Study of SB939 in Subjects With Myelofibrosis

Inclusion Criteria:

1. Must be equal to or greater than 18 years of age
2. Must be diagnosed with PMF or post ET/PV MF with intermediate-1, intermediate -2 or high risk disease according to the IWG prognostic scoring system, or if with low risk disease then with symptomatic splenomegaly that is equal to or greater than 5 cm below left costal margin by physical exam.
3. Must have adequate organ function as demonstrated by the following: • ALT (SGOT) and/or AST (SGPT) equal to or less than 2.5x upper limit of normal (ULN), [unless upon judgment of the treating physician, it is believed to be due to extramedullary hematopoiesis (EMH) related to MF] • Total bilirubin equal to or less than 1.5 x ULN • Serum creatinine equal to or less than 2.5 mg/dL
4. ECOG performance status (PS) of 0, 1, or 2
5. At least 2 weeks from prior MF-directed treatment (till the start of study drug)
6. Treatment-related toxicities from prior therapies must have resolved to Grade equal to or less than 1
7. No other active malignancies.
8. Females of childbearing potential (a sexually mature woman who: 1) has not undergone a hysterectomy or bilateral oophorectomy; or 2) has not been naturally postmenopausal for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months)).must have negative pregnancy test.

Exclusion Criteria:

1. Prolongation of the QTc interval to >470 msec at baseline ECG
2. Known positive status for HIV, or known active hepatitis A, B, or C infection.
3. Any serious medical condition or psychiatric illness that would prevent, (as judged by the treating physician) the subject from signing the informed consent form or any condition, including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study or confounds the ability to interpret data from the study.
4. Pregnant or lactating females.
5. Current use of drugs known to prolong QTc interval.