Highly efficient elimination of melanoma cells expressing melanoma-associated chondroitin sulfate proteoglycan (MCSP) by MCSP/CD3-bispecific single-chain antibody constructs

A61 Bispecific single-chain antibody constructs of the BiTE class can redirect unstimulated human T cells to efficiently lyse target cells at picomolar concentrations. Formation of a proper cytolytic synapse and serial lysis of target cells by BiTE-activated T cells have been demonstrated. A BiTE directed against CD19 (MT103/MEDI-538) produced radiographically confirmed clinical responses in heavily pre-treated non-Hodgkin lymphoma patients, providing clinical proof-of-concept for this novel bispecific antibody format. Another BiTE with specificity for EpCAM (CD326) has shown in animal models eradication of established subcutaneous tumors and primary human tumor tissue.
 Here, we have constructed novel BiTE molecules with dual specificity for CD3 and MCSP by combining MCSP-specific murine single-chain antibodies with a CD3-specific deimmunized murine single-chain antibody. For further reduction of immunogenicity, we have also constructed MCSP-BiTE molecules from fully human single-chain antibodies specific for MCSP and CD3. MCSP is among the best validated and most frequently expressed melanoma surface antigens and has been used as target for both antibody-based therapeutic and diagnostic approaches in melanoma patients.
 MCSP-BiTE molecules showed at picomolar concentrations potent redirected lysis by human T cells of chinese hamster ovary (CHO) cells transfected with human MCSP. Likewise, two melanoma cell lines established from a freshly dissected tumor of a stage III melanoma patient were killed with similar efficacy by T cells redirected through MCSP-BiTE molecules. This was observed in a dose-dependent fashion with allogeneic T cells as well as with autologous T cells of the melanoma patient. Various controls demonstrated that the observed target cell lysis was highly specific for MSCP.
 Given the susceptibility of human melanoma to various T cell therapies, MCSP-BiTE may provide a novel treatment option for melanoma patients. A particular advantage of an MCSP-BiTE over vaccination, adoptive T cell transfer and anti-CTLA-4 approaches is that major immune evasion mechanisms of melanoma cells, such as loss of MHC class I and perturbed antigen processing, do not restrict the activity of BiTE-activated cytotoxic T cells.