From Synthesis to Efficacy: Synthesis, Structural Analysis, and Antiviral Activity of 5‐(Trifluoromethoxy)‐ <scp> 1 <i>H</i> </scp> ‐indole‐2,3‐dione 3‐Thiosemicarbazones

Viral infections are a global cause of serious morbidity and mortality. The recent COVID-19 pandemic further testifies to the continued need to develop novel classes of antiviral agents with broad activity and high selectivity. In this work, 5-(trifluoromethoxy)-1H-indole-2,3-dione 3-(4-phenylthiosemicarbazones) bearing a methyl or ethyl group at position 1- of the indole ring were synthesized. Along with previously synthesized compounds 5a-j, 5l, 6b-d, 6g, 6i, and 6k-n, the novel compounds 5k, 6a, 6e, 6f, 6h, 6j, and 6o were evaluated in cytopathic effect (CPE) reduction assays with a broad range of DNA and RNA viruses. Favorable activity was observed in human embryonic lung (HEL) fibroblast cells infected with herpes simplex virus type 1 (HSV-1, wild type or thymidine kinase-deficient) and type 2, or with vaccinia virus (VV). In general, the ethyl substituted compounds (6a-o) had higher antiviral activity and lower cytotoxicity than the methyl substituted analogues (5a-l). The antiviral potency was markedly enhanced by substitution at position 4- of the phenyl ring, with the analogues bearing a 4-methyl (6c), 4-fluoro (6l), or 4-chloro (6n) moiety exhibiting average EC₅₀ values of 1.6 to 6.6 μM against HSV-1, HSV-2, and VV, besides selectivity indices (i.e., ratio of cytotoxic over antiviral concentration) of 21-40. Within the series bearing a substituent at position 3-, the trifluoromethyl analogue 6d was unique in suppressing human adenovirus type 2 (AdV-2) and coronavirus HCoV-229E. To conclude, our study highlights the versatility and relevance of the 5-(trifluoromethoxy)-1H-indole-2,3-dione 3-(4-phenylthiosemicarbazones) for developing antiviral agents against different virus families.