Volume of Distribution is Unaffected by Metabolic Drug-Drug Interactions.

These results support the widely held founding tenant of pharmacokinetics that clearance and Vss are independent parameters. Knowledge that Vss is unchanged in metabolic drug-drug interactions can be helpful in discriminating changes in clearance from changes in bioavailability (F) when only oral dosing data are available, as we have recently demonstrated. As Vss remains unchanged for intravenous metabolic drug-drug interactions, following oral dosing changes in Vss/F will reflect changes in F alone. This estimation of F change can subsequently be utilized to assess changes in clearance alone from calculations of apparent clearance. Utilization of this simple methodology for orally dosed drugs will have a significant impact on how drug-drug interactions are interpreted from drug development and regulatory perspectives.