Saturable urinary excretion kinetics of famotidine in the dog

An important elimination route of the histamine H2 antagonist famotidine is active tubular secretion via the renal organic cation transport system. To characterize the excretion kinetics of famotidine in-vivo, the relationship between plasma concentration and urinary excretion rate was investigated in the beagle dog over a wide concentration range. The maximum transport capacity and the apparent Michaelis-Menten constant of tubular secretion were estimated. Concentration-dependent renal clearance was determined either after intravenous infusion of high doses of famotidine for a short time or during continuous infusion. From individual experiments only indications of saturation were observed; these could not be quantified. A tubular titration curve, in which the active tubular famotidine secretion was plotted against the plasma concentration, was constructed from the data from all the experiments. Active tubular secretion was calculated for each experiment separately by subtracting the famotidine filtration rate from the total excretion rate. A tubular transport maximum of 2400 +/- 220 micrograms min-1 and an apparent Michaelis-Menten constant for tubular secretion of 26 +/- 4 micrograms mL-1 (76 +/- 12 microM) were estimated from the curve. To the best of our knowledge, this is the first time that saturation of famotidine renal clearance has been fully quantified in-vivo. Considering the low therapeutic plasma concentrations of famotidine (< 0.1 microgram mL-1), these results suggest that clinically the drug has a low interactive potential.