A 58-year-old man presents with a 2.5 cm dark nodular lesion on his back with irregular borders, present for 8 months. Dermoscopy shows asymmetry, multiple colors, and ulceration. Excision biopsy confirms cutaneous melanoma, Clark level IV, Breslow thickness 3.2 mm, with 2 mitoses per mm². Sentinel lymph node biopsy is negative. Which is the drug of choice for adjuvant therapy in this patient?