Abstract 849: Synergistic effect of MSLN-TTC in combination with DNA damage response inhibitors
Authors: Wickstroem Katrine , Hagemann Urs B. , Wengner Antje M. , Kristian Alexander , Ellingsen Christine , Siemeister Gerhard , Bjerke Roger M. , Karlsson Jenny , Ryan Olav B. , Linden Lars , Kreft Bertolt , Mumberg Dominik , Wild Hanno , Ziegelbauer Karl , Cuthbertson Alan 
Published: August 2018

Targeted Thorium-227 Conjugates (TTCs) consist of the alpha particle emitter thorium-227 bound with high affinity by a 3,2-HOPO chelator covalently attached to a tumor-specific antibody. Thorium-227 has a half-life of 18.7 days and decays to the alpha particle emitter radium-223 (half-life of 11.4 days), a calcium-mimetic used in the treatment of CRPC [Henriksen et al. J Nucl Med, 2003]. The mechanism of action for alpha emitters is based on the induction of clustered DNA double strand breaks and G2 cell cycle arrest. In principle, inhibitors of DNA damage response (DDR) should sensitize cancer cells to TTCs. Mesothelin (MSLN) is a 40 kDa membrane-anchored glycoprotein known to be overexpressed in mesothelioma, ovarian, lung, triple-negative breast (TNBC) and pancreatic cancers. This study describes the evaluation of combination treatment with anti-MSLN targeted thorium-227 conjugate (MSLN-TTC) in combination with DDR inhibitors. In vitro cytotoxicity experiments were performed on a set of cancer cell lines from different tissue origins, including Ovcar-3 (ovarian), NCI-H226 (mesothelioma), Capan-2 (pancreas) and mesothelin transfected HT29 (colon) cells. DDR inhibitors targeting Poly [ADP-ribose] polymerase 1 (PARP1), ataxia telangiectasia and Rad3-related (ATR), Ataxia-telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK) included olaparib, BAY 1895344, AZD0156, and VX-984, respectively. Synergistic activity of MSLN-TTC was observed in vitro for all four DDR inhibitors with ATRi BAY 1895344 exhibiting the strongest increase in potency. We further evaluated the in vivo anti-tumor efficacy of MSLN-TTC monotherapy and combination treatment with DDR inhibitors ATRi BAY 1895344 or PARPi olaparib in Ovcar-3 xenografts in nude mice. Monotherapy of the MSLN-TTC yielded a dose dependent and specific tumor growth inhibition as compared with a radiolabeled isotype control. In the combination treatment an increased anti-tumor effect of MSLN-TTC at the lowest dose was observed when combined with a non-efficacious dose of either ATRi BAY 1895344 or olaparib, with a more pronounced effect for the combination of MSLN-TTC with ATRi BAY 1895344, resulting in tumor stasis. The present data support the rationale for combining the MSLN-TTC with DDRi's based on their individual mode of actions and bears strong potential for future cancer therapies in tumor indications characterized by overexpression of mesothelin, such as mesothelioma, ovarian, pancreatic, lung or triple-negative breast (TNBC) cancers.

Citation Format: Katrine Wickstroem, Urs B. Hagemann, Antje M. Wengner, Alexander Kristian, Christine Ellingsen, Gerhard Siemeister, Roger M. Bjerke, Jenny Karlsson, Olav B. Ryan, Lars Linden, Bertolt Kreft, Dominik Mumberg, Hanno Wild, Karl Ziegelbauer, Alan Cuthbertson. Synergistic effect of MSLN-TTC in combination with DNA damage response inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 849.