Abstract 1290: Actinomycin D enhanced immunotoxin RG7787 killing of cancer cells

Immunotoxins are recombinant fusion proteins that contain an antibody fragment directed against a tumor selective surface antigen attached to a protein toxin. RG7787 is a newly developed, mesothelin-targeted immunotoxin designed to be less immunogenic and better tolerated than the previous clinical candidate SS1P. The targeting moiety of RG7787 consists of a humanized anti-mesothelin Fab, while its effector moiety is a truncated portion of Pseudomonas exotoxin A (PE) consisting essentially only of the catalytic domain III fused via a furin cleavable linker to the Fab fragment. The domain III variant used in RG7787 contains mutations that silence all known human B-cell and some T-cell epitopes. RG7787 targets and kills mesothelin-positive tumor cells, which are prevalent in mesothelioma, ovarian, lung, and pancreatic cancers. Safety and immunogenicity of RG7787 are being assessed in these tumor indications in an ongoing phase I trial. In order to further enhance the anti-tumoral activity of RG7787, we screened for clinically used drugs that can synergize with RG7787 in vitro and/or in vivo. Actinomycin D (Act D) is a potent transcription inhibitor and is used for treating a variety of cancers, including gestational trophoblastic neoplasia, Wilms tumor, rhabdomyosarcoma, and Ewing's sarcoma. In this study we found that the combination of Act D with RG7787 greatly stimulated cell killing of mesothelin positive cell lines, such as KB31, KLM1, and Hay. The combination also produced complete regressions of KLM1 pancreatic tumor xenografts in mice. To investigate the mechanism of killing we used an apoptosis RNA array and found that both Act D and RG7787 stimulate apoptotic stress responsive genes including TNFα, TRAILR2 (DR5), NFκB, GAD45A and TP53. The combination also increased cleavage and activation of the pro-apoptotic proteins caspase-3, -9, and PARP. Taken together our data indicate that combining RG7787 and Act D reduces the threshold for activation of apoptosis in cancer cells. Citation Format: Xiu Fen Liu, Laiman Xiang, Marco Prunotto, Gerhard Niederfellner, Ira Pastan. Actinomycin D enhanced immunotoxin RG7787 killing of cancer cells. [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 1290.