A 3RD-GENERATION BISPHOSPHONATE, YM175, INHIBITS OSTEOCLAST FORMATIONIN MURINE COCULTURES BY INHIBITING PROLIFERATION OF PRECURSOR CELLS VIA SUPPORTING CELL-DEPENDENT MECHANISMS

The theory that bisphosphonates inhibit osteoclast formation through t heir effects on osteoblastic cells remains controversial. To confirm t he inhibitory effect of bisphosphonates on osteoclast formation and ga in some insights into the underlying mechanisms, we examined the effec t of disodium dihydrogen (cycloheptylamino)-methylene-bisphosphonate m onohydrate (YM175) on osteoclast-like multinucleated cell (OCL) format ion in various mouse coculture systems. When different origins of oste oclast precursors (bone marrow, spleen, or nonspecific esterase-positi ve cells) were cocultured with the same supporting cells (calvarial os teoblasts), YM175 inhibited OCL formation similarly in all cultures. W hen the same osteoclast precursors (spleen cells) were cocultured with supporting cells of different origin, the results were variable. YM17 5 inhibited OCL formation almost completely in cocultures with calvari al osteoblasts or osteoblastic cell line KS4, while it did not, or onl y slightly, inhibit OCL formation in cocultures with stromal cell line s, Sn or MC3T3-G2/PA6. Temporal addition of YM175 in cocultures of spl een cells with osteoblastic cells revealed that YM175 was effective wh en it was present at an early phase of the culture period. Consistent with this observation, YM175 in the presence of osteoblastic cells inh ibited proliferation of preosteoclastic cells, but did not inhibit the fusion of mononuclear prefusion osteoclasts. In conclusion, the inhib itory effect of YM175 on OCL formation was confirmed in various murine coculture systems, but the effect was dependent on the types of bone- derived cells supporting osteoclastogenesis. The findings suggest that YM175 inhibits osteoclastogenesis by inhibiting the proliferation of osteoclast precursors through its action on supporting cells of osteob last lineage rather than acting directly on osteoclast precursors.