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Highlights
- Sporadic drug resistance was detected in clade 2.3.4.4 b HPAI A (H5N1) viruses isolated from birds in the US, 2022.
- Investigational oral neuraminidase inhibitor AV5080 was most effective at reducing virus enzyme activity.
- Dual mutation N295S + T438N in the neuraminidase conferred reduced inhibition by all neuraminidase inhibitors tested.
- Cap-dependent endonuclease inhibitors baloxavir and AV5116 showed similar antiviral activity in cell culture.
- First report of A (H5N1) virus with PA-I38T and a cluster of PA-A37T viruses displaying reduced baloxavir susceptibility.
Abstract
Clade 2.3.4.4 b highly pathogenic avian influenza (HPAI) A (H5N1) viruses that are responsible for devastating outbreaks in birds and mammals pose a potential threat to public health. Here, we evaluated their susceptibility to influenza antivirals. Of 1015 sequences of HPAI A (H5N1) viruses collected in the United States during 2022, eight viruses (∼0.8%) had a molecular marker of drug resistance to an FDA-approved antiviral: three adamantane-resistant (M2-V27A), four oseltamivir-resistant (NA-H275Y), and one baloxavir-resistant (PA-I38T). Additionally, 31 viruses contained mutations that may reduce susceptibility to inhibitors of neuraminidase (NA) (n = 20) or cap-dependent endonuclease (CEN) (n = 11). A panel of 22 representative viruses was tested phenotypically.
Overall, clade 2.3.4.4 b A (H5N1) viruses lacking recognized resistance mutations were susceptible to FDA-approved antivirals. Oseltamivir was least potent at inhibiting NA activity, while the investigational NA inhibitor AV5080 was most potent, including against NA mutants. A novel NA substitution T438N conferred 12-fold reduced inhibition by zanamivir, and in combination with the known marker N295S, synergistically affected susceptibility to all five NA inhibitors. In cell culture-based assays HINT and IRINA, the PA-I38T virus displayed 75- to 108-fold and 37- to 78-fold reduced susceptibility to CEN inhibitors baloxavir and investigational AV5116, respectively. Viruses with PA-I38M or PA-A37T showed 5- to 10-fold reduced susceptibilities. As HPAI A (H5N1) viruses continue to circulate and evolve, close monitoring of drug susceptibility is needed for risk assessment and to inform decisions regarding antiviral stockpiling.
Published July 24, 2023 in Antiviral Research:
