Izalontamab (SI-B001) is a novel EGFR×HER3 bispecific antibody. This first-in-human phase I study presents the safety and pharmacokinetics of izalontamab.Previously treated patients with locally advanced or metastatic epithelial tumors were enrolled in the dose-escalation or dose-expansion phases. The dose-escalation phase consisted of an accelerated titration and a "3+3" design with 9 dose levels from 0.4 to 28.0 mg/kg. The dose-expansion phase included 5 dose levels from 6.0 to 21.0 mg/kg. Izalontamab was administered intravenously weekly(QW) or every two weeks(Q2W) in a four-week cycle. Available pre-treatment specimens were obtained to explore the relationship between EGFR/HER3 expression and efficacy.60 patients were enrolled. Among the 60 enrolled patients, 49 had non-small cell lung cancer(NSCLC), 6 had nasopharyngeal cancer, 3 had head and neck cancer(HNSCC), and 2 had other types of cancer. The most common treatment-related adverse events were rash (42%), paronychia (25%) and infusion-related reactions (23%). No drug-related death occurred. Izalontamab displayed non-linear pharmacokinetic behavior and clearance at steady state appeared to be approaching a dose-independent value at 6 mg/kg and above. The best response included 2 confirmed partial responses in NSCLC and HNSCC patients; 18 patients had stable disease, including NSCLC(n = 17) and colorectal cancer(n=1). The recommended phase 2 dose for izalontamab was determined as 9-16mg/kg QW weekly.Izalontamab was well tolerated and demonstrated preliminary antitumor activity in patients with locally advanced or metastatic epithelial tumors, supporting it as a promising therapeutic candidate for combination therapies, with a phase 3 study currently underway.