Exploiting the DNA Damaging Activity of Liposomal Low Dose Cytarabine for Cancer Immunotherapy

Abstract

Perhaps the greatest limitation for the continually advancing developments in cancer immunotherapy remains the immunosuppressive tumor microenvironment (TME). The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) axis is an emerging immunotherapy target, with the resulting type I interferons and transcription factors acting at several levels in both tumor and immune cells for the generation of adaptive T cell responses. The cGAS-STING axis activation by therapeutic agents that induce DNA damage, such as certain chemotherapies, continues to be reported, highlighting the importance of the interplay of this signaling pathway and the DNA damage response in cancer immunity/immunotherapy. We have developed a multi-targeted mannosylated cationic liposomal immunomodulatory system (DS) which contains low doses of the chemotherapeutic cytarabine (Ara-C). In this work, we show that entrapment of non-cytotoxic doses of Ara-C within the DS improves its ability to induce DNA double strand breaks in human ovarian and colorectal cancer cell lines, as well as in various immune cells. Importantly, for the first time we demonstrate that the DNA damage induced by Ara-C/DS translates into cGAS-STING axis activation. We further demonstrate that Ara-C/DS-mediated DNA damage leads to upregulation of surface expression of immune ligands on cancer cells, coinciding with priming of cytotoxic lymphocytes as assessed using an ex vivo model of peripheral blood mononuclear cells from colorectal cancer patients, as well as an in vitro NK cell model. Overall, the results highlight a broad immunotherapeutic potential for Ara-C/DS by enhancing tumor-directed inflammatory responses.

Conventional DNA-Damaging Cancer Therapies and Emerging cGAS-STING Activation: A Review and Perspectives Regarding Immunotherapeutic Potential

“…We recently reported that the pyrimidine nucleoside analog cytarabine (Ara-C) activates the cGAS-STING pathway [ 120 ]. We first showed that entrapment of non-cytotoxic doses of Ara-C within a multi-targeted, mannosylated, cationic, liposomal delivery system (DS) greatly improved its biological stability, and that the Ara-C/DS combination had beneficial immunomodulatory properties [ 175 ].…”

“…This prompted us to investigate the immunomodulatory mechanism of Ara-C/DS. We subsequently demonstrated that the DS improves the ability of Ara-C to induce DNA DSB in several human ovarian and colorectal cancer cell lines in vitro, and that this DNA damage leads to the activation of the cGAS-STING signaling axis in an Ara-C dose-dependent fashion [ 120 ]. Interestingly, a similar activity was also observed in immune cells including peripheral blood mononuclear cells (PBMCs) and THP-1-derived macrophages.…”

“…Interestingly, a similar activity was also observed in immune cells including peripheral blood mononuclear cells (PBMCs) and THP-1-derived macrophages. We further demonstrated that Ara-C/DS-mediated DNA damage translated into increased surface expression of various immune ligands on cancer cells, including MHC-1, as well as the NK cell-attracting MHC class 1 chain-related protein A and B (MICA) and UL16-binding proteins 1-62, 5 and 6 (ULBP1-62,5,6) [ 120 ], which are important ligands of the NKG2D receptor [ 176 ]. These immunomodulatory effects were linked with increased priming of cytotoxic lymphocytes ex vivo using PBMC from colorectal cancer patients, and increased NK cell activity in an in vitro co-culture model.…”

“…These immunomodulatory effects were linked with increased priming of cytotoxic lymphocytes ex vivo using PBMC from colorectal cancer patients, and increased NK cell activity in an in vitro co-culture model. Importantly, the doses of Ara-C used in these studies were shown to be non-toxic to the immune cells [ 175 ], and cytostatically reduced cancer cell growth in vitro without directly causing cell death [ 120 ].…”

“…Our own work with the Ara-C/DS follows a similar strategy, albeit with a few important distinctions. We have shown that the multi-targeted nature of the DS leads to DNA damage and cGAS-STING axis activation in both cancer cells and immune cells, with Ara-C doses that are non-cytotoxic to the latter [ 120 , 175 ]. Ultimately, the potential implications and/or benefits of this dual activity requires further investigation.…”