Broad anti-herpesviral activity of α-hydroxytropolones

Herpesviruses are ubiquitous in animals and cause economic losses concomitant with many diseases. Most of the domestic animal herpesviruses are within the subfamily Alphaherpesvirinae, which includes human herpes simplex virus 1 (HSV-1). Suppression of HSV-1 replication has been reported with alpha-hydroxytropolones (alpha HTs), aromatic ring compounds that have broad bioactivity due to potent chelating activity. It is postulated that aHTs inhibit enzymes within the nucleotidyltransferase superfamily (NTS). These enzymes require divalent cations for nucleic acid cleavage activity. Potential targets include the nuclease component of the herpesvirus terminase (pU(L)15C), a highly conserved NTS-like enzyme that cleaves viral DNA into genomic lengths prior to packaging into capsids. Inhibition of pU(L)15C activity in biochemical assays by various aHTs previously revealed a spectrum of potencies. Interestingly, the most potent anti-pU(L)15C alpha HT inhibited HSV-1 replication to a limited extent in cell culture. The aim of this study was to evaluate three different aHT molecules with varying biochemical anti-pU(L)15C activity for a capacity to inhibit replication of veterinary herpesviruses (BoHV-1, EHV-1, and FHV-1) and HSV-1. Given the known discordant potencies between anti-pU(L)15C and HSV-1 replication inhibition, a second objective was to elucidate the mechanism of action of these compounds. The results show that aHTs broadly inhibit herpesviruses, with similar inhibitory effect against HSV-1, BoHV-1, EHV-1, and FHV-1. Based on immunoblotting, Southern blotting, and real-time qPCR, the compounds were found to specifically inhibit viral DNA replication. Thus, aHTs represent a new class of broadly active anti-herpesviral compounds with potential veterinary applications.