Background: GB263T, a novel trispecific antibody directed against EGFR and cMET, adopts the design of two humanized VHH antibodies that recognize two different cMET epitopes, in which a conformational change induced by the first VHH binding exposes the second epitope for the second VHH to bind. Sequential binding of the two VHH antibodies to cMET increases tumor specificity, which results in widening of the safety window. In vitro studies show that GB263T induces better antigen-antibody molecular endocytosis, more completely blocks the signal transduction pathway, and has better ADCC activity compared to a benchmark. In addition, GB263T has demonstrated promising pre-clinical activity in models with various EGFR/cMET alterations. Here, we report the preliminary dose escalation results of the first-in-human phase I/II study of GB263T in patients (pts) with advanced EGFRm NSCLC (NCT05332574). Methods: This multicenter (China and Australia), phase I/II study was conducted using an accelerated titration method followed by a traditional 3+3 design for dose escalation. Pts with EGFRm NSCLC with prior EGFR TKI and platinum-based chemotherapy were enrolled. GB263T was given at 140-1680 mg (5 dose cohorts) IV weekly for the first two 28-day cycles and biweekly thereafter. Objectives were to evaluate the safety and tolerability, determine the maximum tolerated dose (MTD) and the recommended Phase 2 dose (RP2D), and assess the pharmacokinetics, immunogenicity, and preliminary efficacy of GB263T. Results: As of Feb 7, 2023, 6 pts were treated: 140mg (n = 1), 420mg (n = 1), 840mg (n = 3), 1260mg (n = 1). The enrolment of the 1260mg cohort is ongoing. All pts had received previous third-generation EGFR-TKI and platinum-based chemotherapy. Medium treatment duration was 49.5 days (range 2-113). No DLTs were observed. Five (83.3%) pts experienced treatment-related AEs (TRAEs), and all TRAEs were mild (grade 1/2): infusion related reaction (n = 3, 50%), rash (n = 2, 33%), mouth ulcer (n = 1, 17%), fatigue (n = 1, 17%), GGT increased (n = 1, 17%), ALP increased (n = 1, 17%) and direct bilirubin increased (n = 1, 17%). There were no treatment-related discontinuations. Among five response-evaluable pts, three (60%) achieved stable disease (SD). Tumor shrinkage was observed in all three SD pts, with a maximum reduction of 17% in the target lesion. All three SD pts remained on treatment at data cutoff. Conclusions: Results to date demonstrate a favorable safety profile with early signal of clinical activity. Continued dose escalation and clinical investigation of GB263T is ongoing. Updated data will be presented. Clinical trial information: NCT05332574.