Determinants of Immunosuppressive Dose Requirements in Children After Solid Organ Transplantation

Overview

The long-term success of solid organ transplantation is largely dependent on the efficacy of immunosuppressive medication. Unfortunately, for the most important agents the correct drug levels are difficult to attain, with potential severe consequences of drug under- or overexposure. In addition there is a large variation in dose requirements within and between different subjects. Clinical studies have demonstrated that a better control of drug exposure can improve outcome. A large set of patient characteristics appear important in determining dose requirements in adults, in particular genetic variation in genes involved in drug metabolism. In children relative dose requirements are increased compared to adults, but is not known why and the role of pharmacogenetic variation has not been described. Our study aims to describe relative dose requirements in children after solid organ transplantation with the help of clinical and laboratory data obtained during regular hospital visits (retrospective). In addition we will assess their genotype for genes involved in the metabolism of immunosuppressives.

The success of solid organ transplantation is for an important part determined by the potency of the immunosuppressive regime to prevent allograft rejection. Depending on the type of organ transplanted current 5-year graft survival for liver is circa 70% and for kidney almost 80% (1-3). Although acute rejection has become less a problem, analyses of data on the follow-up of solid organ recipients has demonstrated that long-term allograft survival has not improved much in the last decade. The life expectancy of a donor organ appears limited due to phenomena such as chronic, antibody mediated, rejection and the toxic effects of immunosuppressive medication. Unfortunately, the most commonly applied immunosuppressive agents (in particular the calcineurin-inhibitors) possess a narrow therapeutic index with potential severe consequences of drug under- or overexposure. Furthermore, in clinical practice optimal dosing is difficult to achieve due to important inter- and intraindividual variation in drug pharmacokinetics. Consequently attention has shifted towards designing a "tailored immunosuppressive maintenance therapy", aimed at optimization of drug efficacy and the prevention of these unintended and possible deleterious side effects in the individual recipient. For a "perfect fit" this entails a fundamental knowledge of the individual characteristics of the recipient and its graft in relation to the pharmacodynamic and -kinetic properties of the agent. A complex and often interdependent set of factors appears relevant in determining drug exposure. These include recipient characteristics such as age, race, body composition, organ function (intestine-liver-kidney), food and concomitant medication intake, but also graft-related characteristics such as: seize, donor-age, and time after transplantation. Longitudinal and cross-sectional analysis of clinical and genetic co-variables in allograft recipient cohorts has identified several single nucleotide polymorphisms (SNPs) in genes encoding for enzymes and transporters involved in drug metabolism(CYP3A4, CYP3A5, MDR1), which were associated with differences in dosing and toxicity (4-10). In particular carriers of a common polymorphism in CYP3A5 (designated CYP3A5\*1) demonstrate a 25-40% increase in tacrolimus clearance and a 2-3 fold increase in dose requirements. In children the dose requirements per kg bodyweight to attain desired blood concentrations are approximately 2.7 fold higher for patients under 5 years and 1.9 fold higher for patients aged 5 to 12 years when compared to older patients11-14. Age-related differences in expression of enzymes or transporters involved in drug metabolism might play an important role. Fakhoury et al demonstrated an increase in intestinal CYP3A4/5 mRNA and protein expression from childhood to adulthood (15). In addition other factors such as intestinal transit time and length, body composition, protein binding, body metabolism and organ function, might be important in determining dose requirements in children (16-18). The importance of the aforementioned genetic polymorphisms on long-term dose requirements have not been studied in children. Objective: To describe dose requirements of immunosuppressive medication in relation to a set of clinical and non-clinical (genetic) factors in a cohort of pediatric recipients of a solid organ allograft. Methods: A retrospective study in a single center (University Hospitals Leuven) with an active program for pediatric kidney, liver and intestinal transplantation. The study population consists of subjects who received a solid organ transplant (liver, kidney, intestine, combined) between 0 and 18 years of age. As a part of their standard care patients are admitted to the department for a yearly check-up after transplantation. During this admission a set of investigations are performed to evaluate the current medical situation, including an elaborate pharmacokinetic (PK) assessment. Patient characteristics and clinical data (age, bodyweight, sex, type of graft, cause of organ failure, co-medication, laboratory results, PK data) will be retrieved from the patient files concerning these follow-up admissions. In addition patients and/or their parents will be requested permission to collect one extra tube of whole blood (ca 8 ml) for DNA analysis during a standard blood collection at a regular hospital/outpatient visit. After collection patient data will be given a code for further anonymous storage and analysis. DNA SNP analysis will take place at our laboratory for pediatric nephrology with PCR.