Regimen-dependent glucocorticoid effects improve muscle performance without altering CNS physiology in mdx mice.

Duchenne muscular dystrophy (DMD) is a multisystem disorder affecting striated muscle, metabolism and the central nervous system (CNS). Although glucocorticoids remain the standard therapy, muscle-centric evaluations typically fail to capture how dosing regimen and compound selection affect CNS and metabolic phenotypes. Here, we compared daily and weekly dosing of prednisolone and vamorolone in juvenile mdx mice over 6 weeks to determine how these variables influence multisystem outcomes. Multiorgan efficacy and adverse effects were quantified across behavioural, endocrine, metabolic, cardiovascular and muscle domains using behavioural assays, in vivo and functional muscle testing, haemodynamic evaluation and histopathology. Daily glucocorticoid dosing failed to improve muscle function or strength, whereas weekly vamorolone produced the most robust improvements in functional and in vivo muscle strength. Daily prednisolone reduced circulating creatine kinase levels, but this biochemical change did not translate into enhanced muscle function outcomes. Daily regimens also induced severe adrenal cortical atrophy, yet these endocrine alterations were dissociated from CNS stress and anxiety responses, which remained unchanged by treatment. In addition, daily dosing caused pronounced systemic metabolic consequences, whereas weekly regimens substantially attenuated these effects, identifying dosing frequency as a key determinant of safety. Together, these findings demonstrate that glucocorticoid regimen selection fundamentally reshapes the efficacy versus adverse effect profile and underscores the value of integrated multiorgan evaluation in DMD. This work highlights the need to expand therapeutic assessments beyond muscle pathology and raises new questions about how glucocorticoid signalling differentially engages peripheral and central physiological systems. KEY POINTS: Duchenne muscular dystrophy affects multiple physiological systems, yet current glucocorticoid assessments rarely capture the CNS and metabolic effects of dosing regimens. Weekly vamorolone dosing improved muscle strength in mdx mice, while daily prednisolone failed to improve performance despite lowering plasma creatine kinase. Stress- and anxiety-related behaviours remained unchanged, despite adrenal atrophy revealing a disconnect between peripheral endocrine disruption and CNS phenotypes. Daily glucocorticoid regimens triggered endocrine and metabolic side effects, whereas weekly dosing reduced these effects. Prednisolone and vamorolone produced broadly similar systemic side-effect profiles, but dosing frequency was the primary determinant of their severity.