Abstract

Probenecid (PROB) is a clinical probe inhibitor of renal organic anion transporter (OAT) 1 and OAT3 that inhibits in vitro activity of hepatic drug transporters OATP1B1 and OATP1B3. It was hypothesized that PROB could potentially affect the disposition of OATP1B drug substrates. The plasma levels of the OATP1B endogenous biomarker candidates, including coproporphyrin I (CPI), CPIII, hexadecanedioate (HDA), and tetradecanedioate (TDA), were examined in 14 healthy subjects treated with PROB. After oral administration with 1000 mg PROB alone and in combination with furosemide (FSM), AUC_{(0–24 h)} values were 1.39 ± 0.21-fold and 1.57 ± 0.41-fold higher than predose levels for CPI and 1.34 ± 0.16-fold and 1.45 ± 0.57-fold higher for CPIII. Despite increased systemic exposures, no decreases in CPI and CPIII renal clearance were observed (0.97 ± 0.38-fold and 1.16 ± 0.51-fold for CPI, and 1.34 ± 0.53-fold and 1.50 ± 0.69-fold for CPIII, respectively). These results suggest that the increase of CP systemic exposure is caused by OATP1B inhibition. Consistent with this hypothesis, PROB inhibited OATP1B1- and OATP1B3-mediated transport of CPI in a concentration-dependent manner, with IC₅₀ values of 167 ± 42.0 and 76.0 ± 17.2 µM, respectively, in transporter-overexpressing human embryonic kidney cell assay. The inhibition potential was further confirmed by CPI and CPIII hepatocyte uptake experiments. In contrast, administration of PROB alone did not change AUC_{(0–24 h)} of HDA and TDA relative to prestudy levels, although the administration of PROB in combination with FSM increased HDA and TDA levels compared with FSM alone (1.02 ± 0.18-fold and 0.90 ± 0.20-fold vs. 1.71 ± 0.43-fold and 1.62 ± 0.40-fold). Taken together, these findings indicate that PROB displays weak OATP1B inhibitory effects in vivo and that coproporphyrin is a sensitive endogenous probe of OATP1B inhibition. This study provides an explanation for the heretofore unknown mechanism responsible for PROB’s interaction with other xenobiotics.