Resistance to anti-orthopoxviral drug tecovirimat (TPOXX®) during the 2022 mpox outbreak in the US

Background: During the 2022 multinational outbreak of monkeypox virus (MPXV) clade IIb, the antiviral drug tecovirimat (TPOXX®) was deployed in the US on a large scale for the first time ever. The MPXV F13L gene homolog encodes the target of tecovirimat, and single amino acid changes in the F13 protein are known to cause resistance to tecovirimat in orthopoxviruses (OPXV). Methods: Whole genome metagenomic sequencing and amplicon-based sequencing targeting the F13L gene was used to identify nine mutations previously reported to cause resistance in other OPXV along with ten novel mutations that have been identified from the 2022 mpox outbreak. A cytopathic effect assay, previously established at CDC as part of WHO smallpox research, was adapted to MPXV for tecovirimat phenotype testing of virus isolated from mpox patients. Results: As of March 2023, in total, 70 isolates from 40 patients were tested, and 50 of these isolates from 26 patients were found to have a resistant phenotype. Most resistant isolates were associated with severely immunocompromised mpox patients on multiple courses of TPOXX treatment; while isolates with F13 mutations identified by routine surveillance of patients not treated with TPOXX have remained sensitive. Conclusions: These data indicate that tecovirimat resistance is developing in immunocompromised patients treated with TPOXX and that for isolates that we have analyzed, the frequency of resistant viruses remain relatively low (< 1%) compared to the total number of patients treated with TPOXX. These findings inform our understanding of when tecovirimat resistance is likely to occur and highlight the need for additional OPXV therapeutics. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any funding ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The activities in this report were reviewed by the Human Subjects Advisor in the National Center for Emerging and Zoonotic Disease at the Centers for Disease Control and Prevention and determined that it does not meet the regulatory definition of research under provision 45 CFR 46.102(l); the activities fall under public health surveillance and do not require IRB review. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors