Merck & Co.’s Clesrovimab Could Rival Sanofi/AZ’s Beyfortus In RSV

Merck & Co., Inc. hopes to secure a piece of the blockbuster-sized market for products that protect against respiratory syncytial virus (RSV) and appears closer to doing so based on positive Phase IIb/III data results for its prophylactic monoclonal antibody clesrovimab for infants. But the category is a competitive one, and rivals like Sanofi and Pfizer Inc. have had more time on the market to establish a commercial foothold.

The company announced on 23 July that a Phase IIb/III clinical trial evaluating clesrovimab (previously MK-1654) met its primary safety and efficacy endpoints, including reducing medically attended lower respiratory infections caused by RSV through Day 150. Merck only provided top-line results and said it would present detailed data at an upcoming scientific meeting.

The data will serve as the basis of regulatory filings with the US Food and Drug Administration and the European Medicines Agency and other authorities globally, the company said. Merck expects clesrovimab could be available in some markets in the second half of 2025, with the goal of being available for the 2025-2026 RSV season, the company told Scrip.

“We see an opportunity for clesrovimab to be an important new option to help address the ongoing need for multiple and readily available RSV interventions,” Merck said. “Our mAb approach focuses on preventing disease through direct passive immunity in a single dose that would be given to all infants entering their first RSV season.”

While commercial products are already available, the RSV therapeutic market remains in its relative infancy, with the first vaccines only approved in 2023. (Also see "GSK’s Arexvy Is First RSV Vaccine Approved By FDA, But A Second Is Poised To Follow" - Scrip, 3 May, 2023.) Two vaccines, GSK plc’s Arexvy and Pfizer’s Abrysvo, were both approved by the FDA in 2023, while Moderna’s mResvia received approval in May. All three vaccines are approved for adults, with only Pfizer’s Abrysvo being approved by the FDA as a maternal vaccine to protect infants.

Meanwhile, Sanofi and AstraZeneca have made a strong play for the infant market with their monoclonal antibody Beyfortus (nirsevimab), which received approval from the EMA in 2022, followed by FDA approval in July 2023. (Also see "RSV Market Shake Up Starts As Sanofi And AZ’s Beyfortus Wins First Approval" - Scrip, 4 November, 2022.) and (Also see "Sanofi/AstraZeneca Win Beyfortus Approval To Prevent RSV In US Infants" - Scrip, 17 July, 2023.)

Beyfortus generated €547m ($593.8m) last year after its first RSV season on the market, while the vaccines were even bigger sellers. Arexvy generated €1.2bn ($1.55bn) and Abrysvo generated $890m.

A Single Fixed Dose

But Merck said that clesrovimab could have a competitive profile versus Beyfortus. It can be given to all appropriate infants, regardless of weight, and could simplify the dosing for health care providers. Beyfortus is recommended in two different doses based on body weight: 50 mg if less than 5 kg in body weight and 100 mg if greater or equal to 5 kg in body weight.

Clesrovimab is administered as a single dose regardless of weight. Merck Research Laboratories president Dean Li also highlighted the competitive profile of the product during the company’s first quarter sales and earnings call on 25 April.

“Ours is a single fixed dose and has the durability in terms of covering a whole RSV season,” Li said. “And then also we believe that this will be a distinguished monoclonal antibody in its high barrier to resistance.”

The Phase IIb/III trial was a double-blind, randomized, placebo-controlled study in which infants were randomized to receive either a single dose of clesrovimab or placebo. The primary endpoints were the incidence of participants with RSV-associated medically attended lower respiratory infection from Day 1 (post dose) to Day 150 as compared to placebo and safety.