Estimating reduced availability due to first pass elimination from relative total clearance and renal clearance

A simple formula is presented for estimating the systemic availability of an orally administered drug from the relative total clearance (oral clearance) when renal clearance forms an important part of total clearance. Hepatic plasma flow is used in the equation and is represented by an average value taken from the literature. The formula is applied to data for cimetidine discussed in the literature. A further application is demonstrated for a drug under development for which an intravenous formulation was not available. It is possible to estimate the upper and lower limit of availability if some information on the amount of drug absorbed is available.