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Focused ultrasound (FUS) combined with microbubbles is an attractive method to deliver therapeutic agents to brain tissue noninvasively, transiently and locally such as antibodies, gene vectors, liposomal drugs. The objective of this study is to investigate passive delivery outcome of liposomes after ultrasound mediated blood-brain barrier (BBB) disruption. Microbubbles were injected intraveneously through mice tail vein at 0.1 µL/g. Focused ultrasound (center frequency: 1.2819 MHz) was applied to mice brains with 10 ms pulse length and 1 Hz repetition frequency at acoustic power 1.1 W for 60 s. After sonication, 55-nm and 120-nm rhodamine-labeled liposomes were administered to mice intraveneously. Fluorescence imaging of brain coronal sections showed that the spatial distribution of delivered liposomes across BBB was characterized by scattered spots and heterogeneous. However, the quantitative analysis results indicated that passive deliveries of liposomes across BBB were confined, especially for 120-nm liposomes.

Imaging or therapeutic agents larger than the blood-brain barrier's (BBB) exclusion threshold of 400 Da could be delivered locally, non-invasively and reversibly by focused ultrasound (FUS) with circulating microbubbles. The size of agents is an important factor to the delivery outcome using this method. Liposomes are important drug carriers with controllable sizes in a range of nanometers. However, discrepancies among deliveries of intact liposomes with different sizes, especially those larger than 50 nm, across the BBB opened by FUS with microbubbles remain unexplored. In the present study, rhodamine-labeled long-circulating pegylated liposomes with diameters of 55 nm, 120 nm and 200 nm were delivered to mice brains after BBB disruption by pulsed FUS with microbubbles. Four groups of peak rarefactional pressure and microbubble dosages were used: 0.53 MPa with 0.1 μL/g (group 1), 0.53 MPa with 0.5 μL/g (group 2), 0.64 MPa with 0.1 μL/g (group 3) and 0.64 MPa with 0.5 μL/g (group 4). The delivery outcome was observed using fluorescence imaging of brain sections. It was found that the delivery of 55-nm liposomes showed higher success rates than 120-nm or 200-nm liposomes from groups 1-3. The result indicated that it may be more difficult to deliver larger liposomes (>120 nm) passively than 55-nm liposomes after BBB opening by FUS with microbubbles. The relative fluorescence area of 55-nm liposomes to the total area of the sonicated region was statistically larger than that of the 120-nm or 200-nm liposomes. Increasing peak rarefactional pressure amplitude or microbubble dose could induce more accumulation of liposomes in the brain using FUS with microbubbles. Moreover, the distribution pattern of delivered liposomes was heterogeneous and characterized by separated fluorescence spots with cloud-like periphery surrounding a bright center, indicating confined diffusion in the extracellular matrix after extravasation from the microvasculature. These findings are expected to provide useful information for developing FUS with microbubbles as an effective trans-BBB liposomal drug delivery strategy.