Inclusion Criteria:

        Each patient must meet all of the following inclusion criteria to be enrolled in the study:

          -  Age >/= 18 years and are capable of giving informed consent. Voluntary written
             informed consent before performance of any study-related procedure not part of normal
             medical care, with the understanding that consent may be withdrawn by the subject at
             any time without prejudice to future medical care.

          -  Patients must have a pathologically confirmed diagnosis of prostate adenocarcinoma.
             Features of neuroendocrine phenotype are allowed.

          -  Patients must have evidence of castration resistant metastatic disease and eligible
             for Enzalutamide per standard guidelines. Castration resistant non-metastatic disease
             is allowed in phase I study if subject is candidate for Enzalutamide.

          -  Patients must have and ECOG performance status of ≤ 2.(appendix D)

          -  Patients must be on continuous LHRH agonist or antagonist treatment or surgically
             castrated with castrate levels of testosterone (< 20 ng/dl).

          -  Any number of prior chemotherapy regimens are allowed. Chemotherapy naïve patients are
             allowed.

          -  If patient is already on Enzalutamide at a dose of 160mg daily, he is allowed if he
             can have baseline image and PSA within 1 month of the start of entinostat.

          -  Patients may have had androgen synthesis inhibitors or other investigational drugs.
             Patient must have discontinued flutamide or nilutamide or other AR targeted agents
             (including abiraterone) for at least 4 weeks and bicalutamide for at least 6 weeks
             prior to day 1 of treatment.

          -  Patients receiving treatment with bisphosphonates or denosumab must remain on
             treatment during the study.

          -  Patients must not require concurrent radiation or other chemotherapy while receiving
             protocol therapy. Patients may have received previous radiation but must have
             completed radiation at least 4 weeks (8 weeks for radiation to the brain) prior to
             registration.

          -  Patients must have recovered to grade ≤ 1 from all acute toxicity of previous
             radiation, hormonal or chemotherapy treatments.

          -  Patient agrees to use an acceptable method for contraception during the entire study
             treatment period through 4 months after the last dose of entinostat.

          -  Adequate renal function as defined by serum creatinine ≤ 1.5 x ULN. If creatinine >1.5
             x ULN, calculated or measured creatinine clearance must be ≥ 60 mL/minute
             (Cockcroft-Gault).

          -  ANC > 1500/mm³, platelets > 100,000/mm³, Hgb > 9 g/dL.

          -  Total bilirubin ≤ ULN, SGOT (AST) and SGPT (ALT)< 1.5 x ULN. AST and/or ALT may be up
             to 5X ULN in the setting of known liver metastasis.

        Exclusion Criteria:

        Patients meeting any of the following exclusion criteria are not to be enrolled in the
        study:

          -  Patient was treated and discontinued Enzalutamide previously for any reason.

          -  Major surgery within 28 days or serious infection requiring IV antibiotics within 14
             days preceding the first dose of study treatment

          -  Patient has received other investigational drugs within 14 days before enrollment.

          -  Known GI disease or GI procedure that could impact drug absorption in the upper bowel,
             or tolerance of Entinostat. Examples include but are not limited to partial
             gastrectomy, small bowel resection, pancreatectomy, malabsorption or celiac disease

          -  Ongoing nausea or vomiting of any severity without improvement after appropriate
             treatment.

          -  > Grade 1 diarrhea, not controlled with appropriate treatment.

          -  History of uncontrolled sleep apnea syndrome and other conditions that could result in
             excessive daytime sleepiness, such as severe chronic obstructive pulmonary disease
             requiring supplemental oxygen.

          -  Clinical and/or radiographic evidence of cerebral metastases. However, patients who
             have a history of central nervous system (CNS) metastasis but who have no radiographic
             or clinical evidence of residual tumor (eg, following complete surgical resection or
             stereotactic radiosurgery) are not excluded from participation in this study.

          -  Myocardial infarction within 6 months prior to enrollment or has New York Heart
             Association (NYHA) Class III or IV heart failure, uncontrolled angina, severe
             uncontrolled ventricular arrhythmias, or electrocardiographic evidence of acute
             ischemia or active conduction system abnormalities. Prior to study entry, any EKG
             abnormality at Screening has to be documented by the investigator as not medically
             relevant.

          -  Serious medical or psychiatric illness or laboratory abnormality that may increase the
             risk associated with study participation or investigational product administration or
             may interfere with the interpretation of study results and, in the judgment of the
             investigator, would make the patient inappropriate for enrollment in this study.

          -  Any other currently active malignancy (excluding controlled non-melanoma skin cancer).
             Patients are considered NOT to have "currently active" malignancy if they have
             completed any necessary therapy and are considered by their physician to be at less
             than 30% risk of relapse.

          -  Treatment with clinically significant enzyme inducers, such as the enzyme-inducing
             antiepileptic drugs phenytoin, carbamazepine or phenobarbital, or rifampin, rifabutin,
             rifapentine or St. John's Wort within 14 days prior to the first dose of Entinostat
             and during the study.

          -  History of seizure and on active therapy for seizure

          -  Known history of uncontrolled human immunodeficiency virus (HIV) infection. HIV
             positive patients will be allowed if they are on treatment and have an adequate CD4
             count (CD4 count >200). Screening is not required in the absence of clinical findings
             or suspicion.