In another blow to its $525 million buyout of ReViral, Pfizer has ended development of sisunatovir, an oral inhibitor designed to treat respiratory syncytial virus (RSV) in both kids and adults.
Clinical development of sisunatovir faced “ongoing challenges,” including a drug-drug interaction with antacids, a Pfizer spokesperson told Fierce Biotech in an emailed statement.
“While Pfizer has been working diligently to resolve the challenges, we decided to stop the development of sisunatovir to focus our efforts on identifying and advancing the development of other investigational therapeutics that have the greatest potential to prevent and treat RSV disease and other viral respiratory pathogens,” the spokesperson said.
Sisunatovir was being studied in a phase 2/3 study enrolling adults with RSV as well as a phase 1 trial for infants and children with RSV-related pneumonia. Despite a fast-track designation from the FDA, both trials have now been terminated.
The studies were both expected to read out this September.
The trials were “discontinued due to business reasons,” according to ClinicalTrials.gov. The terminations aren’t tied to safety concerns, the Pfizer spokesperson said.
Pfizer picked up the small molecule from its 2022 acquisition of ReViral, a biotech that was working on a clutch of antiviral therapeutics for RSV. At the time, sisunatovir was the most advanced program from the purchase. The candidate is designed to block RSV from fusing to host cells and was found to significantly reduce viral load during a phase 2 challenge study in healthy adults.
After the acquisition, Pfizer had last year terminated a phase 2 study assessing sisunatovir in infants with RSV due to “strategic consideration,” according to ClinicalTrials.gov.
The ReViral buyout included another RSV candidate, known as PF-07923567 or RV-299, that Pfizer discarded in August 2023. The N-protein inhibitor was tossed alongside five other phase 1 assets.
Given the most recent culls, the Big Pharma company will no longer have an investigational oral RSV program, according to the company’s online pipeline, which was last updated July 30. As RSV goes, that just leaves PF-07941314, an early-stage RSV and flu combo vaccine, in the pharma’s investigational pipeline.
Of course, Pfizer touts already-approved Abrysvo, a vaccine for preventing RSV in pregnant people, infants and individuals 60 years and older. That shot is locked in a competition with GSK’s Arexvy and Moderna’s mRESVIA.