Eight months after snaring an inaugural FDA green light, Merck & Co. is bolstering the case for the chief asset in its $11.5 billion Acceleron acquisition.

In the phase 3 ZENITH study, Merck’s activin signaling inhibitor Winrevair, also known as sotatercept, met its primary endpoint of time to first morbidity or mortality event—which included all-cause death, lung transplantation, or hospitalization for at least 24 hours linked to disease worsening—and helped reduce the risk of morbidity or mortality events versus placebo in adults with pulmonary arterial hypertension (PAH) in functional class III or IV.

Patients in the trial’s study drug and control arms both received background PAH therapy, too.

The World Health Organization ranks the severity of patients’ PAH according to functional classes I through IV, with I being the mildest and IV the most severe form of the disease. Winrevair’s original FDA approval in late March leveraged data from the late-phase STELLAR trial, which assessed the drug in PAH patients in functional groups II or III.

In PAH, the blood vessels narrow in the lungs, which causes blood pressure to increase, leading to heart failure. Roughly 40,000 people in the U.S. have the disorder, which often leads to death and strikes more women than men.

Based on the latest results from ZENITH, which enrolled 172 patients, an independent data monitoring committee urged Merck to stop the study early and offer Winrevair to all patients via its SOTERIA open-label extension trial.

With a positive readout in hand, Merck says it plans to present the detailed PAH data at an upcoming medical meeting and submit the results to global regulators. Following Winrevair’s U.S. approval earlier this year, the drug has picked up green lights in an additional 36 countries.

By assessing Winrevair in PAH patients at functional class III or IV, Merck looked at the drug in people at high risk of mortality from their disease. Aside from the primary composite endpoint measuring time to first morbidity or mortality event, the study also assessed Winrevair on secondary outcome measures like overall survival, transplant-free survival and more.

“PAH is a serious, progressive disease with a high incidence of morbidity and mortality, Eliav Barr, M.D., SVP and head of global clinical development and CMO for Merck Research Laboratories, said in a statement.

“These findings are impressive, set a high evidentiary bar for studies of future candidates developed for the treatment of PAH and support the potential of Winrevair to be practice-changing in the management of PAH.”

Winrevair was initially developed by Acceleron and Celgene, with the partners kicking off investigation of the compound in disorders like osteoporosis, anemia and multiple myeloma.

Merck ultimately got its hands on the drug when it closed its $11.5 billion buyout of Acceleron in November of 2021. Given that Bristol Myers Squibb itself acquired the other half behind Winrevair, Celgene, in 2019, Merck is paying BMS royalties on the treatment.

A month after Winrevair snagged approval in the U.S., Merck’s CEO, Rob Davis, was quick to highlight the medication as a prime example of his company’s dealmaking acumen.

“We’ve demonstrated that we can leverage our deep discovery prowess to identify important acquisition targets and then add significant value through our powerful clinical research engine, our regulatory expertise and our commercial scale,” Davis said on a quarterly conference call in April.

To make a name for itself in PAH, Winrevair will need to compete with Johnson & Johnson’s prostacyclin receptor (IP) agonist blockbuster Uptravi, as well as the New Jersey drug giant’s endothelin receptor agonist Opsumit, which itself garnered $2 billion in 2023 revenue.

Meanwhile, just a few days before Merck’s green light, the FDA approved a new combination tablet in Opsynvi, which weds Opsumit with United Therapeutics’ Adcirca.

Still, Winrevair appears to be on good footing to start. Analysts have estimated that the drug could rake in peak sales of at least $2 billion, with J.P. Morgan’s Chriss Schott predicting that haul could even reach between $3 billion and $4 billion.

In the third quarter, Winrevair brought home sales of $149 million, with Merck noting in a release that the drug’s launch is “gaining momentum.”