Clinical Pharmacokinetics of Once-Daily Tacrolimus in Solid-Organ Transplant Patients

Tacrolimus is a pivotal immunosuppressant agent used in solid-organ transplantation. It was originally formulated for oral administration as Prograf^®, a twice-daily immediate-release capsule. In an attempt to improve patient adherence, retain manufacturer market share and/or reduce health care costs, newer once-daily prolonged-release formulations of tacrolimus (Advagraf^® and Envarsus^® XR) and various generic versions of Prograf^® are becoming available. Tacrolimus has a narrow therapeutic index. Small variations in drug exposure due to formulation differences can have a significant impact on patient outcomes. The aim of this review is to critically analyse the published data on the clinical pharmacokinetics of once-daily tacrolimus in solid-organ transplant patients. Forty-three traditional (non-compartmental) and five population pharmacokinetic studies were identified and evaluated. On the basis of the stricter criteria for narrow-therapeutic-index drugs, Prograf^®, Advagraf^® and Envarsus^® XR are not bioequivalent [in terms of the area under the concentration–time curve from 0 to 24 h (AUC_0–24) or the minimum concentration (C _min)]. Patients may require a daily dosage increase if converted from Prograf^® to Advagraf^®, while a daily dosage reduction appears necessary for conversion from Prograf^® to Envarsus^® XR. Prograf^® itself, or generic immediate-release tacrolimus, can be administered in a once-daily regimen with a lower than double daily dose being reported to give 24-h exposure equivalent to that of a twice-daily regimen. Intense clinical and concentration monitoring is prudent in the first few months after any conversion to once-daily tacrolimus dosing; however, there is no guarantee that therapeutic drug monitoring strategies applicable to one formulation (or twice-daily dosing) will be equally applicable to another. The correlation between the tacrolimus AUC_0–24 and C _min is variable and not strong for all three formulations, indicating that trough measurements may not always give a good indication of overall drug exposure. Further investigation is required into whether the prolonged-release formulations have reduced within-subject pharmacokinetic variability, which would be a distinct advantage. Whether the effects of factors that influence tacrolimus absorption and pre-systemic metabolism (patient genotype status; gastrointestinal disease and disorders) and drug interactions differ across the formulations needs to be further elucidated. Most pharmacokinetic comparison studies to date have involved relatively stable patients, and many have been sponsored by the pharmaceutical companies manufacturing the new formulations. Larger randomized, controlled trials are needed in different transplant populations to determine whether there are differences in efficacy and toxicity across the formulations and whether formulation conversion is worthwhile in the longer term. While it has been suggested that once-daily administration of tacrolimus may improve patient compliance, further studies are required to demonstrate this. Mistakenly interchanging different tacrolimus formulations can lead to serious patient harm. Once-daily tacrolimus is now available as an alternative to twice-daily tacrolimus and can be used de novo in solid-organ transplant recipients or as a different formulation for existing patients, with appropriate dosage modifications. Clinicians need to be fully aware of pharmacokinetic and possible outcome differences across the different formulations of tacrolimus.