ANTI EPILEPTIC DRUGS

EPILEPSY

It is a chronic disorder that is characterized by a recurrent seizure.

SEIZURE: It is a violent involuntary spasmodic contraction of skeletal muscle.

Aetiology: 

• In children-idiopathic
•  In young- hypoxia, birth asphyxia, intracranial trauma during birth, metabolic disturbances, infection
• In adult- head injury, alcohol abuse, cerebrovascular disease.

Classification of epilepsy:

 1. Partial seizures: These involve only a portion of the brain, typically part of one lobe of one hemisphere. The symptoms of each seizure type depend on the site of neuronal discharge and on the extent to which the electrical activity spreads to other neurons in the brain.

Simple partial: These seizures are caused by a group of hyperactive neurons exhibiting abnormal electrical activity, which are confined to a single locus in the brain. Simple partial seizures may occur at any age.

Complex partial: These seizures exhibit complex sensory hallucinations, mental distortion, and loss of consciousness. Motor dysfunction may involve chewing movements, diarrhea, and/or urination. Consciousness is altered.

2. Generalized seizures: These seizures may begin locally, producing abnormal electrical discharges throughout both hemispheres of the brain. Primary generalized seizures may be convulsive or nonconvulsive, and the patient usually has an immediate loss of consciousness

• Tonic-clonic: Seizures results in loss of consciousness, followed by tonic (continuous contraction) and clonic (rapid contraction and relaxation) phases. The seizure may be followed by a period of confusion and exhaustion due to the depletion of glucose and energy stores.
• Absence: These seizures involve a abrupt, brief and self-limiting loss of consciousness. The onset generally occurs in patients at 3-5 years of age and lasts until puberty or beyond. The patient stares and exhibits rapid eye-blinking, which lasts for 3-5 seconds. This seizure has a very distinct three-per-second spike and wave discharge seen on electroencephalogram.
• Myoclonic: These seizures consist of short episodes of muscle contractions that may reoccur for several minutes. They generally occur after wakening and exhibit as brief jerks of the limbs. This type of seizures occurs at any age but usually begin around puberty or early adulthood.
• Febrile seizures: This may occur in siblings. Young children may develop seizures with illness accompanied by high fever. The febrile seizures consist of a generalized tonic-clonic convulsion of short duration and do not necessarily lead to a diagnosis of epilepsy.
• Status epilepticus: In status epilepticus, two or more seizures reoccur without recovery of full consciousness between them. These may be partial or primary generalized, convulsive or nonconvulsive. Status epilepticus is life-threatening and requires emergency treatment.

Pathophysiology:

• Excitatory glutamatergic synapses
• Excitatory amino acid neurotransmitter (glutamate, aspartate)
• Abnormal tissues — tumour, AVM, dead area
• Genetic factors
• Role of substantia nigra and GABA

Diagnosis:

• EEG (electroencephalogram)
•  MRI (magnetic resonance imaging)
• CT scan
•  brain mapping

Treatment:

Benzodiazepines: Benzodiazepines bind to GABA inhibitory receptors to reduce firing rate. lorazepam and diazepam are the most often used as an adjunctive therapy for myoclonic as well as for partial and generalized tonic-clonic seizures. Lorazepam has a shorter pharmacokinetic half-life but stays in the brain longer than diazepam. Diazepam is available for rectal administration to interrupt or avoid prolonged generalized tonic-clonic seizures or clusters.

Carbamazepine: Carbamazepine reduces the propagation of abnormal impulses in the brain by blocking sodium channels, thereby inhibiting the generation of repetitive action potentials in the epileptic focus and preventing their spread. Carbamazepine is effective for treatment of partial seizures and secondarily generalized tonic-clonic seizures. It is also used to treat trigeminal neuralgia and in bipolar disease. Carbamazepine is absorbed slowly and erratically following oral administration and may vary from generic to generic, resulting in large variations in serum concentrations of the drug. It induces its own drug metabolism and has an active metabolite. It is a substrate for CYP3A4 with minor metabolism by CYP1A2 and CYC2C8.

Divalproex: Divalproex sodium is a combination of sodium valproate and valproic acid and is reduced to valproate when it reaches the gastrointestinal tract. It was developed to improve gastrointestinal tolerance of valproic acid. All of the available salt forms are equivalent in efficacy (valproic acid and valproate sodium). Commercial products are available in multiple-salt, dosage forms and extended-release formulations. Proposed mechanisms of action include sodium channel blockade, blockade of GABA transaminase, and action at the T-type calcium channels. These many mechanisms provide a broad spectrum of activity against seizures. It is effective for the treatment of partial and primary generalized epilepsies. Valproate inhibits metabolism of the CYP2C9, UGT and epoxide hydrolase systems.

Ethosuximide: Ethosuximide reduces propagation of abnormal electrical activity in the brain, most likely by inhibiting T-type calcium channels. It is effective in treating only primary generalized absence seizures. Use of ethosuximide is limited because of this very narrow spectrum.

Felbamate: Felbamate has a broad spectrum of anticonvulsant action. The drug has multiple proposed mechanisms including 1) blocking voltage-dependent sodium channels, 2) competing with the glycine-coagonist binding site on the N-methyl-D-aspartate (NMDA) glutamate receptor, 3) blocking calcium channels, and 5) potentiation of GABA actions. It is an inhibitor of drugs metabolized by CYP2C19 and b-oxidation. It induces drugs metabolized by CYP3A4. It is reserved for use in refractory epilepsies because of the risk of aplastic anemia and hepatic failure. Fosphenytoin: It is a prodrug and is rapidly converted to phenytoin in the blood, providing high levels of phenytoin within minutes. Fosphenytoin may also be administered intramuscularly (IM). Phenytoin sodium should never be given IM because it can cause tissue damage and necrosis. Fosphenytoin is the drug of choice and standard of care for IM and IV administration. Due to sound-alike and look-alike names, there is a risk for medication error to occur.

Gabapentin: Gabapentin is an analog of GABA. However, it does not act at GABA receptors nor enhance GABA actions, nor is it converted to GABA. Its precise mechanism of action is not known. It is approved as adjunct therapy for partial seizures and for treatment of postherpetic neuralgia. Gabapentin exhibits nonlinear pharmacokinetics due to its uptake by a saturable transport system from the gut. Gabapentin does not bind to plasma proteins and is excreted unchanged through the kidneys. Reduced dosing is required in renal disease.

Lamotrigine: Lamotrigine blocks sodium channels as well as high voltage dependent calcium channels. Lamotrigine is effective in a wide variety of seizure disorders, including generalized seizures, partial seizures, typical absence seizures, and the Lennox-Gastaut syndrome. It is approved for use in bipolar disorder as well. Lamotrigine is metabolized primarily to the N-2 glucuronide through the UGT pathway.

Levetiracetam: Levetiracetam is approved for adjunct therapy of partial onset seizures, myoclonic seizures, and primary generalized tonic-clonic seizures in adults and children. The exact mechanism of anticonvulsant action is unknown. It demonstrates high affinity for a synaptic vesicle protein (SV2A). Side effects most often reported include dizziness, headache, sleep disturbances,  and weakness.

Oxcarbazepine: Oxcarbazepine is a prodrug that is rapidly reduced to the 10-monohydroxy (MHD) metabolite which is responsible for its anticonvulsant activity. MHD blocks sodium channels preventing the spread of the abnormal discharge. Modulation of calcium channels is also a hypothesis. It is approved for use in adults and children with partial onset seizures. Oxcarbazepine is a less potent inducer of CYP3A4 and UGT than carbamazepine. The adverse effects profile is similar to that of other antiepileptic drugs with respect to nausea, vomiting, headache, and visual disturbance.

Phenobarbital: Phenobarbital was synthesized in 1902 and brought to the market in 1912 by Bayer. The primary mechanism of action is the enhancement of inhibitory effects of GABA-mediated neurons.The primary use for phenobarbital in epilepsy is in treatment of status epilepticus. Due to interaction with the cytochrome P450 enzymes as an inducer, and adverse effects of sedation, cognitive impairment, and potential for osteoporosis, this drug should only be considered for chronic therapy once a patient is found to be refractory to many other drugs, and the benefits of therapy outweigh the multiple risks.

Phenytoin: Phenytoin blocks voltage-gated sodium channels by selectively binding to the channel in the inactive state and slowing its rate of recovery. At very high concentrations, phenytoin can block voltage-dependent calcium channels and interfere with the release of monoaminergic neurotransmitters. Phenytoin is effective for treatment of partial seizures and generalized tonic-clonic seizures and in the treatment of status epilepticus. The drug is 90 percent bound to plasma albumin. Phenytoin is an inducer of drugs metabolized by the CYP2C, and CYP3A families and the UGT enzyme system. Phenytoin exhibits saturable enzyme metabolism at a low serum concentration; thus knowledge of zero- order pharmacokinetics and population parameters is important for dosing adjustment. Small increases in a daily dose can produce large increases in the plasma concentration, resulting in drug-induced toxicity. Depression of the CNS occurs particularly in the cerebellum and vestibular system, causing nystagmus and ataxia. The elderly are highly susceptible to this effect. Gingival hyperplasia may cause the gums to grow over the teeth. Long-term use may lead to development of peripheral neuropathies and osteoporosis.

Pregabalin Pregabalin binds to the a2-d site, an auxiliary subunit of voltage-gated calcium channels in the CNS, inhibiting excitatory neurotransmitter release. The exact role this plays in treatment is not known, but the drug has proven effects on partial onset seizures, neuropathic pain associated with diabetic peripheral neuropathy, postherpetic neuralgia, and fibromyalgia. Pregabalin is greater than 90 percent eliminated renally, with no indication of CYP involvement. Drowsiness, blurred vision, weight gain, and peripheral edema have been reported.

Primidone: Primidone has two active metabolites, phenobarbital and phenylethylmalonamide, which have longer half-lives than the parent drug. Due to the nature of the long term adverse effects associated with phenobarbital, this drug should be considered for use only in those patients with refractory epilepsy.

Tiagabine: Tiagabine blocks GABA uptake into presynaptic neurons, permitting more GABA to be available for receptor binding, thus, there is thought to be enhanced inhibitory activity. Tiagabine is effective in decreasing the number of seizures in patients with partial onset epilepsy. Binding to albumin and a1-acid glycoprotein is greater than 95 percent, and metabolism is mainly completed by the CYP3A family of enzymes. Adverse effects include tiredness, dizziness, and gastrointestinal upset. There is some indication in postmarketing surveillance that seizures have occurred in patients who did not have epilepsy when the drug was used. Tiagabine has not been approved for use for any other indication.

Topiramate: Topiramate  possesses several actions that are believed to contribute to its broad spectrum of antiseizure activity. Topiramate blocks voltage-dependent sodium channels; it has been shown to increase the frequency of chloride channel opening by binding to the GABAA receptor. High-voltage calcium currents (L type) are reduced by topiramate. It is a carbonic anhydrase inhibitor and may act at glutamate (NMDA) sites. Topiramate is effective and approved for use in partial and primary generalized epilepsies. It is also approved for treatment of migraine. Topiramate is renally eliminated to a high degree, but it also has inactive metabolites. It inhibits CYP2C19 and is induced by phenytoin, and carbamazepine. Lamotrigine is reported to cause an increase in topiramate concentration. Co-administration of topiramate reduces ethinyl estradiol. Adverse effects include somnolence, weight loss, and paresthesias; renal stones are reported to occur at a higher incidence than in a nontreated population. Glaucoma, oligohidrosis, and hyperthermia have also been reported. The latter are specifically related to the carbonic anhydrase activity.

Zonisamide: Zonisamide is a sulfonamide derivative that has a broad spectrum of action. The compound has multiple effects on neuronal systems thought to be involved in seizure generation. These include blockade of both voltage-gated sodium channels and T-type calcium currents. It has a limited amount of carbonic anhydrase activity. Cross reactivity with other sulfonamides should be reviewed and its use monitored in patients with reported allergies. Zonisamide is approved for use in patients with partial epilepsy. It is metabolized by the CYP3A4 isozyme and may, to a lesser extent, be affected by CYP3A5 and CYP2C19. In addition to the typical CNS adverse effects, zonisamide may cause kidney stones. Oligohidrosis has been reported, and patients should be monitored for increased body temperature and decreased sweating.

Ref:

Epilepsy, seizures, phenytoin