Adjuvant polypeptide vaccines for high-risk uveal melanoma

e21509 Background: Due to biologic differences, uveal melanoma patients are often excluded from clinical trials for cutaneous melanoma; however, many antigens are shared between the two melanoma subtypes. Thus, our vaccine trials at the University of Virginia (UVA) have allowed enrollment of high-risk uveal melanoma patients, who have substantial risk of relapse without FDA-approved adjuvant therapy available. For the present study, we hypothesized that the vaccines would be well-tolerated and immunogenic in this population. We examined the clinical, immunogenicity and safety outcomes of subjects with uveal melanoma enrolled in our adjuvant vaccine clinical studies. Methods: Patients were identified through participation in an adjuvant vaccine study at UVA between 2012-2022. Ten patients received the 6-Melanoma-Helper Peptide (6-MHP) vaccine through Mel-65 and Mel-66 clinical trials (NCT03617328 and NCT04364230) and one patient received the 12-peptide MELITAC vaccine through the Mel-58 clinical trial (NCT01585350). Clinical outcomes included relapse-free (RFS) and overall survival (OS) and trial toxicity reporting. Immunogenicity was assessed by ex vivo interferon-gamma ELISpot assay of peripheral T cells. OS and RFS were reported by Kaplan-Meier survival analysis and compared by log rank test between groups. Results: Of the 11 identified patients, nine were male (82%) and two were female (18%), with a median age of 64 years. All patients were high risk by molecular testing, the majority by Castle Bioscience DecisionDx-UM. Five were PRAME positive, five PRAME negative, and one not reported. In the cohort, three patients had uveal melanoma recurrence and one subject died from metastatic disease during the evaluation period. The 5-year RFS and OS were 71% (95% CI 43-100) and 80% (95% CI 52-100), respectively. Two patients experienced dose-limiting, grade 3 toxicities to the vaccine, requiring early trial discontinuation, including one grade 3 injection-site reaction and one grade 3 uveitis. Nine patients experienced grade 1-2 toxicities, mostly injection-site reactions and constitutional symptoms. Of nine patients with available immunogenicity data, five (55%) had direct antigen-mediated immune response to vaccine. Two patients’ responses are pending. Patients with immunogenic response had a non-significant trend toward better RFS and OS (both p = 0.22). The 3-year RFS and OS were 100% for subjects with T cell response and the RFS and OS were 50% (95% CI 13-100) for patients without T cell response. Two of the relapsed patients did not have an immune response to vaccine, and response data were not available for the third. Conclusions: Melanoma polypeptide vaccines are immunogenic and safe in patients with uveal melanoma. Our early data suggests the possibility of early benefit with direct antigen response, though longer follow up and further study will be needed to investigate these findings.