Discoidin domain receptor 1 (DDR1) is a collagen-activated tyrosine kinase that facilitates cell adhesion, migration, proliferation and matrix remodeling. The collagen/DDR1 axis is also thought to modulate tumor-stromal interaction and potentially affects tumor response to therapy. Mesothelin is a cell-surface protein over-expressed in several human cancers including mesothelioma, ovarian, lung, breast, and pancreatic cancers with limited expression on normal cells. RG7787 is a clinically optimized recombinant immunotoxin (RIT) consisting of a humanized anti-mesothelin Fab fused to domain III of Pseudomonas exotoxin A in which immunogenic B cell epitopes are silenced. To enhance therapeutic efficacy of RITs we conducted a kinome RNAi sensitization screen which identified DDR1 as a potential target. We hypothesized that DDR1 regulates RIT activity and its inhibition might enhance cell killing by RG7787. Knockdown of DDR1 by siRNA or treatment with inhibitor, ‘7rh’, greatly enhanced the cytotoxic activity of RG7787 in several cancer cell lines. Investigation into the mechanism of action showed DDR1 silencing was associated with a decrease in several ribosomal proteins and enhanced inhibition of protein synthesis. Induction of DDR1 expression or stimulation of DDR1 activity by collagen protected cancer cells from RG7787 killing, while DDR1 inhibition enhanced killing by RG7787. Moreover, the combination of immunotoxin and DDR1 inhibitor caused significantly more shrinkage of epithelial A431/H9 and pancreatic KLM1 tumor xenografts than either agent alone. These data demonstrate that DDR1 has a critical role in modulating immunotoxin activity. Inhibition of DDR1 represents a novel strategy to enhance therapeutic efficacy of RITs targeting mesothelin-expressing cancers.

Citation Format: Fatima Ali-Rahmani, David Fitzgerald, Scott Martin, Paresma Patel, Marco Prunotto, Pinar Ormanoglu, Craig Thomas, Ira Pastan. Tyrosine kinase discoidin domain receptor-1 (DDR1) regulates cytotoxicity of recombinant immunotoxin for cancer therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 743.