Pharmacogenetics in solid organ transplantation: current status and future directions

CYP3A polymorphisms and immunosuppressive drugs in solid-organ transplantation

Most immunosuppressive drugs have a narrow therapeutic index and large interpatient variabilities in their pharmacokinetic and pharmacodynamic profiles. Identification of functional single-nucleotide polymorphisms in genes encoding for drug metabolizing enzymes has great potential to improve the drug efficacy and safety profiles, since these genetic factors may be important biomarkers for individualization of immunosuppressive therapy. This article summarizes current knowledge regarding the impact of CYP3A polymorphisms on immunosuppressive drug pharmacokinetics. Many retrospective studies have shown a clear relationship between CYP3A5*1/*3 polymorphism and tacrolimus pharmacokinetics, while the influence of CYP3A5*1/*3 or CYP3A4*/*1B on ciclosporin and sirolimus exposure are still questionable. CYP3A polymorphisms may partially contribute to the clinical variability of the enzyme-mediated drug interactions. Drug-drug interactions may also influence the phenotypic consequence of CYP3A polymorphisms. Population pharmacodynamic/kinetic/genomic modeling was proposed as an emerging and promising approach to quantitatively explore the contribution of genetic polymorphisms to the large interpatient variability in the pharmacokinetic and pharmacodynamic profiles of immunosuppressive drugs. Prospective, randomized studies in large patient populations are needed to further clarify the genetic effects of CYP3A on immunosuppressive drug exposure and response.

Calcineurin inhibitor nephrotoxicity

The use of the calcineurin inhibitors cyclosporine and tacrolimus led to major advances in the field of transplantation, with excellent short-term outcome. However, the chronic nephrotoxicity of these drugs is the Achilles' heel of current immunosuppressive regimens. In this review, the authors summarize the clinical features and histologic appearance of both acute and chronic calcineurin inhibitor nephrotoxicity in renal and nonrenal transplantation, together with the pitfalls in its diagnosis. The authors also review the available literature on the physiologic and molecular mechanisms underlying acute and chronic calcineurin inhibitor nephrotoxicity, and demonstrate that its development is related to both reversible alterations and irreversible damage to all compartments of the kidneys, including glomeruli, arterioles, and tubulo-interstitium. The main question--whether nephrotoxicity is secondary to the actions of cyclosporine and tacrolimus on the calcineurin-NFAT pathway--remains largely unanswered. The authors critically review the current evidence relating systemic blood levels of cyclosporine and tacrolimus to calcineurin inhibitor nephrotoxicity, and summarize the data suggesting that local exposure to cyclosporine or tacrolimus could be more important than systemic exposure. Finally, other local susceptibility factors for calcineurin inhibitor nephrotoxicity are reviewed, including variability in P-glycoprotein and CYP3A4/5 expression or activity, older kidney age, salt depletion, the use of nonsteroidal anti-inflammatory drugs, and genetic polymorphisms in genes like TGF-beta and ACE. Better insight into the mechanisms underlying calcineurin inhibitor nephrotoxicity might pave the way toward more targeted therapy or prevention of calcineurin inhibitor nephrotoxicity.