Seres stops microbiome cancer trial, deprioritizes drug as COVID-19 throttles enrollment

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Seres said enrollment in the study was “meaningfully impacted by COVID-19.” (PDPics/Pixabay)

Seres Therapeutics has stopped enrollment in a metastatic melanoma clinical trial and deprioritized the drug. The microbiome specialist attributed the actions to slow enrollment during the pandemic and the progress of its preclinical oncology pipeline. 

In 2017, Seres joined forces with the Parker Institute for Cancer Immunotherapy (PICI) and MD Anderson Cancer Center to evaluate microbiome therapy SER-401 in combination with an anti-PD-1 checkpoint inhibitor. The planned combination built on MD Anderson research into the role the gut microbiome plays in the efficacy of checkpoint inhibitors in patients with melanoma. 

The collaborators began enrolling metastatic melanoma patients to receive SER-401 or placebo with Bristol Myers Squibb’s Opdivo early in 2019. However, despite running for around two years, the trial never came close to hitting its target of enrolling 30 participants. 

According to, the collaborators stopped enrolling subjects last year after accepting 14 people into the study. Seres confirmed enrollment is over Monday and added details about the decision and next steps.

Enrollment in the study was “meaningfully impacted by COVID-19,” Seres said. With slow enrollment pushing back the anticipated completion date, Seres has stopped recruitment, deprioritized SER-401 and shifted its focus to its preclinical cancer pipeline. Seres said a preliminary analysis of 10 patients, some of whom got placebo, found SER-401 was safe and well tolerated. 

The early cessation of the study, which will continue to work with already enrolled patients, means the wait for clinical data to support the hypothesis that microbiome modulation boosts the efficacy of checkpoint inhibitors goes on. Seres is yet to walk away from the idea, though, and plans to carry on working with MD Anderson and PICI to explore the application of microbiome drugs to cancer. Seres is yet to disclose details of its preclinical cancer pipeline.