JOURNAL OF BONE AND MINERAL RESEARCH Volume 6, Number 11, zyxwvutsrqpo 1991 Mary Ann Liebert, Inc., Publishers zyxwvutsrqp Modulation of PTH-Stimulated Osteoclastic Resorption by Bisphosphonates in Fetal Mouse Bone Explants GABRI VAN DER PLUIJM, CLEMENS W.G.M. LOWIK, HENNY DE GROOT, MARCEL .J. ALBLAS, and SOCRATES E. PAPAPOULOS LIANNE J.A. VAN DER WEE-PALS, OLAV L.M. BIJVOET, ABSTRACT We examined the effects of the bisphosphonates CLMDP, APD, and Me2APDon osteoclastic resorption in the absence and presence of PTH using fetal mouse osteoclast-free bone explants cocultured with fetal liver as a source of osteoclast precursors. Results revealed qualitative and quantitative differencs among the bis- phosphonates tested. With CI,MDP and APD fractional inhibition of resorption (measured as 45Ca release) in the presence of PTH was proportional to that obtained in its absence. In contrast, Me2APD, which is the most potent inhibitor of the three, was found at low concentrations (15 zyxw x lo-' M) to enhance the PTH- stimulated osteoclastic resorption. APD as well, at concentrations that could not inhibit resorption, had a similar effect, but CI,MDP did not. These studies describe a new phenomenon, that low doses of nitrogen- containing bisphosphonates can act synergistically with PTH and enhance osteoclastic resorption. These findings may have clinical implications in the management of patients with increased osteoclastic resorption due to parathyroid overactivity. INTRODUCTION ISPHOSPHONATES zyxwvutsrqp ARE COMPOUNDS that are used for the B suppression of increased bone resorption in such con- ditions as Paget's disease of hypercalcemia of and metastatic bone disease.",8) There is no general agreement, however, about their exact mecha- nism of action on osteoclastic resorption. Their action on bone resorption was initially thought to be due to inhibition of crystal dissolution(P) or to direct in- terference with the metabolism of cells present in bone.(9-1s) Although bisphosphonates have a direct effect on bone cells, it has been impossible to demonstrate any correspondence between such cellular effects and the in- hibitory potency of bisphosphonates on resorption in vivo. (I6) Osteoclastic bone resorption is preceded by a cas- cade of events that starts with the activation of hematopoi- etic stem cells and is followed by proliferation, differentia- tion, chemotaxis, attachment, and subsequent terminal differentiation and fusion of the osteoclast precursors into functioning multinucleated osteoclasts. There are several possible ways in which bisphospho- nates might inhibit osteoclastic resorption. At relatively high concentrations mineral-bound bisphosphonates can inhibit osteoclastic resorption by interfering with the func- tion of mature resorbing osteoclasts, which have ingested bisphosphonate-covered calcified matrix."7-'9' This is es- pecially true for dichloromethylene bisphosphonate (CIzMDP).(L1~L7-'9) The nitrogen-containing bisphosphonates 3-amino- I-hy- droxy propylidene-1 , I-bisphosphonate (APD) and dimeth- ylamino-1-hydroxy propylidene-1 , I-bisphosphonate (Me,APD), which are more potent inhibitors of bone re- sorption than CI,MDP in vivo, exert their action not by toxicity to mature osteoclasts but by interfering with events preceding resorption.(z0~21) They have no direct effect on osteoclast precursors,(17) and they do not disturb the mi- gration of the precursors toward the bone.'"' Therefore it appears that other processes are affected. We previously demonstrated that matrix-bound nitro- gen-containing bisphosphonates specifically inhibit the ter- minal differentiation of tartrate-resistant acid phosphatase Department of Endocrinology, University Hospital, Leiden, The Netherlands. 1203