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The McGRAW Trial

The Microbiome and Cancer: Can Gut Bacteria Help Immunotherapy Destroy Tumors?

Why This Trial

In the war on cancer, we have made serious progress. However some of our best weapons, called checkpoint inhibitors, still only work for 20% to 30% of patients.

In examining this problem, PICI investigator Jennifer Wargo, MD, discovered something interesting. Patients with certain types of gut bacteria and a more diverse gut microbiome overall respond better to checkpoint inhibitors, a type of immunotherapy treatment.

That begged the question: could the right gut bacteria be immunotherapy’s best new ally?

This study, one of the first of its kind, was designed to answer that question.

This trial tested whether altering the gut microbiome of cancer patients can change – or even enhance – their ability to respond to checkpoint inhibitors.

About the Study

In this phase 1b, randomized, blinded clinical trial, all patients were treated with an antibiotic and then with nivolumab, an FDA-approved checkpoint inhibitor that targets the PD-1 protein.

Patients then took either a placebo or SER-401, a pill that contains a mix of select bacteria found to be associated with better checkpoint inhibitor response.

Researchers sequenced the gut of all patients before, during and after treatment with checkpoint inhibitors to record the types and amounts of microbes present.

Researchers also took tissue and blood samples to perform additional lab and imaging tests in order to track changes in a patient’s immune system over time.

Who Was Eligible

  • Patients with metastatic or unresectable melanoma (up to 30)

Treatments Tested

  • Nivolumab
  • SER-401 or a placebo

Where We’re At Now

As of March 2021, this trial is no longer enrolling patients.




Principal Investigators

  • Genevieve Boland, MD, PhD | Massachusetts General Hospital
  • Isabella Glitza, MD, PhD | The University of Texas MD Anderson Cancer Center
  • Kenneth Grossman, MD, PhD | Huntsman Cancer Institute
  • Omid Hamid, MD | The Angeles Clinic
  • Janice Mehnert, MD | Rutgers Cancer Institute
  • Patrick Ott, MD | Dana-Farber Cancer Center
  • Alex Shoushtari, MD | Memorial Sloan Kettering Cancer Center
  • Hussein Tawbi, MD, PhD | The University of Texas MD Anderson Cancer Center


For more information on this trial (NCT03817125), visit