Acute and Delayed Toxicity Studies on the Antiherpesvirus Agents 5-Methoxymethyl-2′-Deoxycytidine and 5-Methoxymethyl-2′-Deoxyuridine

5-Methoxymethyl-2′-deoxycytidine (MMdCyd) and the corresponding deoxyuridine analogue, 5-methoxymethyl-2′-deoxyuridine (MMdUrd) are selective antiherpesvirus agents. MMdCyd (ED50 1.5 μM) is a more potent inhibitor of herpes simplex virus replication than MMdUrd (ED50 30 μM) when maintained in the deoxycytidine form (deamination prevented). The 5′-triphos-phates, MMdCTP and MMdUTP, were synthesized, and incorporation into DNA by mitochondrial DNA polymerase γ was investigated. MMdCTP and MMdUTP were incorporated into DNA in place of dCTP and dTTP, respectively. The effect of MMdCyd and MMdUrd on cell growth (acute toxicity) and prolonged exposure (delayed cytotoxicity) in CEM cells was investigated. The two analogues did not exhibit acute or delayed toxicity (2 weeks exposure) up to 1000 μM. In contrast, at a concentration as low as 0.125 μM of 2′,3′-dideoxycytidine (ddC; control drug), the doubling time of the cells increased after 10 days. At higher concentrations, a very marked increase in doubling time was observed from 6 days onward with ddC treatment. The data suggest that in uninfected cells neither MMdUrd nor MMdCyd are anabolized to the triphosphate form in significant amounts. As a result, little or no MMdCTP or MMdUTP builds up in the mitochondria and thus delayed toxicity is not observed.