Purpose: While patients responding to checkpoint blockade often achieve remarkable clinical responses, there is still significant unmet need due to resistant or refractory tumors. A combination of checkpoint blockade with further T-cell stimulation mediated by 4-1BB agonism may increase response rates and durability of response. A bispecific molecule that blocks the programmed cell death 1 (PD-1)/programmed cell death 1 ligand 1 (PD-L1) axis and localizes 4-1BB costimulation to a PD-L1–positive (PD-L1+) tumor microenvironment (TME) or tumor draining lymph nodes could maximize antitumor immunity and increase the therapeutic window beyond what has been reported for anti–4-1BB mAbs. Experimental Design: We generated and characterized the PD-L1/4-1BB bispecific molecule PRS-344/S095012 for target binding and functional activity in multiple relevant in vitro assays. Transgenic mice expressing human 4-1BB were transplanted with human PD-L1–expressing murine MC38 cells to assess in vivo antitumoral activity. Results: PRS-344/S095012 bound to its targets with high affinity and efficiently blocked the PD-1/PD-L1 pathway, and PRS-344/S095012-mediated 4-1BB costimulation was strictly PD-L1 dependent. We demonstrated a synergistic effect of both pathways on T-cell stimulation with the bispecific PRS-344/S095012 being more potent than the combination of mAbs. PRS-344/S095012 augmented CD4-positive (CD4+) and CD8-positive (CD8+) T-cell effector functions and enhanced antigen-specific T-cell stimulation. Finally, PRS-344/S095012 demonstrated strong antitumoral efficacy in an anti–PD-L1–resistant mouse model in which soluble 4-1BB was detected as an early marker for 4-1BB agonist activity. Conclusions: The PD-L1/4-1BB bispecific PRS-344/S095012 efficiently combines checkpoint blockade with a tumor-localized 4-1BB–mediated stimulation burst to antigen-specific T cells, more potent than the combination of mAbs, supporting the advancement of PRS-344/S095012 toward clinical development. See related commentary by Shu et al., p. 3182

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Background: OX40, a member of the tumor necrosis factor receptor superfamily, is a costimulatory molecule promoting survival and cytokine release by effector T cells and inhibiting human regulatory T-cell activity. OX40 activation has been explored as a potential treatment for solid malignancies, using mainly agonistic anti-OX40 antibodies. These antibodies rely on Fc gamma receptor (FcγR)-mediated crosslinking to activate the OX40 pathway and have shown so far only limited antitumoral efficacy in clinical settings. One underlying hypothesis is that FcγR crosslinking of these antibodies may not allow optimal activation of the OX40 pathway to fully exploit the potential of this target for immunotherapy of cancer. To overcome this limitation, we designed PRS-352/S095025, a strong PD-L1-dependent OX40 agonistic bispecific molecule, that allows combination of OX40 costimulation and PD-L1 blockade. Methods: We generated an Anticalin protein that binds with high affinity to human OX40. Anticalin® proteins are approximately 18-20 kDa protein therapeutics derived from human lipocalins that can be engineered to bind with high affinity and specificity to different targets. The PRS-352/S095025 bispecific fusion protein was obtained by genetic fusion of OX40-targeting Anticalin protein to a PD-L1-targeting monoclonal antibody with a modified IgG4 backbone, allowing an optimal activation of OX40 in the presence of PD-L1, but not FcγRs. Results: We showed that PRS-352/S095025 retains binding to PD-L1 like the anti PD-L1 antibody backbone alone and binds with high affinity to both human and cynomolgus OX40. We showed that PRS-352/S095025 inhibited the PD-1/PD-L1 pathway, as the parental PD-L1 targeting antibody and an approved anti PD-L1 antibody. In addition to its PD-L1 blocking properties, we showed that PRS-352/S095025 activates OX40, and that this activity is dependent on the expression of PD-L1 and not on FcγR, consistent with the desired mechanism of action. Furthermore, we demonstrated that PRS-352/S095025 activity is superior to both PD-L1 benchmark antibodies and to the combination of clinically relevant OX40 and PD-L1 benchmark antibodies. We could also show that PRS352/S095025 strongly stimulated human CD4 T cells. In vivo, we showed that PRS-352/S095025 has a PK profile similar to that of the PD-L1 antibody alone. Conclusions: We provide here the preclinical characterization of the fusion protein PRS-352/S095025, a bispecific molecule composed of a PD-L1 blocking moiety and an Anticalin protein agonizing OX40. In vitro, this molecule showed the desired MoA, PD-L1 blocking and potent OX40 agonism driven by binding to PD-L1, with superior activity to a clinical stage OX40 agonist. Citation Format: Lucia Pattarini, Marina Pavlidou, Aizea Kastresana Morales, Janet Peper-Gabriel, Matthieu Riviere, Didier Demarles, Alix Scholer-Dahirel, Veronique Blanc, Shane Olwill. The anticalin-antibody bispecific PRS-352/S095025 strongly stimulates human CD4+ T cells in a PD-L1-dependent manner [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB220.

Supplementary Table from The PD-L1/4-1BB Bispecific Antibody–Anticalin Fusion Protein PRS-344/S095012 Elicits Strong T-Cell Stimulation in a Tumor-Localized Manner

Supplementary Figure from The PD-L1/4-1BB Bispecific Antibody–Anticalin Fusion Protein PRS-344/S095012 Elicits Strong T-Cell Stimulation in a Tumor-Localized Manner