Masato Horikawa

Which concentration of the inhibitor should be used to predict in vivo drug interactions from in vitro data

When the metabolism of a drug is competitively or noncompetitively inhibited by another drug, the degree of in vivo interaction can be evaluated from the [I]u/Ki ratio, where [I]u is the unbound concentration around the enzyme and Ki is the inhibition constant of the inhibitor. In the present study, we evaluated the metabolic inhibition potential of drugs known to be inhibitors or substrates of cytochrome P450 by estimating their [I]u/Ki ratio using literature data. The maximum concentration of the inhibitor in the circulating blood ([I]max), its maximum unbound concentration in the circulating blood ([I]max,u), and its maximum unbound concentration at the inlet to the liver ([I]in,max,u) were used as [I]u, and the results were compared with each other. In order to calculate the [I]u/Ki ratios, the pharmacokinetic parameters of each drug were obtained from the literature, together with their reported Ki values determined in in vitro studies using human liver microsomes. For most of the drugs with a calculated [I]in,max,u/Ki ratio less than 0.25, which applied to about half of the drugs investigated, no in vivo interactions had been reported or “no interaction” was reported in clinical studies. In contrast, the [I]max,u/Ki and [I]max/Ki ratio was calculated to be less than 0.25 for about 90% and 65% of the drugs, respectively, and more than a 1.25-fold increase was reported in the area under the concentration-time curve of the co-administered drug for about 30% of such drugs. These findings indicate that the possibility of underestimation of in vivo interactions (possibility of false-negative prediction) is greater when [I]max,u or [I]max values are used compared with using [I]in,max,u values.

Reduced Gastrointestinal Toxicity Following Inhibition of the Biliary Excretion of Irinotecan and Its Metabolites by Probenecid in Rats

AbstractPurpose. To ameliorate the late-onset of severe gastrointestinal toxicity provoked by irinotecan (CPT-11), which may be related to the biliary excretion of CPT-11 and/or its metabolites. Methods. Effects of probenecid, an inhibitor of MRP2/ABCC2, on the biliary excretion and mucosal intestinal tissue concentration of CPT-11 and its metabolites were examined in rats. CPT-11-induced late-onset gastrointestinal toxicity was also evaluated. Results. Coadministration of probenecid reduced the biliary excretion of CPT-11, an active metabolite (SN-38) and its glucuronide by half with a concomitant increase in their plasma concentration. When the dose of CPT-11, in the presence of probenecid, was set at half that in its absence, the plasma SN-38 concentration was maintained at the same level as the control, whereas the mucosal intestinal tissue concentration of SN-38 was reduced. Under this condition, CPT-11-induced watery diarrhea, changes in intestinal marker enzymes and body weight reduction were much less in the probenecid-treated group, although the degree of bone marrow suppression was almost the same as that in the control. Conclusions. Coadministration of probenecid with a reduced dose of CPT-11 potently reduces both SN-38 exposure and CPT-11-induced late-onset toxicity in gastrointestinal tissues, possibly by inhibiting the biliary excretion of CPT-11 and/or its metabolites.

Pharmacokinetics and Hepatic Uptake of Eltrombopag, a Novel Platelet-Increasing Agent

Eltrombopag (ELT) is a novel thrombopoietin receptor agonist for the treatment of idiopathic thrombocytopenic purpura. Previous reports indicate that ELT is mainly eliminated in the liver, although its pharmacokinetic profile has not yet been clarified in detail. The purpose of the present study is to investigate the overall elimination mechanism of ELT. After intravenous administration of ELT to rats, approximately 40% of unchanged ELT was excreted into the bile in 72 h, whereas less than 0.02% of the dose was excreted in urine, indicating that liver is the major elimination organ for ELT. The total clearance was much lower than the hepatic blood flow rate and comparable with hepatic uptake clearance obtained from integration plot analysis. Coadministration of rifampicin, an organic anion transporter inhibitor, reduced both total clearance and hepatic uptake clearance of ELT. These results suggest that hepatic uptake is the rate-limiting process in the overall elimination of ELT. To further characterize the uptake mechanism, uptake of ELT by freshly isolated mouse hepatocytes was examined. The ELT uptake showed concentration and energy dependence and was inhibited by various compounds, including not only organic anions but also organic cations. Hepatic uptake clearance in vivo was reduced by coadministration of an organic cation, tetrapentylammonium. Finally, uptake of ELT was observed in human embryonic kidney 293 cells transfected with human hepatic transporters organic anion-transporting polypeptide (OATP) 1B1 and OATP2B1 and organic cation transporter OCT1. These results suggest that multiple transporters, including organic anion transporters and organic cation transporters, are involved in hepatic ELT uptake.

Interaction of Novel Platelet-Increasing Agent Eltrombopag with Rosuvastatin via Breast Cancer Resistance Protein in Humans

Eltrombopag (ELT), an orally available thrombopoietin receptor agonist, is a substrate of organic anion transporting polypeptide 1B1 (OATP1B1), and coadministration of ELT increases the plasma concentration of rosuvastatin in humans. Since the pharmacokinetic mechanism(s) of the interaction is unknown, the present study aimed to clarify the drug interaction potential of ELT at transporters. The OATP1B1-mediated uptake of ELT was inhibited by several therapeutic agents used to treat lifestyle diseases. Among them, rosuvastatin was a potent inhibitor with an IC50 of 0.05 µM, which corresponds to one-seventh of the calculated maximum unbound rosuvastatin concentration at the inlet to the liver. Nevertheless, a simulation study using a physiologically based pharmacokinetic model predicted that the effect of rosuvastatin on the pharmacokinetic profile of ELT in vivo would be minimal. On the other hand, ELT potently inhibited uptake of rosuvastatin by OATP1B1 and human hepatocytes, with an IC50 of 0.1 µM. However, the results of the simulation study indicated that inhibition of OATP1B1 by ELT can only partially explain the clinically observed interaction with rosuvastatin. ELT also inhibited transcellular transport of rosuvastatin in MDCKII cells stably expressing breast cancer resistance protein (BCRP), and was found to be a substrate of BCRP. The interaction of ELT with rosuvastatin can be almost quantitatively explained on the assumption that intestinal secretion of rosuvastatin is essentially completely inhibited by ELT. These results suggest that BCRP in small intestine may be the major target for interaction between ELT and rosuvastatin in humans.