Background Agonist antibodies targeting 4-1BB (CD137) effectively costimulate cytotoxic T cells and are active in preclinical models of cancer. However, clinical development of these agents has been hampered by limited efficacy and/or poor tolerability at active doses.^1,2 To overcome the efficacy and safety limitations of this approach, SGN-BB228, a first-in-class, investigational CD228/4-1BB Antibody Anticalin® bispecific was created. SGN-BB228 targets CD228 (melanotransferrin, p97), a glycosylphosphatidylinositol-anchored membrane protein with limited normal tissue expression, but high and prevalent expression in melanoma, mesothelioma, lung cancer and other tumor types.^3,4 SGN-BB228 is designed to provide a potent costimulatory bridge between tumor-specific T cells and tumor cells, improving and constraining T cell mediated cytotoxicity to tumors, potentially expanding the therapeutic window for 4-1BB agonism.

Methods Here we describe the expression profile of CD228 in cancer and normal tissues and preclinical activity of SGN-BB228 across reporter cell and primary T cell-based assays.

Results SGN-BB228 is comprised of a hinge-stabilized (S228P), Fc-null (FALA) fully human IgG4 antibody specific for CD228 connected to 4-1BB-targeting Anticalin® proteins⁵ via C-terminal heavy-chain fusions. The proposed mechanism of action (MOA) for SGN-BB228 is CD228-conditional clustering of 4-1BB on antigen experienced tumor-specific T cells, resulting in enhanced activation and cellular cytotoxicity. Expression analysis across cancer and normal tissues demonstrates CD228 is a tumor associated antigen prevalent in melanoma, mesothelioma, lung cancer and other tumor types with minimal normal tissue expression. Preclinical testing of SGN-BB228 in vitro shows potent CD228-conditional 4-1BB stimulation and cytotoxic T cell activation across a range of assay systems. In the presence of CD228-expressing tumor cells, but not CD228-negative tumor cells, SGN-BB228 drove dose-dependent amplification of NFkB signaling using a 4-1BB reporter cell system. SGN-BB228 also consistently drove potent CD228-conditional costimulatory activity in assays using primary T cells or whole peripheral blood mononuclear cells (PBMCs) receiving different forms of T cell receptor stimulation. The CD228-conditional activity of SGN-BB228 consistently outperformed a clinical benchmark antibody, even in the presence of antibody-clustering FcgRs expressed by PBMC.

Conclusions Together these data introduce SGN-BB228, a first-in-class, investigational CD228/4-1BB costimulatory Antibody Anticalin® bispecific with potent and CD228-conditional 4-1BB costimulatory activity with therapeutic potential in multiple solid tumor types. These data support future clinical study of SGN-BB228 in a first-in-human Phase 1 trial.

References

1. Segal NH, Logan TF, Hodi FS, et al. Results from an integrated safety analysis of urelumab, an agonist anti-CD137 monoclonal antibody. Clin Cancer Res. 2017;23(8):1929–36.

2. Segal NH, He AR, Doi T, et al. Phase I study of single-agent utomilumab (PF-05082566), a 4-1BB/CD137 agonist, in patients with advanced cancer. Clin Cancer Res 2018;24(8):1816–23.

3. Brown JP, Nishiyama K, Hellström I, Hellström KE. Structural characterization of human melanoma-associated antigen p97 with monoclonal antibodies. J Immunol 1981;127(2):539–546.

4. Smith LM, Nesterova A, Alley SC, Torgov MY, Carter PJ. Potent cytotoxicity of an auristatin-containing antibody-drug conjugate targeting melanoma cells expressing melanotransferrin/p97. Mol Cancer Ther 2006;5(6):1474–1482.

5. Hinner MJ, Aiba RSB, Jaquin TJ, et al. Tumor-localized costimulatory T-cell engagement by the 4-1BB/HER2 bispecific antibody-anticalin fusion PRS-343. Clin Cancer Res. 2019;25(19):5878–89.