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Background. The majority of PV patients are iron deficient at diagnosis [Ginzburg Leukemia 2018]. PV patients are treated with periodic therapeutic phlebotomy (TP) in order to maintain hematocrit levels <45% in an effort to reduce the incidence of thrombotic events [Marchioli NEJM 2013]. Symptomatic iron deficiency represents a challenge in PV as it is commonly present at diagnosis and TP worsens iron deficiency and PV patients can be severely symptomatic from their iron deficiency [Krayenbuehl Blood 2011]. Iron deficiency is defined as insufficient iron stores to meet the needs of cellular homeostasis. We hypothesized that both iron deficiency and expanded erythropoiesis in PV lead to suppression of hepcidin, the body's main negative regulator of iron metabolism, and that hepcidin suppression enhances iron absorption and iron availability for enhanced erythropoiesis in TP-requiring PV patients. We previously demonstrated that PTG-300, a hepcidin-mimetic, caused dose-related anemia in pre-clinical studies. In healthy volunteers PTG-300 decreases serum iron and transferrin saturation (TSAT) by >70% within 12 hours and the effect persists for 3-7 days. In a phase 2 trial in β-thalassemia, PTG-300 also decreased serum iron and TSAT but did not demonstrate off-target effects. The current study aims to compare the iron status in frequent TP-requiring PV patients before, during, and after treatment with PTG-300. Methods. Polycythemia patients who met 2016 WHO criteria for diagnosis were enrolled in the 28-week dose finding part of a Phase 2 trial. All patients required ≥3 phlebotomies with or without concurrent cytoreductive therapy over 6 months prior to enrollment. Patients were given PTG-300 doses of 10, 20, 40, 60 and 80 mg administered subcutaneously weekly in individualized adjustment to maintain hematocrit <45%. Body iron status was quantified by monitoring serum ferritin, serum iron, transferrin saturation (TSAT), mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH). Results. Thirteen subjects were enrolled to date: 7/13 with low risk, mean age 57.4 years (range 31-74). Six receiving TP alone, 6 on concurrent hydroxyurea, 1 on concurrent interferon; TP in the 24 weeks prior to enrollment = 3-9; median time between TP = 42 days. All subjects-maintained hematocrit <45% after appropriate dose adjustment. Mean baseline values were serum ferritin = 14.2 ng/mL (5, 37); serum iron = 33.0 ug/dL (16.8, 107.8); and TSAT = 7.6% (4, 30). During treatment with PTG-300, serum ferritin levels increased progressively toward normal (Figure 1a) reflecting increase in iron stores. TSAT (Figure 1b) and serum iron values increased modestly but remained below normal ranges, reflecting PTG-300's pharmacodynamic effect of inhibiting iron release from intracellular stores. This was associated with increased MCV (Figure 1c) and MCH (Figure 1d) and decreased hematocrit and erythrocyte counts, together suggesting a normalization of iron distribution. Conclusions. The current results indicate that PTG-300 is an effective agent for the controlling hematocrit and reversing iron deficiency. The effect of PTG-300 on PV-related symptoms and those of iron deficiency, are also being evaluated. Continued patient enrollment will enable more definitive conclusions regarding the efficacy and safety of hepcidin mimetic PTG-300 in PV patients with high TP requirements. Disclosures Kremyanskaya: Incyte Corporation: Research Funding; Bristol Myers Squibb: Research Funding; Astex Pharmaceuticals: Research Funding; Constellation Pharmaceuticals: Research Funding; Protagonist Therapeutics: Consultancy, Research Funding. Kuykendall:Novartis: Research Funding; Blueprint Medicines: Research Funding; BMS: Research Funding; Incyte: Research Funding. Yacoub:Agios: Honoraria, Speakers Bureau; Incyte: Speakers Bureau; Hylapharm: Current equity holder in private company; Cara Therapeutics: Current equity holder in publicly-traded company; Ardelyx: Current equity holder in publicly-traded company; Dynavax: Current equity holder in publicly-traded company; Novartis: Speakers Bureau; Roche: Other: Support of parent study and funding of editorial support. Yang:AROG: Research Funding; AstraZeneca: Research Funding; Protagonist: Research Funding; Jannsen: Research Funding. Gupta:Protagonist: Current Employment. Valone:Protagonist: Current Employment. Khanna:Protagonist: Current Employment, Current equity holder in publicly-traded company. Hoffman:Abbvie: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding; Forbius: Consultancy; Protagonist: Consultancy. Verstovsek:Incyte Corporation: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Sierra Oncology: Consultancy, Research Funding; ItalPharma: Research Funding; Celgene: Consultancy, Research Funding; Gilead: Research Funding; Promedior: Research Funding; Protagonist Therapeutics: Research Funding; NS Pharma: Research Funding; CTI Biopharma Corp: Research Funding; Blueprint Medicines Corp: Research Funding; Genentech: Research Funding; PharmaEssentia: Research Funding; Roche: Research Funding; AstraZeneca: Research Funding.