3638 Background: Bone is one of the most common metastatic sites of malignancies. The metastatic tumor cells activate osteoclast activity and promote bone resorption via RANKL/RANK signaling pathway, and lead to osteolytic destruction. JMT103 is an innovative fully human IgG4 monoclonal antibody targeting RANKL. It can block RANKL/RANK signaling pathway, inhibit bone resorption, and protect bones from tumor metastasis. This study aimed to evaluate the safety and tolerability of JMT103 in patients with bone metastasis. Methods: This is a multi-center, open label, dose escalation, phase I clinical trial. The patients (ECOG score: 0-1) with bone metastasis from solid tumor who had not received bisphosphonates within 6 weeks before enrollment and were naïve to denosumab were enrolled. The initial dose was 0.5 mg/kg, sequentially escalated to 1.0, 2.0, and 3.0 mg/kg. JMT103 was injected subcutaneously by accelerated titration in 0.5 and 1.0 mg/kg dose groups, but via traditional "3+3" dose-escalation design in 2.0 and 3.0 mg/kg dose groups. Expansion study was conducted for subjects in 1.0, 2.0, and 3.0 mg/kg dose groups. Specifically, 3 additional doses (q4w) were injected after the end of 12-week single-dose study. The primary endpoints were maximum tolerated dose and safety. The secondary outcome measures included PK profile, preliminary efficacy biomarkers, immunogenicity, and bone mineral density (BMD). Results: From May 2018 to January 2020, 56 patients (13 males, 43 females, mean (SD) age: 55.57 (11.42) years) were enrolled, including bone metastasis from breast cancer (n = 36), gastric cancer (n = 5), lung cancer (n = 4), rectal cancer (n = 3), colorectal cancer (n = 2), or other solid tumors (n = 6). Nineteen patients participated in the dose-escalating study and 37 patients participated in the expansion study. JMT103 showed overall good safety. There were 74 drug-related AEs in all, including grade 3 (DLT hypocalcemia, n = 1; hypophosphatemia, n = 3), grade 2 (n = 15), and grade 1 (n = 55) AEs. The most common drug-related AEs were hypophosphatemia (n = 15), hypocalcemia (n = 12), and hypermagnesemia (n = 6). Median uNTx/Cr decrease from baseline was 76.6% (n = 20). Conclusions: JMT103 shows good safety and tolerability in patients with bone metastasis. Clinical trial information: NCT03550508 .