Abstract LB-061: HER2-targeted PEGylated liposomal doxorubicin (MM-302) efficiently targets the HER2 intermediate cell population in vitro and in vivo

Introduction: MM-302 is an antibody-liposomal drug conjugate designed specifically to target doxorubicin to HER2-overexpressing tumor cells. MM-302 is currently being evaluated in a Phase II trial in HER2 positive metastatic breast cancer (NCT02213744). HER2-positive breast cancer accounts for about 15-20% of breast cancer cases and is defined as IHC 3+ or 2+ and HER2 FISH amplified. A substantial percentage (∼30%) of breast cancer patients show positive HER2 IHC (1+/2+) without HER2 gene amplification (“HER2 intermediate”). This population is not eligible for treatment with currently approved HER2-targeted therapies. The purpose of this study is to investigate the in vitro and in vivo delivery/activity of MM-302 in the HER2 intermediate population. Methods: In vitro binding and viability studies were performed with MM-302, PEGylated liposomal doxorubicin (PLD) and T-DM1 with a panel of cell lines representing a range of HER2 expression. HER2-mediated cellular delivery of MM-302 was investigated in vivo in different HER2 expressing tumor models using a novel PEG immunofluorescent assay herein described. Frozen tumor tissues were stained for PEG, HER2 and cytokeratin, side-by-side with two cell standard arrays: A PEG array, obtained by cell incubation with increasing concentrations of MM-302, and a HER2 array, built with a panel of cell lines at different HER2 expression (from ∼50,000 to over 1,000,000 HER2). Image analysis and subsequent regression of the PEG and HER2 fluorescent intensities from the respective standards allowed for the quantification of the number of liposomes in individual tumor cells at distinct HER2 receptor numbers. Tumor cell apoptosis following MM-302 cellular delivery was measured by immunofluorescence followed by image analysis. Results: MM-302 efficiently bound to, and induced, tumor cell death across a panel of cell lines, with no significant distinction between cell lines expressing 300-400,000 HER2 or above 1,000,000 HER2 (IHC 3+). Conversely, T-DM1, used as control, significantly bound to and induced cell death only to cells above 1,000,000 HER2 (IHC 3+). In vivo evaluation of HER2-mediated cellular delivery via PEG immunofluorescent staining showed that MM-302 can be equally efficiently internalized in tumor cells above 1,000,000 HER2 (IHC 3+) and in the HER2 intermediate expression range. Preliminary analysis on post-treatment patient biopsies collected during a MM-302 Phase I study (NCT01304797) support clinical translation of these preclinical observations. Conclusions: Treatment with MM-302 results in efficient HER2 binding and liposome cellular delivery across a panel of HER2 models that extend beyond the traditional HER2 positive definition. This study suggests that MM-302 may be a promising candidate for the treatment of patients with intermediate HER2 expression who represent a significant unmet medical need. Citation Format: Elena Geretti, Christopher Espelin, Bambang Adiwijaya, Nancy Dumont, Silvia Coma, Zachary Koncki, Minh Pham, Gabriela Garcia, Troy Bloom, Victoria Rimkunas, Joe Reynolds, Karen Campbell, Victor Moyo, Istvan Molnar, Patricia LoRusso, Ian Krop, Kathy Miller, Cynthia Ma, Pamela Munster, Thomas Wickham. HER2-targeted PEGylated liposomal doxorubicin (MM-302) efficiently targets the HER2 intermediate cell population in vitro and in vivo. [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 LB-061.