Acoustic spectroscopy for the characterization of doxorubicin-loaded liposomes

Acoustic spectroscopy is a powerful technique for materials characterization, yet underestimated in pharmaceutical field. In particular, it measures the variation of ultrasound parameters as sound speed and attenuation when an ultrasound wave passes through a materials. Measurements of sound speed or attenuation at a single low-ultrasound frequency in function of temperature are able to detect thermal transitions. Instead, the dependence of attenuation over a broad-range of frequency (generally 1-100 MHz) is employed to obtain information about particle size and volume fraction of samples (Povey M.J.W.,2006). Several works report the use of acoustic spectroscopy for the characterization of the size and thermal transition of disperse systems as suspension, emulsion or microemulsion (Bonacucina G. et al, 2013; Duhkin A.S., 2001). Liposomes are nano-sized double-bilayer particles of large interest in drug delivery. A correspondence in the temperature of phase transitions (Tm) obtained from microcalorimetry and ultrasound spectroscopy for liposomes prepared from different phospholipids was found (Taylor T.M. et al, 2005). In this work attenuation data from high-resolution ultrasonic spectroscopy in function of temperature were compared with heat flow (microcalorimetry) and counts (dynamic light scattering, DLS) in order to highlight the thermal transitions of liposomes made up of a complex mixture of PEGylated and not-PEGylated phospholipids. Moreover, ultrasound spectroscopy was also used to measure the average size of liposomes in comparisonwith DLS, the most common technique for particle size determination.