After missed endpoint, Gossamer and Chiesi pull the plug on PAH drug collaboration

Biotech After missed endpoint, Gossamer and Chiesi pull the plug on PAH drug collaboration By Will Maddox Jul 27, 2026 10:03am Gossamer Bio Chiesi pulmonary arterial hypertension Merck & Co. After signing a deal worth up to $486 million with Italian biopharma Chiesi to develop the pulmonary arterial hypertension candidate seralutinib in 2024, Gossamer Bio has ended the collaboration and licensing agreement. The termination allows Gossamer to regain worldwide development and commercial rights to seralutinib as it prepares to seek FDA approval despite the drug missing the prespecified statistical threshold for the primary endpoint in a phase 3 trial, according to a Monday release. Under the original agreement, Chiesi paid Gossamer $160 million as a development reimbursement. Gossamer was also eligible for up to $146 million in regulatory milestone payments and $180 million in sales milestones. PAH is a rare, rapidly progressing disease in which the blood vessels in the lungs narrow, increasing pressure in the pulmonary arteries and potentially leading to heart failure. Roughly 40,000 people in the United States have the disorder. In February, Gossamer announced that seralutinib produced a placebo-adjusted improvement of 13.3 meters in patients’ six-minute walk distance after 24 weeks. Although the result had a p-value of 0.032, it missed the trial’s prespecified significance threshold of 0.025. Results favored seralutinib on four secondary endpoints, including measures involving intermediate- and high-risk patient subgroups. Gossamer chairman, co-founder and CEO Faheem Hasnain expressed disappointment after the trial missed what he called “the stringent prespecified statistical threshold for our primary endpoint,” but he remained hopeful. “We believe these data clearly demonstrate seralutinib is an active drug in patients with PAH,” Hasnain said in the February release. “These data support the conclusion that seralutinib demonstrated greater activity in patients with more advanced disease.” The termination ends the companies’ agreement to split U.S. profits equally and returns ex-U.S. rights to Gossamer. Chiesi will make a one-time $5 million payment to Gossamer, with no upfront payment required from Gossamer, and the companies will have no other outstanding obligations under the collaboration. Chiesi will be entitled to a capped royalty on worldwide net sales of seralutinib and payments tied to specified regulatory and commercial milestones. Hasnain said regaining the rights will help Gossamer realize seralutinib’s full potential in PAH and other possible indications. “We enter the second half of 2026 with increased clarity on our planned regulatory path, worldwide rights in hand, a healthier balance sheet and real momentum behind us,” he said in Monday’s statement. “There has never been a more exciting time in Gossamer’s history, and we look forward to the milestones ahead.” The company went all-in on seralutinib in 2023, laying off a quarter of its workforce to help fund the phase 3 trial. Before the workforce reduction, Gossamer had experienced a string of setbacks that included the failure of an asthma prospect, the discontinuation of an ulcerative colitis candidate and the end of a lymphoma trial. Despite the phase 3 results and the end of the Chiesi collaboration, Gossamer remains bullish after holding a pre-NDA Type B meeting with the FDA in June. According to Gossamer, the FDA’s official meeting minutes characterized the statistical significance and magnitude of the treatment effect observed in the phase 3 trial as review issues rather than barriers to filing the application. If approved, seralutinib would enter a PAH market that has received considerable attention from Big Pharma. Johnson & Johnson markets Uptravi, Opsumit and the combination therapy Opsynvi for patients with PAH. Merck & Co.’s Winrevair joined the market in 2024, with the company positioning it as the first therapy designed to target an underlying cause of the disease. Encouraged by the FDA meeting, Gossamer plans to submit an NDA in September based on the phase 3 Prosera study, confirmatory evidence from the phase 2 Torrey study and other supporting analyses. If the FDA accepts the application, seralutinib could be eligible for an approval decision in the third quarter of 2027. “This is a defining moment for Gossamer,” Hasnain said in the release. “After years of disciplined execution, we are now closer than ever to bringing forward what we believe can be a first-in-class, important new medicine for PAH patients who need better options.” Gossamer Bio Chiesi pulmonary arterial hypertension Merck & Co. Johnson & Johnson Biotech