Osteoclasts are formed from the monocyte-macrophage lineage in response to receptor activator of nuclear factor kappaB ligand (RANKL) expressed by osteoblasts. Bone is the most common site of breast cancer metastasis, and osteoclasts play roles in the metastasis. The taxane-derived compounds paclitaxel and docetaxel are used for the treatment of malignant diseases, including breast cancer. Here we explored the effects of docetaxel on osteoclastic bone resorption in mouse culture systems. Osteoclasts were formed within 6 days in cocultures of osteoblasts and bone marrow cells treated with 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2). Docetaxel at 10(-8) M inhibited osteoclast formation in the coculture when added for the entire culture period or for the first 3 days. Docetaxel, even at 10(-6) M added for the final 3 days, failed to inhibit osteoclast formation. Osteoprotegerin, a decoy receptor of RANKL, completely inhibited osteoclast formation when added for the final 3 days. Docetaxel at 10(-8) M inhibited the proliferation of osteoblasts and bone marrow cells. RANKL mRNA expression induced by 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2) in osteoblasts was not affected by docetaxel even at 10(-6) M. Docetaxel at 10(-6) M, but not at 10(-8) M, inhibited pit-forming activity of osteoclasts cultured on dentine. Actin ring formation and L: -glutamate secretion by osteoclasts were also inhibited by docetaxel at 10(-6) M. Thus, docetaxel inhibits bone resorption in two different manners: inhibition of osteoclast formation at 10(-8) M and of osteoclast function at 10(-6) M. These results suggest that taxanes have beneficial effects in the treatment of bone metastatic cancers.