A Safety Study of XL019 in Adults With Myelofibrosis

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.