Mocetinostat is an orally administered histone deacetylase (HDAC) inhibitor. Azacitidine is a hypomethylating agent (HMA) used to treat MDS. In this study, patients with intermediate- or high-risk MDS will receive treatment with mocetinostat and azacitidine to evaluate the safety of the study treatment. Safety assessments will include echocardiograms, electrocardiograms and routine safety laboratory studies (hematology and serum chemistry). In addition, clinical response to treatment will be monitored using bone marrow aspirates or biopsies, and other routine methods.

The study will include multiple cohorts of patients. In the first cohort, patients may have previously received treatment with hypomethylating agents such as azacitidine. In later cohorts, prior treatment with this class of anti-cancer agents will be excluded. Later cohorts will include patients that are receiving this class of agents, specifically azacitidine, for the first time.

Inclusion Criteria:

Diagnosis of intermediate- or high-risk (IPSS criteria) myelodysplastic syndrome.

Cohort 1: Any prior treatment, enrollment complete. Cohort 2: Limited or no prior treatment for MDS. Prior treatment should not include hypomethylating agents such as azacitidine or decitabine, or HDAC inhibitors.

ECOG Performance Status 0 or 1.

Exclusion Criteria:

Current or history of small, moderate or large pericardial effusion, tamponade and/or pericarditis.

Significant cardiac abnormalities such as recent myocardial infarction, congestive heart failure ≥ Class 3, or symptomatic, uncontrolled atrial fibrillation, atrial flutter or sinus tachycardia.

Prolonged QT/QTc interval.

Other active cancer excluding basal cell carcinoma or cervical intraepithelial neoplasia.