BACKGROUND: Mpox virus (MPXV) is the causative agent of mpox, a disease characterized by skin lesions, and remains a global threat since the recent 2022 outbreak. Tecovirimat is an antiviral agent that inhibits MPXV replication in vitro and in vivo by targeting the viral phospholipase VP37 protein and blocking viral envelope wrapping. Although tecovirimat has been administered to many mpox patients, the emergence of tecovirimat-resistant MPXVs carrying mutations in the VP37 protein has been reported, particularly in immunocompromised individuals. However, the impact of these respective mutations on drug susceptibility and viral fitness have not been fully characterized. METHODS: Tecovirimat was applied as a selective pressure in vitro to isolate MPXV variants carrying clinically observed VP37 substitutions. Drug susceptibility among the major VP37 mutations was compared, and the cellular fitness of the isolated viruses was evaluated. RESULTS: The VP37-N267D and -Y258C mutants showed markedly reduced susceptibility to tecovirimat compared with the VP37-I372N and -T245I (+I7L-A388S) mutants, while retaining susceptibility to an alternative anti-MPXV agent, brincidofovir. The VP37-I372N, T245I (+I7L-A388S), Y258C, and N267D mutants exhibited viral fitness comparable to that of the parental virus in A549 and Vero cells, as well as in MRC-5 and HaCaT cells. CONCLUSIONS: These findings provide insights into the phenotype of each VP37 mutation and emphasize the importance of developing antivirals with distinct mechanisms of action to effectively combat tecovirimat-resistant MPXV.
Konishi et al. (2026) studied this question.