This is a phase Ib/II, multicenter, single-arm, open-label study to evaluate the efficacy and safety of JMT103 in patients with surgically unsalvageable or refractory giant cell tumor of bone (GCTB).

The objective of the trial is to evaluate the efficacy and safety of JMT103 in patients with GCTB that is surgically unsalvageable or for which the planned surgery is associated with severe morbidity.

This study consists of two parts (phase Ib and II). In the phase Ib part, 6 to 12 subjects will be enrolled, and the safety, pharmacokinetics and pharmacodynamics of JMT103 will be evaluated after treatment for more than 4 weeks. And then the phase II part will be conducted, in which, 125 subjects will be enrolled.

Eligible subjects will receive JMT103 at a dose of 2mg/kg subcutaneously (SC) every 4 weeks (Q4W) with a loading dose of 2mg/kg SC on study days 8 and 15 until disease progression, tumor resection (the pathologic outcome after surgical resection is CR or PR), intolerable toxicity, decision by the participant to discontinue, or decision by the investigator that the subject could no longer benefit from the treatment (whichever occurs first).

Inclusion Criteria:

1. Fully informed and signed informed consent.
2. Male or female, Adults, 18 years and older
3. Histologically confirmed GCTB that is surgically unsalvageable or for which the planned surgery is associated with severe morbidity.
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2.

Exclusion Criteria:

1. Fertile subjects refuse to use effective contraception methods from the signing of the informed consent to 6 months after the last dose;
2. Active dental or jaw condition which requires oral surgery, including tooth extraction;
3. Currently receiving other anti-tumor therapy (radiotherapy, chemotherapy or arterial embolization, etc.);
4. Concurrent treatment with bisphosphonates;
5. Known history of second malignancy within the past 5 years, except for basal cell carcinoma or cervical carcinoma in situ.