A 55-year-old woman presents with symmetric pain and swelling of the metacarpophalangeal and proximal interphalangeal joints for 4 months, morning stiffness lasting 90 minutes, and fatigue. Rheumatoid factor and anti-CCP antibodies are positive, and radiographs show early erosions. According to current rheumatoid arthritis management guidelines, what is the most appropriate initial disease-modifying therapy?