Background: Antiemetic trials typically evaluate CINV control during acute (Day 1), delayed (Days 2-5), and overall (Days 1-5) phases post-chemotherapy; the daily course of events is often unstudied in the delayed phase. In a head-to-head study evaluating NK₁RA-containing regimens, a single dose of NEPA, an oral fixed combination of the NK₁RA netupitant and 5-HT₃RA palonosetron, was non-inferior to a 3-day regimen of APR and GRAN for overall complete response (no emesis/no rescue use) rates (74% NEPA vs 72% APR/GRAN) in 828 patients receiving cisplatin-based highly emetogenic chemotherapy (HEC). This secondary analysis explores daily rates of breakthrough CINV in this study. Methods: This was a double-blind study in chemotherapy-naïve patients with solid tumors. Daily rates of breakthrough CINV, defined as the % of patients with emesis and/or rescue use were calculated with differences between treatment groups evaluated by the Cochran-Mantel-Haenszel method stratified by sex. Results: While daily rates of patients with breakthrough CINV remained stable between 13%-15.1% for APR/GRAN, they declined from 15.5% to 8% over the 5 days for NEPA, with the difference between groups reaching statistical significance on Day 5 (Table). Percent of total patient days with CINV events were 11.7% [NEPA] and 14.0% [APR/GRAN]. The % of patients with ≥ 3 days of breakthrough CINV were 8.5% and 12.3%, respectively. Conclusions: In this study evaluating guideline-recommended antiemetic regimens for HEC, CINV was prevented in most patients during the overall phase yet breakthrough CINV on individual days differed between treatment groups. APR/GRAN showed a relatively constant rate over time, while NEPA rates decreased and patients had fewer total days with breakthrough. This suggests the need for close monitoring of CINV events during the delayed phase.