Background/Purpose: Denosumab (DMAb) has been associated with low incidence of spine and non-spine, including hip, fractures through 10 years of treatment (Bone ASBMR 2015). Questions about bone safety arose in response to the FREEDOM transiliac crest biopsy findings: low numbers of tetracycline labels were observed with low dynamic parameters of remodeling (Reid JBMR 2010). In the FREEDOM Extension, bone biopsies were performed in subjects with 5 years of DMAb treatment and findings were similar to those in subjects with 2 or 3 years of DMAb treatment in FREEDOM (Brown JBMR 2014). We now report bone biopsy findings in subjects with 10 years of DMAb treatment.

Methods: A subset of subjects with 10 years of DMAb exposure (3 years FREEDOM + 7 years Extension) participated in a transiliac bone biopsy substudy. Subjects underwent a tetracycline/demeclocycline labeling procedure prior to their bone biopsy visit (6 months after their last DMAb dose); samples were prepared and analyzed according to standard procedures by the Mayo Clinic as previously described (Reid JBMR 2010). Continuous and categorical variables were summarized using descriptive statistics.

Results:   There were 22 biopsies evaluable for qualitative histology in subjects with 10 years of DMAb exposure; all specimens showed normally mineralized lamellar bone. There was no evidence of pathologic findings, including osteomalacia, woven bone, or marrow fibrosis. There were 21 biopsies evaluable for histomorphometry; these showed that the antiresorptive effects of DMAb were maintained over time.  In addition, indicators associated with bone formation and structure (including osteoid surface, osteoid width, and eroded surface) were generally similar to those at years 2/3 and 5 (Table). As part of the analysis of dynamic parameters, the presence of tetracycline labels was reviewed in all biopsies. The percentage of samples with any tetracycline label in trabecular bone has steadily increased over time from 34% in year 2/3, to 43% in year 5, and 77% in year 10; the percentage of samples with any label in cortical bone has remained steady from 57%, to 64%, and 55%, respectively. Double tetracycline labeling of trabecular or cortical bone was found in 7 (32%) subjects at year 10.

Conclusion: Bone histology showed normal bone microarchitecture, and histomorphometry was consistent with DMAb mechanism of action. There was no evidence of progression in the degree of low remodeling with long-term exposure to DMAb. Table 1: Bone histomorphometry in FREEDOM and its extension