Background: Despite the advances made with CPIs such as aPD-1/aPD-L1, aCTL4 and aLAG-3 there still is a significant unmet medical need for patients with metastatic cancers. As it has been shown that the effect of CPIs are mediated by tumor specific T cells, one strategy for increasing treatment success is to actively induce tumor specific T cells. Here, we report on a first-in-human clinical trial evaluating a personalized neoantigen vaccine (EVX-01) in patients with metastatic melanoma. Methods: Stage-IV metastatic melanoma patients were treated with standard aPD-1 in combination with a personalized tumor-specific neo-antigen vaccine. The vaccine consisted of multiple 15-27mer peptides comprising one or more patient specific neoantigens identified from tumor DNA and mRNA sequencing data by an integrated AI-platform; PIONEERand delivered together with the novel liposomal adjuvant CAF09b to potentiate immune responses. Results: Vaccines were designed and manufactured for each patient in less than 7 weeks. In total, 12 eligible patients received either 500ug (5 patients, reported in PMID: 35036074), 1000ug (3 patients) or 2000ug (4 patients) total peptide load keeping the peptide:CAF09b ratio constant. Four patients had stable disease on aPD-1 treatment for at least 4 months before enrolment in the study, whereas the remaining patients were treatment naïve. All patients received monotherapy with pembrolizumab (11) or nivolumab (1) during the vaccine manufacturing period. The personalized vaccine was shown to be safe and well tolerated with fatigue and injection site reactions being the most frequently reported events and only grade 1-2 events being related to the vaccine. Response was evaluated by RECIST 1.1, according to the investigator’s assessment with 8 patients (67%) having objective response (CR: n = 2, PR: n = 6), of which 1 patients had response initiation before addition of the vaccine. Importantly the vaccine induced neoantigen specific T cells in all patients analyzed. Both CD4+ and CD8+ T cells were detected, with the majority of responses being CD4+ T cell mediated. Interestingly, we report a significant correlation between the PIONEER prediction score and induced immunogenicity. Furthermore, the breadth of the neoantigen recognition seemed to be important for the clinical effect of the vaccine as responders demonstrated T cell recognition of more epitopes within the vaccine compared to non-responders. We found, in our cohort, that the neo-antigen scores effectively separated responders and non-responders, as opposed to the TMB (by FDA guidelines). Conclusions: aPD-1 treatment combined with a peptide-based personalized neo-antigen-based vaccine was shown to be feasible and safe with promising signs of efficacy, warranting further study. Further, the neo-antigen selection method appeared crucial for designing an efficacious vaccine. Clinical trial information: NCT03715985.