Abstract 2647: SGN-CD70A, a novel and highly potent anti-CD70 ADC, induces double-strand DNA breaks and is active in models of MDR+ renal cell carcinoma (RCC) and non-Hodgkin lymphoma (NHL)
Authors: Sandall Sharsti , Anderson Martha , Jonas Mechthild , Nesterova Albina , Miyamoto Jamie , Stone Ivan J. , Zeng Weiping , Law Che-Leung , Lewis Timothy S. 
Published: May 2015

Abstract
CD70 is a member of the tumor necrosis factor superfamily that is aberrantly expressed in several solid tumors and hematologic malignancies, including clear cell and papillary renal cell carcinoma (RCC) and non-Hodgkin lymphoma (NHL). Normal expression of CD70 is limited to stromal cells of the thymic medulla, mature dendritic cells, and activated B and T lymphocytes. Thus, CD70 is an attractive target for antibody-drug conjugate (ADC) based therapy. Using a panel of CD70 positive RCC and lymphoma cell lines and xenograft models, we have previously demonstrated the antitumor activity of SGN-CD70A, a novel ADC that combines a CD70-directed engineered cysteine monoclonal antibody (h1F6ec) with a highly potent, synthetic DNA cross-linking molecule, pyrrolobenzodiazepine (PBD) dimer. The strength of these results led us to develop SGN-CD70A for clinical evaluation in RCC and lymphoma. In this report, we examine the mechanism of action for SGN-CD70A and demonstrate that the formation of double strand breaks (DSB) is an early event that precedes onset of cytotoxicity in RCC and NHL cell lines.
SGN-CD70A is more potent than auristatin-based CD70 ADCs in vitro and in xenograft models, including those that are MDR positive, suggesting that the PBD chemotype may overcome common resistance mechanisms. To define the mechanism(s) of targeted cytotoxicity, we examined DNA damage pathways in Caki-1, 786-0 and UM-RC-3 (RCC, MDR+) and Raji and MHH-PREB-1 (NHL) cell lines. We utilized an immunofluorescence assay to monitor DNA damage foci using antibodies specific to the tumor suppressor p53-binding protein 1 (53BP1), Meiotic recombination 11 homolog (Mre11), and Rad50. Increased amounts of foci were observed within 6 hours of treatment with 2nM PBD to levels observed in cells exposed to 10Gy of ionizing radiation. Similarly, foci were found in SGN-CD70A-treated cells. Further evidence of damage was the co-localization of phosphorylated histone H2A.X (Ser139) to the damage foci and an increase in levels of both phosphorylated Chk1 (Ser317/345) and Chk2 (Thr68) within 4 hours of treatment. The levels of both pChk1 and pChk2 continue to increase after treatment, with peaks at 24-48 hours for pChk1 and 48-72 hours for pChk2. Concomitant, we also observed an increase in both phosphorylated ATM and phosphorylated BRCA1, confirming that SGN-CD70A in vitro activates double strand break response pathways. Ongoing research is examining DNA damage pathway activation in the corresponding xenograft models to confirm our in vitro findings. Furthermore, we are developing assays to examine pH2A.X, pChk1, and pChk2 as potential biomarkers for clinical studies with SGN-CD70A.
Citation Format: Sharsti Sandall, Martha Anderson, Mechthild Jonas, Albina Nesterova, Jamie Miyamoto, Ivan J. Stone, Weiping Zeng, Che-Leung Law, Timothy S. Lewis. SGN-CD70A, a novel and highly potent anti-CD70 ADC, induces double-strand DNA breaks and is active in models of MDR+ renal cell carcinoma (RCC) and non-Hodgkin lymphoma (NHL). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2647. doi:10.1158/1538-7445.AM2014-2647