EFFECTS OF BISPHOSPHONATES APD AND HEBP ON BONE METABOLISM IN-VITRO

Although the effects of the bisphosphonates on resorption have been we ll documented, their effects on bone formation are not as clear, There fore, this investigation was undertaken to elucidate the role played b y bisphosphonates in the regulation of bone formation in vitro, To eva luate bisphosphonate-mediated regulation of bone formation in vitro, t he effects of two drugs, ethane-1-hydroxy,1-diphosphate (Etidronate) ( HEBP), and the second-generation bisphosphonate, disodium-1-hydroxy-1- aminopropylidine-1, phate (Pamidronate) (APD), were assessed in the ch ick periosteal osteogenesis (CPO) model, In this study, drug-induced c hanges in alkaline phosphatase were assessed at the cellular level by means of quantitative fluorescence histochemistry, Cellular proliferat ion was quantified by means of autoradiography ([H-3]thymidine). Miner alization and matrix production were measured morphometrically, wherea s collagen synthesis and degradation were measured biochemically. The data suggest that in addition to their effects on bone resorption, the bisphosphonates have marked and direct effects on bone formation and other parameters of osteogenesis, HEBP may affect cellular proliferati on (75-80% reduction, p < 0.05) in zones distant from bone; alkaline p hosphatase positive cell numbers were increased in the osteoblastic la yer of cells (twofold relative to control, p < 0.05) in 12-day culture s, HEBP, but not APD, prevented mineralization-induced suppression of matrix synthesis in early stages of culture, In 6-day cultures induced to mineralize with beta-glycerophosphate, (GP) cotreatment with HEBP induced a 70% increase in collagen synthesis, In addition, degradation of collagen in the CPO cultures was inhibited by HEBP (25%) and to a lesser extent by APD (8%), Although there were no differences in bone- osteoid areas measured in 12-day cultures treated with various regimen s of bisphosphonate or GP, a clear increase in bone-osteoid area was d etected in 6-day cultures treated with GP and HEBP as compared to GP o nly, This may suggest that initially, osteoblasts may be induced to sy nthesize increased volumes of bone matrix when mineralization is inhib ited (e,g,, with HEBP), but that over time the osteoblasts make the sa me amount of matrix, Taken together, these findings indicate that wher eas the bisphosphonates do have well-documented effects on bone resorp tion, their effects on bone formation may also be important, The data suggest further that these drugs may affect osteogenesis in vitro, in part through their effects on mineralization and perhaps on differenti ation or phenotypic expression in osteoblasts.