In Vitro Methodologies to Assess Potential for Transporter-Mediated Drug–Drug Interactions

Abstract

The importance of membrane transporters and drug-metabolizing enzymes on drug absorption and disposition is widely known. Our current understanding of transporters and enzymes has been achieved largely through the development and utilization of in vitro techniques to study drug transport and metabolism processes. As a result of the increasing sophistication and reliability of these techniques, the FDA has issued guidance documents outlining the type of in vitro tests that can be used to assess whether drug candidates are substrates and/or inhibitors of transporters and/or enzymes. These studies are conducted under the premise that if a drug is a substrate or inhibitor of a transporter, it may interact with other drugs that interact with the same transporter(s). Ultimately, for in vitro drug–drug interactions to be predictive of potential interactions in vivo, in vitro experiments must take into account the drug concentrations expected in vivo. As in vitro tests can be used to waive the conduct of clinical drug–drug interaction studies, they could have a major impact in drug development times and costs and spare healthy individuals from exposure to the unavoidable risks associated with clinical drug–drug interaction studies. Further, since the type and quality of data obtained from in vitro experiments is dependent on the study design used, it is very important for pharmaceutical scientists working in this field to rely on sound and robust experimental protocols to generate reliable and reproducible data. However, the various conditions used for the conduct of these assays in different laboratories have resulted in large interlaboratory variabilities that create much uncertainty regarding published data and the best experimental approaches. In fact, interlaboratory variability may not only reflect the quantitative differences inherent to the repeated performance of biological assays but also differences in the experimental design or strategies used by different laboratories. For example, in running a test to assess whether a compound is a P-gp substrate, some laboratories may use Caco-2 cells while others may use MDR1-MDCK cells. In addition, while two laboratories may use the same cell line (e.g., MDR1-MDCK) to run this test, one laboratory may make the assessment based on the efflux ratio of the substrate drug while another may utilize unidirectional (B-to-A) transport with and without a known inhibitor of P-gp. This chapter will present protocols that have been extensively used for the conduct of in vitro studies; some of which have enabled decision-making on drug–drug interaction studies and thus impacted drug development programs.