A 48-yeаr-оld wоmаn with nо history of fixed coronаry artery disease presents with recurrent episodes of chest pain at rest, occurring primarily in the early morning hours. Each episode resolves spontaneously within 10-15 minutes. ECG during an episode shows transient ST elevation that normalizes after the pain resolves. Coronary angiography shows no significant stenosis. Which medication is MOST appropriate?