Objective: Between 65% and 75% of patients with metastatic breast cancer will have decreased 5-year survival and increased morbidity due to cancer relapse in bone. At this stage of disease treatment is palliative, but tumor-targeted compounds could add to the benefits of anti-resorptive agents, improving clinical outcome. Inhibitor-of-apoptosis proteins (IAPs) are overexpressed in many tumors and second mitochondria-derived activator of caspases (Smac) mimetics have been designed to antagonize IAPs. In this work we explored the use of AT-406. a Smac mimetic, to target the tumor compartment of bone metastases. Methods: Effect of AT-406 on cancer cells apoptosis. expression of IAPs and osteogenic potential was addressed in vitro using the RANK-positive MDA-MB-231 breast cancer cell line. Effect of AT-406 on osteoclastogenesis was determined by inducing the differentiation of the RAW 264.7 mouse monocytic cell line. Osteoclastogenesis was measured by TRAP staining and TRACP 5b quantification. Results: AT-406 increased apoptosis in MDA-MB-231 breast cancer cells in vitro, and activation of RANK-pathway improved this effect. RANKL stimuli induced a strong increase in C-IAP2. AT-406 increased osteoclast differentiation and activity, by up-regulating the osteogenic transcription factor Nfatcl, but also increased the apoptosis of mature osteoclasts in the absence of RANKL. Conclusions: Our results indicate that despite the anti-tumoral effect of AT-406. its use in the context of bone metastatic disease needs to be carefully monitored for the induction of increased bone resorption. We also hypothesize that the combination of AT-406 with anti-RANKL directed therapies could have a beneficial effect, especially in RANK-positive tumors.
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