In this study, we used a wide variety of behavioral, functional, histological, biochemical, imaging, and molecular assays to comprehensively assess the effects of GCs administered for 50, 100, or 180 days to the mdx mice. We found that treatment with GCs resulted in weight loss and an initial, partial improvement in grip strength but a subsequent progressive loss of strength, catabolic effects, and deterioration in functional, histological, and biochemical measures in dystrophin-deficient skeletal and cardiac muscle. Mice in each of the nine drug trials were sacrificed at different ages, and the skeletal muscles were collected for hematoxylin and eosin staining. The muscles were stored in formalin for H&E staining. Serum was also acquired from these mice for use in estimating creatine kinase levels. For H&E staining, the gastrocnemius muscle was stained as previously described. For quantification, five non-overlapping representative fields of the stained tissue sections were imaged under a light microscope, and a digital image for each field was obtained using computer software. The digital images were loaded into Image J with an additional plug-in to count cells. In brief, the total fibers present, total fibers with central nuclei, regenerating fibers, Capromorelin tartrate degenerating fibers, and inflammation were assessed. In this study, we have taken a comprehensive approach to evaluating the effects of chronic administration of GC on the disease phenotype in mdx mice. Our data indicate that continuous administration of GCs significantly improves the disease phenotype early in the disease but that these beneficial effects are eventually lost with continued treatment. In addition, our data indicate that prolonged GC administration significantly decreases heart function and increases heart fibrosis, indicating that prolonged GC treatment is detrimental to dystrophic heart and skeletal muscle and further suggesting that these drugs may not be appropriate positive therapeutic controls for long-term preclinical drug testing in this mouse model. Since body weight is a Sibutramine HCl simple measure of the overall drug effect on the mouse phenotype, we measured body weight and found a significant decrease in the body weight of mdx mice. GCs are known to induce catabolic effects in skeletal muscle by activating the ubiquitin-proteosome pathway. It has previously been demonstrated that muscle-specific E3 ligases are up-regulated in response to GC administration in skeletal muscle. A decrease in body weight and reduction in weight gain with GC treatment are well-documented in the literature: For example, Keeling et al. performed an 84-week trial of twice-weekly oral prednisolone and followed the survival of the treated mdx mice through 104 weeks of age. They found that the treated mice survived longer and had a slower decline in grip strength per gram of body weight than did the untreated mdx mice.