Absorption barriers in the rat intestinal mucosa: 1. Application of an in situ perfusion model to simultaneously assess drug permeation and metabolism.

Modulation of intestinal drug absorption barriers can have a profound impact on the bioavailability of orally administered compounds. With its commonality of use as an absorption model, it is valuable to assess the role of such barriers in the rat intestinal mucosa. In the present study, atenolol and verapamil were concomitantly delivered in the in situ perfused rat intestine in the presence or absence of inhibitors to simultaneously assess the function and modulation of passive diffusion barriers, cytochrome P450 (CYP)3A metabolism and P-glycoprotein (P-gp) efflux. A high performance liquid chromatography-tandem mass spectrometry method measured atenolol, verapamil and the CYP3A-mediated metabolite, norverapamil, with linearity (r(2) > 0.99), precision (CV <or=7.5%) and accuracy (+/-17%). Absorption of parent drug was independent of verapamil concentration; however the formation and disposition of norverapamil were concentration-dependent and saturable. Norverapamil formation decreased (up to 80%) in the presence of CYP3A inhibitors and the fraction of norverapamil observed in the plasma was increased (4.5- to 7.2-fold) in the presence of P-gp inhibitors. These results suggest that in this model of the rat intestinal mucosa, atenolol serves as a marker for diffusion barriers whereas norverapamil formation and disposition are markers of CYP3A and P-gp, respectively.