What happened

Shares of Seres Therapeutics (MCRB 6.30%) were crashing 57.3% lower as of 11 a.m. EDT on Thursday. The steep decline came after the company announced top-line results from a phase 2b clinical study evaluating SER-287 in treating ulcerative colitis (UC).

So what

With the biotech stock losing more than half of its value today, it's no surprise that Seres' clinical results weren't good. The company reported that the primary endpoint of improving clinical remission rates wasn't met.

Four scientists with concerned expressions on their faces.

Image source: Getty Images.

There were three patient cohorts in the clinical study. Sixty-eight patients were given a full induction dose of SER-287. Another 66 patients were given a step-down induction dose. Placebo was administered to the remaining 69 patients. Seres said that there were "no meaningful clinical differences" observed between these three cohorts. 

Seres CEO Eric Shaff acknowledged that "these outcomes were not what we, nor the UC community, were hoping for." Because of the poor results, the company is closing the open-label and maintenance parts of the phase 2b study.

Now what

The main thing to watch with Seres now is its anticipated regulatory filing for approval of SER-109 in treating recurrent Clostridioides difficile infection. The company recently signed a deal with Nestlé Health Science to co-market the drug pending approval.