Gossamer Bio has disclosed that it has halted development of GB5121 and terminated all clinical studies of the drug following a partial clinical hold imposed by the FDA after two patients died in a Phase Ib/II trial.

“A sudden death event and a fatal intracranial hemorrhage,” in addition to atrial fibrillation, occurred among patients enrolled in the Phase Ib/II STAR-CNS trial (NCT05242146), Gossamer disclosed Monday in a regulatory filing.

Investors reacted to the deaths, partial clinical hold, and program termination with a stock selloff that sent Gossamer’s shares down 10% in early afternoon trading, from $1.12 at yesterday’s close to $1.01 as of 3:27 p.m.

STAR-CNS is designed to evaluate GB5121, an oral CNS-penetrant Bruton’s tyrosine kinase (BTK) inhibitor, in patients with primary CNS lymphoma. The trial consists of a Phase Ib dose escalation and dose expansion portion, followed by a Phase II portion intended to determine a recommended Phase II dose of GB5121 by assessing the safety, tolerability, dose-limiting toxicity(ies), maximum tolerated dose, and/or optimal biological dose; as well as preliminary anti-tumor activity and efficacy.

The patient deaths and partial hold were disclosed nearly three weeks after Gossamer said it had paused enrollment in STAR-CNS, citing both the benefit/risk profile observed to date—without disclosing additional details at the time—and the company’s decision to prioritize resources to support the development of its lead pipeline candidate, the pulmonary arterial hypertension drug seralutinib (GB002).

Seralutinib is a tyrosine kinase inhibitor targeting platelet-derived growth factor receptor (PDGFR) α/β, colony stimulating factor 1 receptor (CSF1R), and c-KIT, specifically designed to be delivered via dry powder inhaler.

“We plan to discuss available data with the study’s Data Review Committee to determine next steps,” Gossamer stated in its Form 10-K annual report for 2022, filed March 17. “We expect to report data from this ongoing, open-label clinical trial at relevant medical conferences.”

Gossamer finished 2022 with a net loss of $229.378 million, slightly improved from the $234.004 million net loss for 2021. For the fourth quarter, Gossamer showed a net loss of $55.764 million, vs. $56.284 million in Q4 2021. However, the company saw its cash, cash equivalents, and marketable securities slide by 21%, to $255.678 million from $325.218 million as of December 31, 2021.

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