Clot Retraction Affects the Extent of Ultrasound-Enhanced Thrombolysis in an Ex Vivo Porcine Thrombosis Model

Abstract

Using an FDA-approved contrast agent (Definity®) and thrombolytic drug (rt-PA), we investigated ultrasound-enhanced thrombolysis in two whole-blood clot models. Porcine venous blood was collected from donor hogs and coagulated in two different materials. This method produced clots with differing compositional properties, as determined by routine scanning electron microscopy and histology. Clots were deployed in an ex vivo porcine thrombolysis model, while an intermittent ultrasound scheme previously developed to maximize stable cavitation was applied and acoustic emissions were detected. Exposure of clots to 3.15 μg/mL rt-PA promoted lysis in both clot models, compared to exposure to plasma alone. However, in the presence of rt-PA, Definity®, and ultrasound, only unretracted clots experienced significant enhancement of thrombolysis compared to treatment with rt-PA. In these clots, microscopy studies revealed loose erythrocyte aggregates, a significantly less extensive fibrin network, and a higher porosity, which may facilitate increase penetration of thrombolytics by cavitation.

Ultrasound-mediated drug delivery for cardiovascular disease

“…However, these improvements were modest (54.5 vs 41% with rt-PA/US). Numerous investigators offer reasons for the absence of recanalization improvement in some patients, including inconsistent application of acoustic energy [185] and thrombus immunity to rt-PA lysis due to compositional factors [186,187]. Other investigators cite differences between recanalization and reperfusion, a metric that correlates better with clinical outcome [188].…”

“…While sonothrombolysis investigators have largely discounted thermal mechanisms [189], fibrin degradation product removal, fibrinolytic drug or microbubble penetration due to microstreaming and acoustic radiation force have been proposed as contributing factors (Figure 5). The mechanical and biochemical properties of the occlusive thrombus may also contribute to the degree of US enhancement of lysis [187,223,224]. Further research in this area will continue to refine the acoustic parameters necessary to develop efficient imaging and therapeutic US systems for sonothrombolysis.…”

Kinetics and mechanics of clot contraction are governed by the molecular and cellular composition of the blood

“…9 However, the role of clot contraction in the development of thrombotic vascular occlusion and resistance to endogenous and exogenous thrombolysis has only recently been explored. 10 Clot contraction is driven by platelet-generated contractile forces 5 that are propagated through the platelet-fibrin meshwork 11 and result in the expulsion of serum. 4 Platelets contract because of the interaction of actin 5 and nonmuscle myosin IIa, which is initiated when platelets are activated by various stimuli, including thrombin.…”

Loss of Echogenicity and Onset of Cavitation from Echogenic Liposomes: Pulse Repetition Frequency Independence

“…Stable cavitation of microbubbles facilitates the lysis of clots (Datta et al 2008; Sutton et al 2013b) and enhances the permeability of tissue for drug delivery (Sutton et al 2013a; Hitchcock et al 2010). Stable cavitation is characterized by subharmonic, ultraharmonic, and harmonic acoustic emissions from microbubbles (Eller and Flynn 1969; Neppiras 1969).…”