Rifampin's acute inhibitory and chronic inductive drug interactions: experimental and model-based approaches to drug-drug interaction trial design.

We studied the time course for the reversal of rifampin's effect on the pharmacokinetics of oral midazolam (a cytochrome P450 (CYP) 3A4 substrate) and digoxin (a P-glycoprotein (P-gp) substrate). Rifampin increased midazolam metabolism, greatly reducing the area under the concentration-time curve (AUC(0-∞)). The midazolam AUC(0-∞) returned to baseline with a half-life of ~8 days. Rifampin's effect on the AUC(0-3 h) of digoxin was biphasic: the AUC(0-3 h) increased with concomitant dosing of the two drugs but decreased when digoxin was administered after rifampin. Digoxin was found to be a weak substrate of organic anion-transporting polypeptide (OATP) 1B3 in transfected cells. Although the drug was transported into isolated hepatocytes, it is not likely that this transport was through OATP1B3 because the transport was not inhibited by rifampin. However, rifampin did inhibit the P-gp-mediated transport of digoxin with a half-maximal inhibitory concentration (IC(50)) below anticipated gut lumen concentrations, suggesting that rifampin inhibits digoxin efflux from the enterocyte to the intestinal lumen. Pharmacokinetic modeling suggested that the effects on digoxin are consistent with a combination of inhibitory and inductive effects on gut P-gp. These results suggest modifications to drug-drug interaction (DDI) trial designs.

Effect of the cathepsin K inhibitor odanacatib on bone resorption biomarkers in healthy postmenopausal women: two double-blind, randomized, placebo-controlled phase I studies.

Inhibition of cathepsin K (CatK) is a potential new treatment for osteoporosis. In two double-blind, randomized, placebo-controlled phase I studies, postmenopausal female subjects received odanacatib (ODN), an orally active, potent, and selective CatK inhibitor, once weekly for 3 weeks or once daily for 21 days. Bone turnover biomarkers, safety monitoring, and plasma ODN concentrations were assessed. These studies showed ODN to be well tolerated. Pharmacokinetic (PK) analysis revealed a long half-life (t(1/2); 66-93 h) consistent with once-weekly dosing. Pronounced reductions in C-terminal telopeptide of type I collagen (approximately 62%) and N-terminal telopeptide of type I collagen normalized to creatinine (NTx/Cr) (approximately 62%) at trough (C(168 h)) were seen following weekly administration. Robust reductions in CTx (up to 81%) and NTx/Cr (up to 81%) were seen following daily administration. ODN exhibits robust and sustained suppression of bone resorption biomarkers (CTx and NTx/Cr) at weekly doses > or = 25 mg and daily doses > or = 2.5 mg.