Based on an animal model to improve the antitumor activity of 5-fluorouracil (FUra), a Phase I study of N-(phosphonacetyl)-l-aspartate, methotrexate, FUra, and leucovorin was conducted on 44 patients. Methotrexate was given in an intermediate dose (250 mg/m2) to overcome potential drug resistance, and N-(phosphonacetyl)-l-aspartate was given at a low dose (250 mg/m2) in order to allow escalation of FUra to toxicity. These two drugs were given 24 h before FUra to enhance maximal incorporation of FUra into RNA. Two schedules of administration were used; one every other week and one weekly for 2 weeks. The every other week schedule was well tolerated, with minimal gastrointestinal and hematological toxicity. However, the weekly for 2 weeks schedule was more toxic with increased mucositis, diarrhea requiring therapy, and decreased performance status of 20% in 4 of 6 patients. There were no responders in the every other week schedule. There was one partial response and three patients with stable disease in four evaluable patients on the weekly for 2 weeks schedule. At 24 h post-N-(phosphonacetyl)-l-aspartate-methotrexate treatment, PRPP levels were doubled in bone marrow biopsies, and increased 2.5- to 25-fold in tumor biopsies. We have currently added uridine rescue to this combination with the hope of further escalating the dose of FUra.
This work was supported in part by USPHS Grants N01-CM57732 and PO1-CA25842 awarded by the National Cancer Institute, Department of Health and Human Services, and in part by the Chemotherapy Foundation of New York.