Use of Ppmx-T003 As a Potent Inhibitor of Erythrocytosis in Polycythemia Vera

Iron incorporation through transferrin (Tf) and its receptor, TfR1, plays a critical role in erythropoiesis. We hypothesized that blockade of TfR1 may be a promising strategy to suppress erythrocytosis observed in polycythemia vera (PV). To examine this possibility, we developed PPMX-T003, a monoclonal antibody against human TfR1, through phage display screening. PPMX-T003 was observed to bind to TfR1 with a high affinity(K_d= 3.56×10^-10)and presented strong ability to suppress the growth of PV model cell lines such as HEL (EC₅₀= 0.07 nM). PPMX-T003 induced TfR1 internalization but blocked the recycling of the receptor presumably because it was strongly bound, even in the endosome, which switched the fate of TfR1 from recycling to degradation. Consequently, the PPMX-T003 treatment greatly reduced the intracellular iron concentration, along withcell cycle arrest in the G2/M phase in HEL cells. In contrast, PPMX-T003 failed to block cell proliferation in human umbilical vein endothelial cells that expressed a subtle level of TfR1. These findings suggest that PPMX-T003 is a potent inhibitor for cells whose growth is dependent on iron incorporation by TfR1. This led us to examine whether PPMX-T003 blocked the endogenous erythroid colony (EEC) formation in erythroid progenitor cells in PV. We observed that PPMX-T003, at a concentration of 40 ng/mL, exhibited complete blockade of EEC formation inJAK2 V617F-positive PV patients' cells. Although hydroxycarbamide (HU) and phlebotomy are well-established treatment strategies for the management of hematocrit in PV patients, there is a risk of secondary leukemia associated with HU treatment and side effects such as fatigue and restless leg syndrome due to phlebotomy-induced iron deficiency. Therefore, we suggest PPMX-T003, which may preferentially suppress erythrocytosis without changing the iron level in peripheral blood, as an alternative treatment for PV patients.