Microbubbles in ultrasound-triggered drug and gene delivery

Abstract

Ultrasound contrast agents, in the form of gas-filled microbubbles, are becoming popular in perfusion monitoring; they are employed as molecular imaging agents. Microbubbles are manufactured from biocompatible materials, they can be injected intravenously, and some are approved for clinical use. Microbubbles can be destroyed by ultrasound irradiation. This destruction phenomenon can be applied to targeted drug delivery and enhancement of drug action. The ultrasonic field can be focused at the target tissues and organs; thus, selectivity of the treatment can be improved, reducing undesirable side effects. Microbubbles enhance ultrasound energy deposition in the tissues and serve as cavitation nuclei, increasing intracellular drug delivery. DNA delivery and successful tissue transfection are observed in the areas of the body where ultrasound is applied after intravascular administration of microbubbles and plasmid DNA. Accelerated blood clot dissolution in the areas of insonation by cooperative action of thrombolytic agents and microbubbles is demonstrated in several clinical trials.

Figure 1

Schematic representation of lipid (left column) and polymer (right column) microbubble interaction with ultrasound of increasing intensity (top to bottom).

Figure 2

Optical frame images and streak image corresponding to the oscillation and fragmentation of a contrast agent microbubble, where fragmentation occurs during compression. The bubble has an initial diameter of 3 μm, shown in a. The streak image in h shows the diameter of the bubble as a function of time, and dashed lines indicate the times at which the two-dimensional frame images in a-g were acquired relative to the streak image. (Reproduced with permission from ref. [14], Copyright, 2000, American Institute of Physics).

Figure 3

Optical frames showing initial PB127 microsphere (frame 1), shell fissure and gas escape (frames 4,8 to 11) under ultrasound of 1.7MHz, four cycles, MI of 1.4 and formation of new free bubbles. Two new free bubbles (frames 41, 47 to 60) demonstrating oscillations under ultrasound of 1.7 MHz, four cycles and MI of 0.25. Last displayed frame shows an optical recording taken 40 ms later, demonstrating bubble disappearance due to dissolution. (Reproduced with permission from ref. [146], Copyright, 2005, Elsevier)

Figure 4

Schematic representation of loading strategies of drugs and genes on microbubbles. (A) Non-covalently binding of DNA to the surface of cationic lipid microbubbles. (B) Multilayered structure based on a lipid microbubble sequentially coated with DNA and poly-L-lysine layers. Polymeric microbubbles with central hollow core surrounded by a thick polymeric shell. (C) Polymeric microbubble loaded with hydrophobic drug loaded in the shell phase. (D) Polymeric microbubble with hydrophilic drug loaded in the internal void. (E) Internal structure of polymeric microbubbles: water-phase is dispersed through the polymer matrix, forming upon lyophilization a plurality of cavities distributed over the particle volume. (F) Attachment of liposomes or nanoparticles to the surface of microbubbles through biotin-avidin-biotin bridging system. The drawing is not to scale.

Figure 5

Intravital microscopy of the cremaster muscle in mice following intravascular injection of cationic microbubbles bearing YOYO-1-labeled plasmid. (A) Cremaster microcirculation 1 min after IV injection of microbubbles illustrating stable conjugation of plasmid to microbubbles during microvascular transit. All microbubbles in the field transited unimpeded. (B) Microvessel rupture and peri-vascular hemorrhage with fluorescent DNA deposition at a site of microbubble destruction. (C,D) Deposition of fluorescent plasmid DNA in the perivascular tissue of cremasteric venules following US-mediated destruction of microbubbles without vessel rupture. Scale bar - 20 μm. (Reproduced with permission from ref. [110], Copyright, 2003, Elsevier).