This is a pilot feasibility study designed to investigate the alterations in the gut microbiome that occur during the course of treatment for colorectal cancer

This is a pilot feasibility study designed to investigate the alterations in the gut microbiome that occur during the course of treatment for colorectal cancer (CRC). Three patient cohorts will be followed. Cohort A: patients treated with oral fluoropyrimidine CAP as part of standard of care (SOC) chemotherapy. Cohort B: patients treated with TAS-102 including those receiving it in combination with Y-90 radioembolization as part of a clinical trial. Cohort C: patients receiving CAP plus immunotherapy (pembrolizumab) and bevacizumab as part of a clinical trial. Investigators will replace participants as needed to ensure a minimum of 10 evaluable participants per cohort (or minimum total of 30 evaluable patients). Evaluable participants are defined as patients with two analyzable stool samples including a baseline sample and at least one on-treatment sample to be used in endpoint analysis

Inclusion Criteria:

1. The patient has histologically proven colorectal adenocarcinoma
2. The patient is starting treatment with oral fluoropyrimidine therapy: CAP or TAS-102 as SOC or on a clinical trial. This also includes those patients receiving treatment in the adjuvant setting or post-metastasectomy with no evidence of disease on imaging.
3. Combination of oral fluoropyrimidine with other cancer-directed therapies, including oxaliplatin, bevacizumab, Y-90 radioembolization, or immunotherapy checkpoint inhibitors, is permitted.
4. Combination of CAP with concurrent radiation is permitted, including patients undergoing radiotherapy to a rectal primary or a metastatic site.
5. Male or female patient aged 18 years of age or older at the time of obtaining the signed and dated informed consent (no upper age limit).
6. Be able to read and speak English.
7. Be willing and able to provide written informed consent for the study

Exclusion Criteria:

1. Patient has had prior chemotherapy, biologic or immunotherapy in the previous 2 weeks.
2. Patient has completed a course of antibiotics longer than 2 weeks in preceding six months or a course of antibiotics of any duration in the 4 weeks prior to starting oral chemotherapy. Any patient who requires treatment with antibiotics during the study may be removed at the investigator's discretion.
3. Known HIV positive.

Recruitment will include both patients who will be taking TAS-102 and those taking CAP with or without other cancer-directed therapies, including those receiving CAP in combination with concurrent rectal radiation. Patients with colorectal cancer (CRC) expected to receive CAP or TAS-102 will be screened by the study PI, co-investigators, or study coordinators. The study coordinator may identify potential participants by various mechanisms: direct referral from a treating physician, review of schedules of medical and oncology providers at UCSF and participating sites, or phone calls from patients or referring physicians.

De-identified data will be shared with researchers at other institutions.