Terry O. Matsunaga

Application of Ultrasound to Selectively Localize Nanodroplets for Targeted Imaging and Therapy

Lipid-coated perfluorocarbon nanodroplets are submicrometer-diameter liquid-filled droplets with proposed applications in molecularly targeted therapeutics and ultrasound (US) imaging. Ultrasonic molecular imaging is unique in that the optimal application of these agents depends not only on the surface chemistry, but also on the applied US field, which can increase receptor-ligand binding and membrane fusion. Theory and experiments are combined to demonstrate the displacement of perfluorocarbon nanoparticles in the direction of US propagation, where a traveling US wave with a peak pressure on the order of megapascals and frequency in the megahertz range produces a particle translational velocity that is proportional to acoustic intensity and increases with increasing center frequency. Within a vessel with a diameter on the order of hundreds of micrometers or larger, particle velocity on the order of hundreds of micrometers per second is produced and the dominant mechanism for droplet displacement is shown to be bulk fluid streaming. A model for radiation force displacement of particles is developed and demonstrates that effective particle displacement should be feasible in the microvasculature. In a flowing system, acoustic manipulation of targeted droplets increases droplet retention. Additionally, we demonstrate the feasibility of US-enhanced particle internalization and therapeutic delivery.

Gene Delivery with Ultrasound and Microbubbles

Gene therapy and treatment with siRNA hold potential to treat a wide variety of different diseases. Genetic material is not usually stable and is generally hydrolyzed following intravascular administration. This makes the delivery of genes somewhat problematic since intact genetic material must usually be delivered intracellularly for therapeutic effect. For gene therapy, in most cases the gene construct must reach the cellular intranuclear subcompartment in order to elicit the desired biological effect. Viruses have evolved to deliver DNA and RNA to cells but viral vector-based gene therapy has been associated with effects inherent in biological systems (Marshall, Muruve, Thomas). Non-viral based systems afford the potential to deliver genetic materials without the adverse biological effects of viral-based systems. In most cases, however, non-viral based gene delivery systems have been less effective than viral based systems, yielding lower levels of gene expression (Litzinger). Microbubbles, e.g. acoustically active carriers, in concert with ultrasound (Unger, Zhou) may afford potential for highly effective site directed gene therapy and delivery of other genetic materials such as siRNA (Zhigang).

Synthesis, Characterization, and Calorimetric Studies of a Series of Novel Bioconjugates for Selective Targeting of Microbubbles to GPIIbIIIa Receptors on Vascular Thrombi

Bioconjugate ligands have been used in our laboratory to target microbubbles to glycoprotein receptors for both diagnostic and therapeutic purposes. The purpose of the bioconjugates (Figure 1) are to: 1) anchor the molecule into the microbubble membrane, 2) provide overall flexibility for the ligand to “find” its target [1], and 3) provide a ligand selective for certain receptors [2]. However, in order for ligands to effectively direct the entire microbubble to a selective receptor, the bioconjugate must remain inserted into the microbubble membrane. Failure to do so could result in “free” bioconjugates acting as competitive receptor antagonists to microbubble or liposome binding. We have thus conducted a calorimetric study to determine the efficiency of insertion of a series of bioconjugates varying only in hydrocarbon tail length.