Identification of PAM4 (clivatuzumab)-reactive epitope on MUC5AC: A promising biomarker and therapeutic target for pancreatic cancer
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Donglin Liu1, Chien-Hsing Chang1,2, David V. Gold3, David M. Goldenberg1,2,3
1IBC Pharmaceuticals, Inc., Morris Plains, New Jersey 07950, United States of America
2Immunomedics, Inc., Morris Plains, New Jersey 07950, United States of America
3Garden State Cancer Center, Center for Molecular Medicine and Immunology, Morris Plains, New Jersey 07950, United States of America
Donglin Liu, e-mail: [email protected]
David M. Goldenberg, e-mail: [email protected]
Keywords: MUC5AC, epitope mapping, PAM4, pancreatic cancer, cysteine-rich subdomain
Received: September 01, 2014 Accepted: November 16, 2014 Published: January 19, 2015
PAM4 is a monoclonal antibody showing high specificity for pancreatic ductal adenocarcinoma (PDAC). Humanized PAM4 labeled with 90Y in combination with low-dose gemcitabine has shown promising therapeutic activity, and is being evaluated in a phase III clinical trial. Prior efforts have suggested that PAM4 potentially reacts with MUC5AC, a secretory mucin expressed de novo in early pancreatic neoplasia and retained throughout disease progression. In present study, we provide further evidence validating MUC5AC as the PAM4 antigen, and locate PAM4-reactive epitope within the N-terminal cysteine-rich subdomain 2 (Cys2), thus differentiating PAM4 from most anti-MUC5AC antibodies known to-date. Specifically, we show (i) PAM4-antigen and MUC5AC were co-localized in multiple human cancer cell lines, including Capan-1, BxPC-3, and CFPAC-1; (ii) MUC5AC-specific siRNA prominently reduced the expression of both MUC5AC and PAM4-antigen in CFPAC-1 cells; (iii) PAM4 preferentially binds to the void-volume fractions from Sepharose-CL2B chromatography of Capan-1 culture supernatants, which were revealed by Western blot to display the ladder pattern characteristic of oligomeric MUC5AC; and (iv) the N-terminal Cys2 within several recombinant MUC5AC fragments is essential for binding to PAM4. These findings shed light on the mechanism of PAM4-based diagnosis and treatment for pancreatic cancer, and guide further exploration of its clinical utility.
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