Abstract CT077: Pharmacodynamic characterization of PRS-344/S095012, a PD-L1x4-1BB antibody-Anticalin fusion protein (Mabcalin), in a dose escalation study in patients with advanced solid tumors

Abstract Background: Conditional 4-1BB activation has emerged as an attractive approach to overcome the hepatoxicity or suboptimal efficacy associated with 4-1BB agonistic mAbs. PRS-344/S095012 is a tetravalent PD-L1/4-1BB bispecific molecule designed to simultaneously inhibit PD-1/PD-L1 signaling and provide PD-L1-dependent 4-1BB activation—offering a promising dual mechanism to enhance antitumor immunity while reducing 4-1BB related toxicity. Preclinical data demonstrated PD-L1-dependent 4-1BB trimerization and robust signaling activation (Peper-Gabriel et al., 2022). Here we report the biomarker and pharmacodynamic findings from a first-in-human dose-escalation study of PRS-344/S095012 (NCT05159388) in patients with advanced solid tumors. Methods: 45 patients received PRS-344/S095012 (12-80 mg IV) every 2 weeks. A cohort of 12 patients at the 36mg dose level was pre-treated with Obinutuzumab (OBI) to mitigate anti-drug antibody (ADAs) formation. Plasma and PBMCs were collected pre-dose (Cycle 1 Day 1), and on-treatment (during cycle 1) to measure cytokine levels, Soluble 4-1BB (s4-1BB) as a surrogate for target engagement, and immune cell activation by flow cytometry. DNA sequencing on PBMCs was used to evaluate changes in T cell receptor (TCR) diversity. Baseline tumor biopsies were analyzed by immunohistochemistry (IHC) to evaluate PD-L1 expression and immune infiltrates. Results: PRS-344/S095012 demonstrated a dose dependent induction of s41BB and cytokines within the first treatment cycle, indicative of target engagement and 4-1BB activation. Flow cytometry revealed a marked increase in proliferation and activation markers on both CD8+ and CD4+ T cells, aligning with the mechanism of action. Notably, IFN-γ levels were significantly elevated in cycle 1 at doses ≥36 mg, correlating with the incidence of immune-related adverse events (irAEs). Increased T cell diversity was observed post-treatment in patients experiencing irAEs, consistent with polyclonal T cell activation. OBI pre-treatment did not affect the level of target engagement in the first cycle of treatment (similar induction of s41BB at 36mg with and without OBI) but significantly increased TNF-a, IL-10 and IP-10 levels indicating additive effects of OBI and S095012 on immune activation. In baseline biopsies, immune biomarkers were heterogeneous. However, two patients with objective responses had higher baseline PD-L1 immune cell (IC) score. Conclusion: S095012 elicited a robust, dose-dependent enhancement of immune cell activation, which was further amplified by OBI pre-treatment, increasing the incidence of irAEs. A target with more specific tumor expression than PD-L1 might be better suited for selective 4-1BB agonism to minimize immune toxicities related to systemic immune cell activation. Citation Format: Emiliano Calvo, Elena Garralda, Christiane Jungels, Timothy Price, Sara Wahlroos, Hans Prenen, Sylvie Rottey, John Powderly, Antony Tolcher, Michael Cooper, Matyas Ecsedi, Ibtissam Marchiq, Johanna Verneau, Matthieu Bouaziz, Vanessa Seif, Najah Harouki Crochemore, Abbass Darwich, Aitana Calvo. Pharmacodynamic characterization of PRS-344/S095012, a PD-L1x4-1BB antibody-Anticalin fusion protein (Mabcalin), in a dose escalation study in patients with advanced solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr CT077.