Background: Bispecific antibodies (bsAbs), alike CAR T cells, are not yet successful in solid tumors. Recently we developed a PSMAxCD3 bsAb termed CC-1 in an IgG-based format mediating fully target cell-restricted T cell activation and potent antitumor activity (Zekri et al, EMBO Mol Med, 2020). In prostate cancer (PC), PSMA is not only expressed on the malignant cells themselves, but also on tumor vessels. Targeting the latter improves accessibility of the tumor site for immune effector cells, a critical prerequisite for success. CC-1 is under evaluation in a First in Human trial enrolling patients with castration resistant prostate carcinoma (NCT04104607) and, in combination with checkpoint inhibition, in a phase I trial enrolling patients with squamous cell carcinoma of the lung (NCT04104607). On the basis of the meanwhile available very favorable safety and preliminary efficacy data, CC-1 is now being evaluated as first line treatment in patients with biochemical recurrence (BCR) of PC (NCT05646550), where tumor burden is low and accordingly further reduced side effects and sustained efficacy are expected. Methods: This is an ongoing open label, single center phase I clinical trial evaluating CC-1 without androgen deprivation therapy as first line treatment in men with BCR of PC. Key eligibility criteria include low risk of disease progression (PSA-Doubling time > 1 year after prostatectomy or interval to BCR > 18 months after radiation; ISUP grade < 4; PSA ≥ 0.2 ng/ml and < 5 ng/ml) and no androgen deprivation therapy except in a neoadjuvant/adjuvant setting. The clinical trial consists of (i) a dose escalation part (24-42 patients) using a 3+3 design to determine overall safety, tolerability as well as maximum tolerated dose (MTD) and (ii) a dose expansion part, in which 14 patients are treated at the MTD. CC-1 is applied twice-weekly over 21 days as three-hour infusion in up to six 28-day cycles. In the first cycle, step dosing (day 1: 10µg, day 2: 28µg, day 3 and thereafter: target dose) is performed, and 7 patient cohorts receiving dose levels from 78µg to 600µg are evaluated. Premedication includes acetaminophen, dimetindene and cortisone to minimize the risk of infusion reactions and cytokine release syndrome (CRS). The primary objectives are definition of MTD and recommended phase II dose. Assessment of safety is based on the frequency of adverse events according to CTCAE v5.0. Dose limiting toxicities are defined as ≥ 3° adverse drug reactions (e.g. CRS) with predefined exceptions (e.g. temporary laboratory abnormalities). Efficacy is determined by serial PSA-measurements (PSA-response: ≥50% decrease compared to baseline). Furthermore, percentage of patients with no clinical relapse, no salvage and no subsequent antineoplastic therapy will be assessed. Clinical trial information: NCT05646550.