In vivo temperature controlled ultrasound-mediated intracellular delivery of cell-impermeable compounds

Many chemotherapeutic drugs are characterized by high systemic toxicity and/or suffer from limited bioavailability. Thermosensitive liposomes (TSLs) encapsulating drugs in their aqueous lumen are promising activatable nanocarriers for ultrasound (US)-mediated drug delivery in response to mild hyperthermia. On the other hand, US is known to locally break biological barriers and as a consequence enable internalization of molecules. In this work, a two-step protocol for intracellular delivery of cell-impermeable molecules comprising of US-induced permeabilization followed by temperature-controlled release of the model drug from thermosensitive liposomes has been developed. TSLs containing TO-PRO-3, a cell-impermeable molecule that displays a significant increase in fluorescence upon binding to nucleic acids thus serving as a 'sensor' for internalization have been prepared and characterized in detail. US-mediated permeabilization followed by temperature-controlled release was applied to tumor bearing mice following i.v. injection of TSLs and microbubbles. The efficacy of this approach was evaluated by in vivo fluorescence imaging followed by histological analysis. A 2.4-fold increase of fluorescence signal was observed and intracellular delivery of TO-PRO-3 was confirmed by a characteristic nuclear staining. These results demonstrate the feasibility of novel drug delivery system to tumors comprising of local cell permeabilization by US followed by in situ release of the payload from thermosensitive liposomes. Possible applications include local and controlled intracellular delivery of molecules with otherwise limited bioavailability.