Background: Echogenic immunoliposomes (ELIP) have great potential for targeted ultrasonic enhancement of atheroma/vascular endothelium. These agents also have potential for regional drug and gene delivery. For clinical use, formulations having optimal activity under physiologic conditions need to be established. Methods: ELIP (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and cholesterol in a 69:8:8:15 mol % ratio) were made by lyophilization in the presence of mannitol. Stability (echogenicity as a function of time) was assessed in phosphate buffered saline and human serum (50%), as well as in the presence of bovine serum albumin (BSA) at 5 g/mL, and human IgG (10 mg/mL) (all at room temperature and 37°C). Ultrasound reflectivity was measured with a 20-MHz intravascular ultrasound catheter and quantified by computer-assisted videodensitometry. Results: Protein had a marked effect on stability; the activity (after 3 hrs, room temperature) of liposomes exposed to serum, albumin, globulin, and no protein, was 90, 60, 30 and 10% respectively. Stability at 37°C was lower, but in the presence of serum, stability was quite good (75%) for up to an hour. Conclusions: These data demonstrate that ELIP provide sufficient time for diagnostic imaging under physiologic conditions. Their ability to target molecular structures and potential to enhance drug and gene delivery expand our diagnostic and therapeutic approach to atherosclerotic cardiovascular disease.
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