Peripheral leukocytes as indicators of the enzymatic effects of N-(phosphonacetyl)-L-aspartic acid (PALA) on human L-aspartate transcarbamoylase (ATCase) activity

Abstract

The interaction of N-(phosphonacetyl)-L-aspartic acid (PALA) with L-aspartate transcarbamoylase (ATCase), the putative target for the antineoplastic activity of this drug, has been studied in the blood of patients participating in a phase I trial of PALA. ATCase activity in human blood is most abundant in granulocytes and lymphocytes; comparatively little activity is seen in erythrocytes. Utilizing peripheral leukocytes from patients given infusions of PALA, we find that leukocyte ATCase in inhibited rapidly and strongly. After cessation of therapy the rate of restitution of enzyme activity is slow: half-maximal restoration is achieved in about 280 hours. As a correlate of this gradual recovery of enzymatic activity, nanomolar concentrations of PALA are detectable in the plasma 2 weeks after infusion. The apparent Ki of PALA for leukocyte ATCase with carbamoyl phosphate as the variable substrate is 5 nM. Uptake of PALA into leukocytes in vitro is saturable and occurs at a moderate rate comparable to that measured in murine tumor cells. Correspondingly, inhibitory concentrations of PALA (approximately 10(-7) M) are found in the leukocytes of patients throughout the course of PALA treatment. It is concluded that although leukocytes are not targets for PALA toxicity, they may serve as accessible and pertinent indicators of the enzymatic effects of this new oncolytic drug.