SU6656 was developed as a selective Src kinase inhibitor, but was later found to affect other signalling pathways. We previously observed that this compound could stimulate osteoblast differentiation in vitro. Therefore, we hypothesised that SU6656 could increase bone mass by inhibiting bone resorption through Src inhibition and by stimulating bone formation. To evaluate the effects of this molecule on bone mass, 4-month-old female C57Bl/6J mice received intraperitoneal injections of either 25 mg/kg SU6656 or its vehicle (n=6 per group) every alternate days for 12 weeks. Bone phenotypes were assessed by dexa, microCT, histomorphometry and ELISA assay. Data were analysed by unpaired t test. SU6656 treatment did not affect body weight compared with vehicle treatment. In comparison with vehicle-treated mice, SU6656-treated mice exhibited higher bone mineral density (+4.6%, p=0.013), tibial cortical thickness (+8.5%, p=0.002), tibial cancellous bone volume (+40.9%, p=0.002) and trabecular thickness (+17.6%, p=0.006). SU6656 did not significantly affect serum CTX levels, suggesting that this inhibitor increased bone mass by another mechanism than inhibition of bone resorption. Indeed, SU6656 significantly stimulated mineralising surfaces (1.5-fold, p=0.04 versus vehicle), mineral apposition rate (1.5-fold, p=0.012 versus vehicle) and bone formation rate (2.2-fold, p=0.013 versus vehicle). SU6656 stimulated alkaline phosphatase activity and mineralisation, and potentiated BMP2-induced osteoblast differentiation in vitro, while other selective Src inhibitors (PP2 and CGP77675) did not, indicating that SU6656 did not exert its effects through Src inhibition. SU6656 was shown to display a higher selectivity for Yes (another Src family kinase) than for Src. Interestingly, Dovitinib, a Yes inhibitor, also enhanced osteoblast differentiation in vitro. In conclusion, our findings indicate that SU6656 increases bone mass in mice by stimulating osteoblast differentiation and thus bone formation, possibly through Yes inhibition.
Disclosure: The authors declared no competing interests. This work was supported by the Swiss National Science Foundation.