Bortezomib Protects Osteoblasts from Glucocorticoid-Induced Damage, and Enhances Glucocorticoid-Induced Toxicity Against Osteoclasts and Myeloma Cells.

Introduction: Multiple myeloma is characterized by the accumulation of malignant plasma cells in the bone marrow, and leads most often to bone destruction by osteoclasts and prevention of bone repair by osteoblasts. Bortezomib and glucocorticoids are both powerful anti-myeloma drugs that are used for killing malignant plasma cells in the patients. Furthermore bortezomib has direct anti-osteoclastic and pro-osteoblastic properties that may contribute to bone protection in multiple myeloma, while glucocorticoids have more ambiguous effects on these bone cells and are clearly anti-osteoblastic. Recent clinical trials based on the combination of bortezomib and glucocorticoids drew the attention on the very promising anti-myeloma efficiency of this combination. However, the bone cell response of this combination has not been tested. In order to address this question, we performed an in vitro study, and importantly adapted our in vitro model to mimic the pharmacokinetics of bortezomib and glucocorticoid in the patients.

Methods: Myeloma cell lines, primary human osteoclasts and osteoblast-like cells (MC3T3) were pulse-treated or not with clinically relevant doses of bortezomib (12.5, 25 or 50 nM) for 3 hours. Subsequently, the cells were exposed during a 3-day culture to 1.6 μM prednisolone which approximately corresponds to a dose of 50 mg prednisolone in a patient. The impact of the treatment on the cells was determined by survival, activity and gene expression.

Results: Bortezomib as a single treatment was very efficient in killing sensitive myeloma cells (OPM2) whereas the more resistant cells (U266) were more efficiently killed in combination with prednisolone. The release of TRAP from primary human osteoclasts, a marker of osteoclastic activation, was strongly inhibited by bortezomib treatment alone, but only in combination with prednisolone did it result in killing of osteoclasts. Survival of osteoblast like cells was uninfluenced by treatment with bortezomib alone. In contrast, as shown previously, prednisolone strongly reduced osteoblast survival. Most importantly however, a 3 hr pre-treatment with bortezomib protected the osteoblasts against the detrimental effects of glucocorticoids. Ongoing investigations by Q-PCR indicate that important markers of osteoblast maturation remain high if the osteoblasts were pre-treated with bortezomib prior to prednisolone exposure.

Conclusion: Our study demonstrates in conditions relevant to treatment of myeloma patients, that combining bortezomib and glucocorticoids has a direct synergistic effect against myeloma cells and osteoclasts, and that bortezomib protects directly osteoblasts from the negative impact of glucocorticoids. Thus, the combination of bortezomib and glucocorticoids is not only a powerful treatment of multiple myeloma itself, but also shows promise for treating myeloma bone disease.