New 19-nor-(20S)-1α,25-dihydroxyvitamin D3 analogs strongly stimulate osteoclast formation both in vivo and in vitro

2-Methylene-19-nor-(20S)-1α,25-dihydroxyvitamin D3 (2MD), an analog of 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3], has been shown to strongly induce bone formation both in vitro and in vivo. We have synthesized four substituents at carbon 2 of 2MD (2MD analogs), four stereoisomers at carbon 20 of the respective 2MD analogs (2MD analog-C20 isomers) and four 2MD analogs with an oxygen atom at carbon 22 (2MD-22-oxa analogs) and examined their ability to stimulate osteoclastogenesis and induce hypercalcemia. 2MD analogs were 100 times as potent as 1α,25(OH)2D3 in stimulating the formation of osteoclasts in vitro and in inducing the expression of receptor activator of NF-κB ligand (RANKL) and 25-hydroxyvitamin D3-24 hydroxylase mRNAs in osteoblasts. The osteoclast-inducing activities of 2MD analog-C20 isomers and 2MD 22-oxa analogs were much weaker than those of 2MD analogs. In addition, the activity of a 2MD analog in inducing dentine resorption was much stronger than that of 1α,25(OH)2D3 in the pit formation assay. Affinities to the vitamin D receptor and transcriptional activities of these compounds did not always correlate with their osteoclastogenic activities. Osteoprotegerin-deficient (OPG−/−) mice provide a suitable model for investigating in vivo effects of 2MD analogs because they exhibit extremely high concentrations of serum RANKL. The same amounts of 2MD analogs and 1α,25(OH)2D3 were administered daily to OPG−/− mice for 2 days. The elevation in serum concentrations of RANKL and calcium was much greater in 2MD analog-treated OPG−/− mice than in 1α,25(OH)2D3-treated ones. A 2MD analog was much more potent than 1α,25(OH)2D3 in causing hypercalcemia and in increasing soluble RANKL with enhanced osteoclastogenesis even in wild-type mice. In contrast, the administration of the 2MD analog to c-fos-deficient mice failed to induce osteoclastogenesis and hypercalcemia. These results suggest that new substituents at carbon 2 of 2MD strongly stimulate osteoclast formation in vitro and in vivo, and that osteoclastic bone resorption is indispensable for their hypercalcemic action of 2MD analogs in vivo.