Shares in Arrowhead Pharmaceuticals took a nosedive Wednesday after the FDA placed a hold on a phase 2 clinical trial of the company’s lead drug candidate, which is aimed at treating chronic hepatitis B virus (HBV).

The company said in a press release Tuesday that the FDA has not sent any written communications to Arrowhead regarding the clinical hold. However, the company said it was told verbally by the agency that the hold was prompted by deaths of non-human primates in a separate study that involves EX1, a drug delivery vehicle developed by Arrowhead that is administrated intravenously and targets the liver.

“The FDA did not indicate the clinical hold was based on any human findings,” Arrowhead said in the release. Read the full story here.