The US Food and Drug Administration (FDA) has verbally communicated a clinical hold on two trials of RAPT Therapeutics' zelnecirnon.

They include a Phase IIb study for treating atopic dermatitis and a Phase IIa trial for asthma.

The company is awaiting a formal clinical hold letter from the US regulator.

This decision follows a serious adverse event of liver failure in a patient participating in the atopic dermatitis trial.

The connection between liver failure and the drug zelnecirnon is currently under investigation.

The cause of liver failure is currently unknown, but has been characterised as possibly linked to zelnecirnon.

As a result of the clinical hold, dosing and enrolment of new participants in both the atopic dermatitis and asthma trials have been suspended.

Approximately 350 subjects were enrolled in three trials of zelnecirnon, including the two Phase II trials and a prior Phase Ia/Ib study.

No signs of liver toxicity were reported in other trial participants or non-clinical studies of zelnecirnon.

RAPT is conducting a comprehensive review of this serious adverse event.

The patient in question had a complex medical history, which included a drug allergy to dupilumab, an autoimmune disease necessitating thyroid hormone replacement therapy, the usage of an herbal supplement known to cause liver failure, and a reported Covid-19 infection during the event.

This latest clinical hold does not extend to RAPT's ongoing trial of another drug, tivumecirnon (FLX475), in oncology.

RAPT Therapeutics president and CEO Brian Wong said: “This is an unfortunate and unexpected event, and we are working diligently to get more information on this case.

“Patient safety is our top priority and we will work with the FDA to resolve this as quickly as possible.”