Gossamer Bio’s shares plunged 43.3% as investors reacted negatively to a cautious outlook marked by clinical signaling below the stringent prespecified statistical threshold and broad uncertainty on commercial viability, prompting a significant operational downsizing and cost containment.

• The Phase III PROSERA study showed a modest 13.3 meter placebo-adjusted improvement in 6-minute walk distance with a p-value of 0.032, missing the stricter prespecified 0.025 alpha threshold for primary endpoint significance.
• All four key secondary endpoints favored seralutinib, but overall data leave ambiguity on clinical meaningfulness for regulatory approval.
• The company is pursuing an NDA submission in September 2026 based on PROSERA plus earlier TORREY data, but the timing and success remain contingent on the upcoming Type B pre-NDA FDA meeting.
• Operationally, the company announced a drastic workforce reduction (~50%) and paused other development programs to preserve capital amid uncertainty.
• Engagement with convertible noteholders aims to manage looming 2027 maturities, reflecting financial caution as the PROSERA results introduce significant risk to near-term commercial prospects.