Abstract Background This study investigated the effects of the receptor activator for nuclear factor jB ligand (RANKL) inhibitor, osteoprotegerin (OPG), on tumor-induced allodynia, osteolysis, and bone histol- ogy in the mammary tumor (MRMT-1) rat model for bone cancer pain. Methods Rats (n = 8/group) were inoculated with MRMT-1 or culture medium in the proximal right tibia, injected with OPG or vehicle subcutaneously 2–3 times weekly, evaluated for mechanical allodynia with von Frey paw stimulation, and euthanized on Day 20 for necropsy. Three groups were evaluated starting on Day 5 and received the following interventions begin- ning on Day 1: tumor and OPG, tumor and vehicle, or culture medium and vehicle. Three additional groups received the same interventions but were evaluated starting on Day 3. A seventh group started OPG on Day 8 after tumor inoculation. Results Starting OPG on Day 1 reduced allodynia significantly compared with vehicle injections; pain relief was observed within 5–6 days after tumor inoc- ulation and lasted throughout follow-up. Starting OPG on Day 8 did not reverse allodynia significantly com- pared with the tumor control group. Regardless of treatment start time, OPG treatment reduced osteo- clast number and tartrate-resistant acid phosphatase levels, increased bone mineral density, preserved nor- mal bone volume and integrity on micro-computed tomography, reduced relative tumor volume in the bone, and reduced staining for glial fibrillary acidic protein in the spinal cord. Conclusions RANKL inhibition with OPG reduced bone resorption and bone pain in rats with malignant bone disease; further study is warranted to determine if RANKL inhibition has similar benefits in humans. Keywords Bone neoplasms Æ Mammary neoplasms Æ Animal Æ Osteoprotegerin Æ Pain Æ RANK ligand Æ Rats Abbreviations ANOVA Analysis of variance BMD Bone mineral density DXA Dual X-ray absorptiometry GFAP Glial fibrillary acidic protein MicroCT Micro-computed tomography MRMT-1 Rat mammary tumor cell line OPG Osteoprotegerin RANK Receptor activator for nuclear factor jB RANKL Receptor activator for nuclear factor jB ligand sTRAP5b Tartrate-resistant acid phosphatase form 5b Disclosure Portions of this material were presented at the 96th Annual Meeting of the American Association for Cancer Re- search, Anaheim, California, April 16–20, 200?. M. P. Roudier Æ S. D. Bain SkeleTech, Inc., Bothell, WA, USA W. C. Dougall Department of Cancer Biology, Amgen Washington, Seattle, WA, USA M. P. Roudier (&) Department of Pathology, Amgen Washington, 1201 Amgen Court West, Seattle, WA 98119-3105, USA e-mail: mroudier@amgen.com Clin Exp Metastasis (2006) 23:167–175 DOI 10.1007/s10585-006-9026-x 123 ORIGINAL PAPER Effects of the RANKL inhibitor, osteoprotegerin, on the pain and histopathology of bone cancer in rats Martine P. Roudier Æ Steven D. Bain Æ William C. Dougall Received: 28 April 2006 / Accepted: 31 May 2006 / Published online: 16 August 2006 Ó Springer Science+Business Media B.V. 2006