Optimizing everolimus exposure when combined with calcineurin inhibitors in solid organ transplantation.

The mammalian target of rapamycin (mTOR) inhibitor everolimus is a narrow therapeutic index drug for which optimal exposure levels are essential. The consistent pharmacokinetic profile of everolimus allows trough concentration (C0) measurement to be an appropriate and reliable index for therapeutic drug monitoring (TDM). Exposure-response analyses of data from early fixed-dose trials demonstrated that rates of biopsy-proven acute rejection (BPAR) are significantly higher if everolimus C0 declines below 3 ng/mL, an observation confirmed in subsequent concentration-controlled trials. Evidence for the most favorable upper limit is less clear but with reduced-exposure calcineurin inhibitor (CNI) therapy, an upper limit of 8 ng/mL appears to balance efficacy and safety outcomes. The recommended C0 range is 3-8 ng/mL in kidney, liver and heart transplantation patients, based on LC-MS/MS monitoring in whole blood. Randomized clinical trials based on this target range have demonstrated rates of BPAR comparable to a regimen of mycophenolic acid with standard-exposure CNI. Everolimus exhibits moderate intrapatient pharmacokinetic variability, and it can be challenging to maintain stable concentrations within target range in some individuals. Many factors can influence everolimus exposure for a given dose, including hepatic function, activity of the drug efflux pump P-glycoprotein, the rate of everolimus metabolism, drug-drug interactions (predominantly with CYP3A4 and P-glycoprotein inhibitors, including cyclosporine), intake of fatty food, and patient adherence to the prescribed regimen. Trough concentration levels should be monitored 4-5days after the first dose and after any change in everolimus dose, with additional monitoring in response to any change in concomitant medication or other clinical circumstances which could alter everolimus exposure. Although LC-MS/MS is the gold standard for everolimus monitoring, various immunoassays are widely used due to their relative simplicity and lower cost, and results can show considerable discrepancies with reference methods due to issues such as interassay variability and cross-reactivity. Method standardization will be important in the future to improve the consistency and reproducibility of results between centers. In conclusion, based on an extensive program of clinical trials, the optimal exposure range for everolimus in combination with reduced-exposure CNI therapy has been established and can be achieved in most transplant recipients through careful, planned TDM.