Search

BackgroundAs a major driver of lymphocyte proliferation and activation interleukin 2 (IL-2) is a crucial mediator for antitumor responses. Despite promising activity in a subset of patients, wider therapeutic utility of IL-2 (aldesleukin) has been hampered by severe dose-limiting toxicities, the expansion of immunosuppressive regulatory T cells and a poor pharmacokinetic (PK) profile. Recent engineering efforts, including non-α IL-2 variants, have lowered the toxicity profile, but have yet to induce meaningful antitumor activity in a wider patient population.MethodsWe engineered INBRX-120, a CD8α-targeted Cisleukin™ molecule consisting of an affinity tuned IL-2 (IL2-x) connected to two high affinity CD8α-specific single domain antibodies via an effector-silenced Fc domain. To show that this large affinity differential enables directed IL-2 cis-signaling exclusively on CD8α-expressing tumoricidal effector cell populations, INBRX-120 effects on target cell expansion, activation and antitumor activity were tested in vitro. In vivo antitumor efficacy was evaluated in syngeneic mouse models alone or in combination with programmed cell death protein-1 (PD-1) blockade. Preclinical safety, as well as pharmacodynamic (PD) and PK profiling was carried out in non-human primates.ResultsINBRX-120 effectively expanded and enhanced the cytotoxic capacity of CD8 T cells and natural killer cells towards tumor cells without affecting regulatory T cells in vitro and in vivo. In syngeneic mouse models, INBRX-120 surrogate showed safe, potent, and durable antitumor efficacy alone and in combination with PD-1 blockade. In non-human primates, INBRX-120 expanded and activated CD8α-expressing effector cells, showed a favorable PK profile, and was well tolerated up to a dose of 1 mg/kg.ConclusionsThrough its unique cis-signaling activity on CD8α-expressing effector cells, INBRX-120 overcomes the major limitations of IL-2-based therapy and effectively harnesses IL-2’s potent intrinsic antitumor activity. This novel therapeutic strategy promises safer clinical activity that could induce meaningful antitumor efficacy in a wider set of patients with various cancer indications.

BackgroundNatural Killer (NK) cells play a pivotal role in cancer immunosurveillance due to their potent cytolytic activity and NK cell-centric therapies have emerged as safer alternatives to targeting T cells.1 2 Interleukin 2 (IL-2) drives NK cell expansion and activity, but its therapeutic utility is limited by rapid clearance, expansion of immunosuppressive regulatory T cells, and by severe dose-limiting toxicities.3 INBRX-121 overcomes these liabilities through specific targeting of an affinity-detuned IL-2 variant to cells expressing NKp46.MethodsAn IL-2 variant was engineered to eliminate binding to CD25 and to have attenuated affinity for CD122. This detuned cytokine was fused to a high-affinity single-domain antibody targeting NKp46 to generate INBRX-121. The ability of INBRX-121 to target IL-2-like signaling specifically to NKp46-expressing cells was evaluated in vitro using human lymphocytes by measuring STAT5 signaling and cytotoxic activity in tumor cell co-cultures. Characterization of the pharmacokinetic/pharmacodynamic relationship of INBRX-121 was completed in non-human primates across escalating dose levels, while anti-tumor activity as a monotherapy and in combination with Rituximab or PD-1 checkpoint blockade was tested in Raji xenografts and syngeneic CT-26 mouse models, respectively.ResultsINBRX-121 induces a STAT5 signal equal to that of wild-type IL-2 in human lymphocytes but shows an NK cell-centric activity profile. Cells targeted by INBRX-121 have increased proliferative capacity and improved cytotoxicity in antibody-dependent and -independent tumor cell killing assays. INBRX-121 shows prolonged pharmacokinetic exposure in vivo and is well-tolerated in mice and cynomolgus monkeys. The NKp46-specific IL-2 stimulus in these models results in a robust, dose-dependent NK cell expansion. As predicted by its in vitro activity, INBRX-121 also enhances the cytotoxic capacity of NK cells in vivo measured via elevated intracellular levels of Granzyme B. In a Raji xenograft model, INBRX-121 slows tumor growth as a single agent and synergizes with Rituximab to induce complete tumor regression. Similarly, co-treatment with INBRX-121 improves the incomplete suppression of CT-26 tumor growth by a PD-1 blocking antibody to yield complete responses that show immunological memory upon re-challenge.ConclusionsINBRX-121 offers a unique approach to overcoming the limitations of current IL-2 therapeutics. NKp46-targeting of a detuned IL-2 variant helps to avoid IL-2-mediated toxicity while enhancing the antitumor activities of NK cells. Through its novel therapeutic concept INBRX-121 provides a promising treatment option for multiple cancer indications both as a monotherapy and in combination with a variety of frontline agents.ReferencesShimasaki N, Jain A, Campana D. NK cells for cancer immunotherapy. Nat Rev Drug Discov 2020;19:200–218.Liu S, Galat V, Galat Y, Lee Y, Wainwright D, Wu J. NK cell-based cancer immunotherapy: from basic biology to clinical development. J Hematol Oncol 2021;14:7.Overwijk W, Tagliaferri M, Zalevsky J. Engineering IL-2 to give new life to T Cell immunotherapy. Annu Rev Med 2021;72:281–311.Ethics ApprovalAll animal studies were conducted in accordance with AAALAC regulations and were approved by the IACUC for Explora BioLabs (#SP17-010-013) and BTS Research (20-015 Enrollment 05).

Bispecific T cell engagers that target CD3 on T cells and cell-surface antigens on tumor cells have demonstrated meaningful clinical activity against hematological cancers but have yet to achieve similar efficacy for the treatment of solid tumors. A variety of factors may be contributing to this lack of success in the solid tumor setting, including insufficient exposure of tumor cells to these targeted therapeutics due to target-independent interactions with peripheral T cells. Not only can circulating cells act as a sink for CD3 bispecifics but these interactions can lead to cytokine release syndrome (CRS), a common and dose-limiting toxicity for this class of therapeutics. We have developed a CD3 bispecific platform, CONTRA-MAB® (Constrained T Cell Redirecting and Activating Multispecific Antibody), that combines high-affinity tumor-associated antigen binding single-domain antibodies (sdAbs) with a constrained CD3-targeting VH/VL pair that has been strategically positioned and affinity engineered to minimize T cell activation and cytokine production in the absence of target antigen engagement. INBRX-130 is a CONTRA-MAB® that targets trophoblast glycoprotein (TPBG; 5T4), a cell-surface protein that is upregulated across many solid tumor indications but has restricted expression on normal adult tissues. As determined by flow cytometry, INBRX-130 exhibits specific and high-affinity binding to 5T4-positive cells but shows no detectable binding to T cells, absent target, at concentrations up to one micromolar. Additionally, treatment of peripheral blood mononuclear cells (PBMCs) with high concentration INBRX-130 does not result in production of detectable levels of CRS-associated inflammatory cytokines. Upon binding to 5T4-expressing tumor cells in vitro, INBRX-130 potently induces T cell activation, proliferation, cytokine production, and cytotoxicity, resulting in target cell death. INBRX-130 has favorable pharmacokinetic properties and the capacity to effect regression of 5T4+ tumors in humanized mice, even in models that display minimal initial T-cell infiltrate. INBRX-130 is well-tolerated by cynomolgus monkeys at the maximally administered dose of 2 mg/kg without the requirement of a priming dose. Preliminary in vitro data with CONTRA-MABs® targeting other solid tumor antigens supports the broad applicability of this unique T cell engager platform. CONTRA-MABs® have the potential to have enhanced accumulation in solid tumors relative to other CD3 bispecific antibodies as a result of reduced systemic engagement of T cells and increased tolerability at high doses. These attributes, coupled with the broad tumor biased expression of 5T4 across solid tumors, may enable INBRX-130 to have a greater therapeutic index and provide a significant benefit to patients in need. Citation Format: Katelyn M. Willis, Margaret Haerr, Marchelle Meza, Sae Jeong Ahn, Michael Kaplan, Georgina Ortiz-Barquero, Rajay Pandit, Rafael Abalos, Yao Chen, Samuel Holley, Raul Lozano, Iljin Hwang, Emily Rowell, Brendan P. Eckelman. INBRX-130, a 5T4-targeted CONTRA-MAB, is a potent CD3 bispecific antibody engineered to have minimal off-tumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2912.

IL-2 is a powerful cytokine that enhances the activity of and induces the expansion of tumoricidal effector cells including T and NK cells. Despite promising signs of activity, the clinical utility of IL-2 is limited due to severe treatment-associated toxicities including vascular leak syndrome and the concurrent expansion of immunosuppressive regulatory T cells, which blunts efficacy. We developed a molecular targeted cytokine (MTC) platform that combines an engineered IL-2 variant with reduced affinity for the IL-2 receptor and high affinity single-domain antibodies (sdAb) to restrict IL-2 receptor signaling to cells expressing the target antigen (termed cis-signaling). INBRX-121 is an MTC designed to deliver IL-2 to NK cells using an NKp46-specific sdAb as the targeting moiety. In vitro, INBRX-121 induces dose-dependent STAT5 phosphorylation and proliferation in NK cells with comparable potency to wild-type IL-2, while showing no NKp46-independent activity at concentrations of more than three magnitudes higher than that required for NK cell activity. INBRX-121 enhances NK cell activation and cytotoxic capacity, thereby reducing the NK-to-target cell ratio as well as the target receptor density needed for optimal tumor cell killing. Furthermore, INBRX-121 lowers the threshold for antibody-dependent cellular cytotoxicity (ADCC) and increases maximal target cell killing in combination with ADCC-competent antibodies to an extent that may lessen the need for Fc engineering to enhance binding to the low affinity CD16A variant. In vivo, INBRX-121 demonstrates potent anti-tumor activity with increased intra-tumoral NK cell accumulation and upregulation of NK cell activation markers. INBRX-121 monotherapy reduces primary tumor growth and the formation of metastasis, while combination with ADCC-competent antibodies further increases NK cell activity to convert tumor growth suppression into complete tumor elimination. INBRX-121 is well tolerated in rodents and non-human primates and no overt signs of toxicity have been observed in cynomolgus monkeys up to the highest administered dose of 30 mg/kg. Our MTC platform provides a novel path to overcoming the limitations of IL-2 therapy through the pinpointed delivery of IL-2 activity to target cells of interest. In the case of INBRX-121 this ensures potent enhancement of the naturally broad anti-tumor activity of NK cells, which have distinct advantages over T cell therapeutics. Unlike T cells, tumor recognition by NK cells is independent of MHC-presented antigens and can elicit immediate cytotoxic activity. Further, many of the toxicities associated with T cell therapeutics have not been observed with NK-centric treatments. INBRX-121 therefore provides a promising safe and efficacious treatment option for a broad array of cancer indications both as a monotherapy and in combination with ADCC-enabled antibodies. Citation Format: Heather Kinkead, Florian J. Sulzmaier, Anya Polovina, Nadja Kern, Angelica Sanabria, Jason Ho, Chelsie Macedo, Ryan Henderson, Abrahim Hussain, Rajay Pandit, William Crago, Emily Rowell, Brendan P. Eckelman. INBRX-121, a safe and efficacious molecular targeted cytokine that enhances NK cell-mediated tumor killing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3513.

Cluster of differentiation 47 (CD47) is a transmembrane protein ubiquitously expressed on human cells but overexpressed on many different tumor cells. The interaction of CD47 with signal-regulatory protein alpha (SIRPα) triggers a “don’t eat me” signal to the macrophage, inhibiting phagocytosis. Thus, overexpression of CD47 enables tumor cells to escape from immune surveillance via the blockade of phagocytic mechanisms. We report here the development and characterization of CC-90002, a humanized anti-CD47 antibody. CC-90002 is unique among previously reported anti-CD47 bivalent antibodies that it does not promote hemagglutination while maintaining high-affinity binding to CD47 and inhibition of the CD47–SIRPα interaction. Studies in a panel of hematological cancer cell lines showed concentration-dependent CC-90002-mediated phagocytosis in acute lymphoblastic leukemia, acute myeloid leukemia (AML), lenalidomide-resistant multiple myeloma (MM) cell lines and AML cells from patients. In vivo studies with MM cell line-derived xenograft models established in immunodeficient mice demonstrated significant dose-dependent antitumor activity of CC-90002. Treatment with CC-90002 significantly prolonged survival in an HL-60-disseminated AML model. Mechanistic studies confirmed the binding of CC-90002 to tumor cells and concomitant recruitment of F4-80 positive macrophages into the tumor and an increase in expression of select chemokines and cytokines of murine origin. Furthermore, the role of macrophages in the CC-90002-mediated antitumor activity was demonstrated by transient depletion of macrophages with liposome-clodronate treatment. In non-human primates, CC-90002 displayed acceptable pharmacokinetic properties and a favorable toxicity profile. These data demonstrate the potential activity of CC-90002 across hematological malignancies and provided basis for clinical studies CC-90002-ST-001 (NCT02367196) and CC-90002-AML-001 (NCT02641002).

Background4-1BB is a costimulatory molecule that is predominantly expressed on activated CD8+ T cells and is induced upon T cell receptor mediated activation.1 Within the tumor microenvironment, 4-1BB-expressing T cells are enriched for anti-tumor reactivity 2; thus, 4-1BB agonism provides an opportunity for selective activation of anti-cancer immune effector cells. Early efforts to develop 4-1BB targeted agonists were limited by poor tolerability (Urelumab) or insufficient efficacy (Utomilumab). INBRX-105 is a bispecific antibody that aims to overcome these prior limitations through induction of 4-1BB agonism specifically at sites of PD-L1 expression. Preclinical models have defined pharmacokinetic (PK) and pharmacodynamic (PD) parameters that are correlated with maximal INBRX-105-specific immune responses and antitumor activity.MethodsINBRX-105 was generated by linking 2 humanized single-domain antibody binding domains targeting human PD-L1 and 4-1BB, fused to an effector-silenced human IgG1 constant domain (Fc). A bispecific, anti-mouse PD-L1x4-1BB surrogate molecule, INBRX-105-a, was engineered to match the function and target affinities of INBRX-105. This surrogate was tested for in vivo activity in non-tumor-bearing and MC-38 tumor-bearing animals, including measurements of serum exposure, PD-L1 receptor occupancy, immunophenotyping of peripheral blood and intra-tumoral immune cell populations.ResultsINBRX-105-a was shown to be an appropriate anti-mouse surrogate for INBRX-105 in a variety of in vitro assays. Comparable potencies of activity were demonstrated in a PD-L1 dependent 4-1BB reporter assay, as well as in cytokine induction through co-stimulation of primary T cells. In vivo, INBRX-105-a showed robust induction of mouse CD8+ T effector memory populations (CD8+ TEM) at dose levels that achieved ≥ 96 hours of PD-L1 receptor occupancy. A serum concentration of 800 ng/mL at 96 hours, achieved by a dose of 2 mg/kg in mice, was sufficient to provide the requisite occupancy for maximal pharmacodynamics. CD8+ TEM responses were dependent on 4-1BB agonism and were more efficiently induced by PD-L1 localization, as opposed to 4-1BB multivalent clustering alone. Optimal tumor responses, including complete responses and demonstration of immunological memory, were observed when maximal 4-1BB driven pharmacodynamics were paired with extended PD-1/PD-L1 pathway blockade, provided either by an orthogonal molecule or increased exposure of INBRX-105.ConclusionsPreclinical receptor occupancy and pharmacokinetic determinations have defined a dose of INBRX-105-like activity that induces maximal pharmacodynamics. Additional PD-1 checkpoint inhibition does not change the pharmacodynamic profile of INBRX-105-a, but does allow for optimal efficacy. INBRX-105 is currently being evaluated in patients with advanced solid tumors in a first-in-human trial (NCT03809624).Trial RegistrationINBRX-105 is currently being evaluated in patients with advanced solid tumors in a first-in-human trial (NCT03809624).ReferencesVinay DS, Kwon BS. 4-1BB (CD137), an inducible costimulatory receptor, as a specific target for cancer therapy. BMB Reports; 2014:47:122–129.Ye Q, Song D-G, Poussin M, et al. CD137 accurately identifies and enriches for naturally occurring tumor-reactive T cells in tumor. Clin Cancer Res; 2014:20:44–55.Ethics ApprovalThe care and use of all animals were reviewed and approved by Explora BioLabs’ IACUC # SP17-010-013 and conducted in accordance with AAALAC regulations.

Since their discovery, the matrix metalloproteinase (MMP) family proteases have been considered as therapeutic targets in numerous diseases and disorders. Unfortunately, clinical trials with MMP inhibitors have failed to yield any clinical benefits of these inhibitors. These failures were largely due to a lack of MMP-selective agents; accordingly, it has become important to identify a platform with which high selectivity can be achieved. To this end, we propose using MMP-targeting antibodies that can achieve high specificity in interactions with their targets. Using a scaffold of single-domain antibodies, here we raised a panel of MMP10-selective antibodies through immunization of llamas, a member of the camelid family, whose members generate conventional heavy/light-chain antibodies and also smaller antibodies lacking light-chain and CH1 domains. We report the generation of a highly selective and tightly binding MMP10 inhibitor (K(i) < 2 nm). Using bio-layer interferometry–based binding assays, we found that this antibody interacts with the MMP10 active site. Activity assays demonstrated that the antibody selectively inhibits MMP10 over its closest relative, MMP3. The ability of a single-domain antibody to discriminate between the most conserved MMP pair via an active site–directed mechanism of inhibition reported here supports the potential of this antibody as a broadly applicable scaffold for the development of selective, tightly binding MMP inhibitors.

X-linked inhibitor of apoptosis (XIAP) is a potent antagonist of caspase apoptotic activity. XIAP also functions as an E3 ubiquitin ligase, targeting caspases for degradation. However, molecular pathways controlling XIAP activities remain unclear. Here we report that nitric oxide (NO) reacts with XIAP by S-nitrosylating its RING domain (forming SNO-XIAP), thereby inhibiting E3 ligase and antiapoptotic activity. NO-mediated neurotoxicity and caspase activation have been linked to several neurodegenerative disorders, including Alzheimer’s, Parkinson’s, and Huntington’s diseases. We find significant SNO-XIAP formation in brains of patients with these diseases, implicating this reaction in the etiology of neuronal damage. Conversely, S-nitrosylation of caspases is known to inhibit apoptotic activity. Unexpectedly, we find that SNO-caspase transnitrosylates (transfers its NO group) to XIAP, forming SNO-XIAP, and thus promotes cell injury and death. These findings provide unique insights into the regulation of caspase activation in neurodegenerative disorders mediated, at least in part, by nitrosative stress.

We describe the peptide-binding specificity of the baculoviral IAP repeat (BIR) domains of the human inhibitor of apoptosis (IAP) proteins, X-linked IAP, cellular IAP1 and neuronal apoptosis inhibitory protein (NAIP). Synthetic peptide libraries were used to profile each domain, and we distinguish two types of binding specificity, which we refer to as type II and type III BIR domains. Both types have a dominant selectivity for Ala in the first position of the four N-terminal residues of the peptide ligands, which constitute a core recognition motif. Our analysis allows us to define the signature of type III BIRs that demonstrate a preference for Pro in the third residue of the ligand, resembling the classic IAP-binding motif (IBM). The signature of the type II BIRs was similar to type III, but with a striking absence of specificity for Pro in the third position, suggesting that the definition of an IBM must be modified depending on the type of BIR in question. These findings explain how subtle changes in the peptide-binding groove of IAP BIR domains can significantly alter the target protein selectivity. Our analysis allows for prediction of BIR domain protein-binding preferences, provides a context for understanding the mechanism of peptide selection and heightens our knowledge of the specificity of IAP antagonists that are being developed as cancer therapeutics.

Several of the inhibitor of apoptosis protein (IAP) family members regulate apoptosis in response to various cellular assaults. Some members are also involved in cell signalling, mitosis and targeting proteins to the ubiquitin-proteasome degradation machinery. The most intensively studied family member, X-linked IAP (XIAP), is a potent inhibitor of caspase activity; hence, it is generally assumed that direct caspase inhibition is an important conserved function of most members of the family. Biochemical and structural studies have precisely mapped the elements of XIAP required for caspase inhibition. Intriguingly, these elements are not conserved among IAPs. Here, we review current knowledge of the caspase-inhibitory potential of the human IAPs and show that XIAP is probably the only bona fide caspase inhibitor, suggesting that the other family members never gained the ability to directly inhibit caspase activity.

cIAPs (cellular inhibitor of apoptosis proteins) 1 and 2 are able to regulate apoptosis when ectopically expressed in recipient cells and probably also in vivo. Previous work suggested that this is at least partially due to direct caspase inhibition, mediated by two of the three baculovirus IAP repeat (BIR) domains that are contained in these proteins. In support of this we show that the BIR domains 2 and 3 of the two cIAPs are able to bind caspases-7 and -9. However, we demonstrate that neither of these BIR domains is able to inhibit caspases because of critical substitutions in the regions that target caspase inhibition in the X-linked IAP, a tight binding caspase inhibitor. The cIAP BIR domains can be converted to tight binding caspase inhibitors by substituting these critical residues with XIAP residues. Thus, cIAPs maintain protein scaffolds suitable for direct caspase inhibition but have lost or never acquired specific caspase inhibitory interaction sites. Consequently, although the binding function of the cIAP BIRs may be important for their physiologic function, caspase inhibition is not.

Cluster of differentiation 47 (CD47) is a transmembrane protein ubiquitously expressed on human cells but overexpressed on many different tumor cells. The interaction of CD47 with signal-regulatory protein alpha (SIRPα) expressed on macrophages results in the inhibition of phagocytosis. Thus, tumor cell overexpression of CD47 enables escape from immune surveillance via the blockade of phagocytic mechanisms. We report here the characterization of CC-90002, a humanized anti-CD47 monoclonal antibody (mAb) that is being developed as a potential therapeutic for hematologic malignancies and solid tumors. CC-90002 has a high affinity for binding to CD47 with a subnanomolar dissociation constant (Kd) value. The IC50 required for the blockade of the CD47-SIRPα interaction was also in the subnanomolar range. CC-90002 is unique among previously reported anti-CD47 antibodies for its inability to promote hemagglutination while maintaining high affinity binding to CD47 and inhibition of the CD47-SIRPα interaction. CC-90002 enabled antibody-mediated phagocytosis of a panel of hematological cancer cell lines in vitro, including ALL, multiple myeloma (MM) and acute myeloid leukemia (AML) cells, and primary AML patient samples. The phagocytosis index for this panel (at 1 μg/mL) ranged from approximately 20% to 60% for all the cell lines tested. Antibody concentration-response studies indicated that the CC-90002 effect was concentration-dependent in the ALL and AML cell lines. CC-90002 treatment significantly enabled the phagocytosis of two lenalidomide-resistant MM cell lines and primary AML cells from three patients. Additionally, CC-90002 treatment elicited the phagocytosis of solid tumor lines, including those from ovarian, breast, head and neck, lung, and pancreatic cancers. The in vivo efficacy of CC-90002 was evaluated across cell line-derived and patient-derived xenograft models. Significant dose-dependent antitumor activity was observed with CC-90002 treatment in the MM cell line-derived xenograft models, RPMI 8226 and the parental NCI-H929 and lenalidomide-resistant NCI-H929 (H929/R1). Treatment with CC-90002 demonstrated significant tumor regression in solid tumor xenografts including a cell line-derived model of triple negative breast cancer (TNBC), MDA-MB-231, and a patient-derived TNBC model, AA1126 and significantly prolonged survival in the HL-60-disseminated AML tumor model. Mechanistic studies in the RPMI 8226 xenograft model confirmed binding of CC-90002 to tumor cells and recruitment of F4-80-positive macrophages into the tumor. RPMI 8226 tumor lysates from CC-90002-treated animals demonstrated an increase in select chemokines and cytokines of murine origin. Taken together, the in vitro and in vivo data demonstrate the potential for activity of CC-90002 across both hematological malignancies and solid tumors. CC-90002 is currently in early clinical development. Citation Format: Rama Krishna Narla, Hardik Modi, Lilly Wong, Mahan Abassian, Daniel Bauer, Pragnya Desai, Bonny Gaffney, Pilgrim Jackson, Jim Leisten, Jing Liu, Antonia Lopez-Girona, Maria Romero, WenQing Yang, Brendan P. Eckelman, Quinn Deveraux, Laurie Phillips, Heather K. Raymon, Laure Escoubet, John Boylan, Kandasamy Hariharan. The humanized anti-CD47 monclonal antibody, CC-90002, has antitumor activity in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4694. doi:10.1158/1538-7445.AM2017-4694

Glucocorticoid-induced TNFR-related (GITR, TNFRSF18) is a member of the TNFR superfamily with pleiotropic T cell modulatory activity. We are developing a novel anti-GITR antibody with enhanced agonist activity for the treatment of solid tumors. Our candidate molecule, FPA154, is constructed with single-domain antibodies (sdAbs) in a tetravalent format, with an effector-competent IgG1 Fc domain. Both FPA154 and the mouse-reactive surrogate molecule (cmFPA154, mIgG2a isotype) bind to cell-surface GITR with high affinity. GITR is most highly expressed in vivo on activated and intratumoral Treg, and our data indicates that FPA154 and cmFPA154 potently mediate ADCC activity against Treg expressing high levels of GITR. In contrast, activated effector T cells express modest levels of cell-surface GITR, and FPA154 and cmFPA154 drive GITR-induced NF-κB activation. This activity is independent of Fc-mediated crosslinking, which is normally required for bivalent GITR antibodies to induce GITR signaling. cmFPA154 has potent antitumor activity in several syngeneic mouse tumors, both as a monotherapy treatment and in combination with anti-PD-1. In summary, FPA154 is a promising candidate with multiple mechanisms of action that contribute to generation of an antitumor immune response mediated by different T cell subsets. Citation Format: Susannah D. Barbee, Amanda Chen, Susan Johnson, David I. Bellovin, John C. Timmer, Nebiyu Wondyfraw, Mikayel Mkrtichyan, Amir S. Razai, Kyle S. Jones, Chelsie Y. Hata, Denise Gonzalez, Quinn Deveraux, Brendan P. Eckelman, Luis Borges. Development of FPA154, a novel tetravalent anti-GITR antibody, for the treatment of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 613. doi:10.1158/1538-7445.AM2017-613

B-cell lymphoma is one of the most frequently observed non-cutaneous neoplasms in dogs. For both human and canine BCL, the standard of care treatment typically involves a combination chemotherapy, e.g. "CHOP" therapy. Treatment for human lymphoma greatly benefited from the addition of anti-CD20 targeted biological therapeutics to these chemotherapy protocols; this type of therapeutic has not been available to the veterinary oncologist. Here, we describe the generation and characterization of a rituximab-like anti-CD20 antibody intended as a candidate treatment for canine B-cell lymphoma. A panel of anti-canine CD20 monoclonal antibodies was generated using a mouse hybridoma approach. Mouse monoclonal antibody 1E4 was selected for construction of a canine chimeric molecule based on its rank ordering in a flow cytometry-based affinity assay. 1E4 binds to approximately the same location in the extracellular domain of CD20 as rituximab, and 1E4-based chimeric antibodies co-stain canine B cells in flow cytometric analysis of canine leukocytes using an anti-canine CD21 antibody. We show that two of the four reported canine IgG subclasses (cIgGB and cIgGC) can bind to canine CD16a, a receptor involved in antibody-dependent cellular cytotoxicity (ADCC). Chimeric monoclonal antibodies were assembled using canine heavy chain constant regions that incorporated the appropriate effector function along with the mouse monoclonal 1E4 anti-canine CD20 variable regions, and expressed in CHO cells. We observed that 1E4-cIgGB and 1E4-cIgGC significantly deplete B-cell levels in healthy beagle dogs. The in vivo half-life of 1E4-cIgGB in a healthy dog was ∼14 days. The antibody 1E4-cIgGB has been selected for further testing and development as an agent for the treatment of canine B-cell lymphoma.

ML-IAP (melanoma inhibitor of apoptosis) is a potent anti-apoptotic protein that is strongly up-regulated in melanoma and confers protection against a variety of pro-apoptotic stimuli. The mechanism by which ML-IAP regulates apoptosis is unclear, although weak inhibition of caspases 3 and 9 has been reported. Here, the binding to and inhibition of caspase 9 by the single BIR (baculovirus IAP repeat) domain of ML-IAP has been investigated and found to be significantly less potent than the ubiquitously expressed XIAP (X-linked IAP). Engineering of the ML-IAP-BIR domain, based on comparisons with the third BIR domain of XIAP, resulted in a chimeric BIR domain that binds to and inhibits caspase 9 significantly better than either ML-IAP-BIR or XIAP-BIR3. Mutational analysis of the ML-IAP-BIR domain demonstrated that similar enhancements in caspase 9 affinity can be achieved with only three amino acid substitutions. However, none of these modifications affected binding of the ML-IAP-BIR domain to the IAP antagonist Smac (second mitochondrial activator of caspases). ML-IAP-BIR was found to bind mature Smac with low nanomolar affinity, similar to that of XIAP-BIR2-BIR3. Correspondingly, increased expression of ML-IAP results in formation of a ML-IAP–Smac complex and disruption of the endogenous interaction between XIAP and mature Smac. These results suggest that ML-IAP might regulate apoptosis by sequestering Smac and preventing it from antagonizing XIAP-mediated inhibition of caspases, rather than by direct inhibition of caspases.

Dogma has it that suppression of the programmed cell death pathway by the IAP (inhibitor of apoptosis) proteins is achieved by their direct enzymic inhibition of the chief executioners of the apoptotic process, the caspases. In turn, the IAPs themselves can be neutralized by Smac/DIABLO (second mitochondrial activator of caspases/direct IAP binding protein with low pI), a protein which in healthy cells is thought to be sequestered in the mitochondria, but which, in response to apoptotic stimuli, is released from the mitochondria into the cytosol where it can bind to IAPs, displacing caspases and thus perpetuating the apoptotic signal. While this is an elegant and attractive model, recent studies have suggested that IAPs can also suppress apoptotic cell death independently of their ability to inhibit caspases, and two reports in this issue of the Biochemical Journal reach the interesting conclusion that the cytoprotective IAPs, ML-IAP (melanoma IAP) and ILP-2 (IAP-like protein 2), exert their effects not through direct caspase inhibition, but through the neutralization of Smac/DIABLO. The predicted outcome of these studies is a delicately controlled equilibrium between the activities of IAPs and Smac/DIABLO, leading to a dynamic regulation of the apoptotic threshold.

Plant HKT proteins comprise a family of cation transporters together with prokaryotic KtrB, TrkH, and KdpA transporter subunits and fungal Trk proteins. These transporters contain four loop domains in one polypeptide with a proposed distant homology to K + channel selectivity filters. Functional expression in yeast and Xenopus oocytes revealed that wheat HKT1 mediates Na + -coupled K + transport. Arabidopsis AtHKT1, however, transports only Na + in eukaryotic expression systems. To understand the molecular basis of this difference we constructed a series of AtHKT1/HKT1 chimeras and introduced point mutations to AtHKT1 and wheat HKT1 at positions predicted to be critical for K + selectivity. A single-point mutation, Ser-68 to glycine, was sufficient to restore K + permeability to AtHKT1. The reverse mutation in HKT1, Gly-91 to serine, abrogated K + permeability. This glycine in P-loop A of AtHKT1 and HKT1 can be modeled as the first glycine of the K + channel selectivity filter GYG motif. The importance of such filter glycines for K + selectivity was confirmed by interconversion of Ser-88 and Gly-88 in the rice paralogues OsHKT1 and OsHKT2. Surprisingly, all HKT homologues known from dicots have a serine at the filter position in P-loop A, suggesting that these proteins function mainly as Na + transporters in plants and that Na + /K + symport in HKT proteins is associated with a glycine in the filter residue. These data provide experimental evidence that the glycine residues in selectivity filters of HKT proteins are structurally related to those of K + channels.

Potassium is an important macronutrient in higher plants and K+ transporters play important roles in many physiological processes. Furthermore, sodium (Na+) uptake into plant cells has been suggested to be mediated by physiological K+ transporters. Genes that encode putative molecular mechanisms for K+ and Na+ transport have been isolated. By expression cloning, functional analysis in yeast, Xenopus oocytes, E. coli and plant cell cultures at least three gene families have been identified, that could contribute to K+ uptake into plant cells including K+ channels, the wheat HKT1 gene and the family of AtKUP/HAK/KT transporters. The long-term goal of research in several laboratories is to gain insight into the physiological roles, transport modes, expression patterns and subcellular localizations of cloned K+ and Na+ transporter genes. Advances at analysing the functions of individual genes from these families will be presented.

Sodium (Na+) is toxic to most plants, but the molecular mechanisms of plant Na+ uptake and distribution remain largely unknown. Here we analyze Arabidopsis lines disrupted in the Na+ transporter AtHKT1. AtHKT1 is expressed in the root stele and leaf vasculature. athkt1 null plants exhibit lower root Na+ levels and are more salt resistant than wild-type in short-term root growth assays. In shoot tissues, however, athkt1 disruption produces higher Na+ levels, and athkt1 and athkt1/sos3 shoots are Na+-hypersensitive in long-term growth assays. Thus wild-type AtHKT1 controls root/shoot Na+ distribution and counteracts salt stress in leaves by reducing leaf Na+ accumulation.