Abstract P4-14-02: Neutralization of BCL2/BCL-XL enhances the cytotoxicity of T-DM1 in vivo
Authors: Zoeller JJ , Bronson RT , Sampath D , Leverson J , Brugge JS 
Published: May 2016

Abstract
One of the most recent advances in the treatment of HER2+ breast cancer is the development of the antibody-drug conjugate, T-DM1, composed of trastuzumab (T) linked to the cytotoxic maytansinoid (DM1). T-DM1 has proven clinical benefits for patients with advanced and/or metastatic breast cancer who have progressed on prior HER2-targeted therapies. However, T-DM1 resistance ultimately occurs and represents a major obstacle in the effective treatment of this disease. We previously identified BCL2 upregulation as a critical component and biomarker of the adaptive response to inhibition of PI3K/mTOR or HER2, and thus examined whether BCL2/BCL-XL combinatorial strategies could improve the initial efficacy of T-DM1. Here, we demonstrate that combined inhibition of BCL2/BCL-XL proteins plus T-DM1 significantly enhances the cytotoxicity of T-DM1 in vivo.
The effectiveness of T-DM1 plus BCL2/BCL-XL inhibition was evaluated in two patient-derived xenograft (PDX) models of advanced HER2+ER- resistant disease (PDX8 and PDX12). Animals were randomized into one of four treatment arms: T-DM1, ABT-737, T-DM1 + ABT-737 or vehicle controls. Our initial results after a 14d treatment period indicate that combined treatment with T-DM1 and ABT-737, the dual BCL2/BCL-XL inhibitor, confers an exceptional tumor cell cytotoxic advantage characterized by widespread elimination of the tumor cells.
To evaluate whether ABT-263, the clinically relevant BCL2/BCL-XL inhibitor, mimics ABT-737, we randomized animals into one of four treatment arms: T-DM1 (administered weekly), ABT-263 (administered daily), T-DM1 + ABT-263 or vehicle controls. To minimize thrombocytopenia that is induced by ABT-263, we included a fifth treatment arm that received pulsed treatment of ABT-263 + T-DM1. Notably, unlike continuous treatment, pulsed administration of ABT-263 reduced weight loss to vehicle levels and allowed recovery of platelet counts. Evaluation of pathological responses by H&E staining indicated that T-DM1 + ABT-263 mimics T-DM1 + ABT-737. To better distinguish tumor cells from stromal elements, we used epithelial membrane antigen immunostaining to specifically visualize tumor cells and Trichrome stain to visualize stromal content and scored the tissue sections blindly. ABT-263 had no observable effect. T-DM1 induced a 38.75% and 20% average reduction in tumor cell content in the two PDX models, whereas the combined treatment caused a 74% and 54% average reduction after the 14d treatment period. The loss of tumor cell content was associated with an increased stromal reaction at the tumor bed. T-DM1 treated tumors contained 27.5% and 47.5% average stromal content, whereas combination treated tumors contained 86% and 85.6% average stromal content. Importantly, T-DM1 + pulsed ABT-263 treatment elicited a similar response as continuous treatment in the PDX8 model, but was not as effective in PDX12.
The dramatic improvement in tumor regression observed in these preclinical studies, together with the safety benefits of modified dosing of ABT-263, provides substantial rationale for the clinical investigation of this combination therapy. Furthermore, thorough investigation of treatments that combine anti-apoptotic drugs with tumor-targeted chemotherapeutics could have broad implications in other cancer types.
Citation Format: Zoeller JJ, Bronson RT, Sampath D, Leverson J, Brugge JS. Neutralization of BCL2/BCL-XL enhances the cytotoxicity of T-DM1 in vivo. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-14-02.