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Seres Therapeutics, a pioneer in the race to develop
therapeutic pills from poop, has suffered a key setback with a
drug failing to pass clinical trials.
The results come from a randomized control trial in people with
a recurrent gut infection caused by the bacteria Clostridium
difficile. The drug, “SER-109,” which contains bacterial
spores isolated from stool samples, did not perform much better
than a placebo in phase 2 trials, the
company announced on Friday.
“We are devoted to moving the drug forward,” Roger Pomerantz,
the company’s CEO and president, told reporters and investors
in a teleconference call Friday morning. He characterized the
results as “a bump in the road, but one from which we gain
With SER-109, Seres was toe-to-toe in a race with a competitor,
Minnesota-based Rebiotix, to get FDA approval as the first drug
to treat a version of the C. diff. infection (CDI)
resistant to antibiotics.
The company is also something of a favorite among a crop of
young ventures making drugs containing bacteria.
The company went public
raising $138 million in a surprisingly successful IPO. Then
came a succession of partnerships, including one with Nestlé’s
Health Science division
worth about $120 million. Along the way, the FDA agreed to
label SER-109 as an “orphan
drug” and a “breakthrough
therapy,” official classifications that allowed Seres to
enter a faster track for approval. If the trials had been
successful, Seres could have earned seven-year exclusivity as
the only maker of drugs for antibiotic-resistant CDI.
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A microbial transplant using stool samples — a process that
reboots a person’s gut microbiome with bugs from a healthy
donor — has given some people speedy and lasting relief from
gut infections like CDI and Crohn’s disease.
“The idea behind Seres was to figure out what the active
ingredient is, and get rid of the agents that aren’t active or
potentially harmful,” Seres’s chief scientific officer David
Cook told BuzzFeed News earlier this month.
SER-109 is one of a five drugs Seres is developing for a
variety of conditions, and the farthest ahead in the company’s
This drug was expected to confer the benefits of a poop
transplant without the ick factor, and potentially fewer side
effects: The capsule contains spores from a dozen bacterial
strains isolated from donated stool samples from healthy
The phase 2 study — when the drug is tested for efficacy as
well as safety, and one of the most important steps on the long
road to FDA approval — was conducted with 89 people with
recurrent CDI. Of those, 59 subjects got the drug, and 30 were
given the placebo.
In the group that got the drug, 44% saw a recurrence in their
infection within eight weeks, compared to 53% of the placebo
group, a difference that was not statistically significant, the
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The new results, though disappointing, are unlikely to slow the
momentum of research into microbe-based drugs, Diane Hoffman,
director of the Law and Health Care Program at the University
of Maryland, and chair of a working group studying microbial
transplants, told BuzzFeed News. “I still think it’s really
exciting – maybe we just ruled one thing out,” she said.
The drug’s failure raises an important question about Seres’s
approach: Can a handful of bacterial strains provide the same
benefit as getting the myriad species present in natural stool?
“I have this theory that there’s something potentially unique
about the composition of microorganisms together that produces
the beneficial effect,” Hoffman said. “When you deconstruct
that, you lose some of that benefit.”
Rebiotix, unlike Seres, does not pick the bacteria it puts into
its treatment. Its first product is a suspension of all the
bacteria that are found in a donor stool sample. The drug
crossed the phase-2 mark last year and is expected to begin the
next phase before the end of the year.
“I think it is a big deal,” said Rachel Sachs, a professor of
health and law at Washington University in St. Louis, who has
researched how microbial therapies could be regulated by
agencies. “I would say this is good news for Rebiotix — to the
extent it doesn’t have the same problem.”
Seres Therapeutics’s IPO was in June 2015. An earlier
version of this post misstated the month.