The U.S. Food and Drug Administration (FDA) Center for Drug Evaluation and Research (CDER) has accepted the Biologics License Application (BLA) for nirsevimab for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in newborns and infants entering or during their first RSV season and for children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season. Nirsevimab is being developed jointly by Sanofi and AstraZeneca and, if approved, would be the first protective option for the broad infant population, including those born healthy, at term or preterm, or with specific health conditions. The FDA has indicated they will work to expedite their review.

The Prescription Drug User Fee Act date, the FDA target action date for their decision, is in the third quarter of 2023. About the clinical trials The Phase 2b trial was a randomized, placebo-controlled trial designed to measure the efficacy of nirsevimab against medically attended LRTI through 150 days post-dose. Healthy preterm infants of 29–<35 weeks' gestation (n= 1,453) were randomized (2:1) to receive a single 50mg intramuscular injection of nirsevimab (n= 969) or placebo (n= 484) at the RSV season start.

The primary endpoint was met, significantly reducing the incidence of medically attended LRTI caused by RSV by 70.1% (95% CI: 52.3, 81.2) compared to placebo. The proposed dosing regimen was recommended based on further exploration of the Phase 2b data. When considering the dosing regimen recommended for approval in this study, nirsevimab reduced the incidence of medically attended LRTI caused by RSV by 86.2% (95% CI: 68.0, 94.0) in gestational age =29 to <35 weeks.

The subsequent Phase 3 study, MELODY, applied the recommended dosing regimen. The Phase 3 MELODY (primary cohort) trial was a randomized, placebo-controlled trial conducted across 21 countries designed to determine efficacy of nirsevimab against medically attended LRTI due to RSV confirmed by reverse transcriptase polymerase chain reaction testing through 150 days after dosing, versus placebo, in healthy late preterm and term infants (35 weeks gestational age or greater) entering their first RSV season. The primary endpoint was met, significantly reducing the incidence of medically attended LRTI, such as bronchiolitis or pneumonia, caused by RSV compared to placebo. Infants were randomized (2:1) to receive a single 50mg (in infants weighing <5kg) or 100mg (in infants weighing =5kg) intramuscular injection of nirsevimab or placebo.

Following the analysis of the initial 1,490 infants within the MELODY primary cohort additional infants continued to be enrolled. A total of 3,012 healthy late preterm and term infants (35 weeks gestational age or greater) entering their first RSV season were randomized to receive nirsevimab (n=2,009) or placebo (n=1,003). In the exploratory analysis, a single 50mg (in infants weighing <5kg) or 100mg (in infants weighing =5kg) intramuscular injection of nirsevimab reduced the incidence of medically attended LRTI caused by RSV through 150 days after dosing by 76.4% (95%: CI 62.3, 85.2) compared to placebo, with an acceptable safety profile.

Further, nirsevimab demonstrated a 76.8% (95%: CI 49.4, 89.4) reduction in the risk of RSV LRTI with hospitalization through 150 days after dosing compared to placebo. MEDLEY was a Phase 2/3, randomized, double-blind, palivizumab-controlled trial with the primary objective of assessing safety and tolerability for nirsevimab in preterm infants and infants with congenital heart disease (CHD) and/or chronic lung disease of prematurity (CLD) eligible to receive palivizumab. Between July 2019 and May 2021, approximately 918 infants entering their first RSV season were randomized to receive a single 50mg (in infants weighing <5kg) or 100mg (in infants weighing =5kg) intramuscular injection of nirsevimab or palivizumab.

Safety was assessed by monitoring the occurrence of treatment emergent adverse events (TEAEs) and treatment emergent severe adverse events (TESAEs) through 360 days post-dose. Serum levels of nirsevimab following dosing (on day 151) in this trial were comparable with those observed in the Phase 3 MELODY trial, indicating similar protection in this population to that in the healthy term and late preterm infants is likely. The safety profile of nirsevimab was similar to palivizumab in the MEDLEY Phase 2/3 and consistent with the safety profile in term and preterm infants studied in the MELODY Phase 3 trial compared to placebo.

The most commonly reported adverse reactions were: rash 14 days post-dose, (the majority of which were mild to moderate); pyrexia (fever) within 7 days post-dose; non-serious injection site reactions within 7 days post-dose. Findings from the nirsevimab clinical trial program included a pre-specified pooled analysis of the Phase 3 MELODY trial (primary cohort) and the recommended dose from the Phase 2b trial, in which a relative risk reduction of 79.5% versus placebo (95% CI 65.9, 87.7; P<0.0001) was seen against medically attended LRTI, such as bronchiolitis or pneumonia, caused by RSV in infants born at term or preterm entering their first RSV season. This analysis also showed a relative risk reduction of 77.3% (95% CI 50.3, 89.7; P<0.001) against RSV LRTI hospitalizations.