Osteoclast (OCL) precursors from patients with Paget’s disease (PD) and normal OCL precursors transduced with the measles virus nucleocapsid protein gene (MVNP) are hyperresponsive to 1α,25-dihydroxyvitamin D₃ [1α,25-(OH)₂D₃] and can form OCLs at physiologic concentrations of 1α,25-(OH)₂D₃. This hyperresponsivity to 1α,25-(OH)₂D₃ is due to increased expression of TATA box-associated factor II-17, a potential coactivator of the vitamin D receptor. Hyperresponsivity to 1α,25-(OH)₂D₃ may permit OCL formation in PD patients with low levels of 1α,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-1α-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 vitamin D receptor 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 1α,25-(OH)₂D₃ (41 pg/ml, 10⁻¹⁰m) detected in PD patients by bone marrow cells of patients with PD and MVNP-transduced colony-forming unit-granulocyte macrophage (CFU-GM) cells, which form pagetic-like OCL. Moreover, bone resorption by OCLs derived from MVNP-transduced CFU-GM treated with 10⁻⁹m 1α,25-(OH)₂D₃ was dose-dependently inhibited by TEI-9647 (10⁻⁹m to 10⁻⁶m). Furthermore, 10⁻⁷m TEI-9647 by itself did not cause 1α,25-(OH)₂D₃-dependent gene expression but almost completely suppressed expression of the TATA box-associated factor II-17 and 25-hydroxyvitamin D₃-24-hydroxylase genes induced by 1α,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.