ICI-D1694 AND IDOXURIDINE - A SYNERGISTIC ANTITUMOR COMBINATION

The cytotoxicity of idoxuridine (IdUrd), a thymidine analogue, and ICI D1694 (D1694), a folate-based thymidylate synthase (TS) inhibitor, we re examined individually and in combination in two human tumor cell li nes. MGH-U1 bladder cancer and HCT-I colon cancer cells were grown as monolayer cultures with and without thymidine. The cytotoxicity of the se agents alone and in combination were determined using normal human bone marrow colony-forming unit, granulocyte-macrophage (CFU-GM) as a surrogate for myelosuppression in vivo. Thymidylate synthase inhibitio n, IdUrd incorporation into DNA, and DNA single-strand breaks were mea sured in each cell line and related to cytotoxicity. The cytotoxicity of a 24-h exposure to IdUrd or D1694 increased with drug concentration in each cell line. The drug concentrations producing 50% and 10% clon ogenic survival in MGH-U1 cells, respectively, were 0.006 and 0.009 mu M for D1694 and 13.0 and 81.0 ELM for IdUrd. Those for HCT-L cells, r espectively, were 0.009 and 0.018 mu M for D1694 and 7.5 and 20.5 mu M for IdUrd. The cytotoxicity of IdUrd combined with D1694 was synergis tic in both MGH-U1 and HCT-8 cells as determined by median-effect anal ysis. The addition of thymidine at concentrations of 0.1, 0.3, and 1.0 mu M to the culture medium did not decrease the cytotoxicity of D1694 in either tumor cell line. TS inhibition using the whole cell assay w as observed with only D1694, producing 50% inhibition of TS activity a t 0.002 mu M for MGH-U1 and 0.007 mu M for HCT-8 cells. IdUrd did not inhibit TS activity, nor did it enhance the TS inhibitory effects of D 1694. The incorporation of IdUrd into DNA increased with increasing co ncentrations of D1694. This increased DNA incorporation correlated wit h the increase in DNA single-strand breaks. DNA single-strand breaks p aralleled cytotoxicity. CFU-GM survival, exposed to the same drug conc entrations as those used in the tumor cell lines, revealed that the th erapeutic index was greater for the combination than for either agent alone. These findings suggest that IdUrd plus D1694 is a promising new drug combination, which may have a favorable therapeutic index in viv o.