This is an open-label, multi-center, Phase II trial studying the combination of mocetinostat and gemcitabine in patients who have previously demonstrated disease progression either while, or within six months after, receiving chemotherapy with a gemcitabine-based regimen.

Inclusion Criteria:

• Age ≥ 18 years
• Histologically documented leiomyosarcoma
• Prior systemic therapy with a gemcitabine containing regimen
• ECOG Performance Status of ≤ 1
• Measurable metastatic disease with a target lesion that has increased in size by 20% in maximal dimension either during or within six months after treatment with chemotherapy using a gemcitabine containing regimen
• Adequate organ function within 14 days of study entry
• Patients or their legal representative must be able to read (or have read to them), understand, and sign a written informed consent (approved by the institutional review board) within 14 days prior to start of treatment.

Exclusion Criteria:

• Concurrent, clinically significant, active malignancies (excluding basal cell carcinoma or cervical intraepithelial neoplasia [CIN] in situ or melanoma in situ) (Stage II portion only)
• Patients with baseline QTcF ≥ 480 msec
• Patients with uncontrolled concurrent illness, active infection requiring i.v. antibiotics, or uncontrolled infections, or a fever > 38.5°C on the day of scheduled dosing
• Patients with serious illnesses, medical conditions, or other medical history, including a prior history of pericarditis/pericardial effusion, or abnormal laboratory results, which, in the investigator's opinion, would be likely to interfere with a patient's participation in the study, or with the interpretation of the results
• Patients who have received any investigational drug within 28 days prior to Day 1 of study entry (an investigational drug is one for which there is no approved indication), or who are receiving concurrent treatment with other experimental drugs or anti-cancer therapy
• Pregnant or lactating women. Women of child-bearing potential (WOCBP) must have a negative serum pregnancy test documented within 72 hours prior to starting study drug or a urine pregnancy test shall be done on Day 1 of Cycle 1
• Women of child bearing potential and their partners must use an acceptable method of contraception while enrolled on this study, and for a period of 3 months following study drug treatment. Patients unwilling or unable to follow this guideline will be excluded. Investigators should follow their Institutional standard regarding acceptable methods of contraception.
• Known hypersensitivity to HDAC inhibitors or to any of the components of mocetinostat
• Known hypersensitivity to gemcitabine
• Any condition that will put the patient at undue risk or discomfort as a result of adherence to study procedures. For example, consider requirement to take mocetinostat with water and recommendation to avoid agents that increase gastric pH
• Any condition (e.g., known or suspected poor compliance, psychological instability, geographical location, etc) that, in the judgment of the investigator, may affect the patient's ability to sign the informed consent and undergo study procedures.

through study completion, an average of 3 months