Transporter assays as useful in vitro tools in drug discovery and development.

INTRODUCTION: Drug transporters are transmembrane proteins that facilitate the transfer of substances in and out of cells and play a substantial role in drug absorption, distribution and elimination. During the drug discovery and development phases, in vitro assays are important tools to identify substrates and inhibitors of transporters. AREAS COVERED: This article provides an overview of in vitro transporter assays, discussing their advantages, limitations, and sources of variability. Membrane-based assays take advantage of the location of efflux transporters for measurements of drug interactions. Cell-based systems are functional transporter assays that measure the passage of drugs across cell membranes and transporter proteins. EXPERT OPINION: Use of optimized and validated in vitro transporter assays during drug discovery and development leads to a greater confidence in their results in determining whether a new compound is a substrate or inhibitor. The concept of transporter method suitability provides a general scheme to improve predictability and reduce variability of the in vitro assays by incorporating assay validation, acceptance criteria, and probe substrates and inhibitors. Such a scheme can potentially improve assay reproducibility and allow in vitro transporter assays to aid in defining a test compound as a substrate or inhibitor of efflux and uptake transporters.