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    Seres Therapeutics stock plunges on disappointing drug trial results

    Shares of Seres Therapeutics Inc. lost almost 70 percent of their value Friday after the six-year-old Cambridge biotech company reported disappointing test data for its lead experimental drug.

    Seres, which has amassed a library of bacteria from the human microbiome to use as a drugmaking platform, raised about $134 million in an initial public offering last year to develop a class of “healthy gut” drugs to treat infections and metabolic disorders.

    But the company said Friday that interim findings from a mid-stage clinical study of its lead drug, for an intestinal infection called clostridium difficile, or C. diff, failed to meet its primary goal of reducing the risk of recurrence for up to eight weeks. The infection causes diarrhea, dehydration, and more serious intestinal conditions.

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    The announcement sent Seres stock down $24.83 a share to $10.95 in Friday trading. The stock traded as high as $52 a share last summer.

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    Seres chief executive Roger Pomerantz said that the findings were unexpected in light of positive data from an early-stage clinical trial of the drug candidate, called SER-109. The latest study enrolled 89 patients with recurrent C. diff. All completed a course of antibiotics, after which some were given the Seres pill, while others a placebo.

    “We were obviously surprised by these results because of the earlier study,” Pomerantz said in an interview. “The issue is how these two studies compare, why there was divergence. We’re looking at the root causes, and we’re leaving no stone unturned.”

    Pomerantz said Seres will review the clinical results and make any necessary changes in consultation with Food and Drug Administration officials. Seres in January received a $120 million payment from Nestlé Health Science, an arm of the Swiss food and drink company, as part of a research partnership to develop drugs to treat C. diff, inflammatory bowel disease, ulcerative colitis, and Crohn’s disease.

    Robert Weisman can be reached at robert.weisman@globe.com.