A 65-year-old woman with a BMI of 38 kg/m² and history of type 2 diabetes presents with abnormal uterine bleeding. Endometrial biopsy confirms grade 3 endometrioid adenocarcinoma with deep myometrial invasion (>50%) and lymphovascular space invasion. Staging CT shows no distant metastases. She undergoes total abdominal hysterectomy with bilateral salpingo-oophorectomy and pelvic lymphadenectomy. Pelvic lymph nodes are negative. What is the drug of choice for adjuvant treatment in this patient?