title: Understanding the transport properties of metabolites: case studies and considerations for drug development.
abstract: Recent analyses demonstrated that metabolites are unlikely to contribute significantly to clinical inhibition of cytochrome P450 (P450)-mediated drug metabolism, and that only ∼2% of this type of drug interaction could not be predicted from the parent drug alone. Due to generally increased polarity and decreased permeability, metabolites are less likely to interact with P450s, but their disposition is instead more likely to involve transporters. This commentary presents case studies illustrating the potential importance of transporters as determinants of metabolite disposition, and as sites of drug interactions, which may alter drug efficacy and safety. Many of these examples are hydrophilic phase II conjugates involved in enterohepatic cycling, where modulation of transporter-dependent disposition may alter pharmacokinetics/pharmacodynamics. The case studies suggest that characterization of metabolite disposition, toxicology, and pharmacology should not focus solely on metabolites with appreciable systemic exposure, but should take into consideration major excretory metabolites. A more thorough understanding of metabolite (phase I and II; circulating and excreted) transport properties during drug development may provide an improved understanding of complex drug-drug interactions (DDIs) that can alter drug and/or metabolite systemic and intracellular exposure. Knowledge and capability gaps remain in clinical translation of in vitro and animal data regarding metabolite disposition. To this end, useful experimental and modeling approaches are highlighted. Application of these tools may lead to a better understanding of metabolite victim and perpetrator DDI potential, and ultimately the establishment of approaches for prediction of pharmacodynamic and toxicodynamic consequences of metabolite transport modulation.
authors.affiliation: Drug Disposition, Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, Indiana (M.J.Z.-G.); Department of Pharmacokinetics, Pharmacodynamics and Drug Metabolism, Merck & Co., Rahway, New Jersey (X.C.); Drug Metabolism and Pharmacokinetics, GlaxoSmithKline Inc., Research Triangle Park, North Carolina (J.W.P.); College of Pharmacy, Washington State University, Spokane, Washington (M.F.P.); and Centre for Applied Pharmacokinetic Research, Manchester Pharmacy School, The University of Manchester, Manchester, United Kingdom (A.G.).
authors.forename: Maciej J
authors.initials: MJ
authors.lastname: Zamek-Gliszczynski
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Chu
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Polli
authors.forename: Mary F
authors.initials: MF
authors.lastname: Paine
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Galetin
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D055808:Drug Discovery; D004347:Drug Interactions; D006801:Humans; D008954:Models, Biological; D004364:Pharmaceutical Preparations / Q000097:blood / Q000378:metabolism* / Q000652:urine; D014018:Tissue Distribution
chemical_list: D002352:Carrier Proteins; D004364:Pharmaceutical Preparations; D003577:Cytochrome P-450 Enzyme System