Evaluation of Broad Anti-Herpesviral Activity with a-Hydroxytropolones

Abstract

Herpesviruses are ubiquitous in animals and cause economic losses concomitant with many diseases, including upper respiratory disease, keratitis, abortion, neonatal death, and neurologic disease. The majority of the domestic animal herpesviruses are within the subfamily Alphaherpesvirinae, along with the prototypical human herpes simplex virus 1 (HSV-1). Suppression of HSV-1 replication has been reported with α-hydroxytropolones (αHTs), which are aromatic ring compounds that have broad bioactivity due to potent chelating activity. It is postulated that αHTs inhibit enzymes within the nucleotidyltransferase superfamily (NTS), similarly structured enzymes that require divalent cations for nucleic acid cleavage activity. One potential herpesviral target includes the nuclease of the viral terminase, a highly conserved NTS-like enzyme that cleaves the viral genome for packaging into capsids. Inhibition of the nuclease activity of the viral terminase (pUL15C) by αHTs previously revealed variable potencies, ranging from negligible to marked. Interestingly, the most potent anti-terminase nuclease αHT compounds had limited effect on inhibiting HSV-1 replication. The aim of this study was to evaluate three different αHT molecules with varying in vitro anti-terminase nuclease activity against veterinary herpesviruses (BoHV-1, EHV-1, FHV-1) and HSV-1 to assess for broad inhibitory activity. Additionally, given the discordant potencies between anti-pUL15C and HSV-1 inhibition, a second objective was to elucidate the mechanism of action of these compounds. The results of this research show that αHTs broadly inhibit herpesviruses, with similar inhibitory effect among HSV-1, BoHV-1, EHV-1, and FHV-1 with IC50 values ranging from 30 to ≤ 5 μM. Based on immunoblotting, Southern blotting, and real-time qPCR, the compounds were found to specifically inhibit DNA replication. Thus, αHTs may represent a new class of anti-herpesviral compounds.