Mechanisms of the increase in the permeability of the blood–tumor barrier obtained by combining low-frequency ultrasound irradiation with small-dose bradykinin

Abstract

The research was conducted to study the characteristics of the noninvasive, reversible, targeted opening of the blood-brain barrier (BBB) by use of low-frequency ultrasound (LFU) irradiation and the selective opening of the blood-tumor barrier (BTB) by intracarotid infusion of bradykinin (BK) in small-dose, with the objective of exploring maximum opening of the BTB by combining LFU irradiation with BK infusion. Thus, it provides new therapeutic strategies for targeted transport of macromolecular or granular drugs to the brain. By using the rat C6 glioma model it was shown that extravasation of Evans blue (EB) through the BTB was significantly increased by combining LFU irradiation (frequency = 1.0 MHz, power = 12 mW, duration = 20 s) with intracarotid small-dose BK infusion, compared with utilizing the two methods separately. By transmission electron microscopy (TEM) we observed that this combination significantly increased the number of pinocytotic vesicles of brain microvascular endothelial cells (BMECs) in the BTB. An even more significant increase was observed by using RT-PCR, western blot, immunohistochemistry, and immunofluorescence to detect mRNA and changes of expression of the caveolae structure proteins caveolin-1 and caveolin-2 of BMECs. In summary, this research concludes that LFU irradiation and small-dose BK together selectively enhance the permeability of the BTB and increase the number of pinocytic vesicles of BMECs to a maximum. Significant up-regulation of the level of expression of caveolae structure proteins caveolin-1 and caveolin-2 might be the molecular mechanism of the co-enhanced endocytotic transport by BMECs. Thus, this research provides new therapeutic strategies for targeted transport of macromolecular drugs and the design of drugs.