Sub-millimetre precision of drug delivery in the brain from ultrasound-triggered nanodroplets

Abstract

Drug-loaded nanoscale cavitation agents, called nanodroplets, are an attractive solution to enhance and localize drug delivery, offering increased stability and prolonged half-life in circulation compared to microbubbles. However, the spatial precision with which drug can be released and delivered into brain tissue from such agents has not been directly mapped. Decafluorobutane lipid-shell droplets (206 +/- 6 nm) were loaded with a fluorescent blood-brain barrier (BBB)-penetrating dye (Nile Blue) and vaporized with ultrasound (1.66 MHz, 10 ms pulse length, 1 Hz pulse repetition frequency), generating transient echogenic microbubbles and delivering the encapsulated dye. The distribution and intensity of released fluorophore was mapped in a tissue-mimicking phantom, and in the brain of rats (Sprague Dawley, N = 4, n = 16). The release and distribution of dye was found to be pressure-dependent (0.2-3.5 MPa) and to occur only above the vaporization threshold of the nanodroplets (1.5 +/- 0.25 MPa in vitro, 2.4 +/- 0.05 MPa in vivo). Dye delivery was achieved with sub-millimetre spatial precision, covering an area of 0.4 to 1.5 mm in diameter, determined by the sonication pressure. The distribution and intensity of dye released at depth in the brain followed the axial pressure profile of the ultrasound beam. Nile Blue (354 Da, LogP 2.7) was compared to Nile Red (318 Da, LogP 3.8) and Quantum Dots (CdSe/ZnS, 5 nm diameter) to visualize the role of molecule size and lipophilicity in crossing the intact BBB following triggered release. Acoustic emissions were shown to predict the successful delivery of the BBB-penetrating dye and the extent of the distribution, demonstrating the theranostic capabilities of nanoscale droplets to precisely localize drug delivery in the brain.

Keywords: Cavitation; Focused ultrasound; Microbubbles; Nanoparticles; Phase-change emulsion.