Anti-Myeloma Response to Bortezomib Is Associated with Increased Osteoblast Activity and Bone Formation in Primary Myelomatous SCID-rab Mice.

Recent studies have indicated that certain proteasome inhibitors have bone anabolic activity in vivo. Our clinical study have shown a significant association between anti-myeloma (MM) response of bortezomib (Millennium Pharmaceuticals) and increased serum bone alkaline phosphatase in patients with MM (Zangari et al., BJH 2005, In press). The aim of this study was to investigate the effect of bortezomib on bone remodeling and the association between anti-myeloma response and increased bone formation in our established SCID-rab model for primary MM. In this system rabbit bones were implanted S.C. in unconditioned SCID mice. After 6 weeks, primary MM cells were injected directly into the implanted bone. MM cells from >85% of patients (n>70) were successfully engrafted, grew exclusively in the implanted bone and produced typical disease manifestations including increased osteoclast activity, reduced osteoblast numbers and induction of osteolytic bone disease (Yata & Yaccoby, Leukemia 2004). In the present study, SCID-rab mice were engrafted with MM cells from 5 patients. Following establishment of MM growth, as monitored by weekly measurement of human monoclonal immunoglobulins (hIg) in mice sera (587±276 μg/ml) and by x-rays, mice were injected subcutaneously twice a week with 0.5 mg/kg bortezomib or PBS for 4–8 weeks. Whereas all PBS-treated mice had increased hIg levels during the experimental period, bortezomib treatment resulted in marked reduction of hIg in 2 experiments by 95% and 73% from pretreatment levels, respectively, retardation of myeloma growth in an additional experiment and no response in 2 experiments. Overall, tumor burden in control PBS- and bortezomib-treated mice was increased by 298%±51 and 110%±88 from pretreatment levels, respectively (p<0.03). In control mice the implanted rabbit bone mineral density (BMD) and bone mineral content (BMC) were reduced by 23%±6% and 27%±10% from pretreatment levels, respectively, whereas in bortezomib-responsive mice BMD and BMC were increased by 57%±23% and 79%±60% from pretreatment levels, respectively (p<0.05, bortezomib vs. PBS). In the 2 non-responsive mice BMD and BMC were reduced by 36%±7% and 40%±3% from pretreatment levels, respectively, similar to the matched control mice. The bone anabolic effect of bortezomib could also be visualized on x-rays. Histological examination of a bone from a responsive myeloma revealed increased numbers of osteocalcin-expressing osteoblasts (25±4 vs. 8±3 per mm bone in control mice, p<0.03) and reduced number of multinucleated TRAP-expressing multinucleated osteoclasts (59±6 vs. 20±4 per mm bone in control mice, p<0.008). We conclude that anti-myeloma response to bortezomib is associated with increased osteoblast activity and bone formation in myelomatous bone. Since we previously demonstrated the ability of osteoblasts to inhibit growth of primary MM cells ex vivo, our study suggests an additional mechanism by which bortezomib inhibits myeloma.