Death or clinical worsening cut by 84%
STELLAR randomized 323 patients at 91 sites in 21 countries with WHO Group 1 PAH and with WHO functional class II or III disease to receive either sotatercept or placebo for 24 weeks, with an option for treatment to continue beyond that until the last patient in the study reached 24 weeks on treatment, resulting in an overall median treatment duration of nearly 33 weeks.
In addition to the significant result for the primary endpoint, the 163 patients who received sotatercept had significant improvements, compared with 160 placebo-treated patients, for eight of nine secondary endpoints. The only secondary endpoint with a neutral result was for a measure of cognitive and emotional wellbeing, a parameter that was already at a normal level at baseline in most enrolled patients, Dr. Hoeper explained.
The incidence of either death or an event indicative of clinical worsening during the overall median follow-up of almost 33 weeks was 26.3% among the control patients and 5.5% among those who received sotatercept. This translated into a significant reduction for this endpoint of 84% with sotatercept treatment, compared with placebo.
The rates of treatment-emergent adverse events leading to discontinuation were roughly the same in the control and sotatercept arms, and the incidence of severe or serious treatment-emergent adverse events was higher among the control patients.
The most common adverse event on sotatercept was bleeding events, which occurred in 32% of those on sotatercept and in 16% of the control patients, but the events in the sotatercept arm were “mostly mild,” said Dr. Hoeper. The next most frequent adverse event during sotatercept treatment was appearance of telangiectasias, which occurred in 14% of those on sotatercept and in 4% of control patients.
“It’s an uncommon adverse event profile, but not unexpected for a drug with its mechanism of action,” he said.
Drug binds activin, a pathologic driver of PAH
Sotatercept is an engineered molecule that combines a section of a human immunoglobulin G molecule with a portion of the receptor for activin. This structure allows sotatercept to bind free activin molecules in a patient’s blood, thereby removing a key driver of the pulmonary vascular wall remodeling that is at the pathologic root of PAH.
“Hyperproliferation of blood vessel–wall cells” caused by activin signaling “is perhaps the most important driver of PAH,” Dr. Hoeper said. “Sotatercept allows us for the first time to target the underlying mechanism behind PAH.”
Still ongoing are the HYPERION and ZENITH phase 3 trials of sotatercept. HYPERION is enrolling patients with newly diagnosed or high-risk PAH and is expected to complete in 2028. ZENITH is enrolling patients with more advanced PAH and a higher mortality risk, with results expected in 2026.
Sotatercept has received “Breakthrough Therapy” designation and “Orphan Drug” designation by the Food and Drug Administration, and “Priority Medicines” designation and “Orphan Drug” designation by the European Medicines Agency for the treatment of PAH. One recent review estimated a worldwide PAH prevalence of about 3-4 cases/100,000, which for the United States translates into a total prevalence of perhaps 10,000-15,000 affected people.
STELLAR was funded by Acceleron Pharma, a subsidiary of Merck. Dr. Hoeper is a consultant to Acceleron. Dr. Cooper-DeHoff, Dr. Grapsa, and the authors of the editorial on STELLAR have no relevant disclosures.