IMMU-03. THERAPEUTIC TARGETING OF TUMORIGENIC
EphA2+/EphA3+ BRAIN TUMOR INITIATING CELLS WITH BISPECIFIC ANTIBODY IN HUMAN GLIOBLASTOMA

Abstract Human glioblastoma (hGBM) carries a dismal prognosis and inevitably relapses despite aggressive therapy. Many of the 14 members of the Eph receptor tyrosine kinase family are expressed in hGBM initiating cells (GICs) and constitute potential molecular targets. We hypothesize that multiple members of the EphR family play a critical role in hGBM recurrence. Using a highly specific human EphR antibody panel, we identified differential expression of EphRs in recurrent hGBM (rGBM). We further characterized EphR co-expression along with multiple GIC markers using mass cytometry (CyTOF). Here we show that EphA2 and EphA3 co-expression marks a highly tumorigenic cell population in rGBM that is enriched in GIC marker expression, and exhibits higher in vitro and in vivo self-renewal and proliferation capacity as compared to EphA2+/EphA3-, EphA2-/EphA3+ or EphA2-/EphA3- cells. Knockdown of EphA2 and EphA3 blocks this self-renewal and proliferation capacity, and is marked by increase in the expression of differentiation marker GFAP. Next, we generated and tested a bispecific antibody (BsAb) that co-targets EphA2 and EphA3. In vitro treatment of rGBM with BsAb led to phosphorylation of EphA2 and EphA3, eventually leading to receptor internalization and degradation. The cellular effect of EphA2/A3 blockade was mediated through the down regulation of Akt and MAPK. Intracranial treatment of immune-deficient mice harboring hGBM with BsAb resulted in non-invasive and significantly smaller tumors. Hence, EphA2 and EphA3 co-expression marks an even more potent GIC population in rGBM, and targeting either single EphA2+ or EphA3+ populations alone will allow the remaining GICs to drive tumor recurrence. For the first time, we show that strategic co-targeting of both EphA2 and EphA3 with a BsAb presents a novel and rational therapeutic approach to recurrent GBM, where multiple GIC populations may be driving the intra-tumoral heterogeneity underlying disease progression.