A 48-year-old man with established rheumatoid arthritis has been on methotrexate 20 mg/week for 8 months but shows inadequate response with persistent synovitis in multiple joints and elevated inflammatory markers (CRP 8 mg/dL, ESR 55 mm/hr). He has no contraindications to biologic therapy. What is the drug of choice to add to his methotrexate regimen?