Abstract Introduction The outcome of high-risk stage III melanoma patients was poor with a 5-year overall survival (OS) rate of <50%. Adjuvant ipilimumab (IPI) improved the relapse-free survival (RFS) and OS, and adjuvant anti-PD-1 improved the RFS further. Preclinical data suggested that neoadjuvant therapy may be more effective than adjuvant therapy due to broader immune activation. The OpACIN trial compared neoadjuvant IPI plus nivolumab (NIVO) versus adjuvant IPI plus NIVO, while the subsequent OpACIN-neo trial tested three different dosing schedules of neoadjuvant IPI plus NIVO only. Neoadjuvant IPI plus NIVO induced high pathologic response rates of 74-78%. Here, we present the 36- and 18-months RFS update of the OpACIN and OpACIN-neo trial, respectively. Methods The phase 1b OpACIN trial randomized 20 stage IIIB/IIIC melanoma patients to receive either 4 cycles of adjuvant IPI 3 mg/kg plus NIVO 1 mg/kg or 2 cycles of neoadjuvant IPI plus NIVO at the same dose followed by 2 cycles adjuvant IPI plus NIVO. In the OpACIN-neo trial, 86 patients were randomized to 2 cycles neoadjuvant in arm A: 2x IPI 3 mg/kg plus NIVO 1 mg/kg q3w (n=30), arm B: 2x IPI 1 mg/kg plus NIVO 3 mg/kg q3w (n=30), and arm C: 2x IPI 3 mg/kg q3w followed immediately by 2x NIVO 3 mg/kg q3w (n=26). Pathologic response was defined as <50% viable tumor cells and centrally reviewed by a blinded pathologist. RFS rates were estimated using the Kaplan-Meier method. Results After a median follow-up of 36 months for the OpACIN and 18 months for the OpACIN-neo trial, only 1 of 71 patients (1.4%) with a pathologic response on neoadjuvant therapy had relapsed, versus 15 of 23 patients (65.2%) without a pathologic response. The estimated 3-year RFS rate for the neoadjuvant arm was 80% (95% CI: 59%-100%) versus 60% (95% CI: 36%-100%) for the adjuvant arm in the OpACIN trial. The median RFS was not reached in any of the arms within the OpACIN-neo trial. Estimated 18-months RFS rate was 85% (95% CI: 78%-93%) for all patients; for arm A 90% (95% CI: 80%-100%), for arm B 82% (95% CI: 70%-98%) and for arm C 83% (95% CI: 70%-100%). Translational analyses showed that tumor mutational burden and interferon-γ gene expression score at baseline, both separate and combined, can function as predictors of response. Conclusions OpACIN showed for the first time a potential benefit of neoadjuvant versus adjuvant immunotherapy, while OpACIN-neo confirmed the high pathologic response rates which can be achieved by neoadjuvant IPI plus NIVO. Both trials argue for pathologic response as a surrogate markers for RFS. Clinical trial information: NCT02437279, NCT02977052 Citation Format: Christian U. Blank, Judith M. Versluis, Elisa A. Rozeman, Alexander M. Menzies, Irene L. Reijers, Oscar Krijgsman, Esmée P. Hoefsmit, Bart A. van de Wiel, Karolina Sikorska, Carolien Bierman, Petros Dimitriadis, Maria Gonzalez, Annegien Broeks, Ron M. Kerkhoven, Andrew J. Spillane, John B. Haanen, Winan J. van Houdt, Robyn P. Saw, Hanna Eriksson, Alexander C. van Akkooi, Richard A. Scolyer, Ton N. Schumacher, Georgina V. Long. 36-months and 18-months relapse-free survival after (neo)adjuvant ipilimumab plus nivolumab in macroscopic stage III melanoma patients - update of the OpACIN and OpACIN-neo trials [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3412.