In Duchenne muscular dystrophy (DMD) progressive weakness of respiratory muscles leads to a restrictive pulmonary syndrome that contributes to early morbidity and mortality. Currently no curative treatment exists for DMD. In a Phase II randomized placebo-controlled study (DELPHI) in 21 DMD boys at age 8–16 years, idebenone (450 mg/d) showed trends of efficacy for cardiac and respiratory endpoints. Since the DELPHI study population comprised both glucocorticoid-naive subjects and glucocorticoid-users, we now report a post-hoc analysis investigating the effects of glucocorticoids and idebenone on markers of respiratory weakness, particularly peak expiratory flow (PEF) percent predicted (PEF%p). Baseline values of PEF%p correlated well with the percent predicted values for maximal inspiratory mouth pressure (MIP%p), forced vital capacity (FVC%p), and forced expired volume in 1 sec (FEV1%p). Baseline PEF%p and FVC%p were significantly higher in patients on concomitant glucocorticoids compared to glucocorticoid-naive patients. In the latter subgroup, idebenone caused a 8.0 ± 12.1% improvement in PEF%p, whilst patients on placebo declined by −12.3 ± 17.9% (P
