751. Magnetic Microbubbles: New Carriers for Localized Gene and Drug Delivery

In order to overcome limitations to gene delivery and therapy associated with insufficient vector accumulation at target sites and to increase drug targeting efficiency, we combined the advantages of magnetic drug targeting with microbubble technology. Microbubbles are currently used clinically as contrast agents in ultrasound (US) diagnostics and experimentally as drug or nucleic acid carriers. Localized delivery, drug release and tissue penetration can be achieved by local application of ultrasound leading to microbubble disruption. We have developed perfluoropropane-filled microbubbles containing a high load of superparamagnetic nanoparticles along with an active agent (nucleic acid or drug) in a lipid or denatured albumin shell. For this purpose, magnetic nanoparticles with suitable surface coatings, such as detergents, are required. The magnetic retention of the bubbles at a given flow rate is tremendously improved compared to the same quantity of magnetic nanoparticles in suspension. In cell culture, magnetic microbubbles are sedimented on target cells by magnetic force and bubble disruption and gene delivery is triggered by the application of US of 1 MHz. Biodistribution experiments upon tail vein injection in mice demonstrated the highest nucleic acid deposition when a magnetic field was applied to a target area such as a lung lobe in combination with ultrasound. Similarly, using a dorsal skin chamber model in mice and intravital fluorescence microscopy, we have demonstrated the localized delivery of plasmid DNA, synthetic antisense oligonucleotides and siRNA to the vasculature and surrounding tissue within the chamber by applying a suitable magnetic gradient field to the target area in combination with US. Without magnetic field or US application, no localized delivery was feasible. We have demonstrated the therapeutic potential of the technique with subcutaneous administration of magnetic microbubbles carrying the VEGF gene in a skin flap model. The combination of magnetic field and ultrasound yielded the highest blood perfusion and tissue survival of the skin flap area in comparison to controls with reporter genes or with the VEGF gene in the absence of magnetic field and/or ultrasound. Our results indicate that magnetic microbubbles are highly promising drug carriers that can be remote-controlled within the blood circulation by two independent physical forces. Magnetic microbubbles loaded with appropriate agents can considerably improve drug deposition and the site specificity of delivery. In particular, magnetic microbubbles may offer a combination of therapeutic intervention with molecular imaging (US diagnostics, MRI).