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

The interaction of N-(phosphonacetyl)-L-aspartic acid (PALA) with L-aspartate transcarbamoylase (ATCase), the putative taget for the antineoplastic activity of this drug, has been studied in the blood of patients participating in a phase I tral of PALA. ATCase activity in human blood is most abundant in granulocytes and lymphocytes; comparatively little activit is seen in erythrocytes. Utilizing peripheral leukocytes from patients given infusions of PALA, we find that leukocyte ATCae is inhibited rapidly and strongly. After cessation of therapy the rate of restitution of enzyme activity is slow: half-aximal restoration is achieved in about 280 hours. As a correlate of this gradual recovery of enzymatic activity, nanomolr concentrations of PALA are detectable in the plasma 2 weeks after infusion. The apparent K ₁ of PALA for leukocyte ATCase ith carbamoyl phosphate as the variable substrate is 5nM. Uptake of PALA into leukocytes in vitro is saturable and occursat a moderate rate comparable to that measured in murine tumor cells. Correspondingly, inhibitory concentrations of PALA ~ 10 ^-7 M) are found in the leukocytes of patients throughout the course of PALA treatment. It is concluded that although eukocytes are not targets for PALA toxicity, they may serve as accessible and pertinent indicators of the enzymatic effecs of this new oncolytic drug.