Utilization of diagnostic ultrasound and intravenous lipid-encapsulated perfluorocarbons in non-invasive targeted cardiovascular therapeutics
© Porter et al. 2016
Received: 18 December 2015
Accepted: 4 July 2016
Published: 15 July 2016
Diagnostic ultrasound (DUS) pressures have the ability to induce inertial cavitation (IC) of systemically administered microbubbles; this bioeffect has many diagnostic and therapeutic implications in cardiovascular care. Diagnostically, commercially available lipid-encapsulated perfluorocarbons (LEP) can be utilized to improve endocardial and vascular border delineation as well as assess myocardial perfusion. Therapeutically, the liquid jets induced by IC can alter endothelial function and dissolve thrombi within the immediate vicinity of the cavitating microbubbles. The cavitating LEP can also result in the localized release of any bound therapeutic substance at the site of insonation. DUS-induced IC has been tested in pre-clinical studies to determine what effect it has on acute vascular and microvascular thrombosis as well as nitric oxide (NO) release. These pre-clinical studies have consistently shown that DUS-induced IC of LEP is effective in restoring coronary vascular and microvascular flow in acute ST segment elevation myocardial infarction (STEMI), with microvascular flow improving even if upstream large vessel flow has not been achieved. The initial clinical trials examining the efficacy of short pulse duration DUS high mechanical index impulses in patients with STEMI are underway, and preliminary studies have suggested that earlier epicardial vessel recanalization can be achieved prior to arriving in the cardiac catheterization laboratory. DUS high mechanical index impulses have also been effective in pre-clinical studies for targeting DNA delivery that has restored islet cell function in type I diabetes and restored vascular flow in the extremities downstream from a peripheral vascular occlusion. Improvements in this technique will come from three dimensional arrays for therapeutic applications, more automated delivery techniques that can be applied in the field, and use of submicron-sized acoustically activated LEP droplets that may better permeate the clot prior to DUS activation and cavitation. This article will focus on these newer developments for DUS therapeutic applications.
Although diagnostic ultrasound (DUS) systems and lipid-encapsulated perfluorocarbons (LEP) like Definity (Lantheus Medical) or Sonazoid (GE Healthcare) have been approved only for imaging applications; these two products have significant therapeutic potential for non-invasive targeted thrombolysis and drug delivery. Ultrasound and microbubbles alone as a method of dissolving thrombi was first introduced in 1997  and was predicated upon work published just 1 year earlier by Tachibana and Tachibana demonstrating their potential to augment the effects of lytic therapy . Subsequent in vivo studies demonstrated that ultrasound and microbubbles alone, using low-frequency non-imaging transducers, could recanalize peripheral vascular thrombi without fibrinolytic agents [3–5]. More recently, DUS pressures, despite their short pulse duration, have proven effective at recanalizing intravascular thrombotic occlusions . The effectiveness was related to the use of intermittent high mechanical index impulses that are capable of causing both stable cavitation and inertial cavitation (IC). The intermittent application is necessary for microbubble permeation into the thrombus, and IC appears necessary, as this has been shown to create the fluid jets that erode thrombus both from outside and from within the thrombus infrastructure [6–8]. Subsequently, high mechanical index (MI) impulses from a DUS system have been used in both pre-clinical and clinical studies of acute ST segment elevation myocardial infarction (STEMI) and ischemic stroke, achieving successful coronary and cerebral recanalization with improved microvascular flow without the need of fibrinolytic therapy [9–14]. The DUS sequence has been modified slightly in each of these applications, but it is unclear how much of a modification beyond current diagnostic limits are required to achieve effective thrombolysis and targeted drug delivery. Numerous small animal studies have demonstrated the effectiveness of DUS-guided high MI impulses in targeting DNA [13–16], and more recent studies have demonstrated the potential of DUS to target the delivery of inhibitory RNA to suppress angiogenesis in adenocarcinoma [17, 18]. This review will focus on the data that has been accumulated regarding DUS efficacy in thrombolysis and drug delivery in large animals and how small modifications of current FDA-approved LEP may further improve their clinical potential.
Targeted thrombolysis with DUS
Slight prolongations in pulse duration on a DUS transducer may improve the amount of thrombus dissolution. By increasing pulse duration from <5 to 20 μs on a DUS transducer, a higher epicardial recanalization rate was achieved with DUS-guided therapy added to ½ dose tissue plasminogen activator. Despite the higher epicardial recanalization rate, both short and longer pulse duration high MI impulses were equally effective in ST segment resolution and improvement in wall thickening within the risk area . The guided application of 20-μs pulse duration high MI impulses during an intravenous LEP microbubble infusion, without any fibrinolytic agent, was subsequently shown to produce equivalent epicardial recanalization rates as full dose fibrinolytic therapy in subsequent randomized comparisons in this same porcine model of acute STEMI . Although this slight prolongation of pulse duration appears possible with current DUS transducers, the safety of this longer pulse duration has not been elucidated.
DUS-guided cavitation with commercial available microbubbles: clinical applications in large animal models
Targeted drug and gene delivery with diagnostic ultrasound
Potential large animal application with DUS-targeted gene delivery
Beta cell regeneration
The use of a commercially available LEP and DUS for targeted thrombolysis is now being tested in the first clinical trials , with promising initial results. Targeted IC of LEP has the potential not only to non-invasively and safely dissolve intravascular and microvascular thrombi but could also be effective in targeting gene delivery and has been demonstrated to target the delivery of vascular endothelial growth factors and genes for pancreatic regeneration. Diagnostic transducers have been modified in order to provide radiofrequency feedback to confirm IC has occurred, which may be necessary to confirm that a desired microbubble response has occurred .
One problem with microbubbles is that they are confined to intravascular spaces, and inertial cavitation can only increase subendothelial delivery. The LEP can also be formulated into droplets, even for the lower molecular weight fluorocarbons like octafluoropropane . Since these droplets are nanometer scale, they can cross endothelial membranes and reach interstitial spaces, which may improve targeted delivery of genes into areas of myocardial scar, and improve thrombolysis efficacy by improving clot permeation prior to acoustic activation and inertial cavitation. Studies are ongoing now to explore this new potential for DUS and LEP.
DUS, diagnostic ultrasound; IC, inertial cavitation; LEP, lipid-encapsulated perfluorocarbons; MI, mechanical index; NO, nitric oxide; PWD, pulsed wave Doppler; STEMI, ST segment elevation myocardial infarction
The authors want to thank Carol Gould for her dedicated work in preparing this manuscript and to the Theodore Hubbard Foundation for funding of costs associated with the manuscript preparation.
Funding from the Theodore Hubbard Foundation has helped with costs associated with the manuscript preparation.
Availability of data and supporting materials
TRP has written the entire paper, whereas FX assisted with the manuscript and table and figure preparation. SC assisted with the droplet experiments. All authors read and approved the final manuscript.
Dr. Porter has research interests in both diagnostic and therapeutic applications of perfluorocarbon droplets and microbubbles. His work includes recent publications on acoustic activation of intravenously administered droplets and use of diagnostic ultrasound-induced cavitation of systemically administered microbubbles to treat intracoronary and microvascular thrombi.
Dr. Porter receives research support from Lantheus Medical Imaging, Philips Ultrasound and Astellas Pharma Inc. From BRACCO, Dr. Porter receives educational and research support, and he is also a speaker for Lantheus Medical Imaging.
Ethics approval and consent to participate
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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