Abstract 1251: Use of an attenuated version of a strongly immunogenic, peptide-based vaccine to enhance an anti-cancer immune response against folate receptor-alpha.

Background: FRα is over-expressed in ovarian and breast cancers. E39 (FBP 191-199) is an immunogenic, cytotoxic t lymphocyte (CTL) eliciting peptide derived from FRα used in a cancer vaccine strategy. Due to the in vitro observation of antigen-induced cell death with repeated T cell stimulation by E39, we developed an attenuated form of the E39, J65. We report the use of J65 alone or in combination with E39 to better induce E39-specific CTL and anti- FRα immunity. Methods: T2 stabilization assays were performed using flow cytometry to determine peptide HLA-A2 binding affinity. Interferon-γ (IFNγ) release was measured from peripheral blood mononuclear cell (PMBC) cultures after weekly stimulation with J65X3 followed by re-stimulation with either J65 or E39. Naive PBMCS from healthy donors (HD) were primed with weekly J65X3 or E39X3, re-stimulated with E39 at concentrations of 5 and 25μG/M, and then tested for the ability to lyse FRα-expressing cancer cells. PBMCs from responding donors (RD) were stimulated with E39 or J65, then proliferation was measured by cell counts after 14d. IL-2 secretion was also measured. Results: The affinity of J65 for HLA-A2 was half that of the native E39 (65V130 MCF). After priming with J65X3, IFNγ levels were lower in the re-stimulated J65 culture compared to E39 re-stimulation (43V181 PG/ML). HD cultures demonstrated a higher tumor cell lysis (24.5 & 17.4% V 14.6 & 11.1%) after priming with J65X3 compared to E39X3. In RD cultures, PMBC cell counts were higher in the J65 culture compared with the same donor PMBCS stimulated with E39 (8.2X106V2.4X106 cells) and IL-2 concentrations were lower in the J65 cultures (820V580 PG/ML). Conclusions: In vitro analysis reveals the potential of J65 to induce CTL with the ability to proliferate while avoiding overstimulation. Importantly, these CTL demonstrate enhanced recognition and lysis of FRα-expressing cancer cells. The potential of this weaker, “survival inducing” version of E39 to induce a more robust anti-FRα immune response is currently being assessed in a first in human, Phase 1b clinical trial in ovarian and breast cancer patients. Citation Format: John S. Berry, Alfred F. Trappey, Timothy J. Vreeland, Diane F. Hale, Guy T. Clifton, Alan K. Sears, Nathan M. Shumway, Sathibalan Ponniah, Jarrod P. Holmes, Constantin Ioannides, Elizabeth A. Mittendorf, George E. Peoples. Use of an attenuated version of a strongly immunogenic, peptide-based vaccine to enhance an anti-cancer immune response against folate receptor-alpha. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1251. doi:10.1158/1538-7445.AM2013-1251