What happened

Shares of Assembly Biosciences (NASDAQ:ASMB), a biopharmaceutical company developing a potential new treatment for hepatitis B virus (HBV), are falling after the company presented mid-stage clinical trial results. Investors unimpressed with interim data from two trials with the company's lead candidate, ABI-H0731, have pressed the stock 21.6% lower as of 1:09 p.m. EDT on Monday.

So what 

An estimated 250 million people are infected with HBV, which still lacks a functional cure. Available antiviral therapies can keep the virus suppressed with varying levels of success, but there aren't any cures similar to those for hepatitis C virus. The stock is slipping today because Assembly Biosciences has been labeling its HBV program as a potential cure, but today's presentation suggests ABI-H0731 is just really good at suppressing the virus.

Guy in a suit watching money fall out of his pocket and into a hole in the floor.

Image source: Getty Images.

During study 201, 60% of HBV patients with a viral load already suppressed by standard therapy who received ABI-H0731 achieved circulating levels of viral RNA too low to measure. That didn't happen for any of the patients on standard antivirals.

Study 202 enrolled treatment-naive patients and gave everyone Baraclude, a standard HBV treatment. Adding ABI-H0731 to Baraclude led to significantly stronger reductions to circulating viral DNA and RNA after 12 and 24 weeks, but the combination probably isn't strong enough to completely wipe out the virus after a 24-week round of therapy.

Now what

These were relatively small phase 2 studies, and investors would like to see a larger company with deeper pockets offer to fund the development of ABI-H0731 from here. The combination of Baraclude and ABI-H0731 might be the best-in-class treatment, but the results so far are a long way from an HBV cure that would entice a potential collaborator.

The company's novel antiviral candidates attack viruses by inhibiting their ability to form closed DNA loops, and ABI-H0731 could take a back seat to a new version soon. Data from a healthy volunteer study with a similar HBV candidate, ABI-H2158, should be released around the middle of the year.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.