Ethynylcytidine is a nucleoside antimetabolite.

The cytotoxicity of Ethynylcytidine tends to become stronger as the exposure time becomes longer. The IC50 values of Ethynylcytidine in the five human tumors with 4, 24 and 72 h exposure range from 0.114 to 1.032 μM, 0.015 to 0.067 μM, and 0.008 to 0.058 μM, respectively. The differences in IC50 values between the 24 and 72 h exposure times are not large, and Ethynylcytidine appears to show sufficiently potent cytotoxicity at the 24 h exposure time in all 5 human tumors.

In both OCUM-2MD3 and LX-1 xenografts, tumor regression is noted and a very potent antitumor effect with a tumor growth inhibition rate on day 15 of approximately 90% or even higher is observed at the minimum toxic doses of Ethynylcytidine on all three administration schedules. In particular, the administration of Ethynylcytidine at 6 mg/kg once weekly exhibits a marked tumor-shrinking effect with an IR of 98% against the LX-1 tumor. While Ethynylcytidine treatment on an either 3 or 5 times weekly schedule has a potent antitumor effect with an IR of approximately 85%, the IR of Ethynylcytidine once weekly is less than 60% and its antitumor effect is rather weak.