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

Laboratory investigation

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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.

Abbreviations used in this paper:ACh = acetylcholine; AED = antiepileptic drug; CCD = charged-coupled device; EEG = electroencephalography; HPLC-ED = high-performance liquid chromatography electrochemical detection; RF = radiofrequency; SPD = Subdural Pharmacotherapy Device.

Article Information

Address correspondence to: Nandor Ludvig, M.D., Ph.D., NYU Comprehensive Epilepsy Center, 223 East 34th Street, New York, New York 10016. email: nandor.ludvig@nyumc.org.

Please include this information when citing this paper: published online May 11, 2012; DOI: 10.3171/2012.4.JNS111488.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Top view of the exposed cranium of Monkey 6 during surgical implantation of the SPD. Upper: Photograph taken at the moment when the electrode- and fluid port–integrating strip was slipped under the dura to reach the frontal cortical target site. The broken line indicates the interaural line and the arrow shows the location of the epidural reference electrode. d = dura; e = electrode-contact; f = fluid port; m = midline. Lower: The same area 10 minutes later, after the subdural strip was moved 1.5 cm toward the right frontal bone and the craniotomy was closed with the silicone rubber sheet (arrow). Dental acrylic sealed the gap between the sheet and the bone.

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    Temporal pattern of the fluid-delivery/fluid-removal cycles executed by the SPD dual minipump for up to 11 months. As indicated, one unit of the minipump directed either saline or muscimol into the frontal cortical subdural/subarachnoid space at 12-hour intervals, whereas the other pump unit removed CSF from the same site at 6-hour intervals to wash out accumulating products of host tissue reaction. Inflow and outflow volumes were adjusted to keep the total volume of the treated subdural/subarachnoid compartment constant. The extra flushing cycle, implemented for each 24-hour period, served to ensure adequate cleansing of the treated area. h = hours.

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    Photograph of Monkey 4, taken while the animal explored the test chamber. Note the white protective cap on the animal's head. This cap protected all externalized SPD components, including the minipump, the microcontroller, the RF communication module, and the battery, and could be opened to provide access to these components for maintenance. (Adaptation and encasement of these externalized SPD components for implantation in the chest or abdomen of human patients have yet to be accomplished.)

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    Relationship between a single (120-μl, 1.0-mM) subdural muscimol delivery with the SPD during ongoing seizure activity and the levels of muscimol in the blood, cisternal CSF, and implantation-site CSF. A: Electroencephalography recordings transmitted from the frontal cortex of the implanted monkey before focal seizure induction. B: Acetylcholine-induced focal EEG seizure activity, localized to the ipsilateral ACh-delivery site (Channels [Ch.] 1–3). C: Termination of the focal seizure following the delivery of 1.0 mM muscimol into the seizure focus. D–F: High-performance liquid chromatography electrochemical detection chromatograms of serum, cisternal CSF, and implantation-site CSF samples, respectively, collected in this experiment 35–45 minutes after muscimol delivery. Muscimol (32 μg/ml) is detectable only in the subdural implantation/delivery site (F).

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    Demonstration of the seizure-preventing potency of muscimol (60 μl, 1.0 mM) delivered by the SPD into the frontal cortical seizure focus in Monkey 5. Changes in the power of 4- to 8-Hz EEG waves are shown during 30-minute (1800-second) recording sessions. Upper: Changes in power before and after ACh application in the condition where the SPD performed periodic, subdural saline delivery into the frontal cortex. Note the ACh-induced robust power increase related to EEG seizure activity, and the return to baseline power about 3 minutes after the SPD minipump delivered muscimol into the seizure focus. Lower: Repeat of the same experiment in the condition in which the SPD was switched from periodic saline delivery to periodic muscimol delivery. This treatment completely prevented the development of ACh-induced seizures, as reflected in the lack of increase in the power of the 4- to 8-Hz band. Corresponding maximum power values of the 4- to 8-Hz band are indicated on each panel.

  • View in gallery

    High-performance liquid chromatography electrochemical detection chromatograms of the 1.0-mM muscimol solution used for subdural delivery by the SPD (A), that of a serum sample collected from a monkey before surgery (B), and that of a serum sample collected 120 minutes after the extra delivery of 40 μl of 1.0 mM muscimol in a SPDimplanted monkey, 3 weeks after the start of periodic subdural muscimol delivery by the device (C). Note the muscimol peak in A (122 μg/ml; arrow) and the lack of this peak at the same retention time (indicated with the broken line) in chromatograms B and C.

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    Withdrawal seizures and their prevention in 2 SPD-implanted monkeys. Abrupt discontinuation after 97 and 99 days of subdural delivery of 1.0 mM muscimol (musc.) in Monkeys 3 and 4, respectively, induced Score 4 (generalized) seizures within 10 hours. Restoration of the original muscimol delivery schedule promptly stopped these seizures and normalized both EEG and behavior. Subsequent gradual reduction of the original muscimol concentration to 0.03 mM (Monkey 3; upper) or 0.12 mM (Monkey 4; lower) prevented the development of withdrawal seizures after the discontinuation of muscimol treatment.

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