An ultrasound-responsive nano delivery system of tissue-type plasminogen activator for thrombolytic therapy.

This study is undertaken to design a novel nano-sized delivery system of tissue-type plasminogen activator (t-PA) which has a suppressed thrombolytic activity of t-PA, but recovered the activity only when exposed to ultrasound. Various amounts of ethylenediamine were chemically introduced into gelatin (cationized gelatins) to complex with t-PA. To modify the surface of complexes with polyethylene glycol (PEG), PEG was chemically grafted to the anionic gelatin (PEG-gelatin). The simple mixing with the PEG-gelatin enabled the t-PA-cationized gelatin complex to form a nano-sized delivery complex with PEG chains on the surface. The t-PA activity of PEG-modified complexes was significantly suppressed to be 45% of original t-PA. However, when exposed to ultrasound in vitro, the t-PA activity was fully recovered. A cell culture experiment demonstrated no cytotoxicity of PEG-modified complexes. The body distribution study indicated that the half-life of t-PA in the blood circulation was prolonged about 3 times. In a rabbit thrombosis model, the intravenous administration of PEG-modified complexes followed by ultrasound irradiation resulted in complete recanalization, in remarked contrast to the complex administration alone. It is concluded that the PEG-modified complex is a promising t-PA delivery system to enhance the biological activity at the site necessary only by a local ultrasound irradiation.