Mycophenolate Mofetil for Induction and as a Steroid-Sparing Agent in the Treatment of Idiopathic Inflammatory Myositis: An Open-Label Study

Background Idiopathic inflammatory myositis represents heterogeneous systemic autoimmune disorders characterized by progressive proximal muscle weakness and multisystem manifestations. Traditional corticosteroid therapy precipitates substantial adverse effects during prolonged administration. This prospective, open-label, comparative observational study evaluated clinical outcomes associated with mycophenolate mofetil combination therapy versus corticosteroid monotherapy, examining muscle strength improvements, inflammatory biomarker profiles, pulmonary function parameters, and comparative corticosteroid dose requirements. Methodology This prospective observational study enrolled 40 consecutive participants with a confirmed idiopathic inflammatory myositis diagnosis through non-probability convenience sampling. Group 1 (n=17) received prednisolone monotherapy (mean 17.9±6.71 mg/day); Group 2 (n=23) received mycophenolate mofetil combination therapy (mean 1.19±0.259 g/day) with low-dose corticosteroids. Sample size calculation employed pooled standard deviation σ=16.77 points and minimum clinically significant difference δ=5 points. Participants completed a 24-week observation with assessments at baseline, six, 12, and 24 weeks. The primary outcome measured was Manual Muscle Testing-8 score changes; secondary outcomes encompassed inflammatory biomarkers and pulmonary function parameters. Results Baseline Manual Muscle Testing-8 scores demonstrated significant between-group differences (79.7±6.86 versus 71.3±8.07, p=0.0017). Analysis of covariance adjusting for baseline disease severity demonstrated persistent differences (adjusted mean difference: 4.23 points, 95% CI: 0.97-7.49, p=0.0127). Within-group analysis revealed that mycophenolate mofetil-treated participants achieved mean improvements of 12.5±10.42 points, representing a 17.53±14.61% increase, compared to 3.0±2.84 points, representing a 3.77±3.56% increase in corticosteroid monotherapy (p=0.0004 for percentage comparison). Inflammatory biomarkers demonstrated comparable reductions: creatinine phosphokinase decreased 31.69±13.11% in Group 1 versus 32.85±14.34% in Group 2 (p=0.8022). Pulmonary function assessment in participants with interstitial lung disease (n=11) documented forced vital capacity improvements of 12.26±6.00% versus 11.86±5.58%. Corticosteroid dose reduction of 34.08±19.10% was observed in Group 1 (17.9 to 11.8 mg/day, p<0.0001). Safety surveillance documented no serious adverse events, treatment discontinuations, or deaths, with the composite metabolic adverse event rate significantly lower in mycophenolate combination therapy (8.70% versus 41.18%, p=0.0166). Conclusion This prospective observational study provides preliminary evidence that mycophenolate mofetil combination therapy was associated with statistically significant muscle strength improvements and inflammatory biomarker reductions while being administered with substantially lower cumulative corticosteroid doses compared to prednisolone monotherapy. However, non-randomized treatment allocation with significant baseline disease severity differences introduces confounding by indication, precluding definitive causal inference. These hypothesis-generating observations warrant confirmation through adequately powered, multicenter randomized controlled trials before mycophenolate mofetil can be definitively recommended in evidence-based therapeutic algorithms.