gms | German Medical Science

Objectives: Osteolytic bone destruction is the most common clinical feature of multiple myeloma (MM), characterized by increased osteoclast activity and decreased osteoblast function. Elevated levels of Dickkopf-1 (DKK1), a major inhibitor of Wnt signaling have been shown to promote osteolytic lesions both in myeloma patients and murine models. Thus, regulating DKK1 function or expression may have therapeutic significance in the management of low bone mass disorders. Bortezomib, a proteasome inhibitor has been shown to reduce osteolytic bone resorption, increase bone formation, and help control plasma cell myeloma growth. However, limited studies have been carried out to demonstrate the bone stimulatory effects of bortezomib in non-myeloma bones. Thus, a local delivery approach of bortezomib at different concentrations in a healthy rat model was adopted to gain insight on the underlying mechanism by which it repairs bone damage.

Methods: Sixty-four female Sprague-Dawley rats were randomized into control and test groups (n=16/group) and a 2.5 mm drill hole was created in the metaphysis of their left femur. The defects were either filled with xerogel loaded with different concentrations of bortezomib; 100 µg (XG100b), 500 µg (XG500b) and 2500 µg (XG2500b) or left empty (control group). After 4 weeks, femora were harvested followed by histological, histomorphometric and immunohistochemical (BMP2; bone-morphogenic protein 2, OPG; osteoprotegerin, RANKL; receptor activator of nuclear factor kappa-B ligand, ED1; CD68 antibody and DKK1) analysis. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to assess the distribution of released bortezomib ions. P-value <0.050 was considered significant.

Results and Conclusion: A marked reduced expression of DKK1 was seen in the test group with the lowest concentration of bortezomib (XG100b) and was accompanied with a simultaneous increase in the expression levels of prominent bone formation markers like BMP2, ALP and OPG/RANKL ratio when compared to other groups. Histomorphometric analysis showed a statistically significant increase in bone formation with a simultaneous increase in osteoblast surface over bone surface in the XG100b group when compared to (p=0.000) other groups. Remarkably, the lowest osteoclast activity (Oc/BS; p=0.002) was seen with the highest concentration of bortezomib (2500 µg). ED1 positive cells were found to be highest in the XG2500b group followed by XG500b (p=0.050) and the least being in the XG100b and empty groups (p=0.009) respectively, this suggests a harmful effect of bortezomib at increased concentration. ToF-SIMS analysis also revealed traces of bortezomib only at the highest concentration.

Our data therefore demonstrate that at optimal dosage, bortezomib could lower DKK1 levels thereby annulling the negative feedback on the Wnt signaling pathway. This in turn leads to upregulation of prominent bone formation markers that subsequently lead to new bone formation in the defect area.