Discovery of a 4'-Azido Fleximer Nucleoside Analog With Broad-Spectrum Antiviral Activity Against DENV, EBOV, and Other RNA Viruses.

The ongoing emergence and reemergence of RNA viruses from multiple viral families highlights the urgent need for broad-spectrum antiviral therapeutics. To address this issue, a series of 4'-azido fleximer nucleoside analogs were synthesized and evaluated for broad-spectrum antiviral activity against pathogens of epidemic/pandemic concern. One compound, KAD-039, featuring a 2', 3'-diisobutyrate-5'-ProTide prodrug, exhibited potent antiviral activity across flaviviruses, filoviruses, and coronaviruses, with EC50 values ranging from 1 to 10 µM and minimal cytotoxicity (CC50 > 50 µM). Pharmacokinetic evaluation of KAD-039 demonstrated high plasma stability in dog, monkey, and human plasma, with half-lives ranging from 19.9 to 28 h but exhibited poor stability in human liver microsomes (t1/2 = 6.1 min). To help elucidate the mechanism of action of KAD-039 against these viral families, computational molecular docking studies were performed using the expected triphosphate active form within the cap/GTP-binding pockets of several viral methyltransferases and showed that KAD-039TP docked into all of the viral MTases with similar or higher predicted affinity compared to the natural substrate. In addition, KAD-039TP displayed reduced predicted affinity for the human N7 methyltransferase, suggesting low host toxicity. These results highlight 4'-azido fleximer analogs as promising broad-spectrum antivirals, potentially functioning through viral methyltransferase inhibition, and support further investigation.