Background: In this work, we report a novel targetable ultrasonic contrast agent with the potential to noninvasively define and localize myriad pathological tissues for diagnosis or therapy. The agent is a biotinylated, lipid-coated, perfluorocarbon emulsion that has low inherent echogenicity unless bound to a surface or itself. Methods and Results: In study 1, emulsions with and without biotin were suspended in buffered saline and imaged with a 7.5-MHz linear-array transducer. Neither emulsion manifested significant ultrasonic backscatter until avidin was added. Avidin-induced aggregation produced a marked enhancement in backscatter from the biotinylated but not from the control emulsion. In study 2, porcine fibrin clots in vitro were pretargeted with biotinylated antifibrin monoclonal antibodies and then exposed to avidin and then to biotinylated or control perfluorocarbon emulsions. The basal acoustic reflectivity of clots imaged with a 7.5-MHz linear-array transducer was uniformly low and was increased substantially by exposure to the targeted biotinylated emulsion. In study 3, partially occlusive arterial thrombi were created in dogs and then exposed to antifibrin antibodies and avidin in situ. Biotinylated or control emulsion was administered either in situ or systemically. At baseline, all thrombi were undetectable with a 7.5-MHz linear-array transducer. Thrombi exposed to antifibrin-targeted contrast exhibited increased echogenicity (P<.05); control thrombi remained acoustically undetectable. Conclusions: These data provide the first in vivo demonstration of a site-specific ultrasonic contrast agent and have potential for improved sensitivity and specificity for noninvasive diagnosis of thrombi and other pathological diseases.
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