Receptor activator of nuclear factor kappaB (RANK) is a membrane-bound tumor necrosis factor receptor homologue that mediates signals obligatory for osteoclastogenesis as well as osteoclast activation and survival in vivo. The present study was undertaken to evaluate the efficacy of a soluble murine RANK-human immunoglobulin fusion protein (muRANK.Fc) as a bone resorption inhibitor in vitro and in vivo. The in vitro studies demonstrated the ability of muRANK.Fc to inhibit human parathyroid hormone-related protein (PTHrP)-induced resorption in fetal rat long bone cultures. Short-term administration of muRANK.Fc to normal growing mice resulted in a complete disappearance of osteoclasts from metaphyses of long bones associated with a pronounced increase in calcified trabeculae and bone radiodensity. In a model of humoral hypercalcemia of malignancy in which PTHrP secreted by s.c. xenografts of human lung cancer in nude mice induces extensive osteolysis and severe hypercalcemia, daily administration of muRANK.Fc from time of tumor implantation profoundly inhibited osteoclastic bone resorption and prevented hypercalcemia. muRANK.Fc had no effect on tumor production of PTHrP, because there was no significant difference between circulating human PTHrP levels in muRANK.Fc-treated and vehicle-treated tumor-bearing mice. Moreover, even when treatment was initiated after hypercalcemia was established, muRANK.Fc significantly attenuated further increases in blood ionized calcium. These data demonstrate the potent antiresorptive effects of muRANK.Fc in vivo as well as highlight the potential utility of disrupting RANK signaling as a novel therapeutic approach in humoral hypercalcemia of malignancy and possibly multiple myeloma and skeletal metastases associated with osteolysis.