2571 Background: NY-ESO-1 is an immunogenic "cancer-testis" (CT) antigen, expressed in many cancers including melanoma. Iscomatrix adjuvant (IMX), a saponin-based adjuvant, was formulated with NY-ESO-1 (ESO) protein (ESO/IMX). ESO/IMX was highly immunogenic as defined by ESO specific antibody (Ab), delayed type hypersensitivity (DTH) & T-cell responses in patients (pts) with resected ESO+ve cancer. Disease free survival in vaccine recipients with resected melanoma appeared longer than in controls. A prospective evaluation was subsequently undertaken to determine whether ESO/IMX could mediate antitumor responses in pts with advanced NY ESO-1+ve melanoma. METHODS: 27 pts were vaccinated in a trial designed to assess objective clinical responses, safety, & immunogenicity. Vaccination consisted of 100 μg ESO/IMX administered q 4 wk × 3; this cycle was repeated in pts without symptomatic progression unless they required other treatment. Pts were then eligible to continue vaccination q12 weeks so long as they were responding or stable. RESULTS: ESO/IMX was well tolerated. 13 pts progressed when evaluated after the first cycle at week 11. No objective clinical responses were seen. Vaccine-induced antibody titers to NY-ESO-1 were comparable to those seen in the earlier minimal residual disease trial (ranging to > 1: 390,000). DTH & T-cell responses were less marked. This occurred despite many pts with advanced measurable disease having pre-existing spontaneous ESO immunity ( Table ). DTH responses were seen in 11 pts; 5 had pre-existing responses (2 were boosted) and 6 were induced. The DTH responses were lost by week 33. CONCLUSION: No objective responses were seen. Vaccine-induced immunity appeared to be attenuated in the presence of advanced metastatic disease. Ongoing laboratory studies are investigating the role of regulatory T cells in the suppression of ESO-specific immunity in these pts with a view to adopting strategies to counter regulatory responses e.g. low-dose cyclophosphamide prior to vaccination. [Table: see text] [Table: see text].