Anticonvulsant effect of fosphenytoin in amygdala-kindled rats: comparison with phenytoin

Ureido Functionalization through Amine-Urea Transamidation under Mild Reaction Conditions

Ureido-functionalized compounds play an indispensable role in important biochemical processes, as well as chemical synthesis and production. Isocyanates, and KOCN in particular, are the preferred reagents for the ureido functionalization of amine-bearing compounds. In this study, we evaluate the potential of urea as a reagent to graft ureido groups onto amines at relatively low temperatures (<100 °C) in aqueous media. Urea is an inexpensive, non-toxic and biocompatible potential alternative to KOCN for ureido functionalization. From as early as 1864, urea was the go-to reagent for polyurea polycondensation, before falling into disuse after the advent of isocyanate chemistry. We systematically re-investigate the advantages and disadvantages of urea for amine transamidation. High ureido-functionalization conversion was obtained for a wide range of substrates, including primary and secondary amines and amino acids. Reaction times are nearly independent of substrate and pH, but excess urea is required for practically feasible reaction rates. Near full conversion of amines into ureido can be achieved within 10 h at 90 °C and within 24 h at 80 °C, and much slower reaction rates were determined at lower temperatures. The importance of the urea/amine ratio and the temperature dependence of the reaction rates indicate that urea decomposition into an isocyanic acid or a carbamate intermediate is the rate-limiting step. The presence of water leads to a modest increase in reaction rates, but the full conversion of amino groups into ureido groups is also possible in the absence of water in neat alcohol, consistent with a reaction mechanism mediated by an isocyanic acid intermediate (where the water assists in the proton transfer). Hence, the reaction with urea avoids the use of toxic isocyanate reagents by in situ generation of the reactive isocyanate intermediate, but the requirement to separate the excess urea from the reaction product remains a major disadvantage.

A new method for simultaneous quantification of fosphenytoin, phenytoin and its primary metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin in whole blood by ultra-performance liquid chromatography-tandem mass spectrometry

A method for simultaneous quantification of fosphenytoin (F-PHT), phenytoin (PHT) and its main metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) in whole blood was developed and validated using ultra-performance liquid chromatography-tandem mass spectrometry. Whole blood samples were pretreated by liquid-liquid extraction with acetonitrile and methanol. Chromatographic separation was performed using a CORTECS™ UPLC® C18 (2.1 × 50 mm i.d., particle size 1.6 μm) analytical column, and water containing 10 mM ammonium formate and acetonitrile as the mobile phase. Quantification of the analytes was carried out using mass chromatography with each product ion referenced against phenytoin-d₁₀ as an internal standard. Calibration curves exhibited good linear relationships in a range from 0.005 to 50 μg/ml with correlation coefficients exceeding 0.995. The limits of detection were estimated to be 0.002-0.01 μg/ml. The accuracies and precisions were 96.2-104.3% and 0.7-10.7%, respectively. The recovery efficiencies were in the range of 42.4-59.2%. Matrix effects were observed for PHT and HPPH, with signal suppression ranging from -6.6 to -32.2%. Matrix effect for F-PHT (-5.0 to 8.9%) was less than those for PHT and HPPH. All analytes were stable under different storage conditions. This method was successfully applied for the quantification of F-PHT, PHT and HPPH in rat whole blood samples taken after bolus intravenous administration of F-PHT.

Ethosuximide and phenytoin dose-dependently attenuate acute nonconvulsive seizures after traumatic brain injury in rats

Acute seizures frequently occur following severe traumatic brain injury (TBI) and have been associated with poor patient prognosis. Silent or nonconvulsive seizures (NCS) manifest in the absence of motor convulsion, can only be detected via continuous electroencephalographic (EEG) recordings, and are often unidentified and untreated. Identification of effective anti-epileptic drugs (AED) against post-traumatic NCS remains crucial to improve neurological outcome. Here, we assessed the anti-seizure profile of ethosuximide (ETX, 12.5-187.5 mg/kg) and phenytoin (PHT, 5-30 mg/kg) in a spontaneously occurring NCS model associated with penetrating ballistic-like brain injury (PBBI). Rats were divided between two drug cohorts, PHT or ETX, and randomly assigned to one of four doses or vehicle within each cohort. Following PBBI, NCS were detected by continuous EEG monitoring for 72 h post-injury. Drug efficacy was evaluated on NCS parameters of incidence, frequency, episode duration, total duration, and onset latency. Both PHT and ETX attenuated NCS in a dose-dependent manner. In vehicle-treated animals, 69-73% experienced NCS (averaging 9-10 episodes/rat) with average onset of NCS occurring at 30 h post-injury. Compared with control treatment, the two highest PHT and ETX doses significantly reduced NCS incidence to 13-40%, reduced NCS frequency (1.8-6.2 episodes/rat), and delayed seizure onset: <20% of treated animals exhibited NCS within the first 48 h. NCS durations were also dose-dependently mitigated. For the first time, we demonstrate that ETX and PHT are effective against spontaneously occurring NCS following PBBI, and suggest that these AEDs may be effective at treating post-traumatic NCS.

Delivery of plasmid DNA encoding bone morphogenetic protein-2 with a biodegradable branched polycationic polymer in a critical-size rat cranial defect model

This study investigated the delivery of plasmid DNA (pDNA) encoding bone morphogenetic protein-2 in the form of polyplexes with a biodegradable branched triacrylate/amine polycationic polymer (TAPP) that were complexed with gelatin microparticles (GMPs) loaded within a porous tissue engineering scaffold. More specifically, the study investigated the interplay between TAPP degradation, gelatin degradation, pDNA release, and bone formation in a critical-size rat cranial defect model. The pDNA release kinetics in vitro were not affected by the crosslinking density of the GMPs but depended, rather, on the degradation rates of the TAPPs. Besides the initial release of polyplexes not bound to the GMPs and the minimal release of polyplexes through diffusion or dissociation from the GMPs, the pDNA was likely released as naked pDNA or as part of an incomplete polyplex, after the degradation of fragments of the polycationic polymer. After 30 days, significantly higher amounts of pDNA were released (93%-98%) from composite scaffolds containing naked pDNA or pDNA complexed with P-AEPZ (synthesized with 1-[2-aminoethyl]piperazine, a faster degrading TAPP) compared with those containing pDNA complexed with P-DED (synthesized with N,N-dimethylethylenediamine, a slower degrading TAPP) (74%-82%). Composite scaffolds containing GMPs complexed with TAPP/pDNA polyplexes did not result in enhanced bone formation, as analyzed by microcomputed tomography and histology, in a critical-size rat cranial defect at 12 weeks postimplantation compared with those loaded with naked pDNA. The results demonstrate that polycationic polymers with a slow degradation rate can prolong the release of pDNA from the composite scaffolds and suggest that a gene delivery system comprising biodegradable polycationic polymers should be designed to release the pDNA in an intact polyplex form.

Preliminary explorations of the role of mitochondrial proteins in refractory epilepsy: some findings from comparative proteomics

Approximately 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms that underlie why some patients are responsive and others prove resistant to antiepileptic drugs are poorly understood. Increasing evidence supports a role for altered mitochondrial function in the pathogenesis of epilepsy. To gain greater molecular insight in the pathogenesis of intractable epilepsy, we undertook a global analysis of protein expressions in a pharmacoresistant epileptic model selected by phenytoin in electrical amygdala-kindled rats by using two-dimensional gel electrophoresis coupled with matrix-assisted laser desorption/ionization time of flight (MALDI-TOF-TOF). We identified five increased proteins and 14 decreased proteins including voltage-dependent anion channel 1 (VDAC1) with a 2.82-fold increased level (P < 0.05) and voltage-dependent anion channel 2 (VDAC2) with a 3.97-fold decreased level (P < 0.05) in hippocampus of pharmacoresistant rats. The increased VDAC1 and decreased VDAC2 were confirmed by Western blot analysis and immunohistochemistry. Vascular mitochondria and apoptosis neurons were observed through electron microscopy. Energy contents, the adenine nucleotides, were measured by high-performance liquid chromatography (HPLC). The correlation analyses were carried out between VDAC and the energy charge. These findings indicate that the increase of VDAC1 and the decrease of VDAC2 play an important role during the process and provide new molecular evidence in understanding mechanism of refractory epilepsy.

Antiepileptic Drug Treatment of Nonconvulsive Seizures Induced by Experimental Focal Brain Ischemia

Nonconvulsive seizures (NCSs) after traumatic and ischemic brain injury are often refractory to antiepileptic drug therapy and are associated with a decline in patient outcome. We recently characterized an in vivo rat model of focal brain ischemia-induced NCS and here sought to evaluate potential pharmacological treatments. Electroencephalographic activity was recorded continuously for 24 h in freely behaving rats subjected to permanent middle cerebral artery occlusion (MCAo). Rats were treated with an antiepileptic drug from one of seven different drug classes at ED(50) and 2x ED(50) doses (as reported in other rat seizure models), delivered as a single i.v. injection 20 min post-MCAo. Vehicle-treated rats (n = 9) had an 89% incidence of NCS with an average number of NCS of 8.6 +/- 1.9. The latency to onset of NCS was 32.5 +/- 3.4 min post-MCAo with an average duration of 49.1 +/- 8.2 s/event. The high doses of ethosuximide, gabapentin, fos-phenytoin, and valproate significantly reduced the incidence of NCS (11, 14, 14, and 38%, respectively), whereas midazolam, phenobarbital, and dextromethorphan had no significant effect at either dose. Across treatment groups, there was a low but significant correlation between the number of NCS events per animal and volume of brain infarction (r = 0.352). Antiepileptic drug therapy that prevented the occurrence of NCS also reduced mortality from 26 to 7%. Based on combined effects on NCS, infarction, neurological recovery, and mortality, ethosuximide and gabapentin were identified as having the best therapeutic profile.

Continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat

OBJECTIVE

To explore the effect of continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat.

METHODS

An electrode and cannula complex was implanted in adult male Wistar rats, the electrode being targeted to the left basolateral amygdala. The animals were subjected to a standard kindling procedure. Osmotic minipumps filled with either GABA or mannitol were connected to cannulas and allowed to infuse during 7 days. Kindling experiments measuring after-discharge and seizure thresholds, seizure severity and duration, and behavioral toxicity were performed before, during and after the drug infusion period.

RESULTS

Both after-discharge and seizure thresholds were significantly increased both during and after GABA infusion with respect to pre-infusion controls, while the group receiving mannitol showed no significant differences. Seizure duration was not affected by GABA infusion. Seizures were either completely blocked or fully developed.

CONCLUSIONS

Direct continuous GABA infusion within the epileptogenic focus raises the seizure thresholds without affecting the generalized components in the amygdala kindling model of epilepsy.

A comparison of central brain (cerebrospinal and extracellular fluids) and peripheral blood kinetics of phenytoin after intravenous phenytoin and fosphenytoin

Phenytoin (PHT) is a first-line drug in the treatment of status epilepticus. However, the parenteral PHT formulation is associated with administration difficulties and therefore fosphenytoin (FosPHT), a PHT pro-drug, has been developed. As the peripheral (blood) and central (cerebrospinal fluid [CSF] and brain extracellular fluid [ECF]) kinetic inter-relationship of PHT after i.v. FosPHT administration is unknown we sought to ascertain the relationship and to compare it to that of i.v. PHT. A freely behaving rat model, which allows for the concurrent and temporal sampling of blood (jugular vein), CSF (cisterna magna) and brain ECF (frontal cortex and hippocampus), was used. PHT and FosPHT were administered by i.v. infusion and blood, CSF and microdialysate samples collected at timed intervals up to 6 hours. The pharmacokinetic parameters in plasma of PHT after PHT and FosPHT (30 and 60 mg/kg) administration were indistinguishable. The PHT plasma free fraction (free/total concentration ratio) was 0.25-0.31 and 0.26-0.31 for PHT and FosPHT, respectively. Mean PHT Tmax values for CSF were 9-13 minutes. The equivalent values in the frontal cortex and hippocampal ECF were 29-34 minutes. Cmax values increased dose-dependently and were independent of whether PHT or FosPHT was administered. Furthermore the kinetic profiles of PHT for the frontal cortex and hippocampus were indistinguishable suggesting that PHT distribution in the brain is not brain region specific. Thus, overall, the central and peripheral kinetics of PHT are indistinguishable after PHT and FosPHT.

Effect of phenytoin on sodium and calcium currents in hippocampal CA1 neurons of phenytoin-resistant kindled rats

About 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms explaining why some patients' respond and others prove resistant to antiepileptic drugs are poorly understood. It has been proposed that pharmacoresistance is related to reduced sensitivity of sodium channels in hippocampal neurons to antiepileptic drugs such as carbamazepine or phenytoin. In line with this proposal, a reduced effect of carbamazepine on sodium currents in hippocampal CA1 neurons was found in the rat kindling model of temporal lobe epilepsy (TLE), i.e. a form of epilepsy with the poorest prognosis of all epilepsy types in adult patients. To address directly the possibility that neuronal sodium currents in the hippocampus play a crucial role in the pharmacoresistance of TLE, we selected amygdala-kindled rats with respect to their in vivo anticonvulsant response to phenytoin into responders and nonresponders and then compared phenytoin's effect on voltage-activated sodium currents in CA1 neurons. Furthermore, in view of the potential role of calcium current modulation in the anticonvulsant action of phenytoin, the effect of phenytoin on high-voltage-activated calcium currents was studied in CA1 neurons. Electrode-implanted but not kindled rats were used as sham controls for comparison with the kindled rats. In all experiments, the interval between last kindled seizure and ion channel measurements was at least 5 weeks. In kindled rats with in vivo resistance to the anticonvulsant effect of phenytoin (phenytoin nonresponders), in vitro modulation of sodium and calcium currents by phenytoin in hippocampal CA1 neurons did not significantly differ from respective data obtained in phenytoin responders, i.e. phenytoin resistance was not associated with a changed modulation of the sodium or calcium currents by this drug. Compared to sham controls, phenytoin's inhibitory effect on sodium currents was significantly reduced by kindling without difference between the responder and nonresponder subgroups. Further studies in phenytoin-resistant kindled rats may help to elucidate the mechanisms that can explain therapy resistance.

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.

Anticonvulsant efficacy of gabapentin and levetiracetam in phenytoin-resistant kindled rats

We evaluated the anticonvulsant efficacy of the new antiepileptic drugs (AEDs) gabapentin and levetiracetam 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, three 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 gabapentin and levetiracetam 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 gabapentin and levetiracetam proved to be effective anticonvulsant drugs in the kindling model by significantly increasing the ADT. In addition, both drugs markedly decreased seizure severity recorded at ADT currents, indicating that these drugs affect seizure threshold in the epileptic focus and seizure spread from the focus in the kindling model. When the threshold for secondary generalized seizures (GST) was determined in addition to ADT, gabapentin and levetiracetam strikingly increased this threshold compared to predrug control. In phenytoin nonresponders, gabapentin and levetiracetam significantly increased ADT and GST, which is in line with their proven efficacy in patients with refractory partial epilepsy in whom older AEDs have failed. In phenytoin responders, gabapentin tended to be more efficacious in increasing ADT and GST than in nonresponders, substantiating that the difference between these groups of kindled rats extends to other AEDs. In contrast to gabapentin, levetiracetam was more efficacious in increasing ADT in nonresponders than in responders. The data of this study substantiate that phenytoin nonresponders are a unique model for the search of new AEDs with improved efficacy in refractory partial epilepsy.

Kindling alters the anticonvulsant efficacy of phenytoin in Wistar rats

We have previously shown that subgroups can be selected from large groups of amygdala kindled Wistar rats which either respond consistently or do not respond to the anticonvulsant effect of phenytoin. Phenytoin nonresponders were proposed as a model for pharmaco-resistant temporal lobe epilepsy. In the present study we examined whether the differences of individual rats in response to phenytoin are already present before kindling or are a consequence of kindling. For this purpose, 52 rats were once tested with phenytoin, then kindled, and then repeatedly tested with phenytoin for selection of subgroups. For subgroup selection after kindling, the phenytoin prodrug fosphenytoin was used because of its water solubility and its improved tolerability and absorption after i.p. administration in rats. Before kindling, phenytoin significantly increased the afterdischarge threshold (ADT), i.e. a sensitive measure of focal seizure activity, but there was large individual variation with only 32 of the 52 rats reacting with an ADT increase, while the remaining rats showed either no effect or ADT decreases. After kindling, the selection resulted in 16 rats with consistent ADT increases in response to phenytoin and ten nonresponders (the remaining 26 rats showed variable responses). Unexpectedly, in rats which were responders after kindling, phenytoin exerted no significant anticonvulsant effect before kindling, while kindled nonresponders were very sensitive to phenytoin before kindling, indicating that the kindling process was responsible for the loss of anticonvulsant efficacy in kindled nonresponders and the development of phenytoin's efficacy in kindled responders. The present results substantiate that kindled subgroups of Wistar rats with different response to phenytoin are a valuable source for studying the mechanisms underlying the development of pharmaco-resistant limbic seizures.

Anticonvulsant efficacy of topiramate in phenytoin-resistant kindled rats

PURPOSE

We evaluated the anticonvulsant efficacy of topiramate (TPM), a structurally novel antiepileptic drug (AED), in amygdala kindled rats that had been preselected with respect to their response to phenytoin (PHT).

METHODS

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 PHT prodrug fosphenytoin (FOS) three groups of kindled rats were separated: rats in which consistent anticonvulsant effects were obtained (PHT responders), rats that showed no anticonvulsant response (PHT nonresponders), and rats with variable responses (variable PHT responders). The latter, largest group was used to evaluate at which doses and pretreatment times TPM exerted significant anticonvulsant effects on ADT. For this purpose, TPM was tested at four doses (20, 40, 80, 160 mg/kg i.p.) and two pretreatment times (1 and 4 h). The most effective treatment protocol was then used for TPM testing in PHT responders and nonresponders.

RESULTS

TPM proved to be an effective AED in the kindling model. At 40 mg/kg, significant ADT increases were obtained after both 1 and 4 h after administration. In addition to the effect on focal seizure threshold, seizure severity and duration recorded at ADT were decreased by TPM, indicating that this drug acts on both seizure threshold and seizure spread. In PHT nonresponders, TPM significantly increased ADT, which is in line with its proven efficacy in patients with refractory partial epilepsy in whom phenytoin has failed. However, TPM was more efficacious in increasing ADT in PHT responders than in nonresponders, substantiating that the difference between these groups of kindled rats extends to other AEDs. Repeated testing of kindled rats with TPM indicated that, similar to PHT, there are individual kindled rats without anticonvulsant response to TPM (i.e., TPM nonresponders).

CONCLUSIONS

The data of this study substantiate that PHT nonresponders are a unique model for the search of new AEDs with improved efficacy in refractory partial epilepsy.

Selection of phenytoin responders and nonresponders in male and female amygdala-kindled Sprague-Dawley rats

PURPOSE

We recently described that, by repeated testing of the anticonvulsant phenytoin (PHT), it is possible to select responders and nonresponders from large populations of amygdala-kindled Wistar rats. Whereas responders show marked and reproducible increases of focal seizure threshold (afterdischarge threshold: ADT) on repeated testing of PHT, 75 mg/kg i.p., nonresponders do not show any significant ADT increase after this dose, thus allowing use of these subgroups in the search for mechanisms of pharmacoresistance in temporal lobe epilepsy. In this study, we examined whether PHT responders and nonresponders can also be selected from large groups of kindled rats of the Sprague-Dawley strain.

METHODS

Male and female Sprague-Dawley rats were amygdala kindled, followed by once weekly i.p. testing of PHT.

RESULTS

In contrast to recent experiments in Wistar rats, 75 mg/kg PHT did not induce significant ADT increases in Sprague-Dawley rats, indicating strain differences in response to this drug after kindling. When the dose was lowered to 50 or 25 mg/kg, significant and reproducible ADT increases were obtained in Sprague-Dawley rats of both genders. Therefore these doses were used for selection of responders and nonresponders in a total of 42 rats. Almost 50% of the rats were PHT responders, whereas no rat was a nonresponder when tested in up to six subsequent drug trials. Many rats were variable responders (i.e., showed ADT increases in some but not all trials), which was not due to low or variable drug absorption after i.p. injection.

CONCLUSIONS

The data indicate that, in contrast to Wistar rats, Sprague-Dawley rats are not suited for selection of PHT nonresponders, but rather are quite responsive to this drug. A further difference to the Wistar strain is the truncated dose-response with loss of anticonvulsant efficacy at 75 mg/kg in kindled Sprague-Dawley rats, which may, at least in part, explain the inconsistent results reported on the anticonvulsant efficacy of PHT in this strain in the literature. The lack of anticonvulsant activity after administration of 75 mg/kg may be a result of kindling, because administration of this dose before kindling causes a significant ADT increase in this strain. This kindling-induced alteration of the anticonvulsant activity of PHT is a phenomenon that contrasts Sprague-Dawley with Wistar rats and deserves further investigation.