A 61-year-old man with increasing fatigue, early satiety, and nausea for 5 months vomited dark granular material yesterday. Endoscopy reveals a large ulcerated mass in the gastric fundus. Biopsies are taken and microscopically the mass is composed of spindle cells that are positive for c-Kit with immunohistochemical staining. Mitoses are frequent. Gastrectomy is performed, and the 10-cm circumscribed mass arises from the gastric wall. Which of the following therapies is most likely to be a useful adjunct in the treatment of his disease?