Abstract 565: A novel T-cell costimulatory Humabody® VH therapeutic for PSMA-positive tumors

Agonistic monoclonal antibodies targeting CD137/4-1BB have shown much preclinical promise but their clinical development has been slowed due to a poor therapeutic index, in particular liver toxicity. CB307 is a novel half-life extended bispecific Humabody VH targeting CD137 (4-1BB) and prostate specific membrane antigen (PSMA). The design of CB307 enables agonism of CD137 selectively in the presence of PSMA positive tumour cells and in this way enable tumour selective T cell activation whilst minimising systemic activation. The molecular weight of CB307 is less than 50 kDa (around a third of the size of a standard IgG) and it does not contain an Fc domain, thereby avoiding interaction with Fc receptors. Half life extension is achieved through the inclusion of a VH domain with specificity for human serum albumin (HSA). The identification of CB307 using the Crescendo Mouse™ which develops fully human VH domains in a background devoid of light chains will be described along with characterisation of the key properties of the molecule in in vitro and in vivo models. In dual target binding assays CB307 shows potent co-binding to both PSMA and CD137 targets and mediates CD137 signalling in an NFKB cell reporter assay in the presence of PSMA positive cells but not PSMA negative cells. Co-incubation of primary human T-cells from healthy individuals or cancer patients together with PSMA positive tumour cells and CD3 stimulation induces T-cell activation and cytokine release. In an in vivo model using NSG mice engrafted with human PBMCs the growth of PSMA positive DU145 prostate tumour cells is inhibited by a surrogate bispecific. Together these data support progression of CB307 into clinical development.

Citation Format: James W. Legg, Brian McGuinness, Sophie Archer, Phil Bland-Ward, Verena Brucklacher, Jenny Craigen, Carolyn Edwards, Emma Hames, Jay Majithiya, pavel Pisa, Bhindu Revi, Nikki Royale, Angelica Sette, Yumin Teng, Lorraine Thompson, Wembin Wang, Chris Wilson, Chris Wyre, Chris Rossant. A novel T-cell costimulatory Humabody® VH therapeutic for PSMA-positive tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 565.

Enhancement of antibody-dependent cell-mediated cytotoxicity by endowing IgG with FcαRI (CD89) binding

Fc effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cell-mediated phagocytosis (ADCP) are crucial to the efficacy of many antibody therapeutics. In addition to IgG, antibodies of the IgA isotype can also promote cell killing through engagement of myeloid lineage cells via interactions between the IgA-Fc and FcαRI (CD89). Herein, we describe a unique, tandem IgG1/IgA2 antibody format in the context of a trastuzumab variable domain that exhibits enhanced ADCC and ADCP capabilities. The IgG1/IgA2 tandem Fc format retains IgG1 FcγR binding as well as FcRn-mediated serum persistence, yet is augmented with myeloid cell-mediated effector functions via FcαRI/IgA Fc interactions. In this work, we demonstrate anti-human epidermal growth factor receptor-2 antibodies with the unique tandem IgG1/IgA2 Fc can better recruit and engage cytotoxic polymorphonuclear (PMN) cells than either the parental IgG1 or IgA2. Pharmacokinetics of IgG1/IgA2 in BALB/c mice are similar to the parental IgG, and far surpass the poor serum persistence of IgA2. The IgG1/IgA2 format is expressed at similar levels and with similar thermal stability to IgG1, and can be purified via standard protein A chromatography. The tandem IgG1/IgA2 format could potentially augment IgG-based immunotherapeutics with enhanced PMN-mediated cytotoxicity while avoiding many of the problems associated with developing IgAs.

Abstract 363: Activin receptor-like kinase 1 (Alk1) ligands modulate endothelial cell function in vitro and in vivo

The TGF-ß type-1 receptor activin receptor-like kinase 1 (Alk1) is predominantly expressed on endothelial cells, regulates vascular morphogenesis, and has been implicated in pathological angiogenesis. Alk1 expression is increased in the vasculature of a broad range of tumours and agents that disrupt of Alk1 signalling in tumour endothelial cells may be potential therapeutics for the treatment of a variety of solid tumours. Alk1 has been described as a receptor for the bone morphogenic proteins, BMP9 and BMP10. In order to study the effects of Alk1 signalling in endothelial cells in vitro and in the vasculature in vivo we used a soluble antagonist Alk1-Fc protein consisting of the extracellular domain of the Alk1 protein fused to the Fc domain of IgG1. In vitro, Alk1-Fc inhibited BMP9-dependent Alk1 signalling via phosphorylation and nuclear translocation of SMAD1/5 in endothelial cells and modulated endothelial cell functions such as an ability to form tube- like structures. In mice, physiological levels of BMP9 could be detected in serum by ELISA, consistent with detectable levels of phosphorylated SMAD1/5 in the lung, a site of high Alk1 vascular expression. In mice, the administration of Alk1-Fc effectively blocked SMAD1/5 phosphorylation in the lung, suggesting that in this tissue Alk1 ligands are the predominant driver of lung SMAD1/5 activation and that signalling is efficiently antagonised by Alk1-Fc. Despite suppression of Alk1 signalling in the lung and expression of Alk1 on the vasculature associated with tumours, Alk1-Fc did not have profound effects on tumour size or microvessel density in mice bearing tumours, however administration of Alk1-Fc in neonatal mice did cause defects in angiogenesis of the retinal vasculature demonstrating that in some contexts Alk1 ligands play an important role in regulating the angiogenic process.

Citation Format: Gareth C. Davies, James A. Harper, Richard C. Sainson, Lee Brown, Grace Opoku-Ansah, Alison Smith, Matthew McCourt, James W. Legg. Activin receptor-like kinase 1 (Alk1) ligands modulate endothelial cell function in vitro and in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 363. doi:10.1158/1538-7445.AM2013-363

The role of the CD44 transmembrane and cytoplasmic domains in co-ordinating adhesive and signalling events

CD44 is a widely distributed type I transmembrane glycoprotein and functions as the major hyaluronan receptor on most cell types. Although alternative splicing can produce a large number of different isoforms, they all retain the hyaluronan-binding Link-homology region and a common transmembrane and cytoplasmic domain, which are highly conserved between species. The past decade has seen an extensive investigation of this receptor owing to its importance in mediating cell-cell and cell-matrix interactions in both normal and disease states. Although roles for alternative splicing and variable glycosylation in determining ligand-binding interactions are now well established, the mechanisms by which CD44 integrates structural and signalling events to elicit cellular responses have been less well understood. However, there is now increasing evidence that CD44 is assembled in a regulated manner into membrane-cytoskeletal junctional complexes and, through both direct and indirect interactions, serves to focus downstream signal transduction events.

A novel PKC-regulated mechanism controls CD44 ezrin association and directional cell motility

The dynamic assembly and disassembly of membrane cytoskeleton junctional complexes is critical in cell migration. Here we describe a novel phosphorylation mechanism that regulates the hyaluronan receptor CD44. In resting cells, CD44 is constitutively phosphorylated at a single serine residue, Ser325. After protein kinase C is activated, a switch in phosphorylation results in CD44 being phosphorylated solely at an alternative residue, Ser291. Using fluorescence resonance energy transfer (FRET) monitored by fluorescence lifetime imaging microscopy (FLIM) and chemotaxis assays we show that phosphorylation of Ser291 modulates the interaction between CD44 and the cytoskeletal linker protein ezrin in vivo, and that this phosphorylation is critical for CD44-dependent directional cell motility.

Identification and functional analysis of the ezrin-binding site in the hyaluronan receptor, CD44

ERM (ezrin, radixin and moesin) proteins function as linkers between the actin cytoskeleton and the plasma membrane. In addition to this structural role, these proteins are highly regulatable making them ideal candidates to mediate important physiological events such as adhesion and membrane morphology and to control formation and breakdown of membrane-cytoskeletal junctions. Recently, a direct interaction in vitro has been demonstrated between ERM proteins and the hyaluronan receptor, CD44. We have mapped the ezrin-binding site to two clusters of basic amino acids in a membrane-proximal 9 amino-acid region within the CD44 cytoplasmic domain. To investigate the functional importance of this interaction in vivo, we created a number of mutations within full-length CD44 and expressed these mutants in human melanoma cells. We demonstrate here that mutations within the ezrin-binding site do not disrupt the plasma membrane localization of CD44 and, in addition, that this region is not required to mediate efficient hyaluronan binding. These studies suggest that ERM proteins mediate the outside-in, rather than inside-out, signalling of adhesion receptors.