This was a multicenter, randomized, double-blind, phase III trial conducted in 51 centers in China. Patients aged 18–80 years; with histologically or cytologically confirmed solid tumors and bone metastases within 3 months; Eastern Cooperative Oncology Group performance status of 0–2; life expectancy ≥ 3 months; and adequate organ function were eligible. Exclusion criteria included previous treatment with denosumab or bisphosphonates; previous or ongoing osteomyelitis or osteonecrosis of the jaw, active dental or jaw bone disease requiring oral surgery, an unhealed wound after a dental operation or oral surgery, or a planned invasive dental operation, radiotherapy, or surgery to bones.

The study protocol was approved by the ethics committee in each center and registered at ClinicalTrials.gov (identifier NCT04550949). All patients provided written informed consent before participation.

The baseline characteristics of patients including age, sex, ethnicity, height, weight, Eastern Cooperative Oncology Group performance status, tumor characteristics, and previous SREs were recorded. The treatment period was divided into a 13-week double-blind period and a 40-week open-label period. The safety follow-up period was 20 weeks. A random allocation sequence was generated by a statistician using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). The patients were randomly assigned 1:1 into the QL1206 or denosumab group, stratified based on tumor types (breast cancer, lung cancer, or the others), previous SREs, and current systemic anti-tumor therapy (yes or no) by investigators using an Interactive Web Response System. In the double-blind period, the patients received three doses of QL1206 or denosumab [120 mg subcutaneously every 4 weeks (Q4W)]. Investigators other than the nurses who administer the study drugs were blinded to allocation. The data of the primary endpoint was obtained at the end of the double-blind period. In the open-label period, to evaluate the long-term efficacy and safety of QL1206 and the feasibility of switching from denosumab to QL1206, up to 10 further doses of QL1206 (120 mg subcutaneously Q4W) can be given at the discretion of investigators in both groups. The patients also received daily supplementation of calcium (≥ 500 mg) and vitamin D (≥ 400 U) throughout the treatment period. All cancer-specific therapies were allowed, except for bisphosphonates or unapproved investigational treatments.

The uNTX and s-BALP levels were measured in the central laboratory (Guangzhou Kingmed Diagnostics Group Co., Ltd., Guangzhou, China) at baseline, at Week 2, 5, 13, 25, 37, and 53, and at study treatment discontinuation, using an enzyme-linked immunosorbent assay [19] and the Access Ostase assay (Beckman Coulter Inc., Brea, CA, USA) [20], respectively. SREs, which were defined as pathologic fractures, spinal cord compression, or requirements for radiation or surgery of bones, were assessed throughout the treatment period.

Adverse events (AEs) were evaluated from study treatment initiation to the end of the safety follow-up period according to the National Cancer Institute Common Terminology Criteria for Adverse Events version 5.0. Immunogenicity was evaluated through measuring the serum anti-drug antibody (ADA) levels in the central laboratory (Shanghai Xihua Scientific Co., Ltd., Shanghai, China) at Week 5, 13, 25, and 53, and at study treatment discontinuation, using a semi-quantitative electrochemiluminescent assay. Those with positive ADA results received further tests for the neutralizing antibody (Nab) by a semi-quantitative electrochemiluminescent Nab assay.

For the population PK (popPK) analysis, the data from the phase I trial of QL1206 in healthy subjects (80 for QL1206 and 76 for denosumab) [18] and the present trial were used. In the present study, the serum samples were collected at Week 1, 5, 9, 10, 11, 12, 13, 25, and 53. Drug concentrations were tested in the same laboratory as ADA detection using an enzyme-linked immunosorbent assay.

The primary endpoint was the percentage change in uNTX/uCr from baseline to Week 13. The secondary endpoints included percentage change in uNTX/uCr from baseline to Week 25 and 53, the percentage change in s-BALP from baseline to Week 13, 25, and 53, and the time to first on-study SRE. The safety endpoints were AEs and immunogenicity. The exploratory endpoint was popPK.

It is assumed that the natural log-transformed uNTX/uCr ratio of Week 13 to baseline [=ln(uNTX/uCrWeek13÷uNTX/uCrbaseline)] was equal between the two groups. In total, 598 patients (299 each group) were required for 80% power with a pooled standard deviation of 0.56 and equivalence margins of ± 0.135 based on a 90% two-sided confidence interval (CI). Considering a possible 15% dropout rate, a sample size of 700 patients (350 each group) was needed.

All statistical analyses were performed using SAS version 9.4. For the primary endpoint, according to the Statistical Guideline of Bioequivalence Study of National Medical Products Administration of China [21], a p-value of < 0.05 for two one-sided tests was considered statistically significant. For the secondary endpoints, a two-sided p-value of < 0.05 was considered statistically significant. The analysis set should be as close as possible to the intention-to-treat ideal [22]. Thus, the primary and secondary endpoints were evaluated in the full analysis set (FAS), including all randomized patients receiving at least one dose of the study drug and one efficacy evaluation. The per-protocol set (PPS) was used for the sensitivity analyses, and included patients in the FAS without a major protocol violation. The safety endpoints were summarized by treatment group in the safety set, which included patients receiving at least one dose of the study drug and one post-baseline safety evaluation.

Comparison of the primary endpoint between the two groups was performed using an analysis of covariance on the natural log-transformed uNTX/uCr ratio of Week 13 to baseline. The treatment group and randomization strata were independent variables and the baseline uNTX/uCr was a covariate, with equivalence margins of ± 0.135. Missing data were imputed using the last observation carried forward method for subjects with a post-baseline assessment. Predefined subgroup analyses of the primary endpoint were based on age, sex, and randomization strata. Further, to minimize the center effect, a serial number of centers was included as an additional independent variable in the analysis of covariance model for a post-hoc sensitivity analysis.

Changes in uNTX/uCr at the other timepoints were analyzed using the same methods as the primary endpoint, except missing data were not imputed. The percentage changes in s-BLAP were compared using a van Elteren test adjusted by randomization strata. The median time to the first on-study SRE was estimated by the Kaplan–Meier method and Greenwoods’ formula. Hazard ratios were calculated using a Cox proportional hazards model adjusted by randomization strata.

PopPK was analyzed in the patients receiving at least one dose of the study drug and one drug concentration evaluation in the present study (i.e., popPK analysis set). The PK model was established based on the healthy subjects in the phase I study [18]. PopPK modeling was conducted through NONMEM version 7.4 (Icon Development Solutions, LLC, Ellicott City, MD, USA) using standard model building and evaluation approaches. Covariates, including disease state (healthy subject or patient), treatment group, age, sex, baseline body weight, baseline estimated glomerular filtration rate, baseline aspartate aminotransferase, baseline alanine aminotransferase, baseline alkaline phosphatase, baseline albumin, and ADA were evaluated to determine the association with PK. Nonlinear mixed-effects modeling was used to estimate the PK parameters. The final model was evaluated using visual predictive checks. Based on the established popPK model, the maximum a posteriori Bayesian method was used to estimate the individual PK parameters of patients. PK bioequivalence was also judged using the 90% CI of the ratio of a log-transformed exposure measure (the area under the serum drug concentration-time curve [AUC] from t = 0 to Week 4 after a single dose [AUC0–4 week,1], maximum serum drug concentration after a single dose [Cmax,1], AUC from t = 0 to Week 4 at steady-state [AUC0–4 week,ss], and maximum serum drug concentration at steady-state [Cmax,ss]; equivalence margins: 80–125%).