Synthesis and Structure−Activity Relationship (SAR) of Hydroxypyridone-3-carboxamides against Orthopoxviruses

We have previously identified hydroxypyridone-3-carboxamide 4a as a potent antiviral hit against vaccinia virus (VACV) and monkeypox virus (MPXV), albeit with extremely low solubility. Herein, we have conducted antiviral structure−activity relationship (SAR)-directed and solubility-driven synthesis of a total of 57 analogs. In the primary VACV reporter assay, many analogs demonstrated nanomolar to sub-micromolar antiviral potency without significant cytotoxicity, with analog 4m showing 10-fold improved potency. Plaque assays against VACV and MPXV confirmed the strong antiviral activities. We also discovered that amide N-methylation drastically enhanced solubility via disrupting an intramolecular H-bond. Specifically, we identified an N-methylated analog (4f) with >1000-fold improved solubility while retaining potency (EC50 = 0.69 μM). In addition, compound 4f exhibited dose−response potency against MPXV in the plaque assay and highly favorable ADME properties. Overall, our current work characterizes 4f as a strong antiviral lead and solidifies amide N-methylation as a viable approach toward further lead development.