Updated with new information from the conference call

Arrowhead Research (ARWR - Get Report) just released interim results from a mid-stage study of its experimental RNA interference therapy for hepatitis B. The announcement is short on specific details but the Arrowhead drug, ARC-520, appears to have passed through its first clinical hurdle in hepatitis B patients relatively well.

Arrowhead shares closed Tuesday at $13 ahead of the ARC-520 data announcement. The stock is down 5% to $12.25 in the after-market session, reflecting some uncertainty about the results, perhaps confusion or frustration over why the company chose not to be more forthcoming with actual numbers.

I wrote a preview of the ARC-520 data recently. Here's how Arrowhead describes the ARC-520 results today:

-- Initial blinded data suggest that the magnitude of HBsAg knockdown is similar to non-human primate studies, including the chronically infected chimpanzee reported on previousl

-- Duration of knockdown appears to be substantially more sustained than in non-human primates, with patients in the 2 mg/kg group still demonstrating substantial knockdown after 8 weeks, which is the most recent time point available.

-- HBsAg levels appear to continue to decline in a number of patients at the 8 week time point in the 2 mg/kg group

First thing you should notice is the lack of specific numbers. Arrowhead executives, on a conference call, said the study is still blinded. If the study is blinded, how can Arrowhead report results differentiated between patients dosed with ARC-520 and those on a placebo? Arrowhead says educated guesses are being made based on the blinded responses. Yes, it's a bit unusual, even weird, although I assume Arrowhead assumes hepatitis B patients treated with a placebo would show little or no response at all, making it easy to identify responses to the drug.

In a previous study of ARC-520 involving a chimp infected with hepatitis B, treatment with ARC-520 caused a 0.8 log reduction in HBsAG, which is a measure of the presence of hepatitis B virus in the body. Arrowhead says ARC-520 response in human hepatitis B patients was "similar" to the chimp results but the duration of the knockdown was longer.

In terms of Wall Street expectations, a lot of investors were looking for a 1.0 log reduction in HBsAG from this ARC-520 study, although the company and sell-side analysts were framing a win with a 0.5 to 1.0 log reduction. If ARC-520 accomplished an 0.8 log reduction, as today's announcement suggests, it will likely be viewed positively.

HBsAG levels continue to decline in during the study and Arrowhead says the drug appears safe enough to dose another cohort of hepatitis B patients with a higher 3 mg dose of ARC-520. On its call, Arrowhead expressed confidence in achieving a 1 log reduction in HBsAG with the 3 mg dose.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.