Phenytoin decreases the blood concentrations of sirolimus in a liver transplant recipient: a case report

Drug Interactions and Safe Prescription Writing for Liver Transplant Recipients

Immunosuppression optimization is central to graft function in liver transplant recipients. Post-transplantation patients develop new onset or worsening metabolic syndrome, are prone to atypical infections, and are at higher risk of developing cardiac and brain-related clinical events. In this context, liver transplant recipients are at risk of using multiple comedications alongside immunosuppressants. It is imperative for the transplant physician to understand the various drug-drug interactions that potentially reduce or promote toxicity of immunosuppression, as well as associated synergistic or antagonistic effects on extrahepatic organ systems. This comprehensive review discusses drug-drug interactions in liver transplant recipients and the impact and role of complementary and alternative medicines among individuals on immunosuppression.

Alternatives to rifampicin: A review and perspectives on the choice of strong CYP3A inducers for clinical drug-drug interaction studies

N-Nitrosamine (NA) impurities are considered genotoxic and have gained attention due to the recall of several marketed drug products associated with higher-than-permitted limits of these impurities. Rifampicin is an index inducer of multiple cytochrome P450s (CYPs) including CYP2B6, 2C8, 2C9, 2C19, and 3A4/5 and an inhibitor of OATP1B transporters (single dose). Hence, rifampicin is used extensively in clinical studies to assess drug-drug interactions (DDIs). Despite NA impurities being reported in rifampicin and rifapentine above the acceptable limits, these critical anti-infective drugs are available for therapeutic use considering their benefit-risk profile. Reports of NA impurities in rifampicin products have created uncertainty around using rifampicin in clinical DDI studies, especially in healthy volunteers. Hence, a systematic investigation through a literature search was performed to determine possible alternative index inducer(s) to rifampicin. The available strong CYP3A inducers were selected from the University of Washington DDI Database and their in vivo DDI potential assessed using the data from clinical DDI studies with sensitive CYP3A substrates. To propose potential alternative CYP3A inducers, factors including lack of genotoxic potential, adequate safety, feasibility of multiple dose administration to healthy volunteers, and robust in vivo evidence of induction of CYP3A were considered. Based on the qualifying criteria, carbamazepine, phenytoin, and lumacaftor were identified to be the most promising alternatives to rifampicin for conducting CYP3A induction DDI studies. Strengths and limitations of the proposed alternative CYP3A inducers, the magnitude of in vivo CYP3A induction, appropriate study designs for each alternative inducer, and future perspectives are presented in this paper.

Pharmacokinetic principles of dose adjustment of mTOR inhibitors in solid organ transplanted patients

WHAT IS KNOWN AND OBJECTIVES

mTOR inhibitors possess narrow therapeutic range and substantial pharmacokinetic variability and the consequences from suboptimal dosing are serious. The aim of this review is to summarize the current knowledge about the factors influencing mTOR inhibitors pharmacokinetics and the possibility of using these relationships in order to improve its therapy individualization in solid organ transplanted patients.

METHODS

Literature search from Pubmed and Web of Science databases were performed using Boolean search operators in order to identify relevant studies.

RESULTS AND DISCUSSION

A total of 701 reports were identified from the initial literature search. Out of which 40 studies dealt with relationships between various factors and pharmacokinetics of mTOR inhibitors and with relevance of these associations for dosage optimization.

WHAT IS NEW AND CONCLUSION

The overview of the current covariates for pharmacokinetic variability of mTOR inhibitors has been provided on the level of absorption, distribution and elimination, and consequences of these relationships for dosing optimization has been summarized.

Large-scale identification and analysis of suppressive drug interactions

One drug may suppress the effects of another. Although knowledge of drug suppression is vital to avoid efficacy-reducing drug interactions or discover countermeasures for chemical toxins, drug-drug suppression relationships have not been systematically mapped. Here, we analyze the growth response of Saccharomyces cerevisiae to anti-fungal compound ("drug") pairs. Among 440 ordered drug pairs, we identified 94 suppressive drug interactions. Using only pairs not selected on the basis of their suppression behavior, we provide an estimate of the prevalence of suppressive interactions between anti-fungal compounds as 17%. Analysis of the drug suppression network suggested that Bromopyruvate is a frequently suppressive drug and Staurosporine is a frequently suppressed drug. We investigated potential explanations for suppressive drug interactions, including chemogenomic analysis, coaggregation, and pH effects, allowing us to explain the interaction tendencies of Bromopyruvate.

Drug interactions in transplant patients: what everyone should know

PURPOSE OF REVIEW

Adverse events due to drug-drug interactions remain a challenge in the postsurgical care of transplant recipients. A combination of potent and selective immunosuppressive drugs, which have a narrow therapeutic index, with medications for the treatment of comorbidities such as dyslipidemia, infection, psychiatric conditions, and hypertension, can lead to life-threatening drug-drug interactions.

RECENT FINDINGS

There are a number of important drug-drug interactions which are important for physicians to consider. It is critical to understand the pharmacodynamics and pharmacokinetics of drug-drug interactions, their potential impact on patient care, and the management strategies.

SUMMARY

Close therapeutic drug monitoring and evaluation of drug-specific side effects continue to be an important key to minimize adverse events due to drug-drug interactions.

Immunotherapy in elderly transplant recipients: a guide to clinically significant drug interactions

Currently, >50% of candidates for solid organ transplantation in Europe and the US are aged >50 years while approximately 15% of potential recipients are aged >or=65 years. Elderly transplant candidates are characterized by specific co-morbidity profiles that compromise graft and patient outcome after transplantation. The presence of coronary artery or peripheral vascular disease, cerebrovascular disease, history of malignancy, chronic obstructive lung disease or diabetes mellitus further increases the early post-transplant mortality risk in elderly recipients, with infections and cardiovascular complications as the leading causes of death. Not only are elderly patients more prone to developing drug-related adverse effects, but they are also more susceptible to pharmacokinetic and pharmacodynamic drug interactions because of polypharmacy. The majority of currently used immunosuppressant drugs in organ transplantation are metabolized by cytochrome P450 (CYP) or uridine diphosphate-glucuronosyltransferases and are substrates of the multidrug resistance (MDR)-1 transporter P-glycoprotein, the MDR-associated protein 2 or the canalicular multispecific organic anion transporter, which predisposes these immunosuppressant compounds to specific interactions with commonly prescribed drugs. In addition, important drug interactions between immunosuppressant drugs have been identified and require attention when choosing an appropriate immunosuppressant drug regimen for the frail elderly organ recipient. An age-related 34% decrease in total body clearance of the calcineurin inhibitor ciclosporin was observed in elderly renal recipients (aged >65 years) compared with younger patients, while older recipients also had 44% higher intracellular lymphocyte ciclosporin concentrations. Similarly, using a Bayesian approach, an inverse relationship was noted between sirolimus clearance and age in stable kidney recipients. Ciclosporin and tacrolimus have distinct pharmacokinetics, but both are metabolized by intestinal and hepatic CYP3A4/3A5 and transported across the cell membrane by P-glycoprotein. The most common drug interactions with ciclosporin are therefore also observed with tacrolimus, but the two drugs do not interact identically when administered with CYP3A inhibitors or inducers. The strongest effects on calcineurin-inhibitor disposition are observed with azole antifungals, macrolide antibacterials, rifampicin, calcium channel antagonists, grapefruit juice, St John's wort and protease inhibitors. Drug interactions with mycophenolic acids occur mainly through inhibition of their enterohepatic recirculation, either by interference with the intestinal flora (antibacterials) or by limiting drug absorption (resins and binders). Rifampicin causes a reduction in mycophenolic acid exposure probably through induction of uridine diphosphate-glucuronosyltransferases. Proliferation signal inhibitors (PSIs) such as sirolimus and everolimus are substrates of CYP3A4 and P-glycoprotein and have a macrolide structure very similar to tacrolimus, which explains why common drug interactions with PSIs are comparable to those with calcineurin inhibitors. Ciclosporin, in contrast to tacrolimus, inhibits the enterohepatic recirculation of mycophenolic acids, resulting in significantly lower concentrations and hence risk of underexposure. Therefore, when switching from tacrolimus to ciclosporin and vice versa or when reducing or withdrawing ciclosporin, this interaction needs to be taken into account. The combination of ciclosporin with PSIs requires dose reductions of both drugs because of a synergistic interaction that causes nephrotoxicity when left uncorrected. Conversely, when switching between calcineurin inhibitors, intensified monitoring of PSI concentrations is mandatory. Increasing age is associated with structural and functional changes in body compartments and tissues that alter absorptive capacity, volume of distribution, hepatic metabolic function and renal function and ultimately drug disposition. While these age-related changes are well-known, few specific effects of the latter on immunosuppressant drug metabolism have been reported. Therefore, more clinical data from elderly organ recipients are urgently required.

A comprehensive review of immunosuppression used for liver transplantation

Since liver transplantation was approved for the treatment of end stage liver disease, calcineurin inhibitors (CNI's) have played a critical role in the preservation of allograft function. Unfortunately, these medications cause a variety of Side effects such as diabetes, hypertension and nephrotoxicity which in turn result in significant morbidity and reduced quality of life. A variety of newer immunosuppressants have been evaluated over the last decade in an attempt to either substitute for CNI's or use with reduced dose CNI's while still preserving allograft function However, current data does not recommend complete cessation of CNI's due to unacceptably high rates of allograft rejection. As these medications have their own unique adverse effects, a careful assessment on their risks and benefits is essential, particularly when additive or synergistic effects with CNI's may occur. Furthermore, the impact of these newer medications on the risk of hepatitis C recurrence and progression remains to be elucidated. Controlled trials are urgently required to assist transplant physicians with choosing the optimum immunosuppressive regimen for their patients. This review will discuss commonly used immunosuppressants prescribed in liver transplantation, emerging therapties and where appropriate, the impact of these medications on the recurrence of hepatitis C after liver transplantation.

Altered Cytochrome P450 Metabolism of Calcineurin Inhibitors: Case Report and Review of the Literature

A 19-year-old woman was admitted to receive a kidney transplant from a nonliving donor. At the time of transplantation, she was taking oral phenytoin 300 mg every morning, 100 mg at noon, and 300 mg every evening (total of 700 mg/day) to treat seizures secondary to hemodialysis. Immediately after the transplantation, phenytoin treatment was resumed, and immunosuppressive therapy consisting of antithymocyte globulin, cyclosporine, mycophenolate mofetil, and corticosteroids was started. Her cyclosporine blood levels varied over the first 10 days after transplantation. Cyclosporine was discontinued, and tacrolimus was begun after acute rejection was discovered. The rejection was treated with antithymocyte globulin, plasmapheresis, and intravenous immunoglobulin, and subsequently resolved; however, the patient's blood concentrations of tacrolimus varied widely. Phenytoin is an antiepileptic drug that induces hepatic enzymes, affecting the cytochrome P450 3A family. These enzymes metabolize approximately 50% of all prescribed drugs, including cyclosporine and tacrolimus. According to the Naranjo adverse drug reaction probability scale, this patient's adverse drug reaction probably occurred from altered metabolism of cyclosporine and tacrolimus due to phenytoin therapy. Clinicians must identify drug interactions between metabolic enzyme inducers or inhibitors and drug substrates with narrow therapeutic ranges, closely monitor drug concentrations, and observe patients for clinical signs and symptoms of therapeutic failure or toxicity. In daily practice, clinicians should explore the metabolic characteristics of drugs and their biotransformation pathways to identify patients who require alternative therapy.

Immunosuppressive drug monitoring of sirolimus and cyclosporine in pediatric patients

Sirolimus is primarily used as a rescue agent in pediatric transplant recipients, particularly in cases of cyclosporine or tacrolimus toxicity. Preliminary data indicate a higher apparent oral clearance in younger children (4-10 years of age). Various drug interactions have been described between sirolimus and drugs that are substrates/inhibitors or inducers of CYP3A and the P-glycoprotein transporter. Close monitoring of trough sirolimus blood levels is therefore recommended for pediatric transplant recipients. In de novo adult kidney transplant recipients on triple therapy with cyclosporine, corticosteroids and sirolimus, a therapeutic window of 4-12 microg/l is recommended for sirolimus trough concentrations determined by HPLC or LC/MS-MS. In maintenance adult patients after conversion to a calcineurin inhibitor-free regimen, sirolimus trough concentrations of 5-10 microg/l are proposed in combination with mycophenolate mofetil. These therapeutic ranges may also serve as a guide for pediatric renal transplant recipients. The concept of C2 monitoring still needs to be critically evaluated in pediatric patients. The crucial importance of achieving an adequate cyclosporine exposure early after transplantation has been demonstrated for adult transplant recipients. A cyclosporine concentration taken 2 h after dosing is a good surrogate marker of the AUC0-4h in adults. Various clinical studies have shown that in pediatric patients, the C2 concentration shows a substantially better correlation with cyclosporine exposure compared to the trough level (C0). In an outcome study with pediatric renal transplant recipients, it could be demonstrated that the AUC(0-4h) was a predictor of acute rejection in the first 3 weeks after transplantation, whereas C2 levels showed no significant association. Abbreviated AUC strategies may be preferable for optimization of CsA exposure in pediatric patients.

Clinical relevance of sirolimus drug interactions in transplant patients

Sirolimus, a new immunosuppressant drug; is metabolized by cytochrome P450 3A4 (CYP3A4) and is a substrate of the P-glycoprotein drug efflux pump. The CYP3A4/P-glycoprotein system is mainly localized in the liver and intestine. It is responsible for the severe first pass metabolism of sirolimus with a low bioavailability. Drugs like voriconazole, itraconazole, fluconazole, and erytrhomycin may decrease the metabolic activity of this enzymatic system. This report documents in five patients that coadministration of these antimicrobials with sirolimus increases the blood concentrations of the immunosuppressant. The dose-normalized trough blood concentration showed a mean increase of sevenfold with the coadministration of these drugs. It is essential to monitor the blood sirolimus concentrations and to adjust the sirolimus doses before and after coadministration of these drugs.

Therapeutic drug monitoring of sirolimus: effect of concomitant immunosuppressive therapy and optimization of drug dosing

Sirolimus (SRL) is a new immunosuppressant which shares a common metabolic pathway with several other immunosuppressive agents. This leads to potential pharmacokinetic interactions that might affect SRL blood levels with relevant clinical consequences. As a validated laboratory, 2658 SRL trough samples (corresponding to 495 kidney transplant recipients treated with different immunosuppressive regimens) from more than 40 Italian Transplant Units were analyzed. We found that dose-normalized SRL trough levels were significantly higher in patients treated with cyclosporine (CsA) and SRL (4.15 +/- 2.23 ng/mL/mg SRL), compared with patients treated with mycophenolate mofetil (MMF) and SRL (3.26 +/- 1.86 ng/mL/mg SRL; p < 0.01) or with MMF, steroids and SRL (2.52 +/- 1.73 ng/mL/mg SRL; p < 0.01). Mean intra- and interpatient variabilities were 19% and 47%, respectively. Both parameters are significantly affected by the time postsurgery, with the first week post transplantation being associated with the greatest variability. As additional analysis, a simple dose-adjustment formula has been proposed as a useful tool to guide SRL dose changes. The proposed equation has been able to predict SRL concentration after a dose change in 73% of the tested samples. These findings suggest that different immunosuppressants significantly interfere with SRL bioavailability. Strategies aimed at reducing variability in SRL exposure may have a positive clinical impact.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.