Abstract

P-glycoprotein (P-gp)-mediated drug efflux and cytochrome P450 3A (CYP3A) metabolism within the enterocyte have been implicated as potential biochemical barriers to intestinal drug permeability. The current studies examined the in vitro intestinal permeability of verapamil, a common P-gp and CYP3A substrate, using both disappearance and appearance measurements, and investigated the possible impact of P-gp efflux on the intestinal extraction of verapamil. Bidirectional permeability and metabolism studies were conducted across rat jejunal tissue in side-by-side diffusion chambers and data were modeled using compartmental kinetics. Substantial tissue uptake of verapamil was evident in the in vitro model and resulted in a disappearance permeability coefficient that was approximately 10-fold greater than that determined from verapamil appearance in the receptor chamber. Polarization of the bidirectional transport of verapamil was evident due to P-gp efflux (efflux ratio of 2.5), and significant intestinal extraction of verapamil on passage across the tissue was observed (mucosal to serosal extraction ratio of 0.31 ± 0.04). Surprisingly, the selective P-gp inhibitor, valspodar (PSC833), had an insignificant impact on P-gp-mediated efflux of verapamil; however, selective CYP3A inhibition (afforded by midazolam) increased mucosal to serosal verapamil transport 1.6-fold, presumably through a reduction in intestinal metabolism. Using a four-compartment model, simulations of the impact of P-gp on the intestinal extraction ratio of verapamil demonstrated that for efflux to increase intestinal extraction, a nonlinear relationship must exist between the extent of drug metabolism and the extent of drug transport; the origin of this “nonlinearity” may include saturable drug metabolism, accumulation, and/or distribution.