This is a phase Ⅲ, multi-center, randomized, double-blind, active-controlled study to evaluate the efficacy and safety of narlumosbart (JMT103) in patients with unresectable or surgically difficult giant cell tumor of bone (GCTB). This clinical trial study hypothesizes narlumosbart administration groups are not inferior to active control administration groups.

Inclusion Criteria:

1. Fully informed and signed informed consent;
2. Male or female adults aged ≥ 18 years or skeletally mature adolescents must weigh at least 45 kg and ≥ 12 years of age;
3. Pathologically confirmed giant cell tumor of bone that is surgically unresectable or for which the planned surgery is associated with functional compromise or morbidity;
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2.

Exclusion Criteria:

1. Previous or ongoing osteomyelitis or osteonecrosis of the jaw, non-healed dental/oral surgery, active dental or jaw condition requiring oral surgery, planned invasive dental procedure during the study;
2. Known or suspected diagnosis of other giant cell-rich tumors, brown cell tumor of bone, or Paget's disease;
3. Known diagnosis of malignancy within the past 5 years, except for definitively treated superficial basal cell carcinoma or cervical carcinoma in situ;
4. Bone metabolic disease including hypo-/hyperparathyroidism, hypo-/hyperthyroidism (except for no need treatment subclinical hypothyroidism), hypopituitarism, hyperprolactinemia, Cushing's syndrome, acromegaly, etc.;
5. Active infections requiring systematic treatment within 7 days prior to randomization;
6. Known human immunodeficiency virus (HIV), syphilis infections or viral hepatitis;
7. Current receiving other anti-tumor therapy (such as radiation, chemotherapy, or embolization, etc.);
8. Concurrent bisphosphonate treatment;
9. Use of anti-receptor activator of nuclear factor-κB ligand (RANKL) antibody within six months prior to screening, or no response to previous RANKL antibody treatment;
10. Known allergic/hypersensitive reaction to JMT103, positive control drug, calcium and vitamin D;
11. Pregnant or lactating females; For those of child bearing potential, refusal to use effective contraception methods from signing informed consent to 6 months after last administration.