A 28-year-old primigravida at 24 weeks of gestation is diagnosed with gestational diabetes mellitus on oral glucose tolerance test. She has no prior history of diabetes and is otherwise healthy. Diet and lifestyle modifications are initiated, but after 2 weeks of close monitoring, fasting blood glucose remains 135 mg/dL and postprandial glucose is 165 mg/dL. What is the drug of choice for pharmacological management in this patient?