A 52-year-old woman presents with a 6-month history of symmetrical polyarthritis affecting her hands, wrists, and knees. Laboratory investigations reveal elevated ESR (68 mm/hr), CRP 12 mg/dL, and positive rheumatoid factor (1:640) with anti-CCP antibodies. X-rays show early erosive changes. What is the drug of choice for initiating disease-modifying antirheumatic therapy (DMARD) in this patient?