Anabolic and catabolic bone effects of human parathyroid hormone (1-34) are predicted by duration of hormone exposure.

Parathyroid hormone (PTH)(1-34), given once daily, increases bone mass in a variety of animal models and humans with osteoporosis. However, continuous PTH infusion has been shown to cause bone loss. To determine the pharmacokinetic profile of PTH(1-34) associated with anabolic and catabolic bone responses, PTH(1-34) pharmacokinetic and serum biochemical profiles were evaluated in young male rats using dosing regimens that resulted in either gain or loss of bone mass. Once-daily PTH(1-34) or 6 PTH(1-34) injections within 1 h, for a total daily dose of 80 microg/kg, induced equivalent increases in proximal tibia bone mass. In contrast, 6 PTH(1-34) injections/day over 6 h for a total dose of 80 microg/kg/day or 3 injections/day over 8 h for a total of 240 microg/kg/day decreased tibia bone mass. The PTH(1-34) pharmacokinetics of the different treatment regimens were distinctive. The magnitude of the maximum serum concentrations (Cmax) of PTH(1-34) and area under the curve (AUC) did not predict the catabolic bone outcome. Compared to the anabolic pharmacokinetic profile of a transient increase in PTH(1-34) with rapid decreases in serum calcium and phosphate, the catabolic regimen was associated with PTH(1-34) concentrations remaining above baseline values during the entire 6-h dosing period with a trend toward an increase in serum calcium and a prolonged decrease in phosphate. The pharmacokinetic profiles suggest that the anabolic or catabolic response of bone to PTH(1-34) is determined primarily by the length of time each day that serum concentrations of PTH(1-34) remain above baseline levels of endogenous PTH and only secondarily by the Cmax or AUC of PTH(1-34) achieved.

Novel and selective small molecule stimulators of osteoprotegerin expression inhibit bone resorption.

Osteoprotegerin (OPG), a secreted member of the tumor necrosis factor receptor superfamily, is a potent inhibitor of osteoclast formation and bone resorption. Because OPG functions physiologically as a locally generated (paracrine) factor, we used high-throughput screening to identify small molecules that enhance the activity of the promoter of the human OPG gene. We found three structurally unrelated compounds that selectively increased OPG gene transcription, OPG mRNA levels, and OPG protein production and release by osteoblastic cells. Structural analysis of one compound, a benzamide derivative, led to the identification of four related molecules, which are also OPG inducers. The most potent of these compounds, Cmpd 5 inhibited osteoclast formation and parathyroid hormone-induced calvarial bone resorption. In vivo, Cmpd 5 completely blocked resorptive activity (serum calcium, osteoclast number) in parathyroid hormone-treated rats. Furthermore, Cmpd 5 reduced the ability of a rat breast cancer to metastasize to bone. Finally, the compound also prevented bone loss in a rat adjuvant arthritis model. These results provide proof of the concept that low molecular weight compounds can enhance OPG production in ways that can result in effective therapies.