Background: TAS0313 is a cancer vaccine cocktail containing three long peptides, with a total of 12 cytotoxic T lymphocyte (CTL) epitope peptides. These peptides were derived from eight cancer-associated antigens that are highly expressed in various cancers. We report the results of a phase I part examining the tolerability, safety, potential efficacy, and immunological responses of 9 and 27 mg TAS0313 in patients (pts) with advanced solid tumors. Methods: The enrolled pts had at least one of the following HLA types: HLA-A*02:01, -A*02:06, -A*02:07, -A*11:01, -A*24:02, -A*31:01, or -A*33:03. TAS0313 was subcutaneously administered on Days 1, 8, and 15 of Cycles 1 and 2 and on Day 1 of Cycle 3 or later in 21-day cycles until disease progression, or unacceptable toxicity. Tolerability was assessed in at least six pts during the first cycle. Tumor response was evaluated using RECIST v1.1. The antigen specific CTL and IgG were analyzed post treatment by blood samples. Optional serial tumor biopsies were performed for tumor infiltrating leukocyte (TIL) analysis. CTL, IgG, and TIL were measured by ELISPOT assay, Luminex assay, and IHC (CD8 positive), respectively. Results: Seventeen pts were enrolled in 9mg (n = 10) and 27mg (n = 7) groups. There was no serious adverse drug reaction (ADR) in any patient. All ADRs were of grade 1 or 2, with the most frequent being dermatological injection site reaction, in 7/10 (70%) and 6/7 (86%) pts and pyrexia, in 1/10 (10%) and 2/7 (29%) for the 9 and 27mg groups, respectively. The best overall response was stable disease, in 2/10 (20%) and 2/7 (28%) pts. One patient with cancer of unknown origin received prolonged administration (over 10 months) of the 9mg dose. Increment of CTL and IgG by treatment were detected in 9 and 27mg groups. TIL counts were increased in 2/3(67%) and 3/4(75%) in 9 and 27mg groups compared with pre-treatment samples, respectively. Conclusions: TAS0313 demonstrated safety, tolerability, and immunological responses in pts with advanced solid tumors in the 9 and 27mg groups. A phase II part, evaluating the efficacy of combination therapy with pembrolizumab in pts with urothelial carcinoma and monotherapy in glioblastoma pts, is currently underway. Clinical trial information: JapicCTI-183824.