NEW YORK (GenomeWeb) – Arrowhead Research this week announced that the US Food and Drug Administration ordered a partial hold on a planned Phase IIb trial of ARC-520, the company's RNAi-based drug candidate for hepatitis B.
On the news Monday, shares of Arrowhead plummeted more than 20 percent to around $7.15. But midday Wednesday, the stock had slipped further to $6.59.
ARC-520 comprises two distinct siRNAs targeting highly conserved genomic regions across the major HBV genotypes, and is formulated with a proprietary delivery technology called dynamic polyconjugates. It is currently in a Phase IIa trial testing single doses ranging from 1 mg/kg to 4 mg/kg in chronic HBV patients.
Arrowhead had planned to kick off a Phase IIb study that would test multiple doses of the drug at either 2 mg/kg or 4 mg/kg dose levels, in combination with one of two different direct-acting antivirals.
However, the company disclosed that the FDA has ordered that the Phase IIb begin at the 1 mg/kg dose level instead. The agency also instructed Arrowhead to provide a final study report from the ongoing Phase IIa trial.
"Over the next 30 days, Arrowhead will begin preparations for the multiple-dose Phase 2b study," Arrowhead President and CEO Christopher Anzalone said 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."
The FDA's action marks the latest setback for Arrowhead with ARC-520, which the company has positioned as its flagship product.
In October, Arrowhead released preliminary data from the Phase IIa study that showed unexpectedly modest reductions in HBV surface antigen (s-antigen), a key indicator of active infection.
Arrowhead officials have long maintained that they are shooting for a one-log knockdown in s-antigen with ARC-520, but in the trial patients receiving single 2 mg/kg doses saw substantially weaker reductions.
Investors reacted harshly to the news, especially in light of encouraging statements made earlier by company management, and sent shares of Arrowhead tumbling more than 40 percent to around $7 at the time.
Arrowhead has also been sued for allegedly misleading investors over the ARC-520 data.