Role of freeze-drying in the presence of mannitol on the echogenicity of echogenic liposomes.

Echogenic liposomes (ELIPs) are an excellent candidate for ultrasound activated therapeutics and imaging. Although multiple experiments have established their echogenicity, the underlying mechanism has remained unknown. However, freeze-drying in the presence of mannitol during ELIP preparation has proved critical to ensuring echogenicity. Here, the role of this key component in the preparation protocol was investigated by measuring scattering from freshly prepared freeze-dried aqueous solution of mannitol-and a number of other excipients commonly used in lyophilization-directly dispersed in water without any lipids in the experiment. Mannitol, meso-erythritol, glycine, and glucose that form a highly porous crystalline phase upon freeze-drying generated bubbles resulting in strong echoes during their dissolution. On the other hand, sucrose, trehalose, and xylitol, which become glassy while freeze-dried, did not. Freeze-dried mannitol and other crystalline substances, if thawed before being introduced into the scattering volume, did not produce echogenicity, as they lost their crystallinity in the thawed state. The echogenicity disappeared in a degassed environment. Higher amounts of sugar in the original aqueous solution before freeze-drying resulted in higher echogenicity because of the stronger supersaturation and crystallinity. The bubbles created by the freeze-dried mannitol in the ELIP formulation play a critical role in making ELIPs echogenic.