Physical and biochemical characterization of Albunex, a new ultrasound contrast agent consisting of air‐filled albumin microspheres suspended in a solution of human albumin

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Abstract
Albunex is a new ultrasound contrast agent for medical imaging. The product consists of air-filled albumin microspheres suspended in a solution of 5% (w/v) human albumin. The suspension is sterile, non-pyrogenic and isotonic, with a pH of 7.0 and a viscosity of 1.4 relative to water. The contrast effect is caused by the air-filled microspheres, which range in diameter from 1 to 15 microns, with less than 5% being larger than 10 microns. The product contains a total of about 7 x 10(8) microspheres/ml of suspension. The number concentration of microspheres with diameters between 4 and 10 microns is about 2 x 10(8)/ml. The latter microsphere fraction is assumed to give the main contribution to the ultrasound signal in the left ventricle of the heart after intravenous injection. The air-filled microspheres are prepared by sonication of a heated solution of 5% (w/v) human albumin. During the sonication process, microbubbles of air are formed which become encapsulated in a thin shell of aggregated albumin about 15 nm in thickness. Due to the stabilizing effect of the albumin shell, the air-filled microsphere suspension is stable for at least 2 years when stored refrigerated. The microsphere protein represents about 1.5% of the total protein in the suspension. The remaining protein is soluble albumin molecules which behave like the albumin molecules in the starting material when analysed by a number of biochemical techniques.

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