Antiseizure Drugs By Bohlooli S, PhD School of Medicine, Ardabil University of Medical Sciences
Antiseizure Drugs
By Bohlooli S, PhD
School of Medicine, Ardabil University of Medical Sciences
Introduction Approximately 1% of the world's population has
epilepsy
Epilepsy is a chronic disorder characterized by recurrent seizures
Seizures are finite episodes of brain dysfunction resulting from abnormal discharge of cerebral neurons
The causes of seizures are many infection to neoplasm and head injury
Partial seizuresSimple partial seizuresComplex partial seizuresPartial seizures secondarily generalized
Generalized seizuresGeneralized tonic-clonic (grand mal) seizuresAbsence (petit mal) seizuresTonic seizuresAtonic seizuresClonic and myoclonic seizuresInfantile spasms1
1An epileptic syndrome rather than a specific seizure type; drugs useful in infantile spasms will be reviewed separately.
Classification of seizure types
Drug Development for Epilepsy threshold pentylenetetrazol clonic seizures
in mice Screening of drug for absence seizures
the maximal electroshock test (MES) for generalized tonic-clonic seizures and
complex partial seizures
Limbic seizures induced in rats by the process of electrical kindling screen for predicting efficacy in complex partial
seizures
Drug Development for Epilepsy
New antiseizure drugs are being sought by more rational approaches
Enhancement of GABAergic (inhibitory) transmission
Diminution of excitatory (usually glutamatergic) transmission
Modification of ionic conductances
Molecular targets for antiseizure drugs at the excitatory, glutamatergic synapse
Molecular targets for antiseizure drugs at the inhibitory, GABAergic synapse
BASIC PHARMACOLOGY OF ANTISEIZURE DRUGS Chemistry (five very similar chemical groups)
Barbiturates Hydantoins Oxazolidinediones Succinimides Acetylureas
carbamazepine, valproic acid, and the benzodiazepines
felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin, and zonisamide.
Chemistry
Figure 24-1. Antiseizure heterocyclic ring structure. The "X" varies as follows: hydantoin derivatives, -N-; barbiturates, -C-N-; oxazolidinediones, -O-; succinimides, -C-; acetylureas, -NH2 (N connected to C2). R1, R2, and R3 vary within each subgroup.
Pharmacokinetics The antiseizure drugs exhibit many similar pharmacokinetic
properties selected for oral activity all must enter the central nervous system
with 80-100% of the dose reaching the circulation
Most antiseizure drugs are not highly bound to plasma proteins
cleared chiefly by hepatic mechanisms
Many are converted to active metabolites that are also cleared by the liver
DRUGS USED IN PARTIAL SEIZURES & GENERALIZED TONIC-CLONIC SEIZURES
INTRODUCTION
The classic major drugs : Phenytoin (and congeners) Carbamazepine Valproate Barbiturates
Newer drugs: Lamotrigine, levetiracetam, gabapentin Oxcarbazepine, pregabalin, topiramate Vigabatrin, and zonisamide
PHENYTOIN
the oldest nonsedative antiseizure drug
known for decades as diphenylhydantoin.
PHENYTOIN: chemical A more soluble prodrug of phenytoin,
fosphenytoin, is available for parenteral use
PHENYTOIN; Mechanism of Action
to block sodium channels inhibiting the generation of rapidly
repetitive action potentials A reduction of calcium permeability:
may explain the ability of phenytoin to inhibit a variety of calcium-induced secretory processes
PHENYTOIN: Clinical Use
Effective against: Partial seizures Generalized tonic-clonic seizures
PHENYTOIN:Pharmacokinetics
Absorption is highly dependent on the formulation of the dosage form
Absorption after intramuscular injection is unpredictable
highly bound to plasma proteins Phenytoin accumulates in brain, liver, muscle, and fat. Phenytoin is metabolized to inactive metabolites that
are excreted in the urine The elimination of phenytoin is dose-dependent The half-life of phenytoin varies from 12 hours to 36
hours
Nonlinear relationship of phenytoin dosage and plasma concentrations
PHENYTOIN: Drug Interactions & Interference with Laboratory Tests
Drug interactions are primarily related to : protein binding metabolism
Hypoalbuminemia? has an affinity for thyroid-binding
globulin Inducer of microsomal enzymes
PHENYTOIN: Toxicity Dose-related adverse effects are similar to
other antiseizure drugs in this group Nystagmus occurs early Diplopia and ataxia Sedation Gingival hyperplasia and hirsutism Long-term use :
coarsening of facial features abnormalities of vitamin D metabolism
Idiosyncratic reactions are relatively rare
MEPHENYTOIN, ETHOTOIN No well-controlled clinical trials have
documented their effectiveness Mephenytoin: The incidence of severe
reactions such as Dermatitis Agranulocytosis hepatitis
is higher than for phenytoin Ethotoin: adverse effects and toxicity are
generally less severe than those associated with phenytoin, but the drug appears to be less effective
CARBAMAZEPINE Closely related to imipramine and other
antidepressants Effective in treatment of bipolar depression
CARBAMAZEPINE :Mechanism of Action
similar to that of phenytoin blocks sodium channels inhibits high-frequency repetitive
firing in neurons in culture It also acts presynaptically to
decrease synaptic transmission
CARBAMAZEPINE: Clinical Use
has long been considered a drug of choice for partial seizures generalized tonic-clonic seizures
is not sedative in its usual therapeutic range
very effective in some patients with trigeminal neuralgia
useful in some patients with mania
CARBAMAZEPINE:Pharmacokinetics
The rate of absorption varies widely among patients
Distribution is slow, and the volume of distribution is roughly 1 L/kg
has a very low systemic clearance of approximately 1 L/kg/d
initial half-life of 36 hours observed, decreases to as short as 8-12 hours.
Carbamazepine is completely metabolized in humans to several derivatives
CARBAMAZEPINE: Therapeutic Levels & Dosage
Carbamazepine is available only in oral form
the therapeutic level is usually 4-8 mcg/mL
CARBAMAZEPINE:Drug Interactions
exclusively related to the drug's enzyme-inducing properties: an increased rate of metabolism of other drugs
primidone, phenytoin, ethosuximide, valproic acid, and clonazepam
Other drugs such as propoxyphene, troleandomycin, and valproic acid may inhibit carbamazepine clearance
Anticonvulsants such as phenytoin and phenobarbital decrease steady-state concentrations of
carbamazepine through enzyme induction
CARBAMAZEPINE: Toxicity The most common dose-related adverse effects are
diplopia and ataxia Mild gastrointestinal upsets Unsteadiness Drowsiness : at much higher doses Hyponatremia and water intoxication idiosyncratic blood dyscrasias
fatal cases of aplastic anemia and agranulocytosis The most common idiosyncratic reaction is an
erythematous skin rash
OXCARBAZEPINE closely related to carbamazepine have an improved toxicity profile Its activity resides almost exclusively in the
10-hydroxy metabolite hyponatremia may occur
more commonly with oxcarbazepine than with carbamazepine
PHENOBARBITAL Many consider the barbiturates the drugs of
choice for seizures only in infants clinically useful as antiseizure drugs are :
phenobarbital, mephobarbital, metharbital, and primidone
PHENOBARBITAL : Mechanism of Action
The exact mechanism of action of phenobarbital is unknown enhancement of inhibitory processes diminution of excitatory transmission
selectively suppress abnormal neurons Block sNa+ conductance blocks some Ca2+ currents (L , N type) enhances the GABA receptor-mediated current
blocks excitatory responses induced by glutamate (AMPA)
PHENOBARBITAL : Clinical Use
useful in the treatment of partial seizures generalized tonic-clonic seizures
There is little evidence for its effectiveness in generalized seizures such as absence, atonic attacks, and infantile
spasms it may worsen certain patients with these
seizure types
PRIMIDONE
PRIMIDONE
the mechanism of action is more like that of phenytoin
effective against partial seizures and generalized tonic-clonic seizures
PRIMIDONE :
is completely absorbed Primidone is metabolized
by oxidation to phenobarbital by scission of the heterocyclic ring to form PEMA
efficacious when plasma levels are in the range of 8-12 mcg/mL
The dose-related adverse effects of primidone are similar to those of its metabolite, phenobarbital except drowsiness occurs early
VIGABATRIN irreversible inhibitor of GABA aminotransferase
(GABA-T) useful in the treatment of partial seizures and
West's syndrome Typical toxicities include drowsiness, dizziness,
and weight gain long-term therapy with vigabatrin has been
associated with development of visual field
defects
LAMOTRIGINE a voltage- and use-dependent inactivation of
sodium channels. Blocking actions on voltage-activated Ca2+
channels decreases the synaptic release of glutamate
LAMOTRIGINE: Clinical Use effective for partial seizures
As add on or monotherapy
There is evidence that the drug is also active against: absence and myoclonic seizures in children
Adverse effects include: dizziness, headache, diplopia, nausea,
somnolence, and skin rash
a potentially life-threatening dermatitis will develop in 1-2% of pediatric patients
FELBAMATE
effective in some patients with partial seizures
causes aplastic anemia and severe hepatitis at unexpectedly high rates
a third-line drug for refractory cases a use-dependent block of the NMDA
receptor potentiates GABAA receptor responses effective against the seizures that occur in
Lennox-Gastaut syndrome
GABAPENTIN & PREGABALIN
GABAPENTIN & PREGABALIN modify the synaptic or nonsynaptic release of GABA act presynaptically to decrease the release of
glutamate as an adjunct against:
partial seizures and generalized tonic-clonic seizures
effective in the treatment of neuropathic pain The most common adverse effects are:
somnolence, dizziness, ataxia, headache, and tremor
the drugs are excreted unchanged
LEVETIRACETAM binds selectively to a
synaptic vesicular protein SV2A
modifies the synaptic release of glutamate and GABA
The drug is for the treatment of partial seizures
Two thirds of the drug is excreted unchanged in the urine
TIAGABINE
was "rationally designed" as an inhibitor of GABA uptake
TIAGABINE inhibitor of GABA uptake in both neurons
and glia adjunctive treatment of partial seizures Minor adverse events are dose-related and
include: nervousness, dizziness, tremor, difficulty in
concentrating, and depression Rash is an uncommon idiosyncratic Food decreases the peak plasma
concentration The drug is oxidized in the liver by CYP3A
TOPIRAMATE
a substituted monosaccharide
TOPIRAMATE blocking of voltage-gated sodium channels potentiate the inhibitory effect of GABA depresses the excitatory action of kainate on glutamate
receptors effective against partial and generalized tonic-clonic
seizures has a broader spectrum, with effectiveness against:
Lennox-Gestaut syndrome West's syndrome absence seizures. approved for the treatment of migraine headaches
dose-related adverse effects include somnolence, fatigue, dizziness, cognitive slowing,
paresthesias, nervousness, and confusion Acute myopia and glaucoma Urolithiasis
ZONISAMIDE
a sulfonamide derivative site of action appears to be the
sodium channel act on voltage-gated calcium
channels
ZONISAMIDE
effective against partial and generalized tonic-clonic seizures
useful against infantile spasms Adverse effects include:
drowsiness, cognitive impairment, and potentially serious skin rashes
DRUGS USED IN GENERALIZED SEIZURES
ETHOSUXIMIDE
Ethosuximide has an important effect on Ca2+ currents, reducing the low-threshold (T-type) current
ETHOSUXIMIDE is particularly effective against absence
seizures completely metabolized dose-related adverse effect of ethosuximide
is gastric distress including: pain, nausea, and vomiting
idiosyncratic adverse effects of ethosuximide are extremely uncommon
VALPROIC ACID & SODIUM VALPROATE
was found to have antiseizure properties when it was used as a solvent
VALPROATE: Mechanism of Action
Has an effect on Na+ currents Blockade of NMDA receptor-mediated
excitation is important the effects of valproate on GABA
facilitate glutamic acid decarboxylase inhibitory effect on the GABA transporter GAT-1 inhibits GABA transaminase
is a potent inhibitor of histone deacetylase
VALPROATE: Clinical Use
Very effective against absence seizures
The drug is effective in generalized tonic-clonic seizures
Effective in partial seizures Effective in management of bipolar
disorder and migraine prophylaxis
VALPROATE: Pharmacokinetics
Food may delay absorption 90% bound to plasma proteins Valproate also displaces phenytoin
from plasma proteins the metabolism of several drugs,
including: phenobarbital, phenytoin, and
carbamazepine
VALPROATE: Toxicity The most common dose-related
adverse effects are: nausea, vomiting, abdominal pain and
heartburn weight gain, increased appetite, and
hair loss The idiosyncratic toxicity of valproate
is largely limited to hepatotoxicity thrombocytopenia
OTHER DRUGS USED IN MANAGEMENT OF EPILEPSY
BENZODIAZEPINES Diazepam
Effective against generalized tonic-clonic status epilepticus
Lorazepam appears to be more effective and longer-acting
Clonazepam effetive against absence seizures effective in some cases of myoclonic seizures
Limitations: sedative effect tolerance
ACETAZOLAMIDE inhibition of carbonic anhydrase Mild acidosis in the brain has been used for all types of seizures limited by the rapid development of
tolerance have a special role in epileptic women who
experience seizure exacerbations at the time of menses
CLINICAL PHARMACOLOGY OF ANTISEIZURE DRUGS
Drug Effective Level (mcg/mL)
High Effective Level1(mcg/m
L)
Toxic Level (mcg/mL)
Carbamazepine 4-12 7 > 8
Primidone 5-15 10 < 12
Phenytoin 10-20 18 > 20
Phenobarbital 10-40 35 > 40
Ethosuximide 50-100 80 > 100
Valproate 50-100 80 > 100
Effective plasma levels of six antiseizure drugs
Special Aspects of the Toxicology of Antiseizure Drugs
Teratogenicity Withdrawal Overdose