MSD preparing to take on Beyfortus with RSV antibody

New phase 2b/3 trial results with MSD's experimental respiratory syncytial virus (RSV) antibody clesrovimab have fired a shot across the bows of AstraZeneca and Sanofi, currently predicting blockbuster sales for rival Beyfortus.

In the study, clesrovimab (MK-1654) significantly reduced the incidence of RSV disease and hospitalisations in healthy preterm and full-term infants, putting MSD in a position to potentially file the drug and get approval ahead of the 2025-26 RSV season.

The company said clesrovimab could become the first therapy to protect infants with the same single dose, regardless of their weight, for the duration of their first RSV season. It teased the positive results in July.

Along with Beyfortus (nirsevimab), it is also a potential rival to Pfizer's RSV vaccine Abrysvo, which can be given to women during pregnancy to protect their babies for the first six months after birth.

Globally, RSV infection is a leading cause of hospitalisation for otherwise healthy infants under one year of age.

The 3,600-subject MK-1654-004 trial reported at the IDWeek 2024 congress in Los Angeles tested a single dose of clesrovimab given to healthy preterm and full-term infants from birth up to one year of age.

The antibody achieved a 60% reduction in the incidence of RSV-associated medically attended lower respiratory infections (RTIs) compared to placebo over five months of follow-up, the trial's primary endpoint, whilst also cutting overall RSV hospitalisations by 84%.

Two other secondary endpoints – hospitalisation prompted by RSV lower respiratory infections (LRI) and RSV-associated medically attended lower respiratory infections (MALRI) – were reduced by 91% and 92%, respectively, at the same timepoint.

The efficacy stacks up well against Beyfortus, which reduced hospitalisations by 85% to 90% in its pivotal trials – with the usual caveat about trying to compare trials with different designs. It looks a little stronger on some endpoints, notably MALRI, which was reduced by around 75% with Beyfortus.

The chief executive of MSD (known as Merck & Co in the US and Canada), Robert Davis, said recently that the company is "moving swiftly to bring this important option to market."

Beyfortus made almost $600 million in its first full year on the market in 2023, even though it was held back by manufacturing capacity constraints, and AZ and Sanofi are predicting it will cross the $1 billion threshold this year, helped by new manufacturing capacity including two new filling lines.

Pfizer recorded $890 million in Abrysvo sales last year, although most of that came from its use to prevent RSV infections in older adults, a much larger market.

RSV infections lead to more than three million hospitalisations and almost 60,000 deaths in children under five years of age every year, with around half of fatalities in infants less than six months.

Photo by Yousef Espanioly on Unsplash