Assessing Pleiotropic Effects of a Mixed-mode Perpetrator Drug Rifampicin by Multiple Endogenous Biomarkers in Dogs.

Rifampicin (RIF) is a mixed-mode perpetrator that produces pleiotropic effects on liver cytochrome P450 (CYP) enzymes and drug transporters. To assess the complex drug-drug interaction (DDI) liabilities of RIF in vivo , a known probe substrate midazolam (MDZ) along with multiple endogenous biomarkers were simultaneously monitored in beagle dogs before and after a 7-day treatment period by RIF at 20 mg/kg/day. Confirmed by the reduced MDZ plasma exposure and elevated 4β-hydroxycholesterol (4β-HC, biomarker of Cyp3a activities) level, Cyp3a was significantly induced after repeated RIF doses, and such induction persisted for 3 days after cessation of the RIF administration. On the other hand, increased plasma levels of coproporphyrin-I and III (CP-I and CP-III, biomarkers of organic anion transporting polypeptides 1b (Oatp1b) activities) were observed after the first dose of RIF. Plasma CPs started to decline as RIF exposure decreased, and they returned to baseline 3 days after cessation of the RIF administration. The data suggested the acute (inhibitory) and chronic (inductive) effects of RIF on Oatp1b and Cyp3a enzymes, respectively, and a 3-day washout period is deemed adequate to remove superimposed Oatp1b inhibition from Cyp3a induction. In addition, apparent self-induction of RIF was observed, as its terminal half-life was significantly altered after multiple doses. Overall, our investigation illustrated the need for appropriate timing of modulator dosing to differentiate between transporter inhibition and enzyme induction. As further indicated by the CP data, induction of Oatp1b activities was not likely after repeated RIF administration. Significance Statement Our investigation demonstrated the utility of endogenous biomarkers towards complex DDIs by RIF and successfully determined the optimal timing to differentiate between transporter inhibition and enzyme induction. Based on experimental evidence, Oatp1b inhibition following repeated RIF administration was unlikely, and apparent self-induction of RIF was observed.