Abstract 2471: EGFRvIII T-cell TandAbs are specific and highly potent drug candidates for the treatment of a variety of solid tumors

To harness the cytotoxic capacity of immune effector cells for the treatment of several types of solid tumors, we developed tetravalent bifunctional antibodies that recognize EGFRvIII, the deletion variant III of EGFR. Their second functionality binds with high affinity to CD3, thereby directing T-cells to eliminate EGFRvIII+ cancer cells. The expression of EGFRvIII on various solid tumor types, and its absence from healthy tissues, provides an opportunity to develop cytotoxic antibodies that solely target cancer; these would spare normal tissues, and substantially reduce the side effects associated with EGFR therapy. Using phage display, we identified scFvs that selectively bind to EGFRvIII but not to EGFR. The binding affinities of these highly EGFRvIII-specific antibodies were substantially improved, employing affinity maturation techniques, and achieved KDs in the 100pM range and lower. We engineered a panel of bispecific EGFRvIII/CD3 TandAbs with a broad range of binding and cytotoxic properties. Mono- and bivalent binding, specificity for both EGFRvIII and CD3, T cell-mediated cytotoxic activity, and target-mediated T-cell activation were characterized in a panel of in vitro assays. EGFRvIII/CD3 TandAbs exhibit exquisite specificity towards the EGFRvIII antigen in Western Blot, SPR, ELISA, and FACS assays of EGFRvIII+ cells. No specific binding was observed to recombinant EGFR antigen or to EGFR-expressing cells. Apparent affinities of EGFRvIII/CD3 TandAbs to EGFRvIII were up to 25fold improved relative to the monovalently binding scFvs, and achieved a KD of 11pM for the best binding EGFRvIII/CD3 TandAb. Improvement of the binding affinities in the bivalent TandAb format was largely due to slower dissociation. TandAbs with high affinity for EGFRvIII were most potent in killing assays, displaying cytotoxicity towards EGFRvIII-expressing F98 glioma and CHO cells with EC50 in the range of 1pM-10pM. No cytotoxicity was observed on EGFR+ cells or EGFRvIII- cells up to the maximally-evaluated TandAb concentration of 0.5μM (which is 100000 fold higher than the EC50) demonstrating the high selectivity of EGFRvIII/CD3 TandAbs for the tumor-specific EGFRvIII. High affinity binding to CD3 was necessary for efficacious T cell recruitment as shown by the correlation of CD3-binding and cytotoxic potency of EGFRvIII/CD3 TandAbs. Importantly, in the absence of EGFRvIII+ target cells in vitro, TandAbs did not elicit T cell activation, as demonstrated by their lack of