MCRB Stock Declines As Seres Therapeutics Pauses SER-155 Phase 2 Study And Shrinks Headcount By 30%

The company said that it will continue to seek funding for the SER-155 Phase 2 study.

• Seres said the pause follows the completion of study site evaluation and qualification work with its contract research organization.
• The company said following the pause in additional investment into the SER-155 Phase 2 study, it will shift its operational focus to high-value earlier-stage pipeline programs.
• Seres noted that its cost-cutting measures are expected to extend its cash runway into the third quarter of 2026.

Seres Therapeutics Inc. (MCRB) shares fell more than 21% in Thursday’s pre-market trade after the company announced that it is pausing a planned Phase 2 study, while reducing the headcount by 30% to extend its cash runway.

Seres stated that it has finalized the protocol for the Phase 2 study of SER-155 in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT) with the U.S. Food and Drug Administration (FDA).

“As we pursue earlier stage programs with the greatest opportunity, such as those targeting inflammatory bowel disease (IBD), we are exploring collaborations to provide additional capital to the company,” said Thomas DesRosier and Marella Thorell, co-CEOs of Seres.

Why Did Seres Pause The Study?

Seres said the pause follows the completion of study site evaluation and qualification work with its contract research organization, adding that the Phase 2 study drug substance has already been manufactured.

The company said that following the pause in additional investment into the SER-155 Phase 2 study, it will shift its operational focus to high-value earlier-stage pipeline programs. At the same time, it will continue to seek funding for the Phase 2 study, Seres said.

Extending Cash Runway

Seres noted that its cost-cutting measures, including workforce reductions, are expected to extend its cash runway into the third quarter (Q3) of 2026.

The company said that this will help in supporting the continued development of its programs for inflammatory and immune-related diseases, while also giving it additional time to secure funding for the SER-155 Phase 2 study.

How Did Stocktwits Users React?

Retail sentiment on Stocktwits around Seres Therapeutics trended in the ‘bullish’ territory at the time of writing, with message volumes at ‘high’ levels.

One user on Stocktwits said that Seres’ announcement confirms that “there is no one interested to partner” with the company.

MCRB stock is down 51% year-to-date and 75% over the past 12 months.

For updates and corrections, email newsroom[at]stocktwits[dot]com.<