Therapeutic utility of utilizing low doses of N-(phosphonacetyl)-L-aspartic acid in combination with 5-fluorouracil: a murine study with clinical relevance.

Doses of N-(phosphonacetyl)-L-aspartic acid (PALA) lower than those required for therapeutic activity against the spontaneous murine breast tumor (BALB/c X DBA/8 F1) were found to produce significant depression in uridine triphosphate pools in the tumor. Using such low, nontherapeutic, but biochemically active doses of PALA in combination with 5-fluorouracil (FUra(, it was possible to maintain the dose of FUra at its full maximum tolerated single agent dose. In comparison with the maximum tolerated dose of FUra alone, the combination of FUra plus low-dose PALA produced a significant increase in the level of tumor FUra-containing RNA, only a slight increase in intestinal FUra-containing RNA, and no increase in bone marrow FUra-containing RNA. These biochemical results correlated with the therapeutic findings of significantly increased antitumor activity without an increase in host toxicity. A review of currently reported PALA plus FUra clinical protocols reveals that the experimental parameters which have produced successful therapeutic results in the laboratory (i.e., a low-PALA:high-FUra dosage ratio) have not yet been translated into clinical trial. Final judgment on the clinical efficacy of PALA:FUra combinations must await the results of proposed low-PALA:high-FUra clinical trials.