Modulation of target enzyme associated with the action of antifolates.

The cytotoxicity and molecular effects of antifolate thymidylate synthase inhibitor, ICI-D1694, against human ileocecal carcinoma, were evaluated. The drug concentration for 50% inhibition of cell growth by ICI-D1694 is 73 nM and 3 nM following 2 hr and 72 hr exposure, respectively. The drug induces high level of DNA single strand breaks in a time dependent manner, but subsequent to maximum inhibition of thymidylate synthase. Drug effects can be reversed by thymidine and leucovorin at > 1 microM concentrations. Leucovorin action is primarily at the cell membrane level, competing with the transport and activation of ICI-D1694. Thymidine, however, exerts its competitive effect primarily at the level of thymidylate synthase.