The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. I. The influence of administration vehicles

Translational Considerations in the Development of Intranasal Treatments for Epilepsy

Epilepsy is a common and serious neurological disorder, to which a high proportion of patients continue to be considered "drug-resistant", despite the availability of a host of anti-seizure drugs. Investigation into new treatment strategies is therefore of great importance. One such strategy is the use of the nose to deliver drugs directly to the brain with the help of pharmaceutical formulation to overcome the physical challenges presented by this route. The following review explores intranasal delivery of anti-seizure drugs, covering the link between the nose and seizures, pathways from the nose to the brain, current formulations in clinical use, animal seizure models and their proposed application in studying intranasal treatments, and a critical discussion of relevant pre-clinical studies in the literature.

GluN2C selective inhibition is a target to develop new antiepileptic compounds

OBJECTIVE

Many early-onset epilepsies present as developmental and epileptic encephalopathy associated with refractory seizures, altered psychomotor development, and disorganized interictal cortical activity. Abnormal upregulation of specific N-methyl-d-aspartate receptor (NMDA-R) subunits is being disentangled as one of the mechanisms of severe early-onset epilepsies. In tuberous sclerosis complex (TSC), upregulation of the GluN2C subunit of the NMDA-R with slow deactivation kinetic results in increased neuronal excitation and synchronization.

METHODS

Starting from an available GluN2C/D antagonist, NMDA-R-modulating compounds were developed and screened using a patch clamp on neuronal culture to select those with the strongest inhibitory effect on glutamatergic NMDA currents. For these selected compounds, blood pharmacokinetics and passage through the blood-brain barrier were studied. We tested the effect of the most promising compounds on epileptic activity in Tsc1^+/- mice brain slices with multielectrode array, and then in vivo at postnatal ages P14-P17, comparable with the usual age at epilepsy onset in human TSC.

RESULTS

Using a double-electrode voltage clamp on isolated NMDA currents, we identified the most prominent antagonists of the GluN2C subunit with no effect on GluN2A as a means of preventing side effects. The best compound passing through the blood-brain barrier was selected. Applied in vivo in six Tsc1^+/- mice at P14-P17, this compound reduced or completely stopped spontaneous seizures in four of them, and decreased the background activity disorganization. Furthermore, ictal-like discharges stopped on a human brain sample from an infant with epilepsy due to TSC.

INTERPRETATION

Subunit-selective inhibition is a valuable target for developing drugs for severe epilepsies resulting from an upregulation of NMDA-R subunit-mediated transmission.

Effect of acute and chronic exposure to lovastatin on the anticonvulsant action of classical antiepileptic drugs in the mouse maximal electroshock-induced seizure model

Numerous studies indicate neuroprotective activity of statins, commonly used cholesterol lowering drugs in epilepsy and several other neurological diseases. Promising anti-convulsant and neuroprotective effects of statins, attributed to their anti-excitotoxic and anti-inflammatory action were reported in several animals' seizure models. To determine the effects of acute (single) and chronic (once daily for 7 consecutive days) administration of lovastatin on the protective activity of four classical antiepileptic drugs such as carbamazepine, phenobarbital, phenytoin and valproate in the mouse maximal electroshock seizure model. Seizure activity (maximal electroconvulsions) in mice were generated by alternating current delivered via ear-clip electrodes. Adverse-effect profile of lovastatin combinations with the tested antiepileptic drugs was assessed in the chimney test (motor performance). Total brain concentrations of antiepileptic drugs were evaluated with the fluorescence polarization immunoassay technique as a measure of the pharmacokinetic interaction between drugs. Lovastatin administered acutely or chronically (5-20 mg/kg) did not significantly affect the threshold for electroconvulsions in mice. Acute lovastatin (10 mg/kg) significantly enhanced the anticonvulsant effect of valproate, which was accompanied with a 34% significant increase in total brain concentration of valproate. Acute lovastatin in combination with phenytoin impaired motor performance by notably decreasing the TD₅₀ value of phenytoin. Chronic lovastatin (10 mg/kg) markedly enhanced the anticonvulsant potential of phenytoin. Acute lovastatin increased anticonvulsant action of valproate but also significantly raised level of valproate in brain after combined administration suggesting pharmacokinetic nature of interaction. The combinations of chronic lovastatin combined with phenytoin can potentially enhance the anticonvulsant potency of phenytoin.

Systematic evaluation of rationally chosen multitargeted drug combinations: a combination of low doses of levetiracetam, atorvastatin and ceftriaxone exerts antiepileptogenic effects in a mouse model of acquired epilepsy

Epileptogenesis, the gradual process that leads to epilepsy after brain injury or genetic mutations, is a complex network phenomenon, involving a variety of morphological, biochemical and functional brain alterations. Although risk factors for developing epilepsy are known, there is currently no treatment available to prevent epilepsy. We recently proposed a multitargeted, network-based approach to prevent epileptogenesis by rationally combining clinically available drugs and provided first proof-of-concept that this strategy is effective. Here we evaluated eight novel rationally chosen combinations of 14 drugs with mechanisms that target different epileptogenic processes. The combinations consisted of 2-4 different drugs per combination and were administered systemically over 5 days during the latent epileptogenic period in the intrahippocampal kainate mouse model of acquired temporal lobe epilepsy, starting 6 h after kainate. Doses and dosing intervals were based on previous pharmacokinetic and tolerability studies in mice. The incidence and frequency of spontaneous electrographic and electroclinical seizures were recorded by continuous (24/7) video linked EEG monitoring done for seven days at 4 and 12 weeks post-kainate, i.e., long after termination of drug treatment. Compared to vehicle controls, the most effective drug combination consisted of low doses of levetiracetam, atorvastatin and ceftriaxone, which markedly reduced the incidence of electrographic seizures (by 60%; p<0.05) and electroclinical seizures (by 100%; p<0.05) recorded at 12 weeks after kainate. This effect was lost when higher doses of the three drugs were administered, indicating a synergistic drug-drug interaction at the low doses. The potential mechanisms underlying this interaction are discussed. We have discovered a promising novel multitargeted combination treatment for modifying the development of acquired epilepsy.

Polygonogram and isobolographic analysis of interactions between various novel antiepileptic drugs in the 6-Hz corneal stimulation-induced seizure model in mice

Pharmacotherapy with two antiepileptic drugs in combination is usually prescribed to epilepsy patients with refractory seizures. The choice of antiepileptic drugs in combination should be based on synergistic cooperation of the drugs with respect to suppression of seizures. The selection of synergistic interactions between antiepileptic drugs is challenging issue for physicians, especially, if 25 antiepileptic drugs are currently available and approved to treat epilepsy patients. The aim of this study was to determine all possible interactions among 5 second-generation antiepileptic drugs (gabapentin (GBP), lacosamide (LCM), levetiracetam (LEV), pregabalin (PGB) and retigabine (RTG)) in the 6-Hz corneal stimulation-induced seizure model in adult male albino Swiss mice. The anticonvulsant effects of 10 various two-drug combinations of antiepileptic drugs were evaluated with type I isobolographic analysis associated with graphical presentation of polygonogram to visualize the types of interactions. Isobolographic analysis revealed that 7 two-drug combinations of LEV+RTG, LEV+LCM, GBP+RTG, PGB+LEV, GBP+LEV, PGB+RTG, PGB+LCM were synergistic in the 6-Hz corneal stimulation-induced seizure model in mice. The additive interaction was observed for the combinations of GBP+LCM, GBP+PGB, and RTG+LCM in this seizure model in mice. The most beneficial combination, offering the highest level of synergistic suppression of seizures in mice was that of LEV+RTG, whereas the most additive combination that protected the animals from seizures was that reporting additivity for RTG+LCM. The strength of interaction for two-drug combinations can be arranged from the synergistic to the additive, as follows: LEV+RTG > LEV+LCM > GBP+RTG > PGB+LEV > GBP+LEV > PGB+RTG > PGB+LCM > GBP+LCM > GBP+PGB > RTG+LCM.

Network pharmacology for antiepileptogenesis: Tolerability and neuroprotective effects of novel multitargeted combination treatments in nonepileptic vs. post-status epilepticus mice

Network-based approaches in drug discovery comprise both development of novel drugs interacting with multiple targets and repositioning of drugs with known targets to form novel drug combinations that interact with cellular or molecular networks whose function is disturbed in a disease. Epilepsy is a complex network phenomenon that, as yet, cannot be prevented or cured. We recently proposed multitargeted, network-based approaches to prevent epileptogenesis by combinations of clinically available drugs chosen to impact diverse epileptogenic processes. In order to test this strategy preclinically, we developed a multiphase sequential study design for evaluating such drug combinations in rodents, derived from human clinical drug development phases. Because pharmacokinetics of such drugs are known, only the tolerability of novel drug combinations needs to be evaluated in Phase I in öhealthy" controls. In Phase IIa, tolerability is assessed following an epileptogenic brain insult, followed by antiepileptogenic efficacy testing in Phase IIb. Here, we report Phase I and Phase IIa evaluation of 7 new drug combinations in mice, using 10 drugs (levetiracetam, topiramate, gabapentin, deferoxamine, fingolimod, ceftriaxone, α-tocopherol, melatonin, celecoxib, atorvastatin) with diverse mechanisms thought to be important in epileptogenesis. Six of the 7 drug combinations were well tolerated in mice during prolonged treatment at the selected doses in both controls and during the latent phase following status epilepticus induced by intrahippocampal kainate. However, none of the combinations prevented hippocampal damage in response to kainate, most likely because treatment started only 16-18 h after kainate. This suggests that antiepileptogenic or disease-modifying treatment may need to start earlier after the brain insult. The present data provide a rich collection of tolerable, network-based combinatorial therapies as a basis for antiepileptogenic or disease-modifying efficacy testing.

The novel, catalytic mTORC1/2 inhibitor PQR620 and the PI3K/mTORC1/2 inhibitor PQR530 effectively cross the blood-brain barrier and increase seizure threshold in a mouse model of chronic epilepsy

The mTOR signaling pathway has emerged as a possible therapeutic target for epilepsy. Clinical trials have shown that mTOR inhibitors such as everolimus reduce seizures in tuberous sclerosis complex patients with intractable epilepsy. Furthermore, accumulating preclinical data suggest that mTOR inhibitors may have anti-seizure or anti-epileptogenic actions in other types of epilepsy. However, the chronic use of rapalogs such as everolimus is limited by poor tolerability, particularly by immunosuppression, poor brain penetration and induction of feedback loops which might contribute to their limited therapeutic efficacy. Here we describe two novel, brain-permeable and well tolerated small molecule 1,3,5-triazine derivatives, the catalytic mTORC1/C2 inhibitor PQR620 and the dual pan-PI3K/mTOR inhibitor PQR530. These derivatives were compared with the mTORC1 inhibitors rapamycin and everolimus as well as the anti-seizure drugs phenobarbital and levetiracetam. The anti-seizure potential of these compounds was determined by evaluating the electroconvulsive seizure threshold in normal and epileptic mice. Rapamycin and everolimus only poorly penetrated into the brain (brain:plasma ratio 0.0057 for rapamycin and 0.016 for everolimus). In contrast, the novel compounds rapidly entered the brain, reaching brain:plasma ratios of ∼1.6. Furthermore, they significantly decreased phosphorylation of S6 ribosomal protein in the hippocampus of normal and epileptic mice, demonstrating effective mTOR inhibition. PQR620 and PQR530 significantly increased seizure threshold at tolerable doses. The effect of PQR620 was more marked in epileptic vs. nonepileptic mice, matching the efficacy of levetiracetam. Overall, the novel compounds described here have the potential to overcome the disadvantages of rapalogs for treatment of epilepsy and mTORopathies directly connected to mutations in the mTOR signaling cascade.

Ivabradine attenuates the anticonvulsant potency of lamotrigine, but not that of lacosamide, pregabalin and topiramate in the tonic-clonic seizure model in mice

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are involved not only in synaptic transmission and neuronal excitability under physiological conditions, but also in seizure activity. To determine the influence of ivabradine (an HCN channel inhibitor) on the anticonvulsant potency of four novel antiepileptic drugs (AEDs: lacosamide, lamotrigine, pregabalin and topiramate) in the mouse maximal electroshock-induced seizure (MES) model. Adult male albino Swiss mice were challenged with maximal electroconvulsions (electric current of 25mA delivered via auricular electrodes). Total brain concentrations of AEDs were measured with high-pressure liquid chromatography. Ivabradine (10mg/kg, i.p.) significantly reduced the anticonvulsant potency of lamotrigine by elevating the ED₅₀ value of the AED from 7.48 (6.15-9.11) to 10.07 (8.85-11.45) mg/kg (P<0.05) in the mouse MES model. In contrast, ivabradine (10mg/kg, i.p.) did not significantly affect the anticonvulsant potency of lacosamide, pregabalin or topiramate in the mouse MES model. Additionally, ivabradine had no impact on total brain concentrations of all the studied AEDs in mice. A special caution is advised when combining ivabradine with lamotrigine in epilepsy patients due to the possible pharmacodynamic reduction of the anticonvulsant action of the later drug. The combinations of ivabradine with lacosamide, pregabalin and topiramate seem to be pharmacodynamic and neutral from a preclinical viewpoint.

Abnormal cell-intrinsic and network excitability in the neocortex of serotonin-deficient Pet-1 knockout mice

Neurons originating from the raphe nuclei of the brain stem are the exclusive source of serotonin (5-HT) to the cortex. Their serotonergic phenotype is specified by the transcriptional regulator Pet-1, which is also necessary for maintaining their neurotransmitter identity across development. Transgenic mice in which Pet-1 is genetically ablated (Pet-1(-/-)) show a dramatic reduction (∼80%) in forebrain 5-HT levels, yet no investigations have been carried out to assess the impact of such severe 5-HT depletion on the function of target cortical neurons. Using whole cell patch-clamp methods, two-dimensional (2D) multielectrode arrays (MEAs), 3D morphological neuronal reconstructions, and animal behavior, we investigated the impact of 5-HT depletion on cortical cell-intrinsic and network excitability. We found significant changes in several parameters of cell-intrinsic excitability in cortical pyramidal cells (PCs) as well as an increase in spontaneous synaptic excitation through 5-HT3 receptors. These changes are associated with increased local network excitability and oscillatory activity in a 5-HT2 receptor-dependent manner, consistent with previously reported hypersensitivity of cortical 5-HT2 receptors. PC morphology was also altered, with a significant reduction in dendritic complexity that may possibly act as a compensatory mechanism for increased excitability. Consistent with this interpretation, when we carried out experiments with convulsant-induced seizures to asses cortical excitability in vivo, we observed no significant differences in seizure parameters between wild-type and Pet-1(-/-) mice. Moreover, MEA recordings of propagating field potentials showed diminished propagation of activity across the cortical sheath. Together these findings reveal novel functional changes in neuronal and cortical excitability in mice lacking Pet-1.

Elevated serotonergic signaling amplifies synaptic noise and facilitates the emergence of epileptiform network oscillations

Serotonin fibers densely innervate the cortical sheath to regulate neuronal excitability, but its role in shaping network dynamics remains undetermined. We show that serotonin provides an excitatory tone to cortical neurons in the form of spontaneous synaptic noise through 5-HT3 receptors, which is persistent and can be augmented using fluoxetine, a selective serotonin re-uptake inhibitor. Augmented serotonin signaling also increases cortical network activity by enhancing synaptic excitation through activation of 5-HT2 receptors. This in turn facilitates the emergence of epileptiform network oscillations (10-16 Hz) known as fast runs. A computational model of cortical dynamics demonstrates that these two combined mechanisms, increased background synaptic noise and enhanced synaptic excitation, are sufficient to replicate the emergence fast runs and their statistics. Consistent with these findings, we show that blocking 5-HT2 receptors in vivo significantly raises the threshold for convulsant-induced seizures.

Searching for the ideal antiepileptogenic agent in experimental models: single treatment versus combinatorial treatment strategies

A major unmet medical need is the lack of treatments to prevent (or modify) epilepsy in patients at risk, for example, after epileptogenic brain insults such as traumatic brain injury, stroke, or prolonged acute symptomatic seizures like complex febrile seizures or status epilepticus. Typically, following such brain insults there is a seizure-free interval ("latent period"), lasting months to years before the onset of spontaneous recurrent epileptic seizures. The latent period after a brain insult offers a window of opportunity in which an appropriate treatment may prevent or modify the epileptogenic process induced by a brain insult. A similar latent period occurs in patients with epileptogenic gene mutations. Studies using animal models of epilepsy have led to a greater understanding of the factors underlying epileptogenesis and have provided significant insight into potential targets by which the development of epilepsy may be prevented or modified. This review focuses largely on some of the most common animal models of epileptogenesis and their potential utility for evaluating proposed antiepileptogenic therapies and identifying useful biomarkers. The authors also describe some of the limitations of using animal models in the search for therapies that move beyond the symptomatic treatment of epilepsy. Promising results of previous studies designed to evaluate antiepileptogenesis and the role of monotherapy versus polytherapy approaches are also discussed. Recent data from both models of genetic and acquired epilepsies strongly indicate that it is possible to prevent or modify epileptogenesis, and, hopefully, such promising results can ultimately be translated into the clinic.

Effect of sildenafil on the anticonvulsant action of classical and second-generation antiepileptic drugs in maximal electroshock-induced seizures in mice

PURPOSE

The goal of the present study was to evaluate the effects of sildenafil on the threshold for electrically induced seizures in mice. In addition, interactions between sildenafil and classical and second-generation antiepileptic drugs (AEDs), that is, carbamazepine (CBZ), phenobarbital (PB), phenytoin (PHT), valproate (VPA), lamotrigine (LTG), topiramate (TPM), and oxcarbazepine (OXC) were evaluated.

METHODS

Two electroconvulsive tests were used: maximal electroshock seizure threshold (MEST) and maximal electroshock seizure (MES) tests in mice. Acute adverse effects of the studied combinations were investigated in the chimney test, step-through passive avoidance task, and grip-strength test. Total brain and free plasma concentrations of AEDs were also determined.

RESULTS

Sildenafil raised the threshold for electroconvulsions in a dose-dependent manner. It also increased the anticonvulsant activity of CBZ, VPA, and TPM in the MES test, whereas the activity of the remaining AEDs was not significantly changed. Sildenafil increased total brain and free (protein unbound) plasma CBZ concentrations and total brain VPA concentration. Neither sildenafil nor its coadministration with the studied AEDs affected motor coordination and long-term memory in mice. Interestingly, sildenafil dose-dependently enhanced the skeletal muscle strength in mice, although combinations of sildenafil with AEDs were ineffective in this respect.

CONCLUSIONS

Sildenafil significantly raised the threshold for electroconvulsions in mice without any impairment of motor performance and long-term memory, but it enhanced muscle strength. Treatment of patients on CBZ or VPA with sildenafil may not be recommended due to pharmacokinetic interactions. Coadministration of sildenafil with other AEDs, especially with TPM, seems to be a reasonable choice.

The pharmacokinetics of antiepileptic drugs in rats: consequences for maintaining effective drug levels during prolonged drug administration in rat models of epilepsy

Rodent models of chronic epilepsy with spontaneous recurrent seizures likely represent the closest parallel to the human condition. Such models may be best suited for therapy discovery for pharmacoresistant epilepsy and for antiepileptogenic or disease-modifying therapeutics. However, the use of such rodent models for therapy discovery creates problems with regard to maintaining effective drug levels throughout a prolonged testing period. This is particularly due to the fact that rodents such as rats and mice eliminate most drugs much more rapidly than humans. Thus, knowledge about elimination rate of a test drug in a laboratory species is essential for development of a treatment paradigm that allows maintaining adequate drug levels in the system over the period of treatment. Currently, the most popular models of epilepsy with spontaneous seizures are poststatus epilepticus models of temporal lobe epilepsy in rats. Such models are both used for studies on antiepileptogenesis and drug resistance. For validation of these models, current antiepileptic drugs (AEDs) have to be used. In this article, the elimination rates of these AEDs and their effective plasma levels in rats are reviewed as a guide for developing treatment protocols for chronic drug testing. The advantages and disadvantages of several technologies for drug delivery are discussed, and some examples for calculation of adequate treatment protocols are given. As shown in this review, because of the rapid elimination of most AEDs in rats, it is no trivial task to maintain effective steady-state AED levels in the plasma throughout the day over multiple days to ensure that there will be adequate levels in the system for the purpose of the experiment. However, the use of an adequate dosing regimen that is based on elimination rate is an absolute prerequisite when using rat models for discovery of new antiepileptogenic therapies or therapies for pharmacoresistant epilepsy, because otherwise such models may lead to erroneous conclusions about drug efficacy.

The suppressive effects of oxcarbazepine on mechanical and cold allodynia in a rat model of neuropathic pain

Oxcarbazepine (OCBZ) is a keto analog of carbamazepine (CBZ) and may have similar analgesic properties to CBZ, but studies on its effects in neuropathic pain conditions are rare. In this study, we evaluated the analgesic effects of OCBZ in a rat neuropathic pain model. Male Sprague-Dawley rats were prepared by tightly ligating the left L5 and L6 spinal nerves to produce neuropathic pain. Sixty neuropathic rats were randomly assigned into six groups, and normal saline, a vehicle (polyethylene glycol 400), and OCBZ (10 mg/kg, 20 mg/kg, 30 mg/kg, and 50 mg/kg) were intraperitoneally administered to these individual groups. Mechanical and cold allodynia were observed at preadministration and 15, 30, 60, 90, 120, 150, and 180 min after drug administration and were quantified by measuring withdrawal frequencies to stimuli with von Frey filaments and 100% acetone, respectively. Rotarod performance was measured to detect drug-induced adverse motor effects. In the OCBZ-treated groups, withdrawal frequencies to mechanical and cold stimuli were significantly reduced in a dose-dependent manner (P < 0.05). Only at the largest dose did OCBZ reduce rotarod performance time. These results suggest that OCBZ may be a possible therapeutic consideration in neuropathic pain conditions associated with allodynia and hyperalgesia.

Influence of administration vehicles and drug formulations on the pharmacokinetic profile of lamotrigine in rats

Given that administration vehicles and drug formulations can affect drug bioavailability, their influence on the pharmacokinetic profile of lamotrigine (LTG), a new-generation anti-epileptic drug, was studied in rats. Three different formulations administered intraperitoneally at a dose of 10 mg/kg were used: (1) LTG suspended in a 0.25% methylcelulose solution, (2) LTG dissolved in a 50% propylene glycol solution, and (3) LTG isethionate dissolved in distilled water. Plasma and brain homogenate levels were determined in order to evaluate vehicle-dependent drug absorption. The results demonstrated rapid absorption of LTG when it was administered as an aqueous solution, in contrast to a slower and more erratic absorption after the injection of either the lipophilic solution or the suspension. A plasma peak was achieved 15 min post-dose with the aqueous solution, with a brain peak being achieved 15 min later, while with the other formulations both plasma and brain homogenate peaks were reached 2 h after LTG administration. This study suggests that LTG isethionate dissolved in distilled water is the most suitable formulation for successful LTG pharmacokinetic studies in rats.

Dragon's Blood incense: misbranded as a drug of abuse?

An unknown red substance was being sold and used with other drugs of abuse in Virginia (often being used in conjunction with marihuana). The red substance was identified as Dragon's Blood incense from Daemonorops draco. In bioassays, Dragon's Blood incense exhibited a low, but measurable cytotoxicity in in vitro cell lines. Dragon's Blood incense or Volatilized Dragon's Blood had no adverse effect on mouse motor performance based on the inclined screen and rotorod tests. delta(9)-Tetrahydrocannibinol (THC) produced a dose-related decline in mouse performance on the rotorod test. The combination of Dragon's Blood incense or Volatilized Dragon's Blood with delta(9)-THC did not contribute further to the impairment of the mice on the rotorod. This data suggests that the abuse potential for Dragon's Blood incense alone or in combination with marihuana is minimal.

The new antiepileptic drugs lamotrigine and felbamate are effective in phenytoin-resistant kindled rats

We evaluated the anticonvulsant efficacy of the antiepileptic drugs (AEDs) lamotrigine (LTG) and felbamate (FBM) in amygdala kindled rats that had been preselected with respect to their response to phenytoin. Anticonvulsant response was tested by determining the afterdischarge threshold (ADT), i.e., a sensitive measure for drug effects on focal seizure activity. By repeated testing with the phenytoin prodrug fosphenytoin, 3 groups of kindled rats were separated: rats in which consistent anticonvulsant effects were obtained (phenytoin responders), rats which showed no anticonvulsant response (phenytoin nonresponders), and rats with variable responses (variable phenytoin responders). The latter, largest group was used to evaluate at which doses LTG and FBM exerted significant anticonvulsant effects on ADT 1 h after i.p. drug administration. Effective doses were then used for drug testing in phenytoin responders and nonresponders. Both LTG and FBM proved to be effective anticonvulsant drugs in the kindling model by markedly increasing the ADT. Seizure severity and duration recorded at ADT currents were hardly reduced, indicating that both drugs predominantly affect induction of focal seizures and not seizure spread from the focus. In phenytoin nonresponders, LTG and FBM significantly increased ADT, which is in line with their proven efficacy in patients with refractory partial epilepsy in whom phenytoin has failed. However, LTG and, more markedly, FBM were clearly more efficacious in increasing ADT in phenytoin responders than in nonresponders, substantiating that the difference in phenytoin response between these groups of kindled rats extends to other AEDs. The data in this study reveal that phenytoin nonresponders are a unique model for the search for new AEDs with improved efficacy in refractory partial epilepsy.

Anti-convulsant and adverse effects of the glycineB receptor ligands, D-cycloserine and L-701,324: comparison with competitive and non-competitive N-methyl-D-aspartate receptor antagonists

In this study, the anticonvulsant and adverse effects of compounds that belong to four different categories of systemically available N-methyl-D-aspartate (NMDA) receptor ligands were compared, namely the competitive antagonist CGP 40116, the noncompetitive antagonist MK-801 (dizocilpine), the glycineB receptor antagonist L-701,324, and the glycineB receptor high-efficacy partial agonist D-cycloserine. The maximal electroshock seizures (MES), which are widely used to detect drug efficacy against generalized tonic-clonic seizures in humans, were produced by transcorneal electrical stimulation. Abolition of tonic hind-limb extension was taken as the end-point. The drug-induced motor and long-term memory deficits were quantified by using the inverted screen test and the step-through passive-avoidance test, respectively. All tested compounds exhibited significant anticonvulsant effect. The rank order of potency for the respective compounds was: MK-801 = CGP 40116 > L-701,324 >> D-cycloserine. All of these compounds induced motor impairment at doses close to those found to be anticonvulsant, however, hyperlocomotion and stereotyped behavior occurred only with MK-801. The highest protective indices [PI = TD50 (inverted screen)/ED50 (MES)] were calculated for CGP 40116 and D-cycloserine (2.4 and 2.2, respectively). When tested for memory impairment at one-half the MES ED50, again only MK-801 induced significant memory disruption in the passive avoidance test. In conclusion, these results suggest that glycineB receptor high-efficacy partial agonists and competitive NMDA receptor antagonists may be advantageous to noncompetitive NMDA antagonists and glycineB receptor antagonists as potential antiepileptic drugs.

Effects of single and repeated administration of sulthiame on amygdaloid kindled seizures in rats

In this study, we assessed the anti-convulsive effects of sulthiame (SUL) in amygdaloid (AM) kindled rats. Electrodes were implanted into the left AM of adult male Wistar rats. The animals were kindled at the after-discharge (AD) threshold. Upon completion of kindling, a generalized seizure triggering threshold was determined. The drugs were administered intraperitoneally in rats which reproducibly exhibited generalized convulsions at the near-threshold stimulation. Single administration of SUL (25-200 mg; n = 7-9) reduced the forelimb clonus (FCL) duration, but only the highest dose significantly regressed the secondarily generalized convulsion. During repeated administration of SUL, 50 mg/kg for 8 days, FCL duration was significantly alleviated until the fifth treatment day. With the dose of 200 mg/kg, significant suppression of secondary generalization was noted only until the second test day. On the other hand, significant reductions of FCL and AD duration were preserved afterwards. The anti-convulsive effects of SUL indicated in this study were not comparable to those of other standard anti-epileptic drugs reported from our laboratory.

Intravenous carbamazepine: comparison of different parenteral formulations in a mouse model of convulsive status epilepticus

PURPOSE

A drawback of carbamazepine (CBZ), a major antiepileptic drug (AED) with clinical efficacy against partial and generalized convulsive seizures, is its isolubility in aqueous vehicles, which is generally considered a contraindication to parenteral administration in epileptic patients. However, CBZ can be dissolved in glycofurol, a solvent used clinically as a vehicle for parenteral preparations of drugs such as diazepam (DZP) and phenytoin (PHT). Furthermore, aqueous CBZ solutions can be prepared by complexing CBZ with 2-hydroxypropyl-beta-cyclodextrin (HP beta CD), an inert beta-cyclodextrin derivative believed to have acceptable tolerability for human use. Such solutions of CBZ have been proposed to be suitable for intravenous administration in treatment of convulsive (grand mal) status epilepticus (CSE).

METHODS

A series of five generalized tonic-clonic seizures (GTCS) in 30 min was induced by repeated transauricular electrical stimulation in mice. In this model of convulsive (grand mal) SE, the anticonvulsant potency of intravenous CBZ dissolved in aqueous dilutions of either HP beta CD or glycofurol was evaluated.

RESULTS

In both solutions, CBZ rapidly suppressed seizures after intravenous bolus injection. Potent anticonvulsant activity was obtained as early as 30 s after injection, and peak effects were observed at approximately 3 min. ED50 for blockade of GTCS throughout the 30-min period of repeated electrical stimulation was approximately 7 mg/kg, similar to the potency of DZP in this model. Whereas the HP beta CD/CBZ solutions were tolerated by the animals, with no pronounced behavioral or motor adverse effects, the glycofurol/CBZ solutions induced marked sedation and motor impairment, indicating interactions between drug and solvent. Determination of CBZ in plasma and brain demonstrated that the rapid onset of anticonvulsant action after intravenous bolus injection was related to rapid drug penetration into brain tissue.

CONCLUSIONS

An intravenous formulation of CBZ achieved through complexing with HP beta CD might be suitable for parenteral use in acute clinical conditions such as SE, particularly because CBZ has the advantage of being almost free of respiratory or cardiovascular adverse effects.

Influence of D-cycloserine on the anticonvulsant activity of phenytoin and carbamazepine against electroconvulsions in mice

PURPOSE

D-Cycloserine (DCS) is a high-efficacy partial agonist at the strychnine-insensitive glycine modulatory site within the N-methyl-D-aspartate (NMDA)-receptor/ionophore complex. Previous studies demonstrated that DCS exhibits anticonvulsant activity in a variety of experimental epilepsy models. In this study, we determined the influence of DCS in subprotective doses on the anticonvulsant action of phenytoin (PHT) and carbamazepine (CBZ) in mice.

METHODS

Two electroconvulsive tests were used, i.e., determination of seizure threshold and maximal electroshock seizures. Antiepileptic drug-induced motor and long-term memory deficits were quantified by using the chimney test and the passive-avoidance test, respectively. In addition, plasma levels of PHT and CBZ were measured by fluorescence polarization immunoassay to exclude any pharmacokinetic interactions.

RESULTS

DCS, when used alone in doses of 80 and 160 mg/kg, significantly increased the threshold for electroconvulsive seizures. DCS in a wide range of doses (1.25-40 mg/kg) was combined with either PHT or CBZ and tested in electroconvulsive tests. DCS, at doses of 2.5 and 10 mg/kg, was the most effective in potentiating the threshold-increasing action of PHT; higher doses of DCS (20 and 40 mg/kg) were required to achieve a similar effect of CBZ. In maximal electroshock-induced seizures, DCS (10 mg/kg) augmented the protective action of PHT, but was ineffective at a dose of 40 mg/kg with CBZ. DCS did not potentiate the neurotoxicity produced by PHT and CBZ in the chimney test. Both PHT and CBZ induced impairments of long-term memory; PHT-induced memory adverse effects were counteracted by DCS (10 mg/kg). There was no such effect on CBZ-induced memory impairment, and a worsening influence was observed. Any pharmacokinetic interactions were excluded by measuring total and free plasma levels of both antiepileptic drugs.

CONCLUSION

Our results suggest that combining DCS with PHT and CBZ may be beneficial in treating epileptic seizures.

Behavioral effects of vehicles: DMSO, ethanol, Tween-20, Tween-80, and emulphor-620

Experimental drugs and compounds that do not easily dissolve in water or saline are frequently combined with vehicles like solvents, detergents, or vegetable oils. Yet very little has been reported on the behavioral effects of vehicles. In this study, we assessed the effects of a vegetable oil (emulphor-620), two detergents (Tween-20 and Tween-80), and two solvents [dimethyl sulphoxide (DMSO) and ethanol] on the locomotor activity in CD2F1 male mice. Locomotor activity was monitored for 12 h after vehicle administration (IP). The concentrations for each vehicle were expressed as percent of vehicle in saline (v/v). Emulphor-620 did not affect locomotor activity at any concentration tested (2%, 4%, 8%, 16%, and 32%). Tween-20 significantly decreased locomotor activity at a concentration of 16% and Tween-80 at 32%. DMSO significantly decreased locomotor activity at concentrations of 32% and 64%. In contrast, ethanol produced a biphasic behavioral response: increased activity at a concentration of 16% and decreased activity at a concentration of 32%. These results will facilitate the selection and concentration of vehicles to be used in combination with experimental drugs or test agents.

New injectable aqueous carbamazepine solution through complexing with 2-hydroxypropyl-beta-cyclodextrin: tolerability and pharmacokinetics after intravenous injection in comparison to a glycofurol-based formulation

The poor water solubility of the antiepileptic drug (AED) carbamazepine (CBZ) is generally considered an absolute contraindication to parenteral administration in epileptic patients. However, the water solubility of CBZ can be largely enhanced through formation of an inclusion complex with an amorphous cyclodextrin derivative, 2-hydroxypropyl-beta-cyclodextrin (HP beta CD). We studied tolerability and pharmacokinetics of an aqueous CBZ:HP beta CD solution after intravenous (i.v.) administration in dogs. For comparison, a conventional glycofurol-based solution of CBZ was used. We also administered a commercial liquid formulation of CBZ orally (p.o.). Infusion of CBZ:HP beta CD solutions or HP beta CD "placebo" formulations i.v. was well tolerated by the animals. In contrast, infusion of CBZ:glycofurol solutions and glycofurol placebo solutions induced marked behavioral and cardiovascular adverse effects. Pharmacokinetic studies indicated that glycofurol inhibited CBZ metabolism by decreasing formation of the major CBZ metabolite CBZ-10,11-epoxide (CBZ-E). With infusion of CBZ:HP beta CD 10 ml/min for 12-15 min, resulting in a CBZ dose of CBZ 5 mg/kg body weight, peak CBZ plasma levels of approximately 3.6 micrograms/ml were obtained. This relatively low peak concentration is primarily due to the rapid elimination of CBZ in dogs [half-life (t1/2) < 1 h]. Comparison of peak plasma levels determined after p.o. administration of CBZ to dogs with peak CBZ levels previously determined after p.o. administration in humans indicated that about four times higher doses are needed in dogs to attain the same peak plasma levels as in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Paroxysmal epileptiform discharges in temporal lobe slices after prolonged exposure to low magnesium are resistant to clinically used anticonvulsants

Lowering of extracellular Mg2+ results in various forms of epileptiform activity in different parts of temporal lobe slices [5,22] which contain neocortical areas such as areas Te2 or Te3, the entorhinal cortex (EC), subiculum, hippocampal areas CA1 to CA3 and the dentate gyrus [5,11]. In the EC, the subiculum and Te2/Te3 seizure-like events (SLEs) with tonic and clonic electrographic discharge patterns, negative slow field potentials and ionic changes comparable to those during tonic-clonic seizures in intact animals were observed. After 30 to 120 min of recurrent seizure activity (80 +/- 37 min) the seizure-like events (SLEs) developed into a state of late recurrent discharges (LRDs). Since previous studies had shown that the LRDs do not respond to valproic acid in contrast to a blocking effect of this drug on SLEs, we investigated the effects of the clinically employed anticonvulsants phenytoin, carbamazepine, phenobarbital, midazolam and ethosuximide on LRDs. All these agents were unable to block the LRDs in the EC, subiculum and Te2/Te3. This was found true both for concentrations which can block SLEs and for higher concentrations. Thus we conclude that this activity may represent a model of difficult to treat status epilepticus.

Short- and long-term effects of neonatal diazepam exposure on local cerebral glucose utilization in the rat

The short- and long-term consequences of a neonatal exposure to diazepam (DZP) on the postnatal changes in local cerebral metabolic rates for glucose (LCMRglcs) were studied by the quantitative autoradiographic [14C]2-deoxyglucose method in a total number of 66 brain structures of freely moving rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP, of the dissolution vehicle or of saline from postnatal day 2 (P2) to 21 (P21). The animals were studied at 4 ages, P10, P14, P21 and P60. DZP induced a decrease in LCMRglcs which was restricted to 13 areas at P10, mainly sensory and limbic regions. At P14, the treatment had significant metabolic effects on 48 structures belonging to all functional systems. By P21, 23 brain areas were still affected by the treatment, mainly sensory, limbic and motor areas. At P60, i.e. at about 40 days after the end of drug exposure, LCMRglcs still decreased in 14 brain regions which were mainly sensory and limbic structures. The structures most sensitive to both short- and long-term consequences of the anticonvulsant treatment are mammillary body, limbic cortices and sensory regions. The dissolution vehicle increased LCMRglcs in a few brain regions at P14 and P60, whereas it decreased metabolic levels in 5 brain regions at P21. The results of the present study show that the brain appears to be particularly vulnerable to the treatment at P14, period of active brain growth, whereas by P21, the drug is actively metabolized and a tolerance to the treatment may occur. The long-term effects of the treatment are in good accordance with the well-known effects of DZP on anxiety, sedation and memory. The structures most sensitive to early neonatal DZP exposure are the mammillary body, limbic cortices and sensory regions that all contain a high density of benzodiazepine binding sites.

Effects of early chronic diazepam treatment on incorporation of glucose and beta-hydroxybutyrate into cerebral amino acids: relation to undernutrition

The effects of early chronic diazepam (DZP) exposure on blood glucose and ketone body concentration and glucose and beta-hydroxybutyrate (beta HB) utilization for regional cerebral amino acid biosynthesis were studied in suckling rats. The animals were treated from postnatal day 2 (P2) to 21 (P21) by a daily subcutaneous injection of 10 mg/kg DZP or of the dissolution vehicle and studied at P5, P10, P14 and P21, together with an additional group of food-restricted rats obtained by an increase in litter size. DZP treatment induced a 9-26% decrease in body and brain weight. Undernutrition decreased body weight by 20-24% at all ages whereas brain weight was relatively spared. DZP and N-desmethyldiazepam concentrations decreased with age and were cleared from brain and plasma by 6-8 hrs after the injection. DZP decreased plasma glucose concentrations by 6-12% at P5, P14 and P21, whereas undernutrition did not change plasma glucose concentrations, except for a 7% decrease at P14. DZP treatment had no consequences on circulating concentrations of both ketone bodies while undernutrition increased their concentration by 45-362% at all ages. The conversion of [14C]glucose into cerebral amino acids was reduced by DZP at P5 and P10. The cerebral concentration of neurotransmitter amino acids was not affected by DZP treatment which only increased the amount of neutral amino acids mainly in the cerebellum at P5 and P10. After [U-14C]glucose injection, specific radioactivities of cerebral amino acids were mostly decreased by DZP from P5 to P14 and significantly increased at P21. With [3-14C] beta HB as a precursor, specific radioactivities of neurotransmitter amino acids were increased by DZP. In conclusion, P5 and P10 rats appear to be most sensitive to DZP effects whereas some tolerance to the drug seems to develop by P21. The lack of effects of DZP on blood ketone body concentrations compared to food restriction as well as the relative sparing of brain weight in undernourished rats confirms that the cerebral metabolic consequences of early DZP exposure on brain energy metabolism are mostly direct effects not mediated by sedation-induced undernutrition.

Differences in anticonvulsant potency and adverse effects between dextromethorphan and dextrorphan in amygdala-kindled and non-kindled rats

The anticonvulsant and adverse effects of dextromethorphan, a non-opioid antitussive, and its metabolite dextrorphan were examined in amygdala-kindled rats. Both drugs have repeatedly been proposed to be functional non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, but they also exert effects distinct from antagonism at NMDA receptors, such as blockade of voltage-gated calcium channels and sigma-site mediated actions. Since recent data have demonstrated that kindled rats are more susceptible to the adverse effects of NMDA receptor antagonists than non-kindled rats, the time course, characteristics and severity of adverse effects of dextromethorphan and dextrorphan were also determined in non-kindled animals. Dextromethorphan dose dependently increased the focal seizure threshold (i.e. the threshold for induction of afterdischarges recorded from the amygdala) in fully kindled rats. This anticonvulsant effect was found at relatively low doses (7.5-15 mg/kg i.p.) which were almost free of any adverse effects. At higher doses, dextromethorphan induced motor impairment and seizures, but no phenyclidine (PCP)-like adverse effects, such as hyperlocomotion or stereotypies. In contrast, such adverse effects were seen after dextrorphan, although only infrequently. Dextrorphan was less potent in inducing anticonvulsant but more potent in inducing motor impairing effects than dextromethorphan in kindled rats. In non-kindled rats, the motor impairment induced by dextrorphan was significantly less severe than in kindled rats, whereas no marked differences between kindled and non-kindled rats were found for dextromethorphan. The data indicate that dextromethorphan and dextrorphan differ in their mechanisms of action. Only dextrorphan exerts effects which are characteristic for NMDA receptor antagonism, whereas the potent anticonvulsant effect of dextromethorphan in presumably unrelated to the NMDA receptor complex.

Profile of ucb L059, a novel anticonvulsant drug, in models of partial and generalized epilepsy in mice and rats

The novel anticonvulsant drug ucb L059 ((S)-alpha-ethyl-2-oxo-1-pyrrolidineacetamide) was evaluated in several rodent models of partial and generalized seizures. Ucb L059 (27-108 mg/kg i.p.) increased the thresholds for tonic electroconvulsions and myoclonic and clonic seizures induced by timed i.v. infusion of pentylenetetrazol (PTZ), but was ineffective in the traditional maximal electroshock seizure and s.c. PTZ seizure tests in mice and rats in doses up to 500 mg/kg. The anticonvulsant potency of ucb L059 in seizure threshold tests was similar to that of standard drugs, such as valproate. In amygdala-kindled rats, ucb L059 exerted potent anticonvulsant activity against both focal and secondarily generalized seizures at doses of 13-108 mg/kg. The adverse effects of ucb L059 were quantitated in the open field and in standard tests for motor impairment, such as the rotarod and chimney tests. Ucb L059 exerted only minimal effects on behaviour, e.g. slight hyperactivity, and did not impair muscle activity in the rotarod test in doses up to 1700 mg/kg i.p. The data indicate that ucb L059 is an interesting new anticonvulsant agent with a broad spectrum of activity and high therapeutic index.

Dihydropyridine calcium antagonists in mice: blood and brain pharmacokinetics and efficacy against pentylenetetrazol seizures

Dihydropyridine calcium antagonists are candidate anticonvulsants, but little is known of their penetration into brain. Nifedipine (NFD) and nimodipine (NMD) pharmacokinetics were compared in mouse blood and brain, and their activity against pentylenetetrazol (PTZ) was assessed. After intraperitoneal (i.p.) injection, both dihydropyridines achieved peak blood and brain concentrations in 5 min. Estimated blood and brain elimination half-lives (t1/2) of NMD (16.7 and 22.4 min) were slightly longer than those of NFD (11.2 and 14.7 min). Brain and blood concentrations correlated with both NFD (r = 0.701, p less than 0.001) and NMD (r = 0.572, p less than 0.001). Injection of the dihydropyridines as a suspension (Tween 80) did not alter brain penetration, although systemic absorption was more erratic. NFD (p less than 0.001), NMD (p less than 0.02), and carbamazepine (CBZ, p less than 0.001) i.p. inhibited PTZ-induced seizures. Brain concentrations of PTZ were not altered by NFD pretreatment. Combining NFD and CBZ was less effective than giving NFD alone (p less than 0.005). NFD (p less than 0.002) and NMD (p less than 0.001) inhibited PTZ seizures after 2-week oral dosing, but low dosing was more effective than high dosing (p less than 0.002). NFD and NMD cross the blood-brain barrier (BBB) in mice and inhibit PTZ seizures. A possible therapeutic window was identified, and NFD and CBZ were less effective in combination than singly. A pharmacodynamic interaction may exist, inhibiting effective use of dihydropyridines as adjunctive therapy in epileptic patients.

Trans-2-en-valproate: reevaluation of its anticonvulsant efficacy in standardized seizure models in mice, rats and dogs

The anticonvulsant potency of the trans isomer of 2-en-valproate (trans-2-en-VPA) was determined in standardized models for different seizure types in rodents and dogs. In mice and rats, adverse effects were quantified by the rotarod and chimney tests. Clinically established antiepileptic drugs (valproate, ethosuximide, phenobarbital, carbamazepine, phenytoin, diazepam) were used for comparison. Based on time course studies, drug potencies were determined and compared at the individual time of peak anticonvulsant effect. Potency comparisons were based on administered dosages and, in the case of trans-2-en-VPA and valproate, also on plasma levels determined after administration of anticonvulsant doses. The data show that trans-2-en-VPA exerts anticonvulsant effects against different seizure types, i.e., myoclonic, clonic, and tonic seizures in rodents and (myo)clonic seizures in dogs. In most seizure models, trans-2-en-VPA was more potent than valproate, when both compounds were compared at their individual times of peak effect. Time course and pharmacokinetic studies showed that duration of action and pharmacokinetic characteristics of trans-2-en-VPA and valproate are similar. In the rotarod and chimney tests in mice and rats, trans-2-en-VPA was more potent than valproate. However, because of the higher anticonvulsant potency of trans-2-en-VPA, protective indices calculated from rodent models were similar to those of valproate. Similarly, in dogs trans-2-en-VPA exerted anticonvulsant effects at doses below those which induced sedation and ataxia. In view of the previously reported advantages of trans-2-en-VPA compared to valproate with respect to teratogenic and hepatotoxic effects, the present data substantiate that trans-2-en-VPA might be a valuable alternative to valproate in antiepileptic therapy.

Ralitoline: a reevaluation of anticonvulsant profile and determination of "active" plasma concentrations in comparison with prototype antiepileptic drugs in mice

Ralitoline (RLT) is a new thiazolidinone derivative with potent anticonvulsant activity in different seizure models. During Phase I studies, RLT was well tolerated in human volunteers and showed linear pharmacokinetics in the dose range tested (up to 150 mg). Since RLT will soon be entering clinical Phase II studies, we were interested in obtaining predictive data for effective plasma concentrations in patients. For this purpose, the anticonvulsant potency of RLT was determined in four seizure models in mice, and plasma levels were measured at time of peak drug effect. The four models were the threshold for maximal (tonic extension) electroshock seizures (MES), the threshold for clonic seizures determined by i.v. infusion of pentylenetetrazol (PTZ), the traditional MES test with supramaximal (50 mA) stimulation, and generalized clonic seizures induced by s.c. administration of PTZ. Furthermore, median minimal "neurotoxic" doses (TD50s) were determined by the rotorod and chimney test for calculation of protective indices. All data obtained for RLT were compared with data obtained with standard antiepileptic drugs: phenobarbital, phenytoin, valproate, and diazepam. The onset of anticonvulsant action after i.p. injection of RLT was very rapid, and the peak drug effect was already obtained after 2 min. In the MES models, RLT was the most potent compound. "Active" plasma levels ranged from approximately 300 ng/ml in the MES threshold test to approximately 1,300 ng/ml in the MES test. RLT was also capable of increasing the PTZ threshold, whereas, possibly because of its short duration of action in mice, it was not very active in the s.c. PTZ seizure test.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. IV. Protective indices

Calculation of protective or therapeutic indices is widely used in primary and secondary screening for drugs with selective anticonvulsant activity. The protective index is the median minimal 'neurotoxic' dose, TD50, divided by median effective dose, ED50. TD50s are usually determined by tests, such as the rotarod test or the chimney test, for quantification of 'minimal neurological deficit', such as motor impairment, while median effective doses are commonly determined in the maximal electroshock seizure (MES) test or the s.c. pentylenetetrazol (PTZ) seizure test in mice or rats. For antiepileptic drug development, it has been proposed previously that only compounds with an estimated protective index of at least 5 should proceed to further evaluation. However, various technical, biological and pharmacological factors can influence anticonvulsant or 'neurotoxic' potencies and thereby protective indices. In order to reevaluate the value of protective indexes in the prediction of drugs with selective anticonvulsant action, protective indices were determined for various clinically used antiepileptic drugs in standardized seizure tests, i.e. MES and s.c. PTZ tests in mice and rats, as well as in seizure threshold tests. For most drugs, similar TD50s were determined in the rotarod and chimney test. When protective indices were calculated for the different seizure models, only few drugs reached an index of 5 (some not even reaching an index of 2) in the traditional MES or s.c. PTZ tests in mice and rats. In contrast, using anticonvulsant doses determined by seizure threshold tests, the 5 primary drugs against generalized tonic-clonic seizures, i.e., carbamazepine, phenytoin, phenobarbital, primidone and valproate, had indices of more than 5 in the MES threshold model, while drugs with efficacy against absence and myoclonic seizures, i.e., valproate, ethosuximide and the benzodiazepines, had protective indices of at least 5 in the i.v. PTZ seizure threshold model. The data substantiate that valuable information can be obtained by estimation of protective indices. However, in order to minimize the possibility that an interesting new anticonvulsant compound is overlooked during primary or secondary screening, a protective index of 2 should be considered sufficient in case of traditional MES or s.c. PTZ models with fixed seizure stimulus. Alternatively, seizure threshold models could be used for calculation of protective indices in order to avoid underestimation of anticonvulsant selectivity of test compounds.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. III. Pentylenetetrazole seizure models

Although seizure models using systemic administration of the chemoconvulsant pentylenetetrazol (PTZ) for induction of generalized clonic seizures in rodents are widely employed to identify potential anticonvulsants, the important role of diverse technical, biological and pharmacological factors in interpretation of results obtained with these models is often not recognized. The aim of this study was to delineate factors other than sex, age, diet, climate, and circadian rhythms, which are generally known. For this purpose, experiments with 8 clinically established antiepileptic drugs were undertaken in the following PTZ models: (1) the threshold for different types of PTZ seizures, i.e., initial myoclonic twitch, generalized clonus with loss of righting reflexes, and tonic backward extension of forelimbs (forelimb tonus), in mice; (2) the traditional PTZ seizure test with s.c. injection of the CD97 for generalized clonic seizures in mice; and (3) the s.c. PTZ seizure test in rats. In rats, in addition to evaluating drug effects on generalized clonic seizures, a ranking system was used to determine drug effects on other seizure types. When drugs were dissolved in vehicles which themselves did not exert effects on seizure susceptibility, the most important factors which influenced drug potencies were: (1) bishaped dose-response curves, i.e., a decline in anticonvulsant dose-response at high doses of some drugs, leading to misinterpretations of drug efficacy if only a single high drug dosage is tested; (2) effects of route of PTZ administration (i.v. infusion vs. s.c. injection) on estimation of anticonvulsant potency; (3) species differences in drug metabolism; (4) differences in drug potencies calculated on the basis of administered doses compared to potency calculations based on 'active' drug concentrations in plasma; (5) qualitative and quantitative species differences in drug actions; (6) endpoints used for PTZ tests; (7) misleading predictions from PTZ seizure models. Analysis of anticonvulsant drug actions indicated that myoclonic or clonic seizures induced by i.v. or s.c. PTZ might be suitable for predicting efficacy against myoclonic petit mal seizures in humans, but certainly not to predict efficacy against absence seizures. Tonic seizures induced by PTZ were blocked by drugs, such as ethosuximide, which exert no effect on tonic seizures in humans. In order to reduce the variability among estimates of anticonvulsant activity in PTZ seizure models, the various factors delineated in this study should be rigidly controlled in experimental situations involving assay of anticonvulsant agents.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. II. Maximal electroshock seizure models

Although seizure models using electrical stimulation for the induction of generalized tonic-clonic seizures in rodents are widely employed to identify potential anticonvulsants, the important role of various technical, biological and pharmacological factors in the interpretation of results obtained with these models is often not recognized. The aim of this study was to delineate factors other than sex, age, diet, climate and circadian rhythms, which are generally known. For this purpose, experiments with 8 clinically established antiepileptic drugs were undertaken in the following electroshock seizure models: (1) the maximal (tonic extensor) electroshock seizure threshold (MEST) in mice; (2) the traditional maximal electroshock seizure (MES) test with supra-threshold stimulation in mice; and (3) the MES test with suprathreshold stimulation in rats. When drugs were dissolved in vehicles which did not themselves exert effects on seizure susceptibility, the most important factors which influenced drug potencies were (1) marked differences between drugs and species in terms of peak drug effect, duration of action and the formation of active metabolites; (2) differences in drug potencies calculated on the basis of administered doses compared to potency calculations based on active drug concentrations; (3) the equipment used for seizure induction; (4) marked effects of current strength on results obtained in electroshock seizure models; (5) site of application of the electrical stimulus (transcorneal vs. transauricular). In order to reduce the variability among estimates of anticonvulsant activity, the various factors delineated in this study should be rigidly controlled in experimental situations involving assay of anticonvulsant agents.