Investigating the role of lipid transfer in microbubble mediated drug delivery.

Abstract

Sonoporation, the permeabilization of cell membranes following exposure to microbubbles (MBs) and ultrasound, has considerable potential for therapeutic delivery. To date, engineering of microbubbles for these applications has focused primarily upon optimizing microbubble size and stability, or attachment of targeting species and/or drug molecules. In this work, it is demonstrated that the microbubble coating can also be tailored to directly influence cell permeabilization. Specifically, lipid exchange mechanisms between phospholipid microbubbles and cells can be exploited to significantly increase sonoporation efficiency in vitro. A theoretical analysis of the energy required for pore formation was carried out. From this it was hypothesized that sonoporation could be promoted by transfer of lipid molecules with appropriate carbon chain length and/or shape (cylindrical or conical). Spectral imaging with a hydration-sensitive membrane probe (C-Laurdan) was used to measure changes in the membrane lipid order of A-549 cancer cells following exposure to suspensions of different phospholipids. Two candidate lipids were identified, a short chain length phospholipid (1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)) and a medium chain length lysolipid (1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (16:0 Lyso-PC)). Microbubbles were prepared with matched concentrations, size distributions and acoustic responses. Confocal microscopy was used to measure cell uptake of a model drug (propidium iodide) with and without ultrasound exposure (1MHz, 250kPa peak negative pressure, 1kHz pulse repetition frequency, 10% duty cycle, 15s exposure). Despite significantly decreasing the cell membrane lipid order, DLPC did not increase sonoporation. Microbubbles containing 16:0 Lyso-PC, however, produced a ~5-fold increase in sonoporation compared to control microbubbles. Importantly the Lyso-PC molecules were incorporated into the microbubble coating and did not affect cell permeability prior to ultrasound expsosure. These findings indicate that microbubbles can be engineered to exploit lipid exchange between microbubble shells and cell membranes to enhance drug delivery, an exciting new optimization route that may lead to enhanced therapeutic efficacy of ultrasound mediated treatments.