Osteoclast (OCL) precursors from patients with Paget's disease (PD) and normal OCL precursors transduced with the measles virus nucleocapsid protein gene (MVNP) are hyper-responsive to 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃) and can form OCL at physiologic concentrations of 1,25-(OH)₂D₃. This hyper-responsivity to 1,25-(OH)₂D₃ is due to increased expression of TAF_II-17, a potential coactivator of the vitamin D receptor (VDR). Hyper-responsivity to 1,25-(OH)₂D₃ may permit OCL formation in PD patients with low levels of 1,25-(OH)₂D₃ and play a role in the pathogenesis of PD. Therefore, we tested the effects of a vitamin D antagonist, (23S)-25-dehydro-1-hydroxyvitamin D₃-26,23- lactone (TEI-9647), to determine its potential to inhibit the enhanced OCL formation and bone resorption seen in patients with PD. TEI-9647, by itself, was not a VDR agonist and did not induce OCL formation in vitro, even at 10^-6M. However, it dose-dependently (10^-10M to 10^-6M) inhibited osteoclast formation induced by concentrations of 1,25-(OH)₂D₃ (41pg/ml, 10^-10M) detected in PD patients, by bone marrow cells of patients with PD and MVNP transduced CFU-GM cells, which form pagetic-like OCL. Moreover, bone resorption by OCL derived from MVNP transduced CFU-GM treated with 10^-9 M 1,25-(OH)₂D₃ was dose-dependently inhibited by TEI-9647 (10^-9 M to 10^-6 M). Furthermore, 10^-7M TEI-9647 by itself did not cause 1,25-(OH)₂D₃ dependent gene expression, but almost completely suppressed expression of the TAF_II-17 and 25-hydroxyvitamin D₃-24-hydroxylase (25-OH-D₃-24-hydroxylase) genes induced by 1,25-(OH)₂D₃ treatment of MVNP transduced CFU-GM cells. These results demonstrate that TEI-9647 can suppress the excessive bone resorption and OCL formation seen in marrow cultures from patients with PD.