Seres Therapeutics, a cash-strapped developer of microbiome drugs, will cut staff and pause its lead program as part of a reboot that’ll see the company focus resources on earlier prospects for immune diseases.
The Cambridge, Massachusetts-based company said Thursday it will stop investing in that top program — SER-155, a therapy meant to alleviate complications from stem cell transplants — while seeking funding to move the treatment forward. In the meantime, Seres will prioritize “high-value earlier-stage” programs in immunological conditions and reduce its workforce by about 30%. Those efforts are expected to extend Seres’ cash runway into the third quarter of 2026, the company said in a statement.
“As we shift our operational focus to our promising earlier-stage pipeline, we are now in a position to streamline our organization and cost structure,” added co-CEOs Thomas DesRosier and Marella Thorell, in the statement.
Seres has long been at the forefront of drug research involving the human microbiome, the trillions of microbes colonizing the human body. The company was formed by Flagship Pioneering, went public in 2015 and, since then, helped bring one of the first microbiome drugs to market — a pill called Vowst for a type of tough-to-treat bacterial infection.
But Seres has also dealt with a number of setbacks along the way that have depressed its share price and left it scrambling for cash. An effort to bring an earlier, immunology-focused microbiome drug failed to produce positive results. Seres also struggled to grow Vowst sales, restructured and in 2024, offloaded the medication to its long-time partner, Nestlé Health Science.
That deal helped Seres clear debt payments and advance testing of SER-155. But while it’s since accumulated early data and finalized the protocol for a potential Phase 2 study, Seres has again been on shaky footing. Previous CEO Eric Shaff stepped down last year, paving the way for co-leaders DesRosier and Thorell. At the time, the company was exploring “various deal structures” to finance mid-stage development.
The company received additional breathing room via a $25 million payment from Nestle, but wasn’t able to secure the needed funding in the ensuing months to start Phase 2 testing of SER-155. As of late November, it only had enough cash to operate through the second quarter of 2026.
Now, Seres is pivoting again, choosing instead to halt the SER-155 program altogether while cutting staff and pouring money into different drug programs. Among those candidates is SER-603, which targets immune conditions such as candidate in ulcerative colitis, Crohn's disease, and immune checkpoint-related enterocolitis.
Seres is working with Memorial Sloan Kettering Cancer Center on an investigator-sponsored trial evaluating SER-155. Discussions regarding “potential collaborations” for its other programs are ongoing, the company said.
