Determination of bioavailability and systemically available fractions of drugs undergoing reversible metabolism: application to 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2- (diethylamino)ethyl]benzamide and its sulfide and sulfone metabolites in rats

Mean residence time of drugs administered non-instantaneously and undergoing linear tissue distribution and reversible metabolism and linear or non-linear elimination from the central compartment

Based on disposition decomposition analysis (DDA), equations for the mean residence times (MRT) in the body are derived for a drug and its interconversion metabolite that undergo linear tissue distribution and linear or non-linear elimination from the central compartment after non-instantaneous administration of the drug. The MRT of the drug after non-instantaneous input can be related to the MRT of the drug after intravenous administration, the ratio of the total area under the plasma concentration-time curve of the drug after non-instantaneous administration to that after intravenous administration, the bioavailability of the drug, and the mean input time of the drug. Similar relationships also exist for the MRT of the interconversion metabolite after non-instantaneous input of the drug. The application of these equations to a non-linear reversible metabolic system is illustrated with computer simulations.