SAT-530 Establishment of a Glucocorticoid (GC) Regimen to Induce Both Growth Retardation and Osteoporosis in C57BL/10 Muscular Dystrophy Mouse Models

Glucocorticoids (GCs) are associated with many adverse effects including osteoporosis and growth retardation. These are particular problems in Duchenne muscular dystrophy (DMD) where GCs form the mainstay of treatment. The mechanisms that underlie the undesirable effects of GCs on skeletal development are unclear and there is no proven treatment. We have shown previously (Wood et al. J Endocrinol 2018) that there is no established animal model to investigate GC effects on skeletal development. In this study, we examined this in greater detail and furthermore to mimic the clinical situation in DMD, we also aimed to find a suitable GC regimen to induce both growth retardation and osteoporosis in muscular dystrophy (mdx and mdx:cmah) mice and their C57BL/10 controls (WT). 6-8 male mice in each group were given a GC regimen for 28 days from 4 weeks of age. 5 regimens were trialled; 1) dexamethasone (dex) 2.5mg/kg and 2) dex 5mg/kg via SC injection, 3) soluble prednisolone (pred) 5mg via SC injection, 4) pred 10mg/kg and 5) 20mg/kg in cherry suspension via oral gavage. Regimens 2) and 5) were repeated in mdx and mdx:cmah mice. Control (C) mice received the appropriate vehicle. Bone mass and architecture were assessed by micro-CT and anthropometric measurements were used to assess growth. ELISA was used to analyse serum bone turnover markers and breaking strength was determined by 3-point bending. Regimens 1, 3 and 4 had little effect on any parameter measured. Results shown are for WT mice however the same trends were also seen in mdx and mdx:cmah. In the Pred 20 mice, body weight gain (GC: 12.9% v C: 59.2%, p<0.001), change in crown rump length/day (GC: 0.02cm v C:0.03cm, p=0.05) and growth plate width (GC:122.1 µm v C: 167µm, p=0.07) were all lower compared to control values. Cortical bone fraction (GC; 57.2% v C:63.5%, p<0.001), cortical bone volume (GC:0.48mm(3) v C: 0.57 mm(3), p<0.01), mean polar moment of inertia (GC:0.12mm(4) v C: 0.14mm(4), p<0.01) and serum bone turnover markers P1NP (GC: 159.9 pg/ml v C: 367.6 pg/ml, p=0.02) and CTX (GC: 75.7 pg/ml v 180.4 pg/ml, p=0.03) were also lower in the Pred 20 mice. Trabecular bone parameters, bone density and breaking strength were unchanged. Interestingly, trabecular number (GC: 5.67mm(-1) v C: 5.51mm(-1), p<0.001), was higher and cortical bone fraction (GC:59.4% v C: 65.2%, p<0.001) was lower in dex 5 mice. C57BL/10 mice appear fairly resistant to GC challenge; despite high doses no biomechanical or trabecular architecture changes were noted. Prednisolone 20mg/kg given by oral gavage appears to be the most effective regimen to induce growth retardation and osteoporosis in C57BL10 WT mice and DMD models over a 28-day period.