Different effects of tumor necrosis factor monoclonal antibody and receptor fusion protein on bone metabolism

Tumor Necrosis Factor-alpha (TNF-α) plays a crucial role in bone metabolism. It exists in two forms: soluble TNF-α (sTNF-α) and membrane-bound TNF-α (tmTNF-α). While sTNF-α primarily acts on the TNFRI signaling pathway, tmTNF-α can affect both TNFRI and TNFRII pathways. TNF inhibitors are divided into two categories: TNF-α receptor fusion proteins (such as etanercept) and TNF-α monoclonal antibodies (such as adalimumab). These inhibitors differ in their blocking effects on tmTNF-α and sTNF-α. Etanercept mainly blocks sTNF-α, while adalimumab blocks both tmTNF-α and sTNF-α. Since different TNFRs influence bone metabolism in unique ways, it is hypothesized that different TNF-α inhibitors may have distinct effects on bone metabolism. Therefore, this study aims to investigate the differential effects of etanercept and adalimumab on fracture healing to explore their diverse impacts on bone metabolism.

A rat femoral segmental defect model was established, with subjects divided into three treatment groups: saline control, etanercept, and adalimumab, each administered for 10 weeks. Fracture healing across groups was assessed via gross observation, X-ray imaging, Micro-CT scanning, and histological staining. The expression levels of bone metabolism-related genes, including RANKL, COLI, BMP2, DKK1, Runx2, and Wnt, were analyzed through RT-PCR to evaluate differences among the groups.

(1) Compared to the control group, the etanercept group exhibited notable bone callus resorption and a blurred fracture line. Micro-CT scanning showed a less defined fracture line, reduced callus, and initial replacement of cartilaginous callus with cortical bone in the etanercept group. In contrast, the adalimumab group showed no significant difference from the control, with cortical bone discontinuity and a visible gap at the fracture site. However, there were no statistically significant differences in bone mineral density, bone volume/total volume, trabecular thickness, or trabecular number among the three groups (all P > 0.05). (2) Histological staining indicated sparse and immature trabeculae, few osteocytes, and abundant chondrocytes and fibroblasts in both the control and adalimumab groups. The etanercept group, however, showed a higher quantity of robust trabeculae with numerous osteocytes. Immunohistochemical staining for osteocalcin revealed significantly higher expression in the etanercept group than in both the control and adalimumab groups, with statistically significant differences (P < 0.05 vs. control, P < 0.01 vs. adalimumab), while there was no significant difference between the adalimumab and control groups (P > 0.05). (3) RT-PCR results demonstrated significantly increased expression of COLI, BMP2, and DKK1 and decreased expression of RANKL in the etanercept group compared to the control group (all P < 0.05), with no significant changes in Runx2 and Wnt expression (all P > 0.05). In the adalimumab group, only RANKL expression was significantly decreased compared to the control (P < 0.05), with no other significant changes in COLI, RANKL, Runx2, DKK1, or Wnt expression (all P > 0.05).

Compared to TNF-α monoclonal antibodies, TNF-α receptor fusion proteins may promote fracture healing through the BMP2 and DKK1 signaling pathways, suggesting that TNF-α monoclonal antibodies and receptor fusion proteins have different impacts on bone metabolism, influencing new bone formation and osteoporosis differently.