This is an open label, single arm trial of PTG-300 in subjects with PV who are newly diagnosed or for whom current therapy is not sufficient to control their hematocrit and have hematocrit >48% prior to dosing. The PTG-300 dose and schedule may be adjusted every 2 to 4 weeks to maintain hematocrit <45% with a target of <43%. Subjects may receive PTG-300 treatment for up to 52 weeks.

Inclusion Criteria:

1. Known diagnosis of polycythemia vera.
2. Hematocrit >48% before dosing.
3. Evidence of hematocrit >48% three or more times in the 28 weeks before dosing or five or more times in 52 weeks before dosing (except for newly diagnosed patients).
4. Clinically reasonable alternative causes for erythrocytosis have been evaluated and excluded.
5. Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2.

Exclusion Criteria:

1. Clinically significant thrombosis (e.g., deep vein thrombosis or splenic vein thrombosis) within 3 months of Screening.
2. Active or chronic bleeding within 4 weeks of Screening.
3. Meets the criteria for post-PV myelofibrosis as defined by the International Working Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT).
4. Infection requiring hospitalization or intravenous antimicrobial therapy, or opportunistic infection within 3 months of dosing; any infection requiring systemic antimicrobial therapy within 4 weeks of dosing. Prophylactic antibiotics are allowed.
5. Any serious or unstable medical or psychiatric condition that would prevent, (as judged by the Investigator) the subject from properly providing informed consent or any condition which would jeopardize compliance with the study.
6. Known primary or secondary immunodeficiency.
7. Known positive for active hepatitis B, hepatitis C or human immunodeficiency virus (HIV) infection.
8. Any surgical procedure requiring general anesthesia within 1 month prior to Screening or planned elective surgery during the study.
9. History of invasive malignancies within the last 2 years, except non-melanoma skin cancer and localized curatively treated prostate cancer or cervical cancer.
10. Current or recent history of alcohol dependence or illicit drug use within 1 year prior to Screening.