Zalcitabine is an analog of pyrimidine derived from deoxycytidine with the replacement of the hydroxyl group in position 3’ with a hydrogen. In cells, it is phosphorylated to the active triphosphate form, ddCTP, which acts as a substrate for HIV reverse transcriptase (Ki = 51 nM). It incorporates into viral DNA where it terminates chain elongation when the missing hydroxyl group is encountered. Zalcitabine was the third antiretroviral approved by the FDA for treatment of HIV infection and AIDS.
