2584 Background: CD40 is a master switch for both innate and adaptive immune systems. The clinical development of CD40 agonists has been hampered by dose-limiting toxicity (DLT) due to systemic CD40 activation and peripheral target-mediated drug disposition (TMDD). MP0317 is a CD40-agonistic DARPin (designed ankyrin repeat protein), developed to reduce systemic toxicity. MP0317 is exclusively active in the presence of fibroblast activating protein (FAP) expressed by cancer associated fibroblasts in the tumor microenvironment. This allows reaching serum levels that overcome the TMDD and have the potential to deliver sustained and tumor-localized CD40 activation. Ongoing clinical testing aims at establishing its safety/tolerability profile, PK/PD characteristics, and a recommended dose for combination therapy. Methods: NCT05098405 is a Phase 1, multicenter, open label, dose escalation study followed by a safety expansion part in adult patients with advanced solid tumors. The dose escalation scheme uses an adaptive Bayesian logistic regression model with overdose control. Peripheral blood biomarkers are analyzed by immuno-assays and flow cytometry, and baseline and on-treatment tumor biopsies are characterized by RNA sequencing and immunofluorescence. Eligible patients are enrolled into 9 sequentially escalating dose cohorts of MP0317 (0.03-10 mg/kg), administered IV 3-weekly (Q3W) or 1-weekly (Q1W) until disease progression or unacceptable toxicity. Results: At submission, 23 patients across 6 cohorts completed the study, with no DLT observed. Patient enrolling in higher dose cohorts is ongoing. The most frequent AE was grade 2 infusion related reaction in 5 patients. The 23 patients received ≥2 (range 2-8) doses (range 0.03 – 3 mg/kg) of MP0317 across five Q3W and one Q1W cohorts and completed the 28-day DLT period. PK data confirmed all patients were exposed to MP0317 with broadly dose-dependent Cmax and no sign of accumulation. PD data was derived from patients dosed with 0.03 – 3 mg/kg. Soluble biomarkers (sFAP and sCD40) showed target engagement in periphery with signs of FAP saturation at ≥0.5 mg/kg. Colocalization of MP0317 with FAP and CD40 in the tumor was confirmed in 4 out of 8 evaluable paired tumor biopsies. Whole transcriptome and gene set enrichment analyses showed an upregulation of genes related to B-cell trafficking (CXCL3, CXCR5, CCR6, CCL20) upon treatment. Transient increase of IFNg-induced chemokines (CXCL9, CXCL10) was observed, whereas the pro-inflammatory cytokines (TNFa, IL-2, IL-6, IL-8) were not upregulated. Conclusions: Clinical data and the lack of pro-inflammatory circulating cytokines confirm MP0317 is safe and well-tolerated, while analysis of paired pre- and on-treatment tumor biopsies suggests early evidence of tumor-localized CD40 activation. The current data enables further evaluation in a combination setting. Clinical trial information: NCT05098405 .