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.