Preparation, characterization and in vivo observation of phospholipid-based gas-filled microbubbles containing hirudin.

The objective of this work was to prepare echogenic phospholipid-based gas-filled microbubbles (PGM) and investigate their physical characteristics, echogenicity and loading ability of hirudin under various NaCl concentrations. PGM were prepared by a sonication-lyophilization method. Hirudin was used as a model drug to evaluate the drug encapsulation efficiency of the PGM. PGM loaded with hirudin were prepared by dissolving lyophilized powder with hirudin solution. The morphology, particle size and microbubble concentration of PGM were measured. The hirudin encapsulation efficiency as a function of NaCl concentration was determined. The mean particle size and microbubble concentration of PGM were unchanged by the presence of hirudin for at least 60 min after preparation. Hirudin encapsulation quantity was proportional to the hirudin concentration until saturation occurred at high concentration, and the encapsulation efficiency had an inverse relationship. Hirudin encapsulation efficiency was affected by NaCl concentration. When NaCl concentration was increased from 10 mg mL(-1) to 20 mg mL(-1) in PGM solution, hirudin encapsulation efficiency decreased from 35.8 to 26.7%, and microbubble concentration decreased from 2.7 x 10(8) to 1.7 x 10(8) microbubbles per mL. The PGM were shown easily to be visible in in vivo rabbit liver. There was no difference in echogenicity between the loaded and unloaded bubbles. PGM prepared by the sonication-lyophilization method exhibited satisfactory physical characteristics and loading ability and are suitable for use in imaging and ultrasound-triggered delivery.