| OBJECTIVETo investigate the effects of targeted and reversible disruption of the blood–brain barrier (BBB) by MRI-guided focused ultrasound (FUS) and delivering methotrexate (MTX) to the rabbit brain.METHODSThe brains of 20 rabbits were sonicated by MRI-guided FUS at different exposure times(2s-10s), the sum of sonicated sites was 50,and the sites were devidited into 5 groups( group 2s, 4s, 6s, 8s and 10s),then Evans blue extravasation , contrast-enhanced MRI scans and histologic examination were performed to determine the optimum exposure time for reversible BBB disruption with minimal damage. Five rabbits were sonicated at the optimum exposure time(6s)after MTX was injected intravenously, 1 hour later, the targeted locations were included in the sonicated group, and the nontargeted contralateral counterparts were included in the intravenous (IV) control group. Five other rabbits were not subjected to sonication, and were administered internal carotid artery (ICA) injections of MTX, 1 hour later, the specimens of the counterpart brain tissue were harvested as the ICA group. The MTX concentration in all the specimens was determined by high-performance liquid chromatography (HPLC).RESULTS1. Contrast-enhanced MRI: The MR contrast scans were performed immediately after the sonication and were repeated 4, 8, 12, and 24 hours later. When the exposure time was less than 6s, there was no contrast enhancement could be observed. But when it was 6s or more, all the sonicated sites showed evidence of contrast enhancement immediately after the sonication,and the signal intensity change was still great 4 hours later. When it was 6s, the contrast enhancement could not be detected 8 hours after the sonication, indicating that the BBB was open for less than 8 hours . When the exposure time was 8s, the signal intensity enhancement could still be found 24 hours after the sonication. When sonication was performed for 10s, a continuous and noted contrast enhancement was detected within 24 hours.2. Evans blue staning: Immediately after sonication, no visible blue stain was noted in the brain specimens exposed to 2s or 4s of sonication, however, nearly all the sonicated sites stained positively for EB when the exposure time was 6s or more. EB extravasation was more broadly distributed and darker when sonication was performed for longer durations, and these findings correlated well with the results of MRI contrast enhancement. Furthermore, the EB extravasation decreased as a function of time. Twenty-four hours after sonication, EB staining could only be detected at the sites exposed to sonication for 8s and 10s, and the extent of staining was substantially less.3. Hematoxylin and eosin histologic analysis at the targeted focal locations: Immediately after sonication, the histologic findings of the sonicated sites with exposure time ranging from 2 to 10s showed when the exposure time was 2 and 4s, there was no damage and extravasation in the sonicated sites, and the findings of these sites differed little from those of the nonsonicated sites.When it was 6s, a few scattered erythrocytes were observed, and no obvious signs of parenchymal damage were detected. When it was 8s: pronounced extravasation of erythrocytes around microvessels was detected, and signs of vasodilatation, stasis, and destruction were noted in the vessel walls. Parenchymal damage was mostly in the form of ischemic changes, and slight vacuolation of the neuropil was seen. When it came to10s: multiple areas of blood vessel destruction and extensive erythrocyte extravasations into brain parenchyma were noted, the brain tissue was edematous with fragmented axons, the vacuolation, local tissue necrosis, and neutrophil infiltration were observed.4. MTX concentrations for all the groups :MTX levels in all specimens were determined by using an HPLC. The MTX concentrations in the sonicated group, the IV control group and the ICA group were 7.412±1.471ug/g tissue,0.544±0.084ug/g tissue and 1.984±0.650ug/g tissue respectively. Significant differences were observed between the 3 groups. MTX concentration in the nonsonicated control brain tissue after IV administration of the drug was significantly lower than that after ICA injection (P < 0.05), however, the MTX concentration in the sonicated group was notably higher than that of both the other groups(P < 0.01).CONCLUTION1. MRI-guided focused ultrasound can induce a targeted and reversible disruption of the BBB while the neuronal damage was found to be minimal when the optimum exposure time was 6 s.2. MRI-guided FUS can disrupt the BBB reversibly and deliver the intravenously administered MTX to the targeted brain locations; the application of this method brings about a substantial increase in the drug level in the targeted brain tissue, and is much more effective than drug delivery through the ICA without sonication. This may facilitate the development of improved treatment methods for CNS disorders. |
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