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Insulin-like growth factor 1 (IGF-1) plays an important role in both bone metabolism and breast cancer. In this study, we investigated the effects of the novel IGF-1 receptor tyrosine kinase inhibitor cis-3-[3-(4-methyl-piperazin-l-yl)-cyclobutyl]-1-(2-phenyl-quinolin-7-yl)-imidazo[1,5-a]pyrazin-8-ylamine (PQIP) on osteolytic bone disease associated with breast cancer. Human MDA-MB-231 and mouse 4T1 breast cancer cells enhanced osteoclast formation in RANKL and M-CSF stimulated bone marrow cultures, and these effects were significantly inhibited by PQIP. Functional studies in osteoclasts showed that PQIP inhibited both IGF-1 and conditioned medium induced osteoclast formation by preventing PI-3K/Akt activation without interfering with RANKL or M-CSF signalling. Treatment of osteoblasts with PQIP significantly inhibited the increase in RANKL/OPG ratio by IGF-1 and conditioned medium and totally prevented conditioned medium induced osteoclast formation in osteoblast - BM cell co-cultures, thereby suggesting an inhibitory effect on osteoblast - osteoclast coupling. PQIP also inhibited IGF-1 induced osteoblast differentiation, spreading, migration and bone nodule formation. Treatment with PQIP significantly reduced MDA-MB-231 conditioned medium induced osteolytic bone loss in a mouse calvarial organ culture system ex vivo and in adult mice in vivo. Moreover, once daily oral administration of PQIP significantly decreased trabecular bone loss and reduced the size of osteolytic bone lesions following 4T1 intratibial injection in mice. Quantitative histomorphometry showed a significant reduction in bone resorption and formation indices, indicative of a reduced rate of cancer associated bone turnover. We conclude that inhibition of IGF-1 receptor tyrosine kinase activity by PQIP suppresses breast cancer induced bone turnover and osteolysis. Therefore, PQIP, and its novel derivatives which are currently in advanced clinical development for the treatment of a number of solid tumours, may be of value in the treatment of osteolytic bone disease associated with breast cancer.
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