Quantification of transient increase of the blood–brain barrier permeability to macromolecules by optimized focused ultrasound combined with microbubbles

Abstract

Radioimmunotherapy using a radiolabeled monoclonal antibody that targets tumor cells has been shown to be efficient for the treatment of many malignant cancers, with reduced side effects. However, the blood–brain barrier (BBB) inhibits the transport of intravenous antibodies to tumors in the brain. Recent studies have demonstrated that focused ultrasound (FUS) combined with microbubbles (MBs) is a promising method to transiently disrupt the BBB for the drug delivery to the central nervous system. To find the optimal FUS and MBs that can induce reversible increase in the BBB permeability, we employed minimally invasive multiphoton microscopy to quantify the BBB permeability to dextran-155 kDa with similar molecular weight to an antibody by applying different doses of FUS in the presence of MBs with an optimal size and concentration. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a micro-grinder. About 5 minutes after applying the FUS on the thinned skull in the presence of MBs for 1 minute, TRITC (tetramethylrhodamine isothiocyanate)-dextran-155 kDa in 1% bovine serum albumin in mammalian Ringer’s solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump. Simultaneously, the temporal images were collected from the brain parenchyma ~100–200 μm below the pia mater. Permeability was determined from the rate of tissue solute accumulation around individual microvessels. After several trials, we found the optimal dose of FUS. At the optimal dose, permeability increased by ~14-fold after 5 minutes post-FUS, and permeability returned to the control level after 25 minutes. FUS without MBs or MBs injected without FUS did not change the permeability. Our method provides an accurate in vivo assessment for the transient BBB permeability change under the treatment of FUS. The optimal FUS dose found for the reversible BBB permeability increase without BBB disruption is reliable and can be applied to future clinical trials.

Tumor targeting via EPR: Strategies to enhance patient responses

“…Microbubbles are routinely used as contrast agents for ultrasound imaging. They can, however, also be employed to temporarily increase vessel perfusion and permeability [121,122], thereby improving drug delivery to tumors [123], upon application of ultrasonic waves to induce microbubble oscillation, cavitation or implosion [124]. There are several options for microbubble use to promote drug targeting to pathological sites: either via direct drug delivery (i.e.…”

Approaches to Improve Macromolecule and Nanoparticle Accumulation in the Tumor Microenvironment by the Enhanced Permeability and Retention Effect

“…When sonoporation is controlled with an ultrasound contrasting agent, temporary pores can be produced within the endothelial layer, causing increased vascular permeability which improves EPR [126]. Micro/nano-bubbles (MNBs), which are used as ultrasound contrast agents, have been employed to control temporary increases in tumor vascular leakiness for improved nanomedicine accumulation within tumor tissues [126,136].…”

Drug Penetration into the Central Nervous System: Pharmacokinetic Concepts and In Vitro Model Systems

“…Thus, as described in more detail elsewhere [72,94], lipophilic compounds often show high rates of passive transfer across the BBB but also a high degree of non-specific binding, so that their t 1/2eq,in may be very similar to that of less lipophilic compounds with low rates of passive transfer and a low degree of non-specific binding. An alternative, minimally invasive method for in vivo assessment of BBB permeability in animal models is based on laser-scanning multiphoton fluorescence microscopy of the cerebral microcirculation through a thinned section of the skull bone [96][97][98]. This approach involves infusion of fluorescent solutes into the cerebral circulation and simultaneous collection of temporal images to quantify their rate of tissue accumulation and radial concentration gradient around individual microvessels [96].…”