Synthetic α-Hydroxytropolones Inhibit Replication of Wild-Type and Acyclovir-Resistant Herpes Simplex Viruses

Abstract

cHerpes simplex virus 1 (HSV-1) and HSV-2 remain major human pathogens despite the development of anti-HSV therapeutics as some of the first antiviral drugs. Current therapies are incompletely effective and frequently drive the evolution of drug-resistant mutants. We recently determined that certain natural troponoid compounds such as ␤-thujaplicinol readily suppress HSV-1 and HSV-2 replication. Here, we screened 26 synthetic ␣-hydroxytropolones with the goals of determining a preliminary structure-activity relationship for the ␣-hydroxytropolone pharmacophore and providing a starting point for future optimization studies. Twenty-five compounds inhibited HSV-1 and HSV-2 replication at 50 M, and 10 compounds inhibited HSV-1 and HSV-2 at 5 M, with similar inhibition patterns and potencies against both viruses being observed. The two most powerful inhibitors shared a common biphenyl side chain, were capable of inhibiting HSV-1 and HSV-2 with a 50% effective concentration (EC 50 ) of 81 to 210 nM, and also strongly inhibited acyclovir-resistant mutants. Moderate to low cytotoxicity was observed for all compounds (50% cytotoxic concentration [CC 50 ] of 50 to >100 M). Therapeutic indexes ranged from >170 to >1,200. These data indicate that troponoids and specifically ␣-hydroxytropolones are a promising lead scaffold for development as anti-HSV drugs provided that toxicity can be further minimized. Troponoid drugs are envisioned to be employed alone or in combination with existing nucleos(t)ide analogs to suppress HSV replication far enough to prevent viral shedding and to limit the development of or treat nucleos(t)ide analog-resistant mutants.

Broad anti-herpesviral activity of α-hydroxytropolones

“…1. The 50% inhibitory activities (IC 50 ) against pU L 15C for the three synthetic αHTs have been previously measured and include 0.18 μM for αHT-106, 5.6 μM for αHT-111, and 49.1 μM for αHT-115 (Ireland et al, 2016; Masaoka et al, 2016). Similarly, the in vitro anti-HSV potencies of the three synthetic αHTs were previously determined.…”

“…Given the enzymes’ similar structure, inhibitors of the HIV RNase and integrase were recently screened for activity against HSV-1 and HSV-2 (Tavis et al, 2014). Tropolones, most notably natural and synthetic α-hydroxytropolones (αHTs), were found to be potent anti-HSV inhibitors (Ireland et al, 2016; Tavis et al, 2014). While tropolones are well known metalloenzyme inhibitors due to the high negative charge character on both the carbonyl and adjacent oxygen at physiological pH, αHTs provide an additional contiguous oxygen that makes them particularly good inhibitors of dinuclear metalloenzymes (Bentley, 2008; Piettre et al, 1997).…”

“…To investigate the nature of αHT HSV-1 inhibitory activity, one specific HSV-1 NTS-like enzyme, pU L 15C of the viral terminase, was assessed in the presence of several synthetic β-thujaplicinol derivatives through the analysis of pU L 15C-mediated hydrolysis of short DNA duplexes using dual-probe fluorescence (Masaoka et al, 2016). Interestingly, some of the most potent pU L 15C synthetic αHTs inhibited HSV-1 replication only modestly, while those that strongly inhibited HSV-1 replication had poor anti-terminase activity (Ireland et al, 2016; Masaoka et al, 2016). It follows that although αHTs have anti-pU L 15C activity in biochemical assays, other essential viral activities are likely responsible for their antiviral effects.…”

Troponoids Can Inhibit Growth of the Human Fungal Pathogen Cryptococcus neoformans

“…The ␣-hydroxytropolones are likely to inhibit at least in part through their ability to coordinate divalent cations in metalloenzyme active sites. They are effective inhibitors of HIV, HBV, and herpes simplex viruses, most likely via coordination of the Mg 2ϩ ions in the active sites of enzymes that are members of the nucleotidyl transferase superfamily (18,20,37). There are at least 40 proteins encoded by the C. neoformans genome that likely belong to this class of enzymes (38), and of the various ␣HTs tested, nine (no.…”

“…46), another natural product from T. plicata, has an additional hydroxyl group on the tropolone ring compared with ␤-thujaplicin, and it can inhibit the HIV RNase H (RNaseH) (16). More recently, ␤-thujaplicinol was shown to inhibit hepatitis B virus (HBV) replication by blocking the activity of the virally encoded RNaseH (17,18) and to inhibit herpes simplex virus replication (19,20). The ␣-hydroxytropolones (␣HTs) have a broad range of beneficial effects against other diseases, including malaria, bipolar disorder, and diabetes (21,22).…”