Background: While being highly efficient in hematological malignancies, bispecific antibodies (bsAbs), alike CAR T cells, are yet not successful in solid tumors. Moreover, all available T cell-mobilizing strategies cause side effects that endanger patients and, in case of bsAbs, limit applicable doses and thus efficacy. We report on the clinical development of CC-1, a PSMAxCD3 bsAb, in an IgG-based format that induces fully target cell-restricted T cell activity (Zekri et al, EMBO Mol Med, 2020). Targeting PSMA, which is expressed on malignant cells and on the neovasculature, improves accessibility of the tumor site for immune effector cells as critical prerequisite for success in solid tumors. Notably, CC-1 binds a unique PSMA epitope which is expressed on malignant cells both of prostate carcinoma (PC) and 50% of patients with squamous cell carcinoma (SCC) of the lung. Methods: A FIH trial evaluating CC-1 with pre-emptive Tocilizumab included patients with metastatic castration resistant prostate carcinoma (mCRPC) (NCT04104607) and consisted of two parts. Dose escalation (n=10-66) using a novel intra-individual dose escalation design to rapidly reach the target dose of 826µg to determine safety, tolerability and maximum tolerated dose (MTD) (Labrenz et al, Pharm Stat, 2022). The dose expansion cohort exposed patients to CC-1 at MTD and explored efficacy to define RP2D (n=14). Our second phase I trial enrolls patients with SCC of the lung (NCT04496674) to receive CC-1 in combination with checkpoint inhibition. Based on the meanwhile available very favorable safety and preliminary efficacy data, a third phase I trial was initiated where CC-1 is evaluated as first line treatment in patients with hormone sensitive biochemical recurrence (BCR) of PC (NCT05646550), where tumor burden is low and accordingly lower side effects and long-lasting efficacy are expected. Results: Recruitment in the dose escalation part of the trial in mCRPC has been completed and the target dose was reached and MTD defined without DLT upon treatment of the 9^th and 14^th patient, respectively. 24 patients completed treatment, with the most frequently observed toxicity being cytokine release syndrome (CRS, max. 2°) (88%). Besides grade 1 to 2 hypertension (46%) and xerostomia (8%), no further CC-1 related toxicities were observed. A rapid and profound decline of elevated PSA levels was observed in all but one patient, with up to 60% reduction compared to baseline. So far 5 patients received multiple treatment cycles at MTD-level. One patient with SCC of the lung has been treated and CC-1 was escalated up to 153µg without occurrence of CRS. In the phase I trial in BCR of PC, the first three patients were enrolled in February 2023. Conclusions: CC-1 is a promising compound with a favourable toxicity profile and promising clinical activity. Details on study designs and updated data from the 3 clinical trials will be presented at the meeting. Clinical trial information: NCT04104607, NCT04496674, NCT05646550.