A bispecific antibody that targets the membrane-proximal region of mesothelin and retains high anticancer activity in the presence of shed mesothelin

Mesothelin (MSLN) is a cell-surface protein that is expressed on many cancers, which makes it a popular target for antibody-based cancer therapy. However, MSLN is shed from cancer cells at high levels via proteases that cleave at its membrane-proximal C-terminal region. Shed MSLN accumulates in patient fluids and tumors and can block antibody-based MSLN-targeting drugs from killing cancer cells. A previously established monoclonal antibody (mAb), 15B6, binds MSLN at its protease-sensitive C-terminal region and does not bind shed MSLN. 15B6 variable fragment (Fv)-derived chimeric antigen receptor (CAR) T cells are not inhibited by shed MSLN and kill tumors in mice more effectively than mAb SS1 Fv-derived CAR T cells, which bind an epitope retained in shed MSLN. Here, we have established 15B6 Fv-derived MSLN x CD3 bispecific antibodies (BsAbs) that target MSLN-expressing cancers. We identified our lead candidate, BsAb 5, after screening multiple 15B6-derived BsAb formats in vitro for cytotoxic activity. BsAb 5 activates T cells to kill various cancer cell lines in a MSLN-specific manner. MSLN 296-591 His, a recombinant protein mimicking shed MSLN, does not inhibit 15B6-derived BsAb 5 but completely inhibits humanized SS1-derived BsAb 7. Furthermore, BsAb 5 inhibits and delays tumor growth and is not inhibited by MSLN 296-585 His in mice. Our findings indicate that by targeting the protease-sensitive region of MSLN, BsAb 5 has high MSLN-specific anticancer activity that is not inhibited by shed MSLN. BsAb 5 may be a promising immunotherapy candidate for MSLN-expressing cancers.