Development and characterization of a nano-scale contrast agent

Agents injected parenterally must be less than approximately 8 microm diameter in order to traverse the capillaries in the pulmonary bed, but these agents remain in the vasculature until they are eliminated from the body by a variety of mechanisms. Targeting of cells outside the capillaries requires agent diameters of less than approximately 700 nm to enable escape through the larger-than-usual pores that have been noted in the leaky vasculature of a tumor. The objective of this study was to test the feasibility of creating a surfactant-stabilized nano-bubble with favorable acoustic properties, and identify the key parameters that influence size, yield and stability. Size distribution was characterized using laser light scattering. In vitro acoustic enhancement was assessed by generation of dose and time response curves. We previously developed a successful protocol to generate gas-filled microbubbles (containing perfluorocarbon, sulfur hexafluoride or air) with mean diameter of 1.5 microm, using sonication of carefully selected surfactant mixtures. This presentation describes generation of nano-bubbles with mean diameters ranging from 700 to 450 nm, depending on process variables. In all cases a centrifugation step was employed to separate the nano-sized particles. The in vitro dose response curves gave a maximum of 23-27 dB enhancement compared to buffer in the absence of agent, with the maximum enhancement and presence of shadowing at higher doses being dependent on the fabrication protocol. The effect of sonication time for solutions containing a mixture of the surfactants (Span 60 and Tween 80) was also tested, but was determined not to be an influencing factor. Future studies will involve development of a mathematical model characterizing the mean size as a function of centrifugal force, spin time and initial size distribution. Future work will also include imaging of tumor-bearing mice and measuring imaging potential in vivo in New Zealand white rabbits using power Doppler.