Comparative physiological disposition of N-(phosphonacetyl)-L-aspartate in several animal species after intravenous and oral administration

The physiological disposition of N-(phosphonacetyl)-L-aspartate (NSC 224131; PALA), a potent inhibitor of aspartate transcarbamylase, has been studied in mouse, rat, dog, and monkey after administration of [14C]PALA at 120 mg/sq m i.v. or p.o. Concentrations of PALA equivalents in plasma, urine, and feces were determined radiochemically, and urine was analyzed chromatographically for PALA. The disposition of PALA equivalents in mouse tissues was determined radioautographically. After i.v. administration, PALA was rapidly (half-time, approximately 1 hr) and extensively (up to 80% of the dose) excreted in the urine of all species. Less than 5% was excreted in the feces. Only PALA was found in the urine of all four species, indicating that the metabolism of PALA, if it occurs at all, is insignificant. PALA equivalents were poorly taken up by mouse tumors and tissues, except kidney, bone, and to a lesser extent, skin and lung, and were rapidly and extensively cleared from all except bone. No differences were apparent in the uptake of PALA equivalents by Lewis lung carcinoma (sensitive to PALA treatment) and L1210 lymphocytic leukemia (insensitive). The pharmacokinetics of PALA in the plasma of rat, dog, and monkey, as well as mouse, were inconsistent with deposition of PALA in tissues and more consistent with the probable distribution of PALA into extracellular water. PALA equivalents were eliminate from all species at a rate (half-time, 1 to 1.5 hr) reflecting the rate of urinary excretion of the drug and at a secondary slower rate probably reflecting the rate of release of bound PALA from sites such as aspartate transcarbamylase. PALA was poorly absorbed into the systemic circulation when administered p.o., in that mouse, rat, and monkey excreted less than 5% of the dose in the urine after p.o. administration. These data on the physiological disposition of PALA explain why high doses of the drug have to be administered to achieve therapeutic and toxic effects, despite the inhibitory potency of the drug on aspartate transcarbamylase. They indicate that PALA will be ineffective administered p.o. and might be contraindicated in patients with impaired renal function and that the kinetics of aspartate transcarbamylase-bound drug is probably more important in determining dose scheduling than the kinetics of free PALA.