Epidural pentobarbital delivery can prevent locally induced neocortical seizures in rats: the prospect of transmeningeal pharmacotherapy for intractable focal epilepsy

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

In vivo imaging of epileptic foci in rats using a miniature probe integrating diffuse optical tomography and electroencephalographic source localization

OBJECTIVE

The goal of this work is to establish a new dual-modal brain-mapping technique based on diffuse optical tomography (DOT) and electroencephalographic source localization (ESL) that can chronically/intracranially record optical/electroencephalography (EEG) data to precisely map seizures and localize the seizure-onset zone and associated epileptic brain network.

METHODS

The dual-modal imaging system was employed to image seizures in an experimental acute bicuculline methiodide rat model of focal epilepsy. Depth information derived from DOT was used as constraint in ESL to enhance the image reconstruction. Groups of animals were compared based on localization of seizure foci, either at different positions or at different depths.

RESULTS

This novel imaging technique successfully localized the seizure-onset zone in rat induced by bicuculline methiodide injected at a depth of 1, 2, and 3 mm, respectively. The results demonstrated that the incorporation of the depth information from DOT into the ESL image reconstruction resulted in more accurate and reliable ESL images. Although the ESL images showed a horizontal shift of the source localization, the DOT identified the seizure focus accurately. In one case, when the bicuculline methiodide (BMI) was injected at a site outside the field of view (FOV) of the DOT/ESL interface, ESL gave false-positive detection of the focus, while DOT showed negative detection.

SIGNIFICANCE

This study represents the first to identify seizure-onset zone using implantable DOT. In addition, the combination of DOT/ESL has never been documented in neuroscience and epilepsy imaging. This technology will enable us to precisely measure the neural activity and hemodynamic response at exactly the same tissue site and at both cortical and subcortical levels.

Toward new paradigms of seizure detection

Great effort has been made toward defining and characterizing the pre-ictal state. Many studies have pursued the idea that there are recognizable electrographic (EEG-based) features which occur before overt clinical seizure activity. However, development of reliable EEG-based seizure detection and prediction algorithms has been difficult. In this review, we discuss the concepts of seizure detection vs. prediction and the pre-ictal "clinical milieu" and "EEG milieu". We proceed to discuss novel concepts of seizure detection based on the pre-ictal "physiological milieu"; in particular, we indicate some early evidence for the hypothesis that pre-ictal cell swelling/extracellular space constriction can be detected with novel optical methods. Development and validation of optical seizure detection technology could provide an entirely new translational approach for the many patients with intractable epilepsy.

Long-term behavioral, electrophysiological, and neurochemical monitoring of the safety of an experimental antiepileptic implant, the muscimol-delivering Subdural Pharmacotherapy Device in monkeys

Object

The authors evaluated the extent to which the Subdural Pharmacotherapy Device (SPD), chronically implanted over the frontal cortex to perform periodic, localized muscimol-delivery/CSF removal cycles, affects overall behavior, motor performance, electroencephalography (EEG) activity, and blood and CSF neurochemistry in macaque monkeys.

Methods

Two monkeys were used to adjust methodology and 4 monkeys were subjected to comprehensive testing. Prior to surgery, the animals' behavior in a large test chamber was monitored, and the motor skills required to remove food pellets from food ports located on the walls of the chamber were determined. The monkeys underwent implantation of the subdural and extracranial SPD units. The subdural unit, a silicone strip integrating EEG electrodes and fluid-exchange ports, was positioned over the right frontal cortex. The control unit included a battery-powered, microprocessor-regulated dual minipump and radiofrequency module secured to the cranium. After implantation, the SPD automatically performed periodic saline or muscimol (1.0 mM) deliveries at 12-hour intervals, alternating with local CSF removals at 6-hour intervals. The antiepileptic efficacy of this muscimol concentration was verified by demonstrating its ability to prevent focal acetylcholine-induced seizures. During SPD treatment, the monkeys' behavior and motor performance were again monitored, and the power spectrum of their radiofrequency-transmitted EEG recordings was analyzed. Serum and CSF muscimol levels were measured with high-performance liquid chromatography electrochemical detection, and CSF protein levels were measured with turbidimetry.

Results

The SPD was well tolerated in all monkeys for up to 11 months. The behavioral study revealed that during both saline and muscimol SPD treatment, the monkeys could achieve the maximum motor performance of 40 food-pellet removals per session, as before surgery. The EEG study showed that local EEG power spectra were not affected by muscimol treatment with SPD. The neurochemical study demonstrated that the administration of 1.0 mM muscimol into the neocortical subarachnoid space led to no detectable levels of this compound in the blood and cisternal CSF, as measured 1–125 minutes after delivery. Total protein levels were within the normal range in the cisternal CSF, but protein levels in the cortical-site CSF were significantly higher than normal: 361 ± 81.6 mg/dl. Abrupt discontinuation of 3-month, periodic, subdural muscimol treatments induced withdrawal seizures, which could be completely prevented by gradually tapering off the subdural muscimol concentration from 1.0 mM to 0.12–0.03 mM over a period of 2 weeks. The monkeys' general health and weight were maintained. Infection occurred only in one monkey 9 months after surgery.

Conclusions

Long-term, periodic, transmeningeal muscimol delivery with the SPD is essentially a safe procedure. If further improved and successfully adapted for use in humans, the SPD can be used for the treatment of intractable focal neocortical epilepsy affecting approximately 150,000 patients in the US.

Therapeutic devices for epilepsy

Therapeutic devices provide new options for treating drug-resistant epilepsy. These devices act by a variety of mechanisms to modulate neuronal activity. Only vagus nerve stimulation (VNS), which continues to develop new technology, is approved for use in the United States. Deep brain stimulation of anterior thalamus for partial epilepsy recently was approved in Europe and several other countries. Responsive neurostimulation, which delivers stimuli to 1 or 2 seizure foci in response to a detected seizure, recently completed a successful multicenter trial. Several other trials of brain stimulation are in planning or underway. Transcutaneous magnetic stimulation (TMS) may provide a noninvasive method to stimulate cortex. Controlled studies of TMS are split on efficacy, which may depend on whether a seizure focus is near a possible region for stimulation. Seizure detection devices in the form of shake detectors via portable accelerometers can provide notification of an ongoing tonic-clonic seizure, or peace of mind in the absence of notification. Prediction of seizures from various aspects of electroencephalography (EEG) is in early stages. Prediction appears to be possible in a subpopulation of people with refractory seizures, and a clinical trial of an implantable prediction device is underway. Cooling of neocortex or hippocampus reversibly can attenuate epileptiform EEG activity and seizures, but engineering problems remain in its implementation. Optogenetics is a new technique that can control excitability of specific populations of neurons with light. Inhibition of epileptiform activity has been demonstrated in hippocampal slices, but use in humans will require more work. In general, devices provide useful palliation for otherwise uncontrollable seizures, but with a different risk profile than with most drugs. Optimizing the place of devices in therapy for epilepsy will require further development and clinical experience.

Autoradiographic evidence for the transmeningeal diffusion of muscimol into the neocortex in rats

Electrophysiological and behavioral studies have demonstrated that muscimol administered through the cranial meninges can prevent focal neocortical seizures. It was proposed that transmeningeal muscimol delivery can be used for the treatment of intractable focal neocortical epilepsy. However, it has not been proved that muscimol administered via the transmeningeal route can penetrate into the neocortex. The purpose of the present study was to solve this problem by using combined autoradiography-histology methods. Four rats were implanted with epidural cups over the parietal cortices. A 50 μL mixture of [³H] muscimol and unlabeled muscimol with a final concentration of 1.0mM was delivered through each cup on the dura mater. After a 1-hour exposure, the muscimol solution was removed and replaced with formalin to trap the transmeningeally diffused molecules. Then the whole brain was fixed transcardially, sectioned, with the sections subjected to autoradiography and thionine counterstaining. Results showed that (1) [³H] muscimol diffused through the meninges into the cortical tissue underlying the epidural cup in all rats. (2) [³H] muscimol-related autoradiography grains were distributed in all six neocortical layers. (3) [³H] muscimol-related autoradiography grains were localized to the cortical area underneath the epidural delivery site and were absent in the cerebral cortical white matter and other brain structures. This study provided evidence that muscimol can be delivered via the transmeningeal route into the neocortical tissue in a spatially controlled manner. The finding further supports the rationale of using transmeningeal muscimol for the treatment of intractable focal neocortical epilepsy.

Neuromodulation in Epilepsy

Neuromodulation strategies have been proposed to treat a variety of neurological disorders, including medication-resistant epilepsy. Electrical stimulation of both central and peripheral nervous systems has emerged as a possible alternative for patients who are not deemed to be good candidates for resective procedures. In addition to well-established treatments such as vagus nerve stimulation, epilepsy centers around the world are investigating the safety and efficacy of neurostimulation at different brain targets, including the hippocampus, thalamus, and subthalamic nucleus. Also promising are the preliminary results of responsive neuromodulation studies, which involve the delivery of stimulation to the brain in response to detected epileptiform or preepileptiform activity. In addition to electrical stimulation, novel therapeutic methods that may open new horizons in the management of epilepsy include transcranial magnetic stimulation, focal drug delivery, cellular transplantation, and gene therapy. We review the current strategies and future applications of neuromodulation in epilepsy.

Evolution and prospects for intracranial pharmacotherapy for refractory epilepsies: the subdural hybrid neuroprosthesis

Intracranial pharmacotherapy is a novel strategy to treat drug refractory, localization-related epilepsies not amenable to resective surgery. The common feature of the method is the use of some type of antiepileptic drug (AED) delivery device placed inside the cranium to prevent or stop focal seizures. This distinguishes it from other nonconventional methods, such as intrathecal pharmacotherapy, electrical neurostimulation, gene therapy, cell transplantation, and local cooling. AED-delivery systems comprise drug releasing polymers and neuroprosthetic devices that can deliver AEDs into the brain via intraparenchymal, ventricular, or transmeningeal routes. One such device is the subdural Hybrid Neuroprosthesis (HNP), designed to deliver AEDs, such as muscimol, into the subdural/subarachnoid space overlaying neocortical epileptogenic zones, with electrophysiological feedback from the treated tissue. The idea of intracranial pharmacotherapy and HNP treatment for epilepsy originated from multiple sources, including the advent of implanted medical devices, safety data for intracranial electrodes and catheters, evidence for the seizure-controlling efficacy of intracerebral AEDs, and further understanding of the pathophysiology of focal epilepsy. Successful introduction of intracranial pharmacotherapy into clinical practice depends on how the intertwined scientific, engineering, clinical, neurosurgical and regulatory challenges will be met to produce an effective and commercially viable device.

Advances in the application of technology to epilepsy: the CIMIT/NIO Epilepsy Innovation Summit

In 2008, a group of clinicians, scientists, engineers, and industry representatives met to discuss advances in the application of engineering technologies to the diagnosis and treatment of patients with epilepsy. The presentations also provided a guide for further technological development, specifically in the evaluation of patients for epilepsy surgery, seizure onset detection and seizure prediction, intracranial treatment systems, and extracranial treatment systems. This article summarizes the discussions and demonstrates that cross-disciplinary interactions can catalyze collaborations between physicians and engineers to address and solve many of the pressing unmet needs in epilepsy.

Convection-enhanced delivery in the treatment of epilepsy

Convection-enhanced delivery (CED) is a novel drug-delivery technique that uses positive hydrostatic pressure to deliver a fluid containing a therapeutic substance by bulk flow directly into the interstitial space within a localized region of the brain parenchyma. CED circumvents the blood-brain barrier and provides a wider, more homogenous distribution than bolus deposition (focal injection) or other diffusion-based delivery approaches. A potential use of CED is for the local delivery of antiseizure agents, which would provide an epilepsy treatment approach that avoids the systemic toxicities of orally administered antiepileptic drugs and bystander effects on nonepileptic brain regions. Recent studies have demonstrated that brief CED infusions of nondiffusible peptides that inhibit the release of excitatory neurotransmitters, including omega-conotoxins and botulinum neurotoxins, can produce long-lasting (weeks to months) seizure protection in the rat amygdala-kindling model. Seizure protection is obtainable without detectable neurological or behavioral side effects. Although conventional diffusible antiepileptic drugs do confer seizure protection when administered locally by CED, the effect is transitory. CED is a potential approach for seizure protection that could represent an alternative to resective surgery in the treatment of focal epilepsies that are resistant to orally-administered antiepileptic drugs. The prolonged duration of action of nondiffusible toxins would allow seizure protection to be maintained chronically with infrequent reinfusions.

How do seizures stop?

Although often overshadowed by factors influencing seizure initiation, seizure termination is a critical step in the return to the interictal state. Understanding the mechanisms contributing to seizure termination could potentially identify novel targets for anticonvulsant drug development and may also highlight the pathophysiological processes contributing to seizure initiation. In this article, we review known physiological mechanisms contributing to seizure termination and discuss additional mechanisms that are likely to be relevant even though specific data are not yet available. This review is organized according to successively increasing "size scales"-from membranes to synapses to networks to circuits. We first discuss mechanisms of seizure termination acting at the shortest distances and affecting the excitable membranes of neurons in the seizure onset zone. Next we consider the contributions of ensembles of neurons and glia interacting at intermediate distances within the region of the seizure onset zone. Lastly, we consider the contribution of brain nuclei, such as the substantia nigra pars reticulata (SNR), that are capable of modulating seizures and exert their influence over the seizure onset zone (and neighboring areas) from a relatively great-in neuroanatomical terms-distance. It is our hope that the attention to the mechanisms contributing to seizure termination will stimulate novel avenues of epilepsy research and will contribute to improved patient care.

Technology insight: neuroengineering and epilepsy-designing devices for seizure control

Despite substantial innovations in antiepileptic drug therapy over the past 15 years, the proportion of patients with uncontrolled epilepsy has not changed, highlighting the need for new treatment strategies. New implantable antiepileptic devices, which are currently under development and in pivotal clinical trials, hold great promise for improving the quality of life of millions of people with epileptic seizures worldwide. A broad range of strategies to stop seizures is currently being investigated, with various modes of control and intervention. The success of novel antiepileptic devices rests upon collaboration between neuroengineers, physicians and industry to adapt new technologies for clinical use. The initial results with these technologies are exciting, but considerable development and controlled clinical trials will be required before these treatments earn a place in our standard of clinical care.

Effects of subdural application of lidocaine in patients with focal epilepsy

Antiepileptic drug (AED) delivery directly into the neocortex has recently been shown to be able to both prevent and terminate focal seizures in rats. The present clinical experiment aimed to test the local effects of lidocaine delivered onto the pia mater adjacent to epileptogenic zones in human patients. Administration of lidocaine resulted in a marked diminishment of spike counts on all patients, with a decremental effect of lidocaine on the faster frequency elements of individual spikes and overall testing epochs. The direct cortical application of lidocaine appears to affect local epileptogenic activity in human patients with intractable focal epilepsy.

Histological evidence for drug diffusion across the cerebral meninges into the underlying neocortex in rats

Transmeningeal pharmacotherapy has been proposed to treat neurological disorders with localized pathology, such as intractable focal epilepsy. As a step toward understanding the diffusion and intracortical spread of transmeningeally delivered drugs, the present study used histological methods to determine the extent to which a marker compound, N-methyl-D-aspartate (NMDA), can diffuse into the neocortex through the meninges. Rats were implanted with bilateral parietal cortical epidural cups filled with 50 mM NMDA on the right side and artificial cerebrospinal fluid (ACSF) in the contralateral side. After 24 h, the histological effects of these treatments were evaluated using cresyl violet (Nissl) staining. The epidural NMDA exposure caused neuronal loss that in most animals extended from the pial surface through layer V. The area indicated by this neuronal loss was localized to the neocortical region underlying the epidural cup. These results suggest that NMDA-like, water soluble, small molecules can diffuse through the subdural/subarachnoid space into the underlying neocortex and spread in a limited fashion, close to the meningeal penetration site.

Transmeningeal delivery of GABA to control neocortical seizures in rats

Transmeningeal drug delivery, using an implanted hybrid neuroprosthesis, has been proposed as a novel therapy for intractable focal epilepsy. As part of a systematic effort to identify the optimal compounds and protocols for such a therapy, this study aimed to determine whether transmeningeal gamma-aminobutyric acid (GABA) delivery can terminate and/or prevent neocortical seizures in rats. Rats were chronically implanted with an epidural cup and an adjacent EEG electrode in the right parietal cortex. While the rat was behaving freely, a seizure-inducing concentration of acetylcholine (Ach) was applied into the cup. In a seizure termination study, either artificial cerebrospinal fluid (ACSF) or GABA (0.25, 2.5, 25 or 50mM) was delivered into the exposed neocortical area during an ongoing seizure. In a seizure prevention study, either ACSF or 50mM GABA was delivered into the epidural cup before the application of Ach. Epidural delivery of 50mM GABA completely terminated ongoing Ach-induced EEG seizures and convulsions within 17-437s after its delivery. ACSF and lower concentrations of GABA did not produce this effect, but 25mM GABA reduced seizure severity. However, the used GABA concentration could not prevent the development, or affect the severity, of Ach-induced EEG seizures and convulsions. This study indicates that transmeningeal GABA delivery can be used for terminating neocortical seizures, but to achieve seizure prevention via this route either a more efficient GABA delivery method needs to be developed or other neurotransmitters/pharmaceuticals should be employed for this purpose.