Enhancement of enzymatic fibrinolysis with 2-MHz ultrasound and microbubbles

Background: Microbubbles used for echo-contrast agents accelerate enzymatic fibrinolysis of clots exposed to low-frequency ultrasound (US). It is not known,whether microbubbles are also effective in enhancing high-frequency US-driven enzymatic fibrinolysis. Methods and results: Calibrated whole blood clots were exposed to US, or US and galactose-based microbubbles (Levovist((R))), with or without recombinant tissue plasminogen activator (rt-PA) in an in-vitro flow system. We used low-intensity, 2-MHz, pulsed wave US. Relative weight reduction of clot +/- SD was 30.7 +/- 9.5% after exposure to microbubbles. rt-PA and US, 13.1 +/- 2.6% after exposure to rt-PA and US, 10.9 +/- 3.6% after exposure to microbubbles and US, and 6.1 +/- 1.9% after exposure to US alone. ANOVA demonstrated a significant effect of rt-PA (P = 0.001), microbubbles (P = 0.012), and interaction of both (P = 0.022). Conclusions: The application of galactose-based microbubbles (Levovist((R))) strongly accelerates lysis of clots exposed to 2 MHz, low-intensity US in vitro both with and without rt-PA. The findings suggest a synergy between microbubbles and rt-PA. These methods routinely used for transcranial diagnostic applications have the potential to improve the efficacy of intravenous rt-PA in acute ischemic stroke.