CT26 Tumor-bearing Mice Research Articles

Response to anti-PD-1 and anti-LAG-3 immune checkpoint blockade (ICB) is associated with induction of pro-inflammatory Tregs

Abstract T cells upregulate the immune checkpoint, LAG-3, which is associated with reduced T cell effector function following repeated antigen exposure. Further, high LAG-3 expression on Tregs has been reported in patients with varying cancer types; thus, we hypothesized that the combination of aLAG-3 with another common ICB, aPD-1, would synergize to potently reinvigorate T cells to promote tumor regression and increase survival. Combined aPD-1/aLAG-3 immunotherapy significantly improved the survival of CT26 (BALB/c; colon carcinoma)and MCA-205 (C57BL/6; sarcoma) tumor-bearing mice compared to monotherapy. In-depth analysis of tumor-infiltrating lymphocytes (TIL) revealed that aPD-1/aLAG-3 therapy significantly increased the overall percentage, cytotoxicity (granzyme A), and cytokine production (TNF-a and IFN-g) of CD8+ TIL, and increased CD4+ Teff/Treg ratios compared to aPD-1 or aLAG-3 alone. These effects were enriched in responders to aPD-1/aLAG-3 therapy, which were designated as those exhibiting decreased tumor size on the day of harvest compared to maximum tumor growth post-implant. Surprisingly, Tregs isolated from tumors responding to aPD-1/aLAG-3 displayed increased proliferation, secretion of pro-inflammatory cytokines (TNF-a and IFN-g), and increased LAG-3 expression. In summary, these data suggest that aPD-1/aLAG-3 immunotherapy increased CD8+ TIL exhibiting enhanced effector function, increased CD4+ Teff/Treg ratios, and induced Treg pro-inflammatory responses. Together, these positive immunological changes led to a more immune stimulatory tumor microenvironment capable of supporting tumor regression and significantly improved tumor-free survival.

Gut Microbiome Influences the Efficacy of PD-1 Antibody Immunotherapy on MSS-Type Colorectal Cancer via Metabolic Pathway.

Colorectal cancer (CRC) appears to be rather refractory to checkpoint blockers except the patient with deficient in mismatch repair (dMMR). Therefore, new advances in the treatment of most mismatch repair proficiency (pMMR) (also known as microsatellite stability, MSS) type of CRC patients are considered to be an important clinical issue associated with programmed death 1 (PD-1) inhibitors. In the present study, we evaluated the effects of gut microbiome of MSS-type CRC tumor-bearing mice treated with different antibiotics on PD-1 antibody immunotherapy response. Our results confirmed that the gut microbiome played a key role in the treatment of CT26 tumor-bearing mice with PD-1 antibody. After PD-1 antibody treatment, the injection of antibiotics counteracted the efficacy of PD-1 antibody in inhibiting tumor growth when compared with the Control group (mice were treated with sterile drinking water). Bacteroides_sp._CAG:927 and Bacteroidales_S24-7 were enriched in Control group. Bacteroides_sp._CAG:927, Prevotella_sp._CAG: 1031 and Bacteroides were enriched in Coli group [mice were treated with colistin (2 mg/ml)], Prevotella_sp._CAG:485 and Akkermansia_muciniphila were enriched in Vanc group [mice were treated with vancomycin alone (0.25 mg/ml)]. The metabolites were enriched in the glycerophospholipid metabolic pathway consistent with the metagenomic prediction pathway in Vanc group, Prevotella_sp._CAG:485 and Akkermansia may maintain the normal efficacy of PD-1 antibody by affecting the metabolism of glycerophospholipid. Changes in gut microbiome leaded to changes in glycerophospholipid metabolism level, which may affect the expression of immune-related cytokines IFN-γ and IL-2 in the tumor microenvironment, resulting in a different therapeutic effect of PD-1 antibody. Our findings show that changes in the gut microbiome affect the glycerophospholipid metabolic pathway, thereby regulating the therapeutic potential of PD-1 antibody in the immunotherapy of MSS-type CRC tumor-bearing mice.

Abstract A68: A casein kinase 2 inhibitor disorders myeloid cell differentiation by blocking CCAAT/enhancer-binding protein-alpha signaling pathway in tumor microenvironment

Abstract Myeloid cells are major components of the tumor microenvironment, which are known to suppress antitumor immunity. Casein kinase 2 (CK2), a serine/threonine protein kinase with diverse intracellular protein substrates, regulates several signaling pathways involved in tumor progression and cell differentiation. We examined the efficacy of a novel CK2 inhibitor on modulating the myeloid cells in the tumor microenvironment. Although CK2 inhibitors BMS-595 and BMS-211 moderately inhibited the tumor growth in LLC, 4T1, MC38 and CT26 tumor-bearing mice, these inhibitors drastically enhanced the antitumor efficacy of anti-CTLA4 antibody with 60% to 90% of complete rejection. Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) and macrophages in spleens, and tumor-associated macrophages (TAM) were found to be decreased in LLC tumor-bearing mice treated with BMS-595 for 2 weeks. Murine hematopoietic progenitor cells (HPC) from bone marrow and human progenitor cells from cord blood were cultured with BMS-595 to evaluate the differentiation. BMS-595 treatment dramatically decreased the proportion of granulocytic cells, and this decrease was not caused by the direct killing nor apoptosis. Therefore, the genes regulated by transcription factors, IRF8, C/EBPα and C/EBPβ, which are involved in PMN-MDSC differentiation, were evaluated. Many genes controlled by C/EBPα were downregulated by the BMS-595 treatment in HPCs. Active p42 subunit of C/EBPα was revealed to be reduced by the BMS-595 treatment in HPCs, which are inversely correlated with the increased level of dominant negative p30 subunit of C/EBPα, in both the cytoplasm and nucleus of HPCs. Our results suggest that CK2 inhibition leads to a reduction of immune-suppressive PMN-MDSC and TAM by inhibiting C/EBPα activity, resulting in an enormous augmentation of antitumor efficacy of immunotherapy. Citation Format: Ayumi Hashimoto, Chan Gao, Jerome Mastio, Andrew Kossenkov, Scott I. Abrams, Ashok V. Purandare, Heshani Desilva, Susan Wee, John Hunt, Maria Jure-Kunkel, Dmitry I. Gabrilovich. A casein kinase 2 inhibitor disorders myeloid cell differentiation by blocking CCAAT/enhancer-binding protein-alpha signaling pathway in tumor microenvironment [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr A68.

Monitoring CD8a+ T Cell Responses to Radiotherapy and CTLA-4 Blockade Using [64Cu]NOTA-CD8a PET Imaging

PurposeCurrent response assessment systems for cancer patients receiving immunotherapy are limited. This is due to the associated inflammatory response that may confound the conventional morphological response evaluation criteria in solid tumors and metabolic positron emission tomography (PET) response criteria in solid. Recently, novel PET imaging techniques using radiolabeled antibodies and fragments have emerged as a particularly sensitive and specific modality for quantitative tracking of immune cell dynamics. Therefore, we sought to investigate the utility of Cu-64 labeled F(ab)′2 fragments for in vivo detection of CD8a+ T cells as a prognostic imaging biomarker of response to immunotherapy in an immunocompetent mouse model of colorectal cancer.Procedures[64Cu]NOTA-CD8a was produced by enzymatic digestion of rat-anti-mouse CD8a antibody (clone YTS169.4), purified yielding isolated CD8a-F(ab)′2 fragments and randomly conjugated with the 2-S-(isothiocyanatbenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) chelator. NOTA-CD8a was radiolabeled with Cu-64 and injected into CT26 tumor-bearing mice for longitudinal assessment. To investigate the value of [64Cu]NOTA-CD8a PET imaging for assessment of treatment response, CT26 tumor-bearing mice were subjected to external radiation therapy (XRT) in combination with anti-CTLA-4 therapy. Imaging data was supported by flow cytometry and immunohistochemistry (IHC).ResultsCombination treatment with XRT and anti-CTLA-4 effectively inhibited tumor growth until day 22 post-therapy initiation (p = 0.0025) and increased the overall survival of mice compared to control (p = 0.0017). The [64Cu]NOTA-CD8a tumor-to-heart ratio was increased in XRT + anti-CTLA-4-treated mice on day 8 after initiation of therapy (p = 0.0246). Flow cytometry and IHC confirmed the increase in tumor-infiltrating CD8a+ cells in XRT + anti-CTLA-4-treated mice. Furthermore, [64Cu]NOTA-CD8a PET imaging distinguished responders and non-responders prior to treatment-induced changes in tumor volume among mice.ConclusionIn the present study, we demonstrated that [64Cu]NOTA-CD8a was able to detect treatment-induced changes in CD8a+ infiltration in murine CT26 colon tumors following a common preclinical combination treatment protocol. Overall, [64Cu]NOTA-CD8a exhibited good prognostic and predictive value. We suggest that [64Cu]NOTA-CD8a PET imaging can be used as an early biomarker of response to therapy in preclinical models.

Anti-inflammatory and metabolic reprogramming effects of MENK produce antitumor response in CT26 tumor-bearing mice.

Methionine enkephalin (MENK), an endogenous opioid peptide, has a role in nervous system, immune system, and anticancer therapy. Inflammation, metabolism and cancer are closely intertwined with each other. This study is to identify the correlation of the antitumor effects of MENK with systemic inflammation, liver metabolism, and immune cells as myeloid-derived suppressor cells (MDSCs). We established a subcutaneous CT26 colon carcinoma model and a cyclophosphamide-induced immunosuppressive model subjected to MENK. AML12 and MDSCs were used as in vitro models. The results showed that MENK treatment degraded tumor growth and inhibited proinflammatory cytokines both in tumor tissues and serum. The MENK-treated tumor mice showed normalized liver function with glycolipid metabolic homeostasis. No inhibitory effect on CT26 tumor cell in vitro, but only reduced lipid synthesis in AML12 were presented by MENK. Meanwhile, MENK invigorated immune response in both two animal models by markedly suppressing MDSCs and enhancing T cells response. In vitro MENK-treated MDSCs showed reduced glycolysis and less ROS production, which was mediated by PI3K/AKT/mTOR pathway. Opioid receptor antagonist naltrexone reversed most of the regulation. These results illustrate that MENK preventing development of colon carcinoma might be correlated with the suppression of inflammation, improving metabolism in liver as well as in MDSCs partly through opioid receptor, which brings new elements supporting the adjuvant therapy for tumor by MENK.

Lipophilicity of Bacteriochlorin-Based Photosensitizers as a Determinant for PDT Optimization through the Modulation of the Inflammatory Mediators.

Photodynamic therapy (PDT) augments the host antitumor immune response, but the role of the PDT effect on the tumor microenvironment in dependence on the type of photosensitizer and/or therapeutic protocols has not been clearly elucidated. We employed three bacteriochlorins (F2BOH, F2BMet and Cl2BHep) of different polarity that absorb near-infrared light (NIR) and generated a large amount of reactive oxygen species (ROS) to compare the PDT efficacy after various drug-to-light intervals: 15 min. (V-PDT), 3h (E-PDT) and 72h (C-PDT). We also performed the analysis of the molecular mechanisms of PDT crucial for the generation of the long-lasting antitumor immune response. PDT-induced damage affected the integrity of the host tissue and developed acute (protocol-dependent) local inflammation, which in turn led to the infiltration of neutrophils and macrophages. In order to further confirm this hypothesis, a number of proteins in the plasma of PDT-treated mice were identified. Among a wide range of cytokines (IL-6, IL-10, IL-13, IL-15, TNF-α, GM-CSF), chemokines (KC, MCP-1, MIP1α, MIP1β, MIP2) and growth factors (VEGF) released after PDT, an important role was assigned to IL-6. PDT protocols optimized for studied bacteriochlorins led to a significant increase in the survival rate of BALB/c mice bearing CT26 tumors, but each photosensitizer (PS) was more or less potent, depending on the applied DLI (15 min, 3 h or 72 h). Hydrophilic (F2BOH) and amphiphilic (F2BMet) PSs were equally effective in V-PDT (>80 cure rate). F2BMet was the most efficient in E-PDT (DLI = 3h), leading to a cure of 65 % of the animals. Finally, the most powerful PS in the C-PDT (DLI = 72 h) regimen turned out to be the most hydrophobic compound (Cl2BHep), allowing 100 % of treated animals to be cured at a light dose of only 45 J/cm2.

Abstract PR06: PET imaging of biomarkers in immuno-oncology for response prediction and treatment monitoring

Abstract Background: Cancer immuno-therapy has been established as a powerful way of treating several types of cancer. Expression of biomarkers on tumor and associated immune cells in the tumor microenvironment (TME) seems to reflect clinical outcome. The objective of the current work was to develop a toolbox for non-invasive PET imaging of PD-L1 expression and imaging of CD4+ and CD8a+ tumor-infiltrating lymphocytes for response prediction and treatment monitoring of immune stimulating therapies. Methods: PET radiotracers were generated from intact antibodies (anti-PD-L1, clone 6E11) or antibody fragments (R-anti-M-CD8a+ or R-anti-M-CD4+) and labeled with 89Zr or 64Cu. Syngeneic mouse models were selected based on their respective TME as analyzed by flow cytometry and immunohistochemistry (IHC). PET/CT imaging with 64Cu-NOTA-CD8a-F(ab)’2 in tumor-bearing CT26 mice after treatment with external radiation therapy (XRT) and anti-CTLA-4 was performed for treatment monitoring. Prediction of response to therapy with XRT and anti-PD-L1 was evaluated with 89Zr-DFO-6E11 in 4T1 and CT26 syngeneic mouse models. In vivo imaging data was compared with IHC and flow cytometry. Finally, murine CD3+ T-cells were directly labeled with 89Zr and evaluated in vivo in immunocompetent- and incompetent mice. The labeling efficiency and viability of the cells were evaluated ex vivo, and in vivo cell tracking was performed by longitudinal PET/CT imaging after injection of labeled T-cells in naive immune-competent mice. Results: Optimized protocols for labeling of anti-PD-L1 and fragments directed at CD8a+ and CD4+ with 89Zr or 64Cu were developed with high radiochemical yield and RCP of >99%. PET/CT imaging with all tracers was performed to compare tumor, spleen and lymph node uptake. Blocking of endogenous CD8a+ and PD-L1 was successfully reflected by PET imaging with 89Zr-DFO-CD8a-F(ab)’2 and 89Zr-DFO-6E11, respectively. Fractionated XRT induced infiltration of CD8a+ cells in spleen and CT26 tumors which was detected by in vivo 89Zr-DFO-CD8a-F(ab)’2 PET imaging and ex vivo by IHC analysis. Increased tumor uptake of 64Cu-NOTA-CD8a-F(ab)’2 was seen in CT26 tumor bearing mice treated with fractionated XRT and anti-CTLA-4, and tumor growth was effectively inhibited in treatment groups with high uptake. Imaging with 89Zr-labeled anti-PD-L1 in the CT26 tumor model was able to predict subsequent response to PD-L1 therapy. Finally, longitudinal PET/CT imaging was able to track CD3+ T-cells directly labeled with 89Zr for up to 7 days following intra venous administration in immunocompetent mice. Conclusion: The developed radiotracers enable whole-body visualization of CD8a+ and CD4+ T-cells and PD-L1 expression in various syngeneic mouse models. These radiotracers can be used as tools to predict and monitor immune stimulating therapies in preclinical drug development. Furthermore, PET imaging of direct labeled T-cells is an attractive tool for investigating the distribution of cell-based therapies. Citation Format: Carsten H. Nielsen, Lotte K. Kristensen, Camilla Christensen, Andreas Kjaer. PET imaging of biomarkers in immuno-oncology for response prediction and treatment monitoring [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr PR06. doi:10.1158/1535-7163.TARG-19-PR06

1H NMR-Based Metabolomics Reveals the Antitumor Mechanisms of Triptolide in BALB/c Mice Bearing CT26 Tumors.

Triptolide, the main active ingredient in Tripterygium wilfordii Hook. f. (Celastraceae), has shown promising effects against a variety of tumors. However, the molecular pharmacological mechanisms explaining the action of triptolide remain unknown. In this study, the CT26 colon tumor cell line was inoculated subcutaneously into BALB/c mice, and plasma samples were subjected to 1H NMR metabolomics analysis. The metabolic signature identified five metabolites whose levels were lower and 15 whose levels were higher in CT26 tumor-bearing mice than in normal control mice. Triptolide treatment significantly reversed the levels of nine of these metabolites, including isoleucine, glutamine, methionine, proline, 3-hydroxybutyric acid, 2-hydroxyisovalerate, 2-hydroxyisobutyrate, and low-density lipoprotein/very low-density lipoprotein. Based on the identities of these potential biomarkers, we conclude that the antitumor mechanism of triptolide might rely on correcting perturbations in branched-chain amino acid metabolism, serine/glycine/methionine biosynthesis, and ketone bodies metabolism.

1223P - GI101, a novel triple-targeting bispecific CD80-IgG4-IL2variant fusion protein, elicits synergistic anti-tumour effects in preclinical models

BackgroundDespite mounting interest in combination immunotherapy including immune checkpoint inhibitors (ICIs) to improve anti-tumor responses, clinical translation has been disappointing. To address the significant unmet needs remaining, we designed GI101 comprising the extracellular domain of CD80 acting as a dual checkpoint (CTLA4/PD-L1) inhibitor, together with a long acting IL2v that preferentially binds to the IL2Rβ. MethodsAffinity of GI101 for IL2Rs, CTLA4, and PD-L1 was determined by SPR. Immune cell proliferation was analyzed by CFSE assay and in vivo immune profiling. In vivo efficacy of GI101 were performed in a tumor-bearing mouse model. ResultsGI101 exhibits high affinity to both CTLA4 (Kd, 2.9nM), PD-L1 (Kd, 8.5nM), and preferential binding to IL2Rβ (Kd, 28.4nM). GI101 induces the robust stimulation of in vitro and in vivo CD8+ T and NK cell proliferation without a significant increase in Treg cells. GI101 has high binding affinity to CTLA4 acting as a decoy ligand, thereby enhancing the interaction between endogenous CD80 and CD28, leading to the activation of T cells. GI101 elicits improved restoration of immune functions compared to a CD80-Fc in in vitro settings using human PBMCs co-cultured with PD-L1hi tumor cells. In the CT26 tumor-bearing mice, GI101 was superior at inhibiting tumor growth when compared to a combination of aPD-1/aCLTA4 in association with a profound increase in CD8+ T and NK cells without causing an increase in Tregs in the TME. In addition, a dose-dependent anti-tumor effect was observed in CT26 syngeneic models. Furthermore, isolated mortality was observed in the aPD-1/aCTLA4 combo-treated group whereas GI101-treated group has no evidence for toxicity associated with IL2 activity including vascular leakage syndrome and cytokine storm. ConclusionsThe complementary modes of action of GI101 via dual checkpoint blockade with IL2 activity to enhance the proliferation and activation of Teff and NK cells is projected to translate into superior clinical efficacy and safety as indicated even in ‘cold tumor’ models. GI101 has promising potential to replace first-generation ICIs as a monotherapy or in combination with chemo/radiotherapies and other immunotherapies. Legal entity responsible for the studyGI innovation. FundingGI innovation. DisclosureJ.C. Park: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. K.H. Pyo: Research grant / Funding (institution): Yonsei Medical Center. Y.J. Song: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. W.J. Lee: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. J. Kim: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. H.N. Ji: Full / Part-time employment: GI innovation. S.S. Park: Full / Part-time employment: GI innovation. Y.J. Koh: Full / Part-time employment: GI innovation. K. Lee: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. B. Cho: Research grant / Funding (institution): Yonsei Medical Center. B. Chun: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. Y.M. Oh: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. B. Yang: Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. S.Y. Nam: Leadership role, Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation. M.H. Jang: Leadership role, Shareholder / Stockholder / Stock options, Full / Part-time employment: GI innovation.

Abstract 93: Anti-LAP-TGFb antibodies inhibit tumor growth in a CT26 syngeneic tumor model in combination with radiation therapy

Abstract Anti-LAP-TGFβ antibodies inhibit tumor growth in a CT26 syngeneic tumor model in combination with radiation therapy. TGFβ is a major immunosuppressive cytokine that acts within the TME and is implicated in resistance to checkpoint inhibitors. TGFβ is synthesized as a pro-protein complex in which the mature cytokine is caged within LAP, the latency associated peptide of TGFβ1, and serves to hold TGFβ1 in a latent state. Anti-LAP-TGFβ antibodies have efficacy in mouse models of cancer (Gabriely et al., 2017) and offer a promising new treatment approach in oncology. Radiation is standard of care in many cancer indications and is a known potent inducer of TGFβ. TGFβ is also implicated in resistance to radiation treatment. Thus, combination treatment with anti-LAP antibodies and radiation may yield a novel approach for enhancing therapeutic responses in a population with significant unmet medical need. We have evaluated the anti-tumor effects of targeting TGFβ via an anti-LAP antibody in combination with radiation therapy in a murine syngeneic CT26 colorectal cancer model. We identified a unique class of anti-LAP antibodies, TLS-01, that specifically target LAP on the surface of cells, but do not bind to LAP-TGFβ in the extracellular matrix. We assessed the effects of combination of TLS-01 and radiation on both tumor efficacy and modulation of immune cell subsets within the TME. CT26 tumor bearing mice (tumor size ~300 mm3) were treated with either TLS-01, an isotype control antibody, 12 Gy or 20 Gy of targeted radiation, or a combination of TLS-01 with 12 Gy or 20 Gy of radiation. Seven days after treatment, 3 animals per group were analyzed for tumor infiltrating immune cell subsets and the remaining animals were followed for tumor growth and survival for up to 19 days. Radiation inhibited tumor growth in a dose dependent fashion. Combination treatment with TLS-01 and radiation inhibited tumor growth to a greater extent than was seen with TLS-01 or radiation treatment alone. Radiation treatment dramatically increased the number of CD8+ T cells in the TME and concomitantly increased both the number and immunosuppressive properties of inhibitory cell populations in the TME, including increases in CD73 expression on M-MDSCs and M2 macrophages. Treatment of irradiated mice with TLS-01 reduced Tregs, increased the CD8 / Treg ratio and reduced CD73 expression on immune suppressive cells. Combination of TLS-01 with radiation resulted in enhanced efficacy when compared to either agent alone. Our data provides mechanistic insights underpinning this enhanced efficacy. TLS-01 moderated immunosuppression in the TME by radiation via modulation of multiple immunosuppressive cell types. These data support the therapeutic utility of combination treatment of TLS-01 and radiation therapy in the treatment of solid tumors. Citation Format: Kenneth J. Simon, Stavros Kopsiaftis, Randall Burton, Patricia E. Rao, Jessie M. English, Barbara S. Fox. Anti-LAP-TGFb antibodies inhibit tumor growth in a CT26 syngeneic tumor model in combination with radiation therapy [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 93.

Abstract 2254: Solid tumor activity with a selective PI3Kdelta inhibitor as a single agent and in combination with anti-PD-1

Abstract Inhibitors of PI3Kdelta are approved as monotherapy for the treatment of hematologic malignancies such as follicular B-cell non-Hodgkin lymphoma, chronic lymphocytic leukemia, and small lymphocytic lymphoma. Recent reports suggest that inhibition of PI3Kdelta in mice is effective against solid cancers as well. Mechanistic studies by several groups have suggested a possible adaptive immune-mediated anti-tumor response involving immunosuppressive Tregs and/or MDSCs in the tumor microenvironment. Our structure-guided discovery program led to the identification of a novel heterocycloalkyl purine inhibitor class with excellent isoform and kinome selectivity, as well as good solubility. Optimization included potency enhancements, addressing metabolism and increasing half-life in preclinical species to provide a PI3Kdelta inhibitor with a low once-daily predicted human dose. In FoxP3 reporter mice harboring MC38 tumors treated with inhibitor, we observed reduced peripheral Treg function and an increased ratio of CD8 to Treg in both spleen and tumor-infiltrating lymphocytes. In three-week efficacy studies, modest tumor growth inhibition was found in 4T1, MC38 and CT26 syngeneic tumor models when our inhibitor was dosed as a single agent and we demonstrated a trend towards better efficacy when our inhibitor was dosed in combination with an anti-mPD-1 antibody in CT26 tumor-bearing mice as compared to anti-mPD-1 alone. These preliminary studies confirm the impact of PI3Kdelta inhibition on T-cell subpopulations, and support continued exploration of PI3Kdelta inhibitors for the treatment of solid tumors, either as monotherapy or in combination with other agents. Citation Format: Joey L. Methot, Hua Zhou, Meredith A. McGowan, Benjamin W. Trotter, Michael Altman, Xavier Fradera, Charles Lesburg, Chaomin Li, Stephen Alves, Craig P. Chappell, Renu Jain, Ruban Mangado, Elaine Pinheiro, Sybil M. Williams, Peter Goldenblatt, Armetta D. Hill, Lynsey Shaffer, Dapeng Chen, Robbie L. McLeod, Hyun-Hee Lee, Sanjiv Shah, Jason D. Katz. Solid tumor activity with a selective PI3Kdelta inhibitor as a single agent and in combination with anti-PD-1 [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 2254.

Abstract 1130: PET imaging of tumor-immune status for response prediction and treatment monitoring in syngeneic mouse models

Abstract Introduction: Cancer immuno-therapy has been established as a powerful way of treating several types of cancer and the degree of immune cell infiltration in tumors seems to reflect the clinical outcome. Accordingly, non-invasive PET imaging of key immune cell subsets could become a valuable tool for tailoring cancer immuno-therapy. Therefore, we developed antibody-based PET tracers specific for CD8a+ and PD-L1 enabling visualization of tumor infiltrating T-cells and tumor PD-L1 status in syngeneic mouse models. Furthermore, we evaluated the ability of theses tracers to predict and monitor combinations of immune-stimulating therapies. Experimental procedures: Radiotracers were generated from intact antibodies (anti-PD-L1, clone 6E11, humanized cross-reactive to mouse) or antibody fragments (R-anti-M-CD8a+) and were radiolabeled with 89Zr or 64Cu. Tracer specificity was evaluated by immuno-reactive fraction assays and blocking studies and compared to radiolabeled isotype controls. Syngeneic mouse models were selected based on their respective tumor microenvironment (TME) as analyzed by flow cytometry and immunohistochemistry (IHC). Treatment efficacy of external radiation therapy (XRT) and anti-CTLA-4 was evaluated by baseline and post-therapy scans with 64Cu-NOTA-CD8a-F(ab)’2 in CT26 tumor-bearing mice. Prediction of response to therapy with XRT and anti-PD-L1 was evaluated with 89Zr-DFO-6E11 in various syngeneic mouse models. In vivo imaging data was compared with IHC and flow cytometry. Results: Tracers were highly specific for their targets with an immuno-reactivity >85%. Blocking of endogenous CD8a+ and PD-L1 was successfully reflected by PET imaging with 89Zr-DFO-CD8a-F(ab)’2 and 89Zr-DFO-6E11, respectively. Fractionated XRT induced infiltration of CD8a+ cells in spleen and CT26 tumors which was detected by in vivo 89Zr-DFO-CD8a-F(ab)’2 PET imaging and ex vivo by IHC analysis. Studies with 89Zr-DFO-CD8a-F(ab)’2 PET imaging of combination therapies with fractionated XRT and immune checkpoint inhibitors are ongoing. 89Zr-DFO-6E11 successfully detected various degrees of PD-L1 expression in xenograft mouse models as well as the upregulated PD-L1 expression in syngeneic mouse models following fractionated XRT. Studies with 89Zr-DFO-6E11 PET imaging of combination therapies with fractionated XRT and immune checkpoint inhibitors are ongoing. Conclusions: The developed radiotracers enable whole-body visualization of CD8a+ T cells and PD-L1 expression in various syngeneic mouse models. These radiotracers can be used as tools to predict efficacy of immuno-therapy in preclinical drug development. In addition, they may be valuable in vivo biomarkers of response to immune-therapy in patients. Citation Format: Lotte K. Kristensen, Camilla Christensen, Carsten H. Nielsen, Andreas Kjaer. PET imaging of tumor-immune status for response prediction and treatment monitoring in syngeneic mouse models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1130.

Abstract 3204: Pre-immunization of donor lymphocytes with GITR agonistic antibody enhances antitumor immunity in autologous hematopoietic stem cell transplantation

Abstract Lymphopenia-induced homeostatic proliferation (HP) of T cells following autologous hematopoietic stem cell transplantation (HSCT) skews the T-cell repertoire by engaging tumor-associated antigens, and induces an antitumor immunity. However, the cure by autologous HSCT alone is difficult in case of solid cancers. Glucocorticoid-induced tumor necrosis factor receptor (GITR) is well-known immune-checkpoint molecules. GITR-GITR ligand interaction can provide a co-stimulatory signal to both CD4+ and CD8+ naïve T cells, enhancing proliferation and effector function. Since the tumor-reactive lymphocytes preferentially proliferate during the condition of HP, due to the synergic effect with encountering their cognate antigens in tumor-bearing host, we hypothesize that administration of GITR agonistic antibody (Ab) could increase tumor-responsive T cells in the graft and induce a strong antitumor immunity in HSCT recipients. First, we confirmed that the intraperitoneal administration of GITR Ab into CT26 subcutaneous tumor model mice significantly increased the number of IFN-γ+ lymphocytes in response to CT26 cells in the spleens and regional lymph nodes. Second, to examine the antitumor effect of HSCT, the bone marrow cells and lymphocytes were harvested from CT26 tumor-bearing mice as donor cells, and infused into the lethally-irradiated recipient mice with CT26 inoculation (autologous HSCT model). In the HSCT recipients infused with primed lymphocytes by GITR Ab, the CT26 tumor growth was markedly suppressed compared with the recipients infused with non-primed lymphocytes, and tumors disappeared in all recipient mice infused with primed lymphocytes. An ELISpot assay showed that the number of IFN-γ+ spots in response to CT26 cells was significantly increased in HSCT recipient mice infused with GITR Ab-primed and non-primed donor lymphocytes as compared to wild type mice until 4 weeks after HSCT; while the frequency in recipients with primed lymphocytes was markedly elevated compared with that in mice with non-primed lymphocytes at 2 weeks after HSCT. At present, autologous HSCT is clinically practiced after intensive chemotherapy in patients with lymphomas and solid cancers such as neuroblastoma and sarcoma, the infusion of hematopoietic stem cells and pre-immunized lymphocytes after high dose chemotherapy is an attractive and feasible clinical introduction of HSCT-mediated immunotherapy. Although the subcutaneous tumors of NHOS murine osteosarcoma cells were resistant for HSCT, the infusion of GITR Ab-primed lymphocytes significantly induced stronger antitumor effect than the non-primed lymphocytes did. In conclusion, the combination of HSCT with pre-immunization by GITR Ab can induce a strong antitumor immunity. This therapeutic strategy deserves an evaluation in future clinical trial for solid cancers. Citation Format: Kenta Narumi, Marina Henmi, Chihiro Shibasaki, Aya Hirata, Yukihiro Mizoguchi, Kazunori Aoki. Pre-immunization of donor lymphocytes with GITR agonistic antibody enhances antitumor immunity in autologous hematopoietic stem cell transplantation [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 3204.

Abstract 1133: ImmunoPET and T cell labeling for in vivo monitoring of immunotherapy: Development of CD8 Fab2’ and Fab tracers for immunoPET and direct labeling of T cells for in vivo tracking

Abstract Objectives: Cancer immunotherapy is becoming an increasingly important treatment strategy across a broad spectrum of tumor types. Non-invasive imaging methods for in vivo tracking of T-cells can be used for longitudinal monitoring of tumor infiltrating T-cells to assess the effect of immune checkpoint therapy or cell-based immunotherapies. The objective of the current work was to develop and optimize Fab-based tracers directed at CD8 for PET imaging of T cells. The imaging performance of radiolabeled Fab fragments was compared to Fab2’ fragments in terms of kinetics and image contrast. Furthermore, Zirconium-89 (89Zr) labeled T cells were tracked in a murine transplantation model. Methods: Anti-mouse CD8α antibody was cleaved into Fab2’ by enzymatic cleavage followed by reduction of disulfide bridge into Fab fragments. Fab2’ was conjugated to NCS-DFO for 89Zr labeling, while both Fab2’ and Fab were conjugated to SCN-NOTA for Cupper-64 (Cu-64) labeling. Radiochemical yield, radiochemical purity (RCP) and plasma/buffer stability was assessed by high-performance liquid chromatography. Furthermore, the immunoreactivity and binding affinity was evaluated for the radiolabeled tracers. In vivo comparison of radiolabeled Fab2’ and Fab fragments was performed by longitudinal PET/CT imaging at various time-points after injection in CT26 (murine colon carcinoma) tumor bearing mice. CD3+ T-cells were directly labeled with 89Zr for tracking experiments. The labeling efficiency and viability of the cells was evaluated ex vivo, and in vivo cell tracking was performed by longitudinal PET/CT imaging after injection of labeled T-cells in naive immune-competent mice. Results: The cleavage of anti-CD8 into Fab2’ and subsequent purification by HPLC was optimized to achieve a product with high yield and purity. The optimal Fab2’ reduction conditions were determined followed by conjugation to SCN-NOTA An optimized protocol for 89Zr/64Cu-CD8-Fab2’ and 64Cu-CD8-Fab was developed with high radiochemical yield and RCP of >99%. PET/CT imaging with all tracers was performed to compare tumor, spleen and lymph node uptake. The highest uptake was found 24h post injection, but a clear delineation of lymphoid organs and tumors was evident already after 6h for the single Fab allowing same day imaging. T-cells labeled with 89Zr and cell tracking PET/CT imaging showed homing primarily to the spleen. Conclusions: Different tracers directed at T-cells were successfully developed and evaluated in in vivo PET imaging of T cells. The reduced circulation time of Fab vs. Fab2’ and the use of 64Cu instead of 89Zr allows repeated imaging every 3 days due to the shorter physical half-life (12.7 h) of 64Cu. The study furthermore demonstrates the use of direct radiolabeling of T-cells as an attractive tool for investigating the distribution of cell-based therapies. Citation Format: Camilla Christensen, Lotte K. Kristensen, Carsten H. Nielsen, Andreas Kjær. ImmunoPET and T cell labeling for in vivo monitoring of immunotherapy: Development of CD8 Fab2’ and Fab tracers for immunoPET and direct labeling of T cells for in vivo tracking [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 1133.

Abstract 2794: Expression of LAP, latency-associated peptide of TGFb, on immune cell subsets

Abstract LAP, latency-associated peptide of TGFβ1, is a known marker of regulatory T cells in the tumor microenvironment. TGFβ is a major tolerogenic cytokine that is co-opted by tumors to evade the immune system and is implicated in resistance to checkpoint inhibitors. LAP cages TGFβ in an inactive, latent state until it is activated and released by integrins, MMPs or other activators. Anti-LAP antibodies inhibit the release of TGFβ, inhibit tumor growth in mouse models (Gabriely et al., 2017), and have promise as novel cancer therapeutics. Anti-LAP antibodies are thought to mediate anti-tumor activity both by inhibiting the release of active TGFβ and by reducing the number of LAP+ immunosuppressive cells in the tumor microenvironment (TME). To provide insight into the cellular sources of TGFβ in the TME and further delineate the mechanism of action of anti-LAP antibodies in tumor growth inhibition we determined the expression profile of LAP on the surface of both Treg and other immune cell subsets derived from tumor, spleen, and human PBMCs. LAP expression on the surface of various mouse and human immune cell subsets was determined using flow cytometry. Tumor infiltrating leukocytes were profiled from CT26 and 4T1 syngeneic mouse tumor models. Expression of LAP was detected on a subset of regulatory T cells, M2 macrophages, dendritic cells, G-MDSC and M-MDSC populations in both CT26 and 4T1 tumors suggesting that each of these cell types are a potential source for TGFβ activation in the tumor microenvironment. Striking differences were detected in LAP expression on immune cell subsets in the TME versus spleens of CT26 tumor bearing mice, consistent with TGFβ expression being increased upon immune cell activation. Additionally, CD4+ cells were isolated from human PBMCs from normal healthy donors and activated using CD3/CD28 +IL2 stimulation. Consistent with previous reports, activated Foxp3+ T cells were positive for cell surface LAP. In addition, LAP expression was also assessed on various human macrophage subsets (M1, M2a, M2b and M2c) that were skewed from CD14+ cells isolated from PBMCs. LAP expression was detected in varying amounts on the surface of the different macrophage subsets with M2a macrophages demonstrating the highest expression. Taken together these results demonstrate that LAP is expressed on a variety of immune cells with known immunosuppressive function. The location of the LAP-TGFβ1 complex is of critical biological and clinical importance because, once the mature TGFβ1 cytokine, which has a short half-life in solution, is released, it acts locally, either in an autocrine or near paracrine fashion. These results warrant further research to determine the effectiveness of anti-LAP in inhibiting the release of TGFβ and its effects on immunosuppression in the tumor microenvironment. Citation Format: Stavros Kopsiaftis, Patricia E. Rao, Randall Burton, Jessie M. English, Barbara S. Fox, Kenneth J. Simon. Expression of LAP, latency-associated peptide of TGFb, on immune cell subsets [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 2794.

Abstract 4987: Age-induced changes in anti-tumor immunity alter the tumor immune infiltrate and reduce response to immuno-oncology treatments

Abstract Immuno-oncology research relies heavily on murine syngeneic tumor models. However, whilst the median age for a cancer diagnosis is 65 years or older, for practical purposes the majority of preclinical studies are conducted in young mice, despite the fact that aging has been shown to have a significant impact on the immune response. Using aged mice bearing CT26 tumors, we analysed how aging impacts the immune composition of the tumor, spleen and tumor-draining lymph nodes by flow cytometry. We found many age-related changes between aged (60-72 weeks old) and young (6-8 weeks old) mice, such as a reduction in the naïve T cell population and a decreased CD8/Treg ratio in aged animals. Profiling of co-inhibitory and co-stimulatory receptor expression levels on immune cells in aged versus young mice also identified altered expression profiles in both the periphery and tumor. We hypothesised that these differences may contribute to impaired anti-cancer immune responses in aged mice. To investigate this, we compared the anti-tumor efficacy of immune checkpoint blockade (PD-L1 and CTLA-4) and T-cell costimulation (OX-40) in aged versus young mice. Our data demonstrate that aged mice retained their capacity to generate effective anti-tumor immune responses, albeit often attenuated when compared to the responses observed in young mice. These differences highlight the potential importance of age-related immunological changes in assessing and refining the translational insights gained from preclinical mouse models. Citation Format: Suzanne I. Sitnikova, Michelle Morrow, Viia Valge-Archer, Robert W. Wilkinson, Simon J. Dovedi, Matthew J. Robinson. Age-induced changes in anti-tumor immunity alter the tumor immune infiltrate and reduce response to immuno-oncology treatments [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 4987.

Abstract 4963: A novel intestinal microbiome-derived peptide modulates immune cell activity and the tumor microenvironment

Abstract The composition of the gut microbiota affects cancer development, progression, and response to therapy. A number of commensal bacteria, including Bifidobacterium, have been associated with increased response to immune checkpoint inhibitors in mouse tumor models and in cancer patients. We hypothesized that secreted peptides or proteins are driving the Bifidobacterium-mediated effects. Using our unique bioinformatic-driven discovery platform we nominated putatively secreted Bifidobacterium-derived peptides for evaluation in immune cell effector assays. We demonstrate that several of the peptides induce secretion of pro-inflammatory cytokines (e.g., IL-6, TNF) by in vitro-generated mouse and human dendritic cells, as well as effector cytokine secretion (e.g., IFNγ, IL-2) by mouse splenic T lymphocytes in vitro. To investigate the function of Bifidobacterium-derived peptides in the context of the tumor microenvironment we injected the candidate peptide SG-A directly into the tumor of CT26 tumor-bearing mice and analyzed the immune phenotype using the Nanostring PanCancer Immune Profiling panel and flow cytometry. Both Nanostring as well as flow cytometry analysis showed an increase in CD45+ tumor-infiltrating lymphocytes in SG-A treated tumors. SG-A peptide also induced the upregulation of dendritic cell function genes (CD40, CD83, and CD86) and multiple effector cytokines and chemokines. Re-stimulation of tumor-draining lymph node cells with AH1 peptide (a CT26 tumor-derived antigen) also increased induction of IFNγ in SG-A treated animals (vs. vehicle-treated controls). Collectively, our results demonstrate the utility of the Second Genome discovery platform for leveraging microbiome science to identify novel immunoregulatory factors. This platform offers a promising approach to identify agents with potential for use as therapeutics in cancer immunotherapy. Citation Format: Helena Kiefel, Dhwani Haria, Yuliya Katlinskaya, Divya Ravichandar, Lily McLaughlin, Sunit Jain, Thomas Weinmaier, Shoko Iwai, Todd DeSantis, Toshi Takeuchi, Karim Dabbagh, Kareem Graham. A novel intestinal microbiome-derived peptide modulates immune cell activity and the tumor microenvironment [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 4963.

Abstract 3202: A probody drug conjugate targeting CD166 (ALCAM) enhances preclinical antitumor activity of a probody therapeutic targeting PD-1

Abstract Immune checkpoint blockade therapies have been shown to induce potent and durable anti-tumor immunity in many cancer types. Nevertheless, not all patients benefit from immunotherapy, and immune-related adverse events remain a problem. Recently, it has been demonstrated that Antibody Drug Conjugates (ADCs) are not only capable of killing cancer cells but also can act to induce the immunogenic cell death of tumor cells as well as directly activate dendritic cells. These results provided a rationale to combine ADCs with immunotherapy to enhance the potential of immune checkpoint blockade therapies in a broader population of patients. CytomX Therapeutics has developed a new class of antibodies called Probody™ therapeutics, designed to widen the therapeutic window by minimizing binding to target in healthy tissue while being specifically activated in the tumor microenvironment (TME) by tumor-associated proteases. Probody technology has been evaluated in preclinical studies in several antibody formats, with efficacy and increased safety windows observed for Probody therapeutics targeting the PD-1 pathway, Probody drug conjugates (PDCs) targeting highly expressed tumor antigens, and T-cell engaging bispecific Probody therapeutics. Here we extend our evaluation of the Probody platform to the combination of CX-2009, an investigational PDC targeting human CD166, with an investigational Probody therapeutic targeting PD-1. To evaluate the anti-tumor activity of PDC CX-2009 in a syngeneic mouse model, human CD166 was overexpressed on the surface of the CT-26 murine colon carcinoma cell line. The combination treatment of CX-2009 with a surrogate mouse anti-PD-1 Probody molecule significantly inhibited tumor growth in human CD166 positive CT-26 tumor-bearing mice as compared to CX-2009 or anti-PD-1 Probody molecule alone. Tumor rejection is partially dependent on CD8+ T cells as illustrated by the evidence of a CD8+ memory T cell response in a re-challenge assay, and a reduced activity of CX-2009 alone or in combination with a mouse anti-PD-1 Probody molecule after CD8+ T cell depletion. The immunogenic potential of CX-2009 was further evaluated in multiple in vitro assays using human cancer cells and human PBMCs. In contrast to its cytotoxic activity towards CD166+ tumor cells, CX-2009 spares T cells and may enhance T cell priming. These preclinical data demonstrate the potential utility of a combination of PDC CX-2009 with a Probody therapeutic targeting the PD-1 pathway. Generally, these data highlight the potential to combine ADCs or PDCs with immune checkpoint blockade therapies. PROBODY is a trademark of CytomX Therapeutics, Inc. Citation Format: Erwan Le Scolan, Tiffany Tse, Michael Krimm, Will Garner, Hikmat Assi, Jennifer Razo, Laurie Wong, Kenneth Wong, Victoria Singson, Jennifer Leong, Linnea Diep, Jennifer Richardson, Siew Schleyer, Dylan Daniel, Marcia Belvin, Michael Kavanaugh. A probody drug conjugate targeting CD166 (ALCAM) enhances preclinical antitumor activity of a probody therapeutic targeting PD-1 [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 3202.

Abstract 3258: THOR-707, a novel not-alpha IL-2, elicits durable pharmacodynamic responses in non-human primates and efficacy as single agent and in combination with anti PD-1 in multiple syngeneic mouse models

Abstract Aldesleukin is a recombinant form of IL-2 approved for metastatic melanoma and renal cell carcinoma that induced complete, durable remissions in certain patients. Yet, its use is infrequent because of vascular leak syndrome (VLS), a severe dose-limiting adverse event stemming from the engagement of the high affinity IL-2 receptor (IL-2R) alpha chain in type 2 innate lymphoid cells, eosinophils and vascular endothelial cells. THOR-707 is a site-directed, singly pegylated form of IL-2 completely lacking IL-2R alpha chain engagement yet retaining normal binding to the intermediate affinity IL-2R beta-gamma signaling complex expressed by natural killer (NK) and CD8+ T tumor-killing cells. We studied THOR-707 pharmacokinetics (PK) and pharmacodynamics (PD) in non-human primates (NHP) to evaluate peripheral biomarkers of immune cell activation (expansion of NK, CD8+ T and CD4+ regulatory T (Treg) cells and the induction of pSTAT5 and the proliferation marker Ki67). Here we show that in NHP, THOR-707 elicits the expansion of peripheral CD8+ T cells and the persistence of that response with different administration regimes. We also show that in the mouse syngeneic colon tumor model CT-26, THOR-707 induced Ki67 and the expansion of peripheral NK and CD8+ T cells. Within the tumor, THOR-707 promoted an increase in the numbers of infiltrating tumor-killing NK and CD8+ T cells without expansion of suppressive CD4+ Treg cells. Immunohistochemical (IHC) analysis showed that in THOR-707 treated animals, CD8+ T cells were mostly seen around the edges of the tumor and at areas of necrosis. Evaluation of T cell receptor (TCR) clonality revealed that treatment with THOR-707 increased intra-tumoral T cell diversity compared to the untreated animals. THOR-707 induction of CD8+ T cell tumor infiltration resulted in single agent dose-dependent anti-tumor efficacy, and additive efficacy in combination with PD-1 checkpoint inhibitor, increasing the survival of CT-26 tumor-bearing mice over either group alone. The pharmacodynamics, T cell infiltration and clonality, and efficacy of THOR-707 in two additional syngeneic models, have also been studied. Filing of an investigational new drug application for THOR-707 is expected in the second quarter of 2019 and thereafter initiation of a Phase 1/2 clinical trial in multiple tumor types as a single agent and in combination with an immune checkpoint inhibitor. Citation Format: Ingrid B. Joseph, Lina Ma, Jerod L. Ptacin, Carolina E. Caffaro, Hans R. Aerni, Kristine M. San Jose, Michael J. Pena, Robert W. Herman, Yelena Pavlova, David B. Chen, Ken Bragstad, Shukuan Li, Jasmine Nguyen, Laura K. Shawver, Lilia K. Koriazova, Marcos E. Milla. THOR-707, a novel not-alpha IL-2, elicits durable pharmacodynamic responses in non-human primates and efficacy as single agent and in combination with anti PD-1 in multiple syngeneic mouse models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3258.

Abstract 3915: YH25248, a selective PI3K delta inhibitor, shows synergistic effect with an anti-PD-L1 antibody

Abstract Phosphatidylinositol 3-kinase (PI3K) plays critical roles in numerous intracellular processes. Of the four known subtypes, PI3Kδ subtype is primarily expressed in immune cells and has been reported to promote B cell differentiation and regulatory T (Treg) cell activation. Treg cell-mediated immunosuppression is thought to be a major resistance mechanism for immune checkpoint inhibitors. We therefore hypothesized that an effective PI3Kδ inhibitor could enhance anti-cancer immunity when combined with immune checkpoint inhibitors. Through recent efforts, we have discovered YH25248, an oral PI3Kδ-specific inhibitor. YH25248 effectively inhibits PI3Kδ (IC50 = 10nM) showing more than 100-fold selectivity over α, β, and γ subtypes, and exhibited favorable kinase selectivity in 463 kinase panels. In cellular experiments, phospho-AKT was effectively blocked (IC50 = 7 nM), with basophil activation, a known PI3Kδ specific signal, was also inhibited. YH25248 reduced the activity of Treg cells (CD4+/CD25+) isolated from the splenocytes of CT26 tumor-bearing mice, while sparing conventional T cells (CD4+/CD25-). When YH25248 was orally administered (10 mg/kg) to mice, drug exposure in plasma was reasonable, with a half-life of 4.3 hours, while once-daily oral administration of YH25248 (10 ~ 60 mg/kg) significantly inhibited tumor growth of CT26 colon carcinomas in mouse. When the immune cells of tumors were analyzed by flow cytometry, the number of Treg cells was reduced, CD8 + T cells increased. In this model, YH25248 (30 mg/kg, QD) showed synergistic efficacy when combined with an anti-PD-L1 antibody (200 μg/mice, 10F.9G2 clone). In other syngeneic models using 4T1 or MC38 cells, YH25248 (30 mg/kg, QD) also exhibited similar synergistic efficacy in combination with the anti-PD-L1 antibody. In conclusion, YH25248 is a potent, selective, oral inhibitor of PI3Kδ that reduces tumor growth in syngeneic mouse tumor models by increasing the ratio of CD8+ to Treg cells. The combination of YH25248 and anti-PD-L1 antibody resulted in substantially greater tumor growth inhibition in several syngeneic models. These data support the promising potential of YH25248 for combination therapy with immune checkpoint inhibitors to increase therapeutic response rates. Citation Format: Jinhwi Park, Ho-Woong Kang, Hyun-Mo Koo, Jongsuk Park, Tae-Wang Kim, Se-Woong Oh, Soongyu Choi. YH25248, a selective PI3K delta inhibitor, shows synergistic effect with an anti-PD-L1 antibody [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 3915.