Seres to launch new drug trial to treat hospital infections

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Seres Therapeutics Inc., which last summer lost more than two-thirds of its value after reporting disappointing test results for what would be the first drug based on microbes that inhabit the human body, is ready to give it another go.

The Cambridge biotech said Thursday that it's planning to launch a new clinical trial aimed at winning approval of the treatment in the United States. The drug is based on bacteria from the human microbiome — an army of trillions of microorganisms — to fight a stubborn intestinal infection known as clostridium difficile, or C. diff, the most common hospital-acquired infection.


After the summer findings showed the experimental drug failed to reduce risk of the infection recurring for up to eight weeks, Seres shares sank by nearly 70 percent. But despite the setback, the company has continued to move forward with its drug development programs. It had 127 employees at the end of last year, up from 87 a year earlier, according to regulatory filings.

Seres, which raised $134 million in a high-profile 2015 initial public offering, had been viewed as a pioneer in so-called bugs-to-drugs development after it reported positive results from an early-stage clinical study.

Chief executive Roger Pomerantz said in an interview that he has now reached agreement with Food and Drug Administration regulators on the design of a new mid-stage study that could demonstrate the drug's effectiveness in a larger group of patients and provide the "pivotal" data that would support FDA approval of the treatment, known as SER-109, for US commercial use. The drug has won breakthrough and orphan disease status from the agency, meaning it could be approved without a late-stage study if the data prove strong enough.

"They agreed with us on our analysis as well as our path forward," said Pomerantz, a former head of infectious disease research at drug giant Merck & Co. who has led the development of 10 FDA-approved treatments. "We were able to put our heads down in a root cause analysis" that identified how to improve the drug's effectiveness in a new trial, he said.


Investors were cheered by the news — the company's stock jumped 37 percent to $12.71 in trading Thursday.

Under the new approach, Seres will use a different diagnostic to identify a toxin that disrupts the large intestine and give patients higher doses of its experimental drug, which Pomerantz said has proved safe in the earlier trials. He said the new trial will include 320 patients at dozens of clinics in the United States and Canada. Half will be given SER-109 after they take a standard course of antibiotics, while the remainder will just be given the antibiotics. The earlier midstage study had enrolled just 89 patients.

Seres has amassed a library of bacteria from the human microbiome to use as a drugmaking platform in an effort to develop a class of "healthy gut" drugs to treat infections and metabolic disorders. Its first target, C. diff, causes diarrhea, dehydration, and more serious intestinal conditions.

“This is a totally new field of medicine,” Pomerantz said. “We look at it as a new organ of medicine, only made up of bacteria cells rather than human cells.”

Robert Weisman can be reached at Follow him on Twitter @GlobeRobW.