An in-vivo model for the rapid assessment of skeletal effects of anabolic agents

We recently developed an in vivo model which can be used to rapidly assess the local skeletal effects of anabolic agents. In this model, 160 g Sprague-Dawley (SD) rats were used. A stainless steel cannula was inserted into the marrow cavity of the proximal tibia through the anterior-medial cortex 6 mm distal to the knee joint. The outer opening of the cannula was covered by skin. Agents with known anabolic skeletal effects or vehicle were injected daily for 10 days into the marrow region by a small needle passing through the cannula. Rats were also injected subcutaneously with a fluorescent bone marker to label the newly formed bone. Injection sites were fixed, embedded, and sectioned for histomorphometric analysis of trabecular bone. PTH and PGE2 stimulated a large amount of new trabecular bone formation in regions proximal and distal to the injection site as measured by histomorphometry. Control groups showed minimal bone formation, limited to formation of a thin layer of bony shell immediately surrounding the cannula. The profound anabolic skeletal effects of PTH and PGE2 seen in this Local Injection Model are similar to those seen in systemic injection (i.e. subcutaneous injection in intact or castrated male and female rats) previously reported. This Local Injection Model combines numerous advantages of in vivo models (systemic injection) and in vitro models when assessing agents with anabolic skeletal activities. Compared to conventional in vivo systemic injection models, this model enables detection of anabolic skeletal effects using very small quantities (in μg) of test agents in a short treatment period (< 10 days). Compared to in vitro models, this model allows the drug to be tested under intact bone marrow microenvironment (presence of osteogenic cells and other supporting cells, local stimulatory and inhibitory factors) and under a normal systemic hormonal environment. The model also overcomes issues of bioavailability/metabolism for initial compound screening.