Abstract CT255: Study of PRS-344/S095012 a PD-L1/4-1BB bispecific antibody-Anticalin®-fusion in patients with solid tumors

PRS-344/S095012 is a bispecific antibody-Anticalin® fusion protein targeting PD-L1 and 4-1BB. It is designed to block the PD-1/PD-L1 axis and localize 4-1BB co-stimulation to PD-L1-positive tumor microenvironment (TME) or tumor draining lymph nodes with the aim of maximizing antitumor immunity and increasing the therapeutic window of 4-1BB monoclonal antibodies (mAbs). Multiple in vitro assays have shown that PRS-344/S095012 inhibits PD-1/PD-L1 signaling while simultaneously activating 4-1BB signaling on T cells. This resulted in a synergistic effect on both pathways leading to anti-tumor immune responses. PRS-344/S095012 presents mAb-like pharmacokinetics in vivo and non-clinical mouse models demonstrated dose-dependent efficacy in anti-PD-L1 refractory tumors. In the higher dose range, complete tumor regression was achieved in PD-L1 high and PD-L1 low expressing xenograft mouse models. This first-in-human (FIH), phase 1/2, open-label, multi center, dose escalation and cohort expansion study is designed to determine the safety and tolerability of intravenously administered PRS-344/S095012 in patients with advanced and/or metastatic solid tumors (NCT05159388). Patients with histologically confirmed diagnosis of unresectable, locally advanced, or metastatic solid tumors for which standard treatment options are not available, no longer effective, or not tolerated are eligible. Patients must have Eastern Cooperative Oncology Group (ECOG) status 0 or 1, RECIST 1.1 measurable disease and should have documented progression on prior therapy. Prior therapy with 4-1BB agonists is prohibited. Phase 1 is a dose escalation guided by a Bayesian Logistic Regression Model and a 28-day dose limiting toxicities (DLT) period. Primary objectives are safety and tolerability of PRS-344/S095012; secondary objectives include exploration of antitumor activity and evaluation of pharmacokinetics. Pharmacodynamics and comprehensive biomarker program will be explored. Additional patients may be enrolled to backfill cohort(s)to further explore pharmacokinetic/pharmacodynamic signals. Phase 2 is an expansion with primary objective of anti-tumor activity. First patient was dosed in November 2021 and the study is planned to enroll at sites in Belgium, Spain and Australia. Citation Format: Christiane Jungels, Nuria Kotecki, Emiliano Calvo, Elena Garralda, Timothy Price, Xiaojiang Zahn, Atif Abbas, Lisa Mahnke, Winrich Rauschning, Aizea Morales-Kastresana, Lucia Pattarini, Birgit Bossenmai