Abstract 364: Validation of human osteoclast cultures for studying the mode-of-action and identification of new compounds with the potential of inhibiting the vicious cycle in osteolytic bone metastases

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Abstract When tumor cells home to bone microenvironment they secrete factors that stimulate osteoclast formation, which results in increased bone resorption. This in turn increases the release of factors from bone matrix that stimulate the growth of tumor cells, leading to a vicious cycle characterized by extensive bone loss and enhanced tumor growth in bone. Thus, factors that inhibit osteoclast formation or bone resorption activity of mature osteoclasts may have the potential to inhibit the vicious cycle. The RANKL inhibitor denosumab is approved for treatment of bone metastases from solid tumors, and the cathepsin K inhibitor odanacatib has been shown to suppress bone resorption in breast cancer patients with bone metastases. Human osteoclasts can be generated from bone marrow-derived CD34+ mesenchymal stem cells in the presence of M-CSF and RANKL. In this study we report optimization of separate in vitro culture systems for determining osteoclast differentiation and activity, and validation of denosumab and odanacatib as reference inhibitors of osteoclast differentiation and activity, respectively. CD34+ human osteoclast precursor cells were cultured on bovine bone slices for 7 days. Different concentrations of denosumab (0.01 - 10 μg/ml) were added in the cultures at day 0, and tartrate-resistant acid phosphatase isoform 5b activity (TRACP 5b) was measured from the culture medium collected at day 7 as an index of the number of formed osteoclasts. Osteoclast activity was studied by allowing the formed mature osteoclasts to resorb bone during an additional 3-day culture period. The culture medium was changed and different concentrations of odanacatib (0.001 - 1.0 μM) were added into the cultures at day 7, and the amount of C-terminal cross-linked telopeptides of type I collagen (CTX-I) was measured in the culture medium collected at day 10 to quantitate bone resorption during days 7-10. Denosumab and odanacatib showed strong concentration dependent inhibition of osteoclast differentiation and activity, respectively, with EC50 values of 0.124 μg/ml for denosumab and 0.0433 μM for odanacatib. We conclude that we have validated denosumab as a reference compound of osteoclast differentiation and odanacatib as a reference compound of osteoclast activity in a human in vitro osteoclast culture system. The culture system is a clinically reliable tool for identifying new compounds affecting the vicious cycle of osteolytic bone metastases, and clarifying if these active compounds target directly osteoclast differentiation or activity. Citation Format: Jenni Bernoulli, Jussi M. Halleen, Mari I. Suominen, Johanna Tuomela, Jukka Rissanen, Katja M. Fagerlund. Validation of human osteoclast cultures for studying the mode-of-action and identification of new compounds with the potential of inhibiting the vicious cycle in osteolytic bone metastases. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 364.