Limited sampling strategy of partial area under the concentration-time curves to estimate midazolam systemic clearance for cytochrome P450 3A phenotyping.

Phenotyping is used to determine real time, in vivo drug metabolizing enzyme activity and assess pharmacokinetic-mediated drug-drug interactions (DDIs)1, 2. Of the drug metabolizing enzymes, cytochrome P450 (CYP) 3A plays a substantial role in clinically significant DDIs as more than 50% of the drugs available on the market are subject to CYP3A4 and CYP3A5 pathways3. The preferred CYP3A probe for phenotyping is midazolam2 with intravenous (IV) midazolam exclusively assessing hepatic CYP3A activity1, 4. Systemic (or total body) clearance (CL) of IV midazolam strongly correlates with hepatic CYP3A content (r = 0.93, p < 0.001)5 and is commonly used as a surrogate for hepatic CYP3A activity. Limited sampling strategy is a methodology that estimates systemic CL or area-under-the-concentration-time-curve (AUC) from a small number of plasma samples, ranging from 1 to 4. This method alleviates the cost and inconvenience of intense sampling, while maintaining acceptable precision and minimal bias. Limited sampling strategy has been adopted for clinical use for cyclosporine6, tacrolimus7, and mycophenolic acid8. With regards to IV midazolam, several limited sampling strategies have been examined. Single-point sampling strategies have been proposed, but timepoints vary regarding the optimal post-dose timepoint(s)9–11. Metabolic ratios of 1-hydroxymidazolam to midazolam, as well as two- and three-point limited sampling models (LSMs) have also been suggested11, but conflicting results have been reported12, 13. Katzenmaier et al. proposed an alternative limited sampling strategy of a partial AUC to estimate intestinal and hepatic CYP3A activity with oral midazolam14, 15. These studies recommended a partial AUC from 2 to 4 hours (AUC2–4) to estimate metabolic CL14, 15. In addition, Tai et al. reported a partial AUC0–4 and a partial AUC1–4 could reliably estimate apparent oral CL during conditions of CYP3A induction with rifampin and Ginkgo biloba extract16. Whether this limited sampling strategy is applicable with IV midazolam in estimating hepatic CYP3A activity is unknown. The purpose of this study was to determine if a limited sampling strategy using partial AUCs could estimate systemic CL and thus hepatic CYP3A activity with IV midazolam. The second objective was to perform equivalence testing to determine if the LSMs reproduce the same conclusions (e.g., equivalence or lack of equivalence) as those derived from intense sampling during conditions of CYP3A inhibition and induction/activation.