Antigen presentation deficiency, mesenchymal differentiation, and resistance to immunotherapy in the murine syngeneic CT2A tumor model

Background

The GL261 and CT2A syngeneic tumor lines are frequently used as immunocompetent orthotopic mouse models of human glioblastoma (huGBM) but demonstrate distinct differences in their responses to immunotherapy.

Methods

To decipher the cell-intrinsic mechanisms that drive immunotherapy resistance in CT2A-luc and to define the aspects of human cancer biology that these lines can best model, we systematically compared their characteristics using whole exome and transcriptome sequencing, and protein analysis through immunohistochemistry, Western blot, flow cytometry, immunopeptidomics, and phosphopeptidomics.

Results

The transcriptional profiles of GL261-luc2 and CT2A-luc tumors resembled those of some huGBMs, despite neither line sharing the essential genetic or histologic features of huGBM. Both models exhibited striking hypermutation, with clonal hotspot mutations in RAS genes (Kras p.G12C in GL261-luc2 and Nras p.Q61L in CT2A-luc). CT2A-luc distinctly displayed mesenchymal differentiation, upregulated angiogenesis, and multiple defects in antigen presentation machinery (e.g. Tap1 p.Y488C and Psmb8 p.A275P mutations) and interferon response pathways (e.g. copy number losses of loci including IFN genes and reduced phosphorylation of JAK/STAT pathway members). The defect in MHC class I expression could be overcome in CT2A-luc by interferon-γ treatment, which may underlie the modest efficacy of some immunotherapy combinations. Additionally, CT2A-luc demonstrated substantial baseline secretion of the CCL-2, CCL-5, and CCL-22 chemokines, which play important roles as myeloid chemoattractants.

Conclusion

Although the clinical contexts that can be modeled by GL261 and CT2A for huGBM are limited, CT2A may be an informative model of immunotherapy resistance due to its deficits in antigen presentation machinery and interferon response pathways.

Quinazolinones as allosteric fourth-generation EGFR inhibitors for the treatment of NSCLC

The C797S mutation confers resistance to covalent EGFR inhibitors used in the treatment of lung tumors with the activating L858R mutation. Isoindolinones such as JBJ-4-125-02 bind in an allosteric pocket and are active against this mutation, with high selectivity over wild-type EGFR. The most potent examples we developed from that series have a potential chemical instability risk from the combination of the amide and phenol groups. We explored a scaffold hopping approach to identify new series of allosteric EGFR inhibitors that retained good potency in the absence of the phenol group. The 5-F quinazolinone 34 demonstrated tumor regression in an H1975 efficacy model upon once daily oral dosing at 25 mg/kg.

Milademetan is a highly potent MDM2 inhibitor in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine carcinoma of the skin with 2 etiologies. Merkel cell polyomavirus (MCPyV) integration is present in about 80% of all MCC. Virus-positive MCC (MCCP) tumors have few somatic mutations and usually express WT p53 (TP53). By contrast, virus-negative MCC (MCCN) tumors present with a high tumor mutational burden and predominantly UV mutational signature. MCCN tumors typically contain mutated TP53. MCCP tumors express 2 viral proteins: MCPyV small T antigen and a truncated form of large T antigen. MCPyV ST specifically activates expression of MDM2, an E3 ubiquitin ligase of p53, to inhibit p53-mediated tumor suppression. In this study, we assessed the efficacy of milademetan, a potent, selective, and orally available MDM2 inhibitor in several MCC models. Milademetan reduced cell viability of WT p53 MCC cell lines and triggered a rapid and sustained p53 response. Milademetan showed a dose-dependent inhibition of tumor growth in MKL-1 xenograft and patient-derived xenograft models. Here, along with preclinical data for the efficacy of milademetan in WT p53 MCC tumors, we report several in vitro and in vivo models useful for future MCC studies.

An allosteric inhibitor against the therapy-resistant mutant forms of EGFR in non-small cell lung cancer

Epidermal growth factor receptor (EGFR) therapy using small-molecule tyrosine kinase inhibitors (TKIs) is initially efficacious in patients with EGFR-mutant lung cancer, although drug resistance eventually develops. Allosteric EGFR inhibitors, which bind to a different EGFR site than existing ATP-competitive EGFR TKIs, have been developed as a strategy to overcome therapy-resistant EGFR mutations. Here we identify and characterize JBJ-09-063, a mutant-selective allosteric EGFR inhibitor that is effective across EGFR TKI-sensitive and resistant models, including those with EGFR T790M and C797S mutations. We further uncover that EGFR homo- or heterodimerization with other ERBB family members, as well as the EGFR L747S mutation, confers resistance to JBJ-09-063, but not to ATP-competitive EGFR TKIs. Overall, our studies highlight the potential clinical utility of JBJ-09-063 as a single agent or in combination with EGFR TKIs to define more effective strategies to treat EGFR-mutant lung cancer.

Abstract P203: Milademetan is a potent, murine double minute 2 (MDM2) inhibitor, highly active in TP53 wild-type (p53WT) Merkel cell carcinoma (MCC) cell lines

Background: MCC is a highly aggressive neuroendocrine carcinoma of the skin with a poor overall prognosis. Current treatment options include surgery and radiation therapy for local MCC tumor and checkpoint blockade therapy for advanced disease. However, primary and acquired resistance can reduce response to therapy. Around 80% of all MCCs have integrated copies of Merkel cell polyomavirus (MCV). Virus-Positive MCC (MCCP) tumors typically contain few somatic mutations and express wild type (WT) p53 (TP53). In contrast, Virus-Negative MCC (MCCN) tumors have a high mutational burden, with a predominantly UV mutational signature. MCCP express two viral proteins: MCV small T antigen (ST) and a truncated form of large T antigen (LT). The MCV ST recruits MYCL and EP400 to form the SLaP complex that specifically activates several genes contributing to oncogenesis. A direct target of the SLaP complex is MDM2, an E3 ligase for p53. In p53WT MCCP, SLaP-dependent activation of MDM2 inhibits the tumor suppressive functions of p53. Here, we analyzed the efficacy of milademetan (RAIN-32), a potent, selective, and orally available MDM2 inhibitor, in MCC. Experimental procedures: Established MCCP cell lines with WT (MKL-1, WaGa, PeTa) or mutant p53 (MS-1), were treated with vehicle or several concentrations of milademetan or another MDM2 inhibitor AMG232 and cell viability was analyzed. Similar viability assays were also performed using MKL-1 p53 knockout (KO) cells and two newly established p53WT primary MCC cell lines. The p53 response in MCC cells treated with vehicle, milademetan or AMG232 was assessed by western blot (WB) analysis of p53 and its downstream effectors p21, PUMA and PARP-1. For in vivo testing, an initial tolerability study was conducted with once daily administration of milademetan by oral gavage in NSG mice. Milademetan activity was evaluated in MKL-1 xenograft and patient-derived xenograft (PDX) models in NSG mice. Pharmacodynamic markers of response in tumor samples from mice treated with vehicle or milademetan is analyzed by q-PCR and WB analysis. Summary of Results: Nanomolar concentrations of milademetan reduced cell viability of p53WT but not p53mutantMCCP MCC cell lines. Milademetan treatment increased levels of p21, PUMA and cleaved PARP-1 in MCCP cell lines MKL-1 and WaGa. Using p53 KO MKL-1 cells, we show that the effect of milademetan on MCC cell viability is p53 dependent. In vitro data show that milademetan is more potent than AMG232 in the context of MCC. Tolerability studies show that mice safely tolerate 100 mg/kg of milademetan. Milademetan treatment in the MKL-1 xenograft tumor model shows a dose-dependent response in tumor growth inhibition. In the DFMC-33043 PDX model, milademetan significantly inhibited tumor growth. Conclusion: Milademetan is a promising drug effective against p53WT MCC cell lines, xenograft, and PDX models. Ongoing in vivo testing of the anti-cancer cell activity of milademetan will provide evidence for clinical exploration of milademetan in MCC refractory to current therapies.

Citation Format: Varsha Ananthapadmanabhan, Aine Knott, Kara M. Soroko, Prafulla C. Gokhale, Vijaya Tirunagaru, Robert Doebele, James A. DeCaprio. Milademetan is a potent, murine double minute 2 (MDM2) inhibitor, highly active in TP53 wild-type (p53WT) Merkel cell carcinoma (MCC) cell lines [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P203.

ERK Inhibitor LY3214996-Based Treatment Strategies for RAS-Driven Lung Cancer

RAS gene mutations are the most frequent oncogenic event in lung cancer. They activate multiple RAS-centric signaling networks among them the MAPK, PI3K, and RB pathways. Within the MAPK pathway, ERK1/2 proteins exert a bottleneck function for transmitting mitogenic signals and activating cytoplasmic and nuclear targets. In view of disappointing antitumor activity and toxicity of continuously applied MEK inhibitors in patients with KRAS-mutant lung cancer, research has recently focused on ERK1/2 proteins as therapeutic targets and on ERK inhibitors for their ability to prevent bypass and feedback pathway activation. Here, we show that intermittent application of the novel and selective ATP-competitive ERK1/2 inhibitor LY3214996 exerts single-agent activity in patient-derived xenograft (PDX) models of RAS-mutant lung cancer. Combination treatments were well tolerated and resulted in synergistic (ERKi plus PI3K/mTORi LY3023414) and additive (ERKi plus CDK4/6i abemaciclib) tumor growth inhibition in PDX models. Future clinical trials are required to investigate if intermittent ERK inhibitor-based treatment schedules can overcome toxicities observed with continuous MEK inhibition and-equally important-to identify biomarkers for patient stratification.

Concurrent Dexamethasone Limits the Clinical Benefit of Immune Checkpoint Blockade in Glioblastoma

PURPOSE

Dexamethasone, a uniquely potent corticosteroid, is frequently administered to patients with brain tumors to decrease tumor-associated edema, but limited data exist describing how dexamethasone affects the immune system systemically and intratumorally in patients with glioblastoma (GBM), particularly in the context of immunotherapy.

EXPERIMENTAL DESIGN

We evaluated the dose-dependent effects of dexamethasone when administered with programmed cell death 1 (PD-1) blockade and/or radiotherapy in immunocompetent C57BL/6 mice with syngeneic GL261 and CT-2A GBM tumors. Clinically, the effect of dexamethasone on survival was evaluated in 181 patients with isocitrate dehydrogenase (IDH) wild-type GBM treated with PD-(L)1 blockade, with adjustment for relevant prognostic factors.

RESULTS

Despite the inherent responsiveness of GL261 to immune checkpoint blockade, concurrent dexamethasone administration with anti-PD-1 therapy reduced survival in a dose-dependent manner. Concurrent dexamethasone also abrogated survival following anti-PD-1 therapy with or without radiotherapy in immune-resistant CT-2A models. Dexamethasone decreased T-lymphocyte numbers by increasing apoptosis, in addition to decreasing lymphocyte functional capacity. Myeloid and natural killer cell populations were also generally reduced by dexamethasone. Thus, dexamethasone appears to negatively affect both adaptive and innate immune responses. As a clinical correlate, a retrospective analysis of 181 consecutive patients with IDH wild-type GBM treated with PD-(L)1 blockade revealed poorer survival among those on baseline dexamethasone. Upon multivariable adjustment with relevant prognostic factors, baseline dexamethasone administration was the strongest predictor of poor survival [reference, no dexamethasone; <2 mg HR, 2.16; 95% confidence interval (CI), 1.30-3.68; P = 0.003 and ≥2 mg HR, 1.97; 95% CI, 1.23-3.16; P = 0.005].

CONCLUSIONS

Our preclinical and clinical data indicate that concurrent dexamethasone therapy may be detrimental to immunotherapeutic approaches for patients with GBM.

Combined Targeting of the BRD4-NUT-p300 Axis in NUT Midline Carcinoma by Dual Selective Bromodomain Inhibitor, NEO2734

NUT midline carcinoma (NMC) is a rare, aggressive subtype of squamous carcinoma that is driven by the BRD4-NUT fusion oncoprotein. BRD4, a BET protein, binds to chromatin through its two bromodomains, and NUT recruits the p300 histone acetyltransferse (HAT) to activate transcription of oncogenic target genes. BET-selective bromodomain inhibitors have demonstrated on-target activity in patients with NMC, but with limited efficacy. P300, like BRD4, contains a bromodomain. We show that combining selective p300/CBP and BET bromodomain inhibitors, GNE-781 and OTX015, respectively, induces cooperative depletion of MYC and synergistic inhibition of NMC growth. Treatment of NMC cells with the novel dual p300/CBP and BET bromodomain-selective inhibitor, NEO2734, potently inhibits growth and induces differentiation of NMC cells in vitro; findings that correspond with potentiated transcriptional effects from combined BET and p300 bromodomain inhibition. In three disseminated NMC xenograft models, NEO2734 provided greater growth inhibition, with tumor regression and significant survival benefit seen in two of three models, compared with a lead clinical BET inhibitor or "standard" chemotherapy. Our findings provide a strong rationale for clinical study of NEO2734 in patients with NMC. Moreover, the synergistic inhibition of NMC growth by CBP/p300 and BET bromodomain inhibition lays the groundwork for greater mechanistic understanding of the interplay between p300 and BRD4-NUT that drives this cancer.