Metformin, the most widely prescribed anti-diabetic drug, requires transporters to enter tissues involved in its pharmacologic action, including liver, kidney and peripheral tissues. Although membrane transporters responsible for the liver and kidney uptake of metformin have been identified, the mechanism in which metformin enters peripheral tissues is not completely understood. Organic cation transporter 3 (OCT3, SLC22A3) transports metformin in vitro and is expressed ubiquitously. The goals of this research are to determine the role of OCT3 in metformin disposition and response, identify OCT3 inhibitors that could potentially result in clinical drug-drug interactions when co-administered with metformin, and develop a method to predict competitive and non-competitive inhibitors of transporters.
In this dissertation, we showed that ablation of Oct3 in mice results in dramatic changes in the pharmacokinetics of metformin. In particular, the apparent volume of distribution, clearance and bioavailability of metformin were decreased in Oct3 knockout mice. We also showed that Oct3 knockout mice had reduced response to metformin. Notably, the effect of metformin on glucose tolerance is reduced in mice lacking Oct3, and that a likely explanation for this is reduced metformin accumulation in peripheral sites such as adipose tissue. Analysis of data from healthy volunteers receiving metformin revealed that a reduced function OCT3 genetic variant associated with attenuated response to metformin. In addition, we developed and conducted a high-throughput screen (HTS) against a large library of 2,556 prescription drugs and bioactive compounds for OCT3 inhibitors. We identified 210 potent OCT3 inhibitors, 23 of which could potentially cause drug-drug interactions with metformin. In order to enhance our currently limited understanding of inhibitor-transporter interactions, we developed a methodology combining structure-based and cell-based HTS to rapidly identify competitive and non-competitive inhibitors of OCT1, a paralog of OCT3, which plays a critical role in metformin disposition and response. Our method has broad implications for using combined in silico and HTS methods to not only identify ligands of transporters, but to understand their mechanisms of interaction with transporters.
Our findings have important implications for understanding metformin disposition and pharmacologic response, and pave the way for future studies of OCT3-mediated drug-drug interactions. Additionally, the combined modeling and HTS methodology we developed can accelerate transporter inhibition studies that are mandated by regulatory agencies during drug development.
