Novel vitamin D analogs, 1 alpha, 25-dihydroxyvitamin D-3-26,23-lactam (DLAMs) with a lactam moiety in the side chain, were synthesized and examined for their function in bone. In computer docking simulation, DLAM-1P binds to vitamin D receptor (VDR), and its lactam moiety may interfere with VDR helix-12 folding, In co-cultures of mouse bone marrow cells and osteoblasts, (23S,25S)-DLAM-1P dose-dependently suppressed osteoclast differentiation induced by 1 alpha, 25-dihydroxyvitamin D-3 [1 alpha, 25(OH)(2)D-3]. Its stereoisomer (23R,25R)-DLAM-1P did not affect the osteoclast differentiation. In osteoblasts, (23S,25S)-DLAM-1P suppressed 1 alpha, 25(OH)(2)D-3-induced mRNA expression of the receptor activator of NF-kappa B ligand (RANKL). In an organ culture using mouse calvaria, bone-resorbing activity induced by lot, 25(OH)(2)D-3 was clearly suppressed by (23S,25S)-DLAM-1P. The other analog, (23S,25S)-DLAM-2P, showed a similar activity to (23S,25S)-DLAM-1P. Therefore, DLAMs act on osteoblasts as an antagonist of lot, 25(OH)(2)D-3 to suppress RANKL-dependent osteoclast formation, suggesting them as a novel candidate for the treatment of pathological bone loss. (C) 2008 Published by Elsevier Inc.