Hepcidin is a 25–amino acid peptide synthesized by hepatocytes and is involved in iron homeostasis. The known molecular target of hepcidin is the ferroportin receptor, which functions as a trans-membrane channel for the transport of cellular iron. Hepcidin induces the endocytosis and proteolysis of ferroportin and thereby decreases the delivery of iron to plasma. Hepcidin is prone to aggregation and is difficult to synthesize, characteristics that hinder its future development. The hormone is a highly structured 4-disulfide bonded beta-hairpin that is highly conserved. Protagonist used a proprietary ‘scaffold hopping’ approach VectrixTM to identify a novel peptidic scaffold that matches key pharmacophores of hepcidin. Early hits identified compounds with micromolar affinity. After further optimization PTG-300 was selected as a clinical development candidate. It reduces cell surface expression of ferroportin with an EC50 of 5 nM and causes >80% reduction in serum iron with significant sustained reduction for 48 hr after a single subcutaneous dose of 1 mg/kg in healthy cynomolgus monkeys.
To show erythropoietic efficacy, 1 mg/kg of PTG-300 was injected subcutaneously Q5D for 4 weeks in six week old Hbbth3+ mice. This resulted in a significant hemoglobin increase of 1.75 g/dL. Reticulocyte count and spleen size were also reduced, and an increase in peripheral red cell number was observed. In separate studies in Hbbth3+ mice, flow cytometry studies of bone marrow and spleen erythroid populations from PTG-300 treated animals demonstrated an increase in the relative proportion of mature erythroid cells. Accumulation of tissue iron was also reduced in the liver and spleen. This data support the development of PTG-300 as a novel therapy for the potential treatment of anemia and iron overload disorders associated with hemoglobinopathies. Currently PTG-300 is undergoing Phase 1 clinical trials.