Overview: animal models of osteopenia and osteoporosis

Prior to initiating a clinical trial in a post-menopausal osteoporosis study, it is reasonable to recommence the evaluation of treatment in the 9-month-old ovariectomized female rat. A female rat of this age has reached peak bone mass and can be manipulated to simulate clinical findings of post-menopausal osteoporosis. Ample time exists for experimental protocols that either prevent estrogen depletion osteopenia or restore bone loss after estrogen depletion. More time can be saved by acceleration of the development of the osteopenia by combining ovariectomized (OVX) plus immobilization (IM) models. Methods like serum biochemistry, histomorphometry and densitometry used in humans are applicable in rats. Like most animal models of osteopenia, the rat develops no fragility fractures, but mechanical testing of rat bones substitutes as a predictor of bone fragility. Recent studies have shown that the prevailing activity in cancellous and cortical bone of the sampling sites in rats is remodeling. The problems of dealing with a growing skeleton, the site specificity of the OVX and IM models, the lack of trabecular and Haversian remodeling and the slow developing cortical bone loss have been and can be overcome by adding beginning and pre-treatment controls and muscle mass measurements in all experimental designs, selecting cancellous bone sampling sites that are remodeling, concentrating the analysis of cortical bone loss to the peri-medullary bone and combining OVX and IM in a model to accelerate the development of both cancellous and cortical bone osteopenia. Not to be forgotten is the distal tibia site, an adult bone site with growth plate closure at 3 months and low trabecular bone turnover and architecture similar to human spongiosa. This site would be most challenging to the action of bone anabolic agents. Data about estrogen-deplete mice are encouraging, but the ovariectomized rat model suggests that developing an ovariectomized mouse model as an alternative is not urgent. Nevertheless, the mouse model has a place in drug development and skeletal research. In dealing with drug development, it could be a useful model because it is a much smaller animal requiring fewer drugs for screening. In skeletal research mice are useful in revealing genetic markers for peak bone mass and gene manipulations that affect bone mass, structure and strength. When the exciting mouse glucocorticoid-induced bone loss model of Weinstein and Manolagas is confirmed by others, it could be a significant breakthrough for that area of research. Lastly, we find that the information generated from skeletal studies of nonhuman primates has been most disappointing and recommend that these expensive skeletal studies be curtailed unless it is required by a regulatory agency for safety studies.