Background: Chemotherapy-induced nausea and vomiting (CINV), a major adverse effect of cancer treatment, can attenuate life quality. Multiple international antiemetic guidelines have recommended a 4-drug regimen (corticosteroid, 5-HT3 receptor antagonist, NK-1 receptor antagonist, and the antipsychotic medication olanzapine) for decreasing highly emetogenic CINV. This raised a question regarding whether antiemetic therapy regimens for highly emetogenic chemotherapy could be de-intensified, decreasing cost and side effects from anti-emetogenic agents. Methods: Study A221602 was developed to compare two olanzapine-containing antiemetic regimens, one with the use of an NK-1 receptor antagonist (aprepitant or fosaprepitant) and one without. Patients (pts) received intravenous highly emetogenic chemotherapy as either 1) cisplatin, given on one day, at a dose of ≥ 70 mg/m2, with or without other chemotherapy agent(s) or 2) doxorubicin (60 mg/m2) plus cyclophosphamide (600 mg/m2) on one day. Pts in both arms received 1) a 5-HT₃ receptor antagonist (palonosetron 0.25 mg IV or ondansetron 8-16 mg IV or 16-24 mg PO,) on day one, 2) dexamethasone (12 mg PO, day one followed by 8 mg PO, days 2-4), and 3) olanzapine (10 mg/day PO, days 1 to 4). On Day 1, all agents were given prior to chemotherapy, with the exception that olanzapine could be taken prior to chemotherapy or at bedtime. Additionally, pts were randomized to receive an NK-1 receptor antagonist (fosaprepitant 150 mg IV or aprepitant 130 mg IV; 346 pts) or a matching placebo (344 pts). The primary objective was to compare the proportion of patients with no nausea for 5 days following receipt of their chemotherapy between the two study arms. Results: While there was no suggestion of outcome differences on day 1 between the two study arms, fewer pts (7.4%; upper limit of the 95% confidence interval was 13.5%) without NK-1 receptor antagonists were without nausea for the complete 5-day study period. Conclusions: Per the study design, created to exclude a 10% benefit for NK-1 receptor antagonists, the results did not provide sufficient evidence to reject the hypothesis that the 4-drug regimen was superior to the protocol 3-drug regimen (p=0.24). Support: UG1CA189823; Clinical trial information: NCT03578081.