β-L-1-[5-(E-2-Bromovinyl)-2-(Hydroxymethyl)-1,3-Dioxolan-4-yl)] Uracil (L-BHDU) Inhibits Varicella Zoster Virus Replication by Depleting the Cellular dTTP Pool

ß-L-1-[5-(E-2-bromovinyl)-2-(hydroxymethyl)-1,3-(dioxolan-4-yl)] uracil (L-BHDU) inhibits varicella zoster virus (VZV) replication in cultured cells, human skin, and in SCID-Hu mice with skin xenografts. VZV thymidine kinase converts L-BHDU to monophosphate (MP) and diphosphate (DP) forms, but the triphosphate form was not detected in infected cells and the antiviral mechanism was unknown. Given its similar structure to uridine, we asked if L-BHDU interfered with viral DNA replication via inhibition of the purine and/or pyrimidine biosynthesis pathways. Addition of purines to the medium was unable to restore VZV replication in the presence of L-BHDU. In contrast, excess thymidine and uridine in proportion to L-BHDU restored VZV replication, suggesting that the active form of L-BHDU interfered with pyrimidine biosynthesis. However, addition of thymidine and uridine failed to restore VZV replication in non-dividing cells treated with L-BHDU. Like other herpesviruses, VZV infection increased thymidine triphosphate (dTTP) in confluent cells while L-BHDU treatment decreased the dTTP pool by nearly 4-fold. The active form(s) of L-BHDU did not interfere with cellular metabolism, suggesting viral target(s).