Droplet-assisted ultrasound for brain therapy using CMUT technology – DROPMUT

Rett syndrome (RTT) is a severe neurological disorder with an incidence of 1 in about 15,000, which accounts for up to 10% of severe intellectual disability of genetic origin in women. Current treatments aim to slow the loss of abilities and allow patients to live several dozens of years. RTT is caused mainly by de novo mutations in Mecp2 gene. Recent advances in gene therapy showed promising results in preclinical models. However, previous attempts at Mecp2 gene transfer using adeno-associated virus (AAV) vectors were confounded by limited brain transduction efficiency. In fact, the presence of the blood-brain barrier (BBB) is a major challenge for gene therapy as it prevents the passage of AAV from the vasculature into the brain tissue. The objective of DROPMUT consists in the development and the evaluation of a non-invasive and low-cost ultrasound technology for the treatment of Rett syndrome.
It is now well-established that focused ultrasound in conjunction with the injection of contrast agent microbubbles (i.e., sono-permeabilization) may be used to non-invasively and temporarily disrupt the BBB, allowing localized delivery of systemically administered therapeutic agents to the central nervous system. The method can be used to deliver a wide range of molecules including low molecular weight chemotherapeutic agents, monoclonal antibodies, nucleic acids and AAV to a target site.
Nevertheless, the approach must be controlled since the heterogeneity of the skull makes the prediction of the ultrasound beam challenging and inadequate ultrasound exposures may result in no BBB opening (ineffective therapy) or side effects including permanent brain damages. Passive cavitation detection (PCD) is a way to ensure the safety of the therapeutic protocol by real-time monitoring of microbubble activity. However, for application in human brain, PCD suffers from a lack of sensitivity due to (i) the restricted bandwidth of piezoelectric (PZT) transducers (about 70%–80%) making difficult the recovery of a wide frequency content; (ii) the attenuation induced by the skull reducing the signal amplitude (low-pass filter). In DROPMUT, we propose, for the first time, to exploit the unique properties of a new generation of ultrasonic transducer (Capacitive Micromachined Ultrasonic Transducer, CMUT) to ensure, in real-time, the safety of the ultrasound protocol through wideband PCD. CMUT technology makes possible the recovery of the backscattered signal from microbubbles over a wide frequency range, allowing simultaneous acquisition of specific nonlinear components (e.g., subharmonic + superharmonics) that can be exploited for an advanced and more robust analysis of microbubble cavitation.
Furthermore, the short persistence of regular microbubbles (few minutes) and their size (intravascular agent) limit the efficacy of the ultrasound treatment. Based on our preliminary results, we do support that a new formulation of low-boiling point nanodroplets can be used to increase virus-mediated gene transfer (AAV) into the brain tissue. Nanodroplets are small (<100 nm) and stable contrast agents that could extravasate through the microvasculature and reach an extravascular target (e.g., cells). Once extravasated, nanodroplets can be easily converted into microbubbles using ultrasonic pressure wave and be used for the permeabilization of extravascular neuronal cells.
In summary, the purpose of this project is to develop and validate breakthrough strategies improving safety control and efficiency of ultrasound-assisted AAV delivery into brain. To reach these objectives, the proposal is divided in the 3 following aims: (i) design and characterization of CMUT for safe BBB opening, (ii) formulation of therapeutic ND and optimization of ultrasound sequence for AAV delivery into the brain, (iii) evaluation of the therapeutic benefits of the therapy in preclinical models of Rett syndrome.