A 32-year-old woman undergoing chemotherapy with cisplatin for ovarian cancer develops severe nausea and vomiting on day 1 of treatment. She has no prior history of GERD or gastroparesis. The oncologist prescribes ondansetron 8 mg IV 30 minutes before chemotherapy and every 8 hours for 3 days. Despite this, she continues to experience breakthrough nausea on day 2. Which of the following agents would be the most appropriate addition to her antiemetic regimen?