Inclusion Criteria:

• Female age ≥ 18 years.
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2.
• Histologically confirmed un-resected operable invasive adenocarcinoma of the breast ≥ 0.5 cm.

For Cohort 1

• Estrogen receptor (ER) and/or progesterone receptor (PR) positive ≥ 10%, and no human epidermal growth factor receptor 2 (HER2) amplification or overexpression.

For Cohort 2

Human epidermal growth factor receptor 2 (HER2) amplification with FISH ratio ≥ 2.0 or overexpression by immunohistochemistry 3+ with any ER and/or PR.

For Cohort 3

• Triple negative. Estrogen receptor (ER) and/or progesterone receptor (PR) negative < 10%, and no human epidermal growth factor receptor 2 (HER2) amplification or overexpression.
• Patients must not receive neoadjuvant chemotherapy prior to surgery.
• Patients must have adequate organ and marrow function as defined below:
  • Absolute neutrophil count (ANC) ≥ 1000/mm^3;
  • Platelet count ≥ 75,000/mm^3;
  • Hemoglobin ≥ 9.0 g/dL;
  • Creatinine ≤ 2 x ULN;
  • SGOT (AST) ≤ 2 x ULN;
  • Albumin ≥ 3 g/dL.
• Willing and able to swallow capsules.
• Willing and able to complete the stool and serum testing required for the study.
• Willing to provide blood samples for correlative research purpose.
• Agrees not to take non-dietary probiotics through 8 weeks after receiving the course of study drug (including OTC and prescription).
• Agrees not to take any oral vancomycin, metronidazole, fidaxomicin, rifaximin, nitazoxanide, bezlotoxumab and IVIG through the 8-week follow-up assessment unless newly prescribed by a treating investigator during the course of the study.
• Willing to employ adequate contraception from the time of enrollment through 3 months after the final dose of RBX7455.
  • Note: Adequate contraception methods include birth control pills, barrier device, intrauterine device.
• Capable of understanding the investigative nature, potential risks, and benefits of the study.
• Capable of providing valid informed consent.
• Willing to return to enrolling institution for all study visits (blood draws, etc).
• Willing and able to complete the required Patient Medication Diary.
• Willing and able to meet all study requirements, including attending all assessment visits and telephone calls.
• Women of child bearing potential must have negative pregnancy test within 7 days of enrollment.

Exclusion Criteria:

• Requires systemic antibiotic therapy for other condition.
• Fecal microbiota transplant (FMT) within the past 6 months.
• FMT with an associated serious adverse event related to the FMT product or procedure.
• Co-morbid systemic illnesses or other severe concurrent disease which, in the judgment of the investigator, would make the patient inappropriate for entry into this study or interfere significantly with the proper assessment of safety and toxicity of the prescribed regimens.
• Immunocompromised patients including patients known to be HIV positive or those on chronic steroids > 20 mg prednisone a day or prednisone-equivalent.
  • Note: Must be off systemic steroids at least 90 days prior to enrollment. However, topical steroids, inhalants or steroid eye drops are permitted.
• Receiving any other investigational agent.
• History of inflammatory bowel disease (IBD); e.g., ulcerative colitis, Crohn’s disease, or microscopic colitis.
• Diagnosis of irritable bowel syndrome (IBS) as determined by Rome III criteria.
• History of chronic diarrhea.
• History of celiac disease.
• Currently has a colostomy.
• Intraabdominal surgery related to gastrointestinal tract within the last 60 days.
• Evidence of active, severe colitis.
• History of short gut syndrome or motility disorders.
• Requires the regular use of medications to manage bowel hypermotility.
• Active autoimmune disease that has required systemic treatment in the ≤ 30 days (i.e., with use of disease modifying agents, corticosteroids, or immunosuppressive drugs) prior to randomization.
  • Note: Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment.
• Patients with vitiligo, Graves’ disease, or psoriasis not requiring systemic treatment within the past 30 days are not excluded.
• Pregnancy
• Breast feeding.

Eligibility last updated 2/1/22. Questions regarding updates should be directed to the study team contact.