5-Methoxymethyl-2′-deoxycytidine (MMdCyd) is an anti-metabolite with selective anti-herpes activity and low cytotoxicity. MMdCyd is dependent upon initial activation by the viral-induced deoxythymidine-deoxycytidine (dThd/dCyd) kinase for its activity against herpes simplex virus (HSV). Antiviral activity of MMdCyd is cell-dependent and is influenced by the deaminase content of the cell line used for assays. The antiviral potency against HSV-1 in this study was higher in RK-13 cells (ED50 3–5 μm) than in Vero and HEP-2 cells (ED50 14–26 μm). The potency of MMdCyd increased approximately 20-fold against HSV-1 and twofold against HSV-2 in the presence of tetrahydrodeoxyuridine (H4dUrd; which inhibits both dCyd deaminase and dCMP deaminase) in Vero cells. MdCyd in combination with H4dUrd was effective in preventing the cytopathogenic effect of HSV-1 and decreasing the production of infectious virus particles. The IC99 (concentration required to reduce the yield of infectious virus obtained 72 h after infection by 99% relative to control cultures) was 1.6 μm. In combination with tetrahydrouridine (H4Urd; an inhibitor of Cyd/dCyd deaminase) the potency of MMdCyd was only slightly enhanced (ED50 7–8 μm). Dihydrodeoxyuridine and deoxyuridine reversed the antiviral activity of MMdCyd. The minimum cytotoxic concentration for rapidly dividing cells (RK-13, HEP-2 and Vero) for MMdCyd was greater than 3 mm. H4Urd and H4dUrd were devoid of cytotoxicity and antiviral activity up to 2.12 mm (the highest concentration tested). Diacetyl-MMdCyd (pro-drug form) was approximatewly 20 times less potent than MMdCyd.

5-Methoxymethyl-2' -deoxycytidine (MMdCyd) is a selective anti-herpes agent that is dependent upon initial activation by herpes simplex virus (HSV}-induced deoxythymidine/deoxycytidine kinase. 5-Methoxymethyl-2' -deoxycytidine triphosphate (MMdCTP) was synthesized. The nature of the interaction of MMdCTP and dCTP with the DNA polymerase of Escherichiacoli, HSV-1 and human DNA polymerase ex was determined using specific and optimized assay conditions for each enzyme. MMdCTP was a better substrate for HSV-1 DNA polymerase compared to dCTP. At a nucleotide concentration of 10f.LM MMdCTP utilization was 130% that of an equimolar concentration of dCTP. Under similar conditions, human DNA polymerase ex utilized MMdCTP about as efficiently as dCTP. E. coli DNA polymerase I preferentially utilized dCTP. The IC so values of MMdCTP were 8 x 10-7M and 29 x 10-7M for HSV-1 and human ex DNA polymerase, respectively. MMdCTP is a competitive inhibitor of HSV-1 DNA polymerase with respect to dCTP incorporation (K; = 3.8 x 10-7M). Preferential utilization of MMdCTP and its eventual incorporation into HSV DNA seems to account for the antiviral action ofMMdCyd.

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