FURTHER EXPLORATION INTO THE DRUG-DRUG INTERACTION BETWEEN GEMFIBROZIL AND REPAGLINIDE IN RATS: UPTAKE TRANSPORT

The clinically-relevant drug-drug interaction (DDI) between the dyslipidemia drug gemfibrozil and the antidiabetic repaglinide is well-documented throughout the literature. In humans, repaglinide is predominantly cleared by hepatic metabolism involving cytochrome P450 (P450) 3A4 and 2C8 and UGT1A1 and 1A3. Gemfibrozil and its glucuronide metabolite inhibit CYP2C8 (irreversibly) and UGT1A1, as well as the hepatic uptake transporter OATP1B1. These factors have been implicated in the clinical interaction. A gemfibrozil/repaglinide DDI resulting in increased repaglinide and metabolite plasma exposure (>3-fold) and a vectoral shift in elimination pathways from biliary to urinary excretion following administration of gemfibrozil was previously established in male Sprague-Dawley rats. In that study, negligible effects on liver enzyme activity were observed following gemfibrozil treatment, suggesting that the DDI in rats was unlikely to be caused by P450 or UGT inhibition. In the present study, the potential role of uptake transporter proteins in the interaction was explored in vitro.

Experiments were performed to evaluate the effects of gemfibrozil and gemfibrozil glucuronide on repaglinide transport in rats. Rat primary hepatocytes were cultured for three days and then treated with repaglinide (0.5, 5 and 50 µM) administered in the presence and absence of 100 µM gemfibrozil or gemfibrozil glucuronide. Repaglinide uptake was evaluated at 1, 3, 5, 10, 15 and 30 min. At each time point, incubation media was removed, cells were washed twice with PBS, lysed with organic solvent containing an internal standard, and analyzed for repaglinide uptake by LC-MS/MS against a calibration curve (1 to 5000 nM).

Repaglinide uptake into rat hepatocytes in the absence of gemfibrozil or gemfibrozil glucuronide was extensive, ranging from 2 to 9% at 0.5 µM, 3 to 13% at 5 µM, and 3 to 22 % at 50 µM, respectively, over 30 min incubation. In general, repaglinide uptake into rat hepatocytes increased with both increasing concentration and time. At 50 µM repaglinide, uptake started to plateau at the longest 30 min time point. This may be indicative of saturation as it was not observed at either of the lower concentrations tested. Up to 15 min, the presence of gemfibrozil or gemfibrozil glucuronide had no effect on repaglinide uptake into rat hepatocytes, but effects were observed with both gemfibrozil and gemfibrozil glucuronide at 30 min incubation. In the presence of gemfibrozil, repaglinide uptake plateaued at 30 min at all three repaglinide concentrations. In the presence of gemfibrozil glucuronide, a decrease in repaglinide uptake was observed at 30 min at all three repaglinide concentrations. The observations noted in the coadministration experiment established that gemfibrozil and its glucuronide metabolite had a small impact on repaglinide uptake in rat hepatocytes after 30 min incubation, but neither compound had any effect at the earlier timepoints. However, the observed results do not account for the 4-fold exposure increase or for the change in the vectoral elimination pathway observed in vivo. The results did implicate a time-dependent effect. Consequently, similar experiments incorporating gemfibrozil and gemfibrozil glucuronide pretreatment over longer time points are currently underway.