94. Pooled analysis of nirsevimab resistance through 150 days post dose in preterm and term infants

BACKGROUND: Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract infection (LRTI) and hospitalization in infants. In two global pivotal placebo-controlled studies, nirsevimab, a monoclonal antibody to the RSV prefusion (F) protein with extended half-life, reduced medically attended (MA) RSV LRTI versus placebo throughout the RSV season (MELODY (Primary Cohort)/Study 3 (Proposed Dose) Pool, 79.5% efficacy). Here we summarize resistance analyses of all RT-PCR-confirmed RSV isolates from healthy term and preterm infants through 150 days post dose. METHODS: Infants were randomized 2:1 to receive one intramuscular injection of nirsevimab or placebo, prior to their first RSV season. RT-PCR-confirmed RSV isolates were reflexed for genotypic analyses of RSV F and phenotypic analyses of identified substitutions in a recombinant RSV neutralization susceptibility assay. RESULTS: In the pooled proposed dose analysis of Study 3 (50 mg nirsevimab if < 5 kg at dosing) and MELODY (50 or 100 mg nirsevimab if < 5 kg or ≥5 kg at dosing, respectively), no subject with MA RSV LRTI had an RSV isolate containing nirsevimab resistance-associated substitutions in either treatment group (nirsevimab, RSV A: 0/14 and RSV B: 0/5; placebo, RSV A: 0/35 and RSV B: 0/16). In Study 3 (50 mg nirsevimab if ≥5 kg at dosing), 2/18 subjects in the nirsevimab group and 0/20 subjects in the placebo group with MA RSV LRTI had an RSV isolate harbouring nirsevimab binding site substitutions I64T+K68E+I206M+Q209R (>447-fold) or N208S (>387-fold) that conferred reduced susceptibility to nirsevimab neutralization (nirsevimab, RSV A: 0/9 and RSV B: 2/9; placebo, RSV A: 0/10 and RSV B: 0/10). Subjects with RSV isolates harboring F protein sequence variations that maintained susceptibility to nirsevimab neutralization were balanced between treatment groups with no association with RSV disease severity. No subjects with non-protocol defined MA RSV LRTI cases or hospitalization due to any RSV respiratory illness had an RSV isolate conferring nirsevimab resistance. CONCLUSION: Lack of nirsevimab resistance following immunization at the proposed dose supports efficacy and neutralization activity of nirsevimab against both RSV A and B strains throughout the RSV season. DISCLOSURES: Michael E. Abram, PhD, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Bahar Ahani, BSC, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds David E. Tabor, PhD, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Fiona Fernandes, PhD, AstraZeneca: Stocks/Bonds Deidre Wilkins, BSC, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Anastasia A. Aksyuk, PhD, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Kevin M. Tuffy, MS, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Hong Ji, BSc, AstraZeneca: Stocks/Bonds Christine Blaze, BSc, AstraZeneca: Stocks/Bonds Tyler Brady, MS, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Pamela Griffin, MD, AstraZeneca: Stocks/Bonds Amanda Leach, MD, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Tonya L. Villafana, PhD, MPH, AstraZeneca: Employee|AstraZeneca: Stocks/Bonds Mark T. Esser, PhD, AstraZeneca: Stocks/Bonds.