A 62-year-old man is admitted to the coronary care unit 48 hours after an acute myocardial infarction. He develops frequent ventricular premature beats and two brief runs of non-sustained ventricular tachycardia. His blood pressure is 130/80 mmHg, and serum electrolytes are normal. Which of the following antiarrhythmic agents is most appropriate for suppression of these ventricular arrhythmias in the immediate postinfarction period?