RNA helicases are essential, dynamic proteins that consume ATP to unwind and rearrange RNA. These activities place RNA helicases in roles as central mediators of signaling, especially those pathways dependent on RNA metabolism. Their binding of both ATP and RNA, as well as limited literature examples of small molecule ligands, support the tractability of RNA helicases. We employed structure-based virtual screening to rationally identify ligands that occupy the ATP-binding site of three human DExD/H-box RNA helicases: MDA5, LGP2, and DDX1. Following alignment of the well conserved nucleotide binding pocket for these RNA helicases, we docked and refined the list of potential ligands from the MolPort-2022-03 ligand library of ∼3.7 million members. A chemical lead with favorable solubility emerged from the 144 purchased compounds, which were evaluated in MDA5, LGP2, and DDX1 ATPase assays as well as corresponding SPR assays. It was found to be ATP un-competitive for MDA5 and to have similar affinity for the three RNA helicases. Analogs of the lead compound were designed to optimize the potency and selectivity of the scaffold, yielding both pan-helicase inhibitors and other analogs that are biased toward MDA5 inhibition.

Immune checkpoint blockade (ICB) has demonstrated substantial clinical benefits in cancer treatment. However, many patients fail to achieve durable responses and eventually develop resistance. This study aimed to identify new strategies for predicting immunotherapy response and overcoming ICB resistance via PET imaging in tumor-bearing models. We used a MER proto-oncogene tyrosine kinase (MerTK) targeting radio-tracer, [68Ga]Ga-MerTKi, to noninvasively monitor MerTK expression level with PET/CT scanning in multiple murine cancer models. An ICB-resistant B16F10 melanoma tumor model was established, and we further verified that the combined treatment with MerTK inhibitor UNC2025 and anti-mouse PD-1 recombinant mAb effectively restored the ICB treatment efficacy. We first demonstrated that elevated MerTK expression is strongly correlated to [68Ga]Ga-MerTKi high uptake, poor response to ICB, and the subsequent development of resistance. We found that tumors with low MerTK expression responded well to immunotherapy, while for those with high MerTK expression did not. Importantly, [68Ga]Ga-MerTK PET predicted tumor sensitivity toward ICB therapy before treatment begins, making it a promising strategy to improve patient stratification and ICB treatment optimization. Moreover, the tracer allowed us to monitor MerTK expression level during treatment, which may provide guidance on treatment strategies down the road. Our findings indicate that [68Ga]Ga-MerTKi PET can potentially identify patients who may benefit from ICB therapy alone and who may need combination therapy. This observation supports the use of MerTK PET as a tool to guide more personalized treatment in cancer immunotherapy.

Obvious chemical substitutions could be used to generate powerful new composition-of-matter patents, highlighting the need for patent and antitrust policy reforms that better separate strategic pursuit of exclusivity from true therapeutic innovation.

BACKGROUND The development of drug resistance remains a major challenge in clinical practice for patients with gastrointestinal stromal tumors (GISTs). AIM To investigate whether a cyclin-dependent kinase 6 (CDK6) inhibitor can partially reverse ripretinib-resistant GIST cells and to explore the underlying mechanisms. METHODS Ripretinib-resistant GIST cell lines were established through continuous exposure to increased concentrations of ripretinib. The effects of a CDK6 inhibitor, alone or in combination with ripretinib, on cell viability were evaluated using the Cell Counting Kit-8. Wound healing and invasion indices were used to assess cell migration and invasion capabilities. Cell cycle distribution and apoptosis were analyzed by flow cytometry. The expression levels of genes and proteins related to the cell cycle, apoptosis, and drug resistance pathways were measured by real-time quantitative polymerase chain reaction and western blot, respectively. RESULTS CDK6 was significantly upregulated in ripretinib-resistant GIST cells at both the mRNA and protein levels. The CDK6 inhibitor palbociclib had no significant effect on parental GIST cells but induced G1 phase arrest, promoted apoptosis, and suppressed proliferation in resistant cells. Its combination with ripretinib produced a synergistic effect that reversed ripretinib resistance. Moreover, protein kinase B (AKT) expression and phosphorylation were elevated in resistant cells, and AKT inhibition significantly reduced CDK6 expression. The CDK6 inhibitor also modulated the expression of apoptosis-related proteins (B-cell lymphoma-2-associated X protein, B-cell lymphoma-2, cleaved caspase-9) and cell cycle-related proteins (Cyclin D, p21) in resistant cells. CONCLUSION CDK6 mediates ripretinib resistance in GIST by regulating the AKT signaling pathway. Targeting CDK6 can partially reverse ripretinib-resistant GIST cells. The combination of CDK6 inhibitors and ripretinib represents a promising novel therapeutic strategy for patients with ripretinib-resistant GIST.

Abstract MERTK receptor tyrosine kinase mediates pro-survival signaling and therapeutic resistance and is a potential therapeutic target in acute myeloid leukemia (AML); however, like many therapies, AML interactions with the bone marrow stroma confer resistance to MERTK inhibition, limiting therapeutic efficacy. Rational combination strategies that suppress compensatory signaling in physiologically relevant model systems are needed to overcome microenvironment-driven drug resistance in AML. Here, we demonstrate stromal-mediated ERK activation as a mechanism of resistance to the MERTK inhibitor, MRX-2843. Co-culture with stromal cell lines (Hs27 or Hs5 fibroblasts) or mesenchymal stem cells (MSCs) protected AML cell lines (NOMO-1, OCI-AML5, KASUMI-1) from induction of cell death in response to treatment with MRX-2843 (e.g. no co-culture vs. co-culture: Kasumi-1 + Hs27: 67.6% vs. 31.6% dead, OCI-AML5 + Hs5: 79.4% vs. 36.6%, NOMO-1 + MSC: 77.2% vs. 34.5%). Induction of γH2AX, an indicator of cell death, was also decreased in stromal co-cultures treated with MRX-2843 compared to AML mono-cultures. Mechanistically, MERTK expression was increased in AML cells cultured with stromal cells. ERK phosphorylation was also induced in stromal cell co-cultures and was refractory to inhibition by MRX-2843. Moreover, treatment with the MEK inhibitors PD0329501 or pimasertib in combination with MRX-2843 abrogated ERK phosphorylation and restored induction of γH2AX and anti-leukemia activity in the presence of stromal cells, confirming dependence on MEK/ERK signaling in AML cells for stromal-mediated survival. To better model the impact of the bone marrow microenvironment on anti-leukemia activity, we utilized a novel three-dimensional vascular mesenchymal organoid system that has greater stromal cell complexity and recapitulates many features of the bone marrow observed in human AML (e.g. architecture, cell-cell interactions, cytokine/chemokine production). Combined treatment with MRX-2843 and pimasertib significantly enhanced therapeutic efficacy in organoids established from NOMO-1 and KASUMI-1 AML cell lines, as evidenced by increased γH2AX expression compared to MRX-2843 or pimasertib monotherapies. Notably, these findings were recapitulated in organoids established from a MERTK-expressing patient-derived AML xenograft, reinforcing the translational relevance of the combination therapy. In preliminary dose-finding studies, concurrent treatment with MRX-2843 and pimasertib was well-tolerated in mice. Collectively, these data identify MEK/ERK signaling as a mechanism of stromal-mediated resistance to MERTK inhibition and establish combined treatment with MRX-2843 and a MEK inhibitor as a promising strategy for effective treatment of AML. Citation Format: Chloe Hope, Katherine Minson, Eleana Vasileiadi, Madeline G. Higgins, Alejandro De Janon, Shuichi Takayama, Xiaodong Wang, Steven Frye, H. Shelton Earp, Douglas K. Graham, Deborah DeRykere. MEK inhibition overcomes stromal-mediated resistance to a MERTK targeted therapy in AML co-cultures and vascularized mesenchymal organoids [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6490.

The Hippo signaling pathway prevents unchecked cell growth, coordinates apoptosis, and preserves proper organ function. Dysregulation of this pathway has been implicated in a myriad of diseases, particularly in cancer. The YAP (Yes-associated protein)-TEAD (TEA domain transcription factor) complex, the key transcriptional downstream effector of the Hippo pathway, hence stands out as an appealing target for therapeutic intervention. In this study, we developed a high-throughput screening (HTS) assay leveraging phase separation principles and found that the US Food and Drug Administration-approved clinical drug cobimetinib is a potent inhibitor of the YAP-TEAD complex. Cocrystallization studies of cobimetinib with TEAD showed that cobimetinib bound to the TEAD lipid pocket and disrupted TEAD palmitoylation. Cobimetinib could overcome resistance to mitogen-activated protein kinase kinase 1/2 inhibitors and to the first-line drug sorafenib in vivo. In addition, cobimetinib suppressed tumor growth and tumorigenesis associated with hyperactivated YAP-TEAD activities in a mouse model of lung cancer. Furthermore, it bolstered the efficacy of the first-line drugs sorafenib and lenvatinib in inhibiting both hepatocellular carcinoma tumor growth and tumorigenesis. These findings establish a strategy for identifying and refining inhibitors of the YAP-TEAD complex in the treatment of cancers driven by aberrant YAP-TEAD activity.

mTORC1 integrates growth factor and nutrient signals to regulate cellular metabolism, yet there are no metabolites known to directly regulate mTORC1 activity in cells. Cryo-EM studies revealed that inositol hexakisphosphate (IP6) associates with the FAT domain of mTOR, suggesting that inositol phosphates may directly modulate mTOR activity. We previously showed that higher-order inositol phosphates enhance mTORC1 kinase activity and stability in vitro. Here, we investigated whether inositol phosphate metabolism regulates mTORC1 signaling in pancreatic β-cells. Suppression or acute inhibition of inositol phosphate multikinase (IPMK), as well as knockdown of inositol trisphosphate kinase 1 (ITPK1), selectively reduced cellular IP5 levels without altering IP6 and resulted in impaired basal and insulin-stimulated mTORC1 signaling, particularly under physiological glucose and low growth factor conditions. Combined inhibition of IPMK and ITPK1 nearly abolished IP5 and reduced IP6, demonstrating that these enzymes compensate to supply IP5 for IP6 synthesis. Importantly, depletion of IP5 did not impair PI3K/Akt activation but accelerated termination of the mTORC1 signal, indicating a role for IP5 in stabilizing the active mTORC1 complex. Reduction of inositol phosphate levels did not prevent insulin- or glucose-induced mTORC1 activation, revealing that IP5 primarily regulates signal persistence rather than initiation. Together, these findings identify IP5 as a metabolic regulator that prolong mTORC1 activity in β-cells, providing a mechanism by which cellular metabolic state modulates sustained mTORC1 signaling.

Abstract While many patients with acute myeloid leukemia (AML) have an initial favorable response to treatment with standard of care venetoclax, a BCL2 inhibitor, and azacitidine, a DNA methyltransferase inhibitor, sustained remissions remain elusive and new therapies are urgently needed. We identified MERTK (MER Receptor Tyrosine Kinase) as a potential therapeutic target in AML and developed MRX-2843, a first-in-class dual MERTK/FLT3 kinase inhibitor that is currently being tested in leukemia patients. Here we describe a novel therapy that combines MRX-2843 with venetoclax and azacitidine to provide enhanced therapeutic effects in preclinical AML models. In human AML cell line cultures (KG-1, OCI-AML5, and NOMO-1), treatment with the 3-drug combination (MRX-2843/venetoclax/azacitidine) reduced cell density compared to venetoclax/azacitidine. Mathematical modeling using the fractional product method revealed a synergistic interaction between MRX-2843 and venetoclax/azacitidine in 2 of the 3 cell lines and an additive interaction in the other. In all 3 cell lines, MRX-2843 synergized with venetoclax/azacitidine to increase induction of cell death compared to venetoclax/azacitidine alone. Furthermore, in all 3 cell lines, the 3-drug combination reduced levels of c-MYC protein compared to venetoclax/azacitidine. These findings reveal a potential mechanism of the enhanced therapeutic effects mediated by the 3-drug combination. The triple combination also provided enhanced therapeutic effects against the KG1 cell line in an AML organoid model system that mimics many features of the bone marrow microenvironment, including chemoprotection. These data support addition of MRX-2843 to current standard of care venetoclax/azacitidine to better target bone marrow disease. Indeed, the 3-drug regimen significantly reduced bone marrow disease burden and prolonged survival in immune-compromised mice inoculated with the KG1 AML cell line. After the first treatment cycle (28 days), the fraction of human CD45+ leukemia cells in the bone marrow was significantly reduced in mice treated with the 3-drug combination (8.5±5%, n=4) compared to vehicle (67±8%, n=5, p<0.001), MRX-2843 (41±9%, n=6, p=0.0451), or venetoclax/azacitidine (40%±8%, n=6, p=0.0482). Moreover, mouse survival was significantly prolonged by the triple combination (median survival > 150 days, 59.1% survival after 150 days of treatment) compared to MRX-2843 (median survival = 76.5 days, 0% survival at 150 days, p<0.0001) or venetoclax/azacitidine (median survival = 104.5 days, 4.6% survival at 150 days, p<0.001). Together these findings (i) implicate co-administration of MRX-2843, venetoclax and azacitidine as an effective strategy to treat AML, (ii) reveal a potential mechanistic basis for this strategy, and (iii) support evaluation of this novel 3-drug combination in future clinical trials. Citation Format: Aashis Thapa, Chloe Hope, Edward B. Henderson, Austre Y. Schiaffino Bustamante, Gianna Branella, Alejandro De Janon, Sunil Raikar, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Shuichi Takayama, Deborah DeRyckere, Douglas K. Graham. MERTK inhibitor MRX-2843 sensitizes AML to venetoclax and azacitidine in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3896.

Introduction: Acute myeloid leukemia (AML) is the most aggressive form of leukemia, and therapies that can sustain long-term cures in AML patients remain elusive. Venetoclax, a BCL2 inhibitor, and azacitidine, a DNA methyltransferase inhibitor, are commonly administered together in newly diagnosed AML patients who are not candidates for intensive chemotherapy, including elderly patients. While many patients have initial responses, long-term sustained remission is rare. Hence, there is an urgent need for new therapies that can significantly improve clinical outcomes in AML. We identified MERTK (MER Receptor Tyrosine Kinase) as a novel therapeutic target in AML. MERTK is aberrantly expressed in over 80% of AML patient samples and MERTK inhibition is therapeutically effective against AML cells in vitro and in vivo. Here we describe development of a novel therapy for AML that combines MRX-2843 (an orally bioavailable dual MERTK and FLT3 inhibitor) with venetoclax and azacitidine to provide enhanced therapeutic effects. Methods: MERTK expressing human AML cell lines (NOMO-1, OCI-AML5, KG-1) were treated in vitro with MRX-2843, venetoclax, and/or azacitidine single agents and combinations (venetoclax/azacitidine or MRX-2843/venetoclax/azacitidine) and relative cell numbers were determined using CellTiter-Glo assay. Drug interactions were assessed by mathematical modeling using the fractional product method. Alternatively, cells were stained with Annexin-V and propidium iodide, and apoptotic and dead cells were detected by flow cytometry. For in vivo studies, xenografts were established by intravenous injection of KG-1 cells into NOD-SCID-gamma mice. Mice were randomized to groups (n=10-12 per group) and drug treatments were initiated 24-27 days after leukemia inoculation. MRX-2843 (65 mg/kg) and venetoclax (40 mg/kg) were administered once daily by oral gavage, and azacitidine (2.5 mg/kg) was administered once daily during the first 5 days of each 28-day treatment cycle by intraperitoneal injection. Mice were treated for a total of 150 days encompassing 5 consecutive 28-day cycles. Health status was monitored and survival was determined. Two independent studies were performed. In the second study, bone marrow disease burden (% human CD45+ cells) was assessed on treatment days 28 and 73 using flow cytometry (n=3-6 per group). Results: In cultures of all 3 AML cell lines, treatment with the 3-drug combination (MRX-2843/venetoclax/azacitidine) reduced cell numbers compared to treatment with MRX-2843 monotherapy or standard-of-care venetoclax/azacitidine alone. For instance, in KG-1 cultures, treatment with MRX-2843 or venetoclax/azacitidine reduced cell densities by 55±5% and 60±3%, respectively, compared to vehicle while the triple combination mediated a 96±1% reduction (p<0.0001). Mathematical modeling revealed that the interaction between MRX-2843 and venetoclax/azacitidine was synergistic in 2 of the 3 cell lines and additive in the other. Treatment with the 3-drug combination also enhanced leukemia cell killing compared to MRX-2843 or venetoclax/azacitidine in all 3 cell lines. For example, in OCI-AML5 cultures, treatment with the 3-drug combination induced apoptosis and cell death in 80±1% of cells, compared to 34±2% (p=0.0036) and 71±0.5% (p=0.0212) in MRX-2843-treated and venetoclax/azacitidine-treated cultures, respectively. These findings translated to a mouse AML xenograft model. In 2 independent studies, the triple combination significantly prolonged mouse survival (59.1% survival after 150 days of treatment) compared to MRX-2843 monotherapy (0% survival, 76.5 days median survival, p < 0.0001) or venetoclax/azacitidine (4.6% survival, 104.5 days median survival, p < 0.001). Furthermore, the fraction of leukemic blasts in the bone marrow was significantly reduced after the first treatment cycle in mice treated with the 3-drug combination (8.5±5%, n=4) compared to mice treated with vehicle (67±8%, n=5, p<0.001), MRX-2843 (41±9%, n=6, p = 0.0451) or venetoclax/azacitidine (40%±8%, n=6, p=0.0482). Conclusions: Treatment with MRX-2843 sensitized AML cells to standard-of-care venetoclax/azacitidine, leading to increased AML cell death, more effective targeting of AML cells in the bone marrow, and prolonged survival in mouse models. These findings implicate combined treatment with MRX-2843, venetoclax and azacitidine as an effective therapeutic strategy with potential to improve outcomes for patients with AML.

TYRO3 plays a critical role in platelet aggregation as a platelet response amplifier. Selective inhibition of TYRO3 may provide therapeutic benefits for treating thrombosis and related diseases without increasing bleeding risk. We employed a structure-based approach and discovered a novel and potent TYRO3 inhibitor UNC9426 (12) with an excellent Ambit selectivity score (S50 (1.0 μM) = 0.026) and favorable pharmacokinetic properties in mice. Treatment with UNC9426 reduced platelet aggregation without increasing bleeding time and blocked TYRO3-dependent functions in tumor cells and macrophages, implicating its utility for multiple indications.

The kinase activity of human inositol phosphate multikinase (IPMK) is required for the synthesis of higher-order inositol phosphate signaling molecules, regulation of gene expression, and control of the cell cycle. Here, we report a novel series of highly potent IPMK inhibitors. The first-generation IPMK inhibitor 1 (UNC7437) decreased cellular proliferation and tritiated inositol phosphate levels in metabolically labeled human U251-MG glioblastoma cells. It also impacted the transcriptome of these cells, selectively regulating 993 genes enriched in cancer, epithelial-to-mesenchymal transition (EMT), and inflammatory and viral infection pathways, consistent with anticancer growth activity. Extensive optimization of 1 led to 14 (UNC9750) with improved pharmacokinetic properties. Compound 14 inhibited cellular accumulation of InsP5, the direct product of IPMK kinase activity, while having no effect on either InsP6 or InsP7 levels. These studies suggest that rapid chemical inhibition of IPMK induces a novel InsP5 metabolic signature, providing new biological insights into inositol phosphate metabolism and signaling.

MER tyrosine kinase (MERTK) is highly expressed on the protective and reparative phenotype of microglia, which is in response to neuroregeneration following the neuronal damage induced by multiple sclerosis (MS). A specific imaging tool, which can differentiate anti-inflammatory and immunosuppressive responses of microglia, could be highly beneficial for the early detection and clinical management of MS. To identify potential 18F-radiotracers to image anti-inflammatory responses of microglia, herein a series of fluorinated pyrimidine-5-carboxamide derivatives were prepared from a database of MERTK ligands. Several potent MERTK ligands were discovered with promising selectivity profiles over other off-targets (AXL, TYRO3 and FLT3). A cell-based assay was employed to assess cellular inhibitory MERTK potency, which may be regarded as being particularly relevant to an in vivo imaging situation. This study reports the discovery of several new, potent, and selective fluorinated compounds against MERTK, paving the way for PET tracer development to image protective microglial phenotype in MS patients.

Intracellular bacterial reservoirs contribute to antibiotic treatment failure by fostering metabolically dormant persister cells that are highly tolerant to killing. However, strategies to effectively target intracellular persister cells remain limited. Here we developed a high-throughput screen to identify compounds that modulate the metabolic activity of intracellular Staphylococcus aureus. The identified compound, KL1, increases intracellular bacterial metabolic activity and sensitizes persister populations of S. aureus to antibiotics, without causing cytotoxicity or bacterial outgrowth. KL1 also exhibits adjuvant activity against intramacrophage Salmonella enterica Typhimurium and Mycobacterium tuberculosis, as well as in murine infection models of S. aureus and S. Typhimurium infection. Transcriptomic analysis and further mechanistic studies reveal that KL1 modulates host immune response genes and suppresses the production of reactive species in host macrophages, alleviating a key inducer of antibiotic tolerance. Our findings highlight the potential to target intracellular persisters by stimulating their metabolism. There are two major problems in the field of antimicrobial chemotherapy-antibiotic resistance and antibiotic tolerance. Antibiotic tolerance has been frequently connected with poor treatment outcomes in the clinic. Unlike antibiotic resistance, which permits bacterial growth in the presence of drugs, antibiotic tolerance allows bacteria to withstand multiple antibiotics for prolonged periods. The extended survival of tolerant bacteria further predisposes them to evolve antibiotic resistance over time, underscoring the critical need to address antibiotic tolerance. Host interactions have been shown to induce persister formation in numerous pathogens, with the production of reactive oxygen and nitrogen species heavily implicated in the collapse of bacterial metabolic activity and entry into an antibiotic-tolerant state. Yet, tools to study or target this process remain limited. Here we developed a high-throughput screen to identify compounds that modulate intracellular S. aureus metabolism, leading to the discovery of KL1, a host-directed compound that sensitizes persisters to antibiotic killing.

TAM kinases play dual roles in tumor cells and the innate immune system. While they have redundant functions, the TAM kinases are differentially required in specific contexts. Therefore, inhibition of specific TAM kinases or pairs of TAM kinases will be desirable in different tumor types. We exploited the relatively more diversified back pocket of TAM kinases to modulate the polypharmacology of small molecule inhibitors and discovered several inhibitors with distinct selectivity profiles. The lead compound 45 (UNC8212) displayed potent inhibitory activities toward the TAM family. Its target engagement was confirmed by NanoBRET and cell-based assays. It also had favorable pharmacokinetic properties via intravenous and intraperitoneal routes.

The TAM (TYRO3, AXL, MERTK) family of receptor tyrosine kinases has roles in oncogenesis and innate immunity, but the relative importance of the family members can differ in different contexts and between tumor types or individual tumors. Dual TYRO3 and MERTK inhibition may be advantageous for treatment of diseases or in tumors that are dependent on their coordinated action. Here, we report the discovery of the first potent dual TYRO3/MERTK inhibitor, UNC9435 (44). UNC9435 has 46-fold and 120-fold selectivity of MERTK over AXL and FLT3, respectively, and selectively against a panel of 30 other kinases. TYRO3 and MERTK inhibitory activities were confirmed by NanoBRET assays in HEK293 cells, with <0.51 nM EC50 values for both enzymes and >3000-fold selectivity over AXL. UNC9435 also inhibited TYRO3, MERTK, and downstream oncogenic signaling in cancer cells and reduced colony formation in non-small cell lung cancer cultures, indicating its potential as a novel cancer therapeutic.

The interaction between the membrane (M) protein and the nucleocapsid (N) protein of coronaviruses plays a crucial role in virus assembly and morphogenesis. Previous studies indicate that one M-N interaction occurs between M protein and the carboxy-terminus of N protein. However, the mechanistic details of M-N interactions remain unclear. Here, we present a complex structure of an N protein carboxy-terminal peptide bound to M protein from Pipistrellus bat coronavirus HKU5. The structure shows that the M-N peptide binding site includes a "horizontal" groove located between the carboxy-terminal domain and the transmembrane domain of M protein. Combined with molecular docking and binding analysis, our results provide structural insight into the binding mechanism between M and N proteins of a coronavirus.

The TAM (TYRO3, AXL, MERTK) family receptor tyrosine kinases are important for clearing apoptotic cells and maintaining tissue homeostasis. They promote an immunosuppressive environment during cell clearance, which can be hijacked by tumor cells to promote immune evasion. This study aimed to determine how MERTK inhibition affects immune cells within the tumor microenvironment (TME) in lung cancer using a subcutaneous Kras-mutant murine lung cancer tumor model in wild-type (WT), Axl knock-out (KO), or Mertk KO mice. Tumor growth was inhibited in Mertk KO mice compared to WT mice, while Axl KO mice had no significant change in tumor growth. The anti-tumor effect observed in Mertk KO mice was lost in Mertk KO scid mice, which lack functional T and B cells, suggesting MERTK as an immunotherapeutic target in Kras-mutant lung cancer. MRX-2843 is a novel first-in-class MERTK-selective inhibitor that is currently being tested in phase 1/1b clinical trials. The murine cancer cell line used in this study did not express detectable MERTK and was unresponsive to MRX-2843 in vitro. However, in vivo treatment with MRX-2843 led to dose-dependent inhibition of tumor growth. Conditional knockout of Mertk in LysM expressing (i.e. myeloid) cells resulted in reduced tumor growth compared to tumors in WT mice. These data suggest that MERTK functions in myeloid cells to inhibit anti-tumor immunity, although contributions of homozygous LysM KO could not be eliminated in this study. Additionally, MERTK was only expressed in WT tumor myeloid cells ; myeloid cell MERTK was not detected in the TME in conditional Mertk KO mice. Furthermore, tumors from Mertk KO mice had evidence of increased antigen-presenting capacity with significantly increased numbers of tumor-associated macrophages expressing high levels of MHC-II and increased incidence of dendritic cells. Together these findings suggest that MERTK inhibits anti-tumor immunity by regulating myeloid cell functions and implicate MERTK as an immunotherapeutic target in Kras-mutant non-small cell lung cancer. Dan Yan, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Deborah DeRyckere, Douglas K. Graham. MERTK inhibition regulates tumor-associated myeloid cell phenotypes and potentiates host-versus-lung cancer immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3488.

Phosphate (Pi) serves countless metabolic pathways and is involved in macromolecule synthesis, energy storage, cellular signaling, and bone maintenance. Herein, we describe the coordination of Pi uptake and efflux pathways to maintain mammalian cell Pi homeostasis. We discover that XPR1, the presumed Pi efflux transporter, separately supervises rates of Pi uptake. This direct, regulatory interplay arises from XPR1 being a binding partner for the Pi uptake transporter PiT1, involving a predicted transmembrane helix/extramembrane loop in XPR1, and its hitherto unknown localization in a subset of intracellular LAMP1-positive puncta (named “XLPVs”). A pharmacological mimic of Pi homeostatic challenge is sensed by the inositol pyrophosphate IP8, which functionalizes XPR1 to respond in a temporally hierarchal manner, initially adjusting the rate of Pi efflux, followed subsequently by independent modulation of PiT1 turnover to reset the rate of Pi uptake. These observations generate a unifying model of mammalian cellular Pi homeostasis, expanding opportunities for therapeutic intervention.