Background: INVAC-1 is an optimized DNA plasmid encoding an inactive form of human Telomerase Reverse Transcriptase (hTERT), a universal tumor antigen expressed in most of human tumors with little or no expression in somatic cells. We report here the final results of a First-In-Human Phase I study evaluating INVAC-1 as a single agent in patients (pts) with advanced solid tumors, ended in June 2018. Methods: A two center Phase I trial evaluated INVAC-1 given monthly for a minimum of 3 cycles and up to 9 cycles by intradermal injection followed by electroporation (n = 20) or using a needle-free injection system (n = 6). Primary objectives included safety, tolerability and dose limiting toxicities to identify the maximum tolerated dose and recommended phase 2 dose. Secondary objectives included immune response (assessed by IFN-γ Elispot) and anti-tumor activity. Immuno-monitoring included detection of autoantibodies, lymphocyte phenotyping and inflammatory cytokine levels in blood. Anti-tumor activity was evaluated through RECIST 1.1 adapted to immune response, and plasma circulating tumor DNA (ctDNA). Results: 26 pts with refractory/progressive tumors were enrolled and treated with 3 escalating doses of 100, 400 and 800 µg. 15 pts experienced stable disease according to RECIST. For 11 of them, the treatment was extended, up to 9 months. INVAC-1 was well tolerated with no dose-limiting toxicities. No significant biological signs of autoimmunity were observed. No significant modification in inflammatory plasma cytokines levels was observed after INVAC-1 administration. INVAC-1 triggered de novo or enhanced pre-existing CD4/CD8 specific anti-hTERT response in 63% of pts. This specific anti-hTERT immune response was enhanced ex vivo by adding the immune checkpoint inhibitor nivolumab. ctDNA was evaluated in 17 pts. We observed a ctDNA decrease in 6 cases, a stable level in 5 cases and an increase in 6 cases. Conclusions: Results indicate that INVAC-1 was well tolerated and immunogenic at the doses and schedule tested. Disease stabilization was obtained for the majority of pts (58%) according to RECIST criteria or ctDNA levels. Clinical trial information: NCT02301754