Abstract

The prediction of in vivo drug-drug interactions from in vitro enzyme inhibition parameters remains challenging, particularly when time-dependent inhibition occurs. This study was designed to examine the accuracy of in vitro-derived parameters for the prediction of inhibition of CYP3A by erythromycin (ERY). Chronically cannulated rats were used to estimate the reduction in in vivo and in vitro intrinsic clearance (CL_int) of midazolam (MDZ) after single and multiple doses of ERY; in vitro recovery of CL_int was determined at 1, 2, 3, and 4 days after discontinuation of ERY. Enzyme inhibition parameters (k_inact, K_I, and K_i) of ERY were estimated in vitro by using untreated rat liver microsomes. In vivo enzyme kinetic analysis indicated that single and multiple doses of ERY (150 mg/kg i.v. infusion over 4 h) reduced MDZ CL_int by reversible and irreversible mechanisms, respectively. CYP3A inactivation after multiple doses of ERY treatment reflected metabolic intermediate complex formation without a significant change in hepatic CYP3A2 mRNA. A physiologically based pharmacokinetic model of the interaction between ERY and MDZ predicted a 2.6-fold decrease in CYP3A activity after repeated ERY treatment using in vitro-estimated enzyme inhibition parameters and in vivo degradation half-life of the enzyme (20 ± 6 h). The observed -fold decreases were 2.3-fold and 2.1-fold for the in vitro-estimated CYP3A activity and the in vivo CL_int, respectively. This study demonstrates that in vivo DDIs are predictable from in vitro data when the appropriate model and parameter estimates are available.