Background: HER2 overexpression is common in many malignancies and contributes to tumor growth. Unlike the varied options of anti-HER2 target therapies for breast cancer, there is a huge unmet medical need for HER2 overexpressing non-breast solid tumor (NBST) such as gastric cancer and urothelial cancer. Therefore, we conducted the first study of RC48-ADC, a novel humanized anti-HER2 antibody conjugate, in NBST. Methods: This was an open-label, dose-escalation and expansion study in patients with HER2-overexpression (IHC 2+ and 3+) advanced solid cancers after failure of standard treatment. The dose escalation consisted of accelerated (0.1 and 0.5 mg/kg) and “3+3” titrations (1.0, 2.0, 2.5 and 3.0 mg/kg). In dose expansion stage, patients were given RC48-ADC at 2.0 mg/kg, Q2W. Results: As of Aug 20, 2019, 57 patients (including 47 with gastric cancer and 4 with urothelial cancer) were treated at 0.1 (1 patient), 0.5 (1 patient), 1.0 (3 patients), 2.0 (6 patients in dose escalation and 32 patients in dose expansion), 2.5 (11 patients), and 3.0 mg/kg (3 patients), respectively. Most of them were Stage IV (91.2%) or with metastasis (96.5%). DLT was observed in 1, 2, and 1 patient at 2.0, 2.5, and 3.0 mg/kg, respectively. The MTD was 2.5 mg/kg. Most commonly reported TRAEs were WBC count decreased (66.7%), fatigue (56.1%), neutrophil count decreased (54.4%) and hemoglobin decreased (52.6%). Grade 3/4 TRAEs were reported in 28 patients (49.1%). Confirmed ORR was 21.1% (8/38) for 2.0 mg/kg, and 17.5% (10/57) for all patients. DCR was 52.6% and 49.1%, respectively. PFS was 3.6 months (95% CI: 4.1, 11.3) for 2.0 mg/kg. Subgroup ORR was 20.7% (6/29) at 2.0 mg/kg and 18.2% (2/11) at 2.5 mg/kg for gastric cancer, and 50.0% (2/4) for urothelial cancer. Conclusions: RC48-ADC demonstrated a good safety profile and promising anti-tumor activity in the late stage solid tumor including gastric cancer and urothelial cancer. Response and PFS benefits were clinical meaningful at 2.0 mg/kg and 2.5 mg/kg. Phase 2 pivotal study (NCT03556345) in gastric cancer is ongoing. Clinical trial information: NCT02881190