Trilaciclib, a CDK 4/6 Inhibitor, in Patients Receiving FOLFOXIRI/Bevacizumab for Metastatic Colorectal Cancer (mCRC):

Inclusion Criteria: 1. Age ≥ 18 years of age at the time of signing the informed consent. Patients \> 70 years of age must have a G8 Health State Screening Tool (geriatric screening tool) score \> 14. 2. Proficient mismatch repair/microsatellite stable (pMMR/MSS), histologically or cytologically-confirmed adenocarcinoma of the colon or rectum. Patients with any BRAF or KRAS mutation status (wild type or mutant) are eligible. If historical pMMR/MSS and/or BRAF V600E mutational status are not known, a tumor specimen (archival or fresh biopsy) must be sent for testing and results must be available at the time of randomization in interactive web response system (IWRS). If testing cannot be completed using a standard clinical assay performed institutionally/locally, the tumor specimen may be sent to the Sponsor's designated central laboratory for analysis; only historical KRAS mutational status will be collected (ie, no testing required prior to study entry). Note: Any sample sent for MSS/BRAF analysis will be in addition to that required per Inclusion Criterion 5. 3. Unresectable and measurable or metastatic colorectal cancer per RECIST v1.1 4. ECOG performance status of 0 to 1 5. A formalin-fixed paraffin-embedded (FFPE) tumor specimen (from archival or fresh biopsy) with an associated pathology report documenting pMMR/MSS mCRC must be confirmed to be available to send to the Sponsor for planned retrospective biomarker analyses (tissue requirements are provided in the associated laboratory manual). 6. Hemoglobin ≥ 9.0 g/dL in the absence of RBC transfusion or ESA administration within 14 days prior to first dose of trilaciclib/placebo 7. Absolute neutrophil count (ANC) ≥ 1.5 × 10\^9 /L 8. Platelet count ≥ 100 × 10\^9 /L 9. Estimated glomerular filtration rate (eGFR) ≥ 30 mL/minute/1.73m\^2 10. Total bilirubin ≤ 1.5 × upper limit of normal (ULN) 11. AST, ALT, and alkaline phosphatase ≤ 3 × ULN for patients without liver or bone metastases; AST, ALT and alkaline phosphatase ≤ 5 × ULN in the presence of liver metastases; AST and ALT ≤ 3 x ULN and alkaline phosphatase ≤ 5 × ULN in the presence of bone metastases 12. Resolution of nonhematologic toxicities from prior therapy or surgical procedures to ≤ Grade 1 or baseline (except alopecia) 13. Urine dipstick protein \< 2+. If ≥ 2+ at Screening, then a 24-hour urine collection must be done to demonstrate ≤ 1 g of protein/24 hours 14. Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. Please see Section 17.4 for detailed instructions on methods of contraception requirements. 15. Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Exclusion Criteria: 1. Prior systemic therapy for mCRC. Patients who received adjuvant/neoadjuvant therapy (ie, treatment with curative intent) for colorectal cancer are eligible if it has been ≥ 6 months between the last dose of systemic chemotherapy and the date of informed consent. 2. Any radiotherapy, chemotherapy, immunotherapy, biologic, investigational, or hormonal therapy for cancer treatment (except for adjuvant hormonal therapy for breast cancer or prostate cancer defined as M0 disease or PSA persistence/recurrence without metastatic disease) within 3 weeks prior to the first dose of trilaciclib/placebo. 3. Receipt of any low-dose systemic chemotherapeutic agent (e.g., low-dose methotrexate for rheumatoid arthritis) administered for a nononcologic purpose within 3 weeks prior to the first dose of trilaciclib/placebo. 4. Presence of central nervous system (CNS) metastases/leptomeningeal disease requiring immediate treatment with radiation therapy or steroids (i.e., patient must be off steroids administered for brain metastases for at least 14 days prior to the first dose of trilaciclib/placebo). 5. QTcF interval \> 450 msec (males) or \> 470 msec (females) at screening. For patients with ventricular pacemakers, QTcF \> 500 msec. 6. Personal or family history of long QT syndrome 7. Symptomatic peripheral neuropathy 8. History of interstitial lung disease (ILD) 9. Uncontrolled hypertension (blood pressure ≥ 150/90mm Hg) 10. Clinically significant (i.e., active) cardiovascular disease at the time of signing the informed consent; for example cerebrovascular accidents (≤ 6 months before the first dose of trilaciclib/placebo), myocardial infarction (≤ 6 months before the first dose of trilaciclib/placebo), unstable angina, serious cardiac arrhythmia requiring medication, or uncontrolled symptomatic congestive heart failure \[Class II or higher as defined by the New York Heart Association \[NYHA\] functional classification system\]) 11. Serious, non-healing wound, ulcer, or bone fracture 12. Major surgical procedure, open biopsy, or significant traumatic injury within 28 days prior to study treatment start, or anticipation of the need for major surgical procedure during the course of the study. 13. Known serious active infection (e.g., human immunodeficiency virus \[HIV\], hepatitis B or C, tuberculosis, etc.) 14. Known Gilbert's Syndrome or homozygous for the UGT1A1\*28 allele. UGT1A1 genotyping is not required for this study. 15. Chronic inflammatory bowel disease and/or active intestinal obstruction. Patients should not be treated until the intestinal obstruction has resolved. 16. Previous history of significant/severe hemorrhage, within 1 month before randomization. History of previous abdominal fistula or gastrointestinal perforation within 6 months before randomization 17. Known history of bleeding diathesis or coagulopathy 18. INR \> 1.5 within 14 days prior to starting study treatment. EXEMPTION: patients on full anticoagulation must have an in-range INR (usually between 2 to 3) if INR is used for monitoring. Any anticoagulation therapy must be at stable dosing prior to enrollment. 19. Ongoing or anticipated treatment with potent cytochrome inhibitors CYP450 3A4 (such as ketoconazole) or inducers (such as rifampicin, carbamazepine, phenobarbital, phenytoin or St. John's wort). Irinotecan should not be delivered concurrently. 20. Patients with ongoing or anticipated treatment with sorivudine or its chemically related analogues, such as brivudine. 21. Chronic, daily treatment with high-dose aspirin (\> 325 mg/day) 22. Prior allogeneic or autologous hematopoietic stem cell or bone marrow transplantation 23. Receipt of any live attenuated vaccines within 4 weeks prior to first dose of study treatment 24. Known hypersensitivity to any of the drugs used in this study 25. Pregnant or lactating women 26. Legal incapacity or limited legal capacity 27. Other uncontrolled serious chronic disease or psychiatric condition that in the Investigator's opinion could affect patient safety, compliance, or follow-up in the protocol 28. Any contraindications to the administration of FOLFOXIRI and bevacizumab at the discretion of the investigator.