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.

Single-cell RNA sequencing distinctly characterizes the wide heterogeneity in pediatric mixed phenotype acute leukemia

BACKGROUND

Mixed phenotype acute leukemia (MPAL), a rare subgroup of leukemia characterized by blast cells with myeloid and lymphoid lineage features, is difficult to diagnose and treat. A better characterization of MPAL is essential to understand the subtype heterogeneity and how it compares with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). Therefore, we performed single-cell RNA sequencing (scRNAseq) on pediatric MPAL bone marrow (BM) samples to develop a granular map of the MPAL blasts and microenvironment landscape.

METHODS

We analyzed over 40,000 cells from nine pediatric MPAL BM samples to generate a single-cell transcriptomic landscape of B/myeloid (B/My) and T/myeloid (T/My) MPAL. Cells were clustered using unsupervised single-cell methods, and malignant blast and immune clusters were annotated. Differential expression analysis was performed to identify B/My and T/My MPAL blast-specific signatures by comparing transcriptome profiles of MPAL with normal BM, AML, and ALL. Gene set enrichment analysis (GSEA) was performed, and significantly enriched pathways were compared in MPAL subtypes.

RESULTS

B/My and T/My MPAL blasts displayed distinct blast signatures. Transcriptomic analysis revealed that B/My MPAL profile overlaps with B-ALL and AML samples. Similarly, T/My MPAL exhibited overlap with T-ALL and AML samples. Genes overexpressed in both MPAL subtypes' blast cells compared to AML, ALL, and healthy BM included MAP2K2 and CD81. Subtype-specific genes included HBEGF for B/My and PTEN for T/My. These marker sets segregated bulk RNA-seq AML, ALL, and MPAL samples based on expression profiles. Analysis comparing T/My MPAL to ETP, near-ETP, and non-ETP T-ALL, showed that T/My MPAL had greater overlap with ETP-ALL cases. Comparisons among MPAL subtypes between adult and pediatric samples showed analogous transcriptomic landscapes of corresponding subtypes. Transcriptomic differences were observed in the MPAL samples based on response to induction chemotherapy, including selective upregulation of the IL-16 pathway in relapsed samples.

CONCLUSIONS

We have for the first time described the single-cell transcriptomic landscape of pediatric MPAL and demonstrated that B/My and T/My MPAL have distinct scRNAseq profiles from each other, AML, and ALL. Differences in transcriptomic profiles were seen based on response to therapy, but larger studies will be needed to validate these findings.

Single-cell analysis reveals altered tumor microenvironments of relapse- and remission-associated pediatric acute myeloid leukemia

Acute myeloid leukemia (AML) microenvironment exhibits cellular and molecular differences among various subtypes. Here, we utilize single-cell RNA sequencing (scRNA-seq) to analyze pediatric AML bone marrow (BM) samples from diagnosis (Dx), end of induction (EOI), and relapse timepoints. Analysis of Dx, EOI scRNA-seq, and TARGET AML RNA-seq datasets reveals an AML blasts-associated 7-gene signature (CLEC11A, PRAME, AZU1, NREP, ARMH1, C1QBP, TRH), which we validate on independent datasets. The analysis reveals distinct clusters of Dx relapse- and continuous complete remission (CCR)-associated AML-blasts with differential expression of genes associated with survival. At Dx, relapse-associated samples have more exhausted T cells while CCR-associated samples have more inflammatory M1 macrophages. Post-therapy EOI residual blasts overexpress fatty acid oxidation, tumor growth, and stemness genes. Also, a post-therapy T-cell cluster associated with relapse samples exhibits downregulation of MHC Class I and T-cell regulatory genes. Altogether, this study deeply characterizes pediatric AML relapse- and CCR-associated samples to provide insights into the BM microenvironment landscape.

Constitutively Synergistic Multiagent Drug Formulations Targeting MERTK, FLT3, and BCL-2 for Treatment of AML

PURPOSE

Although high-dose, multiagent chemotherapy has improved leukemia survival rates, treatment outcomes remain poor in high-risk subsets, including acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) in infants. The development of new, more effective therapies for these patients is therefore an urgent, unmet clinical need.

METHODS

The dual MERTK/FLT3 inhibitor MRX-2843 and BCL-2 family protein inhibitors were screened in high-throughput against a panel of AML and MLL-rearranged precursor B-cell ALL (infant ALL) cell lines. A neural network model was built to correlate ratiometric drug synergy and target gene expression. Drugs were loaded into liposomal nanocarriers to assess primary AML cell responses.

RESULTS

MRX-2843 synergized with venetoclax to reduce AML cell density in vitro. A neural network classifier based on drug exposure and target gene expression predicted drug synergy and growth inhibition in AML with high accuracy. Combination monovalent liposomal drug formulations delivered defined drug ratios intracellularly and recapitulated synergistic drug activity. The magnitude and frequency of synergistic responses were both maintained and improved following drug formulation in a genotypically diverse set of primary AML bone marrow specimens.

CONCLUSIONS

We developed a nanoscale combination drug formulation that exploits ectopic expression of MERTK tyrosine kinase and dependency on BCL-2 family proteins for leukemia cell survival in pediatric AML and infant ALL cells. We demonstrate ratiometric drug delivery and synergistic cell killing in AML, a result achieved by a systematic, generalizable approach of combination drug screening and nanoscale formulation that may be extended to other drug pairs or diseases in the future.

Development of constitutively synergistic nanoformulations to enhance chemosensitivity in T-cell leukemia

Advances in multiagent chemotherapy have led to recent improvements in survival for patients with acute lymphoblastic leukemia (ALL); however, a significant fraction do not respond to frontline chemotherapy or later relapse with recurrent disease, after which long-term survival rates remain low. To develop new, effective treatment options for these patients, we conducted a series of high-throughput combination drug screens to identify chemotherapies that synergize in a lineage-specific manner with MRX-2843, a small molecule dual MERTK and FLT3 kinase inhibitor currently in clinical testing for treatment of relapsed/refractory leukemias and solid tumors. Using experimental and computational approaches, we found that MRX-2843 synergized strongly-and in a ratio-dependent manner-with vincristine to inhibit both B-ALL and T-ALL cell line expansion. Based on these findings, we developed multiagent lipid nanoparticle formulations of these drugs that not only delivered defined drug ratios intracellularly in T-ALL, but also improved anti-leukemia activity following drug encapsulation. Synergistic and additive interactions were recapitulated in primary T-ALL patient samples treated with MRX-2843 and vincristine nanoparticle formulations, suggesting their clinical relevance. Moreover, the nanoparticle formulations reduced disease burden and prolonged survival in an orthotopic murine xenograft model of early thymic precursor T-ALL (ETP-ALL), with both agents contributing to therapeutic activity in a dose-dependent manner. In contrast, nanoparticles containing MRX-2843 alone were ineffective in this model. Thus, MRX-2843 increased the sensitivity of ETP-ALL cells to vincristine in vivo. In this context, the additive particles, containing a higher dose of MRX-2843, provided more effective disease control than the synergistic particles. In contrast, particles containing an even higher, antagonistic ratio of MRX-2843 and vincristine were less effective. Thus, both the drug dose and the ratio-dependent interaction between MRX-2843 and vincristine significantly impacted therapeutic activity in vivo. Together, these findings present a systematic approach to high-throughput combination drug screening and multiagent drug delivery that maximizes the therapeutic potential of combined MRX-2843 and vincristine in T-ALL and describe a novel translational agent that could be used to enhance therapeutic responses to vincristine in patients with T-ALL. This broadly generalizable approach could also be applied to develop other constitutively synergistic combination products for the treatment of cancer and other diseases.

Pediatric T-cell acute lymphoblastic leukemia blast signature and MRD associated immune environment changes defined by single cell transcriptomics analysis

Different driver mutations and/or chromosomal aberrations and dysregulated signaling interactions between leukemia cells and the immune microenvironment have been implicated in the development of T-cell acute lymphoblastic leukemia (T-ALL). To better understand changes in the bone marrow microenvironment and signaling pathways in pediatric T-ALL, bone marrows collected at diagnosis (Dx) and end of induction therapy (EOI) from 11 patients at a single center were profiled by single cell transcriptomics (10 Dx, 5 paired EOI, 1 relapse). T-ALL blasts were identified by comparison with healthy bone marrow cells. T-ALL blast-associated gene signature included SOX4, STMN1, JUN, HES4, CDK6, ARMH1 among the most significantly overexpressed genes, some of which are associated with poor prognosis in children with T-ALL. Transcriptome profiles of the blast cells exhibited significant inter-patient heterogeneity. Post induction therapy expression profiles of the immune cells revealed significant changes. Residual blast cells in MRD+ EOI samples exhibited significant upregulation (P < 0.01) of PD-1 and RhoGDI signaling pathways. Differences in cellular communication were noted in the presence of residual disease in T cell and hematopoietic stem cell compartments in the bone marrow. Together, these studies generate new insights and expand our understanding of the bone marrow landscape in pediatric T-ALL.

Siglec-15 Promotes Evasion of Adaptive Immunity in B-cell Acute Lymphoblastic Leukemia

Siglec-15 (Sig15) has been implicated as an immune checkpoint expressed in solid tumor-infiltrating macrophages and is being targeted in clinical trials with mAbs to normalize the tumor immune microenvironment and stimulate antitumor immunity. However, the role of Sig15 in hematologic malignancies remains undefined. Sig15 mRNA and protein expression levels in hematologic malignancies were determined from publicly available databases, cell lines, and primary patient samples. Human B-cell acute lymphoblastic leukemia (B-ALL) cell lines were used to identify signaling pathways involved in the regulation of Sig15 expression. Secreted/soluble Sig15 and cytokine levels were measured from the plasma of children with leukemia and healthy controls. Knockdown and knockout of Siglec15 in a murine model of B-ALL was used to evaluate the effect of leukemia-derived Sig15 on the immune response to leukemia. We observed pathologic overexpression of Sig15 in a variety of hematologic malignancies, including primary B-ALL samples. This overexpression was driven by NFκB activation, which also increased the surface localization of Sig15. Secreted/soluble Sig15 was found to circulate at elevated levels in the plasma of children with B-ALL and correlated with an immune-suppressive cytokine milieu. Genetic inhibition of Sig15 in murine B-ALL promoted clearance of the leukemia by the immune system and a marked reversal of the immune-privileged leukemia bone marrow niche, including expanded early effector CD8⁺ T cells and reduction of immunosuppressive cytokines. Thus, Sig15 is a novel, potent immunosuppressive molecule active in leukemia that may be targeted therapeutically to activate T lymphocytes against leukemia cells.

Significance

We demonstrate that Sig15 is overexpressed in hematologic malignancies driven by NFκB, is required for immune evasion in a mouse model of leukemia, and, for the first time, that it circulates at high levels in the plasma of children with leukemia.

Abstract 2335: Targeting a positive feedback loop of MERTK and STAT3 during macrophage differentiation may provide anti-tumor immune function

MERTK tyrosine kinase expression is upregulated upon monocyte to macrophage differentiation. This receptor tyrosine kinase enables macrophages to efficiently clear apoptotic cells to maintain tissue homeostasis. Activation of MERTK in macrophages during efferocytosis promotes an immunosuppressive phenotype, which is hijacked by tumors to inhibit anti-tumor immunity. Further, this immunosuppressive phenotype in MERTK expressing macrophages in the tumor microenvironment is reversed using MERTK selective inhibitors, suggesting that inhibiting MERTK expression or activity in the tumor microenvironment may be of therapeutic benefit in the treatment of cancer. In an attempt to further understand the mechanism of MERTK upregulation in macrophages, we treated the monocytic leukemia cell line THP1 with PMA to differentiate the cells from a monocytic morphology to an adherent macrophage-like morphology. Following differentiation of the THP-1 cells, MERTK upregulation was confirmed by western blot and flow cytometry. In a similar fashion, treatment with of murine bone marrow derived monocytic cells with PMA induced MERTK expression. Proteomics cytokine array analysis also revealed increased levels of multiple chemokines including MCP-1 and RANTES. Treatment of THP-1 cells with a pan STAT inhibitor in the presence of PMA abrogated the induction of MERTK expression, suggesting a critical role for STAT pathways in the regulation of MERTK expression during monocyte differentiation to macrophages. Specifically, the STAT3 pathway was found to be important in MERTK regulation, as treatment with a STAT3 selective inhibitor was sufficient to abrogate MERTK expression in the presence of PMA treatment. Single cell sequencing of the immune cells in the bone marrow demonstrated Stat3 expression in monocytic lineage cells. Furthermore, both CD11C+Ly6c+CD45+ population and MERTK+CD11C+Ly6c+CD45+ populations were lower in Stat3 -/- bone marrow cells treated with PMA relative to the untreated cells. Collectively, these data suggest that MERTK expression during macrophage maturation may be mediated by STAT3 activation. Previously published data have also demonstrated that STAT3 can be activated downstream of MERTK activation. Thus, we propose that MERTK and STAT3 form a positive feedback loop during macrophage maturation. Treatment with either a MERTK and/or STAT3 inhibitor may interfere with this feedback pathway, potentially reversing an immunosuppressive phenotype in the macrophages in the tumor microenvironment.

Citation Format: Dan Yan, Justus M. Huelse, Swati Sharma Bhasin, Manoj Bhasin, Deborah DeRyckere, Douglas K. Graham. Targeting a positive feedback loop of MERTK and STAT3 during macrophage differentiation may provide anti-tumor immune function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2335.

Machine learning classifier approaches for predicting response to RTK-type-III inhibitors demonstrate high accuracy using transcriptomic signatures and ex vivo data

Motivation

The application of machine learning (ML) techniques in the medical field has demonstrated both successes and challenges in the precision medicine era. The ability to accurately classify a subject as a potential responder versus a nonresponder to a given therapy is still an active area of research pushing the field to create new approaches for applying machine-learning techniques. In this study, we leveraged publicly available data through the BeatAML initiative. Specifically, we used gene count data, generated via RNA-seq, from 451 individuals matched with ex vivo data generated from treatment with RTK-type-III inhibitors. Three feature selection techniques were tested, principal component analysis, Shapley Additive Explanation (SHAP) technique and differential gene expression analysis, with three different classifiers, XGBoost, LightGBM and random forest (RF). Sensitivity versus specificity was analyzed using the area under the curve (AUC)-receiver operating curves (ROCs) for every model developed.

Results

Our work demonstrated that feature selection technique, rather than the classifier, had the greatest impact on model performance. The SHAP technique outperformed the other feature selection techniques and was able to with high accuracy predict outcome response, with the highest performing model: Foretinib with 89% AUC using the SHAP technique and RF classifier. Our ML pipelines demonstrate that at the time of diagnosis, a transcriptomics signature exists that can potentially predict response to treatment, demonstrating the potential of using ML applications in precision medicine efforts.

Availability and implementation

https://github.com/UD-CRPL/RCDML.

Supplementary information

Supplementary data are available at Bioinformatics Advances online.

Constitutively synergistic multiagent drug formulations targeting MERTK, FLT3, and BCL-2 for treatment of AML

Although high-dose, multi-agent chemotherapy has improved leukemia survival rates in recent years, treatment outcomes remain poor in high-risk subsets, including acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) in infants. Development of new, more effective therapies for these patients is therefore an urgent, unmet clinical need. To address this challenge, we developed a nanoscale combination drug formulation that exploits ectopic expression of MERTK tyrosine kinase and dependency on BCL-2 family proteins for leukemia cell survival in pediatric AML and MLL- rearranged precursor B-cell ALL (infant ALL). In a novel, high-throughput combination drug screen, the MERTK/FLT3 inhibitor MRX-2843 synergized with venetoclax and other BCL-2 family protein inhibitors to reduce AML cell density in vitro . Neural network models based on drug exposure and target gene expression were used to identify a classifier predictive of drug synergy in AML. To maximize the therapeutic potential of these findings, we developed a combination monovalent liposomal drug formulation that maintains ratiometric drug synergy in cell-free assays and following intracellular delivery. The translational potential of these nanoscale drug formulations was confirmed in a genotypically diverse set of primary AML patient samples and both the magnitude and frequency of synergistic responses were not only maintained but were improved following drug formulation. Together, these findings demonstrate a systematic, generalizable approach to combination drug screening, formulation, and development that maximizes therapeutic potential, was effectively applied to develop a novel nanoscale combination therapy for treatment of AML, and could be extended to other drug combinations or diseases in the future.

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.

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.

Association of race/ethnicity with innate immune tumor microenvironment of children with B-acute lymphoblastic leukemia

BACKGROUND

Black and Hispanic children with B-acute lymphoblastic leukemia (B-ALL) experience worse outcomes compared with their non-Hispanic white (NHW) counterparts. Immune-based approaches have begun to transform the therapeutic landscape in children with B-ALL. Recent studies identified several alterations in both innate and adaptive immune cells in children with B-ALL that may impact disease risk and outcome. However, the impact of racial/ethnic background on immune microenvironment is less studied, as children of minorities background have to date been severely under-represented in such studies.

METHODS

We performed high-dimensional analysis of bone marrow from 85 children with newly diagnosed B-ALL (Hispanic=29, black=18, NHW=38) using mass cytometry with 40 and 38-marker panels.

RESULTS

Race/ethnicity-associated differences were most prominent in the innate immune compartment. Hispanic patients had significantly increased proportion of distinct mature CD57 +T-bet+DR+ NK cells compared with other cohorts. These differences were most apparent within standard risk (SR) patients with Hispanic SR patients having greater numbers of CD57 +NK cells compared with other cohorts (43% vs 26% p=0.0049). Hispanic and Black children also had distinct alterations in myeloid cells, with a significant increase in a population of non-classical activated HLA-DR +CD16+myeloid cells, previously implicated in disease progression, compared with NHW counterparts. Racial background also correlated with altered expression of inhibitory checkpoint PD-L1 on myeloid cells.

CONCLUSION

There are surprisingly substantial race/ethnicity-based differences in innate immune cells of children with newly diagnosed B-ALL. These differences urge the need to enhance accrual of children from minorities background in immunetherapy trials and may impact their outcome following such therapy.

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 &lt; 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 &lt; 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.

Enrichment of high-risk innate immune cells in Hispanic and Black children with B-acute lymphoblastic leukemia

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Background: Black and Hispanic children with B-acute lymphoblastic leukemia (B-ALL) experience worse outcomes compared to their non-Hispanic white (NHW) counterparts. Immune-based therapies have improved the outcomes of children with B-ALL, however, impact of racial/ethnic background on immune microenvironment is less studied. Methods: BM from 61 children with newly diagnosed B-ALL(Hispanic = 21, Black = 17, NHW = 23) was obtained via the Aflac Biorepository. High-dimensional analysis was performed utilizing single cell mass cytometry with 61 markers to characterize T, NK and myeloid cells. Data was analyzed using Cytobank and high-dimensional visualization platforms such as ViSNE. Clinical data including self-reported race/ethnicity and NCI-risk classification were obtained for all samples. Results: Multi-dimensional analysis was carried out for each cell population to dissect race/ethnicity-associated differences. ViSNE clustering of NK cells identified 3 different NK populations, including a distinct population of mature CD57+ NK cells with Tbethi, HLADRhi, granzymeBhi, CD27- phenotype. The distribution of NK subsets was highly impacted by race/ethnicity. Hispanic (H) patients had higher proportions of CD57+ mature NK cells when compared with other groups, (40 ± 4% vs 33 ± 2%; p = 0.03) with pronounced differences apparent within standard risk (SR) patients. H-SR had higher proportion of CD57+ NK cells compared to other SR patients (mean H-SR 43.4 ± 5.87% vs 26.3 ± 2.87% p = 0.0049). ViSNE clustering of myeloid cells identified 5 clusters based on patterns of cell surface markers, including a distinct CD11c+CD16+DRhi inflammatory/non-classical myeloid population. Further analysis showed that NHW-SR patients have significantly lower proportions of CD16+DR+ myeloid cells compared to Hispanic, Black and NHW-HR patients (mean NHW-SR 3.67 ± 2.56% vs Others 10.8 ± 7.87% p = 0.0394). Notably, a phenotypically similar population has recently been implicated in leukemic progression in preclinical models (Witkowski et al, Cancer Cell 2020). In contrast to innate cells, T cell clusters were broadly comparable between different racial/ethnic groups. Conclusions: These studies provide detailed single-cell proteomic analysis and highlight the impact of racial/ethnic background on immune microenvironment in pediatric B-ALL. Our data identify differences in innate immunity with enrichment of high-risk immune-populations in Hispanic and Black children and depletion of inflammatory myeloid populations in NHW-SR children with B-ALL. These variations may contribute to the observed differences in outcomes and may impact application of immune therapies in racial/ethnic groups.

Obesity-induced galectin-9 is a therapeutic target in B-cell acute lymphoblastic leukemia

The incidence of obesity is rising with greater than 40% of the world’s population expected to be overweight or suffering from obesity by 2030. This is alarming because obesity increases mortality rates in patients with various cancer subtypes including leukemia. The survival differences between lean patients and patients with obesity are largely attributed to altered drug pharmacokinetics in patients receiving chemotherapy; whereas, the direct impact of an adipocyte-enriched microenvironment on cancer cells is rarely considered. Here we show that the adipocyte secretome upregulates the surface expression of Galectin-9 (GAL-9) on human B-acute lymphoblastic leukemia cells (B-ALL) which promotes chemoresistance. Antibody-mediated targeting of GAL-9 on B-ALL cells induces DNA damage, alters cell cycle progression, and promotes apoptosis in vitro and significantly extends the survival of obese but not lean mice with aggressive B-ALL. Our studies reveal that adipocyte-mediated upregulation of GAL-9 on B-ALL cells can be targeted with antibody-based therapies to overcome obesity-induced chemoresistance.

Obesity has been reported to promote tumourigenesis and chemoresistance but the underlying mechanisms are not completely understood. Here, the authors show that adipocytes induce Galectin-9 (GAL-9) expression in B-acute lymphoblastic leukaemia (B-ALL) cells which leads to chemoresistance and antibody-mediated blockade of GAL-9 increases survival in preclinical B-ALL murine models.

Synthetic Matrix Scaffolds Engineer the In Vivo Tumor Immune Microenvironment for Immunotherapy Screening

Immunotherapy has emerged as one of the most powerful anti-cancer therapies but is stymied by the limits of existing preclinical models with respect to disease latency and reproducibility. Additionally, the influence of differing immune microenvironments within tumors observed clinically and associated with immunotherapeutic resistance cannot be tuned to facilitate drug testing workflows without changing model system or laborious genetic approaches. To address this testing platform gap in the immune oncology drug development pipeline, the authors deploy engineered biomaterials as scaffolds to increase tumor formation rate, decrease disease latency, and diminish variability of immune infiltrates into tumors formed from murine mammary carcinoma cell lines implanted into syngeneic mice. By altering synthetic gel formulations that reshape infiltrating immune cells within the tumor, responsiveness of the same tumor model to varying classes of cancer immunotherapies, including in situ vaccination with a molecular adjuvant and immune checkpoint blockade, diverge. These results demonstrate the significant role the local immune microenvironment plays in immunotherapeutic response. These engineered tumor immune microenvironments therefore improve upon the limitations of current breast tumor models used for immune oncology drug screening to enable immunotherapeutic testing relevant to the variability in tumor immune microenvironments underlying immunotherapeutic resistance seen in human patients.

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.

UNC5293, a potent, orally available and highly MERTK-selective inhibitor

Inhibition of MER receptor tyrosine kinase (MERTK) causes direct tumor cell killing and stimulation of the innate immune response. Therefore, MERTK has been identified as a therapeutic target in a wide variety of human tumors. Clinical trials targeting MERTK have recently been initiated, however, none of these drugs are MERTK-specific. Herein, we present the discovery of a highly MERTK-selective inhibitor UNC5293 (24). UNC5293 has subnanomolar activity against MERTK with an excellent Ambit selectivity score (S₅₀ (100 nM) = 0.041). It mediated potent and selective inhibition of MERTK in cell-based assays. Furthermore, it has excellent mouse PK properties (7.8 h half-life and 58% oral bioavailability) and was active in bone marrow leukemia cells in a murine model.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

Risk-associated alterations in marrow T cells in pediatric leukemia

Current management of childhood leukemia is tailored based on disease risk determined by clinical features at presentation. Whether properties of the host immune response impact disease risk and outcome is not known. Here, we combine mass cytometry, single cell genomics, and functional studies to characterize the BM immune environment in children with B cell acute lymphoblastic leukemia and acute myelogenous leukemia at presentation. T cells in leukemia marrow demonstrate evidence of chronic immune activation and exhaustion/dysfunction, with attrition of naive T cells and TCF1⁺ stem-like memory T cells and accumulation of terminally differentiated effector T cells. Marrow-infiltrating NK cells also exhibit evidence of dysfunction, particularly in myeloid leukemia. Properties of immune cells identified distinct immune phenotype–based clusters correlating with disease risk in acute lymphoblastic leukemia. High-risk immune signatures were associated with expression of stem-like genes on tumor cells. These data provide a comprehensive assessment of the immune landscape of childhood leukemias and identify targets potentially amenable to therapeutic intervention. These studies also suggest that properties of the host response with depletion of naive T cells and accumulation of terminal-effector T cells may contribute to the biologic basis of disease risk. Properties of immune microenvironment identified here may also impact optimal application of immune therapies, including T cell–redirection approaches in childhood leukemia.

In high risk pediatric leukemia, naive and stem-like memory T cells are depleted and terminally differentiated effector T cells accumulate.

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 &gt;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&lt;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&lt;0.05), OCI-AML5 (p&lt;0.05), and NB4 (p&lt;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&lt;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&lt;0.01). This effect was more pronounced in cultures treated with the combination therapy (p&lt;0.05-p&lt;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&lt;0.0001). Treatment with doxorubicin and MERTK/ROCKi therapy also decreased OCI-AML5 cell density compared to MERTK/ROCKi therapy alone (p&lt;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.

BPTF regulates growth of adult and pediatric high-grade glioma through the MYC pathway

High-grade gliomas (HGG) afflict both children and adults and respond poorly to current therapies. Epigenetic regulators have a role in gliomagenesis, but a broad, functional investigation of the impact and role of specific epigenetic targets has not been undertaken. Using a two-step, in vitro/in vivo epigenomic shRNA inhibition screen, we determine the chromatin remodeler BPTF to be a key regulator of adult HGG growth. We then demonstrate that BPTF knockdown decreases HGG growth in multiple pediatric HGG models as well. BPTF appears to regulate tumor growth through cell self-renewal maintenance, and BPTF knockdown leads these glial tumors toward more neuronal characteristics. BPTF's impact on growth is mediated through positive effects on expression of MYC and MYC pathway targets. HDAC inhibitors synergize with BPTF knockdown against HGG growth. BPTF inhibition is a promising strategy to combat HGG through epigenetic regulation of the MYC oncogenic pathway.

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.

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 &gt;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&lt;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 &gt;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&lt;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 &gt;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&lt;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&lt;0.001) and decreased spleen weight by 64% (p&lt;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&lt;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&lt;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.

Abstract 2879: TYRO3 is a potential therapeutic target in pediatric acute myeloid leukemia

BACKGROUND: While outcomes for children with acute myeloid leukemia have improved dramatically in recent years, a subset of patients continue to have poor prognosis with &lt;60% 5-year overall survival and the high intensity chemotherapy used to treat these patients is associated with significant short and long-term side effects. New less toxic and more effective treatments are needed. TYRO3, a member of the TAM (TYRO3, AXL, MERTK) family of receptor tyrosine kinases, is overexpressed in many types of cancer and functions to promote tumor cell survival and/or proliferation, metastasis, and resistance to chemotherapy. In addition, higher levels of TYRO3 expression are associated with decreased overall survival in patients with colorectal, hepatocellular or breast cancer. We hypothesize that TYRO3 plays a critical role in survival and proliferation of AML cells and may be a therapeutic target in pediatric AML.

METHODS: The TARGET database was utilized to determine expression of mRNAs encoding the TAM family kinases and their ligands, GAS6 and PROS1, in pediatric AML patient samples. Samples (n=145) were categorized into two groups with “high” expression greater than the median or “low” expression less than or equal to the median and overall survival was compared between the two groups using the log-rank test. Expression of TAM family kinases and ligands was determined in AML cell lines by immunoblot. Derivatives of the NB4 AML cell line with shRNA-mediated inhibition of TYRO3 were generated and the impact of TYRO3 inhibition on MERTK levels was determined by immunoblot.

RESULTS: High levels of TYRO3 or PROS1 mRNAs in patient samples were independently associated with decreased overall survival in children with AML (p&lt;0.001 and p= 0.042, respectively). Patients with leukemias that expressed higher levels of TYRO3 had a median survival of 870 days with only 40% of patients surviving for 10 years. In contrast, 70% of patients with leukemias that expressed lower levels of TYRO3 were long-term survivors. A hazard ratio of 2.60 (1.56-4.33 95% confidence interval, p&lt;0.001) was associated with high levels of TYRO3. Similarly, patients whose leukemias had higher levels of PROS1 expression had a median survival of 1229 days with 46% 10-year overall survival compared to 62% 10-year survival in patients whose tumors express low levels of PROS1. High levels of PROS1 were associated with a hazard ratio of 1.65 (1.01-2.68, p=0.044). TYRO3 protein was detected in all 9 AML cells lines tested; however, MERTK, GAS6, and PROS1 were differentially expressed. Inhibition of TYRO3 in NB4 cells using shRNA resulted in upregulation of MERTK.

CONCLUSIONS: TYRO3 was ubiquitously expressed in all AML cell lines tested and higher levels of TYRO3 and its ligand PROS1 were associated with significantly poorer prognosis in children with AML. These data suggest a role for TYRO3 in pediatric AML and implicate TYRO3 as a therapeutic target in this context.

Citation Format: Sherri K. Smart, Manali Rupji, Bhakti Dwivedi, Jeanne Kowalski, Deborah DeRyckere, Douglas K. Graham. TYRO3 is a potential therapeutic target in pediatric 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 2879.

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.

The Emerging Role of TYRO3 as a Therapeutic Target in Cancer

The TAM family (TYRO3, AXL, MERTK) tyrosine kinases play roles in diverse biological processes including immune regulation, clearance of apoptotic cells, platelet aggregation, and cell proliferation, survival, and migration. While AXL and MERTK have been extensively studied, less is known about TYRO3. Recent studies revealed roles for TYRO3 in cancer and suggest TYRO3 as a therapeutic target in this context. TYRO3 is overexpressed in many types of cancer and functions to promote tumor cell survival and/or proliferation, metastasis, and resistance to chemotherapy. In addition, higher levels of TYRO3 expression have been associated with decreased overall survival in patients with colorectal, hepatocellular, and breast cancers. Here we review the physiological roles for TYRO3 and its expression and functions in cancer cells and the tumor microenvironment, with emphasis on the signaling pathways that are regulated downstream of TYRO3 and emerging roles for TYRO3 in the immune system. Translational agents that target TYRO3 are also described.

Highly Selective MERTK Inhibitors Achieved by a Single Methyl Group

Although all kinases share the same ATP binding pocket, subtle differences in the residues that form the pocket differentiate individual kinases' affinity for ATP competitive inhibitors. We have found that by introducing a single methyl group, the selectivity of our MERTK inhibitors over another target, FLT3, was increased up to 1000-fold (compound 31). Compound 19 was identified as an in vivo tool compound with subnanomolar activity against MERTK and 38-fold selectivity over FLT3 in vitro. The potency and selectivity of 19 for MERTK over FLT3 were confirmed in cell-based assays using human cancer cell lines. Compound 19 had favorable pharmacokinetic properties in mice. Phosphorylation of MERTK was decreased by 75% in bone marrow leukemia cells from mice treated with 19 compared to vehicle-treated mice.

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.

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.

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.

Abstract 1082: MerTK promotes resistance to irreversible EGFR TKIs by activation of the PI3K-AKT pathway in NSCLCs expressing wild-type EGFR

Lung cancer is the leading cause of cancer-related death with poor survival rates worldwide. Non-small cell lung cancer (NSCLC) accounts for 85% of all lung cancers and 60% of these have overexpression of wild-type EGFR (wtEGFR), which portends a poor prognosis. In addition, in a recent clinical trial 6 of 12 lung cancer patients whose tumors acquired a resistance-conferring T790M EGFR mutation during treatment with an EGFR TKI reverted to wtEGFR after treatment with Rociletinib/CO-1686, an irreversible EGFR TKI that is selective for the T790M mutant. These data suggest that targeting wtEGFR may improve treatment outcomes; however, to date single agents that target wtEGFR have not been effective in large scale clinical trials and new ways to target wtEGFR in this context are needed. Our laboratory previously identified the MerTK receptor tyrosine kinase as a potential therapeutic target in NSCLC and developed MRX-2843 as a novel MerTK-selective small molecule tyrosine kinase inhibitor with favorable properties for clinical translation. Irrespective of driver oncogene status, treatment with a MerTK inhibitor yields potent anti-tumor effects in NSCLC cell culture models and blocks tumor growth in xenografts of the MerTK-dependent wtEGFR-expressing A549 cell line. In an attempt to more potently block A549 lung cancer cell proliferation, we screened a library of 378 kinase inhibitors that are in various stages of development and identified synergy between MRX-2843 and multiple irreversible EGFR TKIs, including CO-1686 and Osimertinib/AZD-9291. Further, we found that wtEGFR and MerTK were frequently co-expressed and co-immunoprecipitated with each other in NSCLC cancer cell lysates. Synergistic inhibition of cell expansion was observed in a spectrum of NSCLC cell lines with wtEGFR expression treated with the combination therapy, including H1299 (NRAS mutation), H157 (KRAS mutation), H3122 (ALK fusion), and Colo699 (FGFR1 overexpression). On a mechanistic level, combined treatment with 1μM CO-1686 and 100 nM MRX-2843 dramatically inhibited phosphorylation of both MerTK and EGFR and downstream signaling through the PI3K-AKT and MAPK-ERK pathways, while treatment with equivalent doses of either single agent did not efficiently inhibit MerTK, EGFR or downstream signaling. Additionally, CO-1686 mediated synergistic inhibition of A549 expansion in combination with a PI3K or AKT inhibitor, suggesting a role for PI3K-AKT activation downstream of MerTK contributing to the resistance of wtEGFR NSCLCs to irreversible EGFR TKIs. Taken together, our data provide rationale for a novel strategy for treatment of NSCLC with wtEGFR overexpression by combining MRX-2843 and an irreversible EGFR inhibitor.

Citation Format: Dan Yan, Xiaodong Wang, Stephen V. Frye, Shelton H. Earp, Deborah DeRyckere, Douglas K. Graham. MerTK promotes resistance to irreversible EGFR TKIs by activation of the PI3K-AKT pathway in NSCLCs expressing wild-type EGFR [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1082. doi:10.1158/1538-7445.AM2017-1082

MERTK inhibition in the leukemia microenvironment promotes a therapeutic immune response to acute lymphoblastic leukemia

T cell immunotherapy is effective in some, but not all cancers, and targeting the immunosuppressive, innate immune tumor microenvironment is one key to further advancement. During efferocytosis, activation of MERTK tyrosine kinase receptor in innate immune cells suppresses proinflammatory cytokines, stimulates immunosuppressive cytokines and upregulates co-inhibitory ligands. MERTK has also been validated as a therapeutic target in the tumor microenvironment in solid tumor models. To investigate immunoregulatory roles for MERTK in leukemia, we utilized pharmacologic and genetic inhibition of MERTK in a MERTK-negative syngeneic murine model of acute lymphoblastic leukemia (ALL). Treatment with MRX-2843, a potent, orally bioavailable, small molecule MERTK inhibitor, significantly reduced leukemic burden and prolonged median survival in wildtype mice (38 days post-treatment vs. 24 days in vehicle-treated mice, p&gt;0.01). Similarly, overall survival was increased more than four-fold in Mertk−/− mice transplanted with leukemia (&gt;80 days vs. 21 days in wild-type mice, p&gt;0.0001), suggesting that the observed therapeutic activity is immune-mediated. Indeed, treatment with MRX-2843 abrogated expression of the immune co-inhibitory ligands PD-L1 and PD-L2 on CD11b+ monocytes/macrophages in the ALL microenvironment. Although T cells do not express MERTK, expression of the cognate PD-1 receptor was also reduced on CD8+ T cells and the frequency of FOXP3+CD4+ regulatory T cells was decreased in the leukemia microenvironment following MRX-2843 treatment. These data demonstrate several mechanisms by which MERTK can suppress anti-leukemia immunity and validate MERTK as a novel immunotherapeutic target in ALL.

Discovery of Macrocyclic Pyrimidines as MerTK-Specific Inhibitors

Macrocycles have attracted significant attention in drug discovery recently. In fact, a few de novo designed macrocyclic kinase inhibitors are currently in clinical trials with good potency and selectivity for their intended target. In this study, we successfully engaged a structure-based drug design approach to discover macrocyclic pyrimidines as potent Mer tyrosine kinase (MerTK)-specific inhibitors. An enzyme-linked immunosorbent assay (ELISA) in 384-well format was employed to evaluate the inhibitory activity of macrocycles in a cell-based assay assessing tyrosine phosphorylation of MerTK. Through structure-activity relationship (SAR) studies, analogue 11 [UNC2541; (S)-7-amino-N-(4-fluorobenzyl)-8-oxo-2,9,16-triaza-1(2,4)-pyrimidinacyclohexadecaphane-1-carboxamide] was identified as a potent and MerTK-specific inhibitor that exhibits sub-micromolar inhibitory activity in the cell-based ELISA. In addition, an X-ray structure of MerTK protein in complex with 11 was resolved to show that these macrocycles bind in the MerTK ATP pocket.