Abstract 1206: Preclinical evaluation of targeting Notch-3 in breast cancer
Authors: Hurvitz Sara A. , von Euw Erika , O’Brien Neil , Conklin Dylan , Hu Chuhong , Zhuo Jiaying , Zhao Alice , Calzone Frank , Chen Hsiao-Wang , Dering Judy , Geles Ken , Sapra Puja , Slamon Dennis J. 
Published: July 2016

Abstract
Introduction: Notch-3 overexpression has been implicated in the development of breast cancer (BC) and is associated with poor outcomes. A critical challenge to eliminating treatment resistance in breast cancer likely relates to the presence of cancer stem cells (CSCs) that maintain the ability to differentiate and divide indefinitely. We postulate that targeted eradication of CSCs is possible using a Notch3 antibody drug conjugate (ADC) without irreversibly reducing stem cell viability in vital normal tissues. PF-06650808, is an ADC comprised of a humanized anti-Notch-3 antibody linked to an auristatin-based cytotoxic agent. To better understand the therapeutic index of targeting Notch-3, we evaluated PF-06650808 across a large panel of BC lines and normal cells and correlated response with Notch-3 levels. PF-06650808 was also evaluated in a murine BC xenograft model.
Methods: Response to PF-06650808 and control ADC was evaluated across a panel of BC and normal cell lines by a 2D proliferation assay. Notch-3 mRNA expression was measured by flow cytometry (FC) and RPPA. MDA-MB-468 (triple negative BC, TNBC) tumor bearing mice were randomized into 4 arms of 8 mice and treated with 3 mg/kg PF-06650808 or control-ADC (days 0, 4, 8 & 12), 10 mg/kg docetaxel (q week) or vehicle control.
Results: High expression of Notch-3 was detected in multiple BC cell lines by RPPA and FC. BC cell lines with elevated levels of Notch-3 were sensitive to PF-06650808 (HCC1187, MDA-MB-468, HCC1143, HCC70, EFM-19, HCC202). Responders were also enriched for TNBC. All normal cell lines were resistant to PF-06650808 ADC. When treated with a control ADC against a non-relevant target, all cell lines exhibited IC50s between 5-50ug/ml, indicating that the sub-0.5ug/ml responses seen with the Notch-3 ADC were target-dependent. Durable complete tumor regressions were observed in PF-06650808-treated mice bearing MDA-MB-468 TNBC cell line xenografts.
Conclusions: Sensitivity to a novel anti-Notch-3-ADC is associated with high expression of Notch-3 in BC cell lines. Normal cells are resistant to PF-06650808, possibly predicting a better therapeutic index than seen with other Notch inhibitors. Xenograft studies evaluating the in vivo efficacy of PF-06650808 in a panel of xenografts with varying levels of Notch-3 expression will be presented. Ongoing experiments are exploring the potency of PF-06650808 on CSCs. Our data will help identify breast cancer subtypes most likely to respond to a Notch3-ADC based on high tumor/normal target concentration as well as its effects on CSCs.
Citation Format: Sara A. Hurvitz, Erika von Euw, Neil O’Brien, Dylan Conklin, Chuhong Hu, Jiaying Zhuo, Alice Zhao, Frank Calzone, Hsiao-Wang Chen, Judy Dering, Ken Geles, Puja Sapra, Dennis J. Slamon. Preclinical evaluation of targeting Notch-3 in breast cancer. [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 1206.