Investigating the Role of Lipid Transfer in Microbubble-Mediated Drug Delivery

Abstract

Sonoporation, the permeabilization of cell membranes following exposure to microbubbles 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 the 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-2hydroxy-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 (1 MHz, 250 kPa peak negative pressure, 1 kHz pulse repetition frequency, 10% duty cycle, 15 s 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 exposure. These findings indicate that microbubbles can be engineered to exploit lipid exchange between microbubble shells and cell membranes to enhance drug delivery, a new optimization route that may lead to enhanced therapeutic efficacy of ultrasound-mediated treatments.

Microbubbles Containing Lysolipid Enhance Ultrasound‐Mediated Blood–Brain Barrier Breakdown In Vivo

“…In the previous in vitro study, it was hypothesized that the observed increase in cell membrane permeability was due the incorporation of the lysolipid into the membrane and the corresponding reduction in the energy required for pore formation and/or increase in pore stability. [ 62 ] As mentioned in the introduction, it is possible that a similar mechanism is responsible for the increase in US‐BBBD, either through pore formation facilitating transcellular transport or tight junction disruption. It was, however, not possible to determine this from the histological work in this study and further investigation would be needed, e.g., in an in vitro BBB system.…”

“…[ 62 ] These Lyso‐MBs have been shown to cause an approximately fivefold increase in the cellular uptake of a model drug in vitro compared to MBs containing no lysolipid (Control‐MBs). [ 62 ] Importantly, these Lyso‐MBs were also shown to be nontoxic in the absence of ultrasound. [ 62 ] The detailed mechanisms of US‐BBBD have yet to be conclusively established, but both transcellular transport and tight‐junction disruption have been proposed [ 59 ] and hence lysolipids might be expected to have a similar effect upon BBB permeability; by enabling either endothelial cell membrane poration or disruption of the lipid structures comprising tight junctions.…”

“…[ 62 ] Importantly, these Lyso‐MBs were also shown to be nontoxic in the absence of ultrasound. [ 62 ] The detailed mechanisms of US‐BBBD have yet to be conclusively established, but both transcellular transport and tight‐junction disruption have been proposed [ 59 ] and hence lysolipids might be expected to have a similar effect upon BBB permeability; by enabling either endothelial cell membrane poration or disruption of the lipid structures comprising tight junctions. The aim of this study was to determine whether or not Lyso‐MBs are capable of increasing US‐BBBD when compared to Control‐MBs.…”

“…[60] Recently, however, it has been shown that lipid molecules transfer from MBs to cell membranes both before and during ultrasound exposure, and that this transfer modifies the structural properties of the cell membranes. [61,62] This effect has been exploited to improve drug delivery by developing MBs (Lyso-MBs) containing lysolipids, whose transfer causes an increase in cell membrane permeability. [62] These Lyso-MBs have been shown to cause an approximately fivefold increase in the cellular uptake of a model drug in vitro compared to MBs containing no lysolipid (Control-MBs).…”

“…[61,62] This effect has been exploited to improve drug delivery by developing MBs (Lyso-MBs) containing lysolipids, whose transfer causes an increase in cell membrane permeability. [62] These Lyso-MBs have been shown to cause an approximately fivefold increase in the cellular uptake of a model drug in vitro compared to MBs containing no lysolipid (Control-MBs). [62] Importantly, these Lyso-MBs were also shown to be nontoxic in the absence of ultrasound.…”