Comparative lytic efficacy of rt-PA and ultrasound in porcine versus human clots

Changes in Pulmonary Vascular Resistance and Obstruction Score Following Acute Pulmonary Embolism in Pigs

OBJECTIVES

To investigate the contribution of mechanical obstruction and pulmonary vasoconstriction to pulmonary vascular resistance (PVR) in acute pulmonary embolism (PE) in pigs.

DESIGN

Controlled, animal study.

SETTING

Tertiary university hospital, animal research laboratory.

SUBJECTS

Female Danish slaughter pigs (n = 12, ~60 kg).

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

PE was induced by infusion of autologous blood clots in pigs. CT pulmonary angiograms were performed at baseline, after PE (first experimental day [PEd0]) and the following 2 days (second experimental day [PEd1] and third experimental day [PEd2]), and clot burden quantified by a modified Qanadli Obstruction Score. Hemodynamics were evaluated with left and right heart catheterization and systemic invasive pressures each day before, under, and after treatment with the pulmonary vasodilators sildenafil (0.1 mg/kg) and oxygen (Fio2 40%). PE increased PVR (baseline vs. PEd0: 178 ± 54 vs. 526 ± 160 dynes; p < 0.0001) and obstruction score (baseline vs. PEd0: 0% vs. 45% ± 13%; p < 0.0001). PVR decreased toward baseline at day 1 (baseline vs. PEd1: 178 ± 54 vs. 219 ± 48; p = 0.16) and day 2 (baseline vs. PEd2: 178 ± 54 vs. 201 ± 50; p = 0.51). Obstruction score decreased only slightly at day 1 (PEd0 vs. PEd1: 45% ± 12% vs. 43% ± 14%; p = 0.04) and remained elevated throughout the study (PEd1 vs. PEd2: 43% ± 14% vs. 42% ± 17%; p = 0.74). Sildenafil and oxygen in combination decreased PVR at day 0 (-284 ± 154 dynes; p = 0.0064) but had no effects at day 1 (-8 ± 27 dynes; p = 0.4827) or day 2 (-18 ± 32 dynes; p = 0.0923).

CONCLUSIONS

Pulmonary vasoconstriction, and not mechanical obstruction, was the predominant cause of increased PVR in acute PE in pigs. PVR rapidly declined over the first 2 days after onset despite a persistent mechanical obstruction of the pulmonary circulation from emboli. The findings suggest that treatment with pulmonary vasodilators might only be effective in the acute phase of PE thereby limiting the window for such therapy.

Effect of Thrombin and Incubation Time on Porcine Whole Blood Clot Elasticity and Recombinant Tissue Plasminogen Activator Susceptibility

Deep vein thrombosis is a major source of morbidity and mortality worldwide. Catheter-directed thrombolytics are the frontline approach for vessel recanalization, though fibrinolytic efficacy is limited for stiff, chronic thrombi. Although thrombin has been used in preclinical models to induce thrombosis, the effect on lytic susceptibility and clot stiffness is unknown. The goal of this study was to explore the effect of bovine thrombin concentration and incubation time on lytic susceptibility and stiffness of porcine whole blood clots in vitro. Porcine whole blood was allowed to coagulate at 37°C in glass pipets primed with 2.5 or 15 U/mL thrombin for 15 to 120 min. Lytic susceptibility to recombinant tissue plasminogen activator (rt-PA, alteplase) over a range of concentrations (3.15-107.00 µg/mL) was evaluated using percentage clot mass loss. The Young's moduli and degrees of retraction of the clots were estimated using ultrasound-based single-track-location shear wave elasticity and B-mode imaging, respectively. Percentage mass loss decreased and clot stiffness increased with the incubation period. Clots formed with 15 U/mL and incubated for 2 h exhibited properties similar to those of highly retracted clots: Young's modulus of 2.39 ± 0.36 kPa and percentage mass loss of 8.69 ± 2.72% when exposed to 3.15 µg/mL rt-PA. The histological differences between thrombin-induced porcine whole blood clots in vitro and thrombi in vivo are described.

In Vivo Porcine Aged Deep Vein Thrombosis Model for Testing Ultrasound-based Thrombolysis Techniques

As blood clots age, many thrombolytic techniques become less effective. To fully evaluate these techniques for potential clinical use, a large animal aged-clot model is needed. Previous minimally invasive attempts to allow clots to age in an in vivo large animal model were unsuccessful because of the clot clearance associated with relatively high level of cardiac health of readily available research pigs. Prior models have thus subsequently used invasive surgical techniques with the associated morbidity, animal stress and cost. We propose a method for forming sub-acute venous blood clots in an in-vivo porcine model. The age of the clots can be controlled and varied. By using an intravenous scaffold to anchor the clot to the vessel wall during the aging process, we can show that sub-acute clots can consistently be formed with a minimally invasive, percutaneous approach. The clot formed in this study remained intact for at least 1 wk in all subjects. Therefore, we established a new minimally invasive, large animal aged-clot model for evaluation of thrombolytic techniques.

Assessment of histological characteristics, imaging markers, and rt-PA susceptibility of ex vivo venous thrombi

Venous thromboembolism is a significant source of morbidity and mortality worldwide. Catheter-directed thrombolytics is the primary treatment used to relieve critical obstructions, though its efficacy varies based on the thrombus composition. Non-responsive portions of the specimen often remain in situ, which prohibits mechanistic investigation of lytic resistance or the development of diagnostic indicators for treatment outcomes. In this study, thrombus samples extracted from venous thromboembolism patients were analyzed ex vivo to determine their histological properties, susceptibility to lytic therapy, and imaging characteristics. A wide range of thrombus morphologies were observed, with a dependence on age and etymology of the specimen. Fibrinolytic inhibitors including PAI-1, alpha 2-antiplasmin, and TAFI were present in samples, which may contribute to the response venous thrombi to catheter-directed thrombolytics. Finally, a weak but significant correlation was observed between the response of the sample to lytic drug and its magnetic microstructure assessed with a quantitative MRI sequence. These findings highlight the myriad of changes in venous thrombi that may promote lytic resistance, and imaging metrics that correlate with treatment outcomes.

Accelerated sonothrombolysis with Definity in a xenographic porcine cerebral thromboembolism model

Adjuvant ultrasound at 2 MHz with or without an ultrasound contrast agent improves the rate of thrombus resolution by recombinant tissue plasminogen activator (rt-PA) in laboratory and clinical studies. A sub-megahertz approach can further expand this therapy to a subset of patients with an insufficient temporal bone window, improving efficacy in unselected patient populations. The aim of this study was to determine if a clinical ultrasound contrast agent (UCA), Definity, and 220 kHz pulsed ultrasound accelerated rt-PA thrombolysis in a preclinical animal model of vascular occlusion. The effect of Definity and ultrasound on thrombus clearance was first investigated in vitro and subsequently tested in a xenographic porcine cerebral thromboembolism model in vivo. Two different microcatheter designs (end-hole, multi-side-hole) were used to infuse rt-PA and Definity at the proximal edge or directly into clots, respectively. Sonothrombolysis with Definity increased clot mass loss relative to saline or rt-PA alone in vitro, only when rt-PA was administered directly into clots via a multi-side-hole microcatheter. Combined treatment with rt-PA, Definity, and ultrasound in vivo increased the rate of reperfusion up to 45 min faster than clots treated with rt-PA or saline. In this porcine cerebral thromboembolism model employing retracted human clots, 220 kHz ultrasound, in conjunction with Definity increased the probability of early successful reperfusion with rt-PA.

Using Acoustic Fields to Fabricate ECM-Based Biomaterials for Regenerative Medicine Applications

Ultrasound is emerging as a promising tool for both characterizing and fabricating engineered biomaterials. Ultrasound-based technologies offer a diverse toolbox with outstanding capacity for optimization and customization within a variety of therapeutic contexts, including improved extracellular matrix-based materials for regenerative medicine applications. Non-invasive ultrasound fabrication tools include the use of thermal and mechanical effects of acoustic waves to modify the structure and function of extracellular matrix scaffolds both directly, and indirectly via biochemical and cellular mediators. Materials derived from components of native extracellular matrix are an essential component of engineered biomaterials designed to stimulate cell and tissue functions and repair or replace injured tissues. Thus, continued investigations into biological and acoustic mechanisms by which ultrasound can be used to manipulate extracellular matrix components within three-dimensional hydrogels hold much potential to enable the production of improved biomaterials for clinical and research applications.

Ultrasound-Responsive Cavitation Nuclei for Therapy and Drug Delivery

Therapeutic ultrasound strategies that harness the mechanical activity of cavitation nuclei for beneficial tissue bio-effects are actively under development. The mechanical oscillations of circulating microbubbles, the most widely investigated cavitation nuclei, which may also encapsulate or shield a therapeutic agent in the bloodstream, trigger and promote localized uptake. Oscillating microbubbles can create stresses either on nearby tissue or in surrounding fluid to enhance drug penetration and efficacy in the brain, spinal cord, vasculature, immune system, biofilm or tumors. This review summarizes recent investigations that have elucidated interactions of ultrasound and cavitation nuclei with cells, the treatment of tumors, immunotherapy, the blood-brain and blood-spinal cord barriers, sonothrombolysis, cardiovascular drug delivery and sonobactericide. In particular, an overview of salient ultrasound features, drug delivery vehicles, therapeutic transport routes and pre-clinical and clinical studies is provided. Successful implementation of ultrasound and cavitation nuclei-mediated drug delivery has the potential to change the way drugs are administered systemically, resulting in more effective therapeutics and less-invasive treatments.

In Vitro Thrombolytic Efficacy of Single- and Five-Cycle Histotripsy Pulses and rt-PA

Although primarily known as an ablative modality, histotripsy can increase the efficacy of lytic therapy in a retracted venous clot model. Bubble cloud oscillations are the primary mechanism of action for histotripsy, and the type of bubble activity is dependent on the pulse duration. A retracted human venous clot model was perfused with and without the thrombolytic recombinant tissue plasminogen activator (rt-PA). The clot was exposed to histotripsy pulses of single- or five-cycle duration and peak negative pressures of 0-30 MPa. Bubble activity within the clot was monitored via passive cavitation imaging. The combination of histotripsy and rt-PA was more efficacious than rt-PA alone for single- and five-cycle pulses with peak negative pressures of 25 and 20 MPa, respectively. For both excitation schemes, the detected acoustic emissions correlated with the degree of thrombolytic efficacy. These results indicate that rt-PA and single- or multicycle histotripsy pulses enhance thrombolytic therapy.

In vitro characterization of sonothrombolysis and echocontrast agents to treat ischemic stroke

The development of adjuvant techniques to improve thrombolytic efficacy is important for advancing ischemic stroke therapy. We characterized octafluoropropane and recombinant tissue plasminogen activator (rt-PA)-loaded echogenic liposomes (OFP t-ELIP) using differential interference and fluorescence microscopy, attenuation spectroscopy, and electrozone sensing. The loading of rt-PA in OFP t-ELIP was assessed using spectrophotometry. Further, it was tested whether the agent shields rt-PA against degradation by plasminogen activator inhibitor-1 (PAI-1). An in vitro system was used to assess whether ultrasound (US) combined with either Definity or OFP t-ELIP enhances rt-PA thrombolysis. Human whole blood clots were mounted in a flow system and visualized using an inverted microscope. The perfusate consisted of either (1) plasma alone, (2) rt-PA, (3) OFP t-ELIP, (4) rt-PA and US, (5) OFP t-ELIP and US, (6) Definity and US, or (7) rt-PA, Definity, and US (n = 16 clots per group). An intermittent US insonation scheme was employed (220 kHz frequency, and 0.44 MPa peak-to-peak pressures) for 30 min. Microscopic imaging revealed that OFP t-ELIP included a variety of structures such as liposomes (with and without gas) and lipid-shelled microbubbles. OFP t-ELIP preserved up to 76% of rt-PA activity in the presence of PAI-1, whereas only 24% activity was preserved for unencapsulated rt-PA. The use of US with rt-PA and Definity enhanced lytic efficacy (p < 0.05) relative to rt-PA alone. US combined with OFP t-ELIP enhanced lysis over OFP t-ELIP alone (p < 0.01). These results demonstrate that ultrasound combined with Definity or OFP t-ELIP can enhance the lytic activity relative to rt-PA or OFP t-ELIP alone, respectively.

Effect of Clot Stiffness on Recombinant Tissue Plasminogen Activator Lytic Susceptibility in Vitro

The lytic recombinant tissue plasminogen activator (rt-PA) is the only drug approved by the Food and Drug Administration for treating ischemic stroke. Less than 40% of patients with large vessel occlusions who are treated with rt-PA have improved blood flow. However, up to 6% of all patients receiving rt-PA develop intracerebral hemorrhage. Predicting the efficacy of rt-PA treatment a priori could help guide therapeutic decision making, such that rt-PA is administered only to those individuals who would benefit from this treatment. Clot composition and structure affect the lytic efficacy of rt-PA and have an impact on elasticity. However, the relationship between clot elasticity and rt-PA lytic susceptibility has not been adequately investigated. The goal of this study was to quantify the relationship between clot elasticity and rt-PA susceptibility in vitro. Human and porcine highly retracted and mildly retracted clots were fabricated in glass pipettes. The rt-PA lytic susceptibility was evaluated in vitro using the percent clot mass loss. The Young's moduli of the clots were estimated using ultrasound-based single-track-location shear wave elasticity imaging. The percent mass loss in mildly retracted porcine and human clots (28.9 ± 6.1% and 45.2 ± 7.1%, respectively) was significantly higher (p < 0.05) than in highly retracted porcine and human clots (10.9 ± 2.1% and 25.5 ± 10.0%, respectively). Furthermore, the Young's moduli of highly retracted porcine and human clots (5.33 ± 0.92 and 3.21 ± 1.97 kPa, respectively) were significantly higher (p < 0.05) than those of mildly retracted porcine and human clots (2.66 ± 0.55 and 0.79 ± 0.21 kPa, respectively). The results revealed an inverse relationship between the percent clot mass loss and Young's modulus. These findings motivate continued investigation of ultrasound-based methods to assess clot stiffness in order to predict rt-PA thrombolytic efficacy.