Session: P2A

An exciting recent development in ultrasound microbubble contrast agent technology is their extension from non-specific diagnostic tools to targeted and therapeutic agents. This process involves the addition of large molecules such as ligands or DNA segments to the microbubbles. The aim of this work was to evaluate the effects of these additional components on the acoustic properties and the stability of microbubbles. Novel encapsulated microbubbles were produced in our laboratory with shell materials containing albumin or phospholipid. In each case both targeted, conjugated bubbles and simple unconjugated versions were manufactured. Laboratory made microbubbles were compared to similar commercially produced microbubbles. The attenuation coefficient was measured with a broadband single element transducer using a substitution approach and the backscatter was measured with a Toshiba PowerVisionr 8000. Samples of each bubble type were sized from photographs taken through a standard light microscope. The unconjugated laboratory bubbles were found to behave comparably to their commercial equivalents. For example, albumin based bubbles were of equal stability and echogenicity to Optisonr. After conjugation albumin bubbles showed significantly lower echogenicity after 5 minutes of exposure. Further investigation with bubbles conjugated with plasmid DNA with additional cross-linking of the albumin shell was found to improve both the echogenicity and the stability of the bubbles. The backscatter was no longer significantly different to the unconjugated equivalent [33±6dB (mean±1s.d.) unconjugated versus 38±8dB conjugated at 3MHz with MI=0.2]. While the stability, measured from temporal changes in the attenuation spectra, improved from a half-life of 300±20s for the control sample to 520±10s for conjugated bubbles. For the lipid bubbles improvement in stability was also recorded upon conjugation with PEG-COOH. This work demonstrates that the acoustic characterisation of novel bubbles is crucial in refining the manufacturing process. The results presented show that conjugation of large molecules into the bubble