PERSEPHONE: 6 versus 12 months (m) of adjuvant trastuzumab in patients (pts) with HER2 positive (+) early breast cancer (EBC): Randomised phase 3 non-inferiority trial with definitive 4-year (yr) disease-free survival (DFS) results.

506Background: Adjuvant trastuzumab has significantly improved outcomes for HER2+ EBC pts, using the 12m duration empirically adopted from the pivotal registration trials. A shorter duration could reduce toxicities and cost whilst providing similar efficacy. No reduced-duration trial to date has demonstrated non-inferiority. Methods: PERSEPHONE is a randomised phase 3 non-inferiority trial comparing 6 to 12m trastuzumab, the largest reduced-duration non-inferiority trial internationally. Mapping onto standard UK practice, all HER2+ EBC pts were eligible. Stratification is by ER status, chemotherapy (CT) type, and CT and trastuzumab timing. The primary endpoint is DFS from diagnosis (first relapse or death from any cause). Randomising 4000 pts (1:1) enabled the trial to assess the non-inferiority of 6m (5% 1-sided significance, 85% power), defining non-inferiority as ‘no worse than 3%’ below the 12m arm’s assumed 80% 4-yr DFS. The pre-planned definitive DFS analysis required 500 events. Results: 4089 pts w...