Investigation of endogenous compounds applicable to drug - drug interaction studies involving the renal organic anion transporters, OAT1 and OAT3, in humans.

The present study was a comprehensive analysis of metabolites in plasma and urine specimens from subjects who received probenecid, a potent inhibitor of renal organic anion transporters (OATs). Taurine and glycochenodeoxycholate sulfate (GCDCA-S) could be identified using authentic standards. Probenecid had no effect on the area under the plasma - concentration time curves (AUCp) of taurine and GCDCA-S, whereas it significantly inhibited their urinary excretion in a dose-dependent manner. The renal clearance (CLR) values of taurine and GCDCA-S were decreased by 45% and 60%, 59% and 79%, and 70% and 88% by probenecid at 500, 750, and 1,500 mg, p.o., respectively. The CLR values correlated strongly (r>0.97) between the test compounds (benzylpenicillin, 6β-hydroxycortisol, taurine, and GCDCA-S). Taurine and GCDCA-S were substrates of OAT1 and OAT3, with Km values of 379 ± 58 and 64.3 ± 3.9 μM, respectively. The Ki values of probenecid for the OAT1- and OAT3-mediated uptake of taurine and GCDCA-S (9.49 ± 1.27 and 7.40 ± 0.70 μM, respectively) were similar to those of their typical substrate drugs. The magnitude of the reduction in the CLR of taurine and GCDCA-S by probenecid could be explained reasonably using the geometric mean of unbound probenecid concentration and Ki values. These results suggest that taurine and GCDCA-S can be used as probes for evaluating pharmacokinetic drug - drug interactions involving OAT1 and OAT3, respectively, in humans.