Looking at the blood-brain barrier: molecular anatomy and possible investigation approaches.

Ultrasound for Drug and Gene Delivery to the Brain

Background and Purpose—Ultrasound-enhanced thrombolysis is a promising new approach to facilitate reperfusion therapies for acute ischemic stroke. So far, 3 different ultrasound technologies were used to increase the thrombolytic activity of tissue plasminogen activator (tPA), including transcranial Doppler (TCD), transcranial color-coded duplex (TCCD), and low-frequency ultrasound. We performed a meta-analysis to evaluate the safety and efficacy of ultrasound-enhanced thrombolysis compared to the current standard of care (intravenous tPA). Subjects and Methods—Through Medline, Embase, and Cochrane database search, we identified and abstracted all studies of ultrasound-enhanced thrombolysis in acute cerebral ischemia. Principal investigators were contacted if data not available through peer-reviewed publication were needed. Symptomatic intracerebral hemorrhage (sICH) and recanalization rates were compared between tPA, tPA+TCD±microspheres (μS), tPA+TCCD±μS, and tPA+low-frequency ultrasound. Results—A tota...

Safety and Efficacy of Ultrasound-Enhanced Thrombolysis A Comprehensive Review and Meta-Analysis of Randomized and Nonrandomized Studies

Two clinical trials have used ultrasound to improve tPA thrombolysis in patients with acute ischemic stroke. The Combined Lysis of Thrombus in Brain Ischemia Using Transcranial Ultrasound and Systemic tPA (CLOTBUST) trial reported accelerated recanalisation of the middle cerebral artery (MCA) in patients with symptoms of MCA infarction, which were monitored with 2-MHz transcranial Doppler. In CLOTBUST, there was no increased bleeding as evidenced by cranial computed tomography. The Transcranial Low-Frequency Ultrasound-Mediated Thrombolysis in Brain Ischemia (TRUMBI) trial, which employed magnetic resonance imaging (MRI) before and after tPA thrombolysis, was discontinued prematurely because of an increased number of secondary hemorrhages, possibly related to the use of low frequency 300-kHz ultrasound. The purpose of our work is to help identify possible mechanisms of intracerebral hemorrhage resulting from sonothrombolysis by applying a simulation tool that estimates the pressure levels in the human brain that are produced with different sonothrombolysis devices. A simulation software based on a finite difference time domain (FDTD) three-dimensional (3D) scheme was developed to predict acoustic pressures in the brain. This tool numerically models the wave propagation through the skull and reproduces both ultrasound protocols of CLOTBUST and TRUMBI for analysis of the distribution of acoustic pressure in the brain during stroke treatment. For the simulated TRUMBI trial, we analyzed both a "high" and "low" hypothesis according to published parameters (for high and low amplitude excitations). For these hypotheses, the mean peak rarefactional pressures in the brain were 0.26 +/- 0.2 MPa (high hypothesis) and 0.06 +/- 0.05 MPa (low hypothesis), with maximal local values as high as 1.2 MPa (high hypothesis) and 0.27 MPa (low hypothesis) for configurations modelled in this study. The peak rarefactional pressure was thus higher than the inertial acoustic cavitation threshold in the presence of a standing wave in large areas of the brain, even outside the targeted clot. For the simulated CLOTBUST trial, the maximum peak negative pressure was less than 0.07 MPa. This simulated pressure is below the threshold for both inertial and stable acoustic cavitation but likewise lower than any acoustic pressure that has been reported as sufficient for effective sonothrombolysis. Simulating the pressure field of ultrasound protocols for clinical trials of sonothrombolysis may help explain mechanisms of adverse effects. Such simulations could prove useful in the initial design and optimization of future protocols for this promising therapy of ischemic stroke.

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Simulation of intracranial acoustic fields in clinical trials of sonothrombolysis.

Focused ultrasound activation of systemically administered microbubbles is a noninvasive and localized drug delivery method that can increase vascular permeability to large molecular agents. Yet the range of acoustic parameters responsible for drug delivery remains unknown, and, thus, enhancing the delivery characteristics without compromising safety has proven to be difficult. We propose a new basis for ultrasonic pulse design in drug delivery through the blood–brain barrier (BBB) that uses principles of probability of occurrence and spatial distribution of cavitation in contrast to the conventionally applied magnitude of cavitation. The efficacy of using extremely short (2.3 μs) pulses was evaluated in 27 distinct acoustic parameter sets at low peak-rarefactional pressures (0.51 MPa or lower). The left hippocampus and lateral thalamus were noninvasively sonicated after administration of Definity microbubbles. Disruption of the BBB was confirmed by delivery of fluorescently tagged 3-, 10-, or 70-kDa dextrans. Under some conditions, dextrans were distributed homogeneously throughout the targeted region and accumulated at specific hippocampal landmarks and neuronal cells and axons. No histological damage was observed at the most effective parameter set. Our results have broadened the design space of parameters toward a wider safety window that may also increase vascular permeability. The study also uncovered a set of parameters that enhances the dose and distribution of molecular delivery, overcoming standard trade-offs in avoiding associated damage. Given the short pulses used similar to diagnostic ultrasound, new critical parameters were also elucidated to clearly separate therapeutic ultrasound from disruption-free diagnostic ultrasound.

https://www.pnas.org/content/pnas/108/40/16539.full.pdf

Noninvasive and localized neuronal delivery using short ultrasonic pulses and microbubbles

Background and Purpose— A recent study showed a dramatic increase in cerebral hemorrhage comprising atypical locations with low-frequency ultrasound–mediated recombinant tissue plasminogen activator–thrombolysis in humans. Here, we provide a possible explanation for this phenomenon by a side effect observed in a study using the similar ultrasound device.

Methods— The study was originally undertaken to investigate by transcranial Doppler sonography, positron emission tomography and perfusion MRI whether transcranial application of wide-field low-frequency ultrasound (300 kHz) improves cerebral hemodynamics in patients with cerebral small vessel disease.

Results— Showing no clear positive effect on cerebral hemodynamics in 4 patients and on cerebral perfusion (positron emission tomography) in 2 patients, the study has been terminated early because of a remarkable side effect in the first patient (a 62 year-old man) undergoing perfusion-MRI: detection of frontoparietal extravasation of Gadolinium contrast agent (applied during MRI perfusion imaging preinsonation) on MRI immediately postinsonation.

Conclusions— Abnormal permeability of the human blood-brain barrier can be induced by wide-field low-frequency insonation. The observed excessive bleeding rate with low-frequency sonothrombolysis might thus be attributable to primary blood-brain barrier disruption by ultrasound.

Blood-Brain Barrier Disruption By Low-Frequency Ultrasound

Die medikamentose lokale intraarterielle Fibrinolyse (LIF) ist ein mittlerweile anerkanntes und etabliertes Verfahren. Ungeachtet der Wahl des Fibrinolytikums liegen die Rekanalisationszeiten allerdings bei 1–2 h, so dass zunehmend neue mechanische Techniken erprobt werden, die eine Verkurzung der Rekanalisationszeiten ermoglichen sollen. Durch eine schnellere Rekanalisation kann die Infarktgrose verkleinert werden, was zu einem besseren klinischneurologischen Outcome fuhrt. In der vorliegenden Arbeit werden die Technik der Laserlyse und erste Ergebnisse vorgestellt. Die Patienten wurden im Rahmen der „Safety and Performance Study“ bzw. der Folgestudie („Pivotal Study“) eingeschlossen. Vor der Behandlung wurden NIHSS („National Institute of Health Stroke Scale“) sowie mRS („modified Rankin Scale“) erhoben. Der Grad der Rekanalisation wurde anhand der angiographischen Ergebnisse ermittelt. Die klinischen Verlaufsuntersuchungen erfolgten 24 h, 7 Tage und 30 Tage nach Behandlung. 20 Patienten wurden eingeschlossen. 13 Patienten wurden mit Gefasverschlussen im vorderen und sieben Patienten mit Verschlussen im hinteren Stromkreislauf in die Studie aufgenommen. Entsprechend den Vorgaben der Studie wurden zwolf Behandlungen als technisch komplett mit dem EPAR-System und acht Behandlungen als inkomplett eingestuft. In funf von acht Fallen wurde der Laserkatheter eingefuhrt, aber keine Laserbehandlung durchgefuhrt. Die Rekanalisationsergebnisse im vertebrobasilaren Stromgebiet mit einer 86%igen Rekanalisationsrate bei einer mittleren Laserzeit von 7,6 min (1,3–18,9 min) sind viel versprechend, wahrend die Rekanalisationsrate im vorderen Stromkreislauf lediglich 31% betrug (mittlere Laserzeit von 5,5 min [0–15,6 min]). Neben dem Nachweis des Studienziels Sicherheit und technische Machbarkeit erwies sich diese neue Methode als effektiv in Bezug auf eine rasche Rekanalisation, so dass sie eine weitere Therapieoption zur Behandlung des akuten Schlaganfalls darstellen konnte.

Stefan Kretzer

Papers

Blood-Brain Barrier Disruption By Low-Frequency Ultrasound

Therapie des akuten ischämischen Schlaganfalls mit EPAR (Endovascular Photoacoustic Recanalization)