Tumors with microsatellite instability (MSI) are caused by a deficiency in DNA mismatch repair (dMMR) system that leads to the accumulation of mutations within microsatellite regions. These types of mutations can determine a shift of the translational reading frame, generating Frame Shifted Peptides (FSPs). FSPs are tumor-specific neoantigens. Nous-209 is an off-the-shelf vaccine encoding 209 FSPs shared across dMMR/MSI tumors (Leoni et al., Cancer Res, 2020). The combination of the Nous-209 genetic polyvalent cancer vaccine and pembrolizumab has been demonstrated to be safe, highly immunogenic, with promising early signs of clinical efficacy in 12 patients with 7 durable confirmed PRs, 2 durable SDs and 3 PDs observed (Overman et al., SITC Nov 13, 2021 Poster number: 410). Here, we explore immune correlates potentially associated with the clinical outcome with the aim to address the Nous-209 contribution to the activity of pembrolizumab in this patient population. Several immunological analyses were performed on biological samples collected before and after treatment with Nous-209. Analyses were performed on paired blood and tumor biopsy samples. In periphery, vaccine immunogenicity was demonstrated by ex-vivo interferon-gamma ELISpot assay in 67% of subjects in dose level 1 (n=3), and 100% (n=7) of patients in dose level 2 with a mean of ≈1,500 IFN-γ SFCs/million PBMCs at the peak of the immune response. In 3 patients with partial response whose pre/post tumor biopsies were available, the intratumoral TCR repertoire was expanded and diversified post treatment with Nous-209. Moreover, in one out of these three patients it was possible to track vaccine-induced neoantigen specific TCR in the tumor biopsy post Nous-209 treatment. Overall, these results indicate that neoantigen specific CD8+ T cells, induced by Nous-209, expand and diversify upon treatment, and successfully traffic to and infiltrate the tumor microenvironment to exert anti-tumor activity.

Citation Format: Anna Morena D'Alise, Guido Leoni, Francesca Langone, Elisa Micarelli, Gabriella Cotugno, Michael James Overman, Marwan Fakih, Dung Le, Théa Faivre, Elisa Scarselli. Characterization of immune correlates of clinical activity for Nous-209, an off-the-shelf immunotherapy, with pembrolizumab for treatment of tumors characterized by microsatellite instability (MSI) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB008.