Abstract 2584: The CEA/CD3-bispecific antibody MEDI-565 (MT111) binds a nonlinear epitope present in the full-length but not a short splice variant of CEA

Abstract

Abstract MEDI-565 (MT111) is a novel bispecific single-chain antibody of the BiTE (Bi-specific T-cell engager) class that transiently links carcinoembryonic antigen (CEA) on tumor cells with human CD3 on T cells. Engagement of CD3 and CEA by MEDI-565 induces T cells to kill CEA-positive cancer cells using similar elements and cellular machinery as required for T cells to kill through classic T cell receptor/major histocompatibility complex-peptide signaling. Examination of DNA sequence databases revealed a canonical CEA sequence (full length CEA; NCBI RefSeq NP_004354.2; 702 amino acids) and a previously uncharacterized shorter mRNA splice variant (short CEA; NCBI RefSeq CR749337420 amino acids) that lacks the second and third IgC-like domains and part of the N-terminal IgV-like domain. Expression of full-length CEA mRNA in colorectal, pancreatic, lung, and breast tumors was more widespread than that of short CEA; however, concordant expression of short CEA with the full length version was always observed. Mapping of the MEDI-565 binding region of CEA revealed a nonlinear, conformational epitope in the third immunoglobulin C-like domain. With a combination of deletion and swap variants, site directed mutagenesis, and computational homology modeling, we further identified the critical residues for MEDI-565 binding comprised of Phe292, Thr294, Asn299, Val354, Gly355, Pro356, Glu358, Ile374, and Asn376. CHO cells expressing only the short variant of CEA lacking this epitope failed to bind MEDI-565 and did not undergo T cell-directed lysis in the presence of MEDI-565. These studies suggest that MEDI-565 can discriminate between the full-length and short splice variant of CEA and may broadly target CEA-positive colorectal, pancreatic, lung, and breast tumors without regard for expression of the short splice variant of CEA. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2584.