Gerard Forrest

Concentration-Effect and Concentration-Toxicity Relations with Lamotrigine: A Prospective Study

Summary: This prospective study was designed to ascertain whether measurement of lamotrigine (LTG) concentrations in the epilepsy clinic could be used to predict the onset of complete seizure control or the emergence of adverse effects. LTG was initiated in doses of 25 or 50 mg daily in 69 patients with newly diagnosed or poorly controlled epilepsy and was increased monthly in 50–mg increments until the patient became seizure‐free for at least 6 months or developed adverse effects that abated after a reduction in dosage. LTG and other antiepileptic drug (AED) concentrations were measured at each clinic visit but were not supplied to the investigator examining the patients. Overall, 19 patients either withdrew due to lack of efficacy or defaulted from the clinic. Of the remaining 50 patients, 32 (19 monotherapy, 13 polytherapy) became seizure‐free at widely varying daily LTG doses (median 200 mg, range 25–850 mg) and concentrations (median 3.8 mg/L, range 1.4–18.7 mg/L). Likewise, the 18 patients (5 monotherapy, 13 polytherapy) who experienced intolerable side effects showed substantial variations in daily LTG doses (median 300 mg, range 100–900 mg) and concentrations (median 4.0 mg/L, range 0.4–18.5 mg/L). No useful concentration‐effect or concentration‐toxicity relation with LTG could be demonstrated in this study; therefore, we believe that routine therapeutic drug monitoring with this new AED is not currently indicated.

Neurochemical studies with the novel anticonvulsant levetiracetam in mouse brain

Levetiracetam is a novel antiepileptic agent with a wide spectrum of activity against experimental and clinical seizures. The mechanism of its anticonvulsant action remains to be determined. We have investigated the effects of levetiracetam on several gamma-aminobutyric acid (GABA)-related neurochemical parameters in mouse brain. Adult male mice were randomised into two groups and administered levetiracetam (0-300 mg/kg) intraperitoneally either as a single dose or twice daily for 5 days. Four hours after the final dose, animals were killed and their brains removed. Brain tissues were analysed for concentrations of GABA, glutamate and glutamine and for the activities of GABA-transaminase and glutamic acid decarboxylase. Single dose and repeated levetiracetam treatments were without effect on all of the parameters investigated. The anticonvulsant action of levetiracetam is unlikely to be mediated via an action on the GABAergic system.

Visual field constriction: Accumulation of vigabatrin but not tiagabine in the retina

Background: The antiepileptic drug (AED) vigabatrin (VGB) causes concentric visual field constriction. Anecdotal reports involving tiagabine (TGB) have implied that this may be a class effect of all AEDs with γ-aminobutyric acid (GABA)–related actions. We investigated the pharmacokinetic and pharmacodynamic profiles of VGB and TGB in rat brain and eye. Methods: Adult male rats (n = 8) were administered 0.9% saline (control), VGB (500 or 1,000 mg/kg), or TGB (5, 10, or 20 mg/kg). At 1 (TGB) and 4 hours (VGB) postdosing, the animals were killed, a blood sample was obtained, their brains were dissected into five anatomic regions, and the retina and vitreous humor were isolated from each eye. Samples were analyzed for GABA concentrations and the activity of the enzyme GABA-transaminase (GABA-T). Plasma and tissue drug concentrations were also determined. Results: VGB treatment produced a decrease in the activity of GABA-T and a rise in GABA concentrations in all tissues investigated. This effect was most pronounced in the retina. VGB concentrations were as much as fivefold higher in the retina than in the brain. TGB was without effect on GABA concentrations and activity of GABA-T. TGB concentrations were notably lower in the retina than in the brain. Conclusions: Accumulation of VGB in the retina, with or without an increase in GABA, may be responsible for the visual field constriction reported clinically. In contrast, TGB had no effect on GABA concentrations and did not accumulate in the retina. These results suggest that TGB is unlikely to cause visual field defects in humans.

Vigabatrin, but not gabapentin or topiramate, produces concentration-related effects on enzymes and intermediates of the GABA shunt in rat brain and retina

Summary:  Purpose: The antiepileptic drug (AED) vigabatrin (VGB), which exerts its pharmacologic effects on the γ‐aminobutyric acid (GABA) system, causes concentric visual field constriction in >40% of exposed adults. This may be a class effect of all agents with GABA‐related mechanisms of action. We compared the concentration‐related effects of VGB in rat brain and eye with those of gabapentin (GBP) and topiramate (TPM), both of which have been reported to elevate brain GABA concentrations in humans.

High dose gabapentin in refractory partial epilepsy: clinical observations in 50 patients

Fifty patients with refractory partial seizures took part in a prospective, observational study of adjuvant gabapentin (GBP) in increasing doses. Thirty-three were started on 400 mg GBP daily with further weekly increments of 400 mg until seizures came under control for at least 6 months or to the limit of tolerability. A further 17 patients, not fully controlled on low dose GBP, followed the same regimen. All patients took the drug three times daily. Comparisons were made with seizure numbers during a 3-month baseline during which antiepileptic medication remained unchanged. Overall, 24 of the 50 patients documented a seizure reduction of 50% or more. Fifteen did so at or below 2400 mg GBP daily. Three of these patients became seizure-free. The remaining nine appeared to respond to higher daily doses of GBP (1:2800 mg; 3:3600 mg; 1:4000 mg; 1:4800 mg; 3:6000 mg), with two becoming seizure-free. Side-effects most commonly reported included tiredness, dizziness, headache and diplopia. On GBP doses exceeding 3600 mg daily, three patients developed flatulence and diarrhoea and two more had myoclonic jerks. Mean circulating GBP concentrations (mg/l) at each 1200 mg dose level were as follows: 1200 mg-4.1; 2400 mg-8.6; 3600 mg 13.2; 4800 mg 15.5; 6000 mg-17.2. In six patients, including three taking 6000 mg daily, GBP concentrations continued to rise linearly at each dosage increment. Although limited, our results do not support the suggestion that GBP absorption is saturable. High dose GBP may be effective in controlling seizures in patients with refractory partial epilepsy.

Determination of gabapentin in plasma by high-performance liquid chromatography

A rapid and simple method for determination of the novel antiepileptic compound gabapentin [1-(aminomethyl)cyclohexaneacetic acid] in plasma is described. Blank human plasma was spiked with gabapentin (1.0-10.0 micrograms/ml) and internal standard [1-(aminomethyl)-cycloheptaneacetic acid; 5.0 micrograms/ml]. Individual samples were treated with 2 M perchloric acid, centrifuged and then derivatised with o-phthalaldehyde-3-mercaptopropionic acid. Separation was achieved on a Beckman Ultrasphere 5 microns reversed-phase column with mobile phase consisting of 0.33 M acetate buffer (pH 3.7; containing 100 mg/l EDTA)-methanol-acetonitrile (40:30:30, v/v). Eluents were monitored by fluorescence spectroscopy with excitation and emission wavelengths of 330 and 440 nm, respectively. The calibration curve for gabapentin in plasma was linear (r = 0.9997) over the concentration range 1.0-10.0 micrograms/ml. Recovery was seen to be > or = 90%. The inter- and intra-assay variations for three different gabapentin concentrations were < or = 10% throughout. The lower limit of quantitation was found to be 0.5 microgram/ml. Chromatography was unaffected by a range of commonly employed antiepileptic drugs or selected amino acids.

Neurochemical actions of gabapentin in mouse brain

Gabapentin (GBP) is a recently licensed antiepileptic, drug whose mode of action remains to be fully elucidated. The following studies were designed to investigate the effects of GBP on several gamma-aminobutyric acid (GABA) related neurochemical parameters in mouse brain. GBP (0-75 mg/kg) was administered by intraperitoneal injection either as a single dose or twice daily for 8 days. Animals were sacrificed 4 h after the final administration and their brains removed and analysed for concentrations of GABA, glutamate and glutamine and the activities of GABA-transaminase (GABA-T) and glutamic acid decarboxylase (GAD). Single dose GBP increased brain GABA-T activity and glutamine concentration but was without effect on GAD activity or the concentrations of GABA and glutamate. Following repeated treatment with GBP, brain GABA-T activity was consistently decreased and there was also a decrease in brain glutamate concentration. Repeated drug treatment was without effect on the activity of GAD or on the concentrations of GABA and glutamine. These results suggest that GBP has effects on the GABAergic system which may contribute to its antiepileptic and/or neuroprotective actions.

DIHYDROPYRIDINE CALCIUM ANTAGONISTS IN MICE: BLOOD AND BRAIN PHARMACOKINETICS AND EFFICACY AGAINST PENTYLENETETRAZOL SEIZURES

Summary: 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 < 0.001) and NMD (r = 0.572, p < 0.001). Injection of the dihydropyridines as a suspension (Tween 80) did not alter brain penetration, although systemic absorption was more erratic. NFD (p < 0.001), NMD (p < 0.021, and carbamazepine (CBZ, p < 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 < 0.005). NFD (p < 0.002) and NMD (p < 0.001) inhibited PTZ seizures after 2‐week oral dosing, but low dosing was more effective than high dosing (p < 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.

Effects of anti-epileptic drugs on glutamine synthetase activity in mouse brain.

Glutamine synthetase (GS) is a key enzyme in the regulation of glutamate neurotransmission in the central nervous system. It is responsible for the conversion of glutamate to glutamine, and for the detoxification of ammonia. We have investigated the effects of single and repeated intraperitoneal administration of a range of established and new anti‐epileptic drugs on GS activity in mouse brain. Four hours after the final dose, animals were sacrificed and the brains removed for analysis of GS activity. Both single and repeated doses of phenytoin and carbamazepine were found to reduce enzyme activity (P<0.05). Single doses of phenobarbitone, felbamate and topiramate were without effect, however repeated administration of these drugs dose‐dependently reduced GS activity (P<0.05). Single and repeated doses of sodium valproate, vigabatrin, lamotrigine, gabapentin, tiagabine, levetiracetam and desglycinyl‐remacemide were found to have no effect on GS activity. The reduction in enzyme activity demonstrated is unlikely to be related to the anti‐epileptic actions of these drugs, but may contribute to their toxicity.

Simple high-performance liquid chromatographic method for the measurement of amiloride in body fluids

A simplified rapid high-performance liquid chromatographic (HPLC) procedure has been developed for the measurement of amiloride in plasma or urine. Solid-phase extraction columns provide quick, clean and simple sample preparation, allowing ten samples to be processed in less than 5 min. The HPLC system uses a standard reversed-phase (C18) column with detection by ultraviolet absorption at 365 nm. The assay has been used to define plasma amiloride concentration-time profiles and to quantitate urine amiloride recovery in healthy men after repeated administration at two doses.