Genetic Sost Deletion and Pharmacological Inhibition of Sclerostin Prevent Multiple Myeloma-Induced Bone Loss without Affecting Tumor Growth

In Multiple myeloma (MM) plasma cells grow in the bone marrow and induce localized lytic lesions due to increased bone resorption and suppressed bone formation. High levels of Sclerostin, a potent inhibitor of bone formation, are found in sera of MM patients. The expression of Sost/Scleorstin by osteocytes is also increased in mice bearing MM tumors, suggesting that Sclerostin might play a role in MM-induced bone disease. The goal of this study was to examine the impact of Sost/Sclersotin on tumor growth and MM-induced bone disease.

We first examined the effect of genetic deletion of Sost. Therefore, we generated global Sost KO mice in an immunodeficient background (SCID), which exhibited the expected high bone mass phenotype associated with Sost deficiency. 6-wk-old SostKO or WT control littermates mice were injected intratibially with 10⁵ JJN3 human MM (hMM) cells or saline (n=7-10/group), and sacrificed after 4wks. Sost KO and WT mice injected with hMM had equivalent tumor engraftment as demonstrated by serum human kappa light chain levels. hMM-injected WT mice exhibited ~50% decrease in tibia cancellous bone volume (BV/TV) and trabecular number (Tb.N), and increased trabecular separation (Tb.Sp). In contrast, hMM-injected Sost KO mice displayed no changes in BV/TV or bone architecture. Importantly, X-ray analysis revealed that the number and area of osteolytic lesions was reduced in Sost KO by 60% and 74%, respectively, compared to WT mice.

We next examined the effect of pharmacological inhibition of Sclerostin in an immunocompetent preclinical model of established MM. 6-wk-old C57BLKalwRij mice were injected intratibially with 10⁵ 5TGM1 murine MM cells (mMM) or saline. After 4wks mMM-injected mice had a 2-fold increase in the serum tumor engraftment marker IgG2b. Saline or mMM-injected mice were then treated with a Sclerostin neutralizing antibody (Scl-Ab; 15mg/kg/wk) or control antibody (IgG; n=6-10/group). After 4wks of treatment, serum IgG2b levels were similar in mMM-injected mice receiving Scl-Ab or IgG. mMM-injected mice receiving IgG injections had ~35% decreased BV/TV, Tb.N, and increased Tb.Sp. In contrast, mMM-injected mice receiving Scl-Ab displayed increased trabecular BV/TV (52%), Tb.N (22%), Tb.Th (33%) and decreased Tb.Sp (14%), results that did not differ from saline-injected mice treated with Scl-Ab. Moreover, the number of osteolytic lesions was reduced by 46% in Scl-Ab treated mice when compared to the IgG-treated group. Further, mMM-injected mice treated with IgG or Scl-Ab showed similar increases in the bone resorption markerCTX in the circulation, whereas mMM-injected mice treated with Scl-Ab had a smaller decrease in the bone formation marker P1NP in sera compared to IgG-treated mice (22 vs 45%).

Consistent with the lack of effect of the Scl-Ab on MM tumor growth in vivo, Scl-Ab did not affect the proliferation or viability of MM cells in vitro. We then examined next if Scl-Ab alters the anti-myeloma activity of dexamethasone (DEX), bortezomib (BOR) and the Notch inhibitor GSIXX. As expected, DEX, BOR and GSIXX increased the number of dead mMM and hMM cells in a time- and dose-dependent manner. Importantly, the increase in the number of dead hMM and mMM cells induced by DEX, BOR and GSIXX remained unchanged when Scl-Ab was co-administered.

Taken together, these results demonstrate that increased Scl production by osteocytes inhibits bone formation and contributes to MM-induced bone loss. Further, our data shows that pharmacological inhibition of Scl does not alter tumor growth or the activity of anti-myeloma drugs. These findings provide the rationale for combining Scl-Ab with anti-tumor drugs to simultaneously prevent tumor growth and the bone diseases in MM patients.