Inclusion Criteria:

          -  Provision of signed Informed Consent prior to any screening procedures being performed

          -  Histologically (or cytologically) confirmed diagnosis of adenocarcinoma of the colon
             or rectum, with clinical confirmation of unresectable and/or metastatic disease that
             is measurable according to RECIST 1.1 criteria

          -  Baseline tissue-based KRAS, NRAS, EGFR, BRAF and MEK1 wild-type tumor

          -  Prior treatment with at least one systemic chemotherapy regimen for mCRC, or
             recurrence/progression with development of unresectable or metastatic disease within 6
             months of adjuvant chemotherapy for resected colorectal cancer

          -  Patients who received chemotherapy must have recovered (Common Terminology Criteria
             for Adverse Events [CTCAE] grade =< 1) from the acute effects of chemotherapy except
             for residual alopecia or grade 2 peripheral neuropathy prior to randomization. A
             washout period of at least 21 days is required between last chemotherapy dose and
             randomization (provided the patient did not receive radiotherapy)

          -  Patients who received radiotherapy must have completed and fully recovered from the
             acute effects of radiotherapy. A washout period of at least 14 days is required
             between end of radiotherapy and randomization

          -  Left-sided primary tumor

          -  Prior treatment with, and progression on, anti-EGFR therapy (cetuximab or panitumumab)

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 1

          -  Absolute neutrophil count (ANC) >= 1.5 x 10^9/L

          -  Hemoglobin (Hgb) >= 9 g/dL with or without transfusions

          -  Platelets (PLT) >= 100 x 10^9/L without transfusions

          -  Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) =< 2.5 x upper
             limit of normal (ULN)

          -  Total bilirubin =< 1.5 x ULN and < 2 mg/dL

               -  Note: Patients who have a total bilirubin level > 1.5 x ULN will be allowed if
                  their indirect bilirubin level is =< 1.5 x ULN

          -  Serum creatinine =< 1.5 x ULN, or calculated creatinine clearance (determined as per
             Cockcroft-Gault) >= 50 mL/min at screening

          -  Corrected QT (QTc) interval =< 480 ms (preferably the mean from triplicate
             electrocardiograms [ECGs])

          -  Able to take oral medications

          -  Female patients are either postmenopausal for at least 1 year, are surgically sterile
             for at least 6 weeks, or must agree to take appropriate precautions to avoid pregnancy
             from screening through follow-up if of childbearing potential

               -  Note: Permitted contraception methods include: male condom with spermicide,
                  female condom with spermicide, diaphragm with spermicide, cervical sponge, or
                  cervical cap with spermicide. These should be communicated to the patients and
                  their understanding confirmed. For all females, the pregnancy test result must be
                  negative within 24 hours of starting treatment on study and within 24 hours prior
                  to each cycle. Males must agree to take appropriate precautions to avoid
                  fathering a child from screening through 100 days following the end of therapy

        Exclusion Criteria:

          -  History of a grade 3 or 4 allergic reaction or intolerability attributed to cetuximab

          -  Right-sided or transverse colonic primary tumor

          -  Baseline tissue-based KRAS, NRAS, EGFR, BRAF and MEK1 mutated tumor

          -  Active infection requiring concurrent antibiotic use

          -  Currently using concomitant medications that are strong inhibitors or inducers of
             CYP3A4

          -  Previously completed or withdrawn from this study or any other study investigating an
             ERK1/2 inhibitor

          -  Any known symptomatic brain metastasis

               -  Note: Patients previously treated or untreated for this condition who are
                  asymptomatic in the absence of corticosteroid and anti-epileptic therapy are
                  allowed. Known brain metastases must be stable for >= 4 weeks, with imaging
                  (e.g., magnetic resonance imaging [MRI] or computed tomography [CT])
                  demonstrating no current evidence of progressive brain metastases at screening

          -  Known leptomeningeal disease

          -  Previous or concurrent malignancy within 3 years of study entry, with the following
             exceptions: adequately treated basal or squamous cell skin cancer, superficial bladder
             cancer, prostate intraepithelial neoplasm, carcinoma in-situ of the cervix, or other
             noninvasive or indolent malignancy; other solid tumors treated curatively without
             evidence of recurrence for at least 3 years prior to study entry

          -  Impaired cardiovascular function or clinically significant cardiovascular diseases,
             including any of the following:

               -  History of acute coronary syndromes (including myocardial infarction, unstable
                  angina, coronary artery bypass grafting, coronary angioplasty, or stenting) < 6
                  months prior to screening

               -  Symptomatic chronic heart failure (i.e. grade 2 or higher), history or current
                  evidence of clinically significant cardiac arrhythmia and/or conduction
                  abnormality < 6 months prior to screening except atrial fibrillation and
                  paroxysmal supraventricular tachycardia

               -  The patient has a personal history of any of the following conditions: syncope of
                  cardiovascular etiology, ventricular arrhythmia of pathological origin
                  (including, but not limited to, ventricular tachycardia and ventricular
                  fibrillation), or sudden cardiac arrest

          -  Uncontrolled hypertension defined as persistent elevation of systolic blood pressure
             >= 150 mmHg or diastolic blood pressure >= 90 mm Hg, despite current therapy

          -  The patient has active systemic bacterial infection (requiring intravenous [IV]
             antibiotics at time of initiating study treatment) fungal infection, or detectable
             viral infection (such as known human immunodeficiency virus positivity or with known
             active hepatitis B or C (for example, hepatitis B surface antigen positive.) Screening
             is not required for enrollment. Known history of acute or chronic pancreatitis
             (history of acute pancreatitis with no recurrent events in the prior 24 months are
             permitted)

          -  Impaired gastrointestinal function or disease that may significantly alter the
             absorption of study drug (e.g., ulcerative diseases, uncontrolled vomiting,
             malabsorption syndrome, small bowel resection with decreased intestinal absorption)

          -  Any other condition that would, in the investigator's judgment, contraindicate the
             patient's participation in the clinical study due to safety concerns or compliance
             with clinical study procedures

          -  For patients receiving abemaciclib: The patient has serious and/or uncontrolled
             preexisting medical condition(s) that, in the judgment of the investigator, would
             preclude participation in this study (for example, interstitial lung disease, severe
             dyspnea at rest or requiring oxygen therapy, severe renal impairment [e.g. estimated
             creatinine clearance < 30 ml/min], history of major surgical resection involving the
             stomach or small bowel, or preexisting Crohn's disease or ulcerative colitis or a
             preexisting chronic condition resulting in baseline grade 2 or higher diarrhea)

          -  Major surgery =< 6 weeks prior to starting study drug or failure to recover from side
             effects of such procedure at the discretion of the treating investigator

          -  Pregnant or nursing (lactating) women, where pregnancy is defined as the state of a
             female after conception and until the termination of gestation, confirmed by a
             positive human chorionic gonadotropin (hCG) laboratory test

          -  Medical, psychiatric, cognitive or other conditions that may compromise the patient's
             ability to understand the patient information, give informed consent, comply with the
             study protocol or complete the study