Factors Affecting Ultrasonic Release from eLiposomes

Liposomes containing emulsion droplets (eLiposomes) were studied as ultrasound-responsive liposomal drug carriers. This paper presents the effects of temperature, eLiposome size, and ultrasound parameters on the ultrasonically actuated release of calcein, to test hypotheses concerning the physics of acoustic droplet vaporization with regard to vapor pressure and Laplace pressure. Small (200 nm) eLiposomes containing 100-nm emulsion droplets were formed and compared with large (800 nm) eLiposomes containing 100- or 450-nm droplets. Calcein release was quantified by spectroscopic methods. Various experiments examined the influence of perfluorocarbon (PFC) droplet size, vesicle size, temperature, PFC composition and vapor pressure, insonation time, and insonation frequency. Results showed that eLiposome samples released significantly more calcein than their conventional liposome counterparts. Surprisingly, temperature (which directly controls vapor pressure) did not have a strong effect on ultrasound-induced calcein release. In general, calcein release decreased with decreasing droplet size, as hypothesized based on Laplace pressure. Release decreased with increased ultrasound frequency if the pressure amplitude and exposure time were maintained constant, indicating that the gas-phase nucleation rate may have an important role in rupture of eLiposomes. Interestingly, when ultrasound of the same mechanical index was applied at two frequencies, the amount of release correlated strongly with the mechanical index.