WASHINGTON – A single dose of a novel monoclonal antibody against a respiratory syncytial virus surface protein safely protected preterm infants against severe infections for 150 days during their first winter season in a randomized trial with more than 1,400 children.

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One intramuscular injection of nirsevimab (also known as MEDI8897) administered to infants born at 29-35 weeks’ gestation at the start of the local respiratory syncytial virus (RSV) season (November in the Northern hemisphere) led to a 70% relative reduction in the rate of medically attended lower respiratory tract infections with RSV during the subsequent 150 days, compared with placebo, the study’s primary efficacy outcome, M. Pamela Griffin, MD, said at an annual scientific meeting on infectious diseases.

In a secondary efficacy measure, the rate of hospitalizations for RSV-caused lower respiratory tract infections, a single injection of nirsevimab dropped the incidence by 78%, relative to placebo. Both effects were statistically significant. The rate of total adverse events and serious adverse events was similar in the two treatment arms, reported Dr. Griffin, a clinical development lead with AstraZeneca.

These positive results for a single intramuscular injection of nirsevimab are the first findings from a series of studies aimed at getting the monoclonal antibody onto the U.S. market as a superior alternative to palivizumab (Synagis), which acts in a similar way to block RSV infection (albeit by targeting a different viral surface protein) but which requires administration every 30 days. This need for serial dosing of palivizumab in children younger than 1 year old for complete seasonal protection against RSV is probably a reason why the American Academy of Pediatrics, as well as other medical societies, have targeted using palivizumab only on certain types of high-risk infants: those born before 29 weeks’ gestational age, with chronic lung disease of prematurity, or with hemodynamically significant congenital heart disease (Pediatrics. 2014 Aug;134[2]:415-20). “It’s not feasible for most infants to come for five treatments during RSV season,” Dr. Griffin noted. A tweak in the structure of nirsevimab gives it a much longer blood half-life than palivizumab and allows a single dose to maintain efficacy for 5 months, the duration of RSV season.

“The big advantage of nirsevimab is one dose instead of five,” she said in an interview.

The study randomized 969 preterm infants to nirsevimab and 484 to placebo when the children averaged 3 months old and 4.5 kg. The incidence of the primary endpoint was 2.6% in the nirsevimab-treated infants and 9.5% in those who received placebo. The incidence of hospitalizations associated with an RSV lower respiratory tract infection was 0.8% in the nirsevimab group and 4.1% on placebo. Nirsevimab was equally effective regardless of RSV subtype, infant age, or sex. The rate of hypersensitivity reactions was low, less than 1%, and similar in the two treatment arms, as was the rate of detection of antidrug antibody, 3.8% with placebo and 5.6% with nirsevimab.

Two other large trials are underway to document the performance of nirsevimab in other types of infants. One study is examining the drug’s performance compared with placebo in term infants with a gestational age of at least 36 weeks, while another is comparing nirsevimab against a five-dose regimen of palivizumab in high-risk infants who are recommended to receive palivizumab by local medical societies. In the United States, this would be infants born at less than 29 weeks’ gestation, and those with either hemodynamically significant congenital heart disease or chronic lung disease of prematurity. In these studies, the researchers also will assess the cost effectiveness of nirsevimab relative to the costs for medical care needed by infants who receive comparator treatments, Dr. Griffin said.

The study was funded by AstraZeneca, the company developing nirsevimab. Dr. Griffin is an employee of and shareholder in AstraZeneca.

mzoler@mdedge.com

SOURCE: ClinicalTrials.gov identifier: NCT02878330.

One monoclonal dose gives preterm neonates season-long RSV protection