Abstract 1207: Preclinical development of 2nd generation HER2-directed antibody-drug conjugates
Authors: Lewis Phillips Gail D. , Li Guangmin , Guo Jun , Lau Jeffrey , Yu Shang-Fan , Pillow Thomas , Lee Byoung-Chul , Sadowsky Jack , Schutten Melissa , Fields Carter , Sliwkowski Mark X. 
Published: July 2016

Abstract
The HER2 receptor tyrosine kinase is amplified in approximately 20% of human breast cancer and is associated with poor clinical outcome. The humanized antibodies trastuzumab and pertuzumab are approved for use in both early and metastatic HER2-positive breast cancer, and are most often given with chemotherapy. Antibody-drug conjugates (ADCs) are anti-tumor agents designed to deliver potent cytotoxic drugs selectively to target-expressing tumor cells. Trastuzumab emtansine is a HER2-directed ADC comprised of trastuzumab covalently linked to the microtubule inhibitor DM1, through the stable MCC linker. Trastuzumab emtansine is approved for use in HER2-positive metastatic breast cancer as a single agent in patients who have received prior trastuzumab and a taxane. We are now exploring new HER2-directed ADCs (‘2nd generation ADCs’) with different mechanisms of action (MOA) than trastuzumab emtansine by investigating ADCs utilizing DNA-damaging agents, such as pyrrolobenzodiazepine (PBD) dimers and cyclopropylbenzindole (CBI) dimers, as the cytotoxic drug components. These agents have been conjugated to either trastuzumab or the humanized anti-HER2 antibody 7C2 (hu7C2) using both uncleavable and cleavable linkers. As free drugs and ADCs, the PBDs and CBIs show similar or greater potency in cell proliferation assays in vitro compared to DM1 and trastuzumab emtansine. However, unlike DM1, these agents are not strong substrates of Pgp/MDR1. Moreover, the PBDs and CBIs are active on non-dividing cells, whereas microtubule inhibitors such as DM1 do not affect non-dividing cells. Robust anti-tumor activity was observed in vivo in the fo5 HER2 transgenic tumor transplant model with the 2nd generation ADCs. Efficacious doses resulting in tumor stasis or regression ranged from 0.25-3 mg/kg administered as a single injection. In contrast, doses of trastuzumab emtansine required for stasis/regression in this model are 10 and 15 mg/kg, respectively. Efficacious doses were well-tolerated in the mouse xenograft models. Further tolerability studies of the 2nd generation ADCs were performed in rats. As rats are a non-binding species for trastuzumab and hu7C2, these studies assessed antigen-independent toxicities. Maximum tolerated doses for the different ADCs ranged from 2.5-15 mg/kg administered as a single injection, compared to 46 mg/kg for trastuzumab emtansine (Poon et al., 2013), likely reflecting both the different MOA and greater potency of the cytotoxic agents utilized in the 2nd generation HER2-directed ADCs. Overall, our findings demonstrate robust in vitro and in vivo activity of HER2 ADCs comprised of DNA-active agents, allowing for further development of a HER2-directed ADC distinct from trastuzumab emtansine.
Citation Format: Gail D. Lewis Phillips, Guangmin Li, Jun Guo, Jeffrey Lau, Shang-Fan Yu, Thomas Pillow, Byoung-Chul Lee, Jack Sadowsky, Melissa Schutten, Carter Fields, Mark X. Sliwkowski. Preclinical development of 2nd generation HER2-directed antibody-drug conjugates. [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 1207.