Keros Therapeutics (KROS) shares dropped 70% in recent premarket activity after the company said Thursday that it voluntarily halted dosing in the 3 mg/kg and 4.5 mg/kg treatment arms of its phase 2 TROPOS trial of cibotercept in combination with background therapy in patients with pulmonary arterial hypertension.

The decision was based on a safety review due to the unanticipated observation of pericardial effusion adverse events in the trial, the company said.

The company plans to work with investigators, the US Food and Drug Administration and other relevant regulatory authorities to address the findings as quickly as possible.

Dosing in the 1.5 mg/kg treatment arm of the trial remains ongoing following the completion of a risk and benefit assessment.

Keros expects to present topline data from all treatment arms in the trial in Q2 of 2025.