Distribution of [14C]aztreonam in rat tissue

[14C]aztreonam was administered intramuscularly (50 mg/kg) to male and female rats. Groups of 10 rats (five male and five female) were sacrificed at 0.25, 2, 6, and 24 h after dosing. Blood and various tissues were removed from six rats (three male and three female) in each group for determination of total radioactivity and unchanged aztreonam by liquid scintillation counting and thin-layer radiochromatography. The remaining rats were prepared for whole-body autoradiography. Radioactivity was distributed rapidly to most tissues and was rapidly eliminated from blood into urine and bile. Concentrations of total radioactivity in kidney, liver, small and large intestines and their contents, and urinary bladder were higher than those in serum at all the times examined. Concentrations of unchanged aztreonam in serum, kidney, liver, and lung declined at about the same rate as did that of total radioactivity in the same tissues. The results of whole-body autoradiography essentially confirmed the results of the distribution of [14C]aztreonam as determined by liquid scintillation counting. No major differences between males and females were observed when concentrations in organs common to both were compared.