N-(PHOSPHONACETYL)-L-ASPARTATE SYNERGISTICALLY ENHANCES THE CYTOTOXICITY OF 5-FLUOROURACIL INTERFERON-ALPHA-2A AGAINST HUMAN COLON-CANCER CELL-LINES

Recombinant interferon-alpha (IFN) enhances the cytotoxic effects of t he fluorinated pyrimidine, 5-fluorouracil (5FU), against two human col on cancer cell lines. The aspartate transcarbamylase (ATCase) inhibito r, N-(phosphonacetyl)-L-aspartate (PALA), was studied in combination w ith 5FU/IFN to determine whether further anti-pyrimidine effects would result in greater cytotoxicity. By median effects analysis PALA syner gistically augmented the cytotoxic effects of 5FU/IFN against both hum an colon cancer cell lines. This occurred in the absence of any effect s of 5FU/IFN on ATCase and without further potentiation of the PALA-me diated inhibition of ATCase. To explore the mechanism by which this in teraction occurred, detailed studies of pools of dNTPs were performed. Both 5FU/IFN and PALA/5FU/IFN treatments resulted in early (2-8 hr) d epletion of pools of dTTP, but no effects on pools of dCTP. PALA had n o effect on dTTP pools either alone or in the combination. In contrast , both PALA and PALA/5FU/IFN treatments resulted in later (12-24 hr) d epletion of pools of dCTP. 5FU/IFN treatment had no effect on these po ols. When pools of dCTP and dTTP were repleted by treatment with cytid ine or thymidine, 20 muM, however, there was only partial reversal of cytotoxicity induced by 5FU/IFN + PALA, suggesting that the synergy ob served did not result solely from a sequential anti-pyrimidine effect. The incorporation of 5FU into RNA was also studied; PALA enhanced the incorporation of [6-H-3]5FU into RNA by 83-150%, but not into DNA, su ggesting an alternative mechanism of drug interaction.