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Metastatic breast cancer is the overwhelming cause of breast cancer mortality and is still incurable. The rapid development of immunotherapy is an exciting new area of research in metastatic breast cancer. However, the extreme immunosuppressive tumor environment poses a major challenge. A better understanding of how the immune system can be harnessed against metastatic cancer is required to improve patient outcomes. We previously showed that expression of the receptor tyrosine kinase Ron in the host, rather than Ron's tumor expression, contributed to tumor-associated immunosuppression and duo inhibition of Ron and CTLA-4 significantly reduced metastatic outgrowth. However, the actual mechanism remains unclear. The present study provides evidence that the N-terminal truncated isoform, short-form Ron (SF-Ron), is the major contributor in suppressing the anti-tumor immune responses and promoting metastatic outgrowth. Genetic deletion of host SF-Ron nearly eliminated breast cancer metastasis in mice, lead to systemic immune-activation, increased recruitment of lymphocytes to the site of metastasis, and augmented tumor-specific T-cell responses. Lack of SF-Ron also leads to the accumulation of CD4+ T-cells in the metastatic lungs and endowed with anti-tumor potential. Importantly, mice treated with small molecule Ron kinase inhibitor that targets both Ron and SF-Ron, produced significantly higher, active, tumor-specific CD8+ T-cells. Our study indicates that blocking Ron, especially the SF-Ron, remodels the metastatic lung microenvironment to enhance anti-tumor immunity. This study sheds light on the potential non-redundant roles of full-length and SF-Ron isoforms in mediating breast cancer metastasis and anti-tumor immune responses; and highlights the relevance of combining Ron inhibitors with immunotherapeutic agents to potentially improve treatment efficacy for metastatic breast cancer patients. Citation Format: Shu-Chin Alicia Lai, Harika Gundlapalli, Huseyin A Ekiz, Alana L Welm. Inhibition of short-form ron eliminates breast cancer metastases through an immune-mediated mechanism [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-24.

Development of chemoresistance in breast cancer patients greatly increases mortality. Thus, understanding mechanisms underlying breast cancer resistance to chemotherapy is of paramount importance to overcome this clinical challenge. Although activated Notch receptors have been associated with chemoresistance in cancer, the specific Notch ligands and their molecular mechanisms leading to chemoresistance in breast cancer remain elusive. Using conditional knockout and reporter mouse models, we demonstrate that tumor cells expressing the Notch ligand Dll1 is important for tumor growth and metastasis and bear similarities to tumor-initiating cancer cells (TICs) in breast cancer. RNA-seq and ATAC-seq using reporter models and patient data demonstrated that NF-κB activation is downstream of Dll1 and is associated with a chemoresistant phenotype. Finally, pharmacological blocking of Dll1 or NF-κB pathway completely sensitizes Dll1+ tumors to chemotherapy, highlighting therapeutic avenues for chemotherapy resistant breast cancer patients in the near future., Although activated Notch receptors have been associated with chemoresistance in cancer, the role of specific Notch ligands remain elusive. Here, the authors show that in breast cells the Notch ligand DLL1 is expressed in cells with a cancer stem cell phenotype and promote doxorubicin resistance in part through NF-kB, as well as metastasis.

Background Oestrogen Receptor 1 (ESR1) mutations are frequently acquired in oestrogen receptor (ER)-positive metastatic breast cancer (MBC) patients who were treated with aromatase inhibitors (AI) in the metastatic setting. Acquired ESR1 mutations are associated with poor prognosis and there is a lack of effective therapies that selectively target these cancers. Methods We performed a proteomic kinome analysis in ESR1 Y537S mutant cells to identify hyperactivated kinases in ESR1 mutant cells. We validated Recepteur d’Origine Nantais (RON) and PI3K hyperactivity through phospho-immunoblot analysis, organoid growth assays, and in an in vivo patient-derived xenograft (PDX) metastatic model. Results We demonstrated that RON was hyperactivated in ESR1 mutant models, and in acquired palbociclib-resistant (PalbR) models. RON and insulin-like growth factor 1 receptor (IGF-1R) interacted as shown through pharmacological and genetic inhibition and were regulated by the mutant ER as demonstrated by reduced phospho-protein expression with endocrine therapies (ET). We show that ET in combination with a RON inhibitor (RONi) decreased ex vivo organoid growth of ESR1 mutant models, and as a monotherapy in PalbR models, demonstrating its therapeutic efficacy. Significantly, ET in combination with the RONi reduced metastasis of an ESR1 Y537S mutant PDX model. Conclusions Our results demonstrate that RON/PI3K pathway inhibition may be an effective treatment strategy in ESR1 mutant and PalbR MBC patients. Clinically our data predict that ET resistance mechanisms can also contribute to CDK4/6 inhibitor resistance.

Despite being the hallmark of cancer that is responsible for the highest number of deaths, very little is known about the biology of metastasis. Metastatic disease typically manifests after a protracted period of undetectable disease following surgery or systemic therapy, owing to relapse or recurrence. In the case of breast cancer, metastatic relapse can occur months to decades after initial diagnosis and treatment. In this review, we provide an overview of the known key factors that influence metastatic recurrence, with the goal of highlighting the critical unanswered questions that still need to be addressed to make a difference in the mortality of breast cancer patients.

BackgroundPatients with PD-L1-positive metastatic triple-negative breast cancers (mTNBC) who have been treated with atezolizumab+nab-paclitaxel had a clinically meaningful overall survival extension of 9.5 months compared to nab-paclitaxel alone, although overall survival in overall population was not statistically significant. Unlike many other cancers, immunotherapy for breast cancer has had limited success, due to the fact that there are very few T cells in the tumor microenvironment of mTNBC patients. Identifying ways to boost immunotherapy responses could change the paradigm of mTNBC, a disease still difficult to treat. The highly proliferative nature of tumor cells, along with infiltration of myeloid cells into the tumors, leads to depletion of nutrients such as functional/natural amino acids. This metabolically stressful milieu causes activation of nutrient stress pathways, autophagy, and repressed immune responses. A key meditator of this nutrient stress pathway is a cytoplasmic Ser/Thr protein kinase called General Control Nonderepressible 2 (GCN2), also called EIF2AK4. GCN2 switches on following reduction of amino acids, and its activity results in T cell inactivation, T cell death, regulatory T cell expansion, and the potentiation of myeloid-derived suppressor cells (MDSCs).MethodsWe have developed and synthesized a series of novel small molecule immunotherapeutic agents that reversibly bind to GCN2 kinase, competitively block the ATP site, and elicit pharmacological responses in immune cells and in breast cancer cells.ResultsGCN2 cell-free kinase binding, kinome selectivity, pGCN2, pEIF2α, ATF-4 phosphorylation inhibition assays were performed. We confirmed on-target efficacy and tested the potency of our lead GCN2 inhibitor HCI-1046. HCI-1046 demonstrated potent activity, with an IC50 of 36 nM in inhibiting GCN2 kinase and exhibited cellular efficacy with an IC50 of 0.1 to 1.0 μM range. Our preliminary results support the hypothesis that the inhibition of GCN2 reinstates anti-tumor immunity and blocks tumor progression in breast cancer models. In vivo PK studies of HCI-1046 in rodents showed excellent PK properties; 55% oral bioavailability, low clearance, and >5 hour half-life.ConclusionsThus, HCI-1046 is nominated as a pre-clinical agent. Additional data regarding evaluation of the effects of HCI-1046 on the MDSC-suppressive function on T cells using ELISpot assays with breast cancer patient samples, and mouse model efficacy studies will be discussed.ReferencesEkiz HA, Lai SA, Gundlapalli H, Haroun F, Williams MA, Welm AL. Inhibition of RON kinase potentiates anti-CTLA-4 immunotherapy to shrink breast tumors and prevent metastatic outgrowth. Oncoimmunology 2018;7(9):e1480286.Toogood PL. Small molecule immuno-oncology therapeutic agents. Bioorg Med Chem Lett 2018;28(3):319–329.Ravindran R, Loebbermann J, Nakaya HI, Khan N, Ma H, Gama L, Machiah DK, Lawson B, Hakimpour P, Wang YC, Li S, Sharma P, Kaufman RJ, Martinez J, Pulendran B. The amino acid sensor GCN2 controls gut inflammation by inhibiting inflammasome activation. Nature 2016;531(7595):523–527.Brazeau JF, Rosse G. Triazolo[4,5-d]pyrimidine derivatives as inhibitors of GCN2. ACS Med Chem Lett 2014;5(4):282–3.

We created the PDX Network (PDXNet) Portal (https://portal.pdxnetwork.org/) to centralize access to the National Cancer Institute-funded PDXNet consortium resources (i.e., PDX models, sequencing data, treatment response data, and bioinformatics workflows), to facilitate collaboration among researchers, and to make resources easily available for research. The portal includes sections for resources, analysis results, metrics for PDXNet activities, data processing protocols, and training materials for processing PDX data.The initial portal release highlights PDXNet model and data resources, including 334 new models across 33 cancer types. Tissue samples of these models were deposited in the NCI’s Patient-Derived Model Repository (PDMR) for public access. These models have 2,822 associated sequencing files from 873 samples across 307 patients, which are hosted on the Cancer Genomics Cloud powered by Seven Bridges and the NCI Cancer Data Service for long-term storage and access with dbGaP permissions. The portal also includes results from standardized analysis workflows on PDXNet sequencing files and PDMR data (2,594 samples from 463 patients across 78 disease types). These 15 analysis workflows for whole-exome and RNA-Seq data are freely available, robust, validated, and standardized.The model and data lists will grow substantially over the next two years and will be continuously updated as new data are available. PDXNet models support multi-agent treatment studies, determination of sensitivity and resistance mechanisms, and preclinical trials. The PDXNet portal is a centralized location for these data and resources, which we expect to be of significant utility for the cancer research community.

Background Metastatic breast cancer (MBC) is incurable, with a 5-year survival rate of 28%. In the USA, more than 42,000 patients die from MBC every year. The most common type of breast cancer is estrogen receptor-positive (ER+), and more patients die from ER+ breast cancer than from any other subtype. ER+ tumors can be successfully treated with hormone therapy, but many tumors acquire endocrine resistance, at which point treatment options are limited. There is an urgent need for model systems that better represent human ER+ MBC in vivo, where tumors can metastasize. Patient-derived xenografts (PDX) made from MBC spontaneously metastasize, but the immunodeficient host is a caveat, given the known role of the immune system in tumor progression and response to therapy. Thus, we attempted to develop an immune-humanized PDX model of ER+ MBC. Methods NSG-SGM3 mice were immune-humanized with CD34+ hematopoietic stem cells, followed by engraftment of human ER+ endocrine resistant MBC tumor fragments. Strategies for exogenous estrogen supplementation were compared, and immune-humanization in blood, bone marrow, spleen, and tumors was assessed by flow cytometry and tissue immunostaining. Characterization of the new model includes assessment of the human tumor microenvironment performed by immunostaining. Results We describe the development of an immune-humanized PDX model of estrogen-independent endocrine resistant ER+ MBC. Importantly, our model harbors a naturally occurring ESR1 mutation, and immune-humanization recapitulates the lymphocyte-excluded and myeloid-rich tumor microenvironment of human ER+ breast tumors. Conclusion This model sets the stage for development of other clinically relevant models of human breast cancer and should allow future studies on mechanisms of endocrine resistance and tumor-immune interactions in an immune-humanized in vivo setting.

A new study in Nature determines metastatic tropism in xenograft mouse models. This results in a metastasis map for 21 tumor types, the utility of which is demonstrated by identifying lipid metabolism to be uniquely altered in breast cancer cell lines that metastasize to the brain.

Pancreatic ductal adenocarcinoma (PDA) was responsible for ~ 44,000 deaths in the United States in 2018 and is the epitome of a recalcitrant cancer driven by a pharmacologically intractable oncoprotein, KRAS1–4. Downstream of KRAS, the RAF→MEK→ERK signaling pathway plays a central role in pancreatic carcinogenesis5. However, paradoxically, inhibition of this pathway has provided no clinical benefit to patients with PDA6. Here we show that inhibition of KRAS→RAF→MEK→ERK signaling elicits autophagy, a process of cellular recycling that protects PDA cells from the cytotoxic effects of KRAS pathway inhibition. Mechanistically, inhibition of MEK1/2 leads to activation of the LKB1→AMPK→ULK1 signaling axis, a key regulator of autophagy. Furthermore, combined inhibition of MEK1/2 plus autophagy displays synergistic anti-proliferative effects against PDA cell lines in vitro and promotes regression of xenografted patient-derived PDA tumors in mice. The observed effect of combination trametinib plus chloroquine was not restricted to PDA as other tumors, including patient-derived xenografts (PDX) of NRAS-mutated melanoma and BRAF-mutated colorectal cancer displayed similar responses. Finally, treatment of a patient with PDA with the combination of trametinib plus hydroxychloroquine resulted in a partial, but nonetheless striking disease response. These data suggest that this combination therapy may represent a novel strategy to target RAS-driven cancers. Targeted inhibition of RAF–MEK–ERK signaling induces autophagy through the LKB1–AMPK axis, creating a therapeutic vulnerability that can be exploited for treating patients with pancreatic cancer and potentially other RAS-mutant tumors.

Model systems that recapitulate the complexity of human tumors and the reality of variable treatment responses are urgently needed to better understand cancer biology and to develop more effective cancer therapies. Here we report development and characterization of a large bank of patient-derived xenografts (PDX) and matched organoid cultures from tumors that represent some of the greatest unmet needs in breast cancer research and treatment. These include endocrine-resistant, treatment-refractory, and metastatic breast cancers and, in some cases, multiple tumor collections from the same patients. The models can be grown long-term with high fidelity to the original tumors. We show that development of matched PDX and PDX-derived organoid (PDxO) models facilitates high-throughput drug screening that is feasible and cost-effective, while also allowing in vivo validation of results. Our data reveal consistency between drug screening results in organoids and drug responses in breast cancer PDX. Moreover, we demonstrate the feasibility of using these patient-derived models for precision oncology in real time with patient care, using a case of a triple negative breast cancer with early metastatic recurrence as an example. Our results uncovered an FDA-approved drug with high efficacy against the models. Treatment with the PDxO-directed therapy resulted in a complete response for the patient and a progression-free survival period more than three times longer than her previous therapies. This work provides valuable new methods and resources for functional precision medicine and drug development for human breast cancer.Graphical Abstract

Background Targeted therapies for triple-negative breast cancer (TNBC) are limited; however, the epidermal growth factor receptor (EGFR) represents a potential target, as the majority of TNBC express EGFR. The purpose of these studies was to evaluate the effectiveness of two EGFR-targeted antibody-drug conjugates (ADC: ABT-414; ABBV-321) in combination with navitoclax, an antagonist of the anti-apoptotic BCL-2 and BCL-XL proteins, in order to assess the translational relevance of these combinations for TNBC. Methods The pre-clinical efficacy of combined treatments was evaluated in multiple patient-derived xenograft (PDX) models of TNBC. Microscopy-based dynamic BH3 profiling (DBP) was used to assess mitochondrial apoptotic signaling induced by navitoclax and/or ADC treatments, and the expression of EGFR and BCL-2/XL was analyzed in 46 triple-negative patient tumors. Results Treatment with navitoclax plus ABT-414 caused a significant reduction in tumor growth in five of seven PDXs and significant tumor regression in the highest EGFR-expressing PDX. Navitoclax plus ABBV-321, an EGFR-targeted ADC that displays more effective wild-type EGFR-targeting, elicited more significant tumor growth inhibition and regressions in the two highest EGFR-expressing models evaluated. The level of mitochondrial apoptotic signaling induced by single or combined drug treatments, as measured by DBP, correlated with the treatment responses observed in vivo. Lastly, the majority of triple-negative patient tumors were found to express EGFR and co-express BCL-XL and/or BCL-2. Conclusions The dramatic tumor regressions achieved using combined agents in pre-clinical TNBC models underscore the abilities of BCL-2/XL antagonists to enhance the effectiveness of EGFR-targeted ADCs and highlight the clinical potential for usage of such targeted ADCs to alleviate toxicities associated with combinations of BCL-2/XL inhibitors and systemic chemotherapies.

Immunotherapy has potential to prevent and treat metastatic breast cancer, but strategies to enhance immune-mediated killing of metastatic tumors are urgently needed. We report that a ligand-independent isoform of Ron kinase (SF-Ron) is a key target to enhance immune infiltration and eradicate metastatic tumors. Host-specific deletion of SF-Ron caused recruitment of lymphocytes to micrometastases, augmented tumor-specific T-cell responses, and nearly eliminated breast cancer metastasis in mice. Lack of host SF-Ron caused stem-like TCF1+ CD4+ T cells with type I differentiation potential to accumulate in metastases and prevent metastatic outgrowth. There was a corresponding increase in tumor-specific CD8+ T cells, which were also required to eliminate lung metastases. Treatment of mice with a Ron kinase inhibitor increased tumor-specific CD8+ T cells and protected from metastatic outgrowth. These data provide a strong preclinical rationale to pursue small-molecule Ron kinase inhibitors for the prevention and treatment of metastatic breast cancer. Significance: The discovery that SF-Ron promotes antitumor immune responses has significant clinical implications. Therapeutic antibodies targeting full-length Ron may not be effective for immunotherapy; poor efficacy of such antibodies in trials may be due to their inability to block SF-Ron. Our data warrant trials with inhibitors targeting SF-Ron in combination with immunotherapy. This article is highlighted in the In This Issue feature, p. 2945

The bone is the third most common site of metastasis for a wide range of solid tumors including lung, breast, prostate, colorectal, thyroid, gynecologic, and melanoma, with 70% of metastatic prostate and breast cancer patients harboring bone metastasis.1 Unfortunately, once cancer spreads to the bone, it is rarely cured and is associated with a wide range of morbidities including pain, increased risk of fracture, and hypercalcemia. This fact has driven experts in the fields of bone and cancer biology to study the bone, and has revealed that there is a great deal that each can teach the other. The complexity of the bone was first described in 1889 when Stephen Paget proposed that tumor cells have a proclivity for certain organs, where they "seed" into a friendly "soil" and eventually grow into metastatic lesions. Dr. Paget went on to argue that although many study the "seed" it would be paramount to understand the "soil." Since this original work, significant advances have been made not only in understanding the cell-autonomous mechanisms that drive metastasis, but also alterations which drive changes to the "soil" that allow a tumor cell to thrive. Indeed, it is now clear that the "soil" in different metastatic sites is unique, and thus the mechanisms that allow tumor cells to remain in a dormant or growing state are specific to the organ in question. In the bone, our knowledge of the components that contribute to this fertile "soil" continues to expand, but our understanding of how they impact tumor growth in the bone remains in its infancy. Indeed, we now appreciate that the endosteal niche likely contributes to tumor cell dormancy, and that osteoclasts, osteocytes, and adipocytes can impact tumor cell growth. Here, we discuss the bone microenvironment and how it impacts cancer cell seeding, dormancy, and growth. © 2018 American Society for Bone and Mineral Research.

B cell lymphoma is the most common hematologic malignancy in the United States. Although treatment options have greatly improved in the past several decades, outcomes for patients with relapsed B cell lymphoma remain poor. Chimeric antigen receptor (CAR) T cells have recently entered the clinic with promise to address the gap in effective therapies for patients relapsed B cell lymphoma. However, antigen loss and poor CAR T cell persistence has been shown to drive resistance to the widely approved CD19-targeted CAR in some patients, demonstrating the need for additional therapies. Here, we demonstrate CD229-targeted CAR T cell therapy as a promising option for the treatment of relapsed B cell lymphoma, addressing an important group of patients with typically poor outcomes. CD229 is an immune-modulating receptor expressed on the surface of B cells that we recently found to be highly expressed in the plasma cell neoplasm multiple myeloma (Radhakrishnan et al. 2020). We utilized semi-quantitative PCR and flow cytometry to assess whether CD229 is also expressed on malignant B cells earlier in development as found in B cell lymphoma. Expression analysis revealed the presence of CD229 in a panel of 11 B cell lymphoma cell lines and 45 primary B cell lymphoma samples comprising several subsets of disease including aggressive B cell lymphomas such as diffuse large B cell lymphoma (DLBCL), mantle cell lymphoma (MCL) and Burkitt lymphoma as well as indolent subtypes of B cell lymphoma including chronic lymphoblastic leukemia (CLL) and follicular lymphoma. Of note, CD229 was found to be overexpressed on primary B cell lymphoma cells when compared to autologous normal B cells. Given the high levels of CD229 expression throughout all B cell lymphoma subtypes analyzed, we generated CD229 CAR T cells in order to determine whether CAR T cell therapy is an effective way to target CD229 expressing B cell lymphoma cells. CD229 CAR T cells exhibited robust cytotoxicity when cocultured with B cell lymphoma cell lines and primary samples characterized by significant production of TH1 cytokines IL-2, TNF and IFNγ and rapid loss of B cell lymphoma cell viability when compared to control CAR T cells lacking an antigen binding scFv domain (∆scFv CAR T cells). In vivo analysis revealed effective tumor control in NSG mice carrying B cell lymphoma cell lines JeKo-1 (MCL) and DB (DLBCL) when treated with CD229 CAR T cells versus ∆scFv CAR T cells. Finally, we sought to determine the efficacy of CD229 CAR T cells in the context of CD19 CAR T cell therapy relapse. Here, a 71-year-old patient with CLL had an initial response when treated with CD19 CAR T cells but quickly relapsed only 2 months after treatment. Malignant cells from the CLL patient retained CD229 expression as identified by flow cytometry and an ex vivo coculture with CD229 CAR T cells revealed robust killing of CLL cells by CD229 CAR T cells. Transfer of antigen from target cell to CAR T cell by trogocytosis was recently suggested to drive relapse following CAR T cell therapy by decreasing antigen on tumor cells and promoting CAR T cell fratricide (Hamieh et al. 2019). We cocultured CD19 and CD229 CAR T cells with primary CLL cells and assessed CD19 and CD229 expression as well as CAR T cell viability by flow cytometry. In contrast with CD19 CAR T cells, CD229 CARs did not strip their target antigen from the surface of CLL cells. The transfer of CD19 from CLL cells to CD19 CAR T cells resulted in poor CAR T cell viability while CD229 CAR T cell viability remained high following coculture. In summary, we demonstrate that CD229 is a promising therapeutic target in B cell lymphoma due to its high levels of expression throughout many subtypes of disease. CD229 CAR T cells effectively kill B cell lymphoma cells in vitro and control growth of aggressive B cell lymphomas in vivo. Finally, CD229 CAR T cells are effective against primary CLL cells from patients that have relapsed from CD19 CAR T cell therapy and do no exhibit antigen loss by trogocytosis. Taken together, these data suggest that CD229 CAR T cell therapy may be a promising option to address the poor outcomes for patients with relapsed B cell lymphoma. Disclosures No relevant conflicts of interest to declare.

Patient derived xenografts (PDX) are valuable, clinically-relevant models of cancer. Their close genomic, phenotypic, and temporal association with patient tumors makes them well-suited for pre-clinical and co-clinical studies that assess the potential of new therapeutics. However, PDX models are not amenable to large-scale drug sensitivity studies or real-time precision oncology due to their relatively slow growth, high cost, and low throughput. To address this, we established and characterized organoid lines from breast cancer patient tumors and PDXs and evaluated their fidelity to the original tumors and their utility in high-throughput therapeutic studies. Results of comparisons between in vitro and in vivo patient-derived models of breast cancer will be presented, along with extension of this technology to inform patient care in an IRB-approved study of precision oncology. Our work demonstrates that breast cancer organoid models are a powerful parallel resource to PDX models, especially useful for efficient determination of drug responses, and are amenable to informing real-time patient care in a functional precision oncology setting. Citation Format: Alana L. Welm. Using patient-derived models for functional precision oncology in advanced breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr SY16-02.

Patient-derived tumor xenograft (PDX) mouse models have emerged as an important oncology research platform to study tumor evolution, mechanisms of drug response and resistance, and tailoring chemotherapeutic approaches for individual patients. The lack of robust standards for reporting on PDX models has hampered the ability of researchers to find relevant PDX models and associated data. Here we present the PDX models minimal information standard (PDX-MI) for reporting on the generation, quality assurance, and use of PDX models. PDX-MI defines the minimal information for describing the clinical attributes of a patient's tumor, the processes of implantation and passaging of tumors in a host mouse strain, quality assurance methods, and the use of PDX models in cancer research. Adherence to PDX-MI standards will facilitate accurate search results for oncology models and their associated data across distributed repository databases and promote reproducibility in research studies using these models. Cancer Res; 77(21); e62–66. ©2017 AACR.

Cancer immunotherapy continues to make headway as a treatment for advanced stage tumors, revealing an urgent need to understand the fundamentals of anti-tumor immune responses. Noteworthy is a scarcity of data pertaining to the breadth and specificity of tumor-specific T cell responses in metastatic breast cancer. Autochthonous transgenic models of breast cancer display spontaneous metastasis in the FVB/NJ mouse strain, yet a lack of knowledge regarding tumor-bound MHC/peptide immune epitopes in this mouse model limits the characterization of tumor-specific T cell responses, and the mechanisms that regulate T cell responses in the metastatic setting. We recently generated the NetH2pan prediction tool for murine class I MHC ligands by building an FVB/NJ H-2q ligand database and combining it with public information from six other murine MHC alleles. Here, we deployed NetH2pan in combination with an advanced proteomics workflow to identify immunogenic T cell epitopes in the MMTV-PyMT transgenic model for metastatic breast cancer. Five unique MHC I/PyMT epitopes were identified. These tumor-specific epitopes were confirmed to be presented by the class I MHC of primary MMTV-PyMT tumors and their T cell immunogenicity was validated. Vaccination using a DNA construct encoding a truncated PyMT protein generated CD8 + T cell responses to these MHC class I/peptide complexes and prevented tumor development. In sum, we have established an MHC-ligand discovery pipeline in FVB/NJ mice, identified and tracked H-2Dq/PyMT neoantigen-specific T cells, and developed a vaccine that prevents tumor development in this metastatic model of breast cancer.

Great progress has been made in cancer therapeutics. However, metastasis remains the predominant cause of death from cancer. Importantly, metastasis can manifest many years after initial treatment of the primary cancer. This is because cancer cells can remain dormant before forming symptomatic metastasis. An important question is whether metastasis research should focus on the early treatment of metastases, before they are clinically evident ("overt"), or on developing treatments to stop overt metastasis (stage IV cancer). In this commentary we want to clarify why it is important that all avenues of treatment for stage IV patients are developed. Indeed, future treatments are expected to go beyond the mere shrinkage of overt metastases and will include strategies that prevent disseminated tumor cells from emerging from dormancy.

Human cancer is a complex and heterogeneous collection of diseases that kills more than 18 million people every year worldwide. Despite advances in detection, diagnosis, and treatments for cancers, new strategies are needed to combat deadly cancers. Models of human cancer continue to evolve for preclinical research and have culminated in patient-derived systems that better represent the diversity and complexity of cancer. Still, no model is perfect. This Perspective attempts to address ways that we can improve the clinical translatability of models used for cancer research, from the point of view of researchers who mainly conduct cancer studies in vivo.

Patient-derived xenografts (PDXs) are resected human tumors engrafted into mice for preclinical studies and therapeutic testing. It has been proposed that the mouse host affects tumor evolution during PDX engraftment and propagation, affecting the accuracy of PDX modeling of human cancer. Here, we exhaustively analyze copy number alterations (CNAs) in 1,451 PDX and matched patient tumor (PT) samples from 509 PDX models. CNA inferences based on DNA sequencing and microarray data displayed substantially higher resolution and dynamic range than gene expression-based inferences, and they also showed strong CNA conservation from PTs through late-passage PDXs. CNA recurrence analysis of 130 colorectal and breast PT/PDX-early/PDX-late trios confirmed high-resolution CNA retention. We observed no significant enrichment of cancer-related genes in PDX-specific CNAs across models. Moreover, CNA differences between patient and PDX tumors were comparable to variations in multiregion samples within patients. Our study demonstrates the lack of systematic copy number evolution driven by the PDX mouse host.

Background: Endocrine therapy resistance is common and is a leading cause of breast cancer-related death. Thus, the search for therapeutic targets based on mechanistic insights into endocrine therapy resistance continues. Kinases are important drug targets and regulators in cells, many of which are involved in tumorigenesis and the development of treatment resistance. Mass spectrometry-based kinome analysis has been impeded by the low abundance of individual kinases. Here we utilized kinase inhibitor-conjugated beads to enrich and thereby sensitively profile the kinome of estrogen receptor-positive (ER+) breast cancer patient-derived xenograft (PDX) tumors under estradiol-deprivation treatment. Experimental design and methods: We harvested tumor samples from 20 breast PDX lines with various degrees of estradiol (E2) dependence in ovariectomized SCID-beige mice with and without E2 supplementation (n=3 per PDX line per arm). The kinases in the tumor lysates were enriched using kinase inhibitor pulldown (KIP) beads and the tightly bound kinases were quantified by mass spectrometry. To identify candidates, we selected for kinases that were highly E2-regulated in the E2-dependent PDX lines but constitutively expressed in E2-independent PDX lines. Survival analysis of candidate kinases in patients with ER+ breast cancer was performed using the METABRIC dataset. Results: Each PDX line had a unique yet reproducible kinome. To seek kinases with consistent relationships with estrogen dependence, we sought kinases that were statistically differentially expressed in tumors between E2 supplied and deprived conditions. Noticeably, membrane-associated tyrosine-and threonine-specific cdc2-inhibitory kinase (PKMYT1), a WEE family kinase known to have estrogen response elements (EREs) in its regulatory region, was significantly decreased after E2 deprivation in E2-dependent PDXs (log2 fold change = -7.86, p Conclusion: Here, we analyzed the kinomes of 20 ER+ breast cancer PDX tumors with or without E2 by mass spectrometry. We discovered that PKMYT1 is a marker of hormone independent growth and poor outcome. Ongoing experiments that study the effects of PKMYT1 inhibition in both ER-dependent and independent circumstances will be presented. Citation Format: Anran Chen, Beom-Jun Kim, Doug W Chan, Purba Singh, Lacey E Dobrolecki, Jonathan T Lei, Shunqiang Li, Alana L Welm, Michael T Lewis, Matthew J Ellis. Kinome profiling of ER+ breast cancer PDXs identifies PKMYT1 as a marker of hormone independent growth and poor outcome [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD8-02.

Background: Predicting the risk of relapse in patients with non-metastatic breast cancer is important for medical decisions and patient counseling. For patients who receive neoadjuvant chemotherapy, pathologic response and especially pathologic complete response (pCR) has been associated with risk of relapse; however this association is imperfect. Our prior work in patient-derived xenograft (PDX) models indicated that tumor engraftment in mice correlated with risk of recurrence. To understand further the prognostic utility of PDX, we designed a prospective clinical trial to determine the correlation between PDX generation with residual disease following neoadjuvant chemotherapy and relapse. Preliminary data are presented. Methods: Women with newly diagnosed non-metastatic hormone receptor low-positive (HR-low, ER and/or PR ≤ 10%) or negative breast cancer planned to receive systemic chemotherapy prior to definitive surgery were eligible. Tumor tissue at diagnosis was orthotopically implanted in NOD/SCID mice. The primary objective of the study is to correlate the ability of a tumor to engraft in mice with pathologic responses and clinical outcomes. Results: Between 12/2016 and 2/2020, 58 patients enrolled (triple negative breast cancer (TNBC), n=37; HR-low or negative/Her2+, n=11; or HR-low/Her2-, n=8; mixed, n=1). PDXs were successfully established from 16 patients. Patients uniformly received intensive preoperative chemotherapy per standard of care. Among the 12 patients whose tumors engrafted in mice (PDX(+)) and underwent surgery, the pCR rate was 41.7%. In the subgroup of patients with postoperative follow up >6 months (n=9), 3 patients had achieved a pCR, 1 of whom recurred; and 6 patients had residual disease, 3 of whom recurred (overall relapse rate 44.4%). All patients who relapsed (TNBC n=3; HR-low/Her2- n=1), experienced a very early relapse ( Among the 38 patients whose tumors did not engraft (PDX(-)) and underwent surgery, the overall pCR rate was 60.6%. In the subgroup of patients with postoperative follow up >6 months (n=30), 18 patients had achieved a pCR of whom none recurred, and 12 had residual disease of whom 1 patient (with TNBC) had locoregional recurrence 35.1 months after her definitive surgery (relapse rate 3.3%). She underwent repeat surgery followed by radiation therapy and 5.5 months after her recurrence, she remains disease-free. Achievement of pCR was not statistically different between the PDX(+) and PDX(-) group. In the entire cohort, the presence of residual disease was associated with high risk of relapse (22.2%) as compared to the achievement of pCR (4.8%, p = 0.16). However, successful tumor engraftment was associated not only with a higher risk of relapse (44.4% vs 3.3%, p = 0.0068, odds ratio 20.45), but also an early and exceptionally aggressive relapse. Conclusion. Our functional studies not only identify a patient population with a high risk of relapse with greater precision than the achievement of pCR, but also identify patients whose relapse has a particularly aggressive natural history. We established models that recapitulate this aggressive disease and in-depth genomic studies are underway. These studies will lead us to better patient stratification on the basis of risk of relapse and novel therapeutic strategies focused on patients with exceptionally aggressive breast cancers that our current diagnostic methods cannot identify. Citation Format: Christos Vaklavas, Zhengtao Chu, Rachel E Factor, Alana L Welm, Bryan E Welm, Cindy B Matsen. Towards personalized medicine - patient-derived breast tumor grafts as predictors of relapse and response to therapy. Preliminary results [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD7-01.

Purpose: Recent studies have demonstrated that short-form Ron (sfRon) kinase drives breast tumor progression and metastasis through robust activation of the PI3K pathway. We reasoned that upfront, concurrent inhibition of sfRon and PI3K might enhance the antitumor effects of Ron kinase inhibitor therapy while also preventing potential therapeutic resistance to tyrosine kinase inhibitors (TKI). Experimental Design: We used patient-derived breast tumor xenografts (PDX) as high-fidelity preclinical models to determine the efficacy of single-agent or dual Ron/PI3K inhibition. We tested the Ron kinase inhibitor ASLAN002 with and without coadministration of the PI3K inhibitor NVP-BKM120 in hormone receptor–positive [estrogen receptor (ER)+/progesterone receptor (PR)+] breast PDXs with and without PIK3CA gene mutation. Results: Breast PDX tumors harboring wild-type PIK3CA showed a robust response to ASLAN002 as a single agent. In contrast, PDX tumors harboring mutated PIK3CA demonstrated partial resistance to ASLAN002, which was overcome with addition of NVP-BKM120 to the treatment regimen. We further demonstrated that concurrent inhibition of sfRon and PI3K in breast PDX tumors with wild-type PIK3CA provided durable tumor stasis after therapy cessation, whereas discontinuation of either monotherapy facilitated tumor recurrence. Conclusions: Our work provides preclinical rationale for targeting sfRon in patients with breast cancer, with the important stipulation that tumors harboring PIK3CA mutations may be partially resistant to Ron inhibitor therapy. Our data also indicate that tumors with wild-type PIK3CA are most effectively treated with an upfront combination of Ron and PI3K inhibitors for the most durable response. Clin Cancer Res; 21(24); 5588–600. ©2015 AACR.

Although the RAF>MEK>ERK MAP kinase pathway is key to the development of many RAS-mutated cancers, efforts to target this pathway with pharmacologic inhibitors have failed to deliver clinical benefit to patients. Here we show that inhibition of RAS>RAF>MEK>ERK signaling in RAS-mutated cancer cell lines elicits autophagy, a process of cellular recycling that protects cancer cells from the potentially cytotoxic effects of RAF>MEK>ERK pathway inhibition. Furthermore, combined inhibition of MEK1/2 plus autophagy displays synergistic antiproliferative effects against cell lines in vitro and promotes regression of xenografted patient-derived tumors in mice. Finally, treatment of a cancer patient with the combination of trametinib plus hydroxychloroquine resulted in a partial, but nonetheless striking, disease response. These data suggest that this combination therapy may represent a novel strategy to target RAS-driven malignancies such as melanoma, lung, and pancreatic cancer. Citation Format: Conan Kinsey, Soledad Camolotto, Amelie Boespflug, Katrin Gullien, Amanda Truong, Mona Foth, Jill Shea, Michael Seipp, Jeffrey Yap, Lance Burrell, David Lum, Jonathan Whisenant, Courtney Cavalier, Kaitren Rehbein, Stephanie Cutler, Kajsa Afotler, Alana Welm, Bryan Welm, Courtney Scaife, Eric Snyder, Martin McMahon. Targeting MEK1/2 inhibitor resistance in RAS-mutated cancers [abstract]. In: Proceedings of the AACR Special Conference on Melanoma: From Biology to Target; 2019 Jan 15-18; Houston, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(19 Suppl):Abstract nr IA25.

Patient-Derived Xenografts (PDXs) are preclinical models largely used to study tumor biology and drug response. Recent literature highlighted the possibility that growth of human tumors in a mouse microenvironment imposes a selection driving mouse-specific genetic evolution of PDXs, which may compromise their reliability as human cancer models. Conversely, independent studies observed a conservation of the genomic landscape during PDX engraftment and passaging. We noticed that PDX genetic evolution was particularly evident in studies based on copy number aberration (CNA) inferred from gene expression data, while it was negligible when DNA-based CNA profiles were employed. Therefore, in a joint international effort of the EurOPDX and PDXNet consortia, we assembled a dataset of 37 hepatocellular and 54 gastric carcinoma tumor or PDX samples with matched RNA-based and DNA-based CNA profiles. We found that DNA-based CNA profiles invariably yield higher concordance between patient's tumor and derived PDXs than those inferred from RNA. RNA-based profiles displayed poor concordance with matched DNA-based profiles, and much lower resolution, so that they missed many focal copy number events detected by DNA-based methods. These results revealed that CNA measurements cannot be accurately estimated by expression data and that a systematic reassessment of CNA dynamics in PDXs based on DNA data is required. To this aim, we generated CNA profiles by low-pass whole genome sequencing (WGS) of 87 colorectal and 43 breast cancer triplets, each composed of matched patient's tumor (PT) and PDX at early (PDX-early) and later (PDX-late) passage. In this way, for each tumor type, we generated three perfectly matched PT, PDX-early and PDX-late cohorts and performed CNA recurrence analysis by GISTIC in each cohort. The hypothesis was that if the mouse host induces a selective pressure capable of shaping the CNA landscape during PDX engraftment and propagation, GISTIC analysis would highlight systematic and progressive changes, from the PT to the PDX-early cohort, and then to the PDX-late cohort. Notably instead, the CNA profiles of the PT and PDX-early/late cohorts were virtually indistinguishable, with no progressive accumulation or loss of CNA during PDX passage and only minor changes not functionally related or associated to cancer-driver or actionable genes. These results were not consequence of insufficient capture of the CNA repertoire, since the GISTIC profiles recapitulated those generated by TCGA for colorectal and breast cancer. In summary, our analyses highlighted that while RNA-based CNA inferences have inadequate resolution and accuracy to study genomic evolution in PDXs, DNA-based CNA profiles confirm retention of CNAs in PTs and PDXs, excluding a systematic mouse driven selection via copy number changes. Ultimately, these results support the robustness of PDXs as preclinical models for predicting drug response. Citation Format: Jessica Giordano, Xing Yi Woo, Anuj Srivastava, Zi-Ming Zhao, Michael W. Lloyd, Roebi de Bruijn, Yun-Suhk Suh, Francesco Galimi, Andrea Bertotti, Adam Lafferty, Alice C. O'Farrell, Elodie Modave, Diether Lambrechts, Petra ter Brugge, Violeta Serra, Elisabetta Marangoni, Rania El Botty, Jong-Il Kim, Han-Kwang Yang, Charles Lee, Dennis A. Dean, Brandi Davis-Dusenbery, Yvonne A. Evrard, James H. Doroshow, Alana L. Welm, Bryan E. Welm, Michael T. Lewis, Bingliang Fang, Jack Roth, Funda Meric-Bernstam, Meenhard Herlyn, Michael Davies, Li Ding, Shunqiang Li, Ramaswamy Govindan, Jeffrey A. Moscow, Carol J. Bult, Claudio Isella, Livio Trusolino, Annette T. Byrne, Jos Jonkers, Jeffrey H. Chuang, Enzo Medico, EurOPDX consortium & PDXNET consortium. Absence of mouse-specific tumor evolution in patient-derived cancer xenografts [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1118.

Pancreatic ductal adenocarcinoma was responsible for ~43,000 deaths in the USA in 2017 and is the epitome of a recalcitrant cancer driven by a pharmacologically intractable oncoprotein, KRAS. Although the clinical picture remains grim, the mechanisms by which key alterations in tumor suppressors and proto-oncogenes contribute to PDA have been dissected. Downstream of KRAS, the RAF→MEK→ERK signaling pathway plays a central role in pancreatic carcinogenesis. However, to date, pharmacologic inhibition of this pathway has provided no clinical benefit to PDA patients. Here we show that inhibition of KRAS→RAF→MEK→ERK signaling in PDA cell lines elicits autophagy, a process of cellular recycling that protects pancreatic cancer cells from the potentially cytotoxic effects of KRAS pathway inhibition. Furthermore, combined inhibition of MEK1/2 plus autophagy displays synergistic antiproliferative effects against PDA cell lines in vitro, and promotes regression of xenografted patient-derived PDA tumors in mice. Finally, treatment of a KRAS-mutated PDA patient on a compassionate basis with the combination of trametinib plus hydroxychloroquine resulted in a partial but nonetheless striking disease response. These data suggest that this combination therapy may represent a new strategy to target RAS-driven cancers such as PDA. Citation Format: Conan G. Kinsey, Soledad A. Camolotto, Amelie M. Boespflug, Katrin P. Gullien, Mona Foth, Jill E. Shea, Michael T. Seipp, Jeffrey T. Yap, Lance D. Burrell, David H. Lum, Jonathan R. Whisenant, Weldon Gilcrease, Courtney C. Cavalieri, Kaitrin M. Rehbein, Stephanie L. Cutler, Kajsa E. Affolter, Alana L. Welm, Bryan E. Welm, Courtney L. Scaife, Eric L. Snyder, Martin McMahon. Protective autophagy elicited by RAF→MEK→ERK inhibition suggests a treatment strategy for RAS-driven cancers [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr A45.

Background: Estrogen Receptor (ER) positive breast cancer is the most common subtype (70%) and associated with the most deaths from the disease. Almost all deaths from breast cancer are caused by metastasis. While metastatic ER-positive breast cancer is a leading cause of cancer-related death, metastasis studies have largely focused on ER-negative breast cancer. A major reason for such imbalance is the lack of physiologically relevant mice models for studying ER-positive breast cancer. The patient derived xenograft (PDX) technique provides us with a powerful means to study highly clinically relevant human ER-positive breast cancer in mice, but most studies using PDX models still focus on orthotopic rather than metastatic tumor growth. Here we describe studies of the metastatic process using PDX models to more comprehensively understand the value of these PDX lines for therapeutic modelling. These findings will empower us to develop therapeutics designed to control systemic disease, not just orthotopic tumor growth. Experimental design and methods: We have studied 14 ER positive PDX lines in ovariectomized SCID beige mice with and without estradiol supplementation. We have analyzed both orthotopic tumor and a variety of metastatic sites including liver, lung, brain and bone. Data is being generated using next-generation DNA and RNA sequencing, mass spectrometry-based proteomics. We use integrated bioinformatics pipelines to shed light on the metastatic process. Results: We have generated macro-metastatic maps that record sites with observable metastatic nodules of each PDX line in mice. IHC of Liver, bone, brain, lung of each mouse also generated micro-metastatic maps. The most common organ of metastasis in mice is the lung. In some PDX lines, the sites of metastasis differ between groups with and without estradiol supply. The ongoing proteogenomic profiling of orthotopic and metastatic tumors reveals potential molecular drivers of metastasis in ER+ breast cancer. Conclusion: Here, we describe an approach to the systematic investigation of the ER+ breast cancer metastasis process. This approach will allow us to identify druggable targets that may interrupt the metastatic process and/or inhibit tumor growth at metastatic sites. Citation Format: Anran Chen, Purba Singh, Beom-Jun Kim, Lacey E Dobrolecki, Alana L Welm, Michael T Lewis, Matthew J Ellis. Proteogenomic analysis of estrogen modulated breast cancer metastasis [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-20.

Metastasis is the biggest challenge in treating breast cancer, and it kills >40,000 breast cancer patients annually in the US. Aberrant expression of the RON receptor tyrosine kinase in breast tumors correlates with poor prognosis and has been shown to promote metastasis. However, the molecular mechanisms that govern how RON promotes metastasis, and how to block it, are still largely unknown. We sought to determine critical effectors of RON using a combination of mutational and pharmacologic strategies. High-throughput proteomic analysis of breast cancer cells upon activation of RON showed robust phosphorylation of ribosomal protein S6. Further analysis revealed that RON strongly signals through mTORC1/p70S6K, which is mediated predominantly by the PI3K pathway. A targeted mutation approach to modulate RON signaling validated the importance of PI3K/mTORC1 pathway for spontaneous metastasis in vivo. Finally, inhibition of mTORC1 with an FDA-approved drug, everolimus, resulted in transient shrinkage of established RON-dependent metastases, and combined blockade of mTORC1 and RON delayed progression. These studies have identified a key downstream mediator of RON-dependent metastasis in breast cancer cells and revealed that inhibition of mTORC1, or combined inhibition of mTORC1 and RON, may be effective for treatment of metastatic breast cancers with elevated expression of RON., Metastasis: Druggable target mediates metastasis-promoting signal A surface protein linked to metastasis in various cancers promotes the spread and migration of breast tumors by activating a druggable signaling pathway. Alana Welm and colleagues from the University of Utah, Salt Lake City, USA, engineered human breast cancer cells to conditionally overexpress RON, a receptor known to drive metastatic disease. They searched for proteins turned on by high RON levels, and found that one, called ribosomal protein S6 (rpS6), became highly decorated with phosphate groups, a sign of activation. Further analyses showed that an intermediary—the mechanistic target of rapamycin complex 1 (mTORC1)—relayed the activation signal to rpS6. The researchers thus inhibited mTORC1, both genetically and with an approved drug called everolimus, and observed transient shrinkage of metastatic breast tumors in mice, while combined blockade of mTORC1 and RON delayed metastatic progression.

The advent of immune checkpoint blockade as a new strategy for immunotherapy has changed the outlook for many aggressive cancers. Although complete tumor eradication is attainable in some cases, durable clinical responses are observed only in a small fraction of patients, underlining urgent need for improvement. We previously showed that RON, a receptor tyrosine kinase expressed in macrophages, suppresses antitumor immune responses, and facilitates progression and metastasis of breast cancer. Here, we investigated the molecular changes that occur downstream of RON activation in macrophages, and whether inhibition of RON can cooperate with checkpoint immunotherapy to eradicate tumors. Activation of RON by its ligand, MSP, altered the gene expression profile of macrophages drastically and upregulated surface levels of CD80 and PD-L1, ligands for T-cell checkpoint receptors CTLA-4 and PD-1. Genetic deletion or pharmacological inhibition of RON in combination with anti-CTLA-4, but not with anti-PD-1, resulted in improved clinical responses against orthotopically transplanted tumors compared to single-agent treatment groups, resulting in complete tumor eradication in 46% of the animals. Positive responses to therapy were associated with higher levels of T-cell activation markers and tumor-infiltrating lymphocytes. Importantly, co-inhibition of RON and anti-CTLA-4 was also effective in clearing metastatic breast cancer cells in lungs, resulting in clinical responses in nearly 60% of the mice. These findings suggest that RON inhibition can be a novel approach to potentiate responses to checkpoint immunotherapy in breast cancer.

Context College of American Pathologists (CAP) and the American Society of Clinical Oncology have emphasized the need to reduce preanalytic variables for evaluating predictive biomarker expression in breast cancer. Postoperative x-ray of excised breast tissue is commonplace, yet is a variable that has not been investigated previously. We asked whether such radiation affects expression of relevant biomarkers. Design A previous study found that human breast cancers grown in mice demonstrate the same immunohistochemical and molecular profiles as the original tumors. Thirteen patient-derived xenografts were harvested fresh and divided for specimen radiography and a matched nonirradiated control, while following CAP/ASCO guidelines for cold ischemia time and fixation. Samples were processed in a tissue microarray for immunohistochemistry. Estrogen receptor (ER), progesterone receptor (PR), p53, and Ki67 staining was evaluated using an optimized scoring algorithm performed on digitally scanned slides. Samples were also scored manually by a blinded pathologist using the H-score method, and HER2 by the CAP/ASCO 2013 protocol. Histologic scores were compared by analysis of variance. Results There was no significant difference in quantity or intensity of staining between irradiated and nonirradiated samples for estrogen receptor (P=0.28), p53 (P=0.96), and Ki67 (P=0.94). A small but statistically significant difference was observed for PR (P=0.0058). HER2 staining was similarly unchanged in the 1 tumor exhibiting 3+ staining. Conclusions Our study demonstrates that x-ray of breast carcinomas does not significantly affect the expression of predictive biomarkers, with the exception of PR for unclear reasons. It also highlights the utility of the patient-derived xenograft model for biomarker studies.

With an increasing body of literature covering RON receptor tyrosine kinase function in different types of human cancers, it is becoming clear that RON has prominent roles in both cancer cells and in the tumor-associated microenvironment. RON not only activates several oncogenic signaling pathways in cancer cells, leading to more aggressive behavior, but also promotes an immunosuppressive, alternatively activated phenotype in macrophages and limits the antitumor immune response. These two unique functions of this oncogene, the strong correlation between RON expression and poor outcomes in cancer, and the high tolerability of a new RON inhibitor make it an exciting therapeutic target, the blocking of which offers an advantage toward improving the survival of cancer patients. Here, we discuss recent findings on the role of RON signaling in cancer progression and its potential in cancer therapy.

Patient-derived xenograft (PDX) models of a growing spectrum of cancers are rapidly supplanting long-established traditional cell lines as preferred models for conducting basic and translational pre-clinical research. In breast cancer, to complement the now curated collection of approximately 45 long-established human breast cancer cell lines, a newly formed consortium of academic laboratories, currently from Europe, Australia, and North America, herein summarizes data on over 500 stably transplantable PDX models representing all three clinical subtypes of breast cancer (ER+, HER2+, and “Triple-negative” (TNBC)). Many of these models are well-characterized with respect to genomic, transcriptomic, and proteomic features, metastatic behavior, and treatment response to a variety of standard-of-care and experimental therapeutics. These stably transplantable PDX lines are generally available for dissemination to laboratories conducting translational research, and contact information for each collection is provided. This review summarizes current experiences related to PDX generation across participating groups, efforts to develop data standards for annotation and dissemination of patient clinical information that does not compromise patient privacy, efforts to develop complementary data standards for annotation of PDX characteristics and biology, and progress toward “credentialing” of PDX models as surrogates to represent individual patients for use in pre-clinical and co-clinical translational research. In addition, this review highlights important unresolved questions, as well as current limitations, that have hampered more efficient generation of PDX lines and more rapid adoption of PDX use in translational breast cancer research.

Targeting HER2 is one of the greatest successes in oncology, and has resulted in the generation of a wide array of HER2-targeting agents. Our genomic approaches are revealing other mechanisms of HER2 activation, such as our discovery of activating HER2 mutations in different cancer types. From initial breast cancer and SUMMIT trial data, the pan-HER drug neratinib as monotherapy showed initial clinical response in ER+ breast cancer, but with frequent early relapse. This study investigates the preclinical efficacy of anti HER2 agents alone or in combination with endocrine therapy agents or in combination with CDK4/6 inhibitors using ER+/HER2 mutant cell lines and ex vivo HER2 mutant patient derived xenograft (PDX) model. ER+ breast cancer cell lines (T47D and MCF7) stably expressing HER2V777L, and ER+/HER2 mutant PDX model (HER2G778_P780 dup) were used to examine HER2 signaling. We found that MCF7/T47D cells expressing HER2V777L and HER2G778_P780 dup PDX tumors showed strongly activated autophosphorylation of HER2 and increased expression of CDK4, CDK6, phospho-Rb, and cyclin D1 as compared to MCF7/T47D cells expressing HER2WT or ER+/non-HER2mut PDX model, suggesting that HER2 mutations preferentially depend on CDK4/6 signaling for cell growth. Additionally, we showed that activating MCF7 HER2V777L cause resistance to endocrine therapy treatment (fulvestrant IC50 >5μM). Further, we show that neratinib alone is effective at higher concentrations (IC50 < 2μM) in MCF7/HER2V777L cells. Next we showed that abemaciclib alone exhibited moderate activity against MCF7 HER2V777L cells (IC50 < 0.4μM) and additional activity in combination with neratinib (IC50 < 0.06μM) was seen. Moreover, ex vivo HER2G778_P780 dup cells are relatively resistant to fulvestrant alone (IC50 < 0.2μM), neratinib alone (IC50 < 0.006μM), abemaciclib alone (IC50 < 0.04μM), and neratinib in combination with abemaciclib (IC50 < 0.005μM), suggesting that patients harboring ER+/HER2-mutant tumors may benefit from neratinib in combination with abemaciclib. Therefore, we propose that simultaneous targeting of both HER2 and the CDK4/6 axis is required for maximal inhibition of ER+ breast cancers harboring HER2 activating mutations. Citation Format: Vaishnavi Devarakonda, LaTerrica Williams, Sinem Seker, Jonathan T. Lei, Purba Singh, Airi Han, Meenakshi Anurag, Kimberly R. Holloway, Alana L. Welm, Matthew J. Ellis, Shyam M. Kavuri. Evaluating preclinical efficacy of anti-HER2 drug combinations using ER+/HER2 mutant models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 850.

Pancreatic ductal adenocarcinoma was responsible for ~43,000 deaths in the USA in 2017 and is the epitome of a recalcitrant cancer driven by an, as yet, pharmacologically intractable oncoprotein, KRAS. Although the clinical picture remains grim, the mechanisms by which key alterations in tumor suppressors and proto-oncogenes contribute to PDA have been dissected. Downstream of KRAS, the RAF→MEK→ERK signaling pathway plays a central role in pancreatic carcinogenesis. However, to date, pharmacological inhibition of this pathway has provided no clinical benefit to PDA patients. Here we show that inhibition of KRAS→RAF→MEK→ERK signaling in PDA cell lines elicits autophagy, a process of cellular recycling that protects pancreatic cancer cells from the potentially cytotoxic effects of KRAS pathway inhibition. Furthermore, combined inhibition of MEK1/2 plus autophagy displays synergistic anti-proliferative effects against PDA cell lines in vitro, and promotes regression of xenografted patient-derived PDA tumors in mice. Finally, treatment of a PDA patient with the combination of trametinib plus hydroxychloroquine resulted in a partial, but nonetheless striking disease response. These data suggest that this combination therapy may represent a novel strategy to target RAS-driven cancers such as PDA. Citation Format: Conan G. Kinsey, Soledad A. Camolotto, Amelie M. Boespflug, Katrin P. Guillen, Mona Foth, Amanda Truong, Sophia S. Schuman, Jill E. Shea, Michael T. Seipp, Jeffrey T. Yap, Lance D. Burrell, David H. Lum, Jonathan R. Whisenant, G. Weldon Gilcrease, Courtney C. Cavalieri, Kaitrin M. Rehbein, Stephanie L. Cutler, Kajsa E. Affolter, Alana L. Welm, Bryan E. Welm, Courtney L. Scaife, Eric L. Snyder, Martin McMahon. Protective autophagy elicited by RAF®MEK®ERK inhibition suggests a treatment strategy for pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2183.

In the version of this article initially published, the label over the bottom schematic in Fig. 1a was “pH > 5.0”; it should have been “pH < 5.0”. Further, the original article misspelt the surname of Katrin P. Guillen as “Gullien”. These errors have been corrected in the print, PDF and HTML versions of the article.

Triple-negative breast cancer (TNBC) is associated with poor survival. Chemotherapy is the only standard treatment for TNBC. The prevalence of BRCA1 inactivation in TNBC has rationalized clinical trials of poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors. Similarly, the overexpression of epidermal growth factor receptor (EGFR) rationalized anti-EGFR therapies in this disease. However, clinical trials using these 2 strategies have not reached their promise. In this study, we used EGFR as a target for radioimmunotherapy and hypothesized that EGFR-directed radioimmunotherapy can deliver a continuous lethal radiation dose to residual tumors that are radiosensitized by PARP inhibitors and chemotherapy. Methods:We analyzed EGFR messenger RNA in published gene expression array studies and investigated EGFR protein expression by immunohistochemistry in a cohort of breast cancer patients to confirm EGFR as a target in TNBC. Preclinically, using orthotopic and metastatic xenograft models of EGFR-positive TNBC,we investigated the effect of the novel combination of 177 Lu-labeled anti-EGFR monoclonal antibody, chemotherapy, and PARP inhibitors on cell death and the survival of breast cancer stem cells. Results: In this first preclinical study of anti-EGFR radioimmunotherapy in breast cancer, we found that anti-EGFR radioimmunotherapy is safe and that TNBC orthotopic tumors and established metastases were eradicated in mice treated with anti-EGFR radioimmunotherapy combined with chemotherapy and PARP inhibitors. We showed that the superior response to this triple-agent combination therapy was associated with apoptosis and eradication of putative breast cancer stem cells. Conclusion: Our data support further preclinical investigations toward the development of combination therapies using systemic anti-EGFR radioimmunotherapy for the treatment of recurrent and metastatic TNBC. Refereed/Peer-reviewed

Background microRNAs are promising candidate breast cancer biomarkers due to their cancer-specific expression profiles. However, efforts to develop circulating breast cancer biomarkers are challenged by the heterogeneity of microRNAs in the blood. To overcome this challenge, we aimed to develop a molecular profile of microRNAs specifically secreted from breast cancer cells. Our first step towards this direction relates to capturing and analyzing the contents of exosomes, which are small secretory vesicles that selectively encapsulate microRNAs indicative of their cell of origin. To our knowledge, circulating exosome microRNAs have not been well-evaluated as biomarkers for breast cancer diagnosis or monitoring. Methods Exosomes were collected from the conditioned media of human breast cancer cell lines, mouse plasma of patient-derived orthotopic xenograft models (PDX), and human plasma samples. Exosomes were verified by electron microscopy, nanoparticle tracking analysis, and western blot. Cellular and exosome microRNAs from breast cancer cell lines were profiled by next-generation small RNA sequencing. Plasma exosome microRNA expression was analyzed by qRT-PCR analysis. Results Small RNA sequencing and qRT-PCR analysis showed that several microRNAs are selectively encapsulated or highly enriched in breast cancer exosomes. Importantly, the selectively enriched exosome microRNA, human miR-1246, was detected at significantly higher levels in exosomes isolated from PDX mouse plasma, indicating that tumor exosome microRNAs are released into the circulation and can serve as plasma biomarkers for breast cancer. This observation was extended to human plasma samples where miR-1246 and miR-21 were detected at significantly higher levels in the plasma exosomes of 16 patients with breast cancer as compared to the plasma exosomes of healthy control subjects. Receiver operating characteristic curve analysis indicated that the combination of plasma exosome miR-1246 and miR-21 is a better indicator of breast cancer than their individual levels. Conclusions Our results demonstrate that certain microRNA species, such as miR-21 and miR-1246, are selectively enriched in human breast cancer exosomes and significantly elevated in the plasma of patients with breast cancer. These findings indicate a potential new strategy to selectively analyze plasma breast cancer microRNAs indicative of the presence of breast cancer. Electronic supplementary material The online version of this article (doi:10.1186/s13058-016-0753-x) contains supplementary material, which is available to authorized users.

Ewing sarcoma is a tumor of the bone and soft tissue caused by the expression of a translocation-derived oncogenic transcription factor, EWS/FLI. Overt metastases are associated with a poor prognosis in Ewing sarcoma, but patients without overt metastases frequently harbor micrometastatic disease at presentation. This suggests that the metastatic potential of Ewing sarcoma exists at an early stage during tumor development. We have therefore explored whether the inciting oncogenic event in Ewing sarcoma, EWS/FLI, directly modulates tumor cell features that support metastasis, such as cell adhesion, cell migration, and cytoarchitecture. We used an RNAi-based approach in patient-derived Ewing sarcoma cell lines. Although we hypothesized that EWS/FLI might induce classic metastatic features, such as increased cell adhesion, migration, and invasion (similar to the phenotypes observed when epithelial malignancies undergo an epithelial-to-mesenchymal transition during the process of metastasis), surprisingly, we found the opposite. Thus, EWS/FLI expression inhibited the adhesion of isolated cells in culture and prevented adhesion in an in vivo mouse lung assay. Cell migration was similarly inhibited by EWS/FLI expression. Furthermore, EWS/FLI expression caused a striking loss of organized actin stress fibers and focal adhesions and a concomitant loss of cell spreading, suggesting that EWS/FLI disrupts the mesenchymal phenotype of a putative tumor cell-of-origin. These data suggest a new paradigm for the dissemination and metastasis of mesenchymally derived tumors: these tumors may disseminate via a "passive/stochastic" model rather than via an "active" epithelial-to-mesenchymal type transition. In the case of Ewing sarcoma, it appears that the loss of cell adhesion needed to promote tumor cell dissemination might be induced by the EWS/FLI oncogene itself rather than via an accumulation of stepwise mutations.

Metastasis is the cause of death for nearly all types of cancer, including breast cancer. An exciting new area of research in metastatic breast cancer centers on immune therapy. Although new immune checkpoint blockade therapies have provided benefit for a fraction of patients tested so far, the majority of patients still do not respond to these drugs. A better understanding of how the immune system can be harnessed against metastatic breast cancer is required in order to improve patient outcomes in this area. We previously discovered that macrophage Ron receptor tyrosine kinase promotes breast cancer metastasis by inhibiting CD8+ cytotoxic T lymphocyte activity. We hypothesized that dual blockade of Ron activity and existing immune checkpoint molecules would unleash a more effective CD8+ T cell response to control or eliminate metastatic breast cancer. Our strategy would simultaneously disable tumor-mediated immune evasion mechanisms on both the innate and adaptive immune systems. To test the potential of combination therapy, the Ron inhibitor BMS-777607 and/or the immune checkpoint blocking agents anti-PD1 or anti-CTLA4, were administered in our mouse mammary tumor model to examine the effects on breast tumor progression. To examine Ron-specific effects of BMS-777607, we also examined the effect of genetic deletion of host Ron signaling activity in combination with immunotherapy. The anti-tumor immune response was comprehensively examined by multi-color flow cytometry and immunohistochemical staining for tumor-infiltrating lymphocytes (TILs). Our data suggest that the combination of Ron inhibition with immune checkpoint blockade may be an effective therapy in breast cancer. Citation Format: Shu Chin Alicia Lai, Atakan Ekiz, Alana Welm. Ron kinase inhibition to improve immunotherapy for breast cancer metastasis [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A30.

Bone destruction occurs in aging and numerous diseases, including osteoporosis and cancer. Many cancer patients have bone osteolysis that is refractory to state-of-the-art treatments, which block osteoclast activity with bisphosphonates or by inhibiting the receptor activator of nuclear factor κB ligand (RANKL) pathway. We previously showed that macrophage-stimulating protein (MSP) signaling, which is elevated in about 40% of breast cancers, promotes osteolytic bone metastasis by activation of the MSP signaling pathway in tumor cells or in the bone microenvironment. We show that MSP signals through its receptor, RON tyrosine kinase, expressed on host cells, to activate osteoclasts directly by a previously undescribed pathway that is complementary to RANKL signaling and converges on proto-oncogene, non-receptor tyrosine kinase SRC (SRC). Genetic or pharmacologic inhibition of RON kinase blocked cancer-mediated bone destruction and osteoporosis in several mouse models. Furthermore, the RON kinase inhibitor BMS-777607/ASLAN002 altered markers of bone turnover in a first-in-human clinical cancer study, indicating the inhibitor's potential for normalizing bone loss in patients. These findings uncover a new therapeutic target for pathogenic bone loss and provide a rationale for treatment of bone destruction in various diseases with RON inhibitors.

Bone destruction occurs in aging and numerous diseases, including osteoporosis and cancer. Many cancer patients have bone osteolysis that is refractory to state-of-the-art treatments, which block osteoclast activity with bisphosphonates or by inhibiting the RANKL pathway. We previously showed that macrophage-stimulating protein (MSP) signaling, which is elevated in approximately 40% of breast cancers, promotes osteolytic bone metastasis, but it was unknown whether this was due to activation of the MSP signaling pathway in the tumor cells or in the bone microenvironment. Here we show that MSP signals through host expression of its receptor, RON tyrosine kinase, to directly activate osteoclasts by a previously undescribed pathway that is complementary to RANKL signaling and converges on SRC. Genetic or pharmacologic inhibition of RON kinase blocked cancer-mediated bone destruction and osteoporosis in several mouse models. Furthermore, the RON kinase inhibitor BMS-777607/ASLAN002 altered markers of bone turnover in a first-in-human clinical cancer study, indicating potential for normalizing bone loss in patients. These findings uncover a new therapeutic target for pathogenic bone loss and provide a rationale for treatment of bone destruction in various diseases with RON inhibitors. Citation Format: Kelsi Andrade, Jaime Fornetti, Ling Zhao, Scott C. Miller, Lor R. Randall, Neysi Anderson, Susan E. Waltz, Mark McHale, Alana L. Welm. RON kinase: a target for treatment of cancer-induced bone destruction and osteoporosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-209. doi:10.1158/1538-7445.AM2017-LB-209

Cell lines grown in 2-dimensional (2D) plastic surfaces have historically been the main models to identify cancer cell vulnerabilities. In contrast, malignant tumors in vivo grow as 3-dimensional (3D) cellular masses and manifest remarkably different drug-response patterns compared to cell lines. We have shown that breast cancer (BrCa) cells in 3D extracellular matrices acquire distinct phenotypic properties, compared to the 2D counterparts. Notably, serum-free culture medium supplemented with defined cocktails of developmental morphogens was sufficient to support the growth of BrCa cell lines in 3D, but not in 2D conditions; indicating a key role of 3D architecture in in vivo growth regulation. CRISPR/Cas9-based gene editing screens in BrCa cells indicated that distinct gene subset govern the growth in 3D vs. 2D conditions. Using similar culture conditions, we expanded several molecularly annotated BrCa patient-derived 3D Organoids (3D PDOs), adapted them into a high-throughput miniaturized drug screening platform, and interrogated them against a panel of ~500 target-annotated kinase inhibitors and FDA-approved anti-cancer agents. This pharmacological screen revealed high sensitivity to distinct classes of agents (eg. PI3K inhibitors), but also variable responses to others (eg. EGFR inhibitors); and provided insight into potentially driver vs. passenger mutations in the respective patients. Co-culture of PDOs with non-malignant stromal cells from bone metastatic niche reduced sensitivity to some classes of drugs (eg. PLK inhibitors). Importantly, long-term (up to 3-4 weeks) treatment of 3D organoids did not eradicate the cells in culture (in contrast to conventional 2D assays), but generated a cell subpopulation which remained viable for the duration of the experiment, reminiscent of the residual disease after initial clinical partial responses to a drug. This residual disease phenotype was also confirmed in vivo, when the respective PD-Xenografts were treated with the same agents. The majority of transcripts with differential expression in residual PDX tumor cells were also concordantly up- or down-regulated in the respective 3D PDO model. These transcripts included canonical regulators of cell cycle, chromatin signaling and epigenetic mechanisms, cytoskeletal function and metabolic pathways; as well as other transcripts not previously linked to drug resistance in conventional 2D models. To our knowledge this is the first description of an in vitro personalized cancer model that simulates both molecularly and phenotypically the post-treatment residual disease in vivo. Our high-throughput functional platform can potentially complement genomic approaches in personalized medicine and help discover novel drugs that overcome tumor drug resistance. Citation Format: Eugen Dhimolea, Ricardo De Matos Simoes, Yoko DeRose, Pallavi Awate, Xiang Weng, Huihui Tang, Aedin Culhane, Alana Welm, Constantine Mitsiades. High-throughput patient-derived 3-dimensional organoid cultures as personalized models to assess drug response and post-treatment residual disease [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5016. doi:10.1158/1538-7445.AM2017-5016

The MSP/RON signaling pathway favors the conversion of micrometastatic lesions to overt metastases by suppressing antitumor immune responses. The loss of RON functions in the host potentiates tumor-specific CD8+ T-cell responses, hence inhibiting the outgrowth of metastatic cancer cells. Thus, RON inhibitors may potentially prevent the outgrowth of micrometastases in cancer patients.

Research models that replicate the diverse genetic and molecular landscape of breast cancer are critical for developing the next-generation therapeutic entities that can target specific cancer subtypes. Patient-derived tumorgrafts, generated by transplanting primary human tumor samples into immune-compromised mice, are a valuable method to model the clinical diversity of breast cancer in mice, and are a potential resource in personalized medicine. Primary tumorgrafts also enable in vivo testing of therapeutics and make possible the use of patient cancer tissue for in vitro screens. Described in this unit are a variety of protocols including tissue collection, biospecimen tracking, tissue processing, transplantation, and three-dimensional culturing of xenografted tissue, which enable use of bona fide uncultured human tissue in designing and validating cancer therapies.

Laboratory models that accurately replicate human tumor initiation and characteristics are integral to advancing knowledge in cancer research. However, comparative studies between commonly employed laboratory models and human tumors have demonstrated that some models have molecular and genomic alterations dissimilar to the cancer type they attempt to replicate. In contrast, several recent comparative studies suggest that because patient-derived tumors grown in mice maintain many of the important characteristics of the original tumor, they represent an important tool for the development of new cancer therapeutics. Detailed in this overview are the advantages and disadvantages of the most commonly used cancer models for mechanistic and therapeutic research, with an emphasis on the advances made in the production and use of patient-derived tumorgrafts.

Aim Breast cancer is the second leading cause of cancer death in women, yet only a few antigens are available for directing immune therapies to a patient’s breast tumor. In this study we establish a new system for identifying tumor-specific antigens presented by the class I HLA of breast tumors. Our approach relies on generating a breast cancer HLA peptide-ligand reference database coupled with isolation of peptides from the HLA of patient-derived breast cancer xenografts (PDX). This HLA PDX approach can rapidly identify targets for existing and emerging cancer immune therapies. Methods A tumor-specific HLA reference peptide ligand library was constructed using 3 breast epithelial tumor cell lines and 1 healthy breast epithelial line. Cells were transfected to secrete HLA-A∗02:01, secreted HLA was affinity purified, HLA-bound peptides were eluted, separated by HPLC, and HLA-A∗02:01 breast cancer peptides were analyzed by mass spectroscopy. To validate the peptides as relevant targets for human breast tumors in vivo, HLA was extracted from PDX-derived tumors and the presence of specific peptides were confirmed in these primary tumors. Results In 20 PDX tumor lines, 7 of the tumors were A∗02:01, half of which expressed detectable A∗02:01. When evaluated for candidate peptide:HLA expression, the HCI-015 triple negative PDX xenograft expressed high levels of the Macrophage Inhibitory Factor 19–27 (MIF) peptide, a peptide that is expressed solely in breast cancer cell lines and primary tumors but not healthy tissues. We are currently expanding HCI-015 PDX for additional HLA extractions. Conclusions These data suggest that PDX tumors can be used as surrogates to human tumors for the direct identification of tumor-specific antigens. The use of a peptide library generated from several breast cancer cell lines allows peptides to be identified with increased sensitivity. Furthermore, expression of the candidate MIF 19-27 peptide on HCI-015 provides compelling evidence that certain tumor-antigens are conserved among cell lines, tumors, and in vivo models of breast cancer. Such antigens provide a unique therapeutic window that can be exploited for targeted immunotherapies.

Twenty to thirty percent of breast cancer patients eventually develop metastasis despite receiving state-of-the-art therapies. Clearly, identifying better ways to eliminate micro-metastatic tumor cells is paramount to eradicating death from breast cancer. We now know that every tumor is unique and that each tumor is heterogenous. Tumors hijack multiple, redundant pathways to survive and grow, and evolve resistance under selective pressure from therapy. Moreover, micro-metastatic cancer cells may be dormant and resistant to therapy. Thus, traditional approaches to treat cancer and prevent metastatic recurrences don't work for everyone. Stimulating the immune system to kill cancer cells (known as immunotherapy) carries strong appeal: the response can be individualized; the immune system is effective against diverse pathogens (in this case, tumor antigens); and immunity can evolve and retain memory for long-term control of disease. The challenge is that by the time tumors are clinically detectable they are already ‘invisible’ to the immune system due to a number of factors: (1) natural selection for tumor cells that can escape immune control; (2) immune tolerance to self- and tumor-antigens; and (3) the strongly immunosuppressive environment in tumors, which renders effector cells inactive. A better understanding of how tumors achieve immune evasion will allow development of strategies to promote more effective anti-tumor immunity. We have determined that Ron protein kinase activity in the host immune system is a major contributor to metastasis. This finding was unanticipated; the expected result was that blocking Ron signaling in tumor cells would inhibit tumor progression, which was the basis for development of Ron inhibitors by the pharmaceutical industry. Our work now shows that Ron inhibitors reduce metastasis through dual effects on tumor cells and on the immune system: Approximately 40% of human breast tumors upregulate the Ron ligand, which is secreted from the tumor, activates Ron on macrophages, and polarizes them to an immune suppressive state. Inhibiting Ron, either genetically or with drugs, blocks metastasis by activating an anti-tumor immune response mediated by CD8+ cytotoxic T lymphocytes (CTLs). Thus, inhibition of Ron boosts anti-tumor immunity and prevents metastasis. These data suggest that inhibition of Ron in the tumor microenvironment might provide a new, complementary strategy to ongoing immunotherapy. Thus, Ron inhibitors might serve as macrophage ‘checkpoint inhibitors’ to complement checkpoint inhibition in T-cells. Citation Format: Alana L. Welm. Immune control of breast cancer metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr IA15.