Nano-sized ultrasound contrast agent: salting-out method

Tumor imaging by ultrasound is greatly enhanced by the use of ultrasound contrast agents (UCAs), stabilized, gas-filled bodies. They are generally less than 7 microm to pass freely through the capillary bed. Development of a nano-sized agent would enable them to extravasate through the leaky pores of angiogenic vessels. We describe the development of an echogenic, nano-sized polylactic acid UCA by adaptation of a salting-out method. The viscosity of the initial colloidal suspension (concentration and molecular weight of protective colloid [polyvinyl alcohol (PVA)] and concentration of polymer) was key in determining particle size and polydispersity (increasing viscosity increased both). Addition of the porogens ammonium carbonate and camphor, required to produce hollow echogenic capsules, also increased the size (eg, 5 wt% PVA, mean solid nanocapsule size 386 +/- 25 nm, polydispersity index 0.367 +/- 0.14, and mean UCA size 640 +/- 18 nm, polydispersity index 0.308 +/- 0.027). Viscosity had the opposite effect on echogenicity of the resultant nano-UCA, which ranged from 21.69 +/- 0.78 dB for 2% PVA to 12.1 +/- 0.8 dB for 10% PVA. The UCA prepared with 10% PVA, however, had a longer half-life in the ultrasound beam (t(1/2) > 15 minutes vs t(1/2) < 10 minutes), suggesting a thicker shell. Optimization will require compromise among size, echogenicity, and stability.