Inclusion Criteria:

          -  Willing and able to give informed consent

          -  Histologically confirmed prostate cancer (not exclusive of adenocarcinoma)

          -  mCRPC that has progressed on at least 1 therapy progression (defined as Prostate
             Cancer Working Group 2 [PCWG2] or at investigators' discretion) approved for treatment
             of mCRPC, one of which must include abiraterone acetate and/or enzalutamide

          -  Metastatic tumor that has been biopsied

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-2

          -  Willing to undergo biopsy of a metastatic lesion at the time of progression

          -  Patients must have ongoing therapy to maintain serum testosterone < 50 ng/dL

          -  Absolute neutrophil count > 1,500/uL during screening evaluation

          -  Platelet count > 100,000/uL during screening evaluation

          -  Hemoglobin > 9 g/dL during screening evaluation

          -  Total bilirubin within the reference range during screening evaluation

          -  Alanine aminotransferase (ALT) within the reference range during screening evaluation

          -  Aspartate aminotransferase (AST) within the reference range during screening
             evaluation

          -  Creatinine < (1.5 mg/dL) during screening evaluation (> 1.5 is allowed if epidermal
             growth factor receptor [EGFR] > 45 mL/min/1.73 m^2)

          -  International normalized ratio (INR) < 1.3 (or < 3 if on warfarin or other
             anticoagulants) during screening evaluation

          -  Left ventricular ejection fraction (LVEF) >= 45% as measured by echocardiogram during
             screening evaluation

          -  Electrocardiogram (EKG) without clinically significant abnormality

        Exclusion Criteria:

          -  A history of retinal vein occlusion (RVO) or risks factors for RVO

          -  A history of retinal pigment epithelial detachment (RPED) or risk factors for RPED

          -  Clinically significant abnormality on ophthalmologic examination during screening
             evaluation

          -  Clinically significant cardiovascular disease including:

               -  LVEF < 45% measured by echocardiogram

               -  History of acute coronary syndromes (including myocardial infarction and unstable
                  angina), coronary angioplasty, or stenting within 6 months

               -  Uncontrolled angina within 3 months

               -  New York Heart Association (NYHA) class III or IV congestive heart failure

               -  Clinically significant abnormality on EKG

               -  History of clinically significant ventricular arrhythmias (e.g., ventricular
                  tachycardia, ventricular fibrillation, torsades de pointes)

               -  Patients with intra-cardiac defibrillators or permanent pacemakers

          -  Presence of a comorbid disease or medical condition that would impair the ability of
             the patient to receive or comply with the study protocol

          -  History of interstitial lung disease or pneumonitis

          -  Use of any medication or herbal products that may have hormonal anti-prostate cancer
             activity and/or are known to modulate PSA levels (e.g., saw palmetto) or systemic
             corticosteroids greater than the equivalent of 10 mg of prednisone per day within 4
             weeks of enrollment

          -  Prior use of trametinib or other mitogen activated protein kinase (MAPK) inhibitor in
             any context

          -  Known or suspected brain metastasis or active leptomeningeal disease or spinal cord
             compression

          -  Gastrointestinal disorder affecting absorption (e.g., gastrectomy, active peptic ulcer
             disease within last 3 months, inflammatory bowel disease)

          -  Any major surgery, extensive radiotherapy, chemotherapy with delayed toxicity,
             biologic therapy, or immunotherapy within 30 days of enrollment and/or daily or weekly
             chemotherapy without the potential for delayed toxicity within 14 days of enrollment

          -  Hospitalization within 30 days of enrollment for cancer related events

          -  History of another malignancy within the previous 5 years other than curatively
             treated non-melanoma skin cancer

          -  Use of an investigational agent within 4 weeks of enrollment

          -  Use of any medications known to affect the serum androgen level

          -  Any condition or reason that, in the opinion of the investigator, interferes with the
             ability of the patient to participate in the trial, which places the patient at undue
             risk, or complicates the interpretation of safety data