EphA3 As a Target For Monoclonal Antibody Therapy For Acute Leukemia

Eph receptor tyrosine kinases interact with cell-surface ephrin ligands to direct cell growth, migration and cell positioning. High expression of EphA3 in a number of cancers has been linked to progression through facilitation of invasiveness and metastatic spread. EphA3 protein, which was originally described in leukemia, has also reported to be expressed in sarcomas, lung cancer, melanoma and glioblastoma. Using a Humaneered® derivative of the IIIA4 anti-EphA3 monoclonal antibody, KB004 (KaloBios Pharmaceuticals Inc), a clinical trial has commenced which targets the EphA3 protein in leukemia. The EphA3-specific monoclonal antibody, IIIA4, binds and activates human and mouse EphA3 with similar affinities, binding is followed by internalization of receptor-antibody complexes. We have shown high expression of EphA3 in the LK63 pre-B ALL and no expression of EphA3 in the Reh, a similar pre-B ALL cell line. We have examined the effect of the IIIA4 antibody in LK63 and Reh NOD/SCID xenograft models. In the LK63 xenograft model, administration of the IIIA4 antibody led to inhibition of tumour growth and decrease spread from bone marrow to the spleen and other organs and increased the latency of the disease, however no reduction in engraftment was observed in Reh xenograft model, suggesting that the effect was directed against the leukemic cells rather than the stromal and vascular elements. To further analyse the function of EphA3 expression, LK63 EphA3 knock down and EphA3-transfected Reh cells were tested in the xengraft model. Similar to LK63, antibody-treated Reh EphA3 xenografts showed reduction in the bone marrow engraftment and increased the latency of the disease. In contrast, LK63 EphA3 knock down xenografts showed minimal difference between treated and control group but notably showed a significant reduction in the splenic engraftment compared to the normal LK63 model. These results are consistent with IIIA4-mediated effects on the growth of EphA3-positive leukemic cells in the bone marrow and a secondary effect on the subsequent spread of the leukemic cells to extra-medullary sites and provides evidence that EphA3 is a functional therapeutic target in acute leukemias.