Vaporizing emulsion droplets may aid in endosomal rupture as a drug delivery route to the cytosol. Upon insonation, emulsion droplets formed from perfluorocarbon liquids may vaporize with sufficient expansion to disrupt liposomal or endosomal membranes. Emulsion droplets of perfluorohexane (PFC6) or perfluoropentane (PFC5) were prepared as free droplets in calcein or as droplets encapsulated within liposomes containing calcein. Folate-stimulated endocytosis created an experimental model, wherein calcein was self-quenched until released from the vesicles. Upon release, calcein was diluted below its self-quenching concentration and its release quantified by fluorescence. In this experimental model, folated emulsions or folated eLiposomes were incubated with folate-starved HeLa cells. Samples were exposed to two seconds of 20-kHz ultrasound (US) at 1 W/cm(2). Fluorescence microscopy identified released intracellular calcein. Upon insonation, both free emulsion samples and eLiposome samples produced calcein release to the cytosol. Calcein fluorescence was more intense in samples containing PFC5 compared to PFC6. Insonation of samples without emulsion droplets produced no cytosolic delivery. Likewise, cells that took up emulsion droplets but were not exposed to US did not exhibit fluorescence throughout the cell. These results suggest that vaporizing emulsion droplets are internalized into the cells and can produce endosomal escape of a therapeutic payload.
Keywords: Cytosol; drug delivery; eLiposome; liquid emulsion nanodroplet; perfluorocarbon emulsion; ultrasound.