$Arrowhead Research(ARWR)$ Shares of Arrowhead Research Corp (NASDAQ: ARWR) fell more than 20 percent in Monday’s premarket following news that the U.S. FDA verbally informed the company in a preliminary call of a partial clinical hold, under which the company is cleared to begin a modified multiple-dose study of ARC-520 in patients with chronic hepatitis B infection.

The FDA requested that the company start the multiple-dose study at 1 mg/kg of ARC-520 rather than the proposed parallel study design of 2 and 4 mg/kg, and requested additional information be provided to the agency.

The additional information includes a final study report from the single-dose Phase 2a study in patients who received 1-4 mg/kg ARC-520, which is ongoing and has not reported any serious adverse events or evidence of end organ toxicity to date.

“Over the next 30 days, Arrowhead will begin preparations for the multiple-dose Phase 2b study,” said Arrowhead President and CEO, Dr. Christopher Anzalone. “We will work closely with the FDA throughout this process while we continue to seek approval to proceed with other planned studies in Asia and Europe.”