Observations on the viability of C6-glioma cells after sonoporation with low-intensity ultrasound and microbubbles

Ultrasound (US) and microbubbles can be used to facilitate cellular uptake of drugs through a cavitationinduced enhancement of cell membrane permeability. The mechanism is, however, still incompletely understood. A direct contact between microbubbles and cell membrane is thought to be essential to create membrane perturbations lasting from seconds to minutes after US exposure of the cells. A recent study showed that the effect may even last up to 8 h after cavitation (with residual permeability up to 24 h after cavitation). In view of possible membrane damage, the purpose of this study was to further investigate the evolution of cell viability in the range of the 24-h temporal window. Furthermore, a description of the functional changes in tumor cells after US exposure was initiated to obtain a better understanding of the mechanism of membrane perturbation after sonication with microbubbles. Our results suggest that US does not reduce cell viability up to 24 h post-exposure. However, a perturbation of the entire cell population exposed to US was observed in terms of enzymatic activity and characteristics of the mitochondrial membrane. Furthermore, we demonstrated that US cavitation induces a transient loss of cell membrane asymmetry, resulting in phosphatidylserine exposure in the outer leaflet of the cell membrane.

I. Introduction

IN the field of ultrasound (US)-mediated local drug delivery, US is used to enhance the delivery and activity of therapeutic agents such as nucleic acids, proteins, and chemotherapeutic drugs. Its advantages over other types of radiation are that the US beam can reach tissues deep inside the body and that it can be focused in a small region of interest without harming tissue within its beam path [1], [2].