Principles and experimental considerations for in vitro transporter interaction assays.

Drug transporters are now universally acknowledged as important determinants of the absorption, distribution, metabolism and excretion of both endogenous and exogenous compounds. Altered transporter function, whether due to genetic polymorphism, DDIs, disease, or environmental factors such as dietary constituents, can result in changes in drug efficacy and/or toxicity due to changes in circulating or tissue levels of either drugs or endogenous substrates.Prediction of whether and to what extent the biological fate of a drug is influenced by drug transporters, therefore, requires in vitro test systems that can accurately predict the risk and magnitude of clinical DDIs. While these in vitro assessments appear simple in theory, practitioners recognize that there are multiple factors that can influence experimental outcomes. A better understanding of these variables, including test compound characteristics, test systems, assay formats, and experimental design will enable clear, actionable steps and translatable outcomes that may avoid unnecessary downstream clinical engagement. This chapter will delineate the role of these variables in improving in vitro assay outcomes.

Case study 6. Transporter case studies: in vitro solutions for translatable outcomes.

Assessing the interactions of a new drug candidate with transporters, either as a substrate or as an inhibitor, is no simple matter. There are many clinically relevant transporters, as many as nine to be evaluated for an FDA submission and up to eleven for the EMA as of 2013. Additionally, it is likely that if a compound is a substrate or inhibitor of one transporter, it will be so for other transporters as well. There are practically no specific substrates or inhibitors, presumably because the specificities of drug transporters are so broad and overlapping, and even fewer clinically relevant probes that can be used to evaluate transporter function in humans. In the case of some transporters, it is advisable to evaluate an NCE with more than one test system and/or more than one probe substrate in order to convince oneself (and regulatory authorities) that a clinical drug interaction study is not warranted. Finally, each test system has its own unique set of advantages and disadvantages. One has to really appreciate the nuances of the available tools (test systems, probe substrates, etc.) to select the best tools for the job and design the optimal in vitro experiment. In this chapter, several examples are used to illustrate the successful interpretation of in vitro data for both efflux and uptake transporters. Some data presented in this chapter is unpublished at the time of compilation of this book. It has been incorporated in this chapter to provide a sense of complexities in transporter kinetics to the reader.