Evaluation of the Antiherpes Simplex Virus Activities of Thiourea-Based Compounds

The emergence of drug-resistant viruses, particularly in immunocompromised individuals, has necessitated the development of novel antiviral drugs. We previously identified compound 147B3 as an inhibitor of herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV). Although 147B3 exhibits cytotoxicity, it inhibits the function of infected cell protein 4 (ICP4), a viral transcription factor essential for HSV-1 replication. In this study, we evaluated five commercially available compounds (1C6, 1C6L, 1H6, 1H6L, and 2D10) that had structural similarity to 147B3, exhibited potent anti-HSV-1 activity with reduced cytotoxicity. Among these, 1C6L and 1H6L are structural analogs of 1C6 and 1H6, respectively. An HSV-1 strain resistant to 147B3 carrying a mutation in ICP4 exhibited resistance to all the five compounds, suggesting a shared mechanism of action involving ICP4. Among these compounds, 1H6L had the highest selective index against HSV-1. Notably, these compounds did not reduce early or late protein expression of HSV-1, unlike the parent compound 147B3. Although viral genome replication occurred in the presence of 1H6L, it prevented virion release into the culture supernatant. The cell fraction analysis revealed that 1H6L reduced the size of the cytoplasmic HSV-1 genome. These findings suggest that 1H6L shares certain aspects of its mechanism of action with the parent compound 147B3 but may also inhibit multiple steps in the HSV-1 life cycle.