458 Background: Saccharomyces cerevisiae (yeast) has been genetically modified to express CEA protein and employed as a heat-killed immune-stimulating, vector-based vaccine. Preclinical studies have shown that yeast CEA vaccine can induce a strong CEA-specific T-cell immune response (IR) and anti-tumor activity. Methods: Patients (Pts) were enrolled in this phase I trial at 3 dose levels: 4, 16, and 40 yeast units (each unit =107 yeast particles). The vaccine was administered equally at 4 sites subcutaneously in bilateral inguinal and anterior chest wall regions. Vaccine was administered at 2 week intervals for 3 months, then monthly. Eligible pts were required to have a serum CEA > 5 ng/ml or > 20% CEA+ positive tumor block and no autoimmune history. An expansion cohort of 10 pts was enrolled to focus on IR. Pts had re-staging scans at 3 months, then bimonthly. Peripheral blood was collected for analysis of IR including the Effector/Regulatory T-cell ratio, ELISPOT assay, changes in the myeloid-derived suppressor cells (MDSC) and natural killer cells (NK). Results: 25 pts with progressive metastatic CEA-expressing carcinoma were enrolled; 22 had colorectal adenocarcinoma. Vaccine was well tolerated with no dose limiting toxicities. The most common adverse event was grade 1/2 injection site reaction. Overall, 7 patients had stabilization or declines in serum CEA after treatment. Of them, 5 pts (3 with colorectal cancer) had stable disease beyond 3 months and 1 is still on-going (14 +, 8, 8, 4.5 and 4 months). No anti-CEA antibodies were detected. Post vs. pre-vaccination: a) five out of 9 evaluable pts showed evidence by ELISPOT of CEA-specific T-cell IRs b) 8/16 pts had increased and 8/16 pts had decreased CD4 Effector/Treg ratio and c) 6/13 pts had increased and 2/13pts had decreased NK frequency. Conclusions: Saccharomyces cerevisiae-CEA demonstrated an acceptable safety profile. Although this is an advanced disease population of pts which is not ideal for immune-based therapy, CEA serum stabilizations and CEA-specific IRs were seen in some pts. Randomized studies are required to determine the clinical benefit of this vaccine in a more appropriate patient population for vaccine therapy.