Seres Therapeutics Sinks 74% After Lead Drug Fails in Study

  • Results from phase 2 trial weren’t statistically significant
  • Drug had been granted breakthrough therapy status by FDA

Seres Therapeutics Inc. shares lost three quarters of their value Friday after the biotechnology company said its lead experimental drug failed in a clinical trial testing it against hard-to-treat intestinal infections.

The shares slumped 74 percent to $9.47 at 11:53 a.m. in New York. The stock had gained 4 percent this year as of Thursday’s close.

Seres was testing its drug in a trial of 89 people who had recurrent Clostridium difficile, or C. diff, infections, which inflame the colon and can cause diarrhea and dehydration that can be deadly in vulnerable patients. While patients getting Seres’s drug, known as SER-109, had lower rates of infection recurrence, the results weren’t statistically significant, meaning they could have occurred by chance. An earlier trial had shown strong results, leading the U.S. Food and Drug Administration to designate SER-109 as a breakthrough therapy.

“These are unexpected clinical results in view of the positive data in our prior” trial, Chief Executive Officer Roger Pomerantz said in a statement.

In the trial, 44 percent of patients on the drug saw their C. diff infections recur, while 53 percent of those on placebo did. The most common side effects were diarrhea, abdominal pain and flatulence.

C. diff infections are often treated with antibiotics, researches have also examined therapies like fecal transplants. Seres describes its treatment as an “ecology of bacterial spores enriched and purified from healthy, screened human donors” designed to rebalance the gut microbiome.

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