Selective glucocorticoid receptor modulation as a therapeutic option in dystrophic muscle

Abstract

Duchenne Muscular Dystrophy is a devastating neuromuscular condition defined by the absence of the integral sarcolemma-associated protein, dystrophin, and characterised by myofibre necrosis, muscle regeneration, inflammatory infiltration, and premature satellite cell senescence. The anti-inflammatory properties of glucocorticoids, mediated predominantly through monomeric glucocorticoid receptor inhibition of transcription factors such as NF-kappaB (transrepression), are postulated to be an important mechanism for their beneficial effects in Duchenne Muscular Dystrophy. The chronic use of supraphysiological doses of glucocorticoids in Duchenne Muscular Dystrophy is associated with well-recognised glucocorticoid-related side effects such as growth suppression, reduced bone mineral density and increased fracture rates, muscle atrophy, and metabolic disturbances such as increased adiposity, impaired glucose tolerance, and insulin resistance. It has been postulated that these detrimental effects are mediated through direct glucocorticoid homodimer interactions with glucocorticoid response elements (transactivation). Compound A is a non-steroidal selective glucocorticoid receptor modulator with the ability to bind the glucocorticoid receptor and induce transrepression, without promoting glucocorticoid receptor dimerization (Section 1.7.3). It has the theoretical potential to attenuate and modulate the inflammatory signature in dystrophic muscle without the associated adverse systemic effects. These properties were tested in the mdx mouse, a model of Duchenne Muscular Dystrophy. The in vitro cell studies performed in Chapter 3 confirm the anti- NF-kappaB activity of Compound A in H-2Kb-tsA58 mdx myoblasts and myotubes and demonstrate the absence of glucocorticoid receptor-mediated transactivation of genes that represent the basis for hypothalamic-pituitary-adrenal axis suppression. The in vivo studies involving the mdx mouse model in Chapters 4 to 7 provide information pertaining to the bioactivity, potential toxicity, as well as the extent and nature of positive benefits of Compound A treatment in dystrophic muscle. Intraperitoneal but not oral Compound A at doses up to 7.5mg/kg/day had significant positive effects on muscle strength and inflammation in mdx mice. When administered via the intraperitoneal route at 7.5mg/kg/day, Compound A caused detrimental effects on muscle mass and voluntary locomotor activity in WT mice. Intraperitoneal Compound A treatment in mdx mice from a postnatal age of 18 days increased the absolute and normalised forelimb and hindlimb grip strength, attenuated cathepsin-B activity in forelimb and hindlimb muscles, decreased serum creatine kinase levels, and reduced Interleukin-6, Chemokine (C-C motif) ligand 2, Interferon-gamma, Tumour Necrosis Factor, and Interleukin-12p70 cytokine levels in gastrocnemii muscles. Compared with Compound A, prednisolone treatment in mdx mice increased osteopontin (Spp1) gene expression and osteopontin protein levels, and had less favourable effects on expression of genes that determine muscle mass including Foxo1, Foxo3, Fbxo32, and Trim63, as well as hepatic Igf1. In conclusion, the results of these studies confirm the integral role of glucocorticoid receptor-mediated transrepression in improving pathological features in the mdx mouse model. Selective glucocorticoid receptor modulation by Compound A may represent a more effective therapeutic strategy in Duchenne Muscular Dystrophy, with advantages over glucocorticoids. However, this therapeutic potential may be limited by its associated toxicity profile. Show more