Fibrin targeting of tissue plasminogen activator-loaded echogenic liposomes

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Abstract
We recently reported entrapment of tissue-plasminogen activator (tPA) into echogenic liposomes (ELIP) with retention of echogenicity and thrombolytic effect. Integral to the potential of this agent for ultrasound-detectable local drug delivery is the specific binding of tPA–ELIP to clots. tPA contains fibrin-binding sites; we hypothesized that tPA when associated with ELIP, will maintain fibrin binding properties, rendering further manipulation for targeting of the tPA–ELIP unnecessary. We demonstrated strong fibrin binding of the ELIP-associated tPA. Fibrin binding for ELIP-associated tPA was twice that of free tPA. This strong affinity for fibrin was confirmed using echogenicity analysis of porcine clots in vitro. Both objective (mean gray scale analysis) and subjective (visual estimation by two experienced echocardiographers) evaluation of the clots showed enhanced highlighting of clots treated with tPA–ELIP when compared to control. The findings in this study represent new approaches for fibrin-targeted, ultrasound-directed and enhanced local delivery of a thrombolytic agent.