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Approximately 70% of breast cancer cases are Estrogen Receptor (ER) positive. In many cases, these patients are well-served by endocrine therapies such as tamoxifen and aromatase inhibitors, however the emergence of endocrine-resistant disease in up to 40% of these patients is a major clinical concern. This necessitates treatment with more toxic chemotherapies which are often successful at delaying tumor progression, but are not curative. Recent work from our group and others has demonstrated that Amphiregulin, an Epidermal Growth Factor Receptor (EGFR) ligand, is a critical effector of ER signaling in both normal development and breast cancer pathogenesis, and is expressed in both endocrine-sensitive and endocrine-resistant breast cancer. Like other EGFR ligands, Amphiregulin is proteolytically processed at the cell surface to release a soluble EGFR-binding signaling domain and a residual cell-associated stalk. In this project, we are developing antibodies that selectively recognize the residual transmembrane Amphiregulin cleavage product with the goal of developing antibody-drug conjugates with which to selectively target tumor cells with high levels of Amphiregulin shedding. We have generated six humanized antibodies that recognize the cell-associated neo-epitope revealed following the TACE-mediated proteolytic release of Amphiregulin. Using both flow Cytometry and immunofluorescence, we demonstrate that these antibodies selectively recognize cleaved over full-length Amphiregulin in estrogen receptor positive breast cancer cells. Conjugation of these antibodies with a pH-sensitive dye, pHrodo, demonstrates robust and rapid antibody internalization in live cells. Conjugation of these antibodies with monomethyl auristatin E results in the disruption of the microtubule network of breast cancer cells followed by cell death. We propose that these agents may have utility in the treatment of breast and other cancers in which Amphiregulin expression and processing play a prominent role. This study is supported by the Department of Defense Breast Cancer Research Program (W81XWH-14-1-0294). Citation Format: Edmund C. Jenkins, Kelly S. Levano, Eric H. Jung, Matthew Levy, Paraic A. Kenny. Antibody-drug conjugates to target cell surface TACE-cleaved amphiregulin in breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1692. doi:10.1158/1538-7445.AM2015-1692

Objectives: Our objective is to determine the effect of PI3K activity in mediating ligand-independent estrogen signaling and endocrine sensitivity in Uterine Papillary Serous Carcinoma (UPSC). Methods: Drug effect on cell proliferation was calculated via Calcusyn in patient-derived UPSC cell lines ARK 1 and 2. PI3K mutation was analyzed after laser capture microdissection of tumor DNA in 7 consecutive patients with UPSC. Protein expression, via Western Blot, was correlated prospectively with clinical parameters. Results: Table 1 shows cells line baseline characteristics and IC50 to drugs. Fulvestrant, an ER antagonist, rendered the cells significantly more resistant to taxol in ARK1 and 2(p=0.035, 0.021 respectively). The concomitant decrease in pERS167 and pAKTS473 with Fulvestrant treatment is independent of baseline ER expression and PI3K mutation. In ARK2, a cell line that is ER-, disrupting AKT signaling via the PI3K inhibitor GDC-0941, or with the lapatinib is synergistic with Fulvestrant with Combination Index (CI)50 of 0.441 and 0.229, respectively. Independent of HER2 amplification or PI3K mutation, lapatinib and GDC-0941 exhibited synergistic cytotoxicity in both UPSC cell lines (CI50 of 0.577, ARK 1 and 0.233, ARK2). Finally, PI3K mutation and pAKTS473 expression was analyzed in 7 tumor samples. While none harbored PI3K mutation, patients who are chemotherapy resistant had significantly lower expression of baseline pAKTS473 in their tumor samples than those who are chemosensitive, similar to our cell line data. Conclusions: PI3K pathway dysregulation, either via upstream erbB amplification, or downstream constitutive activation of AKT, may be important in mediating endocrine and taxol sensitivity in UPSC. pAKTS473 may be an important biomarker of drug sensitivity. The combination of PI3K inhibitor and lapatinib is synergistic and warrant further therapeutic investigation. Baseline characteristics and drug sensitivities in UPSC cell linesARK1ARK2PI3KCA mutationfExon 9NoERα (WB)YesNopERα167 (WB)YesNopHER (WB)NoYesIC50 nM (mean ± STD)GDC-094193.1±5.9271±116.5LapatinibNo effect135.6±29.2FulvestrantNo effectNo effectTaxol4.6±1.97.8±5.6Cisplatin1103±65.72632.6±1797.9 Citation Format: Kelly S. Levano, Alicia Rodriguez-Gabin, Gary L. Goldberg, Susan B. Horwitz, June Y. Hou. The PI3K inhibitor GDC-0941 is synergistic with lapatinib, and mediates endocrine sensitivity in uterine papillary serous carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4687. doi:10.1158/1538-7445.AM2014-4687

About 70% of breast cancers are estrogen receptor positive (ER+) and current treatments target either ER activity (Tamoxifen and Fulvestrant) or the production of estrogen (aromatase inhibitors). These treatments have led to significant increases in disease-free and overall survival, but after initial response to these agents, many ER+ cancers progress to endocrine resistance. There have been several attempts to understand the molecular mechanisms that lead to acquired resistance, but a clear understanding is still lacking. We hypothesize that endocrine resistant cells in the tumor arise by acquiring compensatory somatic mutations, and that some of these driver mutations responsible for resistance are in oncogenes or tumor suppressor genes. To test this we performed whole exome sequencing on three of MCF7 breast cancer cell sublines that are resistant to either tamoxifen, fulvestrant or exemestane - drugs commonly used in the clinic to treat ER+ breast cancers. An Illumina TruSEQ adaptor-based exonic library was prepared using genomic DNA from the resistant and control cell lines, and multiplexed on an Illumina HiSeq 2000 sequencer (100bp paired-end reads). Reads were aligned to human genome build 19 using BWA, variants were called using SAMTools, somatic mutations in endocrine cell lines were determined with VarScan, and amino acid changes were annotated using ANNOVAR. We have identified several non-synonymous mutations in cancer-relevant genes and are currently working to validate these targets. Studying these genes will increase our understanding of how tumors overcome the requirement for estrogen signaling and ultimately, we hope that comparing tumor profiles of patients with endocrine resistance disease will lead to therapeutics targeted to these secondary mutational lesions. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-343. doi:1538-7445.AM2012-LB-343