Effective cancer immunotherapy combining mRNA-encoded bispecific antibodies that induce polyclonal T cell engagement and PD-L1-dependent 4-1BB costimulation.

Background: Immune checkpoint inhibitors have revolutionized cancer therapy, but many patients fail to respond or develop resistance, often due to reduced T cell activity. Costimulation via 4-1BB has emerged as a promising approach to enhance the effector function of antigen-primed T cells. Bispecific T cell-engaging (TCE) antibodies are an effective way to provide tumor-specific T cell receptor-mediated signaling to tumor-infiltrating lymphocytes. mRNA-based delivery of bispecific antibodies, offer a novel approach to enhance tumor-specific immune responses while minimizing adverse effects.

Methods: Two bispecific antibodies were generated: the EGFR x CD3 TCE antibody (LiTE) and the PD-L1 x 4-1BB costimulatory antibody (LiTCo), which was further fused to a high FcRn albumin variant (Albu-LiTCo). The mRNA encoding these bispecific antibodies contains an N1-methylpseudouridine modified nucleoside and regulatory sequences to ensure proper expression and stability. A series of assays and cell-based analyses were performed to characterize both antibodies. The efficacy of the mRNA-encoded bispecific antibodies was evaluated in xenograft tumor models expressing EGFR.

Results: We investigated the combined effect of two mRNA-encoded Fc-free bispecific antibodies with complementary mechanisms of action: an EGFR-targeting TCE and a half-life extended PD-L1 x 4-1BB costimulatory antibody. The mRNAs encoding both bispecific LiTE and Albu-LiTCo, showed similar binding specificity and function to their protein analogues. Pharmacokinetic studies demonstrated sustained expression of both bispecific antibodies following intravenous administration of the mRNAs formulated using a polymer/lipid-based nanoparticle (LNP) but different pharmacokinetic profiles, shorter for the TCE and longer for the PD-L1 x 4-1BB. When administered as a mRNA-LNP combination (Combo), the growth of EGFR-positive tumors in immunocompetent mice was significantly inhibited, resulting in tumor regression in 20% of cases with no associated toxicity. Histological analysis confirmed increased T cell infiltration in the tumors treated with LITE and Combo. Repeated administration resulted in sustained production of bispecific antibodies with different exposure cycles and potent antitumor activity with a favorable safety profile.

Conclusions: These results highlight the potential of combining two mRNA-encoded bispecific antibodies with different mechanisms of action and programmable half-life for cancer immunotherapy.