Selected 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 >.

          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 are eligible.

          3. Unresectable and measurable or evaluable disease 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.

          6. Adequate organ function

        Selected 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

          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. Prior allogeneic or autologous hematopoietic stem cell or bone marrow transplantation