A 48-year-old Indian man with a 10-year history of type 2 diabetes presents with velvety, hyperpigmented plaques over the neck and axillae. He is on metformin and glibenclamide. Recent investigations: fasting glucose 156 mg/dL, HbA1c 8.1%, fasting insulin 32 mIU/L. On skin examination, the plaques have exaggerated skin markings and are non-pruritic. There are no oral mucosal lesions. Abdominal imaging is unremarkable. Which of the following interventions is most likely to improve his acanthosis nigricans?