Enhancement of ultrasound-induced apoptosis and cell lysis by echo-contrast agents

To determine the effects of echo-contrast agents (ECAs) on ultrasound (US)-induced apoptosis and cell lysis, human myelomonocytic lymphoma U937 cells in suspension were exposed to 1 MHz continuous waves US for 1 min at an intensity of 0.5, 1.0, 2.0 or 4.0 W/cm² with or without non-shell type ECA, Levovist™ (2 mg/ml), and shell type, Optison™ (1 μl/ml) or YM454 (1 μl/ml). Levovist™ minimally enhanced the US-induced apoptosis at 1.0 W/cm² while Optison™ and YM454 did at 2.0 and 4.0 W/cm², as detected by flow cytometry. Cell lysis was also augmented when Levovist™ was combined with US at 2.0 W/cm², and when Optison™ was combined with US at 2.0 and 4.0 W/cm². YM454 showed the highest rate of enhanced cell lysis at 1.0, 2.0 and 4.0 W/cm². Therefore, this study shows that Optison™ and YM454 are effective in augmenting the US-induced cell killing, but not Levovist™. Another result indicates that cavitation plays a role in the augmented effects and that inertial cavitation appears necessary for Optison™ and YM454 to effect their actions. In addition, results show that the rate of apoptosis is lower in the presence of ECAs with higher free radical scavenging activity, suggesting a possible role for free radicals in apoptosis. These findings suggest that some ECAs have potential to be adjuncts in cases wherein augmented US-induced cell killing is needed, such as in cancer therapy with US.