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 in many cancers, which makes it a popular target for Ab-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 patients' fluids and tumors and can block Ab-based MSLN-targeting drugs from killing cancer cells. A previously established mAb, 15B6, binds MSLN at its protease-sensitive C-terminal region and does not bind shed MSLN. Moreover, 15B6 variable fragment (Fv)-derived chimeric antigen receptor T cells are not inhibited by shed MSLN and kill tumors in mice more effectively than mAb SS1 Fv-derived chimeric antigen receptor T cells, which bind an epitope retained in shed MSLN. In this study, we have established 15B6 Fv-derived MSLN × CD3 bispecific antibodies (BsAb) 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.

Tofacitinib to prevent anti-drug antibody formation against LMB-100 immunotoxin in patients with advanced mesothelin-expressing cancers.

Background: LMB-100 is a mesothelin (MSLN)-targeting recombinant immunotoxin (iTox) carrying a Pseudomonas exotoxin A payload that has shown promise against solid tumors, however, efficacy is limited by the development of neutralizing anti-drug antibodies (ADAs). Tofacitinib is an oral Janus Kinase (JAK) inhibitor that prevented ADA formation against iTox in preclinical studies. Methods: A phase 1 trial testing LMB-100 and tofacitinib in patients with MSLN-expressing cancers (pancreatic adenocarcinoma, n=13; cholangiocarcinoma, n=1; appendiceal carcinoma, n=1; cystadenocarcinoma, n=1) was performed to assess safety and to determine if tofacitinib impacted ADA formation. Participants were treated for up to 3 cycles with LMB-100 as a 30-minute infusion on days 4, 6, and 8 at two dose levels (100 and 140 µg/kg) while oral tofacitinib was administered for the first 10 days of the cycle (10 mg BID). Peripheral blood was collected for analysis of ADA levels, serum cytokines and circulating immune subsets. Results: The study was closed early due to occurrence of drug-induced pericarditis in 2 patients. Pericarditis with the combination was not reproducible in a transgenic murine model containing human MSLN. Two of 4 patients receiving all 3 cycles of treatment maintained effective LMB-100 levels, an unusual occurrence. Sustained increases in systemic IL-10 and TNF-α were seen, a phenomenon not observed in prior LMB-100 studies. A decrease in activated T cell subsets and an increase in circulating immunosuppressive myeloid populations occurred. No radiologic decreases in tumor volume were observed. Discussion: Further testing of tofacitinib to prevent ADA formation is recommended in applicable non-malignant disease settings. Clinical trial registration: https://www.clinicaltrials.gov/study/NCT04034238.

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.