Noninvasive ultrasonic induction of cerebrospinal fluid flow enhances intrathecal drug delivery

Strategies for enhanced gene delivery to the central nervous system

The delivery of genes to the central nervous system (CNS) has been a persistent challenge due to various biological barriers. The blood-brain barrier (BBB), in particular, hampers the access of systemically injected drugs to parenchymal cells, allowing only a minimal percentage (<1%) to pass through. Recent scientific insights highlight the crucial role of the extracellular space (ECS) in governing drug diffusion. Taking into account advancements in vectors, techniques, and knowledge, the discussion will center on the most notable vectors utilized for gene delivery to the CNS. This review will explore the influence of the ECS - a dynamically regulated barrier-on drug diffusion. Furthermore, we will underscore the significance of employing remote-control technologies to facilitate BBB traversal and modulate the ECS. Given the rapid progress in gene editing, our discussion will also encompass the latest advances focused on delivering therapeutic editing in vivo to the CNS tissue. In the end, a brief summary on the impact of Artificial Intelligence (AI)/Machine Learning (ML), ultrasmall, soft endovascular robots, and high-resolution endovascular cameras on improving the gene delivery to the CNS will be provided.

Ultrasonic cerebrospinal fluid clearance improves outcomes in hemorrhagic brain injury models

Impaired clearance of the byproducts of aging and neurologic disease from the brain exacerbates disease progression and severity. We have developed a noninvasive, low intensity transcranial focused ultrasound protocol that facilitates the removal of pathogenic substances from the cerebrospinal fluid (CSF) and the brain interstitium. This protocol clears neurofilament light chain (NfL) - an aging byproduct - in aged mice and clears red blood cells (RBCs) from the central nervous system in two mouse models of hemorrhagic brain injury. Cleared RBCs accumulate in the cervical lymph nodes from both the CSF and interstitial compartments, indicating clearance through meningeal lymphatics. Treating these hemorrhagic brain injury models with this ultrasound protocol reduced neuroinflammatory and neurocytotoxic profiles, improved behavioral outcomes, decreased morbidity and, importantly, increased survival. RBC clearance efficacy was blocked by mechanosensitive channel antagonism and was effective when applied in anesthetized subjects, indicating a mechanosensitive channel mediated mechanism that does not depend on sensory stimulation or a specific neural activity pattern. Notably, this protocol qualifies for an FDA non-significant risk designation given its low intensity, making it readily clinically translatable. Overall, our results demonstrate that this low-intensity transcranial focused ultrasound protocol clears hemorrhage and other harmful substances from the brain via the meningeal lymphatic system, potentially offering a novel therapeutic tool for varied neurologic disorders.

Advances and future perspectives of intranasal drug delivery: A scientometric review

Intranasal drug delivery is as a noninvasive and efficient approach extensively utilized for treating the local, central nervous system, and systemic diseases. Despite numerous reviews delving into the application of intranasal drug delivery across biomedical fields, a comprehensive analysis of advancements and future perspectives remains elusive. This review elucidates the research progress of intranasal drug delivery through a scientometric analysis. It scrutinizes several challenges to bolster research in this domain, encompassing a thorough exploration of entry and elimination mechanisms specific to intranasal delivery, the identification of drugs compatible with the nasal cavity, the selection of dosage forms to surmount limited drug-loading capacity and poor solubility, and the identification of diseases amenable to the intranasal delivery strategy. Overall, this review furnishes a perspective aimed at galvanizing future research and development concerning intranasal drug delivery.

Evaluation of advective solute infiltration into porous media by pulsed focused ultrasound-induced acoustic streaming effects

PURPOSE

Acoustic streaming induced by applying transcranial focused ultrasound (FUS) promotes localized advective solute transport in the brain and has recently garnered research interest for drug delivery and enhancement of brain waste clearance. The acoustic streaming behavior in brain tissue is difficult to model numerically and thus warrants an in vitro examination of the effects of using different sonication parameters, in terms of frequency, intensity, and pulse duration (PD).

METHODS

Melamine and polyvinyl alcohol (PVA) foams were used to mimic the porous brain tissue, which contains leptomeningeal fenestrations and perivascular space, while agar hydrogel was used to emulate denser neuropil. FUS was delivered to these media, which were immersed in a phosphate-buffered saline containing toluidine blue O dye, across various frequencies (400, 500, and 600 kHz; applicable to transcranial delivery) in a pulsed mode at two different spatialpeak pulse-average intensities (3 and 4 W/cm2).

RESULTS

Image analysis showed that the use of 400 kHz yielded the greatest dye infiltration in melamine foam, while sonication had no impact on infiltration in the agar hydrogel due to the dominance of diffusional transport. Using a fixed spatial-peak temporal-average intensity of 0.4 W/cm2 at 400 kHz, a PD of 75 ms resulted in the greatest infiltration depth in both melamine and PVA foams among the tested range (50-150 ms).

CONCLUSION

These findings suggest the existence of a specific frequency and PD that induce greater enhancement of solute/fluid movement, which may contribute to eventual in vivo applications in promoting waste clearance from the brain.

In vivo distribution of cerebrospinal fluid tracer in human upper spinal cord and brain stem

BACKGROUNDIntrathecal injection is an attractive route through which drugs can be administered and directed to the spinal cord, restricted by the blood-spinal cord barrier. However, in vivo data on the distribution of cerebrospinal fluid (CSF) substances in the human spinal cord are lacking. We conducted this study to assess the enrichment of a CSF tracer in the upper cervical spinal cord and the brain stem.METHODSAfter lumbar intrathecal injection of a magnetic resonance imaging (MRI) contrast agent, gadobutrol, repeated blood samples and MRI of the upper cervical spinal cord, brain stem, and adjacent subarachnoid spaces (SAS) were obtained through 48 hours. The MRI scans were then analyzed for tracer distribution in the different regions and correlated to age, disease, and amounts of tracer in the blood to determine CSF-to-blood clearance.RESULTSThe study included 26 reference individuals and 35 patients with the dementia subtype idiopathic normal pressure hydrocephalus (iNPH). The tracer enriched all analyzed regions. Moreover, tracer enrichment in parenchyma was associated with tracer enrichment in the adjacent SAS and with CSF-to-blood clearance. Clearance from the CSF was delayed in patients with iNPH compared with younger reference patients.CONCLUSIONA CSF tracer substance administered to the lumbar thecal sac can access the parenchyma of the upper cervical spinal cord and brain stem. Since CSF-to-blood clearance is highly individual and is associated with tracer level in CSF, clearance assessment may be used to tailor intrathecal treatment regimes.FUNDINGSouth-Eastern Norway Regional Health and Østfold Hospital Trust supported the research and publication of this work.

Influence of focused ultrasound on locoregional drug delivery to the brain: Potential implications for brain tumor therapy

INTRODUCTION

Efficient delivery of therapeutics across the blood-brain barrier (BBB) for the treatment of central nervous system (CNS) tumors is a major challenge to the development of safe and efficacious therapies. Locoregional drug delivery platforms offer an improved therapeutic index by achieving high drug concentrations in the target tissue with negligible systemic exposure. Intrathecal (intraventricular) [IT] and convection-enhanced delivery [CED] are two clinically relevant methods being employed for various CNS malignancies. Both of these standalone platforms suffer from passive post-administration distribution forces, sometimes limiting the desired distribution for tumor therapy. Focused ultrasound and microbubble-mediated blood-brain barrier opening (FUS-BBBO) is a recent modality used for enhanced drug delivery. It is postulated that coupling of FUS with these alternative delivery routes may provide benefits. Multimodality FUS may provide the desired ability to increase the depth of parenchymal delivery following IT administration and provide a means for contour directionality with CED. Further, the transient enhanced permeability achieved with FUS-BBBO is well established, but drug residence and transit times, important to clinical dose scheduling, have not yet been defined. The present investigation comprises two discrete studies: 1. Conduct a comprehensive quantitative evaluation to elucidate the effect of FUS-BBBO as it relates to varying routes of administration (IT and IV) in its capacity to facilitate drug penetration within the striatal-thalamic region. 2. Investigate the impact of combining FUS-BBBO with CED on drug distribution, with a specific focus on the temporal dynamics of drug retention within the target region.

METHODS

Firstly, we quantitatively assessed how FUS-BBBO coupled with IT and IV altered fluorescent dye (Dextran 2000 kDa and 70 kDa) distribution and concentration in a predetermined striatal-thalamic region in naïve mice. Secondly, we analyzed the pharmacokinetic effects of using FUS mediated BBB disruption coupled with CED by measuring the volume of distribution and time-dependent concentration of the dye.

RESULTS

Our results indicate that IV administration coupled with FUS-BBBO successfully enhances delivery of dye into the pre-defined sonication targets. Conversely, measurable dye in the sonication target was consistently less after IT administration. FUS enhances the distribution volume of dye after CED. Furthermore, a shorter time of residence was observed when CED was coupled with FUS-BBBO application when compared to CED alone.

CONCLUSION

1. Based on our findings, IV delivery coupled with FUS-BBBO is a more efficient means for delivery to deep targets (i.e. striatal-thalamic region) within a predefined spatial conformation compared to IT administration. 2. FUS-BBBO increases the volume of distribution (Vd) of dye after CED administration, but results in a shorter time of residence. Whether this finding is reproducible with other classes of agents (e.g., cytotoxic agents, antibodies, viral particles, cellular therapies) needs to be studied.

Ultrasound meets the cell membrane: for enhanced endocytosis and drug delivery

Endocytosis plays a crucial role in drug delivery for precision therapy. As a non-invasive and spatiotemporal-controllable stimulus, ultrasound (US) has been utilized for improving drug delivery efficiency due to its ability to enhance cell membrane permeability. When US meets the cell membrane, the well-known cavitation effect generated by US can cause various biophysical effects, facilitating the delivery of various cargoes, especially nanocarriers. The comprehension of recent progress in the biophysical mechanism governing the interaction between ultrasound and cell membranes holds significant implications for the broader scientific community, particularly in drug delivery and nanomedicine. This review will summarize the latest research results on the biological effects and mechanisms of US-enhanced cellular endocytosis. Moreover, the latest achievements in US-related biomedical applications will be discussed. Finally, challenges and opportunities of US-enhanced endocytosis for biomedical applications will be provided.

Intrathecal delivery of Macromolecules: Clinical status and emerging technologies

The proximity and association of cerebrospinal fluid (CSF) and the intrathecal (IT) space with deep targets in the central nervous system (CNS) parenchyma makes IT injection an attractive route of administration for brain drug delivery. However, the extent to which intrathecally administered macromolecules are effective in treating neurological diseases is a question of both clinical debate and technological interest. We present the biological, chemical, and physical properties of the intrathecal space that are relevant to drug absorption, distribution, metabolism, and elimination from CSF. We then analyze the evolution of IT drug delivery in clinical trials over the last 20 years. Our analysis revealed that the percentage of clinical trials assessing IT delivery for the delivery of biologics (i.e., macromolecules, cells) for treatment of chronic conditions (e.g., neurodegeneration, cancer, and metabolic diseases) has steadily increased. Clinical trials exploring cell or macromolecular delivery within the IT space have not evaluated engineering technologies, such as depots, particles, or other delivery systems. Recent pre-clinical studies have evaluated IT macromolecule delivery in small animals, postulating that delivery efficacy can be assisted by external medical devices, micro- or nanoparticles, bulk biomaterials, and viral vectors. Further studies are necessary to evaluate the extent to which engineering technologies and IT administration improve CNS targeting and therapeutic outcome.

Non-invasive enhancement of intracortical solute clearance using transcranial focused ultrasound

Transport of interstitial fluid and solutes plays a critical role in clearing metabolic waste from the brain. Transcranial application of focused ultrasound (FUS) has been shown to promote localized cerebrospinal fluid solute uptake into the brain parenchyma; however, its effects on the transport and clearance of interstitial solutes remain unknown. We demonstrate that pulsed application of low-intensity FUS to the rat brain enhances the transport of intracortically injected fluorescent tracers (ovalbumin and high molecular-weight dextran), yielding greater parenchymal tracer volume distribution compared to the unsonicated control group (ovalbumin by 40.1% and dextran by 34.6%). Furthermore, FUS promoted the drainage of injected interstitial ovalbumin to both superficial and deep cervical lymph nodes (cLNs) ipsilateral to sonication, with 78.3% higher drainage observed in the superficial cLNs compared to the non-sonicated hemisphere. The application of FUS increased the level of solute transport visible from the dorsal brain surface, with ~ 43% greater area and ~ 19% higher fluorescence intensity than the unsonicated group, especially in the pial surface ipsilateral to sonication. The sonication did not elicit tissue-level neuronal excitation, measured by an electroencephalogram, nor did it alter the molecular weight of the tracers. These findings suggest that nonthermal transcranial FUS can enhance advective transport of interstitial solutes and their subsequent removal in a completely non-invasive fashion, offering its potential non-pharmacological utility in facilitating clearance of waste from the brain.

Global sonication of the human intracranial space via a jumbo planar transducer

Contrary to conditioning a Focused Ultrasound (FUS) beam to sonicate a localized region of the human brain, the goal of this investigation was to explore the prospect of distributing homogeneous ultrasound energy over the entire brain space with a large cranium-wide ultrasound beam. Recent ultrasound preclincal studies utilizing large or whole brain stimulation regions create a demand for expanding the treatment envelope of transcranial pulsed-low intensity ultrasound towards Global Brain Sonication (GBS) for potential human investigation. Here, we conduct ultrasound field characterizations when transmitting pulsed ultrasound through human skull specimens using a 1-3 piezocomposite planar transducer operating at 464 kHz with an active single-element surface of 30 × 30 cm. Through computational simulation and hydrophone scanning methodology, ultrasound wave behavior and dose homogeneity in the brain space were evaluated under various trajectories of sonication using the planar transducer. Clinically relevant pulse parameters used for transcranial therapeutic ultrasound applications were used in the experiments. Simulations and empirical testing revealed that dose homogeneity and acoustic intensity over the brain space are influenced by sonication trajectory, skull lens effects, and acoustic wave reflections. The transducer can emit a spatial peak pulse average intensity of 4.03 W/cm² (0.24 MPa) measured in the free-field at 464 kHz with electrical power of 1 kW. The simulation showed that approximately 99 % of the cranial volume was exposed with <30 % of the maximum external acoustic intensity being transmitted into the skull. The transmission loss across all sonication trajectories is similar to previously reported FUS studies. A marker for GBS dose homogeneity is introduced to score the ultrasound pressure field uniformity in the intracranial space. Results of this study identify the initial challenges of exposing the entire human brain space with ultrasound using a large cranium-wide sonication beam intended for global brain therapeutic modulation.

High-frequency ultrasound-guided intrathecal injections in a young mouse model: Targeting the central nervous system in drug delivery

BACKGROUND

Intrathecal injections provide important access to the central nervous system for delivery of anesthetic, analgesic or chemotherapeutic drugs that do not otherwise cross the blood-brain barrier. The administration of drugs via this route in animal models is challenging due to an inability to visualize the small target space during injection. Successful drug delivery therefore requires expertise in indirectly assessing vertebral and spinal cord anatomy and gaining advanced procedural skills. These factors are especially compounded in small animals such as mice (the most common mammalian model) and in investigations modeling pediatric drug delivery, where the animal is even smaller.

NEW METHOD

To address these issues, we have developed a method in which high-frequency ultrasound imaging is used to visualize and target the lumbar intrathecal space for injections. The technique is demonstrated in mice as young as postnatal day 16. To evaluate the method, a gadolinium-based magnetic resonance imaging (MRI) contrast agent was injected intrathecally, and subsequent brain delivery was verified post-injection by MRI.

RESULTS

Successful intrathecal injections of the MRI contrast agent showed distribution to the brain. In this study, we achieved a targeting success rate of 80% in 20 animals.

COMPARISON WITH EXISTING METHODS AND CONCLUSION

We expect that the new method will be convenient for drug delivery to the central nervous system in rodent research and provide higher reliability than unguided approaches, an essential contribution that will enable intrathecal delivery in pediatric mouse models.

Applications of focused ultrasound-mediated blood-brain barrier opening

The blood brain barrier (BBB) plays a critically important role in the regulation of central nervous system (CNS) homeostasis, but also represents a major limitation to treatments of brain pathologies. In recent years, focused ultrasound (FUS) in conjunction with gas-filled microbubble contrast agents has emerged as a powerful tool for transiently and non-invasively disrupting the BBB in a targeted and image-guided manner, allowing for localized delivery of drugs, genes, or other therapeutic agents. Beyond the delivery of known therapeutics, FUS-mediated BBB opening also demonstrates the potential for use in neuromodulation and the stimulation of a range of cell- and tissue-level physiological responses that may prove beneficial in disease contexts. Clinical trials investigating the safety and efficacy of FUS-mediated BBB opening are well underway, and offer promising non-surgical approaches to treatment of devastating pathologies. This article reviews a range of pre-clinical and clinical studies demonstrating the tremendous potential of FUS to fundamentally change the paradigm of treatment for CNS diseases.

A tool for monitoring cell type-specific focused ultrasound neuromodulation and control of chronic epilepsy

Focused ultrasound (FUS) is a powerful tool for noninvasive modulation of deep brain activity with promising therapeutic potential for refractory epilepsy; however, tools for examining FUS effects on specific cell types within the deep brain do not yet exist. Consequently, how cell types within heterogeneous networks can be modulated and whether parameters can be identified to bias these networks in the context of complex behaviors remains unknown. To address this, we developed a fiber Photometry Coupled focused Ultrasound System (PhoCUS) for simultaneously monitoring FUS effects on neural activity of subcortical genetically targeted cell types in freely behaving animals. We identified a parameter set that selectively increases activity of parvalbumin interneurons while suppressing excitatory neurons in the hippocampus. A net inhibitory effect localized to the hippocampus was further confirmed through whole brain metabolic imaging. Finally, these inhibitory selective parameters achieved significant spike suppression in the kainate model of chronic temporal lobe epilepsy, opening the door for future noninvasive therapies.

High Incidence of Intracerebral Hemorrhaging Associated with the Application of Low-Intensity Focused Ultrasound Following Acute Cerebrovascular Injury by Intracortical Injection

Low-intensity transcranial focused ultrasound (FUS) has gained momentum as a non-/minimally-invasive modality that facilitates the delivery of various pharmaceutical agents to the brain. With the additional ability to modulate regional brain tissue excitability, FUS is anticipated to confer potential neurotherapeutic applications whereby a deeper insight of its safety is warranted. We investigated the effects of FUS applied to the rat brain (Sprague-Dawley) shortly after an intracortical injection of fluorescent interstitial solutes, a widely used convection-enhanced delivery technique that directly (i.e., bypassing the blood-brain-barrier (BBB)) introduces drugs or interstitial tracers to the brain parenchyma. Texas Red ovalbumin (OA) and fluorescein isothiocyanate-dextran (FITC-d) were used as the interstitial tracers. Rats that did not receive sonication showed an expected interstitial distribution of OA and FITC-d around the injection site, with a wider volume distribution of OA (21.8 ± 4.0 µL) compared to that of FITC-d (7.8 ± 2.7 µL). Remarkably, nearly half of the rats exposed to the FUS developed intracerebral hemorrhaging (ICH), with a significantly higher volume of bleeding compared to a minor red blood cell extravasation from the animals that were not exposed to sonication. This finding suggests that the local cerebrovascular injury inflicted by the micro-injection was further exacerbated by the application of sonication, particularly during the acute stage of injury. Smaller tracer volume distributions and weaker fluorescent intensities, compared to the unsonicated animals, were observed for the sonicated rats that did not manifest hemorrhaging, which may indicate an enhanced degree of clearance of the injected tracers. Our results call for careful safety precautions when ultrasound sonication is desired among groups under elevated risks associated with a weakened or damaged vascular integrity.