- 5-Fluoro-2-deoxycytidine (FdCyd), a fluoropyrimidine nucleoside analog, has a short (10-minute) half-life and is rapidly degraded in vivo by cytidine deaminase. However, coadministration with tetrahydrouridine (THU), an inhibitor of cytidine/deoxycytidine deaminase, has been shown to increase the AUC of the parent compound more than 4-fold. Increased FdCyd exposure allows it to be taken up intracellularly and converted to its triphosphate, which is incorporated into DNA and inhibits the action of the enzyme DNA methyltransferase (DNMT). Inhibition of DNMT, and in turn DNA methylation, can result in the re-expression of tumor suppressor genes.