The study consists of two phases: The first portion of the study is a Phase 1 dose escalation study to determine the maximum tolerated dose and the dose limiting toxicities of SB1518 when given as a single agent orally once daily in subjects with Chronic Idiopathic Myelofibrosis (CIMF) regardless of their JAK2 mutational status. The second portion of the study is a Phase 2 study to define the efficacy and safety profile of single agent SB1518 at the recommended dose in subjects with CIMF.

Inclusion Criteria

• Subjects with CIMF (including post ET/PV MF) requiring therapy, including:
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2
• Men of reproductive potential who can agree to practice effective contraception during the entire study period and for one month after the last study treatment
• Women of child-bearing potential who have a negative pregnancy test within 14 days prior to the first dose of study drug and can agree to practice effective contraception during the entire study period and for one month after the last study treatment, unless documentation of infertility exists
• Subjects who are able to understand and willing to sign the informed consent form

Exclusion Criteria

• Subjects with uncontrolled inter-current illness including, but not limited to, ongoing active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements as judged by treating physician. Subjects receiving antibiotics for infections that are under control may be included in the study unless the antibiotic is a CYP3A4 inducer/inhibitor
• Subjects known to be HIV-positive
• Subjects with known active hepatitis A, B, or C, or latent hepatitis B
• Women who are pregnant or lactating
• Subjects with prior radiation therapy to more than 20% of the hematopoietic marrow (prior radiation to spleen is allowed)