Intra-articular Methotrexate-Loaded Microsponge as an Adjuvant Strategy for Rheumatoid Arthritis: Localized Treatment with a Systemic Impact

Rheumatoid arthritis (RA) is a debilitating autoimmune disease characterized by chronic synovial inflammation and progressive joint destruction. Methotrexate (MTX) remains the gold standard in RA therapy, yet systemic administration often provides suboptimal joint targeting and causes dose-limiting toxicity. This study investigates intra-articular (IA) administration of an MTX-loaded Microsponge (MTX-MSP) as a localized strategy to enhance drug retention while minimizing systemic exposure. MTX-MSP is synthesized using hyaluronic acid-based cross-linking and MTX loading. Drug release and carrier mass loss were evaluated in a pathological human synovial fluid (HSF), and rheological and Fourier transform infrared spectroscopy analyses assess viscoelastic behavior and drug-carrier interactions. Biocompatibility and anti-inflammatory activity are tested in primary RA fibroblast-like synoviocytes, while therapeutic efficacy is evaluated in a collagen-induced arthritis rat model. MTX-MSP provides sustained release in HSF for 14 days, minimizes burst effects, and preserves structural integrity. Rheological profiling confirms injectability and interaction with the pathological synovial fluid, enhancing the elastic response. <i>In vitro</i>, MTX-MSP reduces IL-6, TNF-α, and IL-1β expression at gene and protein levels, outperforming free MTX. <i>In vivo</i>, weekly IA injections improve histological scores in treated and contralateral joints, suggesting systemic immunomodulation. MTX-MSP thus achieves prolonged release, anti-inflammatory efficacy, and reduced systemic toxicity, representing a promising IA formulation for RA management.