This is a Phase 2 study with an open-label dose escalation phase followed by a blinded withdrawal phase and an open label extension. The study is designed to monitor the PTG-300 safety profile and to obtain preliminary evidence of efficacy of PTG-300 for the treatment of phlebotomy-requiring polycythemia vera.

Phase 2 study in approximately sixty subjects previously diagnosed with Polycythemia Vera who require phlebotomy on a routine basis. There is a 28 week dose finding phase to identify a dose that maintains hematocrit <45%. Subjects who successfully complete the dose finding phase will be entered into a 12 week randomized withdrawal phase to confirm the response. Subsequently patients will enter into an up to 3 year open label extension to investigate long term safety.

Main Inclusion Criteria: All subjects must meet ALL of the following inclusion criteria to be enrolled.

1. Male and female subjects aged 18 years or older.
2. Meet revised 2016 World Health Organization (WHO) criteria for the diagnosis of polycythemia vera.
3. Records of all phlebotomies performed for at least 28 weeks (preferably up to 52 weeks) before dosing are available.
4. Subjects who are not receiving cytoreductive therapy must have been discontinued from any prior cytoreductive therapy for at least 24 weeks before screening and have recovered from any adverse events due to cytoreductive therapy.
5. Subjects receiving cytoreductive therapy with hydroxyurea, interferon, or ruxolitinib must have received cytoreductive therapy for at least 24 weeks and be on a stable dose or have a decreasing dose (Medical Monitor approval required) for at least 8 weeks before dosing and with no planned change in dose.

Main Exclusion Criteria: Subjects must meet NONE of the following exclusion criteria to be enrolled:

1. Active or chronic bleeding within 4 weeks of screening.
2. Meets the criteria for post-PCV myelofibrosis as defined by the International Working Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT).
3. Known primary or secondary immunodeficiency.
4. Any surgical procedure requiring general anesthesia within 1 month prior to screening or planned elective surgery during the study.

Part 1 open label, Part 2 blinded, Part 3 open label