Ultrasound frequency and microbubble properties can be tuned to modulate the blood-brain barrier phenotype

Introduction:: Focused ultrasound (FUS) combined with microbubbles (MB) provides a physical method to transiently open the blood-brain barrier (BBB) and improve drug delivery in the brain. It is hypothesized that the observed changes in the BBB permeability are related to direct oscillatory force, radiation force, and indirect shear force exerted by the microbubble oscillation. However, there is limited consensus on how microbubble properties and FUS frequency contribute to their interaction with the vessel wall.


Materials and Methods:: To overcome the challenges associated with the direct visualization and quantification of the microscale MB dynamics in the brain we developed a mathematical model of MB dynamics inside an elastic and porous vessel. Using this model, we study the impact of a range of system parameters on the MB dynamics, including excitation frequency (0.5-5MHz), MB shell properties, and MB diameter (1-2 µm). These data are then used to generate hypotheses that are tested under controlled in vivo experiments. In our experiments, the microbubble dynamics and kinetics are monitored with passive cavitation detection while changes in BBB phenotype are assessed by DCE-MRI (K^trans values), bulk transcriptome analysis, and spatial protein expressions.


Results, Conclusions, and Discussions:: The mathematical model suggests that MB with a more elastic shell have stronger oscillation under the same acoustic pressure. To test this model prediction, we conducted FUS-BBB opening experiments using MB with a more elastic lipid shell (Definity) and MB with a more rigid albumin shell (Optison) of the same size (2 µm). The experiments with Definity revealed stronger harmonic emission (1.7-fold, p< 0.0001), K^trans values (2.5-fold, p=0.028), and a higher down-regulation tight junction protein, Claudin 5, (4-fold, p=0.01) after treatment as compared to Optison, indicating that MBs with a more elastic shell are more potent in opening the BBB. Moreover, our model suggests that MB exerts a 3-fold higher level of shear force on the vessel wall when excited with 1.5MHz versus 0.5MHz for moderate changes in MB oscillation amplitude. To test this hypothesis, we compared 0.5MHz and 1.5MHz frequencies with Definity bubble using the same focal pressure (175kPa PNP; the insertion loss from the skull is accounted). Interestingly, we observed a similar level of K^trans for both groups but significant upregulation of transcripts that are associated with mechanosensation and the expression of molecules in the vessels that are sensitive to shear stress (1.54-fold, p=0.048), ICAM-1, at the 1.5MHz group compared to 0.5MHz. For all of our experiments, less than 1% broadband emissions were observed during sonication. Together, our findings indicate that microbubble shell properties and ultrasound excitation frequency can be fine-tuned to modulate BBB permeability and activate mechanosensitive genes.


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