Figure 4.

Schematic of liposomal cGAMP structure and therapeutic strategy. a) 2’3’-cGAMP is encapsulated in cationic liposomes created from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and cholesterol using thin film rehydration, freeze thawing, and membrane extrusion. A polyethylene glycol(PEG)-containing lipid (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]; DSPE-PEG(2000)) is optionally included in the liposome preparation to create a PEG coating that improves liposome stability. b) In a therapeutic setting, melanoma tumor-bearing hosts are injected with free or liposomal cGAMP, where cells, for example antigen-presenting cells (APCs), in the tumor microenvironment take up liposomal cGAMP concurrent with melanoma cell antigens. (Inset) Free cGAMP has limited transport into the cytosol due to the presence of two negative charges that limit its permeability through the negatively charged cell membrane. cGAMP encapsulated in cationic liposomes shows improved cell membrane binding and uptake. Once internalized into the endosomal compartment, cationic liposomes facilitate the release of cGAMP into the cytosol, where cGAMP binds to the stimulator of interferon genes (STING) adaptor molecule, leading to type I interferon production by the APC (Adapted from Koshy et al. (2017).¹³⁶ Copyright 2017 John Wiley & Sons, Inc.)