxicities. Many factors may affect the pharmacokinetic characteristics of immunosuppressive agents, potentially reducing treatment effectiveness. Absorption and metabolism of immunosuppressive drugs are influenced by patient genotype and comedications, while comorbidities (ie, diabetes and cystic fibrosis) are responsible for altered pharmacokinetics. Dose individualization in transplant recipients is performed according to their health status, graft function, and drug therapeutic range. With respect to the last issue, therapeutic drug monitoring (TDM) plays a crucial role in achieving optimal immunosuppression, improving the efficacy of drugs, and lowering toxic effects. Pharmacokinetic analysis allowed the identification of specific parameters, such as plasma or blood levels, immediately before dosing (C(min) or trough levels) or 2 hours after administration (C(2)), which are significantly related to tissue exposure to the drug. More recently, studies have investigated treatment individualization by evaluating drug pharmacogenetics based on the expression level or mutations of their molecular targets, including calcineurin for cyclosporine and tacrolimus, and inosine monophosphate dehydrogenase for mycophenolic acid. Although no conclusive data may be drawn from these preliminary trials, further studies are underway to address the role of pharmacogenetics in clinical decision making.     

Chronopharmacology for deoxyspergualin: toxicity and efficacy in the rat.

BACKGROUND: Deoxyspergualin (DSG), a potent immunosuppressive drug, has been clinically used as a rescue therapy for acute graft rejection. However, DSG has severe adverse effects that limits its use. Here, we examined the optimized therapeutic protocol for DSG using chronopharmacological profiles of it. METHODS: First, a dosing time-dependent difference in the toxicity and efficacy of DSG was determined using a heterotopic heart transplantation model. Second, chronokinetics of DSG were examined to reveal the mechanism for dosing time-dependent differences in the effect. RESULTS: In rats treated with DSG at the active period, bone marrow suppression and damage of small intestine were significantly severe. However, no significant difference was found in cardiac allograft survival and pharmacokinetics according to the timing of drug administration. CONCLUSIONS: The toxicity of DSG varied with the dosing time, whereas its efficacy did not. The chronopharmacological approach may provide merits for immunosuppressive therapy with DSG in clinical organ transplantation.     

Pharmacogenetics in solid organ transplantation: current status and future directions

Tailoring of immunosuppressive drug therapy to the specific requirements of the individual patient to optimize efficacy and minimize toxicity remains one of the biggest challenges in solid organ transplantation. Pharmacogenetic and pharmacogenomic research, studying the effects of genetic polymorphisms on drug disposition and action, holds promise to produce useful clinical tools for individualizing immunosuppressive therapy. In the past years, many interesting studies have been reported, assessing the impact of single nucleotide polymorphisms of genes encoding drug metabolizing enzymes, drug transporters and—to lesser extent—pharmacological target molecules, on pharmacokinetics and pharmacodynamics of immunosuppressive drugs like tacrolimus, cyclosporine, sirolimus, mycophenolic acid, and corticosteroids. Currently, we still are in the early phases of this exciting research, and the question whether pharmacogenetic profiling will eventually become a useful clinical tool remains to be answered.     

Reflex sympathetic dystrophy syndrome in renal transplanted patients under immunosuppression with tacrolimus

Reflex sympathetic dystrophy syndrome (RSDS), which probably has a multifactorial etiology, may appear after kidney transplantation. Its clinical manifestations include severe periarticular pain with inflammatory signs, especially in knees and ankles, causing functional disability. Symptoms develop during the first 3 months after transplantation and usually disappear 3 to 6 months later without sequelae. In renal transplant recipients it has previously been related to immunosuppressive treatment with cyclosporine. Therefore we had suggested that introducing tacrolimus could be a therapeutic option. We now present four cases of RSDS in kidney transplant recipients treated with tacrolimus. All but one patient were receiving tacrolimus monotherapy, excluding other drugs that might have been involved to cause the syndrome. It is also interesting that one of our cases develop RSDS long after transplantation when immunosuppressive treatment was changed. Symptoms correlated with an increase in alkaline phosphatase and with bone scintigraphy findings. All patients recovered without sequels 3 to 6 months afterward. In conclusion, RSDS is a relevant osteoarticular complication in patients receiving either anticalcineurinic drug (CyA or tacrolimus), even under monotherapy or with a low steroid dose.     

Ganciclovir Dosing Strategies and Development of Cytomegalovirus Resistance in a Kidney Transplant Recipient: A Case Report

In renal transplant recipients, delayed graft function and accompanying renal impairment may lead to therapeutic underexposure of valganciclovir. We describe a case of a cytomegalovirus (CMV)-seronegative kidney transplant recipient from a CMV-seropositive donor, whose course was complicated during valganciclovir prophylaxis by CMV disease, ultimately progressing to ganciclovir, foscarnet, and cidofovir resistance. Assessments and adjustments for renal dysfunction, according to both Cockgroft-Gault and Modification of Diet in Renal Disease study equations, are described. Therapy was complicated by outpatient parenteral therapy with pump-administered antiviral therapy, which may have led to drug underexposure and the fostering of antiviral resistance. Suppression was ultimately achieved in conjunction with reduction in immunosuppressive therapy, CMV immunoglobulin, and initiation of leflunomide. At-risk recipients may benefit from 24 hour creatinine clearance assessments, direct creatinine clearance measurement, or therapeutic drug monitoring. Optimal dosing strategies in recipients with impaired kidney function remain undefined, with limited pharmacokinetic data to date.     

Minimization of immunosuppression: transplant immunology

Induction of tolerance, which obviates the need for maintenance immunosuppression following organ transplantation remains elusive. In cardiac transplantation, ongoing immunosuppressive therapy is essential to ensure long-term graft survival. Although drug regimens have substantially improved in recent years, their adverse effects continue to cause significant morbidity and affect quality of life. Newer immunosuppressive therapies have been effective at reducing allograft rejection rates in the short term but long term outcomes have changed little in the last two decades. Minimization of immunosuppression requires appreciation of the potential consequence. High risk patients in particular need to be identified and excluded from low intensity immunosuppressive regimens. A variety options exist for lowering of immunosuppression and steroid weaning has now become common practice with about 40% of all cardiac transplant recipients remaining steroid free in the long term. Minimization of calcineurin inhibitor exposure may be achieved with concurrent use of the more potent anti-proliferative agents mycophenolate mofetil and sirolimus. Patients require close monitoring for rejection during weaning. In addition to the conventional clinical parameters which include therapeutic drug monitoring, endomyocardial biopsy and echocardiography, newer techniques for monitoring hold future promise. These include detection of circulating alloantibodies and quantitative measurement of the net state of immunosuppression (Cylex). However, the efficacy of these modalities requires further investigation.     

Cyclosporine: progress, problems, and perspectives

Since it much heralded debut, cyclosporine appears to have gained a major role in organ transplantation. The drug appears to have gained widespread acceptance as the immunosuppressive treatment of choice for high-risk cadaveric renal transplant recipients as well as for extrarenal organ graft recipients who depend upon a successful allograft for survival. Most centers have utilized either HPLC or RIA assays for cyclosporine levels in blood or serum to maximize immunosuppressive therapy with minimal toxicity during the critical period of induction therapy. In most patients, the balance between therapeutic and toxic effects of cyclosporine favors the immunosuppressive efficacy of the drug. Long-term consequences of cyclosporine therapy and innovative future treatment protocols will define the size of the role cyclosporine will play in the future as well as the interaction with uncast groups, such as highly presensitized patients.     

Molecular‐targeted therapy in advanced renal cell carcinoma based on pharmacokinetics,pharmacodynamics and pharmacogenetics: A proposed strategy

In advanced renal cell carcinoma, a transition of drug therapies from cytokines to molecular‐targeted drugs and immune‐oncology drugs provides more clinical benefits to patients, while adequate management is required for various and sometimes serious adverse events. At present, the relationship between the pharmacokinetics of many drugs and their effectiveness or adverse events has been elucidated, and therapeutic drug monitoring is being applied to some immunosuppressive, anti‐epileptic and antibacterial drugs in daily clinical practice. Most of the molecular‐targeted drugs used in patients with renal cell carcinoma are orally active, and are affected by absorption and disposition, which can be different for each individual. The monitoring of the circulating drug concentration could be beneficial to patients by providing information for the adjustment of drug dose and the maintenance of a therapeutic plasma concentration range. Genetic polymorphisms are known to be involved in pharmacokinetics, and cause individual differences in clinical efficacy and adverse events. Therefore, a more scientific strategy should be used in regard to the treatment of patients with advanced renal cell carcinoma treated with molecular‐targeted drugs by accumulating evidence on pharmacodynamics and pharmacogenetics.     

Introduction to genetic processes in transplantation

Recent investigations have explored how analyses of genetic DNA from organ donors and recipients may be helpful during organ allocation so as to reduce graft rejection or improve dosing of immunosuppressive medications. This discussion reviews those data and the processes by which genetic coding controls the production of protein that is important in cellular structure and function during donor and recipient care. Changes in gene structure (polymorphisms or mutations) may occur spontaneously or as a result of cell interactions with environmental factors, and may be associated with recognized diseases or, potentially, could reduce or worsen graft rejection.     

Gene polymorphisms and prostate cancer: the evidence

OBJECTIVE

Prostate cancer is still the most frequent noncutaneous male malignancy and is the second most common cause of cancer death. Genetic factors have been extensively studied in different countries. In addition, numerous genome–wide association studies have been performed in developed countries. Genetic tests will be applied in the near future for diagnosis, therapeutic, and prognostic significance. Therefore, we reviewed the association of several important pathways and genes with critical functions in prostate cancer development or progression.

MATERIALS AND METHODS

We performed a PubMed® search using several key words such as prostate cancer, names of important genes with critical function, and polymorphisms. Then, we reviewed retrieved articles as well as relevant articles from 1997 to 2009.

RESULTS

There are conflicting results of studies on some gene polymorphisms in association with prostate cancer. Most of the inconsistent results have been reported in studies investigating the vitamin D receptor gene polymorphism in association with prostate cancer. Genes related to angiogenesis and cell adhesion genes are more promising. Following results of future studies, the use of antibodies blocking over‐expressed genes or proteins may be supported in patients with prostate cancer.

CONCLUSIONS

The difference between the results of studies on gene polymorphisms in prostate cancer may be explained partly by ethnic differences, limited sample size, and other risk or protective factors modifying these effects. Genome‐wide studies are currently performed in developed countries and extensive use of this type of analysis may merit consideration in other countries. Furthermore, future studies are needed to further investigate environmental and diet factors interactions with genetic factors.