Vincent Rönnpagel

• Introduction: The liver plays a major role in drug pharmacokinetics, serving as the primary site for drug metabolism. Existing in vitro and ex vivo models for hepatic metabolism are laborintensive or still offer limited representation of the complexity of hepatic transporters and metabolizing enzymes. Membrane transporter proteins, including solute carriers (SLCs), can substantially affect hepatic uptake and, consequently, the hepatic clearance of drugs by directly influencing drug disposition and the risk of drug-drug interactions (DDIs). Organic cation transporter 1 (OCT1) is a polyspecific transporter and member of the SLC22 family (SLC22A1), which is predominantly expressed in hepatocytes. OCT1 is known to significantly influence the hepatic uptake and pharmacokinetics of several commonly clinically used drugs such as morphine, sumatriptan, tramadol, and metformin, as well as endogenous compounds like thiamine. Aim: The aim of this study was to develop and then apply new methods to analyze the role of OCT1 in hepatic drug metabolism and DDIs. An ex vivo mouse liver perfusion model and an in vitro multi-substrate cocktail for OCT1 were developed and applied to analyze the contribution of OCT1 to the first-pass effect. Additionally, these methods should be applied to analyze the variability in the risks of DDI depending on the victim drug. Results: Using the mouse liver perfusion, we were able to simulate the first-pass of codeine and proguanil, and could show that OCT1 plays a key role in the uptake of morphine. We also confirmed in this model that OCT1 deficiency affects morphine and morphine-3-glucuronide concentrations in the perfusate following both morphine and codeine administration. Furthermore, we demonstrated that OCT1 deficiency alters the concentrations of both proguanil and cycloguanil after proguanil administration. We also applied the liver perfusion model to show that OCT1 deficiency affects the hepatic uptake of berberine. However, the data suggest that another, yet unidentified, transporter also contributes to its hepatic uptake, as inhibition with desipramine additionally substantially altered berberine concentrations. These results may explain the lack of effects of OCT1 deficiency on berberine plasma concentrations in humans. Using the cocktail, we were able to analyze substrate-specific differences in OCT1 transport and inhibition between paralogs and orthologs. We were able to identify two groups of inhibitors, which have closely correlated inhibitory potencies depending on the victim drug. The first group included fenoterol, verapamil, quinidine, and ipratropium, and the second sumatriptan, trimethoprim, and metformin. Conclusion: We successfully established, validated, and applied a mouse liver perfusion model and an OCT1 multi-substrate cocktail. These methods enable prediction of first-pass effects and substrate-specific DDI potentials, respectively, which are important for pre-clinical drug development. In the future, both methods can be combined, which should reduce the time, costs, and number of animals required for the analysis of the substrate-specific effects of OCT1.