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Advances in immunostimulatory and anti-immunosuppressive therapeutics have revolutionized cancer treatment. However, novel immunotherapeutics with these dual functions are not frequently reported. Here we describe the creation of a heterodimeric bifunctional fusion molecule, HCW9218, constructed using our soluble tissue factor (TF)-based scaffold technology. This complex comprises extracellular domains of the human transforming growth factor-β (TGF-β) receptor II and a human interleukin-15 (IL-15)/IL-15 receptor α complex. HCW9218 can be readily expressed in CHO cells and purified using antibody-based affinity chromatography in a large-scale manufacturing setting. HCW9218 potently activates mouse natural killer (NK) cells and CD8+ T cells in vitro and in vivo to enhance cell proliferation, metabolism, and antitumor cytotoxic activities. Similarly, human immune cells become activated with increased cytotoxicity following incubation with HCW9218. This fusion complex also exhibits TGF-β neutralizing activity in vitro and sequesters plasma TGF-β in vivo. In a syngeneic B16F10 melanoma model, HCW9218 displayed strong antitumor activity mediated by NK cells and CD8+ T cells and increased their infiltration into tumors. Repeat-dose subcutaneous administration of HCW9218 was well tolerated by mice, with a half-life sufficient to provide long-lasting biological activity. Thus, HCW9218 may serve as a novel therapeutic to simultaneously provide immunostimulation and lessen immunosuppression associated with tumors.

Natural killer (NK) cells are a promising cellular therapy for cancer, with challenges in the field including persistence, functional activity, and tumor recognition. Briefly, priming blood NK cells with recombinant human (rh)IL-12, rhIL-15, and rhIL-18 (12/15/18) results in memory-like NK cell differentiation and enhanced responses against cancer. However, the lack of available, scalable Good Manufacturing Process (GMP)–grade reagents required to advance this approach beyond early-phase clinical trials is limiting. To address this challenge, we developed a novel platform centered upon an inert tissue factor scaffold for production of heteromeric fusion protein complexes (HFPC). The first use of this platform combined IL-12, IL-15, and IL-18 receptor engagement (HCW9201), and the second adds CD16 engagement (HCW9207). This unique HFPC expression platform was scalable with equivalent protein quality characteristics in small- and GMP-scale production. HCW9201 and HCW9207 stimulated activation and proliferation signals in NK cells, but HCW9207 had decreased IL-18 receptor signaling. RNA sequencing and multidimensional mass cytometry revealed parallels between HCW9201 and 12/15/18. HCW9201 stimulation improved NK cell metabolic fitness and resulted in the DNA methylation remodeling characteristic of memory-like differentiation. HCW9201 and 12/15/18 primed similar increases in short-term and memory-like NK cell cytotoxicity and IFNγ production against leukemia targets, as well as equivalent control of leukemia in NSG mice. Thus, HFPCs represent a protein engineering approach that solves many problems associated with multisignal receptor engagement on immune cells, and HCW9201-primed NK cells can be advanced as an ideal approach for clinical GMP-grade memory-like NK cell production for cancer therapy.

Immunotherapeutics that aid in boosting natural immune defenses against cancers have revolutionized cancer treatment. Previously, we reported a novel heterodimeric bifunctional fusion molecule, HCW9218, designed using soluble tissue factor (TF)-based scaffold technology comprising extracellular domains of the human transforming growth factor-β (TGF-β) receptor II and a human interleukin (IL)-15/IL-15 receptor α complex which exhibited both immune cell stimulatory and TGF-β neutralizing properties. Herein, we showed in two different syngeneic murine tumor models (B16F10, 4T1) that subcutaneous treatment with HCW9218 induces a proliferative burst of CD8+ T cells and NK cells in blood and a subsequent infiltration of these cells into established tumors. In vivo imaging of 4T1 tumor-bearing mice after treatment showed that HCW9218 was present both in lymph nodes and established tumors up to 24hr following treatment. Comprehensive analysis of tumor infiltrating lymphocytes (TILs) showed that HCW9218 mediated antitumor activity by expanding TCF+TIM3− ‘progenitor exhausted’ (Tpex) CD8+ T cells in tumors. Sphingosine-1-phosphate receptor blockade resulted in decreased tumor infiltration of CD8+ Tpex in B16F10 and 4T1 tumor-bearing mice indicating that these cells originate from tumor draining lymph nodes (TdLN). Increased ‘terminally exhausted‘ TCF-1−TIM3+ (Tex) CD8+ TILs were also observed in tumors of HCW9218-treated mice indicating increased antitumor activity. Tumor transplantation experiments further confirmed the mechanism of HCW9218 antitumor activity by increasing influx of CD45.1+ CD8+ T cells into transplanted tumors from CD45.2+ mice. Additionally, HCW9218 enhanced the therapeutic efficacy of PD-L1 treatment by increasing the infiltration of activated/memory CD8+ T cells into B16F10 tumors in mice leading to significant reduction in tumor volume. Collectively, the results of this study demonstrated that HCW9218 treatment of mice bearing solid tumors resulted in modulating the TdLN immune landscape and invigorating T cells for enhanced checkpoint blockade therapy. HCW9218 are currently in two clinical trials (clinicaltrials.org: NCT05322408, NCT05304936) against chemo-resistant/refractory solid tumors. Citation Format: Varghese George, Pallavi Chaturvedi, Niraj Shrestha, Leah Kanakraj, Crystal Gilkes, Nicole Encalada, Meng Wang, Xiaoyun Zhu, Bai Liu, Peter Rhode, Hing C. Wong. Bifunctional immunotherapeutic HCW9218 facilitates recruitment of immune cells from tumor draining lymph nodes to promote antitumor activity and enhance checkpoint blockade efficacy in solid tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4441.

Background: A major mechanism of action for therapeutic antibodies is antibody-dependent cell-mediated cytotoxicity (ADCC). ALT-803 is an interleukin-15 superagonist complex that enhances ADCC against human carcinoma cells in vitro and exerts an antitumor activity in murine, rat, and human carcinomas in vivo. The authors investigated the ability of ALT-803 to modulate ADCC mediated by the humanized IgG1 monoclonal antibody (mAb) NEO-201 against human carcinoma cells. Materials and Methods: ALT-803 modulating activity on ADCC mediated by NEO-201 was evaluated on several NEO-201 ligand-expressing human carcinoma cells. Purified human natural killer (NK) cells from multiple healthy donors were treated with ALT-803 before their use as effectors in ADCC assay. Modulation of NK cell phenotype and cytotoxic function by exposure to ALT-803 was evaluated by flow cytometry and gene expression analysis. Results: ALT-803 significantly enhanced ADCC mediated by NEO-201. ALT-803 also upregulated NK activating receptors, antiapoptotic factors, and factors involved in the NK cytotoxicity, as well as downregulated gene expression of NK inhibiting receptors. Conclusions: These findings indicate that ALT-803 can enhance ADCC activity mediated by NEO-201, by modulating NK activation and cytotoxicity, suggesting a possible clinical use of ALT-803 in combination with NEO-201 for the treatment of human carcinomas.

Purpose: IL15 induces the activation and proliferation of natural killer (NK) and memory CD8+ T cells and has preclinical antitumor activity. Given the superior activity and favorable kinetics of ALT-803 (IL15N72D:IL15RαSu/IgG1 Fc complex) over recombinant human IL15 (rhIL15) in animal models, we performed this first-in-human phase I trial of ALT-803 in patients with advanced solid tumors. Patients and Methods: Patients with incurable advanced melanoma, renal cell, non–small cell lung, and head and neck cancer were treated with ALT-803 0.3 to 6 μg/kg weekly intravenously or 6 to 20 μg/kg weekly subcutaneously for 4 consecutive weeks, every 6 weeks. Immune correlates included pharmacokinetics, immunogenicity, and lymphocyte expansion and function. Clinical endpoints were toxicity and antitumor activity. Results: Twenty-four patients were enrolled; 11 received intravenous and 13 received subcutaneous ALT-803. Of these patients, nine had melanoma, six renal, three head and neck, and six lung cancer. Although total lymphocyte and CD8+ T-cell expansion were modest, NK cell numbers rose significantly. Neither anti–ALT-803 antibodies nor clinical activity were observed. Overall, ALT-803 was well tolerated, with adverse effects including fatigue and nausea most commonly with intravenous administration, whereas painful injection site wheal was reported most commonly with subcutaneous ALT-803. Conclusions: Subcutaneous ALT-803 produced the expected NK cell expansion and was well tolerated with minimal cytokine toxicities and a strong local inflammatory reaction at injection sites in patients with advanced cancer. These data, together with compelling evidence of synergy in preclinical and clinical studies, provide the rationale for combining ALT-803 with other anticancer agents. Clin Cancer Res; 24(22); 5552–61. ©2018 AACR.

Natural killer (NK) cells are innate cytotoxic lymphocytes that play a fundamental role in the immunosurveillance of cancers. NK cells of cancer patients exhibit impaired function mediated by immunosuppressive factors released from the tumor microenvironment (TME), such as transforming growth factor (TGF)-β1. An interleukin (IL)-15 superagonist/IL-15 receptor α fusion complex (IL-15SA/IL-15RA; ALT-803) activates the IL-15 receptor on CD8 T cells and NK cells, and has shown significant anti-tumor activity in several in vivo studies. This in vitro study investigated the efficacy of IL-15SA/IL-15RA on TGF-β1-induced suppression of NK cell-cytotoxic function. IL-15SA/IL-15RA inhibited TGF-β1 from decreasing NK cell lysis of four of four tumor cell lines (H460, LNCap, MCF7, MDA-MB-231). IL-15SA/IL-15RA rescued healthy donor and cancer patient NK cell-cytotoxicity, which had previously been suppressed by culture with TGF-β1. TGF-β1 downregulated expression of NK cell-activating markers and cytotoxic granules, such as CD226, NKG2D, NKp30, granzyme B, and perforin. Smad2/3 signaling was responsible for this TGF-β1-induced downregulation of NK cell-activating markers and cytotoxic granules. IL-15SA/IL-15RA blocked Smad2/3-induced transcription, resulting in the rescue of NK cell-cytotoxic function from TGF-β1-induced suppression. These findings suggest that in addition to increasing NK cell function via promoting the IL-15 signaling pathway, IL-15SA/IL-15RA can function as an inhibitor of TGF-β1 signaling, providing a potential remedy for NK cell dysfunction in the immunosuppressive tumor microenvironment.

This mini review provides an overview and rationale for creating IL-15-based fusion protein complexes to be used as targeted immunotherapeutic agents. IL-15 stimulates proliferation and activation of CD8+ T and natural killer cells which result in augmentation of their anti-tumor activities. We have created ALT-803, an IL-15 superagonist complex which exhibits longer serum half-life, longer retention in lymphoid tissues, and better immunostimulatory and anti-tumor activities compared to native IL-15. When used alone or in combination with other immunotherapeutic molecules in various mouse tumor models, ALT-803 effectively reduces tumor burden and prolongs survival by stimulating the innate and adaptive arms of the immune system. To evaluate whether ALT-803 could be used as a protein scaffold to create IL-15-based tumor cell-specific molecules, we genetically fused it with a single chain anti-CD20 antibody derived from the variable regions of rituximab. This novel fusion protein exhibits enhanced anti-tumor activity compared to rituximab while maintaining IL-15 immunostimulating properties. Thus, ALT-803 may be exploited as a versatile scaffold to produce multivalent targeted fusion proteins to improve the anti-tumor efficacy of other therapeutic agents in the clinic.

BackgroundAllogeneic Natural Killer (NK) cells are emerging as a safe and effective modality for the treatment of cancer, overcoming several limitations associated with adoptive T cell therapies. Cytokine induced memory-NK cells offer several advantages over conventional NK cells, including enhanced functional persistence, efficacy, and metabolic fitness. Additionally, unlike iPSC and cord blood derived NK cells, they do not require engineering to enable functionality. Here we describe the use of WU-PRIME, a GMP-grade fusion protein complex to generate memory NK cells, and WU-EXPAND, a feeder cell free expansion system to expand memory-NK cells and create WU-NK-101. Further cryopreservation enables the large-scale, off-the-shelf manufacture of memory NK for cancer immunotherapy, with high anti-tumor activity.MethodsNK cells derived from healthy donor leukopheresate were either activated with WU-PRIME and then expanded with WU-EXPAND to form WU-NK-101 or immediately expanded with WU- EXPAND as controls and then cryopreserved. We compared NK cell expansion as well as post- thaw NK cell functionality as assessed by cytokine secretion and short-term and long-term anti- tumor functionality, long-term persistence in NSG mice, as well as anti-tumor activity in vivo.ResultsNK cells activated with WU-PRIME followed by WU-EXPAND (WU-NK-101), expand robustly in large-scale reactions, over 250-fold in 14 days. The cells maintain durable expression of CD25 after expansion, as well as several other hallmarks of the memory-NK phenotype as assessed by mass cytometry. As compared to cells expanded with WU-EXPAND only, WU-NK-101 cells have improved in vitro activity against K562 cells, as well as AML cell lines (TF-1, THP-1, and HL-60). Notably, this functionality is maintained long-term upon repeated challenge. In vivo, WU-NK-101 cells, compared to expanded NK cells have improved in vivo persistence (figure 1; 50,290 v. 9,623, pAbstract 188 Figure 1NK cell persistence in tumor-bearing mice. 10e6 cryopreserved NK cells were injected into K562 tumor-bearing mice, and supported with 50,000IU human IL-2 every other day. After 9 days, blood was harvested by cheek bleed and assessed for NK cells (hCD45+, CD56+, CD3) in the blood by flow cytometry.ConclusionsThe data demonstrate that WU-NK-101 generated using a feeder cell-free expansion system has a memory phenotype and improved in vitro and in vivo anti-tumor activity compared to conventional NK cells. This activation and expansion platform will enable the development and clinical translation of multiple allogeneic NK cell therapies.

It has been reported that the Natural killer (NK) cell with FCGR3A (CD16a) V genotype is associated with enhanced clinical response to IgG1 monoclonal Ab (mAb) therapy such as trastuzumab, rituximab and cetuximab (1,2), suggesting a role of antibody-dependent cell-mediated cytotoxicity (ADCC) induced by NK cells. NK cells express three types of polymorphism of CD16; FcγRIIIa-158 VV, VF, FF, which are derived from the genotype of FCGR3A. It is a clinical challenge to improve the outcome in patients with FCGR3A 158FF genotype whose NK cells have lower affinity to mAb and mediate poor ADCC activity. The IL-15 superagonist/IL-15Rα-Fc fusion complex (ALT-803) activates the IL-15 receptor on CD8 T cells and NK cells, inducing their expansion, cytotoxity, and ADCC against B cell lymphoma (3, 4, 5). Here, we examined the effect of ALT803 on NK cell-mediated ADCC activity by the the anti-HER2 IgG1 mAb trastuzumab in HER2+ cell lines (SKBR3, BT474, MDA-MB-453). In addition, we used the anti-epidermal growth factor receptor(EGFR) IgG1 mAb cetuximab in EGFR positive TNBC cell lines (MDA-MB-231, SUM149, BT549). Finally, we examined the anti-PD-L1 IgG1 mAb avelumab was used for PD-L1 positive breast cancer cell lines (MDA-MB-231, BT549). Trastuzumab, cetuximab, and avelumab all significantly increased NK cell-induced lysis via ADCC. ALT803 significantly further increased both NK induced lysis and ADCC activity in all the cell lines. There was a significant positive correlation for the mean of ADCC lysis induced by NK cells from three FF (21%), three VF (33%), three VV (45%) donors. ALT803 significantly increased the mean of ADCC lysis by NK cells from all donors of each genotype to the same extent. ALT803 increased the expression of NK cell-activating receptors and cytotoxic granules regardless of the genotype of NK cell FCGR3A in terms VV, VF, or FF. We further examined the potential of ALT803 for NK cell-cytotoxicity suppressed by TGF-β1 which is one of the main barriers to immunity in the tumor microenvionment (TME). NK cells treated with TGF-β1 showed lower expression of activating receptors and cytotoxic granules, culminating in decreased lysis of MDA-MB231. ALT803 inhibited TGF-β1 from down-regulating the expression of NK cell-activating receptors and cytotoxic granules, and from suppressing the cytotoxicity of NK cells to MDA-MB231. In conclusion, the IL-15 superagonist ALT803 can potentially increase the clinical benefit of ADCC-based mAb therapy for breast cancer patients, regardless of the genotype of FCGR3A. Moreover, ALT803 prevented NK cell-cytotoxity from TGF-β1-induced suppression, providing a rationale for ALT803 therapy to overcome TME-mediated immunosuppression. References (1) Gavin et. al. JAMA Oncol.2017;3(3) (2) Musolino et. al. J Clin Oncol.2008;26(33) (3) Xu et. al. Cancer Res.2013;73(10) (4) Kim et. al. Oncotarget.2016;7(13) (5) Rosario et. al. Clin. Cancer Res. 2016; 22(3) Citation Format: Fujii R, Wong HC, Schlom J, Hodge JW. An IL-15 superagonist enhances antibody-dependent cell-mediated cytotoxicity against breast cancer cells regardless of FCGR3A (CD16) genotype and rescues NK cell from TGF-β1-induced immunosuppression [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-05-04.

Purpose: Pancreatic adenocarcinoma is a highly aggressive cancer, currently treated with limited success and dismal outcomes. New diagnostic and treatment strategies offer the potential to reduce cancer mortality. Developing highly specific noninvasive imaging probes for pancreatic cancer is essential to improving diagnostic accuracy and monitoring therapeutic intervention. Experimental Design: A bispecific heterodimer was synthesized by conjugating an anti-tissue factor (TF) Fab with an anti-CD105 Fab, via the bio-orthogonal “click” reaction between tetrazine (Tz) and trans-cyclooctene (TCO). The heterodimer was labeled with 64Cu for PET imaging of nude mice bearing BXPC-3 xenograft and orthotopic pancreatic tumors. Results: PET imaging of BXPC-3 (TF/CD105+/+) xenograft tumors with 64Cu-labeled heterodimer displayed significantly enhanced tumor uptake (28.8 ± 3.2 %ID/g; n = 4; SD) at 30 hours postinjection, as compared with each of their monospecific Fab tracers (12.5 ± 1.4 and 7.1 ± 2.6 %ID/g; n = 3; SD). In addition, the activity–concentration ratio allowed for effective tumor visualization (tumor/muscle ratio 75.2 ± 9.4 at 30 hours postinjection.; n = 4; SD). Furthermore, 64Cu-NOTA-heterodimer enabled sensitive detection of orthotopic pancreatic tumor lesions with an uptake of 17.1 ± 4.9 %ID/g at 30 hours postinjection and tumor/muscle ratio of 72.3 ± 46.7. Conclusions: This study demonstrates that dual targeting of TF and CD105 provided synergistic improvements in binding affinity and tumor localization of the heterodimer. Dual-targeted imaging agents of pancreatic and other cancers may assist in diagnosing pancreatic malignancies as well as reliable monitoring of therapeutic response. Clin Cancer Res; 22(15); 3821–30. ©2016 AACR.

Many signal transduction inhibitors are being developed for cancer therapy target pathways that are also important for the proper function of antitumor lymphocytes, possibly weakening their therapeutic effects. Here we show that most inhibitors targeting multiple signaling pathways have especially strong negative effects on T-cell activation at their active doses on cancer cells. In particular, we found that recently approved MEK inhibitors displayed potent suppressive effects on T cells in vitro. However, these effects could be attenuated by certain cytokines that can be administered to cancer patients. Among them, clinically available IL15 superagonists, which can activate PI3K selectively in T lymphocytes, synergized with MEK inhibitors in vivo to elicit potent and durable antitumor responses, including by a vaccine-like effect that generated resistance to tumor rechallenge. Our work identifies a clinically actionable approach to overcome the T-cell–suppressive effects of MEK inhibitors and illustrates how to reconcile the deficiencies of signal transduction inhibitors, which impede desired immunologic effects in vivo. Cancer Res; 76(9); 2561–72. ©2016 AACR.

Purpose: Anti-CD20 monoclonal antibodies (mAb) are an important immunotherapy for B-cell lymphoma, and provide evidence that the immune system may be harnessed as an effective lymphoma treatment approach. ALT-803 is a superagonist IL-15 mutant and IL-15Rα–Fc fusion complex that activates the IL-15 receptor constitutively expressed on natural killer (NK) cells. We hypothesized that ALT-803 would enhance anti–CD20 mAb-directed NK-cell responses and antibody-dependent cellular cytotoxicity (ADCC). Experimental Design: We tested this hypothesis by adding ALT-803 immunostimulation to anti-CD20 mAb triggering of NK cells in vitro and in vivo. Cell lines and primary human lymphoma cells were utilized as targets for primary human NK cells. Two complementary in vivo mouse models were used, which included human NK-cell xenografts in NOD/SCID-γc−/− mice. Results: We demonstrate that short-term ALT-803 stimulation significantly increased degranulation, IFNγ production, and ADCC by human NK cells against B-cell lymphoma cell lines or primary follicular lymphoma cells. ALT-803 augmented cytotoxicity and the expression of granzyme B and perforin, providing one potential mechanism for this enhanced functionality. Moreover, in two distinct in vivo B-cell lymphoma models, the addition of ALT-803 to anti-CD20 mAb therapy resulted in significantly reduced tumor cell burden and increased survival. Long-term ALT-803 stimulation of human NK cells induced proliferation and NK-cell subset changes with preserved ADCC. Conclusions: ALT-803 represents a novel immunostimulatory drug that enhances NK-cell antilymphoma responses in vitro and in vivo, thereby supporting the clinical investigation of ALT-803 plus anti-CD20 mAbs in patients with indolent B-cell lymphoma. Clin Cancer Res; 22(3); 596–608. ©2015 AACR.

IL15, a potent stimulant of CD8+ T cells and natural killer (NK) cells, is a promising cancer immunotherapeutic. ALT-803 is a complex of an IL15 superagonist mutant and a dimeric IL15 receptor αSu/Fc fusion protein that was found to exhibit enhanced biologic activity in vivo, with a substantially longer serum half-life than recombinant IL15. A single intravenous dose of ALT-803, but not IL15, eliminated well-established tumors and prolonged survival of mice bearing multiple myeloma. In this study, we extended these findings to demonstrate the superior antitumor activity of ALT-803 over IL15 in mice bearing subcutaneous B16F10 melanoma tumors and CT26 colon carcinoma metastases. Tissue biodistribution studies in mice also showed much greater retention of ALT-803 in the lymphoid organs compared with IL15, consistent with its highly potent immunostimulatory and antitumor activities in vivo. Weekly dosing with 1 mg/kg ALT-803 in C57BL/6 mice was well tolerated, yet capable of increasing peripheral blood lymphocyte, neutrophil, and monocyte counts by >8-fold. ALT-803 dose-dependent stimulation of immune cell infiltration into the lymphoid organs was also observed. Similarly, cynomolgus monkeys treated weekly with ALT-803 showed dose-dependent increases of peripheral blood lymphocyte counts, including NK, CD4+, and CD8+ memory T-cell subsets. In vitro studies demonstrated ALT-803–mediated stimulation of mouse and human immune cell proliferation and IFNγ production without inducing a broad-based release of other proinflammatory cytokines (i.e., cytokine storm). Based on these results, a weekly dosing regimen of ALT-803 has been implemented in multiple clinical studies to evaluate the dose required for effective immune cell stimulation in humans. Cancer Immunol Res; 4(1); 49–60. ©2015 AACR.

Background Neuroblastoma (NB) is the most common malignant extracranial solid tumor in children. Despite significant survival improvements in multiple pediatric malignancies, children with relapsed neuroblastoma (NB) have dismal outcomes. ROR1 is highly expressed on the majority of NB (Rebagay/Yan Front Oncol, 2012). Our group has successfully expanded functional and active peripheral blood NK cells (exPBNK) with irradiated feeder cells and electroporated CAR mRNA to exPBNK (Chu/Cairo, et al, Can Imm Res 2015). N-803 (ALT-803) is a novel IL15 superagonist complex where mutant IL-15N72D is bound to an IL-15RαSu-Fc fusion protein with higher potency and longer half-life compared to IL-15 (Han/Wong, Cytokine, 2011). Objectives To determine if anti-ROR1 CAR engineered NK cells can efficiently target ROR1+ NB and if the cytotoxicity can be further enhanced in combination with N-803. Methods ExPBNK cells were expanded with lethally irradiated K562-mbIL21cells (Denman/Lee Plos One, 2012) and isolated as previously described (Chu/Cairo, Can Imm Res, 2015). ExPBNK cells were electroporated with anti-ROR1-CAR mRNA (ROR1-CAR was generously provided by Riddell S) using maxcyte electroporator. ALT-803 was generously provided by Altor BioScience. CHLA-255 and CHLA-136 were generously provided by Seeger R. In vitro cytotoxicity of anti-ROR1 CAR NK against NB cell lines were examined at different E:T ratios. Granzyme B, Perforin and IFN-γ levels were evaluated by ELISA assays. High dimentionsal CyTOF analysis was performed to character the immunophenotying of anti-ROR1-CAR-NK. NB xenografted NSG mice were generated and tumor burden was monitored as we previously described (Chu/Cairo, Oncoimmunology 2017). Results Anti-ROR1-CAR was efficiently expressed on exPBNK after CAR mRNA-electroporation at 24 hrs. Anti-ROR1-CAR-NK were effective in killing NB cells even at very low E:T ratio compared to mock NK with enhanced granzyme B, perforin and interferon- release (p Conclusion anti-ROR1-CAR NK had significantly enhanced cytotoxicity against ROR1+ NB cells in vitro and in vivo using human NB xenografted NSG mice compared to controls. ALT-803 further enhanced the anti-tumor effect of anti-ROR1 CAR NK (supported by W81XWH-17-1-0420).

Shedding of the human NKG2D ligand MIC (MHC class I-chain-related molecule) from tumor cell surfaces correlates with progression of many epithelial cancers. Shedding-derived soluble MIC (sMIC) enables tumor immune escape through multiple immune suppressive mechanisms, such as disturbing natural killer (NK) cell homeostatic maintenance, impairing NKG2D expression on NK cells and effector T cells, and facilitating the expansion of arginase I+ myeloid suppressor cells. Our recent study has demonstrated that sMIC is an effective cancer therapeutic target. Whether targeting tumor-derived sMIC would enhance current active immunotherapy is not known. Here, we determined the in vivo therapeutic effect of an antibody co-targeting sMIC with the immunostimulatory IL-15 superagonist complex, ALT-803, using genetically engineered transplantable syngeneic sMIC+ tumor models. We demonstrate that combined therapy of a nonblocking antibody neutralizing sMIC and ALT-803 improved the survival of animals bearing sMIC+ tumors in comparison to monotherapy. We further demonstrate that the enhanced therapeutic effect with combined therapy is through concurrent augmentation of NK and CD8 T cell anti-tumor responses. In particular, expression of activation-induced surface molecules and increased functional potential by cytokine secretion are improved greatly by the administration of combined therapy. Depletion of NK cells abolished the cooperative therapeutic effect. Our findings suggest that administration of the sMIC-neutralizing antibody can enhance the anti-tumor effects of ALT-803. With ALT-803 currently in clinical trials to treat progressive solid tumors, the majority of which are sMIC+, our findings provide a rationale for co-targeting sMIC to enhance the therapeutic efficacy of ALT-803 or other IL-15 agonists.

Glioblastoma is the most aggressive primary central nervous system malignancy with a poor prognosis in patients. Despite the need for better treatments against glioblastoma, very little progress has been made in discovering new therapies that exhibit superior survival benefit than the standard of care. Immunotherapy has been shown to be a promising treatment modality that could help improve clinical outcomes of glioblastoma patients by assisting the immune system to overcome the immunosuppressive tumor environment. Interleukin-15 (IL-15), a cytokine shown to activate several effector components of the immune system, may serve as an excellent immunotherapeutic candidate for the treatment of glioblastoma. Thus, we evaluated the efficacy of an IL-15 superagonist complex (IL-15N72D:IL-15RαSu-Fc; also known as ALT-803) in a murine GL261-luc glioblastoma model. We show that ALT-803, as a single treatment as well as in combination with anti-PD-1 antibody or stereotactic radiosurgery, exhibits a robust antitumor immune response resulting in a prolonged survival including complete remission in tumor bearing mice. In addition, ALT-803 treatment results in long-term immune memory against glioblastoma tumor rechallenge. Flow cytometric analysis of tumor infiltrating immune cells shows that ALT-803 leads to increased percentage of CD8+-cell infiltration, but not the NK cells, and IFN-γ production into the tumor microenvironment. Cell depletion studies, in accordance with the flow cytometric results, show that the ALT-803 therapeutic effect is dependent on CD4+ and CD8+ cells. These results provide a rationale for evaluating the therapeutic activity of ALT-803 against glioblastoma in the clinical setting.

Effector CD8+ T cells conditioned with IL12 during activation mediate enhanced antitumor efficacy after adoptive transfer into lymphodepleted hosts; this is due in part to improved IL7 responsiveness. Therefore, we hypothesized that increasing the intensity or type of lymphodepletion would deplete more IL7-consuming host cells and improve the persistence and antitumor activity of IL12-conditioned CD8+ T cells. Using cyclophosphamide, fludarabine, and total body irradiation (TBI, 6 Gy) either individually or in combination, we found that combined lymphodepletion best enhanced T-cell engraftment in mice. This improvement was strongly related to the extent of leukopenia, as posttransfer levels of donor T cells inversely correlated to host cell counts after lymphodepletion. Despite the improvement in engraftment seen with combination lymphodepletion, dual-agent lymphodepletion did not augment the antitumor efficacy of donor T cells compared with TBI alone. Similarly, IL7 supplementation after TBI and transfer of tumor-reactive T cells failed to improve persistence or antitumor immunity. However, IL15 or IL2 supplementation greatly augmented the persistence and antitumor efficacy of donor tumor-reactive T cells. Our results indicate that the amount of host IL7 induced after single agent lymphodepletion is sufficient to potentiate the expansion and antitumor activity of donor T cells, and that the efficacy of future regimens may be improved by providing posttransfer support with IL2 or IL15. Significance: The relationship between lymphodepletion and cytokine support plays a critical role in determining donor T-cell engraftment and antitumor efficacy. Cancer Res; 78(11); 3067–74. ©2018 AACR.

Dual-targeted imaging agents have shown improved targeting efficiencies in comparison to single-targeted entities. The purpose of this study was to quantitatively assess the tumor accumulation of a dual-labeled heterobifunctional imaging agent, targeting two overexpressed biomarkers in pancreatic cancer, using positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging modalities. A bispecific immunoconjugate (heterodimer) of CD105 and tissue factor (TF) Fab′ antibody fragments was developed using click chemistry. The heterodimer was dual-labeled with a radionuclide (64Cu) and fluorescent dye. PET/NIRF imaging and biodistribution studies were performed in four-to-five week old nude athymic mice bearing BxPC-3 (CD105/TF+/+) or PANC-1 (CD105/TF−/−) tumors xenografts. A blocking study was conducted to investigate the specificity of the tracer. Ex vivo tissue staining was performed to compare TF/CD105 expression in tissues with PET tracer uptake to validate in vivo results. PET imaging of 64Cu-NOTA-heterodimer-ZW800 in BxPC-3 tumor xenografts revealed enhanced tumor uptake (21.0 ± 3.4 %ID/g; n = 4) compared to the homodimer of TRC-105 (9.6 ± 2.0 %ID/g; n=4; p < 0.01) and ALT-836 (7.6 ± 3.7 %ID/g; n=4; p < 0.01) at 24 h post-injection. Blocking studies revealed that tracer uptake in BxPC-3 tumors could be decreased by four-fold with TF blocking and two-fold with CD105 blocking. In the negative model (PANC-1), heterodimer uptake was significantly lower than that found in the BxPC-3 model (3.5 ± 1.1 %ID/g; n=4; p < 0.01). The specificity was confirmed by the successful blocking of CD105 or TF, which demonstrated the dual targeting with 64Cu-NOTA-heterodimer-ZW800 provided an improvement in overall tumor accumulation. Also, fluorescence imaging validated the PET imaging, allowing for clear delineation of the xenograft tumors. Dual-labeled heterodimeric imaging agents, like 64Cu-NOTA-heterodimer-ZW800, may increase the overall tumor accumulation in comparison to single-targeted homodimers, leading to improved imaging of cancer and other related diseases.

Background The overall survival rate for pediatric osteosarcoma (OS), neuroblastoma (NB) and glioblastoma (GBM) in children with relapsed/refractory and metastatic disease remains dismal. Our group has successfully expanded peripheral blood Natural Killer cells (exPBNK) with irradiated feeder cells (Chu/Cairo, et al, Can Imm Res 2015). ALT-803 (N-803) is a superagonist of an IL-15 variant bound to an IL-15RαSu-Fc fusion with enhanced IL-15 biological activity (Zhu et al. 2009 J Immunol). Tim-3 was found expressed on active NK cells and its expression in cancer patientsO NK cells correlated with disease stage and prognosis. Objective To determine if the combination of N-803 and TIM-3 blockage significantly enhances exPBNK cell cytotoxicity against OS, GBM and NB Method PBMCs were expanded with lethally irradiated K562-mbIL21-41BBL cells. CD56+CD3− exPBNK cells were isolated using Miltenyi NK cell isolation kit. N-803 was generously provided by Altor BioScience. ExPBNK were cultured with 3.5ng/ml N-803 and 10 or 20ug/ml anti-TIM-3 antibody. NK proliferation, NK receptors expression and cytotoxicity were examined (Chu/Cairo, oncoimmunology, 2017). Results N-803 increased NK activating receptorO expression: NKG2D, NKp30, NKp44, and NKp46. N-803 significantly enhanced exPBNK in vitro cytotoxicity against OS, NB and GBM (p Conclusions The combination of N-803 and anti-Tim-3 blockage significantly enhanced exPBNK in-vitro cytotoxicity against OS, NB and GBM cells. In-vivo effects of the combination are under investigation.

Background Neuroblastoma is the most common malignant extracranial solid tumor in children (Park/Eggert Pediatr Clin North Am., 2011). Despite significant survival improvements in multiple pediatric malignancies, children with relapsed neuroblastoma (NB) (London/Matthay, Journal of Clinical Oncology, 2011) continue to have dismal outcomes. Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is highly expressed in the majority of neuroblastoma (Rebagay/Yan Front Oncol, 2012). ALT-803 is a novel IL-15 superagonist complex where mutant IL-15N72D is bound to an IL-15RαSu-Fc fusion protein (Xu/Wong, Cancer Res, 2013). ALT-803 (N-803) has been shown to have superior biological activity, higher potency, and a longer half-life compared to IL-15 (Han/Wong, Cytokine, 2011), and is currently being tested in several clinical trials. Objectives We hypothesized that ROR1-CAR NK cells by mRNA electroporation will be highly effective against neuroblastoma alone, and in combination with ALT-803. Methods PBMCs were isolated and expanded ex vivo with lethally irradiated K562-mbIL21cells (Denman/Lee Plos One, 2012). ExPBNK cells were isolated using Miltenyi NK cell isolation kits as previously described (Chu/Cairo, Can Imm Res, 2015). ExPBNK cells (aNK) were electroporated with anti-ROR1-CAR mRNA with a 4-1BB signaling domain which is synthesized in vitro (ROR1-CAR was generously provided by Riddell S, MD, Fred Hutch, WA). N-803 was generously provided by Altor BioScience. CHLA-136, CHLA-255 cells were generously provided by Seeger R.C. MD, UCLA). Functional cytotoxic activity of anti-ROR1-CAR NK cells against neuroblastoma cell lines were examined via LDH-based and luciferase-based assays at different E:T ratios. IFN-g levels were evaluated by ELISA assays. Results Flow cytometry demonstrated that ROR1 is highly expressed in different neuroblastoma cell lines (CHLA-136, CHLA-255, SKN-BE-(2)). Anti-ROR1-CAR was expressed on up to 90% of anti-ROR1 mRNA-transfected NK cells at 24 hrs. Anti-ROR1-CAR expressing NK cells had significantly higher in vitro cytotoxicity compared to mock NK cells at 6 hours (p Conclusion These data demonstrated that N-803 significantly increases in vitro cytotoxicity of anti-ROR1-CAR NK cells against ROR1 neuroblastoma cells. In vivo studies using human NB xenograft mouse model are currently in progress.

NK cells, lymphocytes of the innate immune system, are important for defense against infectious pathogens and cancer. Classically, the CD56dim NK cell subset is thought to mediate antitumor responses, whereas the CD56bright subset is involved in immunomodulation. Here, we challenge this paradigm by demonstrating that brief priming with IL-15 markedly enhanced the antitumor response of CD56bright NK cells. Priming improved multiple CD56bright cell functions: degranulation, cytotoxicity, and cytokine production. Primed CD56bright cells from leukemia patients demonstrated enhanced responses to autologous blasts in vitro, and primed CD56bright cells controlled leukemia cells in vivo in a murine xenograft model. Primed CD56bright cells from multiple myeloma (MM) patients displayed superior responses to autologous myeloma targets, and furthermore, CD56bright NK cells from MM patients primed with the IL-15 receptor agonist ALT-803 in vivo displayed enhanced ex vivo functional responses to MM targets. Effector mechanisms contributing to IL-15-based priming included improved cytotoxic protein expression, target cell conjugation, and LFA-1-, CD2-, and NKG2D-dependent activation of NK cells. Finally, IL-15 robustly stimulated the PI3K/Akt/mTOR and MEK/ERK pathways in CD56bright compared with CD56dim NK cells, and blockade of these pathways attenuated antitumor responses. These findings identify CD56bright NK cells as potent antitumor effectors that warrant further investigation as a cancer immunotherapy.

Purpose: ALT-801 is a bifunctional fusion protein comprising interleukin-2 (IL-2) linked to a soluble, single-chain T-cell receptor domain that recognizes a peptide epitope (aa264–272) of the human p53 antigen displayed on cancer cells in the context of HLA-A*0201 (p53+/HLA-A*0201). We evaluated the safety, pharmacokinetics, and pharmacodynamics of ALT-801 in p53+/HLA-A*0201 patients with metastatic malignancies. Experimental Design: p53+/HLA-A*0201 patients were treated with ALT-801 on a schedule of four daily 15-minute intravenous infusions, then 10 days rest and four more daily infusions. Cohorts of patients were treated at 0.015, 0.040, and 0.080 mg/kg/dose. Results: Four, 16, and 6 patients were treated at the 0.015, 0.04, and 0.08 mg/kg cohorts, respectively. Two dose-limiting toxicities (a grade 4 transient thrombocytopenia and a myocardial infarction) in the 0.08 mg/kg cohort established the maximum tolerated dose (MTD) at 0.04 mg/kg. Patients treated at the MTD experienced toxicities similar to those associated with high-dose IL-2 but of lesser severity. The serum half-life of ALT-801 was 4 hours and ALT-801 serum recovery was as expected based on the dose administered. ALT-801 treatment induced an increase of serum IFN-γ but not TNF-α. Response assessment showed 10 subjects with stable disease at at least 11 weeks, and in one who had melanoma metastasis, there is an ongoing complete absence of identifiable disease after resection of radiographically identified lesions. Conclusion: This first-in-man study defines an ALT-801 regimen that can be administered safely and is associated with immunologic changes of potential antitumor relevance. Clin Cancer Res; 17(24); 7765–75. ©2011 AACR.

Background: The prognosis of children with relapsed/refractory B-cell (CD20+) non-Hodgkin lymphoma (B-NHL), including Burkitt lymphoma (BL), is dismal due to chemo-radiotherapy resistance (Cairo et al, JCO, 2012, Cairo et al, BJH, 2018). Rituximab has been widely used in frontline treatment of B-NHL, however, some patients retreated with rituximab will relapse which limits patient treatment options (Goldman/Cairo, Leukemia, 2013). Several strategies for overcoming rituximab-resistance are currently being evaluated, including engineering of NK cells with chimeric antigen receptors (Chu/Cairo et al, Can Imm Res 2015), as well as second-generation engineered anti-CD20 antibodies (Tiwari/Cairo et al BJH 2015). To enhance NK based therapy, our group has successfully expanded the functionally activated peripheral blood NK cells (exPBNK) with irradiated feeder cells (Chu/Cairo, et al, Can Imm Res 2015). To enhance NK cell-based targeted therapy, we had successfully engineered expanded NK cells with chimeric antigen receptors (Chu/Cairo, et al, Can Imm Res 2015) and combined expanded NK with an anti-CD20 targeted IL-15 fusion protein N-820 (2B8T2M) targeting rituximab-sensitive and -resistant BL (Chu/Cairo, et al, ASH 2017). N-820 was generated by fusing ALT-803 to four single-chain antibody domains of rituximab (Liu/Wong, et al, JBC, 2016). ALT-803 is a superagonist of an IL-15 variant bound to an IL-15RαSu-Fc fusion with enhanced IL-15 biological activity (Zhu et al. 2009 J Immunol), longer half-life and increased potency (Han, et al. Cytokine. 2011). N-820 displayed tri-specific binding activity through its recognition of CD20 on tumor cells, activated NK cells to enhance antibody-dependent cellular cytotoxicity (ADCC), and induced apoptosis of B-lymphoma cells (Liu/Wong, et al, JBC, 2016). Objective: To investigate how N-820 modulates the crosstalk between BL tumor cells and expanded NK cells by monitoring cytokines, chemokines and growth factors released and the anti-tumor effects of N-820 on NK cells in BL xenografts. Method: PBMCs were expanded with lethally irradiated K562-mbIL21-41BBL cells and isolated as we have previously described with K562-mbIL15-41BBL (Chu/Cairo, et al, Can Imm Res 2015). ALT-803 and N-820 were generously provided by Altor Bioscience. Equal doses of N-820, Rituximab (R), ALT-803, R+ALT-803, obinutuzumab (obinu, generously provided by Christian Klein, PhDfrom Roche) were used for comparison. IgG was used as control. Rituximab-sensitive Raji and -resistant BL cells Raji-2R and Raji-4RH were used as target cells. NK cells were sorted by Beckman Coulter MoFlo XDP high-speed sorter and NK purity was confirmed by flow cytometry. Cytokines, chemokines and growth factors mRNA levels in NK cells were monitored by real-time PCR. Secreted cytokines, chemokines and growth factors were examined by standard ELISAs. Raji-2R-Luc cells were injected into NSG mice. ExPBNK+N-820 and controls were injected to the xenografted NSG mice. The tumor burden was measured with the IVIS-200 system. Results: We found that ALT-803 and N-820 at equal doses significantly enhanced the expression of NK activating receptors such as NKG2D, NKp30, NKp44, and CD16 on exPBNK compared to IgG controls. Additionally, N-820 significantly enhanced exPBNK cytotoxicity against Raji, Raji-2R and Raji-4RH cells compared to the controls IgG, R, ALT-803, R+ALT-803, or obinu (p Conclusions: N-820 significantly enhanced exPBNK cytotoxicity, IFN-g, granzyme B, GM-CSF release from exPBNK in-vitro, inhibited MDC release from BL and increased anti-tumor activity against BL xenografts in NSG mice. Disclosures Jeng: Altor: Employment. Alter:NANTCell / Altor Bioscience: Employment. Rhode:NANTCell / Altor Bioscience: Employment, Patents & Royalties: Altor Bioscience. Lee:NantKwest: Employment. Lee:Merck, Sharp, and Dohme: Consultancy; Courier Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; CytoSen Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding. Wong:Altor: Employment, Equity Ownership, Patents & Royalties: Altor Bioscience. Cairo:Janssen: Research Funding.

This abstract is being presented as a short talk in the scientific program. A full abstract is printed in the Proffered Abstracts section (PR05) of the Conference Proceedings. Citation Format: John Wrangle, Vamshidar Velchetti, Manish Patel, Samantha Suriano, Marzena Swiderska-syn, Kate Anderton, James Ravenel, Elisabeth Hill, Amy Rock, Hing Wong, Mark Rubinstein. Safety and activity of the IL-15/sIL-15Rα complex ALT-803 in combination with the anti-PD1 mAb nivolumab in metastatic non-small cell lung cancer [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr B34.

Background: Despite the unprecedented efficacy of immunotherapy with PD-1/PD-L1 blockade in non-small cell lung cancer, treatment failure is common and an area of unmet clinical need. IL-2/IL-15Rbg agonists, which potently augment the activity of CD8+ T and NK cells, have shown the ability to induce complete and durable responses in select cancers. However, this class of drugs has been neither combined with PD-1 targeted immunotherapies nor examined extensively in NSCLC. We hypothesized that the addition of an IL-15 superagonist (ALT-803) would be safe and improve the efficacy of anti-PD-1 mAb (nivolumab) in metastatic NSCLC patients in second-line therapy. Methods: In this phase Ib trial, patients with metastatic NSCLC who were eligible to initiate or to continue receiving nivolumab (anti-PD-1 mAb) after first-line therapy were enrolled to trial, including patients with disease relapsed on or refractory to PD-1 blockade. Nivolumab was administered in combination with ALT-803 at four dose levels. The primary objective of the study was to define safety and tolerability and a recommended phase II dose of ALT-803 in combination with nivolumab, with secondary objectives including immune correlates and clinical parameters including response rate. Results: In a first-in-human combination, 21 NSCLC patients were treated at 4 dose levels of ALT-803 in combination with the approved dose of nivolumab. No dose-limiting toxicities were observed, and the most common adverse events were fever, flu-like symptoms, and injection site reactions. The recommended phase II dose (RP2D) of therapy is weekly subcutaneous ALT-803 at 20 mcg/kg in combination with intravenous nivolumab 240 mg every 14 days. Multiple objective responses were observed, including when ALT-803 was added to a failing PD-1 mAb regimen. Discussion: The recommended phase II dose of ALT-803 plus nivolumab was determined and the regimen can safely be administered in the outpatient setting. Responses among patients with PD-1 mAb relapsed disease with the addition of ALT-803 to their regimen demonstrate for the first time evidence of antitumor activity for a new class of agents for NSCLC and in patients with disease progression on PD-1/PD-L1 pathway blockade. This abstract is also being presented as Poster B34. Citation Format: John Wrangle, Vamshidar Velchetti, Manish Patel, Samantha Suriano, Marzena Swiderska-syn, Kate Anderton, James Ravenel, Elisabeth Hill, Amy Rock, Hing Wong, Mark Rubinstein. Safety and activity of the IL-15/sIL-15Rα complex ALT-803 in combination with the anti-PD1 mAb nivolumab in metastatic non-small cell lung cancer [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr PR05.

Immunotherapy is an effective modality for cancer treatment. IL-15 is a cytokine that stimulates CD8+ T cell and NK cell survival, metabolism, and anti-tumor immunity. We hypothesized that IL-15 therapy will augment anti-cancer immune responses in concert with tumor-targeting therapeutic mAbs. Pre-clinical studies have shown that the IL-15 receptor super agonist complex, ALT-803, enhanced the anti-tumor effects of anti-CD20 mAb against iNHL in vitro and in vivo (Rosario M et al., Clin Ca Res, 2016). Here, we conducted the first clinical study combining ALT-803 with a tumor-targeting mAb, in patients with rel/ref iNHL. A total of 21 iNHL patients [N=16 FL (FLIPI low: 4, int: 4, high: 8), N=4 MZL, N=1 SLL] were treated. Median age was 64 years (range 54-81), median number of prior therapies was 2 (range 1-18), and 5 patients were refractory to prior anti-CD20 mAb therapy. Treatment included an induction cycle of 4 weekly doses of rituximab (375 mg/m2 intravenously, IV) and ALT-803. The ALT-803 dose levels were 1, 3, and 6 mcg/kg IV followed by 6, 10, 15, and 20 mcg/kg subcutaneously (SC, N=3 per dose level). In patients with SD, PR or CR, consolidation therapy with ALT-803 (at the same dose level) and rituximab was then administered every 8 weeks for 4 cycles. No dose limiting toxicities were observed, and the recommended phase 2 dose was 20 mcg/kg SC. Overall ALT-803 was well tolerated with no grade 4 or 5 adverse events. Grade 3 AEs (regardless of attribution to ALT-803) included transient hypertension (14%), anemia (5%), nausea (5%), chills (5%), fever (5%), neutropenia (5%), and hyperglycemia (5%). Patients who received ALT-803 SC had a self-limited injection site reaction. To date, best ORR was 10/21 (48%), with 7 CR, 3 PR, 8 SD (all with decreased tumor volume), and 3 PD. Of the 12 patients treated with SC ALT-803, only 1 patient had PD, with the remainder receiving ongoing consolidation. In the 5 rituximab-refractory patients we observed 1 CR, 2 SD (45% and 36% tumor volume decrease), and 2 PD. PK analysis showed that SC ALT-803 administration resulted in a lower Cmax and prolonged blood levels compared to IV. At the 15 and 20 mcg/kg SC dose levels of ALT-803, NK cells were significantly increased by percentage (mean peak 30+5%, 6-fold, P Citation Format: Todd A. Fehniger, Brian T. Hess, Veronika Bachanova, Michelle Becker-Hapak, Ethan McClain, Melissa Berrien-Elliott, Julia Wagner, Nancy L. Bartlett, Brad Kahl, Neha Mehta-Shah, Amanda F. Cashen, Feng Gao, Kyle Conradi, Amy D. Rock, Emily K. Jeng, Liza Hernandez, Jack O. Egan, Peter R. Rhode, Hing C. Wong. First-in-human phase I combination of the IL-15 receptor super agonist complex ALT-803 with a therapeutic (anti-CD20) monoclonal antibody (mAb) for patients with relapsed or refractory indolent non-Hodgkin lymphoma (iNHL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT146.

The clinical promise of cancer immunotherapy relies on the immune system recognizing and eliminating tumor cells identified as non-self. However, the tumor microenvironment (TME) can greatly impede a tumor targeted immune response via increasing immunosuppressive cells or hampering T and NK cell maturation, recruitment, and function through numerous pathways, including upregulation of immune checkpoints such as PD-L1. Hence, there is an unmet clinical need to develop effective therapeutic strategies that can reprogram the TME to restore tumor immune recognition and reverse immune evasion. We recently demonstrated that Entinostat, a class I histone deacetylase (HDAC) inhibitor, reverses tumor immune escape to T cell-mediated lysis. We hypothesize that the immune-mediated tumor elimination promoted by the IL-15/IL-15Rα superagonist ALT803 in combination with PD-L1 checkpoint blockade or a therapeutic adenoviral vaccine targeting CEA (Ad-CEA) will be augmented by the epigenetic reprograming of the TME induced by Entinostat. In preclinical studies, ALT803 has been shown to exhibit potent antitumor activity in multiple murine models of cancer through the expansion of NK and CD8+ T cells with high effector function. Here, we demonstrate that Entinostat increased extracellular expression of immune-relevant proteins on murine colon and breast carcinoma cells in vitro. Additionally, the frequency of T cells with an activated phenotype were increased in non-tumor bearing mice. In the MC38-CEA murine model of colon carcinoma, the addition of Entinostat augmented the antitumor activity promoted by ALT803 plus Ad-CEA resulting in increased survival. Furthermore, in the 4T1 murine model of triple-negative breast cancer, the combination of Entinostat with ALT803 and a monoclonal antibody targeting PD-L1 significantly reduced primary tumor weight relative to ALT803 plus anti-PD-L1 therapy and resulted in a significant reduction of the number of 4T1 tumor-forming cells in the lung. The immune mechanism associated with antitumor efficacy of these multivalent therapies was examined in both the periphery and TME. Overall, these studies provide a rationale for combining Entinostat with multivalent immunotherapy combinations, including cytokines, antibodies targeting PD-L1, and therapeutic cancer vaccines. Citation Format: Kristin C. Hicks, Karin M. Knudson, Frank R. Jones, Peter Ordentlich, Shahrooz Rabizadeh, Hing C. Wong, James W. Hodge, Jeffrey Schlom, Sofia R. Gameiro. Epigenetic reprogramming of the tumor microenvironment by entinostat increases tumor sensitivity to multivalent immunotherapy combinations with an IL-15 superagonist plus vaccine or immune checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1740.

Background: ALT-803 is a novel IL-15 superagonist complex consisting of an IL-15 mutant (IL-15N72D) bound to an IL-15 receptor α/IgG1 Fc fusion protein. It has been reported that ALT-803 exerted a significant anti-tumor activity in murine myeloma, breast, colon carcinomas, in rat bladder cancer, and in human bladder, ovarian carcinomas in vivo. ALT-803 was also found to enhance ADCC against wide range of human carcinoma cells in vitro. NEO-201 is a novel humanized IgG1 monoclonal antibody (mAb) which was found to react against a variety of cultured human carcinoma cell lines and tumor tissues. NEO-201 can mediate ADCC against human carcinoma cells. Methodology: We investigate the ability of ALT-803 to modulate the ADCC mediated by NE0-201, employing NK cells isolated from normal donors as effectors. Human NK cells were treated with ALT-803 at different concentrations for 48h prior to be used as effector cells, and human carcinoma cell lines expressing the NE0-201 antigen were utilized as targets in an in vitro non-radioactive ADCC assay. The ability of ALT-803 to affect the phenotype of NK cells and to modulate NK cells gene expression was evaluated by flow cytometry and by using the Nanostring analysis respectively. Results: The treatment with ALT-803 significantly enhanced the ADCC activity mediated by NEO-201 against NEO-201 positive carcinoma cells. The effect ALT-803 was dose-dependent and achieved statistical significance at all doses tested compared to vehicle control treatment. Treatment of NK cells with ALT-803 enhanced ADCC activity also from donors with minimal ADCC activity and lowered the effective dose of NEO-201 required to initiate the ADCC response compared to untreated NK cells. ADCC activity can be blocked by using anti-CD16 and anti-TIM3 blocking antibody. Phenotypic analysis of NK cells treated with 25 ng/ml of ALT-803 for 48h demonstrated that ALT-803 can enhance the expression of TIM3 and NKG2D and the mean fluorescence intensity (MFI) of granzyme B and CD107a in CD16/CD56 positive NK cells. Nanostring analysis of human NK cells treated with ALT-803 at different concentrations for 48h, showed that ALT-803 was able to modulate mRNA expression of 62 genes (1.6 log2 fold change compared to vehicle control was considered significant). ALT-803 up-regulated the mRNA expression of 43 genes, including NK activating receptors, factors involved in the NK cytotoxicity, cytokines and their receptors, and down-regulated the mRNA expression of 19 genes, including NK inhibiting receptors and factors involved in the activation of apoptosis. Conclusions: ALT-803 can enhance ADCC activity mediated by NEO-201 against human carcinoma cells. The enhancement of the ADCC activity may be in part due to the increase in the expression of TIM3, NKG2D, granzyme B, and CD107a positive NK cells, as well as to the modulation of transcripts that are involved in the NK activation and cytotoxicity. Citation Format: Massimo Fantini, Justin M. David, Hing C. Wong, Christina M. Annunziata, Philip M. Arlen, Kwong Yok Tsang. ALT-803 enhances antibody-dependent cell-mediated cytotoxicity (ADCC) mediated by NEO-201 against human carcinoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 619.

Background: The prognosis of patients with relapsed or progressed B-cell (CD20+) non-Hodgkin lymphoma (B-NHL), including Burkitt lymphoma (BL), is dismal due to chemo-radiotherapy resistance (Cairo et al, JCO, 2012, Cairo et al, BJH, 2016). Rituximab has been widely used in frontline treatment of B-NHL, however, some patients retreated with rituximab relapse which limit patient treatment options (Goldman/Cairo, Leukemia, 2013). Several strategies for overcoming rituximab-resistance are currently being evaluated, including engineering immune cells with chimeric antigen receptors (Chu/Cairo et al, Can Imm Res 2015), as well as second-generation engineered anti-CD20 antibodies (Tiwari/Cairo et al BJH 2015). Nature Killer (NK) cells are bone marrow-derived cytotoxic lymphocytes that play a major role in the rejection of tumors. A variety of activating and inhibitory receptors on the NK cell surface are engaged to regulate NK cell activities. However, NK therapy is limited by several factors, including small numbers of active NK cells in unmodified peripheral blood, lack of tumor targeting specificity, and multiple mechanisms of tumor escape of NK cell immunosurveillance. Our group has successfully expanded functional and active peripheral blood NK cells (exPBNK) with irradiated feeder cells (Chu/Cairo, et al, Can Imm Res 2015). ALT-803 (Altor BioScience Corporation) is a superagonist of an IL-15 variant bound to an IL-15RαSu-Fc fusion with enhanced IL-15 biological activity (Zhu et al . 2009 J Immunol), longer half-life and increased potency (Han, et al . Cytokine . 2011). Recently, a new therapeutic molecule 2B8T2M was generated by fusing ALT-803 to four single-chains of the tumor-targeting monoclonal antibody, rituximab (Liu/Wong, et al, JBC, 2016). 2B8T2M displayed tri-specific binding activity through its recognition of the CD20 molecule on tumor cells, activated NK cells to enhance antibody-dependent cellular cytotoxicity (ADCC), and induced apoptosis of B-lymphoma cells (Liu/Wong, et al, JBC, 2016). Objective: To examine if 2B8T2M significantly enhances the cytotoxicity of exPBNK against rituximab-sensitive and -resistant BL cells. Method: PBMCs were expanded with lethally irradiated K562-mbIL21-41BBL cells (Denman/Dean Lee, PLoS One, 2012). CD56+CD3- exPBNK cells were isolated using Miltenyi NK cell isolation kit. ALT-803 and 2B8T2M were generously provided by Altor BioScience Corporation. NK receptors expression and cytotoxicity were examined as we previous described (Chu/Cairo, et al, Can Imm Res 2015). IFN-γ and granzyme B levels were examined by standard enzyme-linked immunosorbent assays. Equal doses of Rituximab, ALT-803, Rituximab+ALT-803, obinutuzumab (obinu) were used for comparison. IgG was used as controls. Rituximab-sensitive and -resistant BL cells Raji, Raji-2R and Raji-4RH, were used as target cells. Results: Both ALT-803 and 2B8T2M at equal doses significantly enhanced NK activating receptors such as NKG2D, NKp30, NKp44, and CD16 on exPBNK cells compared to IgG controls. 2B8T2M significantly enhanced exPBNK cytotoxicity against rituximab-sensitive Raji cells compared to the controls IgG, Rituximab, ALT-803, Rituximab+ALT-803, obinu (2B8T2M vs IgG vs ALT803 vs Rituximab vs ALT803+Rituximab vs obinu=98.34%+0.24% vs 56.5+1.83% vs 62.8+1.1% vs 69.52+1.79% vs 81.85+1.83% vs 81.23+1.8%, p Conclusions: 2B8T2M significantly enhanced exPBNK activating receptor expression and in vitro cytotoxicity against rituximab-sensitive and -resistant BL cells. The in vivo functions of 2B8T2M with exPBNK against rituximab-sensitive and -resistant BL cells using humanized NSG models are under investigation. Download : Download high-res image (138KB) Download : Download full-size image Disclosures Alter: Altor BioScience: Employment, Equity Ownership. Jeng: Altor Bioscience: Employment, Equity Ownership. Rhode: Altor BioScience: Employment, Equity Ownership. Lee: Intellia Therapeutics, Inc: Consultancy; Courier Therapeutics, Inc: Membership on an entity's Board of Directors or advisory committees; Miltenyi: Speakers Bureau; Cyto-Sen Therapeutics, Inc: Other: Founder, Vice President, Medical Director. Wong: Altor BioScience: Employment, Equity Ownership. Cairo: Jazz Pharmaceuticals: Speakers Bureau.

510 Background: The current standard of care for patients with high risk NMIBC is a TURBT or biopsy followed by a 6-week induction course of intravesical BCG and supplementary maintenance therapies every 3 months thereafter (Lamm 2000). While clinical response is significantly improved with BCG, 50% of patients are still expected to recur within 12 months ( Sfakianos 2014). The pursuit of novel agents to prevent progression and recurrence of NMIBC remains critical. This phase Ib clinical trial evaluates the safety and tolerability of ALT-803, an IL-15 superagonist, plus BCG in BCG-naïve NMIBC patients. Methods: A dose escalation 3+3 design was employed to evaluate intravesical ALT-803 plus 50 mg BCG in BCG-naïve patients with intermediate or high-risk Ta, T1 or Tis stage NMIBC. Patients received intravesical ALT-803 in conjunction with BCG weekly for 6 consecutive weeks (induction) and then encouraged (but not required) to receive maintenance BCG alone as per standard practices. Patients had a routine cystoscopy and voided urinary cytology (VUC) every 3 months for 2 years to determine response. Negative cystoscopy, VUC and/or biopsy yielded a complete response (CR). No cohort (100, 200 or 400 μg/instillation ALT-803) experienced any dose limiting toxicities. Results: All patients are disease-free (CR) at 24 months; no patients experienced disease recurrence or progression. Adverse events consistent with SoC BCG treatment (hematuria and urinary tract pain) were reported in all cohorts. One patient experienced a grade 3 adverse event of hypertension that resolved the same day. No grade 4 toxicities or DLTs were observed. Clinical trial information: NCT02138734. Conclusions: Intravesical ALT-803 plus BCG treatment is well tolerated in BCG-naïve patients with NMIBC. All patients are disease-free 24 months after treatment with BCG and ALT-803. A randomized phase II trial is currently underway.[Table: see text]

TPS544 Background: BCG unresponsive NMIBC includes patients with persistent high-grade disease or recurrence within 6 months of receiving at least two courses of BCG; or T1 high-grade disease at the first evaluation following BCG induction alone. The recommendation after failing BCG in patients suitable for surgery is cystectomy or for unwilling/unfit patients, salvage chemotherapy or immunotherapy (administered with limited success). This highlights a critical need for novel preservation therapies to facilitate a better quality of life and reduce health-care costs for patients who are unresponsive to BCG (Correa 2015, Dalbagni 2006, Dinney 2013, Kamat 2017). Altor BioScience has initiated a phase II clinical study in BCG unresponsive patients to expand on promising data collected from a compassionate use patient who remains disease free more than 2.5 years after ALT-803 plus BCG treatment for refractory NMIBC (Huang 2017). Methods: This is a Phase II, open-label, single-arm, multicenter study of intravesical BCG plus ALT-803 in patients with BCG unresponsive high grade NMIBC. Group A will enroll patients who have histologically confirmed presence of CIS [with or without Ta or T1 disease]. Group B will enroll patients who have histologically confirmed high-grade Ta or T1 disease (in the absence of CIS). All patients will receive BCG plus ALT-803 weekly for 6 consecutive weeks. A cystoscopy will be performed at Week 12. Patients with no disease or low-grade Ta will receive a maintenance course of therapy (3 weekly instillations of BCG plus ALT-803). Patients with residual CIS and/or high-grade Ta will receive a re-induction course of therapy. Presence of Ta will require a TURBT. Patients with greater than or equal to T1 disease or new CIS will be deemed treatment failures. Patients with a Complete Response (CR) or low-risk disease at Months 6, 9 and 12 are eligible for continued BCG plus ALT-803 maintenance. Patients will be followed for recurrence, progression, and survival. The primary endpoint is to assess complete response (CR; absence of lesions on cystoscopy or negative, for cause, biopsies along with negative urine cytology) of CIS at six months. Enrollment is underway. Clinical trial information: NCT03022825.

32 Background: Patients retreated with rituximab often relapse which limit patient treatment options (Goldman/Cairo, Leukemia, 2013). Our group has successfully expanded functional and active peripheral blood NK cells (exPBNK) to target BL (Chu/Cairo, et al, Can Imm Res, 2015). 2B8T2M was generated by fusing ALT-803, an IL-15 superagonist, to four single-chains of rituximab (Liu/Wong, et al, JBC, 2016). 2B8T2M displayed tri-specific CD20 binding activity, activated NK cells to enhance antibody-dependent cellular cytotoxicity, and induced apoptosis of B-lymphoma cells (Liu/Wong, et al, JBC, 2016). Methods: ALT-803 and 2B8T2M were generously provided by Altor BioScience Corporation. NK expansion, NK receptors expression and cytotoxicity were examined as we previous described (Chu/Cairo, et al, Can Imm Res 2015). IFNg and granzyme B levels were examined by ELISA assays. Equal doses of IgG, Rituximab, ALT-803, Rituximab+ALT-803, obinutuzumab (obinu, generously provided by Christian Klein, PhD from Roche) were used for comparison. Results: 2B8T2M significantly enhanced exPBNK cytotoxicity against rituximab-sensitive Raji cells compared to the controls IgG, Rituximab, ALT-803, Rituximab+ALT-803, obinu (p < 0.001, E:T = 1:1). 2B8T2M also significantly enhanced exPBNK cytotoxicity against rituximab-resistant Raji-2R cells (p < 0.001, E:T = 1:1) and resistant Raji-4RH cells (p < 0.001, E:T = 1:1). Furthermore, 2B8T2M significantly enhanced IFN-g and granzyme B production from exPBNK against Raji, Raji-2R and Raji-4RH compared to IgG (p < 0.001), rituximab (p < 0.001), ALT-803 (p < 0.001), Rituximab+ALT-803 (p < 0.001), and obinutuzumab (p < 0.001). Conclusions: 2B8T2M compared to rituximab, ALT-803 or obinutuzumab significantly enhanced exPBNK in vitro cytotoxicity against rituximab-sensitive and –resistant BL cells. The in vivo functions of 2B8T2M with exPBNK using humanized NSG models are under investigation.

We have previously reported that a single-chain T cell receptor/IL-2 fusion protein (scTCR-IL2) exhibits potent targeted antitumor activity in nude mice bearing human tumor xenografts that display cognate peptide/HLA complexes. In this study, we further explore the mechanism of action of this molecule. We compared the biological activities of c264scTCR-IL2, a scTCR-IL2 protein recognizing the aa264-272 peptide of human p53, with that of MART-1scTCR-IL2, which recognizes the MART-1 melanoma antigen (aa27-35). In vitro studies showed that c264scTCR-IL2 and MART-1scTCR-IL2 were equivalent in their ability to bind cell-surface IL-2 receptors and stimulate NK cell responses. In mice, MART-1scTCR-IL2 was found to have a twofold longer serum half-life than c264scTCR-IL2. However, despite its shorter serum half-life, c264scTCR-IL2 showed significantly better antitumor activity than MART-1scTCR-IL2 against p53(+)/HLA-A2(+) tumor xenografts. The more potent antitumor activity of c264scTCR-IL2 correlated with an enhanced capacity to promote NK cell infiltration into tumors. Similar differences in antigen-dependent tumor infiltration were observed with activated splenocytes pre-treated in vitro with c264scTCR-IL2 or MART-1scTCR-IL2 and then transferred into p53(+)/HLA-A2(+) tumor bearing recipients. The data support a model where c264scTCR-IL2 activates immune cells to express IL-2 receptors. Following stable interactions with cell-surface IL-2 receptors, c264scTCR-IL2 fusion molecule enhances the trafficking of immune cells to tumors displaying target peptide/HLA complexes where the immune cells mediate antitumor effects. Thus, this type of fusion molecule could be used directly as a targeted immunotherapeutic or in adoptive cell transfer approaches to activate and improve the anti-cancer activities of immune cells by providing them with pre-selected antigen recognition capability.

To date, there is no effective therapy for triple-negative breast cancer (TNBC), which has a dismal clinical outcome. Upregulation of tissue factor (TF) expression leads to increased patient morbidity and mortality in many solid tumor types, including TNBC. Our goal was to employ the Fab fragment of ALT-836, a chimeric anti-human TF mAb, for PET imaging of TNBC, which can be used to guide future TNBC therapy. ALT-836-Fab was generated by enzymatic papain digestion. SDS-PAGE and FACS studies were performed to evaluate the integrity and TF binding affinity of ALT-836-Fab before NOTA conjugation and 64Cu-labeling. Serial PET imaging and biodistribution studies were carried out to evaluate the tumor targeting efficacy and pharmacokinetics in the MDA-MB-231 TNBC model, which expresses high levels of TF on the tumor cells. Blocking studies, histological assessment, as well as RT-PCR were performed to confirm TF specificity of 64Cu-NOTA-ALT-836-Fab. ALT-836-Fab was produced with high purity, which exhibited superb TF binding affinity and specificity. Serial PET imaging revealed rapid and persistent tumor uptake of 64Cu-NOTA-ALT-836-Fab (5.1 ± 0.5 %ID/g at 24 h post-injection; n = 4) and high tumor/muscle ratio (7.0 ± 1.2 at 24 h post-injection; n = 4), several-fold higher than that of the blocking group and tumor models that do not express significant level of TF, which was confirmed by biodistribution studies. TF specificity of the tracer was also validated by histology and RT-PCR. 64Cu-NOTA-ALT-836-Fab exhibited prominent tissue factor targeting efficiency in MDA-MB-231 TNBC model. The use of a Fab fragment led to fast tumor uptake and good tissue/muscle ratio, which may be translated into same-day immunoPET imaging in the clinical setting to improve TNBC patient management.

We previously have generated a single-chain T cell receptor-cytokine fusion protein (264scTCR/IL-2) comprising interleukin-2 genetically linked to a soluble HLA-A2.1-restricted TCR recognizing a peptide of human p53 protein. In this report, we show that 264scTCR/IL-2 inhibits the growth of primary tumors derived from the A375 (p53+/HLA-A2.1+) human melanoma and exhibits significantly better antitumor activity than recombinant human IL-2 alone. Moreover, treatment with 264scTCR/IL-2 results in tumor growth retardation in mice bearing large A375 tumors and other p53+/HLA-A2.1+ human tumors but does not affect tumor outgrowth of HLA-A2.1-negative tumors. This suggests that antigen targeting plays a substantial role in the efficacy of 264scTCR/IL-2 against p53+/HLA-A2+ tumors. Further, the antitumor activity of 264scTCR/IL-2 was found to be likely mediated by NK cell activation and tumor infiltration. A biologically active chimeric version of the molecule (c264scTCR/IL-2) also exhibits favorable pharmacokinetic properties required of a clinical candidate for this novel class of potent antitumor activities and targeted anticancer immunotherapeutics.

Antibody-based targeted immunotherapy has shown promise as an approach to treat cancer. However, many known tumor-associated antigens are not expressed as integral membrane proteins and cannot be utilized as targets for antibody-based therapeutics. In order to expand the limited target range of antibodies, we have constructed a soluble single-chain T-cell receptor (TCR) fusion protein designated 264scTCR/IL-2. This fusion protein is comprised of a three-domain HLA-A2-restricted TCR specific for a peptide epitope of the human p53 tumor suppressor protein, which is overexpressed in a broad range of human malignancies. The 264scTCR/IL-2 fusion protein has been expressed at high levels in mammalian cells, and milligram quantities have been purified. MHC-restricted antigen-specific binding properties are maintained in the single-chain, three-domain TCR portion of the fusion protein, and the IL-2 portion retains bioactivity similar to that of free recombinant IL-2. Moreover, this fusion protein is capable of conjugating target and effector cells, remains intact in the blood and substantially increases the half life of the IL-2 portion of the molecule. Finally, the 264scTCR/IL-2 fusion protein can be used to stain tumor cells and is capable of reducing lung metastases in an experimental model of metastasis. Thus, TCR-based fusion proteins may provide a novel class of targeted immunotherapeutics for cancer.

TPS510 Background: IL-15 is one of the most promising new candidates in cancer immunotherapy. Similar to IL-2, IL-15 stimulates natural killer (NK) and CD8+ T cells, however IL-15 may be a more effective and safer therapy as it does not trigger regulatory T cells or induce capillary leak syndrome. Preclinical data also show that IL-15 is effective in eradicating murine pancreatic cancers. However, the use of rhIL-15 in the clinic however has been limited by its short half-life (30 minutes) and cytokine related adverse effects, such as fever and hypotension. Compared with IL-15, ALT-803, a novel IL-15 superagonist complex (IL-15N72D:IL-15RαSu/Fc), has a prolonged serum half-life and is capable of potently inducing NK and CD8+ T cell proliferation in vivo. Pancreatic cancer remains one of the most common causes of cancer death and novel therapies are needed. This study represents the first clinical experience of ALT-803 in the treatment of pancreatic cancer. Methods: This is a Phase Ib/II, multi-institution study of ALT-803 in combination with gemcitabine and nab-paclitaxel in patients with advanced pancreatic cancer. The study involves a phase Ib dose escalation phase to determine the MTD using a standard 3+3 design. Patients will receive gemcitabine 1,000 mg/m2 and nab-paclitaxel 125 mg/m2 D1, 8, 15 and ALT-803 at assigned dose on D2, 9, 16 of a 28d cycle. Primary endpoints are safety and tolerability. Phase II will be a two-stage expansion phase using a Simon two-stage design with a primary endpoint of survival at 8.5 months. Secondary endpoints include response rate, progression free survival, and overall survival. Response will be assessed q8 weeks by RECIST v1.1. Major inclusion criteria include: locally advanced (Phase Ib only) or metastatic pancreatic adenocarcinoma, adequate organ function, PS ECOG 0-2, and 0 or 1 (Phase Ib only) line of prior therapy for advanced pancreatic cancer. Prior nab-paclitaxel is not allowed Current enrollment: Enrollment to the first dose level is ongoing. Complete accrual of phase Ib is expected within nine months. Clinical trial information: NCT02559674.

Meeting abstracts IL-15 activates and induces the proliferation of CD8+ T cells and NK cells. ALT-803 (Altor Bioscience, Miramar, FL) is a superagonist of IL-15 consisting of 2 molecules of IL-15 N-to-D substituted at position 72, bound to 2 molecules of the "sushi" domain of IL-15 alpha receptor

Intravesical Bacillus Calmette-Guerin (BCG) has been shown to induce a specific immunologic response (i.e., activation of IL-2 and effector T-cells), while preclinical studies using ALT-803 (mutated IL-15 analogue combined with IL-15Rα-Fc fusion) have shown promising results by prolonging the agent's half-life and stimulating CD8+ T-cells. Based on these results, we hypothesized that the intravesical administration of ALT-803 along with BCG will generate an immunologic response leading to significant bladder tumor burden reduction. Using a well-established carcinogen induced rat non-muscle invasive bladder cancer (NMIBC) model, we studied the effects of intravesical ALT-803 with and without BCG. Rat tissues were evaluated to document treatment response. Intravesical ALT-803 was safe and well tolerated alone and in combination with BCG. As a single treatment agent, ALT-803 reduced tumor burden by 35% compared to control whereas BCG alone only reduced tumor burden by 15%. However, the combination of ALT-803 plus BCG reduced tumor burden by 46% compared to control. Immune monitoring suggested that the antitumor response was linked to the production and secretion of IL-1α, IL-1β and RANTES, which in turn, induced the proliferation and activation of NK cells. Lastly, tumoral responses of the combinational treatment were associated with 76% reduction in angiogenesis, which is significantly higher than when assessed with either agent alone. The enhanced therapeutic index seen with this duplet provides justification for the development of this regimen for future clinical trials.

ALT-803, a complex of an interleukin (IL)-15 superagonist mutant and a dimeric IL-15 receptor αSu/Fc fusion protein, was found to exhibit significantly stronger in vivo biologic activity on NK and T cells than IL-15. In this study, we show that a single dose of ALT-803, but not IL-15 alone, eliminated well-established 5T33P and MOPC-315P myeloma cells in the bone marrow of tumor-bearing mice. ALT-803 treatment also significantly prolonged survival of myeloma-bearing mice and provided resistance to rechallenge with the same tumor cells through a CD8+ T-cell–dependent mechanism. ALT-803 treatment stimulated CD8+ T cells to secrete large amounts of IFN-γ and promoted rapid expansion of CD8+CD44high memory T cells in vivo. These memory CD8+ T cells exhibited ALT-803–mediated upregulation of NKG2D (KLRK1) but not PD-1 (PDCD1) or CD25 (IL2RA) on their cell surfaces. ALT-803–activated CD8+ memory T cells also exhibited nonspecific cytotoxicity against myeloma and other tumor cells in vitro, whereas IFN-γ had no direct effect on myeloma cell growth. ALT-803 lost its antimyeloma activity in tumor-bearing IFN-γ knockout mice but retained the ability to promote CD8+CD44high memory T-cell proliferation, indicating that ALT-803–mediated stimulation of CD8+CD44high memory T cells is IFN-γ–independent. Thus, besides well-known IL-15 biologic functions in host immunity, this study shows that IL-15–based ALT-803 could activate CD8+CD44high memory T cells to acquire a unique innate-like phenotype and secrete IFN-γ for nonspecific tumor cell killing. This unique immunomodulatory property of ALT-803 strongly supports its clinical development as a novel immunotherapeutic agent against cancer and viral infections. Cancer Res; 73(10); 3075–86. ©2013 AACR.

Interleukin (IL)-15-N72D superagonist-complexed with IL-15RαSushi-Fc fusion protein (IL-15SA/IL-15RαSu-Fc; ALT-803) has been reported to exhibit significant anti-tumor activity in murine myeloma, rat bladder cancer, and murine glioblastoma models. In this study, we examined the immunomodulatory and anti-tumor effects of IL-15SA/IL-15RαSu-Fc in tumor-free and highly metastatic tumor-bearing mice. Here, IL-15SA/IL-15RαSu-Fc significantly expanded NK and CD8+ T cells. In examining natural killer (NK) cell subsets, the greatest significant increase was in highly cytotoxic and migrating (CD11b+, CD27hi; high effector) NK cells, leading to enhanced function on a per-cell basis. CD8+ T cell subset analysis determined that IL-15SA/IL-15RαSu-Fc significantly increased IL-15 responding memory (CD122+, CD44+) CD8+ T cells, in particular those having the innate (NKG2D+, PD1-) phenotype. In 4T1 breast tumor-bearing mice, IL-15SA/IL-15RαSu-Fc induced significant anti-tumor activity against spontaneous pulmonary metastases, depending on CD8+ T and NK cells, and resulting in prolonged survival. Similar anti-tumor activity was seen in the experimental pulmonary metastasis model of CT26 colon carcinoma cells, particularly when IL-15SA/IL-15RαSu-Fc was combined with a cocktail of checkpoint inhibitors, anti-CTLA-4 and anti-PD-L1. Altogether, these studies showed for the first time that IL-15SA/IL-15RαSu-Fc (1) promoted the development of high effector NK cells, (2) enhanced function of NK cells, and (3) played a vital role in reducing tumor metastasis and ultimately survival, especially in combination with checkpoint inhibitors. Citation Format: Peter S. Kim, Anna R. Kwilas, Wenxin Xu, Sarah Alter, Emily K. Jeng, Hing C. Wong, Jeffrey Schlom, James W. Hodge. IL-15 superagonist/IL-15RαSushi-Fc fusion complex (IL-15SA/IL-15RαSu-Fc; ALT-803) markedly enhances specific subpopulations of NK and memory CD8+ T cells, and mediates potent anti-tumor activity of murine breast and colon carcinomas. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3245.

470 Background: Novel agents are necessary to treat NMIBC to avoid recurrence and progression. This phase Ib clinical trial evaluated the safety and toleraibility of ALT-803, an IL-15 superagonist, plus BCG in patients with BCG-naïve NMIBC. Methods: This is a Phase Ib trial using the 3+3 design to evaluate intravesical ALT-803 plus BCG 50 mg in BCG-naïve NMIBC patients, who would normally be eligible for intravesical BCG alone. The initial dose of ALT-803 was 100 μg/instillation with 2 dose-escalations allowed (200 μg/instillation and 400 μg/instillation) if dose limiting toxicities (DLTs) were not evident. Patients received intravesical ALT-803 in conjunction with BCG weekly for 6 consecutive weeks (Induction Phase). Patients then had routine follow-up with cystoscopy and voided urinary cytology (VUC) every 3 months for 2 years equating to confirmatory response assessment. Negative cystoscopy, VUC and/or biopsy yielded a complete response (CR). When appropriate for high-risk patients, physicians and patients were encouraged to receive maintenance BCG alone as per common practice patterns. Results: No patient in 100 μg/instillation of ALT-803/BCG and 200 μg/instillation of ALT-803/BCG reported an AE. One patient in 400 μg/instillation/BCG cohort developed a urinary tract infection requiring delay of intravesical treatment by 1 week. No grade 3/4 toxicities were noted. All patients have completed therapy without DLTs. To date, 4 patients have 12 month follow-up and are CR. The remaining 5 patients have no evidence of disease. However, follow-ups are currently approximately 3 months. Corollary immune studies are still pending. Conclusions: Intravesical ALT-803 plus BCG was safe and tolerable in patients with BCG-naive NMIBC. Further evaluation in expansion cohorts in a phase II trial is currently underway. Clinical trial information: NCT02138734.

Upregulation of tissue factor (TF) expression leads to increased patient morbidity and mortality in many solid tumor types. The goal of this study was to develop a PET tracer for imaging of TF expression in pancreatic cancer.ALT-836, a chimeric antihuman TF monoclonal antibody, was conjugated to 2-S-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) and labeled with (64)Cu. To compare the TF binding affinity of ALT-836 and NOTA-ALT-836, flow cytometry analysis was performed in 3 pancreatic cancer cell lines with different expression levels of TF (from low to high: PANC-1, ASPC-1, and BXPC-3). PET, biodistribution, blocking, and histology studies were performed on pancreatic tumor-bearing mice to evaluate the ability and specificity of (64)Cu-NOTA-ALT-836 to target TF in vivo.There was no difference in TF binding affinity between ALT-836 and NOTA-ALT-836. (64)Cu-labeling was achieved with high yield and specific activity. Serial PET revealed that the uptake of (64)Cu-NOTA-ALT-836 in BXPC-3 tumors (high TF expression) was 5.7 ± 1.8, 10.4 ± 0.8, and 16.5 ± 2.6 percentage injected dose per gram at 4, 24, and 48 h after injection, respectively (n = 4), significantly higher than that in the PANC-1 and ASPC-1 tumors. Biodistribution data as measured by γ-counting were consistent with the PET findings. Blocking experiments and histology further confirmed the TF specificity of (64)Cu-NOTA-ALT-836.Herein we report the first successful PET imaging of TF expression. Persistent and TF-specific uptake of (64)Cu-NOTA-ALT-836 was observed in pancreatic cancer models.

451 Background: Novel agents are necessary to treat BCG-resistant NMIBC to avoid radical cystectomy (RC). This phase I clinical trial evaluated the safety and activity of ALT-801, a recombinant humanized TCR-IL-2 fusion protein in BCG-resistant NMIBC. Methods: This is a Phase I trial using the 3+3 design to evaluate intravenous (IV) ALT-801 plus IV G 1000 mg/m2 in BCG-resistant high-risk NMIBC patients (pts) defined as high grade Ta, T1 or carcinoma in situ, size > 4 cm or multi-focal tumors. BCG-intolerant pts, those who refused or were unfit to undergo RC were also eligible. Initially, patients received ALT-801 monotherapy; an amendment added G. Pts received induction of 2 cycles, with a 13-day rest between cycles. Each cycle consisted of 4 doses of ALT-801 on Day 3, Day 5, Day 8, and Day 15 and 2 doses of G, one each on Day 1 and Day 8. Pts who have a biopsy-proven complete response (CR) after induction received one maintenance cycle and underwent response assessment. The initial dose of ALT-801 was 0.08 mg/kg with 2 step-down doses allowed if dose limiting toxicities (DLTs)- 0.06 mg/kg and 0.04 mg/kg. Results: 2 pts in cohort one received ALT-801 alone at 0.08 mg/kg per dose, a 3rd pt received G and ALT-801 at 0.08 mg/kg per dose. Grade ≥ 3 hepatotoxicity in the 3rd patient led to a step-down to 0.06 mg/kg dose. One pt in the 0.06 mg/kg dose experienced a DLT (Grade ≥ 3 hepatotoxicity) and the cohort was expanded to 6 pts with no further DLTs. Other attributed adverse events were: anorexia, pruritus, rash, edema, fatigue, chills. For the 0.06 mg/kg ALT-801 + G regimen, 6 pts received up to 2 cycles of induction and 4 pts received the maintenance cycle. All pts have completed therapy without further DLTs. CR was observed in 3 pts, which was durable in 2 pts lasting ≥ 18 months. Preliminarily immune studies have shown transient IFN-γ and IL-6 but not TNF-α and IL-10 induction after ALT-801 dosing. Conclusions: ALT-801 plus gemcitabine was feasible in BCG-resistant NMIBC and demonstrated immune responses and potential durable clinical activity. Further evaluation in expansion cohorts in a phase Ib/II trial is planned. Clinical trial information: NCT01625260.