The purpose of this study is to evaluate the safety and tolerability of the JAK2 inhibitor XL019 administered orally in adults with Polycythemia Vera.

Inclusion Criteria:

• The subject has a diagnosis of polycythemia vera (PV), and has failed, or is intolerant of, standard therapies or refuses to take standard medications.
• 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.
• Subjects who have received phlebotomy due to PV must have documented phlebotomy history for 12 weeks prior to enrollment.
• 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 other diagnosis of malignancy or evidence of other malignancey 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 received treatment for PV within 14 days prior to first dose of XL019
• 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.