In silico, in vitro and in situ models to assess interplay between CYP3A and P-gp.

The bioavailability, fraction of dose that reaches systemic circulation, of orally administered drugs is often limited by both physical barriers of the intestine (e.g., unstirred-water and mucosal layers, epithelial tight junctions) as well as biochemical barriers such as cytochromes P450 (CYP) and P-glycoprotein (P-gp). Highly expressed in intestine and liver, CYP and P-gp can limit the systemic-availability of parent-drug by metabolism and efflux, respectively, by means of similarly large and flexible active sites that accommodate a variety of structurally-diverse, lipophilic molecules over a wide-range of molecular weights. Consequently, many molecules that are substrates for CYP3A4 also demonstrate affinity for P-gp and numerous studies have reported that for these dual-substrates, CYP3A4 and P-gp afford an interplay that affects bioavailability and clearance in a manner that is non-linear. Several in vitro and in situ models of metabolism and permeability, including transfected cell lines, isolated tissues and perfused organs as well as computational models including physiologically-based pharmacokinetic models of such co-expressing systems have demonstrated this phenomenon of CYP3A/Pgp interplay. Furthermore, recent availability of ligand bound X-ray co-crystal structures of the CYP3A4 and P-gp binding sites coupled with computational docking techniques and other validated in silico models, provide medicinal chemists with tools to inform structural-design modifications that can modify the interaction with one or both proteins. This article provides a review of relevant in silico, in vitro, ex vivo and in situ models that allow for investigation of the extent to which clearance or bioavailability can be affected by CYP/P-gp interplay.