This phase II trial studies the side effects and best dose of capecitabine when given together with pembrolizumab and bevacizumab, and investigates how well they work in treating patients with microsatellite stable colorectal cancer that has spread to nearby tissues or lymph nodes, has spread to other places in the body, or that cannot be removed by surgery. Monoclonal antibodies, such as pembrolizumab and bevacizumab, may interfere with the ability of tumor cells to grow and spread. Drugs used in chemotherapy, such as capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving capecitabine together with pembrolizumab and bevacizumab may work better in treating patients with colorectal cancer.

PRIMARY OBJECTIVES:

I. To determine the recommended phase 2 dose (RP2D)/maximum tolerated dose (MTD) of capecitabine when administered with pembrolizumab and bevacizumab. (Safety Lead-In Cohort) II. To evaluate the overall response rate (ORR) to pembrolizumab plus capecitabine and bevacizumab (complete or partial response rate per Response Evaluation Criteria in Solid Tumors [RECIST] 1.1) in subjects with metastatic or locally advanced unresectable microsatellite stable (MSS)/mismatch repair proficient (pMMR) colorectal carcinoma (CRC) that is stable or progressing on fluorouracil (5FU)-based therapy. (Phase II Expansion Cohort)

SECONDARY OBJECTIVES (Phase II Expansion Cohort):

I. To determine the safety and tolerability of pembrolizumab in combination with capecitabine and bevacizumab.

II. To evaluate ORR per immune-related RECIST (irRECIST). III. To evaluate duration of response (DOR), disease control rate (DCR) and progression-free survival (PFS) per RECIST 1.1 and irRECIST and overall survival (OS).

EXPLORATORY OBJECTIVES:

I. Correlation of outcomes to line of therapy; stable disease or progression on a prior regimen containing infusional 5-FU or capecitabine; prior exposure to bevacizumab; and primary tumor location.

II. To explore baseline immune profiles via PD-L1, and multiplex Immunohistochemistry (IHC) for identification of potentially predictive biomarkers in patients with metastatic or locally advanced, unresectable CRC treated with pembrolizumab-based combination therapy.

III. To characterize the change in the populations of tumor-infiltrating immune cells (TIICs) by IHC induced by pembrolizumab-based combination therapy in paired pre- and on-treatment tumor biopsies from patients with metastatic or locally advanced, unresectable MSS / pMMR CRC.

IV. To determine the change in T cell repertoire via next generation sequencing (NGS) within blood and tumor biopsy samples induced by pembrolizumab-based combination therapy in patients with metastatic or locally advanced, unresectable MSS/pMMR CRC.

V. To establish human immune system (HIS) patient-derived xenograft (PDX) models from pre-treatment biopsies to a) analyze change in immune cell profiles HIS PDX models using the same techniques as described for corresponding patients, above and b) correlate response to pembrolizumab-containing therapy in patients and HIS PDX.

OUTLINE:

An initial safety lead-in will be performed to define the maximum tolerated dose (MTD)/recommended phase 2 dose (RP2D). Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1, bevacizumab IV over 30-90 minutes on day 1, and capecitabine orally (PO) twice daily (BID) on days 1-14. Treatment repeats every 3 weeks for up to 35 courses in the absence of disease progression or unacceptable toxicity.

After completion of study treatment, patients are followed up at 30 days, every 9 weeks until disease progression, start of new anti-cancer therapy, death, or end of the study whichever comes first

Inclusion Criteria:

1. Have histologically confirmed, locally advanced unresectable or metastatic (stage IV) colorectal adenocarcinoma

2. Have locally confirmed MSS or pMMR CRC; MSS is defined as 0-1 allelic shifts among 3-5 tumor microsatellite loci using a PCR-based assay; pMMR is defined as presence of protein expression of 4 MMR enzymes (DNA mismatch repair protein Mlh1 (MLH1), DNA mismatch repair protein Msh2 (MSH2), mutS homolog 6 (MSH6) and Mismatch repair endonuclease postmeiotic segregation increased 2 (PMS2) by immunohistochemistry.

3. Have stable disease or progression on a prior regimen containing infusional 5-FU or capecitabine according to the interpretation of the treating provider

4. Be willing and able to provide written informed consent/assent for the trial

5. Be 18 years of age on day of signing informed consent

6. Have measurable disease based on RECIST 1.1

7. Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion (phase II dose expansion cohort only); newly-obtained is defined as a specimen obtained up to 6 weeks (42 days) prior to initiation of treatment on day 1; subjects for whom newly-obtained samples cannot be provided (e.g. inaccessible or subject safety concern) may submit an archived specimen only upon agreement from the Sponsor

8. Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) performance scale

9. Demonstrate adequate organ function performed within 10 days of treatment initiation as defined below:

   1. Absolute neutrophil count (ANC) >= 1,500 /mcL
   2. Platelets >= 100,000 / microliter (mcL)
   3. Hemoglobin >= 9 g/dL or >= 5.6 mmol/L without transfusion or erythropoietin (EPO) dependency (within 7 days of assessment)
   4. Serum creatinine =< 1.5 X upper limit of normal (ULN) OR measured or calculated creatinine clearance >= 60 mL/min for subject with creatinine levels > 1.5 X institutional ULN. Glomerular filtration rate (GFR) can also be used in place of creatinine or creatinine clearance (CrCl)). Creatinine clearance should be calculated per institutional standard
   5. Serum total bilirubin =< 1.5 X ULN OR direct bilirubin =< ULN for subjects with total bilirubin levels > 1.5 ULN. Patients with Gilbert's disease may be included if their direct bilirubin is ≤ 1.5 X ULN.
   6. Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase (SGOT)) and alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase (SGPT)) =< 2.5 X ULN OR =< 5 X ULN for subjects with liver metastases
   7. Albumin >= 2.5 mg/dL
   8. International normalized ratio (INR) or prothrombin time (PT) =< 1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants
   9. Activated partial thromboplastin time (aPTT) =< 1.5 X ULN unless subject is receiving anticoagulant therapy; as long as PT or PTT is within therapeutic range of intended use of anticoagulants

10. Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication; if the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required

11. Female subjects of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 120 days after the last dose of study medication

12. Male subjects of childbearing potential must agree to use an adequate method of contraception, starting with the first dose of study therapy through 120 days after the last dose of study therapy

    • Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject

Exclusion Criteria:

1. Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of treatment

2. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment

3. Has a known history of active tuberculosis (TB) (Bacillus Tuberculosis)

4. Hypersensitivity to pembrolizumab or any of its excipients

5. Hypersensitivity/intolerance to capecitabine, Infusional 5-Fluorouracil (5-FU), or bevacizumab

6. Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study day 1 or who has not recovered (i.e., =< grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier

7. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study day 1 or who has not recovered (i.e., =< grade 1 or at baseline) from adverse events due to a previously administered agent

   • Note: Subjects with =< grade 2 neuropathy are an exception to this criterion and may qualify for the study
   • Note: If subject received major surgery, they must have recovered adequately from the toxicity and/or complications from the intervention prior to starting therapy

8. Has a known additional malignancy that is progressing or requires active treatment; exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer

9. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis; subjects with previously treated brain metastases may participate provided they are stable (without evidence of progression by imaging for at least four weeks prior to the first dose of trial treatment and any neurologic symptoms have returned to baseline), have no evidence of new or enlarging brain metastases, and are not using steroids for at least 7 days prior to trial treatment; this exception does not include carcinomatous meningitis which is excluded regardless of clinical stability

10. Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease modifying agents, corticosteroids or immunosuppressive drugs); replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment

11. Has known history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or current pneumonitis/interstitial lung disease.

12. Has an active infection at the time of cycle 1 day 1 requiring systemic therapy

13. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator

14. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial

15. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment

16. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent

17. Has a known history of human immunodeficiency virus (HIV) (HIV 1/2 antibodies)

18. Has known active hepatitis B (e.g., hepatitis B surface antigen (HBsAg) reactive) or hepatitis C (e.g., hepatitis C virus (HCV) ribonucleic acid [RNA] [qualitative] is detected)

19. Requires therapeutic anticoagulation with warfarin at baseline. Patients must be off warfarin or warfarin-derivative anti-coagulants for at least 7 days prior to starting study drug, however, therapeutic or prophylactic therapy with low-molecular weight heparin is allowed

20. Has history of known coagulopathy that increases risk of bleeding or a history of clinically significant hemorrhage within 12 months of start of study drug

21. Known bleeding risk including serious hemorrhage or hemoptysis within the last 3 months; major surgery within the past 8 weeks or minor surgery within the past 4 weeks

22. Has known gastrointestinal bleeding or any other hemorrhage/bleeding event Common Terminology Criteria for Adverse Events (CTCAE) grade > 3 within 6 months of start of study drug

23. Has greater than 1+ proteinuria on a urine dipstick or equivalent routine laboratory analysis will require further testing with a urine protein to creatinine ratio (UPCR); UPCR must be calculated as follows: UPCR = protein concentration (mg/dL)/creatinine (mg/dL); if the UPCR >= 1, then the patient will not be eligible for study entry; however, if urinalysis or equivalent routine laboratory analysis shows no protein, then UPCR testing is not required

24. Has a history of non-healing wounds or ulcers, or bone re-fractures within 3 months of fracture

25. Has a history of arterial thromboembolism within 12 months of start of study drug

26. Has inadequately controlled hypertension (defined as systolic blood pressure greater than 150 mm Hg or diastolic blood pressure greater than 95 mm Hg). The use of anti-hypertensive medications to control blood pressure is permitted. Retesting is permitted.

27. Has a history of hypertensive crisis or hypertensive encephalopathy within 6 months prior to planned start of study drug

28. Has had clinically significant cardiovascular disease within 12 months of planned start of study drug, including myocardial infarction, unstable angina, grade 2 or greater peripheral vascular disease, cerebrovascular accident, transient ischemic attack, congestive heart failure, or arrhythmias not controlled by outpatient medication, percutaneous transluminal coronary angioplasty/stent

29. Has a known history of abdominal fistula, gastrointestinal perforation, or intra-abdominal abscess within 6 months prior to planned start of study drug

30. Known reversible posterior leukoencephalopathy syndrome (RPLS)

31. Difficulty swallowing, malabsorption, active partial or complete bowel obstruction, or other chronic gastrointestinal disease or conditions that may hamper compliance and/or absorption of capecitabine

32. Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of study drug. Administration of killed vaccines is allowed.