Background: In advanced ACC, no significant progress has been made since introduction of mitotane and cisplatin-based therapy. EO2401 (EO) was designed to activate existing commensal memory T cells cross-reacting with tumor associated antigens (TAAs). EO includes microbial-derived, synthetically produced peptides corresponding to HLA-A2 restricted epitopes with molecular mimicry to three TAAs upregulated in ACC, IL13Rα2, BIRC5 and FOXM1, with the CD4 helper peptide UCP2 and the adjuvant Montanide. Pre-clinically EO generates strong immune responses and cross-reactive CD8 cells recognizing the TAAs. Methods: This Phase 1/2 trial (NCT04187404) investigated EO + nivolumab (N) (EO + N = EN) in pts with ACC. Cohort 1 lead-in established safety of EN. Cohort 2 includes pts with metastatic ACC, with (C2a), or without (C2b) prior systemic therapy. EN was given 4 times q2 w, followed by boosters q4 w until PD (iRECIST). Results: 33 pts with ACC started study treatment: C2a 26 pts (58% 1; 42% 2 prior lines), C2b 7 pts. Median age 47; 24% men; ECOG 0/52%, 1/42%, 2/6%; 61% ≥2 organs involved by metastases (61% liver, 76% lung). EN was well tolerated. The combination safety profile was consistent with the profile of N monotherapy except for higher local administration site reactions (any erythema/pain/induration in 35% of pts). Overall (n = 33), best RECIST response was PR 12%, SD 24%, PD 45%, NE 18%; median PFS was 1.9 mo (range 0.4-7.6+); median survival not reached, and survival rate at 6-mo 63% (median FU 4.9 mo, range 0.9-12.0). Strong CD8 T cell ELISPOT responses against the vaccine peptides (9/9 pts) and cross-reactivity against targeted TAAs (8/8 evaluable pts) was observed. Tetramer staining of specific CD8 cells for all 3 peptides was detected in 7/8 tested. When investigated, positive staining against BIRC5 was detected as early as 4 w after the first vaccination. In C2a, a group of pts (n = 10) with SD at the first CT (incl. 4 pts with PR) seemed to fare well; all investigated tumor samples in this group showed a low level of TMB, low MSI, and low PDL1 expression. In contrast, 10 pts had PD < 2mo and died < 6mo. There was no correlation between clinical benefit and a large panel of cytokines/chemokines. However, post-hoc analysis identified several clinical factors (prior mitotane, ECOG ≤ 1, ACC 1st diagnosis > 9 mo, max lesion ≤125 mm, ≤3 organs involved, lymphocytes ≤ grade 1) that excluded 90% of pts without benefit to EN. In the post-hoc selected group (n = 14) with median FU 6.9 mo (12 pts censored) the DCR was 64% (4 PR, 5 SD), 6-mo PFS was 42% and 6-mo survival rate 93%. Conclusions: EO2401 in combination with nivolumab was well tolerated and induced a specific immune response in all tested pts. In addition, efficacy was seen in a subpopulation of pts with ACC defined by clinical parameters in a post-hoc analysis. A randomized phase 2 study based on the findings of Cohort 2a is being planned. Clinical trial information: NCT04187404.