ATG-101, a tetravalent PD-L1×4-1BB bispecific antibody, augments anti-tumor immunity through PD-L1 blockade and PD-L1-directed 4-1BB activation

Abstract Immune checkpoint inhibitors (ICIs) have transformed the landscape of cancer treatment. However, only a minority of patients achieve a profound and long-lasting response, and many patients are innately resistant or acquire resistance to ICI therapy. In addition, hepatotoxicity and suboptimal efficacy have hampered the clinical development of agonistic antibodies targeting 4-1BB, a promising immune stimulating target. To effectively target 4-1BB and treat diseases resistant to ICIs, we engineered a tetravalent "2 + 2" PD-L1×4-1BB bispecific antibody (BsAb), ATG-101. ATG-101 binds to PD-L1 and 4-1BB concurrently, with a greater affinity for PD-L1, and potently activates 4-1BB positive T cells when crosslinked with PD-L1 positive cells. ATG-101 also activates exhausted T cells upon PD-L1 crosslinking, indicating a possible role in reversing T-cell dysfunction and exhaustion. ATG-101 revealed potent antitumor activity in numerous in vivo tumor models, including those resistant to ICIs or that have progressed following ICI treatment. ATG-101 greatly increased the proliferation of CD8+ T cells, the infiltration of effector memory T cells, and the ratio of CD8+ T/Treg cells in the tumor microenvironment (TME), rendering an immunologically "cold" tumor "hot". ATG-101 is well-tolerated and does not induce hepatotoxicity in non-human primates, even at high doses. According to a computational semi-mechanistic pharmacology model simulation, 4-1BB/ATG-101/PD-L1 trimer formation and PD-L1 receptor occupancy are both maximized at around 2 mg/kg of ATG-101, providing guidance regarding a clinically optimal biological dose. In summary, by localizing to PD-L1-rich TME and activating 4-1BB-positive immune cells in a PD-L1 crosslinking-dependent manner, ATG-101 safely inhibits tumor growth in ICI resistant or refractory animal models.