Chiesi exits $486M lung-drug partnership, leaving Gossamer to take seralutinib to the FDA alone

Chiesi is walking away from a $486 million partnership over a lung-hypertension drug that missed its main trial goal, leaving small-cap Gossamer Bio to take it to the FDA on its own with a one-time $5 million exit payment in hand.

Under the original 2024 agreement, Italy's Chiesi paid $160 million in development reimbursement to Gossamer, with up to $146 million in regulatory milestones and $180 million in sales milestones still on the table. Chiesi held U.S. profit-share rights and ex-U.S. commercialization rights. On Monday, Gossamer announced that Chiesi had terminated the deal. Chiesi keeps a capped royalty on worldwide net sales of seralutinib and pays Gossamer $5 million, with milestone payments tied to specified regulatory and commercial events. Gossamer regains worldwide development and commercialization rights.

Seralutinib is an inhaled drug aimed at pulmonary arterial hypertension, or PAH, a rare, progressive disease of the lung arteries that affects roughly 40,000 people in the United States. The drug is the centerpiece of Gossamer's pipeline, and losing its development partner is a real blow to a company that was relying on Chiesi's deep pockets to carry it through late-stage development.

The partnership collapsed over the readout from PROSERA, Gossamer's late-stage Phase 3 trial. The result, reported in February, was a mixed bag. On the primary endpoint, seralutinib improved patients' six-minute walk distance by 13.3 meters versus placebo after 24 weeks. That is a real signal. The p-value was 0.032, below the conventional 0.05 bar for statistical significance, but above the 0.025 threshold the trial had been designed in advance to clear. That bar is called the prespecified alpha. The trial missed it.

What the trial did show was stronger subgroup activity. In the intermediate- and high-risk subgroup of 234 patients, the placebo-adjusted improvement was 20.0 meters, with a p-value of 0.0207, and three of four key secondary endpoints hit p-values below 0.0125. In the smaller North American subgroup of 75 patients, the placebo-adjusted gain was 25.9 meters, though that p-value of 0.0573 was not statistically significant. The safety profile was generally well tolerated and consistent with prior experience.

Gossamer chairman and CEO Faheem Hasnain said the data "clearly demonstrate seralutinib is an active drug" and that the regained worldwide rights would let the company "realize the full potential" of the program in PAH and other indications.

The harder question is what comes next. Gossamer said it plans to meet with the FDA to discuss the path forward. Filing on a Phase 3 trial that missed its prespecified primary endpoint, even one that cleared the conventional 0.05 bar, is a steeper ask than filing on a clean win. The subgroup signals will matter; the FDA will want to understand which patients drove the benefit and whether the magnitude holds up across the broader population. The meeting date has not been disclosed.

For now, Gossamer has a worldwide asset, a $5 million exit payment from Chiesi, and a royalty overhang on any future sales. The next concrete milestone is the FDA meeting itself. The agency will set the terms under which seralutinib can reach patients.