• To evaluate the safety, tolerability, maximum tolerated dose, and dose-limiting toxicities of XL019 as a single agent when orally administered in subjects with PMF, post-PV MF, or post-ET MF. [ Time Frame: Assessed at periodic visits ] [ Designated as safety issue: Yes ]
  

• Determine plasma pharmacokinetics, evaluate pharmacodynamic correlates, and estimate renal elimination of XL019 [ Time Frame: Assessed at periodic visits ] [ Designated as safety issue: No ]
  
• Evaluate preliminary efficacy of XL019 as a single agent when administered orally [ Time Frame: Assessed at periodic visits ] [ Designated as safety issue: No ]
  

Drug: XL019
gelatin capsules supplied in 5-mg, 25-mg, and 100-mg strengths

Inclusion Criteria:

• The subject has primary myelofibrosis (PMF), post-polycythemia vera MF, or post-essential thrombocythemia MF and requires therapy, including subjects who have received prior MF-directed therapy and relapsed or subjects with refractory disease; or if newly diagnosed, then with intermediate or high risk according to the Lille scoring system.
• The subject is unwilling to undergo or is not a candidate for peripheral stem cell/bone marrow transplant.
• The subject is ≥18 years old.
• The subject has an Eastern Cooperative Oncology Group (ECOG) performance status of ≤2.
• The subject has adequate organ function.
• The subject has the capability of understanding the informed consent document and has signed the informed consent document.
• Sexually active subjects (male and female) must use medically acceptable methods of contraception during the course of the study.
• Female subjects of childbearing potential must have a negative pregnancy test at screening.
• The subject has had no diagnosis of malignancy or evidence of other malignancy for 2 years prior to screening for this study (except non-melanoma skin cancer or in situ carcinoma of the cervix).

Exclusion Criteria:

• The subject has uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, hypertension, symptomatic congestive heart failure, unstable angina pectoris, myocardial infarction within 3 months, or cardiac arrhythmias.
• The subject is pregnant or breastfeeding.
• The subject is known to be positive for the human immunodeficiency virus (HIV).
• The subject is unable or unwilling to abide by the study protocol or cooperate fully with the investigator or designee.