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 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.

Serum uridine levels in patients receiving N-(phosphonacetyl)-L-aspartate

Serum uridine levels of N-(phosphonacetyl)-L-aspartate-treated patients from Phase 1 and continuing trials receiving 1000 to 2000 mg/sq m/day were measured by reverse-phase high-pressure liquid chromatography. For the five patients studied, the predose serum levels ranged from 2.6 to 64. microM, well within the normal range of 1.9 to 8.9 microM. All serum uridine levels decreased in the first 24 hr, but the drop was quite variable (7 to 65%). Uridine levels in all five patients remained being 37 to 85% at differing time points. The only patient to show a daily decrease was the only responder and the patient with the largest decrease. N-(Phosphonacetyl)-L-aspartate appears to have a limited reductive effect on human serum uridine levels.

Application of a simple competitive protein-binding assay technique to the pharmacokinetics of N-(phosphonacetyl)-L-aspartate in humans

A competitive protein-binding assay for N-(phosphonacetyl)-L-aspartate (PALA) using aspartate transcarbamylase as the receptor protein and [14C]PALA as the radioactive ligand is described here and has been applied to study the pharmacokinetics of PALA in humans. A protein-free ultrafiltrate of plasma, prepared by centrifugation of 1-ml samples through Amicon Centriflo membrane cones, was used in the assay, which had a maximun sensitivity of 0.7 microM PALA in plasma. At this level, the coefficient of variation was less than 10%. Comparison of the competitive binding assay to a gas chromatographic-mass spectrometric technique shows that the two methods yield equivalent results in the concentration range of 1 microM to 1 mM. However, the competitive binding assay possesses practical advantages because of its simplicity and the ease with which multiple samples may be assayed. PALA disappearance from plasma was studied in seven patients and was found to be consistent with a two-compartment open model. The t1/2 alpha (elimination half-life for initial phase) and t1/2 beta (elimination half-life for terminal phase) were 0.93 +/- 0.73 (S.D.) hr and 4.82 +/- 1.48 hr, respectively. The cumulative urinary excretion of PALA int two patients was 70 and 90% of the administered dose 16 hr after the infusion was completed.

An overview of the clinical pharmacology of N-phosphonacetyl-L-aspartate (PALA), a new antimetabolite

N-Phosphonacetyl-L-aspartic acid (PALA) is new synthetic antimetabolite which inhibits de novo pyrimidine biosynthesis. Its significant activity against Lewis lung carcinoma, B16 melanoma, and glioma 26 suggested that it might be useful in the treatment of human solid tumors. Phase I trials revealed that dose-limiting toxicity included skin reactions, diarrhea, and stomatitis. Pharmacologic studies demonstrated rapid renal excretion of more than 70% of the unmetabolized drug in 24 h. Peak plasma levels correlated with dose of PALA administered. Partial responses to PALA were seen in one patient with melanoma, one with chondrosarcoma, and one with colon carcinoma. The potential for PALA's use in combination chemotherapy, particularly with 5-fluorouracil, is discussed.

Phase I trial of N-(phosphonacetyl)-L-aspartate

A Phase I clinical trial of N-(phosphonacetyl)-L-aspartate, an antimetabolite which inhibits a key enzyme in the de novo pathway of pyrimidine biosynthesis, was conducted. N-(Phosphonacetyl)-L-aspartate was given as an i.v. 15-min infusion once daily for five days; cycles of treatment were repeated every three weeks. Thirty-four patients received treatment. Dose-limiting toxicity was observed at 1500 to 2000 mg/sq m/day and was manifested by skin rash, diarrhea, and stomatitis. Rash and diarrhea usually began during the first week of treatment and persisted up to Day 17 of a cycle of therapy. No consistent hematopoietic, hepatic, or renal toxicity was observed. One partial response in a patient with colon carcinoma was seen and continues at more than eight months. Stable disease was observed in three patients with colon carcinoma, two patients with hypernephroma, one patient with pancreatic carcinoma, and one patient with melanoma. The predictability and reversibility of toxicity and the suggestion of antitumor activity in humans are observations which support the further evaluation of N-(phosphonacetyl)-L-aspartate in Phase II studies.

Ectopic production of human chorionic gonadotropin: a case report

The ectopic production of the beta-subunit of human chorionic gonadotropin (hCG) is described in a patient with an anaplastic carcinoma. After chemotherapy the marker decreased in a logarithmic fashion to undetectable levels but the neoplasm progressed and the patient died. The specificity of the beta-subunit of hCG is discussed. Discordance of the marker and clinical disease is pointed out, and several possible explanations are outlined. The lack of specificity of the beta-subunit of hCG and the discordance that it may exhibit means that its use in diagnosing and following disease progression may be limited.