Arrowhead Research Corp. saw its stock plummet 24 percent Monday after the Food and Drug Administration placed a partial clinical hold on one of its drugs.

The Pasadena biopharmaceutical firm announced that while it had been verbally cleared to begin a modified study of its drug ARC-520 in patients with chronic hepatitis B infection, the FDA asked it not to test a higher dosage of the drug. It may now only proceed with a test of a lower dose than proposed. Shares fell $2.16 to close the day at $6.90.

The FDA also requested a final report from an ongoing Phase 2a study of ARC-520 on patients. That trial has not reported any serious adverse events or evidence of end organ toxicity to date, according to the company.

“Over the next 30 days, Arrowhead will begin preparations for the multiple-dose Phase 2b study,” said President and Chief Executive Dr. Christopher Anzalone in a statement. “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.”

Arrowhead, which develops targeted drugs that silence disease-causing genes, has seen the value of its shares rise in recent weeks on speculation the company was a takeover target for Gilead Sciences Inc. of Foster City.

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