The study of a cystic fibrosis treatment is on hold after a study in rats showed the drug caused lung inflammation.

The Pasadena, California-based biopharmaceutical company is developing ARO-ENaC to treat patients with cystic fibrosis. The therapy aims to reduce production in the lungs’ airways of a protein associated with airway dehydration and reducing the transport of mucus in patients with the disease.

A clinical trial, dubbed AROENaC1001, is now on hold after the preclinical data in rats flagged the inflammation concerns. Arrowhead said they were voluntarily and immediately halting new patient screening, enrollment and further dosing of the medicine while they await more data from the rat study and another primate toxicology study.

“This is difficult news for cystic fibrosis patients, who are in need of new therapeutic options, but we place above all else our obligation to ensure the safety of those enrolled in our clinical trials,” Arrowhead President and CEO Christopher Anzalone, Ph.D., said in the statement.

The trial included 92 healthy volunteers and patients with cystic fibrosis to assess safety and tolerability. The main goal was to assess the number of participants who experience adverse events that could be linked to treatment.

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