Delivery of all-trans-retinoic-acid (ATRA) in liposomes modulated MDSCs and T-cell activities in the tumor microenvironment.

e15644 Background: All-Trans-Retinoid-Acid (ATRA) is a naturally occurring vitamin A metabolite that participates in many biological processes. Beside its highly potent effect of promoting terminal differentiation of acute promyelocytic leukemia blasts into mature granulocytes, there have been many other studies suggesting its activity on the myeloid derived suppressor cells (MDSCs) and tumor specific CD8+ T cells in animal models as well as using clinical samples. But the use of ATRA as an immune-oncological agent in solid tumor therapy has been limited by the very poor solubility of the compound, its fast metabolism, and very limited exposure achieved after oral administration. Methods: We prepared a new dosage form by encapsulating ATRA inside PEGylated liposomes. The liposomes were shown to accumulate inside solid tumor tissues and deliver more ATRA with longer duration. Results: The effect and dose response of the liposomal ATRA on CT26 murine tumor growth were examined, as well as specific molecular signatures concerning tumor infiltrating myeloid cells. Notably, there was significant higher expression of CD86 and lower expression of PD-L1. These myeloid cells had very low inhibitory effect on ex vivo activated T cells, while on the other hand could promote specific antigen presentation to amplify CD8+ T cells. Furthermore, the liposomal ATRA was also shown to synergize with anti-PD-1 treatment to result in more CD8 T cell distribution in the tumor tissues. Conclusions: These data may suggest an exciting opportunity for targeting MDSCs using liposomal ATRA for combination with T cell based therapeutics in cancer immunotherapy.