Discovery of Novel Macrocyclic MERTK/AXL Dual Inhibitors

MERTK and AXL are members of the TAM (TYRO3, AXL, MERTK) family of receptor tyrosine kinases that are aberrantly expressed and have been implicated as therapeutic targets in a wide variety of human tumors. Dual MERTK and AXL inhibition could provide antitumor action mediated by both direct tumor cell killing and modulation of the innate immune response in some tumors such as nonsmall cell lung cancer. We utilized our knowledge of MERTK inhibitors and a structure-based drug design approach to discover a novel class of macrocyclic dual MERTK/AXL inhibitors. The lead compound 43 had low-nanomolar activity against both MERTK and AXL and good selectivity over TYRO3 and FLT3. Its target engagement and selectivity were also confirmed by NanoBRET and cell-based MERTK and AXL phosphorylation assays. Compound 43 had excellent pharmacokinetic properties (large AUC and long half-life) and mediated antitumor activity against lung cancer cell lines, indicating its potential as a therapeutic agent.

TAM family kinases as therapeutic targets at the interface of cancer and immunity

Novel treatment approaches are needed to overcome innate and acquired mechanisms of resistance to current anticancer therapies in cancer cells and the tumour immune microenvironment. The TAM (TYRO3, AXL and MERTK) family receptor tyrosine kinases (RTKs) are potential therapeutic targets in a wide range of cancers. In cancer cells, TAM RTKs activate signalling pathways that promote cell survival, metastasis and resistance to a variety of chemotherapeutic agents and targeted therapies. TAM RTKs also function in innate immune cells, contributing to various mechanisms that suppress antitumour immunity and promote resistance to immune-checkpoint inhibitors. Therefore, TAM antagonists provide an unprecedented opportunity for both direct and immune-mediated therapeutic activity provided by inhibition of a single target, and are likely to be particularly effective when used in combination with other cancer therapies. To exploit this potential, a variety of agents have been designed to selectively target TAM RTKs, many of which have now entered clinical testing. This Review provides an essential guide to the TAM RTKs for clinicians, including an overview of the rationale for therapeutic targeting of TAM RTKs in cancer cells and the tumour immune microenvironment, a description of the current preclinical and clinical experience with TAM inhibitors, and a perspective on strategies for continued development of TAM-targeted agents for oncology applications.

Inhibiting efferocytosis reverses macrophage-mediated immunosuppression in the leukemia microenvironment

Background

Previous studies show that the spleen and bone marrow can serve as leukemia microenvironments in which macrophages play a significant role in immune evasion and chemoresistance. We hypothesized that the macrophage driven tolerogenic process of efferocytosis is a major contributor to the immunosuppressive leukemia microenvironment and that this was driven by aberrant phosphatidylserine expression from cell turnover and cell membrane dysregulation.

Methods

Since MerTK is the prototypic efferocytosis receptor, we assessed whether the MerTK inhibitor MRX2843, which is currently in clinical trials, would reverse immune evasion and enhance immune-mediated clearance of leukemia cells.

Results

We found that inhibition of MerTK decreased leukemia-associated macrophage expression of M2 markers PD-L1, PD-L2, Tim-3, CD163 and Arginase-1 compared to vehicle-treated controls. Additionally, MerTK inhibition led to M1 macrophage repolarization including elevated CD86 and HLA-DR expression, and increased production of T cell activating cytokines, including IFN-β, IL-18, and IL-1β through activation of NF-κB. Collectively, this macrophage repolarization had downstream effects on T cells within the leukemia microenvironment, including decreased PD-1+Tim-3+ and LAG3+ checkpoint expression, and increased CD69+CD107a+ expression.

Discussion

These results demonstrate that MerTK inhibition using MRX2843 altered the leukemia microenvironment from tumor-permissive toward immune responsiveness to leukemia and culminated in improved immune-mediated clearance of AML.

TNK2/ACK1-mediated phosphorylation of ATP5F1A (ATP synthase F1 subunit alpha) selectively augments survival of prostate cancer while engendering mitochondrial vulnerability

The challenge of rapid macromolecular synthesis enforces the energy-hungry cancer cell mitochondria to switch their metabolic phenotypes, accomplished by activation of oncogenic tyrosine kinases. Precisely how kinase activity is directly exploited by cancer cell mitochondria to meet high-energy demand, remains to be deciphered. Here we show that a non-receptor tyrosine kinase, TNK2/ACK1 (tyrosine kinase non receptor 2), phosphorylated ATP5F1A (ATP synthase F1 subunit alpha) at Tyr243 and Tyr246 (Tyr200 and 203 in the mature protein, respectively) that not only increased the stability of complex V, but also increased mitochondrial energy output in cancer cells. Further, phospho-ATP5F1A (p-Y-ATP5F1A) prevented its binding to its physiological inhibitor, ATP5IF1 (ATP synthase inhibitory factor subunit 1), causing sustained mitochondrial activity to promote cancer cell growth. TNK2 inhibitor, (R)-9b reversed this process and induced mitophagy-based autophagy to mitigate prostate tumor growth while sparing normal prostate cells. Further, depletion of p-Y-ATP5F1A was needed for (R)-9b-mediated mitophagic response and tumor growth. Moreover, Tnk2 transgenic mice displayed increased p-Y-ATP5F1A and loss of mitophagy and exhibited formation of prostatic intraepithelial neoplasia (PINs). Consistent with these data, a marked increase in p-Y-ATP5F1A was seen as prostate cancer progressed to the malignant stage. Overall, this study uncovered the molecular intricacy of tyrosine kinase-mediated mitochondrial energy regulation as a distinct cancer cell mitochondrial vulnerability and provided evidence that TNK2 inhibitors can act as "mitocans" to induce cancer-specific mitophagy.Abbreviations: ATP5F1A: ATP synthase F1 subunit alpha; ATP5IF1: ATP synthase inhibitory factor subunit 1; CRPC: castration-resistant prostate cancer; DNM1L: dynamin 1 like; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; Mdivi-1: mitochondrial division inhibitor 1; Mut-ATP5F1A: Y243,246A mutant of ATP5F1A; OXPHOS: oxidative phosphorylation; PC: prostate cancer; PINK1: PTEN induced kinase 1; p-Y-ATP5F1A: phosphorylated tyrosine 243 and 246 on ATP5F1A; TNK2/ACK1: tyrosine kinase non receptor 2; Ub: ubiquitin; WT: wild type.

Discovery and characterization of a functional scFv for CCR2 inhibition via an extracellular loop

The chemokine receptor CCR2 plays a key role in cellular migration and inflammatory processes. While tremendous progress has been made in elucidating CCR2 function and inhibition, the majority of approaches target its N-terminal domain and less is known about the function of the remaining extracellular loops and their potential as targets. Here, we used phage display to identify an antibody-derived scFv (single chain variable fragment) clone that specifically targets the second extracellular epitope of CCR2 (ECL2) for inhibition. Using in silico molecular docking, we identified six potential primary binding conformations of the novel scFv to the specified CCR2 epitope. In silico molecular dynamic analysis was used to determine conformational stability and identify protein-protein interactions. Umbrella sampling of a range of configurations with incrementally increasing separation of scFv and target generated by force pulling simulations was used to calculate binding energies. Downstream characterization by ELISA showed high binding affinity of the ECL2-scFv to CCR2. Furthermore, we showed that blocking the second extracellular loop inhibits macrophage migration and polarized macrophages towards M1 inflammatory cytokine production as potently as lipopolysaccharide (LPS). These studies highlight the applicability of epitope-specific targeting, emphasize the importance of in silico predictive modeling, and warrant further investigation into the role of the remaining epitopes of CCR2.

Abstract A35: MERTK inhibition induces an anti-leukemia dendritic cell – T cell axis while TYRO3 inhibition protects by a separate mechanism

The TAM family receptor tyrosine kinases TYRO3, AXL and MERTK are potential therapeutic targets in a variety of cancers. In our previous studies, MERTK inhibition in the leukemia microenvironment significantly prolonged survival in a syngeneic B-cell acute leukemia (B-ALL) model, implicating MERTK as a promising immuno-oncology target. Strikingly, Mertk-/- mice were almost completely protected against leukemia. Here, we probed the mechanisms of anti-leukemia immunity mediated by MERTK inhibition and evaluated roles for TYRO3 and AXL in the leukemia microenvironment. Single cell RNA sequencing and flow cytometry studies revealed an increase in CD8α+ dendritic cells (DCs) with enhanced antigen-presentation capacity in naïve and leukemia-bearing Mertk-/- mice. These cells were also increased in leukemic wild type (WT) mice treated with the MERTK inhibitor MRX-2843 (currently in phase I/Ib clinical trials). Additionally, CD8+ T cells expressing high levels of the T cell exhaustion marker Tox were decreased in mice treated with MRX-2843, implicating a CD8α+ DC – T cell axis in the anti-leukemia immune response stimulated by MERTK inhibition. Indeed, combined depletion of CD8+ T cells and CD8α+ DCs completely abrogated the anti-leukemia response in Mertk-/- mice, while immunity remained partially intact in mice with selective depletion of CD8+ T cells. Similarly, protection from leukemia was abrogated in Mertk-/- scid mice, which lack functional B and T cells and have defects in DC function. These data demonstrate a critical immunosuppressive role for MERTK in DCs of the leukemia microenvironment. Similar to Mertk-/- mice, B-ALL growth was almost completely prevented in Tyro3-/- mice, while Axl-/- did not impact leukemogenesis, implicating TYRO3, but not AXL, as an additional immunotherapeutic target. However, in contrast to Mertk-/- mice, CD8α+ DCs with enhanced antigen-presentation capacity were not significantly increased in Tyro3-/- mice compared to WT, indicating differences in the underlying mechanisms by which MERTK and TYRO3 contribute to the immunosurveillance of leukemia cells. Indeed, in vivo depletion experiments confirmed differential roles for MERTK and TYRO3 in the leukemia microenvironment. In contrast to Mertk-/- mice, selective depletion of CD8+ T cells completely abrogated protection from leukemia in Tyro3-/- mice, indicating a mechanism less dependent on DCs. Together, these findings reveal novel and distinct mechanistic insights into the immunosuppressive roles for MERTK and TYRO3 in the leukemia microenvironment, demonstrate a critical role for MERTK in DC activity, and validate a similar mechanism mediated by MRX-2843. Thus, these studies provide strong rationale for development of MERTK and/or TYRO3-targeted immunotherapies for treatment of acute leukemia.

Citation Format: Justus M Huelse, Swati S Bhasin, Kristen M Jacobsen, Beena E Thomas, Madison L Chimenti, Travon A Baxter, Xiaodong Wang, Stephen V Frye, Curtis J Henry, H. Shelton Earp, Manoj Bhasin, Deborah DeRyckere, Douglas K Graham. MERTK inhibition induces an anti-leukemia dendritic cell – T cell axis while TYRO3 inhibition protects by a separate mechanism [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr A35.

A cryptic transactivation domain of EZH2 binds AR and AR's splice variant, promoting oncogene activation and tumorous transformation

Enhancer of Zeste Homolog 2 (EZH2) and androgen receptor (AR) are crucial chromatin/gene regulators involved in the development and/or progression of prostate cancer, including advanced castration-resistant prostate cancer (CRPC). To sustain prostate tumorigenicity, EZH2 establishes non-canonical biochemical interaction with AR for mediating oncogene activation, in addition to its canonical role as a transcriptional repressor and enzymatic subunit of Polycomb Repressive Complex 2 (PRC2). However, the molecular basis underlying non-canonical activities of EZH2 in prostate cancer remains elusive, and a therapeutic strategy for targeting EZH2:AR-mediated oncogene activation is also lacking. Here, we report that a cryptic transactivation domain of EZH2 (EZH2TAD) binds both AR and AR spliced variant 7 (AR-V7), a constitutively active AR variant enriched in CRPC, mediating assembly and/or recruitment of transactivation-related machineries at genomic sites that lack PRC2 binding. Such non-canonical targets of EZH2:AR/AR-V7:(co-)activators are enriched for the clinically relevant oncogenes. We also show that EZH2TAD is required for the chromatin recruitment of EZH2 to oncogenes, for EZH2-mediated oncogene activation and for CRPC growth in vitro and in vivo. To completely block EZH2's multifaceted oncogenic activities in prostate cancer, we employed MS177, a recently developed proteolysis-targeting chimera (PROTAC) of EZH2. Strikingly, MS177 achieved on-target depletion of both EZH2's canonical (EZH2:PRC2) and non-canonical (EZH2TAD:AR/AR-V7:co-activators) complexes in prostate cancer cells, eliciting far more potent antitumor effects than the catalytic inhibitors of EZH2. Overall, this study reports a previously unappreciated requirement for EZH2TAD for mediating EZH2's non-canonical (co-)activator recruitment and gene activation functions in prostate cancer and suggests EZH2-targeting PROTACs as a potentially attractive therapeutic for the treatment of aggressive prostate cancer that rely on the circuits wired by EZH2 and AR.

MERTK activation drives osimertinib resistance in EGFR-mutant non-small cell lung cancer

Acquired resistance is inevitable in non-small cell lung cancers (NSCLCs) treated with osimertinib (OSI), and the mechanisms are not well defined. The MERTK ligand GAS6 promoted downstream oncogenic signaling in EGFR-mutated (EGFRMT) NSCLC cells treated with OSI, suggesting a role for MERTK activation in OSI resistance. Indeed, treatment with MRX-2843, a first-in-class MERTK kinase inhibitor, resensitized GAS6-treated NSCLC cells to OSI. Both GAS6 and EGF stimulated downstream PI3K/AKT and MAPK/ERK signaling in parental cells, but only GAS6 activated these pathways in OSI-resistant (OSIR) derivative cell lines. Functionally, OSIR cells were more sensitive to MRX-2843 than parental cells, suggesting acquired dependence on MERTK signaling. Furthermore, MERTK and/or its ligands were dramatically upregulated in EGFRMT tumors after treatment with OSI in both xenograft models and patient samples, consistent with induction of autocrine/paracrine MERTK activation. Moreover, treatment with MRX-2843 in combination with OSI, but not OSI alone, provided durable suppression of tumor growth in vivo, even after treatment was stopped. These data identify MERTK as a driver of bypass signaling in treatment-naive and EGFRMT-OSIR NSCLC cells and predict that MRX-2843 and OSI combination therapy will provide clinical benefit in patients with EGFRMT NSCLC.

Abstract 240: MERTK inhibition induces an anti-leukemia dendritic cell - T cell axis while TYRO3 inhibition protects through a separate mechanism

The TAM family receptor tyrosine kinases TYRO3, AXL and MERTK are potential therapeutic targets in a variety of cancers. In previous studies, inhibition of MERTK decreased PD-1 checkpoint proteins in the leukemia microenvironment and prolonged survival in a syngeneic BCR-ABL+/Arf-/- B-cell acute leukemia model, implicating MERTK as a promising immune-oncology target in leukemia. Strikingly, Mertk-/- mice were largely protected from leukemia. In our current studies, Tyro3-/- almost completely prevented development of leukemia, comparable to Mertk-/-, while Axl-/- mice died with similar timing to wild type (WT) mice (20-40 days). These data demonstrate differential roles for TAM kinases in the anti-leukemia immune response. Depletion studies were conducted to evaluate potential roles for T cells and dendritic cells (DCs) in anti-leukemia immunity in Mertk-/- mice. Selective depletion of CD8+ T cells abrogated protection from leukemia in Mertk-/- mice, but survival was still prolonged relative to WT. Thus, while CD8+ T cells were required for complete protection from leukemia, the anti-leukemia response remained partially intact even in the absence of CD8+ T cells, implicating an innate immune mechanism. Indeed, combined depletion of CD8+ T cell and CD8α+ DC subsets completely abrogated the anti-leukemic effects in Mertk-/- mice, revealing a critical immunosuppressive role for MERTK in DCs in the leukemia microenvironment. In contrast to Mertk-/- mice, selective depletion of CD8+ T cells completely abrogated protection from leukemia in Tyro3-/- mice, indicating a mechanism less dependent on DCs. Similarly, single cell RNA sequencing revealed CD8+ DCs with a more mature and antigen-presenting phenotype in Mertk-/- mice compared to WT, while antigen-presenting DCs were not increased in Tyro3-/- mice. Single cell sequencing data also suggest induction of an anti-leukemic DC - T cell axis in WT leukemic mice treated with the MERTK-selective inhibitor MRX-2843. DCs were nearly absent in leukemic bone marrow from saline-treated mice and were dramatically increased in response to treatment with MRX-2843. Treatment with MRX-2843 also decreased the incidence of CD8+ T cells expressing high levels of Tox, which has been associated with T cell exhaustion. These changes coincided with decreased leukemic blasts, even in the context of established disease.Together, our findings support a model whereby MERTK inhibition promotes DC function and CD8+ T cell activity, leading to anti-leukemia immunity. In contrast, anti-leukemia immunity in response to TYRO3 inhibition is less dependent on DCs. Differential roles for the TAM kinases in the leukemia microenvironment provide rationale for development of MERTK and/or TYRO3 targeted immunotherapies to treat acute leukemia.

Citation Format: Justus M. Huelse, Swati S. Bhasin, Beena E. Thomas, Madison L. Chimenti, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Manoj Bhasin, Deborah DeRyckere, Douglas K. Graham. MERTK inhibition induces an anti-leukemia dendritic cell - T cell axis while TYRO3 inhibition protects through a separate mechanism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 240.

Abstract 3990: MERTK is a potential therapeutic target in Ewing sarcoma

Ewing sarcoma (EWS) is the second most common pediatric bone tumor, and outcomes remain poor in patients with advanced or relapsed disease. In addition, current treatments rely on multi-modal therapy that has significant short- and long-term side effects. New, less toxic and more effective treatments are urgently needed. One potential therapeutic target is the receptor tyrosine kinase MERTK. MERTK is overexpressed in numerous cancers where it promotes tumor cell survival, metastasis, and resistance to cytotoxic and targeted therapies. We demonstrated expression and phosphorylation of MERTK protein in 5 of 5 EWS family cell lines tested. Stimulation with the MERTK ligand GAS6 resulted in activation of downstream oncogenic signaling pathways, including JAK/STAT and MAPK/ERK. Moreover, publicly available data from CRISPR-based library screens suggest that EWS cell lines are particularly dependent on MERTK. Thus, therapeutic strategies targeting MERTK may be particularly effective for treatment of EWS. To explore the therapeutic impact of MERTK inhibition in various cancers, we developed MRX-2843, a first-in-class MERTK-selective tyrosine kinase inhibitor that is currently in clinical trials. Treatment with MRX-2843 decreased phosphorylation of MERTK in a dose dependent manner, with an IC50 of 123 nM (95% CI; 53-180 nM) in A673 cells, and reduced downstream STAT6 signaling. In addition, MRX-2843 had potent anti-tumor activity against all 5 EWS cell lines, leading to reduced expansion and decreased cell density in culture with IC50 values ranging from 178 - 297 nM. Of note, inhibition of MERTK phosphorylation correlated with anti-tumor activity in both the A673 and TC106 cell lines, implicating MERTK inhibition as a mechanism of MRX-2843 anti-tumor activity. Together these data validate MERTK as a promising therapeutic target in EWS and support development of MRX-2843 for treatment of EWS, with potential to directly inform and enable a clinical trial in pediatric patients and, ultimately, to improve both outcomes and quality of life for patients with this disease.

Citation Format: Sherri K. Smart, Tsz Y. Yeung, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Douglas K. Graham, Deborah DeRyckere. MERTK is a potential therapeutic target in Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3990.

Abstract 3339: MRX-2843, a dual MERTK and FLT3 inhibitor, mediates synergistic anti-leukemia activity in combination with BCL-2 inhibitors in acute myeloid leukemia and early T-cell precursor acute lymphoblastic leukemia

While overall outcomes have improved for patients with acute leukemia, high-risk subsets including acute myeloid leukemia (AML) and relapsed/refractory early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) continue to have poor prognosis. New therapies are urgently needed. Both MERTK tyrosine kinase and the anti-apoptotic protein BCL-2 have been implicated as therapeutic targets in AML and ETP-ALL. We developed MRX-2843, a novel small molecule MERTK and FLT3 inhibitor currently in clinical trials in patients with leukemia. The BCL-2 inhibitor venetoclax is FDA-approved for treatment of AML and has clinical activity in relapsed/refractory T-ALL. Here, we investigated the impact of treatment with MRX-2843 in combination with BCL-2 inhibitors in preclinical models. Human AML and ETP-ALL cell lines were treated with MRX-2843 and/or a BCL-2 inhibitor for 48-72 hours and relative cell numbers were determined using CellTiter-Glo reagent. Synergy was assessed by mathematical modeling using the response additivity and fractional product methods. Combined treatment with MRX-2843 and venetoclax provided enhanced therapeutic efficacy compared to MRX-2843 or venetoclax alone. The interaction between drugs was dose-dependent and synergistic in AML cell lines. For instance, in KG-1 cells combined treatment with an IC50 concentration of MRX-2843 and an IC15 concentration of venetoclax reduced cell density by 88 ± 4.0% and the combination was significantly more effective than MRX-2843 or venetoclax alone (p < 0.0001, 2-way ANOVA). Moreover, the 88% reduction in cell density in cultures treated with the combination was significantly greater than the 58 ± 1.6% reduction expected for an additive interaction (p < 0.0001). Robust therapeutic activity and dramatic synergy were also observed in NOMO-1 and OCI-AML5 cell cultures treated with the combination and the interaction between drugs was additive or synergistic in Loucy ETP-ALL cells. Enhanced therapeutic efficacy and synergistic interactions were also observed in AML cell cultures treated with MRX-2843 and navitoclax, a BCL-2 and BCL-XL inhibitor, implicating BCL-2 inhibition as a mechanism of synergy. In a high-throughput screen, MRX-2843 mediated synergistic anti-leukemia activity in combination with venetoclax in all 7 AML and both ETP-ALL cell lines tested. Synergy was optimal when MRX-2843 and venetoclax were administered in a 1:20 ratio. Our data (i) implicate combined treatment with MRX-2843 and a BCL-2 inhibitor, such as venetoclax, as a promising new strategy for treatment of both AML and ETP-ALL, (ii) define optimized dosing strategies for MRX-2843 and venetoclax combination therapy, and (iii) support further evaluation of MRX-2843 in combination with venetoclax in murine models and potentially in upcoming clinical trials.

Citation Format: Aashis Thapa, Juhi Jain, Ryan J. Summers, James M. Kelvin, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Erik C. Dreaden, Deborah DeRyckere, Douglas K. Graham. MRX-2843, a dual MERTK and FLT3 inhibitor, mediates synergistic anti-leukemia activity in combination with BCL-2 inhibitors in acute myeloid leukemia and early T-cell precursor acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3339.

Abstract P3-15-01: Patients and Researchers Together (PART); a patient-centered tumor tissue collection PARTnership between patients and researchers to increase tissue donations for breast cancer research

Introduction: Translational research is essential to the success of every cancer center. Increasingly doctors ask patients to donate tissue to study all aspects of the disease to improve cancer treatments and quality of life. Patients are key stakeholders and partners in the success of the translational research process. The University of North Carolina (UNC), Lineberger Comprehensive Cancer Center (LCCC) researchers would like to increase the number of patients that donate tissue to support research and value patients who donate. Securing the right tissues at the right time, and processing and storing them properly is essential for research that leads to breakthrough discoveries. Purpose: LCCC supports the inclusion of patient advocates in all cancer center research. Therefore, the UNC Breast SPORE patient advocates initiated a program called Patients and Researchers Together (PART) to ensure patients work closely with researchers to drive the best research forward. The PART-Tissue program aims to develop a process to increase the donation and usage of patient donated tissues. This program is built on significant patient engagement that leads to a bi-directional patient and researcher PARTnership. We believe patients will be more willing to donate tissue if they are informed about the research process, understand the impact research has to improve patient’s lives, and are valued as PARTners of LCCC. Objectives: The first objective is to promote the excellence of LCCC translational research to all stakeholders, including patients and the community. Second; create a patient-centered tissue donation program that will meet researcher needs across LCCC, and Third; ensure the tissue collection process is primarily focused on recognizing patient’s voluntary contributions while minimizing patient burden. Methods: We first created a multi-stakeholder committee to ensure the participation of key stakeholders in all activities. We developed a process to identify relevant materials to assist in tissue donation at LCCC. The project focused on a Lineberger breast cancer tissue collection protocol (LCCC9819). We discussed patient, researcher and clinician/surgeon needs that reflect the vision of the PART program. We identified the need for several types of informational materials for patients, developed them with multi-stakeholder involvement and evaluated their accuracy, readability, understandability, and health literacy. We assessed the current and future needs of LCCC researchers by conducting interviews and surveys. We also engaged patient advocates as PARTners throughout this project. Results: We initiated the PART-Tissue program in August 2020 and we have engaged multiple stakeholders throughout the project. We evaluated LCCC9819 from the patient, research and clinical perspectives. We identified a series of informational materials needed for patients to introduce them to the value of tissue donation and to introduce them to the LCCC9819 protocol. We developed two introductory materials and a study summary. We conducted interviews and developed a survey to collect information about researcher needs at LCCC. In pilot interviews, we noticed a high need for fresh tissue, which requires real-time coordination between clinicians, clinical research coordinators, surgeons, pathologists and researchers. We also developed a website to connect directly with patients and the community about the importance of tissue donation that can lead to discoveries in cancer risk reduction and treatment. The goal is to develop a process where patients feel encouraged to donate tissue samples and are appreciated as partners with researchers who use the tissue to improve the lives of cancer patients.

Citation Format: Patricia A. Spears, Vernal Branch, Jennifer A. Potter, Missy Van Lokeren, Nasrin H. Babadi, Adriana S. Beltran, Laurie Betts, Coleen Crespo, Barbara Dean, Amy L. Garrett, Laura T. Jensen, Gary L. Johnson, Hayley Morris, Philip Spanheimer, Charles M. Perou, Lisa A. Carey, H. Shelton Earp. Patients and Researchers Together (PART); a patient-centered tumor tissue collection PARTnership between patients and researchers to increase tissue donations for breast cancer research [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-15-01.

Targeting MERTK and AXL in EGFR Mutant Non-Small Cell Lung Cancer

MERTK and AXL are members of the TAM family of receptor tyrosine kinases and are abnormally expressed in 69% and 93% of non-small cell lung cancers (NSCLCs), respectively. Expression of MERTK and/or AXL provides a survival advantage for NSCLC cells and correlates with lymph node metastasis, drug resistance, and disease progression in patients with NSCLC. The TAM receptors on host tumor infiltrating cells also play important roles in the immunosuppressive tumor microenvironment. Thus, MERTK and AXL are attractive biologic targets for NSCLC treatment. Here, we will review physiologic and oncologic roles for MERTK and AXL with an emphasis on the potential to target these kinases in NSCLCs with activating EGFR mutations.

Abstract 1109: A novel strategy to cope with osimertinib resistance in non-small cell lung cancer by treatment with a PIM kinase inhibitor in combination with a MERTK-selective kinase inhibitor

Osimertinib is currently the preferred treatment for EGFR-mutated non-small cell lung cancer (NSCLC) patients due to its superior therapeutic efficacy and prolonged overall survival compared to earlier generation EGFR tyrosine kinase inhibitors, but durable responses to osimertinib treatment are rare due to acquired drug resistance. Thus, there is an urgent need for novel strategies to treat osimertinib-resistant NSCLC. Recently, we found that treatment with MRX-2843, a novel MERTK-selective kinase inhibitor currently in Phase I clinical trials, resulted in dose-dependent inhibition of cell expansion and colony formation in an osimertinib-resistant (osiR) H4006 derivative cell line. An unbiased screen of 378 kinase inhibitors was carried out to identify compounds that synergized with MRX-2843 to inhibit expansion of an osiR derivative of the EGFR-mutated H4011 cell line. Treatment with 1µM PIM kinase inhibitor SGI-1776 or 100nM MRX-2843 alone reduced cell density by 5±3% and 44±7%, respectively, while treatment with MRX-2843 and SGI-1776 combined mediated an 82±0.4% decrease. Synergy was also observed in H4006 osiR and H1650 osiR derivative cell lines. Furthermore, treatment with PIM447, a structurally distinct PIM kinase inhibitor, and MRX-2843 decreased cell expansion more effectively than either agent alone. Mechanistically, treatment with a PIM kinase inhibitor in combination with MRX-2843 decreased downstream PI3K-AKT and MAPK-ERK signaling more effectively than single agents. Additionally, combined treatment with MRX-2843 and SGI-1776 prevented colony formation, while single agents had limited effect. In all, these data indicate that combining MRX-2843 and a PIM TKI may control osimertinib resistant tumor growth, providing a potential treatment strategy for osimertinib resistant EGFR-mutated NSCLC patients for whom the choices are still limited.

Citation Format: Dan Yan, Zikang Tan, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, III, Deborah DeRyckere, Douglas K. Graham. A novel strategy to cope with osimertinib resistance in non-small cell lung cancer by treatment with a PIM kinase inhibitor in combination with a MERTK-selective kinase inhibitor [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 1109.

Cistrome analysis of YY1 uncovers a regulatory axis of YY1:BRD2/4-PFKP during tumorigenesis of advanced prostate cancer

Castration-resistant prostate cancer (CRPC) is a terminal disease and the molecular underpinnings of CRPC development need to be better understood in order to improve its treatment. Here, we report that a transcription factor Yin Yang 1 (YY1) is significantly overexpressed during prostate cancer progression. Functional and cistrome studies of YY1 uncover its roles in promoting prostate oncogenesis in vitro and in vivo, as well as sustaining tumor metabolism including the Warburg effect and mitochondria respiration. Additionally, our integrated genomics and interactome profiling in prostate tumor show that YY1 and bromodomain-containing proteins (BRD2/4) co-occupy a majority of gene-regulatory elements, coactivating downstream targets. Via gene loss-of-function and rescue studies and mutagenesis of YY1-bound cis-elements, we unveil an oncogenic pathway in which YY1 directly binds and activates PFKP, a gene encoding the rate-limiting enzyme for glycolysis, significantly contributing to the YY1-enforced Warburg effect and malignant growth. Altogether, this study supports a master regulator role for YY1 in prostate tumorigenesis and reveals a YY1:BRD2/4-PFKP axis operating in advanced prostate cancer with implications for therapy.

FOXA1 and adaptive response determinants to HER2 targeted therapy in TBCRC 036

Inhibition of the HER2/ERBB2 receptor is a keystone to treating HER2-positive malignancies, particularly breast cancer, but a significant fraction of HER2-positive (HER2+) breast cancers recur or fail to respond. Anti-HER2 monoclonal antibodies, like trastuzumab or pertuzumab, and ATP active site inhibitors like lapatinib, commonly lack durability because of adaptive changes in the tumor leading to resistance. HER2+ cell line responses to inhibition with lapatinib were analyzed by RNAseq and ChIPseq to characterize transcriptional and epigenetic changes. Motif analysis of lapatinib-responsive genomic regions implicated the pioneer transcription factor FOXA1 as a mediator of adaptive responses. Lapatinib in combination with FOXA1 depletion led to dysregulation of enhancers, impaired adaptive upregulation of HER3, and decreased proliferation. HER2-directed therapy using clinically relevant drugs (trastuzumab with or without lapatinib or pertuzumab) in a 7-day clinical trial designed to examine early pharmacodynamic response to antibody-based anti-HER2 therapy showed reduced FOXA1 expression was coincident with decreased HER2 and HER3 levels, decreased proliferation gene signatures, and increased immune gene signatures. This highlights the importance of the immune response to anti-HER2 antibodies and suggests that inhibiting FOXA1-mediated adaptive responses in combination with HER2 targeting is a potential therapeutic strategy.

Combined Associations of a Polygenic Risk Score and Classical Risk Factors With Breast Cancer Risk

We evaluated the joint associations between a new 313-variant PRS (PRS313) and questionnaire-based breast cancer risk factors for women of European ancestry, using 72 284 cases and 80 354 controls from the Breast Cancer Association Consortium. Interactions were evaluated using standard logistic regression and a newly developed case-only method for breast cancer risk overall and by estrogen receptor status. After accounting for multiple testing, we did not find evidence that per-standard deviation PRS313 odds ratio differed across strata defined by individual risk factors. Goodness-of-fit tests did not reject the assumption of a multiplicative model between PRS313 and each risk factor. Variation in projected absolute lifetime risk of breast cancer associated with classical risk factors was greater for women with higher genetic risk (PRS313 and family history) and, on average, 17.5% higher in the highest vs lowest deciles of genetic risk. These findings have implications for risk prevention for women at increased risk of breast cancer.

Inherited predisposition to breast cancer in the Carolina Breast Cancer Study

The Carolina Breast Cancer Study (CBCS) phases I-II was a case-control study of biological and social risk factors for invasive breast cancer that enrolled cases and controls between 1993 and 1999. Case selection was population-based and stratified by ancestry and age at diagnosis. Controls were matched to cases by age, self-identified race, and neighborhood of residence. Sequencing genomic DNA from 1370 cases and 1635 controls yielded odds ratios (with 95% confidence limits) for breast cancer of all subtypes of 26.7 (3.59, 189.1) for BRCA1, 8.8 (3.44, 22.48) for BRCA2, and 9.0 (2.06, 39.60) for PALB2; and for triple-negative breast cancer (TNBC) of 55.0 (7.01, 431.4) for BRCA1, 12.1 (4.18, 35.12) for BRCA2, and 10.8 (1.97, 59.11) for PALB2. Overall, 5.6% of patients carried a pathogenic variant in BRCA1, BRCA2, PALB2, or TP53, the four most highly penetrant breast cancer genes. Analysis of cases by tumor subtype revealed the expected association of TNBC versus other tumor subtypes with BRCA1, and suggested a significant association between TNBC versus other tumor subtypes with BRCA2 or PALB2 among African-American (AA) patients [2.95 (1.18, 7.37)], but not among European-American (EA) patients [0.62 (0.18, 2.09)]. AA patients with pathogenic variants in BRCA2 or PALB2 were 11 times more likely to be diagnosed with TNBC versus another tumor subtype than were EA patients with pathogenic variants in either of these genes (P = 0.001). If this pattern is confirmed in other comparisons of similarly ascertained AA and EA breast cancer patients, it could in part explain the higher prevalence of TNBC among AA breast cancer patients.

Outcomes of Hormone-Receptor Positive, HER2-Negative Breast Cancers by Race and Tumor Biological Features

Background

Black women have higher hormone receptor positive (HR+) breast cancer mortality than White women. Early recurrence rates differ by race, but little is known about genomic predictors of early recurrence among HR+ women.

Methods

Using data from the Carolina Breast Cancer Study (phase III, 2008-2013), we estimated associations between race and recurrence among nonmetastatic HR+/HER2-negative tumors, overall and by PAM50 Risk of Recurrence score, PAM50 intrinsic subtype, and tumor grade using survival curves and Cox models standardized for age and stage. Relative frequency differences (RFD) were estimated using multivariable linear regression. To assess intervention opportunities, we evaluated treatment patterns by race among patients with high-risk disease.

Results

Black women had higher recurrence risk relative to White women (crude hazard ratio = 1.81, 95% confidence interval [CI] = 1.34 to 2.46), which remained elevated after standardizing for clinical covariates (hazard ratio = 1.42, 95% CI = 1.05 to 1.93). Racial disparities were most pronounced among those with high PAM50 Risk of Recurrence score (5-year standardized recurrence risk = 18.9%, 95% CI = 8.6% to 29.1% in Black women vs 12.5%, 95% CI = 2.0% to 23.0% in White women) and high grade (5-year standardized recurrence risk = 16.6%, 95% CI = 11.7% to 21.5% in Black women vs 12.0%, 95% CI = 7.3% to 16.7% in White women). However, Black women with high-grade tumors were statistically significantly less likely to initiate endocrine therapy (RFD = −8.3%, 95% CI = −15.9% to −0.6%) and experienced treatment delay more often than White women (RFD = +9.0%, 95% CI = 0.3% to 17.8%).

Conclusions

Differences in recurrence by race appear greatest among women with aggressive tumors and may be influenced by treatment differences. Efforts to identify causes of variation in cancer treatment are critical to reducing outcome disparities.

Abstract 6388: MERTK and ROCK inhibitors mediate synergistic AML cell death and enhance therapeutic activity in combination with cytotoxic chemotherapy

Survival rates are suboptimal for adult and pediatric patients with acute myeloid leukemia (AML). Cytotoxic chemotherapies have short and long-term side effects and are often contraindicated for elderly patients. Targeted agents can reduce toxicity compared to chemotherapy; however, resistance to single-agents often develops, thus combination therapies may provide more durable responses. To increase efficacy and reduce exposure to cytotoxic agents, a targeted combination therapy (MERTK/ROCKi) was utilized alone and with doxorubicin or etoposide, frontline cytotoxic chemotherapeutics routinely used for AML treatment. MERTK is aberrantly expressed in >80% of AML patient samples and MERTK inhibitors are in clinical development. Both MERTK and rho-associated, coiled-coil-containing protein kinases 1 and 2 (ROCK1/2) regulate actin/microtubule dynamics and cell cycle progression and shMERTK or siROCK1 knockdown induces apoptosis in AML cells. Here, analysis of The Cancer Genome Atlas database revealed poorer overall survival in patients with higher levels of ROCK1 mRNA (p<0.01, n=172). MERTK inhibitors (MRX-2843 or UNC3997) and ROCK1/2 inhibitors (RKI-1447 or GSK269962A) synergized to reduce cell density in 4 of 6 AML cell lines tested and had an additive effect in the remaining cell lines. Flow cytometric analysis of cells stained with popro-1-iodide and propidium iodide (PI) dyes revealed synergistic induction of cell death in KG-1 (p<0.05), OCI-AML5 (p<0.05), and NB4 (p<0.1) cell cultures and an additive effect in Kasumi-1 cultures. Cells treated with the combination therapy failed to expand, even when cultured in the absence of inhibitor(s), and cell density was reduced in response to treatment with the combination therapy compared to single agents (p<0.01). Flow cytometric analysis of DNA content revealed an increased fraction of cells with G2/M DNA content in cultures treated with MRX-2843 (p<0.01). This effect was more pronounced in cultures treated with the combination therapy (p<0.05-p<0.0001) and was accompanied by an accumulation of cells with slightly reduced PI staining relative to untreated G2 cells. Thus, the combination therapy mediates anti-leukemia activity by multiple mechanisms, including abrogation of cell cycle progression and induction of cell death. Addition of MERTK/ROCKi therapy to doxorubicin or etoposide additively decreased cell density relative to chemotherapy alone in OCI-AML5 and KG-1 cultures (p<0.0001). Treatment with doxorubicin and MERTK/ROCKi therapy also decreased OCI-AML5 cell density compared to MERTK/ROCKi therapy alone (p<0.01). These data indicate that combination therapies targeting MERTK and ROCK1/2, alone or in conjunction with chemotherapy, may be particularly effective for treatment of AML and support further studies in animal models.

Citation Format: Dawn E. Barnes, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Barnes. MERTK and ROCK inhibitors mediate synergistic AML cell death and enhance therapeutic activity in combination with cytotoxic chemotherapy [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 6388.

Abstract 1882: MERTK drives residual tumor growth in EGFR-mutated non-small cell lung cancer cells treated with osimertinib

Osimertinib (OSI) was recently FDA-approved as a front-line agent for newly diagnosed EGFRMT non-small cell lung cancer (NSCLC). However, unmet clinical needs have arisen in conjunction with OSI use, including understanding mechanisms of OSI resistance and developing novel approaches to prevent or reverse resistance and/or enhance OSI efficacy in responsive patients. To address these issues, osimertinib-resistant (osiR) derivatives of five EGFRMT NSCLC cell lines were generated and roles for MERTK, a receptor tyrosine kinase that has been implicated as a potential therapeutic target in NSCLC, were characterized. PI3K-AKT and MAPK-ERK signaling pathways were activated in osiR cells, even when EGFR was not active. Treatment with the MERTK ligands GAS6 or PROS1 stimulated AKT, ERK, and ribosomal S6 phosphorylation in parental cells treated with OSI and in osiR cells, implicating MERTK as a mediator of resistance to OSI. Downstream signaling was responsive to both EGF and GAS6 stimulation in parental cells but was only activated by GAS6 in osiR cells. OSI blocked EGF-dependent signaling through AKT, ERK and S6 in parental cells in the absence of GAS6, but combined treatment with OSI and MRX-2843, a novel MERTK inhibitor currently in Phase I clinical trials, was required to block signaling in the presence of GAS6. However, treatment with MRX-2843 alone had little impact on downstream signaling in the presence of activated EGFR. Thus, MERTK is not the dominant driver of downstream signaling in parental cells. In contrast, treatment with MRX-2843 alone was sufficient to inhibit downstream signaling in osiR cells and osiR cells were also more sensitive to treatment with MRX-2843 in clonogenic assays. Thus, osiR cells have increased dependence on MERTK kinase activity relative to parental cells. Interestingly, EGFR and MERTK co-precipitated from parental cell lysates and GAS6 stimulation enhanced this interaction. In contrast, MERTK and EGFR interaction was not detected in osiR cells, suggesting a more complex interplay between these two receptors. MERTK and the ligand PROS1 were dramatically upregulated in EGFRMT tumors treated with OSI in vivo, consistent with a role for autocrine MERTK activation in osiR tumor growth. Indeed, treatment with OSI alone or in combination with MRX-2843 was sufficient to block tumor growth in vivo, but when treatment was stopped, tumors treated with OSI alone started to grow, while treatment with the combination resulted in durable suppression of tumor growth. Together these data implicate MERTK as a mediator of resistance to OSI and suggest that combining MRX-2843 and OSI therapy will control tumor growth.

Citation Format: Dan Yan, Justus Huelse, Rebecca Parker, Zikang Tan, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK drives residual tumor growth in EGFR-mutated non-small cell lung cancer cells treated with osimertinib [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 1882.

Abstract C045: Outcomes of hormone-receptor positive, HER2-negative breast cancers by race and tumor biologic features

Background: Black women have higher breast cancer mortality than white women, particularly within the hormone receptor positive, human epidermal growth factor receptor 2 negative (HR+/HER2-) clinical subtype. Interactions between tumor biology and treatment factors are complex and racial disparities in early events (such as 5-year recurrence risk) are not well characterized. Methods: Using data from the Carolina Breast Cancer Study (Phase 3, 2008-2013), a large population-based study enrolling 50% black and 50% white women from 44 counties in North Carolina, we estimated associations between race and recurrence among non-metastatic HR+/HER2- tumors, overall and within subgroups defined by PAM50 Risk of Recurrence score (ROR-PT, calculated from subtype, proliferation level, and tumor size), PAM50 intrinsic subtype, and tumor grade. We also compared treatment patterns by race among HR+/HER2- patients with high-risk disease. Relative frequency differences (RFD), interpretable as the percentage difference between index and referent groups, were estimated using multivariable linear regression. Recurrence risk was estimated using survival curves standardized for age and stage and inverse probability-weighted Cox models. Results: Among 1,775 eligible women, black women had higher recurrence risk relative to white women (crude hazard ratio: 1.8, 95% confidence interval [CI]: 1.3, 2.5), which remained elevated after standardizing for age and clinical covariates (hazard ratio: 1.4, CI: 1.0, 1.9). In stratified analyses, racial disparities persisted among women with high ROR-PT score (5-year standardized recurrence risk 18.9% in black vs. 12.5% in white women) and in high grade patients (5-year standardized recurrence risk 16.6% in black vs. 12.0% in white women). Black women with high grade tumors were significantly less likely to initiate endocrine therapy (RFD: -8.4%, CI: -15.9, -1.0) and experienced treatment delay more often than white women (RFD: +10.4%, CI: 1.8, 19.0). Conclusions: Racial disparities in HR+/HER2- breast cancer recurrence persist within high-risk subgroups. Efforts to identify treatment inequities and other causes of variation in cancer treatment are critical to reducing outcome disparities.

Note: This abstract was not presented at the conference.

Citation Format: H. Shelton Earp, Andrew F Olshan, Lisa A Carey, Charles M Perou, Melissa A Troester, Halei C. Benefield, Katherine E Reeder-Hayes, Hazel B Nichols, Benjamin C Calhoun, Michael I Love, Xuezheng Sun, Erin L Kirk, Joseph Geradts, Katherine A Hoadley, Stephen R Cole. Outcomes of hormone-receptor positive, HER2-negative breast cancers by race and tumor biologic features [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr C045.

Integrating biology and access to care in addressing breast cancer disparities: 25 years' research experience in the Carolina Breast Cancer Study

Purpose of Review

To review research on breast cancer mortality disparities, emphasizing research conducted in the Carolina Breast Cancer Study, with a focus on challenges and opportunities for integration of tumor biology and access characteristics across the cancer care continuum.

Recent Findings

Black women experience higher mortality following breast cancer diagnosis, despite lower incidence compared to white women. Biological factors, such as stage at diagnosis and breast cancer subtypes, play a role in these disparities. Simultaneously, social, behavioral, environmental, and access to care factors are important. However, integrated studies of biology and access are challenging and it is uncommon to have both data types available in the same study population. The central emphasis of Phase 3 of the Carolina Breast Cancer Study, initiated in 2008, was to collect rich data on biology (including germline and tumor genomics and pathology) and health care access in a diverse study population, with the long term goal of defining intervention opportunities to reduce disparities across the cancer care continuum.

Summary

Early and ongoing research from CBCS has identified important interactions between biology and access, leading to opportunities to build greater equity. However, sample size, population-specific relationships among variables, and complexities of treatment paths along the care continuum pose important research challenges. Interdisciplinary teams, including experts in novel data integration and causal inference, are needed to address gaps in our understanding of breast cancer disparities.

Integrating access to care and tumor patterns by race and age in the Carolina Breast Cancer Study, 2008-2013

PURPOSE

Understanding breast cancer mortality disparities by race and age is complex due to disease heterogeneity, comorbid disease, and the range of factors influencing access to care. It is important to understand how these factors group together within patients.

METHODS

We compared socioeconomic status (SES) and comorbidity factors in the Carolina Breast Cancer Study Phase 3 (CBCS3, 2008-2013) to those for North Carolina using the 2010 Behavioral Risk Factor Surveillance Study. In addition, we used latent class analysis of CBCS3 data to identify covariate patterns by SES/comorbidities, barriers to care, and tumor characteristics and examined their associations with race and age using multinomial logistic regression.

RESULTS

Major SES and comorbidity patterns in CBCS3 participants were generally similar to patterns in the state. Latent classes were identified for SES/comorbidities, barriers to care, and tumor characteristics that varied by race and age. Compared to white women, black women had lower SES (odds ratio (OR)  6.3, 95% confidence interval (CI) 5.2, 7.8), more barriers to care (OR 5.6, 95% CI 3.9, 8.1) and several aggregated tumor aggressiveness features. Compared to older women, younger women had higher SES (OR 0.5, 95% CI 0.4, 0.6), more barriers to care (OR 2.1, 95% CI 1.6, 2.9) and aggregated tumor aggressiveness features.

CONCLUSIONS

CBCS3 is representative of North Carolina on comparable factors. Patterns of access to care and tumor characteristics are intertwined with race and age, suggesting that interventions to address disparities will need to target both access and biology.

Bimodal age distribution at diagnosis in breast cancer persists across molecular and genomic classifications

PURPOSE

Female breast cancer demonstrates bimodal age frequency distribution patterns at diagnosis, interpretable as two main etiologic subtypes or groupings of tumors with shared risk factors. While RNA-based methods including PAM50 have identified well-established clinical subtypes, age distribution patterns at diagnosis as a proxy for etiologic subtype are not established for molecular and genomic tumor classifications.

METHODS

We evaluated smoothed age frequency distributions at diagnosis for Carolina Breast Cancer Study cases within immunohistochemistry-based and RNA-based expression categories. Akaike information criterion (AIC) values compared the fit of single density versus two-component mixture models. Two-component mixture models estimated the proportion of early-onset and late-onset categories by immunohistochemistry-based ER (n = 2860), and by RNA-based ESR1 and PAM50 subtype (n = 1965). PAM50 findings were validated using pooled publicly available data (n = 8103).

RESULTS

Breast cancers were best characterized by bimodal age distribution at diagnosis with incidence peaks near 45 and 65 years, regardless of molecular characteristics. However, proportional composition of early-onset and late-onset age distributions varied by molecular and genomic characteristics. Higher ER-protein and ESR1-RNA categories showed a greater proportion of late age-at-onset. Similarly, PAM50 subtypes showed a shifting age-at-onset distribution, with most pronounced early-onset and late-onset peaks found in Basal-like and Luminal A, respectively.

CONCLUSIONS

Bimodal age distribution at diagnosis was detected in the Carolina Breast Cancer Study, similar to national cancer registry data. Our data support two fundamental age-defined etiologic breast cancer subtypes that persist across molecular and genomic characteristics. Better criteria to distinguish etiologic subtypes could improve understanding of breast cancer etiology and contribute to prevention efforts.

The role of endothelial MERTK during the inflammatory response in lungs

As a key homeostasis regulator in mammals, the MERTK receptor tyrosine kinase is crucial for efferocytosis, a process that requires remodeling of the cell membrane and adjacent actin cytoskeleton. Membrane and cytoskeletal reorganization also occur in endothelial cells during inflammation, particularly during neutrophil transendothelial migration (TEM) and during changes in permeability. However, MERTK’s function in endothelial cells remains unclear. This study evaluated the contribution of endothelial MERTK to neutrophil TEM and endothelial barrier function. In vitro experiments using primary human pulmonary microvascular endothelial cells found that neutrophil TEM across the endothelial monolayers was enhanced when MERTK expression in endothelial cells was reduced by siRNA knockdown. Examination of endothelial barrier function revealed increased passage of dextran across the MERTK-depleted monolayers, suggesting that MERTK helps maintain endothelial barrier function. MERTK knockdown also altered adherens junction structure, decreased junction protein levels, and reduced basal Rac1 activity in endothelial cells, providing potential mechanisms of how MERTK regulates endothelial barrier function. To study MERTK’s function in vivo, inflammation in the lungs of global Mertk^-/- mice was examined during acute pneumonia. In response to P. aeruginosa, more neutrophils were recruited to the lungs of Mertk^-/- than wildtype mice. Vascular leakage of Evans blue dye into the lung tissue was also greater in Mertk^-/- mice. To analyze endothelial MERTK’s involvement in these processes, we generated inducible endothelial cell-specific (iEC) Mertk^-/- mice. When similarly challenged with P. aeruginosa, iEC Mertk^-/- mice demonstrated no difference in neutrophil TEM into the inflamed lungs or in vascular permeability compared to control mice. These results suggest that deletion of MERTK in human pulmonary microvascular endothelial cells in vitro and in all cells in vivo aggravates the inflammatory response. However, selective MERTK deletion in endothelial cells in vivo failed to replicate this response.

Data-Driven Construction of Antitumor Agents with Controlled Polypharmacology

Controlling which particular members of a large protein family are targeted by a drug is key to achieving a desired therapeutic response. In this study, we report a rational data-driven strategy for achieving restricted polypharmacology in the design of antitumor agents selectively targeting the TYRO3, AXL, and MERTK (TAM) family tyrosine kinases. Our computational approach, based on the concept of fragments in structural environments (FRASE), distills relevant chemical information from structural and chemogenomic databases to assemble a three-dimensional inhibitor structure directly in the protein pocket. Target engagement by the inhibitors designed led to disruption of oncogenic phenotypes as demonstrated in enzymatic assays and in a panel of cancer cell lines, including acute lymphoblastic and myeloid leukemia (ALL/AML) and nonsmall cell lung cancer (NSCLC). Structural rationale underlying the approach was corroborated by X-ray crystallography. The lead compound demonstrated potent target inhibition in a pharmacodynamic study in leukemic mice.

TAM Family Receptor Kinase Inhibition Reverses MDSC-Mediated Suppression and Augments Anti-PD-1 Therapy in Melanoma

Myeloid cell receptor tyrosine kinases TYRO3, AXL, and MERTK and their ligands, GAS6 and PROTEIN S, physiologically suppress innate immune responses, including in the tumor microenvironment. Here, we showed that myeloid-derived suppressor cells (MDSC) dramatically upregulated TYRO3, AXL, and MERTK and their ligands [monocytic MDSCs (M-MDSC)>20-fold, polymorphonuclear MDSCs (PMN-MDSC)>15-fold] in tumor-bearing mice. MDSCs from tumor-bearing Mertk^-/-, Axl^-/- , and Tyro3^-/- mice exhibited diminished suppressive enzymatic capabilities, displayed deficits in T-cell suppression, and migrated poorly to tumor-draining lymph nodes. In coimplantation experiments using TYRO3^-/-, AXL^-/-, and MERTK^-/- MDSCs, we showed the absence of these RTKs reversed the protumorigenic properties of MDSCs in vivo Consistent with these findings, in vivo pharmacologic TYRO3, AXL, and MERTK inhibition diminished MDSC suppressive capability, slowed tumor growth, increased CD8⁺ T-cell infiltration, and augmented anti-PD-1 checkpoint inhibitor immunotherapy. Mechanistically, MERTK regulated MDSC suppression and differentiation in part through regulation of STAT3 serine phosphorylation and nuclear localization. Analysis of metastatic melanoma patients demonstrated an enrichment of circulating MERTK⁺ and TYRO3⁺ M-MDSCs, PMN-MDSCs, and early-stage MDSCs (e-MDSC) relative to these MDSC populations in healthy controls. These studies demonstrated that TYRO3, AXL, and MERTK control MDSC functionality and serve as promising pharmacologic targets for regulating MDSC-mediated immune suppression in cancer patients.

Abstract 3037: Stromal cell activation of MAPK signaling pathways mediates resistance to MERTK inhibition in acute myeloid leukemia

MERTK is a TAM family (TYRO-3, AXL, MERTK) receptor tyrosine kinase that is aberrantly expressed in >80% of primary acute myeloid leukemia (AML) samples. MERTK inhibition mediated by the small molecule tyrosine kinase inhibitor (TKI), MRX-2843, induced apoptosis in leukemia cell cultures and prolonged survival in mouse xenograft models of AML, but was not curative. In these models, treatment efficiently reduced peripheral disease burden but was less effective in the bone marrow, suggesting a role for the bone marrow microenvironment in therapeutic resistance. To evaluate the role of the bone marrow stromal niche in resistance to MERTK inhibition by MRX-2843, Kasumi-1 and OCI-AML5 AML cell lines were cultured with the fibroblast-like Hs27 cell line and induction of apoptosis and cell death was measured by flow cytometry after treatment with MRX-2843 or vehicle. Co-culture with Hs27 cells significantly reduced AML cell death in response to MRX-2843 compared to leukemia cells alone (e.g. Kasumi-1 31.9% vs 61.2%, p<0.05). To identify parallel signaling pathways that may be activated under cytokine rich bone marrow stromal conditions and mediate resistance to MRX-2843, a unbiased phospho-kinase array was performed to identify targets differentially activated by co-culture and/or MRX-2843. Kasumi-1 cells were treated with MRX-2843 in the presence or absence of stromal co-culture, then cell lysates were incubated with human phospho-kinase arrays and proteins were quantitated by densitometry. Five kinases were differentially inhibited by >1.5-fold in the presence or absence of stromal cells, including numerous MAPK pathway components - p38α, ERK1/2, EGFR, TOR, and HSP27. Changes in ERK1/2 phosphorylation (pERK) were confirmed by immunoblot in Kasumi-1 and OCI-AML5 cell cultures. In the absence of co-culture MRX-2843 potently inhibited pERK at doses as low as 100nM, while in co-culture even a 3-fold higher dose did not fully inhibit pERK, consistent with a potential role in resistance. To determine whether the observed upregulation and persistent activation of ERK1/2 has functional significance, Kasumi-1 cells were treated with MRX-2843 and the MEK1/2 TKI pimasertib in the presence or absence of stromal co-culture. Treatment with MRX-2843 or pimasertib alone did not significantly affect survival of AML cells in co-culture (31.3% MRX-2843, 28.1% pimasertib vs 22.7% vehicle, p=ns), but treatment with a combination of MRX-2843 and pimasertib resulted in significant induction of apoptosis compared to vehicle or single TKIs (56.7%, p<0.05). Similar results were observed in OCI-AML5 cell cultures and in combination with the MEK1/2 inhibitor PD0325901. Together these results indicate a critical role for stromal cell-mediated activation of ERK1/2 in resistance to MRX-2843 and demonstrate the utility of translational strategies targeting both MERTK and MAPK pathways to overcome resistance.

Citation Format: Katherine A. Minson, Eleana Vasileiadi, Madeline G. Huey, Xiadong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. Stromal cell activation of MAPK signaling pathways mediates resistance to MERTK inhibition in acute myeloid leukemia [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 3037.

Abstract 2192A: Combined MERTK and ROCK1/2 inhibition as a potential therapeutic strategy for AML

Acute myeloid leukemia (AML) has a 5-year overall survival rate under 26% in adults and current cytotoxic chemotherapies are toxic, can cause long-term side effects and are often contraindicated for elderly patients. Targeted therapies may have reduced toxicity compared to chemotherapy; however, resistance to single-agents often develops and combination therapies may provide more durable responses. MERTK is aberrantly expressed in >80% of AML patient samples and is a promising therapeutic target. MRX-2843 is a novel small molecule dual MERTK and FMS-like tyrosine kinase 3 (FLT3) inhibitor that is currently in clinical trials. Here, we report synergistic anti-leukemia activity mediated by MRX-2843 and rho-associated, coiled-coil-containing protein kinases 1 and 2 (ROCK1/2) inhibitors. Both MERTK and ROCK1/2 have been implicated in actin/microtubule dynamics and cell cycle progression and previous publications demonstrated induction of apoptosis in AML cells in response to RNAi-mediated MERTK or ROCK1 inhibition. ROCK1/2 inhibitors RKI-1447 or GSK269962A synergized with sub-therapeutic doses of MRX-2843 to reduce cell density as indicated by decreased Presto Blue staining in cultures of 4 of 6 AML cell lines tested and had an additive effect in the remaining 2 cell lines. Similar interactions were observed between RKI-1447 and UNC3997, a MERTK-selective TKI with 15-fold weaker FLT3 activity. Flow cytometric analysis of cells stained with popro-1-iodide and propidium iodide (PI) dyes revealed synergistic induction of cell death in the KG-1, OCI-AML5, and NB4 cell lines and an additive effect in Kasumi-1 cultures. In addition, cells that survived treatment exhibited subsequent defects in expansion, even when they were cultured without inhibitor(s), and these effects were significantly more pronounced in cells treated with the combination therapy compared to single agents. Flow cytometric analysis of PI-stained permeabilized cells revealed an increased fraction of cells in G2/M phase in cultures treated with MRX-2843. This effect was more pronounced in cultures treated with the combination therapy and was accompanied by accumulation of a population with slightly reduced DNA content relative to untreated G2 cells, suggestive of a defect in late S-phase. Thus, the combination therapy mediates anti-leukemia activity by multiple mechanisms, including abrogation of cell cycle progression and induction of cell death. In addition, a preliminary analysis of The Cancer Genome Atlas database revealed significantly poorer overall survival in patients with higher levels of ROCK1 expression (p=0.00147, n=172). Together, these data suggest that combination therapies targeting MERTK and ROCK1/2 may be particularly effective for treatment of AML and support further studies to test the effects of this novel strategy in animal models.

Citation Format: Dawn E. Barnes, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. Combined MERTK and ROCK1/2 inhibition as a potential therapeutic strategy for AML [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 2192A.

Abstract 1314: MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia

Background: Early T-precursor acute lymphoblastic leukemias (ETP-ALL) account for 15% of pediatric T-cell ALL (T-ALL) cases and are characterized by an immature phenotype, resistance to therapy, and high relapse rates. MERTK receptor tyrosine kinase is ectopically expressed in ~50% of T-ALLs, particularly those with an immature T cell phenotype, suggesting a role in ETP-ALL. The anti-apoptotic protein B-cell lymphoma-2 (BCL-2) is specifically expressed in immature T cell precursors, is preferentially expressed in ETP-ALL compared to other T-ALLs, is essential for ETP-ALL cell survival, and is regulated downstream of MERTK in acute leukemia cells. Thus, combination therapies targeting these two proteins may be particularly effective to treat ETP-ALL.

Methods: MERTK and BCL-2 mRNA expression was assessed in T-ALL patient samples using publicly available data. Loucy and PEER ETP-ALL cell lines were cultured with vehicle or MRX-2843, a dual MERTK/FLT3 inhibitor, alone or in combination with the BCL-2 inhibitor venetoclax. Phosphorylated and total MERTK were assessed by immunoblot. Cells were stained with PoPro-1-iodide and propidium iodide dyes and analyzed by flow cytometry to assess cell death. Relative cell numbers were assessed using Presto Blue reagent. Orthotopic xenografts were established in NSG or NSGS mice using luciferase-expressing Loucy cells or an ETP-ALL patient sample and leukemia burden was monitored by bioluminescence imaging or flow cytometry. MRX-2843 or saline vehicle were administered orally once daily. Median survival was determined by Kaplan-Meier analysis.

Results: MERTK and BCL-2 mRNAs were expressed at significantly higher levels in ETP-ALL patient samples relative to other T-ALLs. MRX-2843 mediated a dose-dependent decrease in phosphorylated MERTK and induced dose-dependent cell death (43.2% vs 16% in vehicle-treated cultures, p<0.01) in PEER cells. Moreover, in a patient-derived ETP-ALL xenograft model, treatment with MRX-2843 reduced peripheral blood disease burden by 83% (p<0.001) and decreased spleen weight by 64% (p<0.001) compared to vehicle-treated mice. Furthermore, treatment with MRX-2843 prolonged median survival by 11 days compared to control mice (n=8/group, p=0.0016). In a cell line-derived xenograft model, MRX-2843 reduced disease burden by 60% (n=10/group, p<0.0001) compared to vehicle. Treatment with a combination of MRX-2843 and venetoclax decreased cell density more effectively than either single agent in cultures of the Loucy and PEER cell lines (p<0.05).

Conclusions: MERTK and BCL-2 are preferentially expressed in ETP-ALL relative to T-ALL and MRX-2843 has robust therapeutic activity in cell culture and xenograft models of ETP-ALL. These data validate MRX-2843 as a novel agent with potential for clinical application in patients with ETP-ALL. Combined targeting of MERTK and BCL-2 may be particularly effective.

Citation Format: Ryan J. Summers, Katherine A. Minson, Eleana Vasileiadi, Xiaodong Wang, Steven V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia [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 1314.

Abstract 4765: MERTK is a potential therapeutic target in osimertinib-resistant non-small cell lung cancer

Non-small cell lung cancer (NSCLC) remains a global problem causing more deaths in both men and women than any other cancer worldwide with an urgent need for more efficacious treatments. During the last decade the therapeutic landscape for NSCLC has been profoundly changed with the discovery of activating mutations in Epidermal Growth Factor Receptor (EGFR) (EGFRMT). Osimertinib, a 3rd generation EGFR TKI, was recently FDA-approved as a front-line agent for newly diagnosed EGFRMT NSCLCs due to its superior efficacy relative to earlier-generation EGFR TKIs in the international FLAURA trial. However, unmet clinical needs have arisen in conjunction with osimertinib use, including understanding mechanisms of osimertinib resistance and developing novel approaches to prevent or reverse resistance and/or enhance osimertinib efficacy in responsive patients. Our group previously identified MERTK receptor tyrosine kinase as a potential therapeutic target in NSCLC and developed MRX-2843, a novel small molecule inhibitor with dual MERTK and FLT3 activity, which is currently in Phase I clinical trials. Here, we report upregulation of MERTK and its ligand PROS1 in xenograft tumors derived from the EGFRMT H4006 NSCLC cell line and treated with osimertinib relative to vehicle-treated tumors. MERTK expression was also increased in osimertinib-resistant derivatives of the H4006 and H4011 cell lines generated by culture in escalating doses of osimertinib. However, overexpression of exogenous MERTK in H4006 cells was not sufficient to confer osimertinib resistance. In contrast, stimulation with TAM kinase ligands GAS6 or PROS1 protected H4006 cells from osimertinib treatment, as indicated by restoration of AKT, ERK, and ribosomal S6 phosphorylation in the presence of osimertinib. Together these data implicate MERTK as a mediator of resistance to osimertinib. Indeed, combined treatment with osimertinib and MRX-2843 effectively blocked PI3K-AKT and MAPK-ERK signaling and mediated synergistic inhibition of colony formation in osimertinib-resistant H4006 cells. Thus, MERTK inhibition may be an effective therapeutic strategy to re-sensitize osimertinib-resistant NSCLCs to EGFR TKI treatment.

Citation Format: Dan Yan, Justus Huelse, Rebecca Parker, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas Graham. MERTK is a potential therapeutic target in osimertinib-resistant non-small cell lung 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 4765.

Beyond growth signaling: apoptotic sensor MERTK activates AKT by a novel mechanism

Canonically the oncogenic kinase AKT is activated by growth signals. Our work suggests apoptotic materials, abundant in tumors, also contribute to AKT activation by stimulating MERTK that in turn phosphorylates Y26 in the AKT PH domain. pY26 reverses binding of an AKT endogenous, WW-domain containing inhibitor, SAV1, allowing AKT responsiveness to classic growth signals. This novel mechanism may contribute to drug resistance.

SGC-GAK-1: A Chemical Probe for Cyclin G Associated Kinase (GAK)

We describe SGC-GAK-1 (11), a potent, selective, and cell-active inhibitor of cyclin G-associated kinase (GAK), together with a structurally related negative control SGC-GAK-1N (14). 11 was highly selective in an in vitro kinome-wide screen, but cellular engagement assays defined RIPK2 as a collateral target. We identified 18 as a potent RIPK2 inhibitor lacking GAK activity. Together, this chemical probe set can be used to interrogate GAK cellular biology.

Inhibition of MERTK Promotes Suppression of Tumor Growth in BRAF Mutant and BRAF Wild-Type Melanoma

Molecularly-targeted agents have improved outcomes for a subset of patients with BRAF-mutated melanoma, but treatment of resistant and BRAF wild-type tumors remains a challenge. The MERTK receptor tyrosine kinase is aberrantly expressed in melanoma and can contribute to oncogenic phenotypes. Here we report the effect of treatment with a MERTK-selective small molecule inhibitor, UNC2025, in preclinical models of melanoma. In melanoma cell lines, treatment with UNC2025 potently inhibited phosphorylation of MERTK and downstream signaling, induced cell death, and decreased colony formation. In patient-derived melanoma xenograft models, treatment with UNC2025 blocked or significantly reduced tumor growth. Importantly, UNC2025 had similar biochemical and functional effects in both BRAF-mutated and BRAF wild-type models and irrespective of NRAS mutational status, implicating MERTK inhibition as a potential therapeutic strategy in tumors that are not amenable to BRAF-targeting and for which there are limited treatment options. In BRAF-mutated cell lines, combined treatment with UNC2025 and the BRAF inhibitor vemurafenib provided effective inhibition of oncogenic signaling through ERK, AKT, and STAT6, increased induction of cell death, and decreased colony-forming potential. Similarly, in NRAS-mutated cell lines, addition of UNC2025 to cobimetinib therapy increased cell death and decreased colony-forming potential. In a BRAF-mutated patient-derived xenograft, treatment with combined UNC2025 and vemurafenib was well-tolerated and significantly decreased tumor growth compared with vemurafenib alone. These data support the use of UNC2025 for treatment of melanoma, irrespective of BRAF or NRAS mutational status, and suggest a role for MERTK and targeted combination therapy in BRAF and NRAS-mutated melanoma.

MerTK as a therapeutic target in glioblastoma

Background

Glioma-associated macrophages and microglia (GAMs) are components of the glioblastoma (GBM) microenvironment that express MerTK, a receptor tyrosine kinase that triggers efferocytosis and can suppress innate immune responses. The aim of the study was to define MerTK as a therapeutic target using an orally bioavailable inhibitor, UNC2025.

Methods

We examined MerTK expression in tumor cells and macrophages in matched patient GBM samples by double-label immunohistochemistry. UNC2025-induced MerTK inhibition was studied in vitro and in vivo.

Results

MerTK/CD68+ macrophages increased in recurrent tumors while MerTK/glial fibrillary acidic protein-positive tumor cells did not. Pharmacokinetic studies showed high tumor exposures of UNC2025 in a syngeneic orthotopic allograft mouse GBM model. The same model mice were randomized to receive vehicle, daily UNC2025, fractionated external beam radiotherapy (XRT), or UNC2025/XRT. Although median survival (21, 22, 35, and 35 days, respectively) was equivalent with or without UNC2025, bioluminescence imaging (BLI) showed significant growth delay with XRT/UNC2025 treatment and complete responses in 19%. The responders remained alive for 60 days and showed regression to 1%-10% of pretreatment BLI tumor burden; 5 of 6 were tumor free by histology. In contrast, only 2% of 98 GBM mice of the same model treated with XRT survived 50 days and none survived 60 days. UNC2025 also reduced CD206+ macrophages in mouse tumor samples.

Conclusions

These results suggest that MerTK inhibition combined with XRT has a therapeutic effect in a subset of GBM. Further mechanistic studies are warranted.

The Prognostic Significance of Low-Frequency Somatic Mutations in Metastatic Cutaneous Melanoma

Background: Little is known about the prognostic significance of somatically mutated genes in metastatic melanoma (MM). We have employed a combined clinical and bioinformatics approach on tumor samples from cutaneous melanoma (SKCM) as part of The Cancer Genome Atlas project (TCGA) to identify mutated genes with potential clinical relevance. Methods: After limiting our DNA sequencing analysis to MM samples (n = 356) and to the CANCER CENSUS gene list, we filtered out mutations with low functional significance (snpEFF). We performed Cox analysis on 53 genes that were mutated in ≥3% of samples, and had ≥50% difference in incidence of mutations in deceased subjects versus alive subjects. Results: Four genes were potentially prognostic [RAC1, FGFR1, CARD11, CIITA; false discovery rate (FDR) < 0.2]. We identified 18 additional genes (e.g., SPEN, PDGFRB, GNAS, MAP2K1, EGFR, TSC2) that were less likely to have prognostic value (FDR < 0.4). Most somatic mutations in these 22 genes were infrequent (< 10%), associated with high somatic mutation burden, and were evenly distributed across all exons, except for RAC1 and MAP2K1. Mutations in only 9 of these 22 genes were also identified by RNA sequencing in >75% of the samples that exhibited corresponding DNA mutations. The low frequency, UV signature type and RNA expression of the 22 genes in MM samples were confirmed in a separate multi-institution validation cohort (n = 413). An underpowered analysis within a subset of this validation cohort with available patient follow-up (n = 224) showed that somatic mutations in SPEN and RAC1 reached borderline prognostic significance [log-rank favorable (p = 0.09) and adverse (p = 0.07), respectively]. Somatic mutations in SPEN, and to a lesser extent RAC1, were not associated with definite gene copy number or RNA expression alterations. High (>2+) nuclear plus cytoplasmic expression intensity for SPEN was associated with longer melanoma-specific overall survival (OS) compared to lower (≤ 2+) nuclear intensity (p = 0.048). We conclude that expressed somatic mutations in infrequently mutated genes beyond the well-characterized ones (e.g., BRAF, RAS, CDKN2A, PTEN, TP53), such as RAC1 and SPEN, may have prognostic significance in MM.

MERTK inhibition alters the PD-1 axis and promotes anti-leukemia immunity

MERTK is ectopically expressed and promotes survival in acute lymphoblastic leukemia (ALL) cells and is thus a potential therapeutic target. Here we demonstrate both direct therapeutic effects of MERTK inhibition on leukemia cells and induction of anti-leukemia immunity via suppression of the coinhibitory PD-1 axis. A MERTK-selective tyrosine kinase inhibitor, MRX-2843, mediated therapeutic anti-leukemia effects in immunocompromised mice bearing a MERTK-expressing human leukemia xenograft. In addition, inhibition of host MERTK by genetic deletion (Mertk-/- mice) or treatment with MRX-2843 significantly decreased tumor burden and prolonged survival in immune-competent mice inoculated with a MERTK-negative ALL, suggesting immune-mediated therapeutic activity. In this context, MERTK inhibition led to significant decreases in expression of the coinhibitory ligands PD-L1 and PD-L2 on CD11b+ monocytes/macrophages in the leukemia microenvironment. Furthermore, although T cells do not express MERTK, inhibition of MERTK indirectly decreased PD-1 expression on CD4+ and CD8+ T cells and decreased the incidence of splenic FOXP3+ Tregs at sites of leukemic infiltration, leading to increased T cell activation. These data demonstrate direct and immune-mediated therapeutic activities in response to MERTK inhibition in ALL models and provide validation of a translational agent targeting MERTK for modulation of tumor immunity.

ZFX Mediates Non-canonical Oncogenic Functions of the Androgen Receptor Splice Variant 7 in Castrate-Resistant Prostate Cancer

Androgen receptor splice variant 7 (AR-V7) is crucial for prostate cancer progression and therapeutic resistance. We show that, independent of ligand, AR-V7 binds both androgen-responsive elements (AREs) and non-canonical sites distinct from full-length AR (AR-FL) targets. Consequently, AR-V7 not only recapitulates AR-FL's partial functions but also regulates an additional gene expression program uniquely via binding to gene promoters rather than ARE enhancers. AR-V7 binding and AR-V7-mediated activation at these unique targets do not require FOXA1 but rely on ZFX and BRD4. Knockdown of ZFX or select unique targets of AR-V7/ZFX, or BRD4 inhibition, suppresses growth of castration-resistant prostate cancer cells. We also define an AR-V7 direct target gene signature that correlates with AR-V7 expression in primary tumors, differentiates metastatic prostate cancer from normal, and predicts poor prognosis. Thus, AR-V7 has both ARE/FOXA1 canonical and ZFX-directed non-canonical regulatory functions in the evolution of anti-androgen therapeutic resistance, providing information to guide effective therapeutic strategies.

Abstract A35: MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia

Background: Early T-precursor acute lymphoblastic leukemia (ETP-ALL) is a subclass of T-cell ALL (T-ALL) accounting for 15% of pediatric T-ALL cases and characterized by an immature phenotype, resistance to therapy, and high rates of induction failure and relapse (Wood B et al., Blood 2009). MERTK receptor tyrosine kinase is not expressed in normal T cells but is ectopically expressed in 40-50% of T-ALLs, particularly those with an immature T-cell phenotype (Graham DK et al., Clin Cancer Res 2006), suggesting a role in ETP-ALL. One potential role is regulation of the anti-apoptotic protein B-cell lymphoma-2 (BCL-2). BCL-2 is specifically expressed in double-negative T-cell precursors and is preferentially expressed in ETP-ALL compared to T-ALL (Chonghaile et al., Cancer Discovery 2014). Moreover, ETP-ALL cells are dependent on BCL-2 for survival. Our previous studies demonstrated regulation of BCL-2 and BCL-2 family members downstream of MERTK in B-ALL and acute myeloid leukemia cells (Linger RM et al., Blood 2013; Lee-Sherick AB et al., Oncotarget 2015). This interplay between MERTK and BCL-2 and their association with an immature T-ALL phenotype suggest that combination therapies targeting these two proteins may be particularly effective to treat ETP-ALL.

Methods: Publicly available mRNA expression data were used to assess MERTK and BCL-2 expression in T-ALL cell lines and patient samples. MERTK and BCL-2 protein expression were determined by immunoblot. ETP-ALL cell lines were cultured with vehicle or MRX-2843, a dual MERTK/FLT3 kinase inhibitor. MERTK protein was immunoprecipitated from cell lysates and phosphorylated and total proteins were assessed by immunoblot. Alternatively, cells were stained with PoPro-1-iodide and propidium iodide dyes and analyzed by flow cytometry to assess cell death. Orthotopic xenografts were established in NSGS mice using an ETP-ALL patient sample, and leukemia burden in peripheral blood (%hCD45+) was monitored by flow cytometry. After engraftment (1.86 +/- 0.43% peripheral blasts), mice were treated once daily with 75 mg/kg MRX-2843 or saline vehicle administered orally. Mice were euthanized when symptoms of advanced leukemia were evident and median survival was determined by Kaplan-Meier analysis.

Results: MERTK mRNA was expressed at significantly higher levels in ETP-ALL cell lines and patient samples relative to other T-ALLs. Similarly, MERTK protein was ubiquitously expressed in ETP-ALL cell lines (n=2), ETP-ALL patient samples (n=2), and a near-ETP-ALL patient sample (n=1). In contrast, only 60% of other T-ALL cell lines (n=5) expressed MERTK. BCL-2 mRNA was expressed at significantly higher levels in ETP-ALL patient samples relative to other T-ALLs and BCL-2 protein was expressed in 2 of 2 ETP-ALL cell lines and 2 of 3 ETP-ALL and near-ETP-ALL patient samples. Treatment with MRX-2843 mediated a dose-dependent decrease in phosphorylated MERTK in ETP-ALL cells and induced dose-dependent cell death in the ETP-ALL cell lines PEER (43.2% vs 16% in vehicle-treated cultures, p&lt;0.01) and Loucy (36.6% vs 19.2%, p&lt;0.05). Moreover, in a xenograft model of ETP-ALL, treatment with MRX-2843 for 29 days reduced peripheral blood disease burden by 53.9% (p&lt;0.001) and decreased spleen weight by 64% (p&lt;0.001) compared to vehicle-treated controls. Furthermore, mice treated with MRX-2843 survived for a median of 41 days post-treatment compared to 29 days for control mice (n=8/group, p=0.003).

Conclusions: MERTK and BCL-2 are preferentially expressed in ETP-ALL relative to T-ALL and MRX-2843, a dual MERTK/FLT3 kinase inhibitor, has robust therapeutic activity in cell culture and xenograft models of ETP-ALL. These data validate MRX-2843 as a novel agent with potential for clinical application in patients with ETP-ALL. In particular, combination therapy targeting MERTK and BCL-2 may be an effective therapeutic option for ETP-ALL.

Citation Format: Ryan J. Summers, Katherine A. Minson, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK and BCL-2 as potential therapeutic targets in early T-precursor acute lymphoblastic leukemia [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr A35.

MERTK Mediates Intrinsic and Adaptive Resistance to AXL-targeting Agents

The TAM (TYRO3, AXL, MERTK) family receptor tyrosine kinases (RTK) play an important role in promoting growth, survival, and metastatic spread of several tumor types. AXL and MERTK are overexpressed in head and neck squamous cell carcinoma (HNSCC), triple-negative breast cancer (TNBC), and non-small cell lung cancer (NSCLC), malignancies that are highly metastatic and lethal. AXL is the most well-characterized TAM receptor and mediates resistance to both conventional and targeted cancer therapies. AXL is highly expressed in aggressive tumor types, and patients with cancer are currently being enrolled in clinical trials testing AXL inhibitors. In this study, we analyzed the effects of AXL inhibition using a small-molecule AXL inhibitor, a monoclonal antibody (mAb), and siRNA in HNSCC, TNBC, and NSCLC preclinical models. Anti-AXL-targeting strategies had limited efficacy across these different models that, our data suggest, could be attributed to upregulation of MERTK. MERTK expression was increased in cell lines and patient-derived xenografts treated with AXL inhibitors and inhibition of MERTK sensitized HNSCC, TNBC, and NSCLC preclinical models to AXL inhibition. Dual targeting of AXL and MERTK led to a more potent blockade of downstream signaling, synergistic inhibition of tumor cell expansion in culture, and reduced tumor growth in vivo Furthermore, ectopic overexpression of MERTK in AXL inhibitor-sensitive models resulted in resistance to AXL-targeting strategies. These observations suggest that therapeutic strategies cotargeting both AXL and MERTK could be highly beneficial in a variety of tumor types where both receptors are expressed, leading to improved survival for patients with lethal malignancies. Mol Cancer Ther; 17(11); 2297-308. ©2018 AACR.

Identification of Clonal Hematopoiesis Mutations in Solid Tumor Patients Undergoing Unpaired Next-Generation Sequencing Assays

PURPOSE

In this era of precision-based medicine, for optimal patient care, results reported from commercial next-generation sequencing (NGS) assays should adequately reflect the burden of somatic mutations in the tumor being sequenced. Here, we sought to determine the prevalence of clonal hematopoiesis leading to possible misattribution of tumor mutation calls on unpaired Foundation Medicine NGS assays.

EXPERIMENTAL DESIGN

This was a retrospective cohort study of individuals undergoing NGS of solid tumors from two large cancer centers. We identified and quantified mutations in genes known to be frequently altered in clonal hematopoiesis (DNMT3A, TET2, ASXL1, TP53, ATM, CHEK2, SF3B1, CBL, JAK2) that were returned to physicians on clinical Foundation Medicine reports. For a subset of patients, we explored the frequency of true clonal hematopoiesis by comparing mutations on Foundation Medicine reports with matched blood sequencing.

RESULTS

Mutations in genes that are frequently altered in clonal hematopoiesis were identified in 65% (1,139/1,757) of patients undergoing NGS. When excluding TP53, which is often mutated in solid tumors, these events were still seen in 35% (619/1,757) of patients. Utilizing paired blood specimens, we were able to confirm that 8% (18/226) of mutations reported in these genes were true clonal hematopoiesis events. The majority of DNMT3A mutations (64%, 7/11) and minority of TP53 mutations (4%, 2/50) were clonal hematopoiesis.

CONCLUSIONS

Clonal hematopoiesis mutations are commonly reported on unpaired NGS testing. It is important to recognize clonal hematopoiesis as a possible cause of misattribution of mutation origin when applying NGS findings to a patient's care.See related commentary by Pollyea, p. 5790.

Factor XIIIA-expressing inflammatory monocytes promote lung squamous cancer through fibrin cross-linking

Lung cancer is the leading cause of cancer-related deaths worldwide, and lung squamous carcinomas (LUSC) represent about 30% of cases. Molecular aberrations in lung adenocarcinomas have allowed for effective targeted treatments, but corresponding therapeutic advances in LUSC have not materialized. However, immune checkpoint inhibitors in sub-populations of LUSC patients have led to exciting responses. Using computational analyses of The Cancer Genome Atlas, we identified a subset of LUSC tumors characterized by dense infiltration of inflammatory monocytes (IMs) and poor survival. With novel, immunocompetent metastasis models, we demonstrated that tumor cell derived CCL2-mediated recruitment of IMs is necessary and sufficient for LUSC metastasis. Pharmacologic inhibition of IM recruitment had substantial anti-metastatic effects. Notably, we show that IMs highly express Factor XIIIA, which promotes fibrin cross-linking to create a scaffold for LUSC cell invasion and metastases. Consistently, human LUSC samples containing extensive cross-linked fibrin in the microenvironment correlated with poor survival.

Lung squamous carcinomas (LUSC) are poorly molecularly characterized, but sub-populations show promising response to immune checkpoint inhibitors. Here, the authors identify a subset of LUSC characterized by infiltration of inflammatory monocytes, where metastasis is linked to Factor XIIIA promoting fibrin cross-linking.

Tumor-secreted Pros1 inhibits macrophage M1 polarization to reduce antitumor immune response

Tyro3, Axl, Mer (TAM) receptor tyrosine kinases reduce inflammatory, innate immune responses. We demonstrate that tumor-secreted protein S (Pros1), a Mer/Tyro3 ligand, decreased macrophage M1 cytokine expression in vitro and in vivo. In contrast, tumor cells with CRISPR-based deletion of Pros1 failed to inhibit M1 polarization. Tumor cell-associated Pros1 action was abrogated in macrophages from Mer- and Tyro3- but not Axl-KO mice. In addition, several other murine and human tumor cell lines suppressed macrophage M1 cytokine expression induced by IFN-γ and LPS. Investigation of the suppressive pathway demonstrated a role for PTP1b complexing with Mer. Substantiating the role of PTP1b, M1 cytokine suppression was also lost in macrophages from PTP1b-KO mice. Mice bearing Pros1-deficient tumors showed increased innate and adaptive immune infiltration, as well as increased median survival. TAM activation can also inhibit TLR-mediated M1 polarization. Treatment with resiquimod, a TLR7/8 agonist, did not improve survival in mice bearing Pros1-secreting tumors but doubled survival for Pros1-deleted tumors. The tumor-derived Pros1 immune suppressive system, like PD-L1, was cytokine responsive, with IFN-γ inducing Pros1 transcription and secretion. Inhibition of Pros1/TAM interaction represents a potential novel strategy to block tumor-derived immune suppression.

Abstract P6-08-01: Race and recurrence by PAM50 intrinsic subtype and ROR-PT score: The Carolina breast cancer study

Compared with white women, black women have higher breast cancer mortality. Racial differences have been reported in frequency of PAM50 intrinsic subtypes, and PAM50 intrinsic subtype is associated with prognosis. However, there is a paucity of data from large population-based studies with adequate representation of black women to evaluate its role in racial differences in breast cancer outcomes. We studied breast cancer recurrence in relation to PAM50 intrinsic subtype using data from the Carolina Breast Cancer Study Phase III (CBCS3), a large population-based prospective cohort study oversampled black women.

The current analysis included 472 black and 463 white women with stage I-III tumor and available Nanostring gene expression data for PAM50 subtype and risk of recurrence score (ROR-PT, a score calculated based on subtype, proliferation level, and tumor size; categorized as low, medium, and high). Cox proportional hazard regression was used to estimate the association of recurrence with PAM50 intrinsic subtype and ROR-PT, overall and stratified by race, adjusted for tumor size, grade, and lymph node status.

During a median follow-up of 5.78 years (range=0.39-8.22), 110 recurrent cases were identified (68 blacks and 42 whites). Consistently with previous data, basal-like tumors had the highest risk for recurrence (reference=luminal A, adjusted hazard ratio [HR]=2.85, 95% confidence interval [CI]=1.44-5.66), followed by luminal B (adjusted HR=2.35, 95%CI=1.18-4.69) and Her2-enriched (adjusted HR=2.19, 95%CI=1.01-4.74). High-ROR-PT score was also associated with increased risk in both black (adjusted HR=1.55, 95%CI=0.93-2.60) and white (adjusted HR=2.07, 95%CI=1.04-4.11). Recurrence rate for black women with medium/low ROR-PT scores was suggested higher than that for white women with similar scores (adjusted HR=1.54, 95%CI=0.69-3.42).

Our results demonstrate that PAM50 subtype and ROR-PT score are associated with breast cancer recurrence in both black and white women. Given the higher relative frequency of poor prognosis subtypes in black women, these findings help explain prognosis disparities by race.

Citation Format: Sun X, Reeder-Hayes KE, Kirk EL, Olsson L, Tse C-K, Bell MB, Earp HS, Carey LA, Perou CM, Olshan AF, Troester MA. Race and recurrence by PAM50 intrinsic subtype and ROR-PT score: The Carolina breast cancer study [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-08-01.

The small-molecule MERTK inhibitor UNC2025 decreases platelet activation and prevents thrombosis

Essentials Signaling by Gas6 through Tyro3/Axl/Mer receptors is essential for stable platelet aggregation. UNC2025 is a small molecule inhibitor of the Mer tyrosine kinase. UNC2025 decreases platelet activation in vitro and thrombus formation in vivo. UNC2025's anti-platelet effect is synergistic with inhibition of the ADP receptor, P2Y12 .

SUMMARY

Background Growth arrest-specific protein 6 signals through the TAM (TYRO-3-AXL-MERTK) receptor family, mediating platelet activation and thrombus formation via activation of the aggregate-stabilizing αIIb β3 integrin. Objective To describe the antithrombotic effects mediated by UNC2025, a small-molecule MERTK tyrosine kinase inhibitor. Methods MERTK phosphorylation and downstream signaling were assessed by immunoblotting. Light transmission aggregometry, flow cytometry and microfluidic analysis were used to evaluate the impact of MERTK inhibition on platelet activation and stability of aggregates in vitro. The effects of MERTK inhibition on arterial and venous thrombosis, platelet accumulation at microvascular injury sites and tail bleeding times were determined with murine models. The effects of combined treatment with ADP-P2Y1&12 pathway antagonists and UNC2025 were also evaluated. Results and Conclusions Treatment with UNC2025 inhibited MERTK phosphorylation and downstream activation of AKT and SRC, decreased platelet activation, and protected animals from pulmonary embolism and arterial thrombosis without increasing bleeding times. The antiplatelet effect of UNC2025 was enhanced in combination with ADP-P2Y1&12 pathway antagonists, and a greater than additive effect was observed when these two agents with different mechanisms of inhibition were coadministered. TAM kinase signaling represents a potential therapeutic target, as inhibition of this axis, especially in combination with ADP-P2Y pathway antagonism, mediates decreased platelet activation, aggregate stability, and thrombus formation, with less hemorrhagic potential than current treatment strategies. The data presented here also demonstrate antithrombotic activity mediated by UNC2025, a novel translational agent, and support the development of TAM kinase inhibitors for clinical applications.

Abstract A140: MERTK mediates intrinsic and adaptive resistance to AXL-targeting agents

The TAM family of receptor tyrosine kinases (RTKs) has been discovered to play a predominant role in promoting the growth, survival, and metastatic spread of several tumor types. AXL and MERTK are two TAM family RTKs that are overexpressed in head and neck squamous cell carcinoma (HNSCC), triple-negative breast cancer (TNBC), and non-small cell lung cancer (NSCLC), malignancies that are highly metastatic and lethal. The AXL receptor is the most well-characterized TAM receptor and has been found to mediate resistance to both conventional and targeted cancer therapies. Since AXL is overexpressed in aggressive tumor types, cancer patients are currently being enrolled in clinical trials testing AXL inhibitors. In the current study, we analyzed the efficacy of AXL inhibitors—both small molecule and monoclonal antibody therapy—in HNSCC, TNBC, and NSCLC preclinical models. We observed limited efficacy of anti-AXL targeting strategies across these different models, which was attributed to the upregulation of MERTK. MERTK was robustly overexpressed in cell lines and patient-derived xenografts treated with AXL inhibitors. Inhibition of MERTK sensitized HNSCC, TNBC, and NSCLC preclinical models to AXL inhibitors, leading to a more potent blockade of downstream signaling, decreased expansion of tumor cells in culture, and reduced tumor growth in vivo. Furthermore, ectopic overexpression of MERTK in AXL inhibitor-sensitive models resulted in resistance to AXL-targeting strategies. These results suggest that cotargeting both AXL and MERTK may be highly beneficial in a variety of tumor types where both receptors are expressed and may therefore prolong antitumor effects and improve the survival of patients with lethal malignancies.

Citation Format: Nellie K. McDaniel, Christopher T. Cummings, Toni M. Brand, Mari Iida, Justus Hulse, Hannah E. Pearson, Rachel A. Orbuch, Olivia J. Ondracek, Kurtis D. Davies, Parkash Gill, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Randall J. Kimple, Paul M. Harari, Deborah DeRyckere, Douglas K. Graham, Deric L. Wheeler. MERTK mediates intrinsic and adaptive resistance to AXL-targeting agents [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A140.