Abstract P6-15-01: Triple negative breast cancer is vulnerable to Pan-HER, an antibody mixture simultaneously targeting EGFR, HER2 and HER3
Authors: Choi DS , Qian W , Davila-Gonzalez D , Ensor JE , Lantto J , Kragh M , Horak ID , Chang JC 
Published: May 2016

Abstract
Background: Triple negative breast cancer (TNBC) is a highly heterogeneous and aggressive subtype of cancer, lacking expression of estrogen and progesterone receptors as well as human epidermal growth factor receptor (HER) 2 protein. Limited standard therapeutic options, absence of effective targeted therapies, and early metastatic spread have contributed to poor prognosis and outcomes associated with this disease. Although overexpression of EGFR has been reported in nearly 80% TNBC, EGFR-targeted therapy has yielded little clinical benefit, and the outcome is still under debate. In conjunction, we also found mixed effects of EGFR-targeted therapy on TNBC xenograft tumors despite significant target engagement, suggesting that tumor heterogeneity and compensating mechanisms may contribute to the variable drug responses to the EGFR-targeted therapy. Recently, we reported superior anti-cancer effects of Pan-HER, a mixture of antibodies targeting the HER family members EGFR, HER2 and HER3 on various types of cancer by overcoming drug resistance and tumor heterogeneity. To this end, we hypothesized that Pan-HER can effectively inhibit tumor growth in TNBC by inhibiting tumor heterogeneity and drug resistance.
Objective: The goal of this study is to test the effect of Pan-HER antibody mixture (Sym013) on tumor growth and recurrence of 14 patient-derived (PDX) TNBC orthotopic xenograft tumor models and to investigate molecular biomarkers which can predict drug response to Pan-HER.
Methods: We evaluated in-vivo anti-tumor effects of Pan-HER (50 mg/kg, i.p. three times/week, 10 doses in total for 3 weeks) over vehicle on tumor growth and tumor recurrence on 14 PDX TNBC models with known expression levels of EGFR and HER3 (n=3/group). HER family proteins and related downstream molecules (Akt, Erk, Stat3, FAK) in the tumor tissues were evaluated by Western blot assay and immunohistochemistry analysis. Additionally, using dCHIP and ingenuity pathway analysis, we compared microarray data from the tested cohorts and other TNBC PDX models with known HER family receptor status.
Results: We found that Pan-HER alone effectively inhibited tumor growth in all 14 PDX models and showed statistical significance (p=0.0103) when compared to the vehicle groups. Among these, one PDX model, BCM-3186, showed substantial tumor reduction and additional two (MC1 and BCM-4913) showed complete response with no recurrence after the last treatment of Pan-HER. The significant anti-tumor effects of Pan-HER were positively correlated with inhibition of phosphorylation and expression of EGFR, HER3, Akt, Erk, and FAK, but not Stat3, and this was consistent in all PDX models tested. Additionally, the microarray and the pathway enrichment analyses suggest that loss of PTEN expression and up-regulation of FAK and RAS pathways may be the predictive markers for the Pan-HER drug response in TNBC.
Conclusion: Our in-vivo data suggest that simultaneous targeting of the three HER family receptors is a potential new approach for treatment of TNBC. Further confirmation of our in-vivo results will warrant a phase I clinical trial and lend support to single agent Pan-HER as a viable treatment strategy for TNBC patients in the clinic.
Citation Format: Choi DS, Qian W, Davila-Gonzalez D, Ensor JE, Lantto J, Kragh M, Horak ID, Chang JC. Triple negative breast cancer is vulnerable to Pan-HER, an antibody mixture simultaneously targeting EGFR, HER2 and HER3. [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 P6-15-01.