A comprehensive study into the mineralization mechanisms of giant cell tumors of bone in the spine during denosumab treatment

Giant cell tumors of bone are rare but aggressive, locally destructive tumors. They typically affect young people, significantly reducing their quality of life and increasing mortality rates. Giant cell tumors of bone are composed of osteoclast-like giant cells that respond to increased secretion of receptor activator of nuclear factor kappa-B ligand by stromal cells, triggering osteolysis. For over a decade, denosumab, a monoclonal antibody targeting this receptor activator, has been approved to facilitate surgical resection or in the setting of inoperable tumors. Although variable degrees of tumor mineralization have been observed in clinical applications of this drug, the nature of this newly formed mineralized tissue has yet to be determined. To characterize both mineralization and collagen organization in the newly formed bone, extensive analyses were conducted on four post-treatment giant cell tumor vertebral samples. Quantitative backscattered imaging, electron probe microanalysis, and a novel method for determining the alignment order of collagen fibrils using second harmonic generation were used to evaluate bone composition and structure. Additionally, histological staining and mass spectroscopy were employed to investigate the biological mechanisms of mineralization and matrix formation. Our results concluded that denosumab treatment of giant cell tumor of bone in the spine is associated with the formation of woven bone demonstrating increased mineral density in a matrix of disorganized collagen fibrils characterized by increased collagen III. These observations were associated with patient age and extension of treatment, as well as the patient’s mineralization response to denosumab. This study provides the first comprehensive material-based analysis of bone formed during denosumab treatment in giant cell tumors of bone, offering new insight into its impact on bone quality. These findings contribute to a deeper understanding of denosumab’s effects on bone, informing future patient management and surgical decision-making.