546. Targeted Non-Viral Gene Delivery Using Microbubbles and Focused Ultrasound
Authors: Ahad Rahim, Sarah L. Taylor, Nigel L. Bush, Gail R. ter Haar, Jeff C. Bamber, Colin D. Porter
Journal: Molecular Therapy (2006)

Ultrasound/microbubble-mediated gene delivery has the potential to target delivery to tissue deep in the body by the ability to target the ultrasound beam subsequent to administration of the vector. Application of this technology in vivo would be minimally invasive and relatively straightforward given that ultrasound and microbubble contrast agents are well-documented utilities in the clinic. Gene delivery in the focal plane of an ultrasound beam offers the potential for spatially targeting delivery. No assessment of the spatial localisation of gene delivery in the focal plane has been conducted and so we perform high-resolution analysis of the spatial distribution and relate this to measurements of the acoustic field to evaluate the efficiency and targeting ability of this technology. The gene delivery efficiency after exposure of cells in the focal plane was compared to cells exposed in the area close to the 1MHz transducer face (an area we refer to as the 'homogeneous near-field') in a custom built tank. We first characterised the acoustic field in the focal plane and in the near homogeneous field and showed a marked difference between the two areas. Gene delivery efficiency when cells were exposed to equivalent acoustic pressures in the homogeneous near-field or in the focal plane were similar, and cell viability remained high. High-resolution analysis showed spatial distribution of gene delivery after exposure in the focal plane was different from that in the homogeneous near-field. In the latter, gene delivery was confined to the area directly adjacent to the transducer face and was relatively homogeneous across the face of the transducer, consistent with the dosimetry at this field position. Gene delivery in the focal plane was highest close to the focus but decreased with increasing distance, broadly consistent with the dosimetry profile but with some notable discrepancies. Thus, gene delivery at the focal distance was spatially targeted. We then examined the relationship between pulse amplitude and gene delivery efficiency. The general trend was an increase in gene delivery efficiency with increasing pressure above a threshold of 0.1MPa, below which negligible gene delivery was observed. However, whilst monotonic increases in gene delivery efficiency were observed as pressure increased from 0.1MPa to 0.3MPa, and again from 0.3MPa to 1.4MPa, there was a marked discontinuity at 0.3MPa and apparently marks the boundary between two different phenomena in terms of acoustic pressure but was not evident in terms of radial distance. This observation suggests that a factor other than pressure amplitude that varies continuously with distance is important. Other non-viral approaches are unable to target delivery at the resolution achieved at the focal plane of the ultrasound field without using more invasive approaches. Given the improved understanding of the acoustic pressure-dependence of microbubble-mediated gene delivery that our data provide it becomes possible to determine the resolution of targeting within a given area by manipulating transducer design to generate the required ultrasound field.