Key Inclusion Criteria (for Part 3):

• A histologically or cytologically confirmed diagnosis of colorectal adenocarcinoma which is metastatic (measurable disease).
• Documented KRAS- or BRAF- tumor mutation.
• Previously treated with or unsuitable for treatment with 5-Fluorouracil (5-FU), oxaliplatin, irinotecan and, if available, bevacizumab.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2.
• Additional criteria exist.

Key Exclusion Criteria (for Part 3):

• Uncontrolled or symptomatic brain metastases (if the patient has brain metastases and is on steroids, the steroid dose must have been stable for at least 30 days).
• History of central serous retinopathy, retinopathy visible at baseline that would be considered a risk factor for central serous retinopathy or retinal vein occlusion.
• Concomitant malignancies or previous malignancies with less than a 2-year disease free interval at the time of enrollment; patients with adequately resected basal or squamous cell carcinoma of the skin, carcinoma in situ of the cervix, or stage A low grade prostate cancer may enroll irrespective of the time of diagnosis.
• Prior treatment with a MEK inhibitor.
• Treatment with prior chemotherapy, anticancer immunotherapy, monoclonal antibodies or other protein or peptide therapeutics within 21 days of the first dose of study drug.
• Treatment with a small molecule targeted agent or anticancer hormonal therapy within 14 days of the first dose of study drug.
• Treatment with prior radiotherapy within 28 days of initiating study drug (if the radiation portal covered ≤ 10% of the bone marrow reserve, the patient may be enrolled irrespective of the end date of radiotherapy).
• Major surgery within 4 weeks or minor surgery within 7 days prior to the first dose of study drug.
• Known positive serology for the human immunodeficiency virus (HIV), hepatitis C, and/or active hepatitis B.
• Additional criteria exist.