Background: Adenocarcinoma is the most common histologic type of solid tumors and can arise from almost anywhere in the body. Labyrinthin is a novel tumor-specific protein expressed on the cell surface of the majority of adenocarcinomas of various cancer types. Several therapeutic strategies targeting labyrinthin are under development. We hypothesize that vaccination against labyrinthin can elicit strong immune responses against the labyrinthin-positive adenocarcinomas in cancer patients. LabVax 3(22)-23 is a novel anti-tumor vaccine that contains 4 synthetic labyrinthin-based peptides designed to elicit both B-cell and T-cell responses. Sargramostim, a recombinant granulocyte macrophage colony-stimulating factor (GM-CSF), is given as an immunostimulator with LabVax 3(22)-23 to boost the antitumor immune response. Preclinical studies showed that LabVax 3(22)-23 significantly inhibited tumor growth that was augmented by GM-CSF without any significant toxicity in C57/BL6 transgenic mice expressing human PD-1/PD-L1 implanted with the murine colon adenocarcinoma cell line MC-38-huPD-L1. This single institution, first-in-human, phase I trial (UCDCC#296) evaluates LabVax 3(22)-23 and adjuvant sargramostim in patients with labyrinthin-positive metastatic or recurrent adenocarcinoma of any primary tumor site after all standard-of-care therapies. Methods: The primary objective is to assess the toxicity of LabVax 3(22)-23 and adjuvant sargramostim according to NCI CTCAE V5.0. The primary endpoint is dose limiting toxicity (DLT), which is defined as grade ≥ 2 allergic and autoimmune reaction, grade ≥ 3 injection site reaction, any grade 3 toxicities lasting >1 week, or any grade ≥ 4 toxicities. With a sample size of 10 patients, there is 89-97% of chance to observe ≥1 DLT if the true event rate is 20-30%. A secondary endpoint is the preliminary assessment of tumor response rate by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. Exploratory objectives measure the effect of LabVax 3(22)-23 on various immune responses (cytokines, anti-labyrinthin antibody production) and the correlation between the level of labyrinthin expression and the efficacy of LabVax 3(22)-23. Eligible patients are required to have labyrinthin expression on their tumor cells by immunohistochemistry and adequate organ function. Patients receive sargramostim subcutaneously and LabVax 3(22)-23 intradermally on weeks 1, 2, 4, 8, and 12 in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed every 3 months for 1 year. Two patients are enrolled at the time of submission. Clinical trial information: NCT05101356.