Mycophenolate, clinical pharmacokinetics, formulations, and methods for assessing drug exposure

Abstract

This article summarizes part of a consensus meeting about mycophenolate (MPA) therapeutic drug monitoring held in Rome under the auspices of The Transplantation Society in November 2008 (Clin J Am Soc Nephrol. 2010;5:341-358). This part of the meeting focused on the clinical pharmacokinetics of MPA and included discussion on how to measure MPA (active drug) exposure and the differences between the currently available formulations. Because of variability in the dose-concentration relationship, MPA exposure should be measured and doses should be adjusted accordingly to achieve optimal clinical outcomes. Suggested therapeutic exposures derived for MPA from mycophenolate mofetil (MMF) may differ to those that could be useful for MPA from enteric-coated mycophenolate sodium (EC-MPS), particularly if limited sampling strategies or single concentration, especially trough concentrations, is used, as the concentration-time profiles of MPA from the 2 formulations are quite different. The 2 MPA formulations cannot be considered as bioequivalent. The area under the concentration-time curve (AUC 0-12) is considered the criterion standard for monitoring of MPA, which is a reflection of exposure to the drug over the entire dosing period. If a limited sampling protocol coupled with multilinear regression or Bayesian estimation is used to estimate this parameter, it should be used only for the population in which the model has been developed and should preferably include at least one time point after 4 hours (preferably around 8 or 9 hours after MMF dosing). If a single time point is to be used as a surrogate for an AUC 0-12, trough concentration of MPA may be the most practical but, from a pharmacokinetic standpoint, is not the most informative time point to choose. Because limited sampling strategies to estimate MPA exposure from EC-MPS have not yet been well developed and fully evaluated, nor have accurate Bayesian estimators been reported, AUC 0-12 measurement is still necessary to obtain reliable estimates of MPA exposure in patients treated with EC-MPS. The measurement of MPA trough concentrations should not be used at all for MPA exposure assessment following administration of EC-MPS. Because limited sampling strategies to estimate MPA exposure from EC-MPS have not yet been well developed and fully evaluated, nor have accurate Bayesian estimators been reported, AUC 0-12 measurement is still necessary to obtain reliable estimates of MPA exposure in patients treated with EC-MPS. The measurement of MPA trough concentrations should not be used at all for MPA exposure assessment following administration of EC-MPS. Lower (or higher) than expected total MPA exposure in patients with severe renal impairment may still indicate sufficient free MPA exposure. Mycophenolate free exposure measurement/estimation is likely to be beneficial in patients with severe renal impairment (creatinine clearance b25 mL/min) to guide dosage estimation, especially because renal function changes over time after transplant, while recognizing that robust prospective studies to show the clinical advantage of measuring free MPA exposure are still required. Lower total measured MPA exposure in patients with hypoalbuminemia may still indicate sufficient free MPA exposure. Mycophenolate free concentration measurement and estimation of exposure are likely to be beneficial in patients with a serum albumin less than or equal to 31 g/L to guide interpretation of MPA exposure. A 1.5-g twice-daily starting dose of MMF rather than a 1-g twice-daily starting dose of MMF is more likely to achieve the minimum target MPA exposure in adult transplant recipients receiving concomitant cyclosporine therapy. Because the cyclosporine dose is progressively tapered following transplantation, MPA exposure should be measured repeatedly and MMF should be doses adjusted accordingly to achieve optimal clinical outcome. Mycophenolate exposure should be measured in the first week after transplant, then each week for the first month, each month until month 3, and subsequently every 3 months up to 1 year with appropriate dosage adjustment, as AUC is likely to increase over time. After 1 year, if dosage requirement has stabilized, MPA exposure can be assessed each time the immunosuppressive regimen is changed or a potentially interacting drug is introduced or withdrawn. Assessment of UGT1A9 single nucleotide polymorphisms (-275TNA, -2152CNT, -440CNT, -331TNC) should be considered before transplantation to assist in dosing decisions to achieve optimal MPA exposure immediately after transplant. Consideration of the points summarized above should lead to more effective dosage adjustment based on sound applied pharmacokinetic and pharmacodynamic principles.