Anti-(herpes simplex virus) activity of 4 '-thio-2 '-deoxyuridines: a biochemical investigation for viral and cellular target enzymes

The antiviral activity of several nucleoside analogues is often limited by their rapid degradation by pyrimidine nucleoside phosphorylases. In an atte mpt to avoid this degradation, several modified nucleosides have been synth esized. A series of 4'-thio-2'-deoxyuridines exhibits an anti-[herpes simpl ex virus (HSV)] activity significantly higher (20-600 times) than that show n by the corresponding 4'-oxy counterpart, We investigated the mode of acti on of these compounds and we found that: (i) several 4'-thio-2'-deoxyuridin es are phosphorylated to the mono- and diphosphates by HSV-1 thymidine kina se (TK) more efficiently than their corresponding 4'-oxy counterpart; (ii) both are inhibitors of cellular thymidylate synthase; (iii) 4'-thio-2'-deox y uridines are resistant to phosphorolysis by human thymidine phosphorylase ; (iv) both 4'-oxy- and 4'-thio-2'-deoxyuridines are phosphorylated to deox yribonucleotide triphosphate in HSV-1-infected cells and are incorporated i nto viral DNA; (v) 4'-thio-2'-deoxyuridines are better inhibitors than thei r 4'-oxy counterparts of [H-3]thymidine incorporation in HSV-l-infected cel ls; (vi) 4'-thio-2'-deoxyuridines are not recognized by HSV-1 and human ura cil-DNA glycosylases. Our data suggest that 4'-thio-2'-deoxyuridines, resis tant to pyrimidine phosphorylase, can be preferentially or selectively phos phorylated by viral TK in HSV-infected cells, where they are further conver ted into triphosphate by cellular nucleotide kinases. Once incorporated int o viral DNA, they are better inhibitors of viral DNA synthesis than their c orresponding 4'-oxy counterpart, either because they are not recognized, an d thus not removed, by viral uracil-DNA glycosylase, or because they prefer entially interfere with viral DNA polymerase.