Lecture 47 - Pharmacology of Anticonvulsant Drugs Flashcards
Mechanism of action of anticonvulsant drugs:
stabilize and reduce neuronal excitability (reduce E/I balance)
1. Decrease sodium influx, prolong inactivation of Na+ channels (following the opening and subsequent closing of the channel)
2. Reduction of calcium influx (this is critical for absence seizures)
3. Enhance GABA-mediated neuronal inhibition
4. Antagonism of excitatory transmitters (i.e., glutamate)
5. Other targets (i.e., Levetiracetam)
Decrease sodium influx, prolong inactivation of Na+ channels examples
carbamazepine oxcarbazepine
phenytoin
lacosamide
lamotrigine
valproate
Reduction of calcium influx (this is critical for absence seizures)
ethosuximide
lamotrigine
valproate
Enhance GABA-mediated neuronal inhibition
barbiturates (activate the GABAA receptor)
benzodiazepines (activate the GABAA receptor)
valproate (increases GABA levels)
gabapentin (increases GABA release)
vigabatrin (inhibits GABA transaminase)
tiagabine (inhibits GAT-1)
Antagonism of excitatory transmitters (i.e., glutamate)
felbamate (antagonist of NMDA receptors)
topiramate (antagonist of kainate/AMPA receptors)
Why there are few drugs targeting K channels to treat seizures?
because HERG is also targeted –> heart problems
Molecular targets at the excitatory (glutamatergic) synapse
Presynaptic targets
§ Na+ channels
§ Ca2+ channels
Post-synaptic targets
§ NMDA receptors
§ AMPA receptors
Molecular targets at the inhibitory (GABAergic) synapse
Presynaptic targets
§ GABA transporter (GAT-1)
§ GABA transaminase (GABA-T)
Post-synaptic targets
§ GABAA receptors
§ GABAB receptors (?)
Drugs used to treat focal seizures and generalized tonic-clonic seizures
phenytoin, carbamazepine, oxcarbazapine, lacosamide, phenobarbital, primidone, diazepam, clonazepam, gabapentin, pregabalin, vigabatrin, tiagabine, felbamate, topiramate
A number of antiseizure drugs have a common
heterocyclic ring structure.
X group:
-N- hydantoin derivatives (phenytoin)
-C-N- barbiturates (phenobarbital)
-C- succinimides (ethosuximide)
Hydantoins
phenytoin (Dilantin)
– oldest non-sedative antiseizure drug (introduced in 1938)
– mechanism of action: binds and stabilizes the inactivated state of Na+ channels (not isoform selective thus can target sodium channels in the brain as well as other parts of the body)
– other drugs in this class with a similar mechanism of action:
* fosphenytoin (Cerebyx): injectable phosphate prodrug
* ethotoin (fewer side effects, but less effective than phenytoin)
* mephenytoin (more toxic than phenytoin)
Mechanism of sodium channel activation
Phenytoin and other anticonvulsants (e.g., carbamazepine, valproate) act by binding and stabilizing the inactivated state of Na+ channels.
Hydantoins: phenytoin (Dilantin): pharmacokinetics
- Phenytoin elimination kinetics are dose-dependent. This leads to non- linear pharmacokinetics.
- As blood levels of phenytoin increase, the liver enzymes responsible for metabolizing the drug become saturated.
- Small increases in the drug dose can lead to dramatic increases in the drug concentration in the blood.
- Therapeutic plasma level: 7.5-20 μg/mL (a higher level can be toxic).
Hydantoins: phenytoin (Dilantin): drug interactions
- Phenytoin can be displaced from plasma proteins by other drugs (e.g., Valproate), leading to an increase in its plasma concentration.
- Phenytoin induces liver cytochrome P450 enzymes, thereby increasing the rate of metabolism of other drugs (e.g., carbamazepine).
Hydantoins: phenytoin (Dilantin): toxicity
- arrhythmia
- visual: nystagmus (involuntary eye movements), diplopia (blurred vision)
- ataxia
- GI symptoms
- sedation (only at high doses)
- gingival hyperplasia, hirsutism (growth of facial hair)
- hypersensitivity reactions (skin rash)
Iminostilbenes
carbamazepine (Tegretol) and oxcarbazepine (Trileptal)
Carbamazepine
– structure: tricyclic compound (used to treat bipolar depression)
– 3D structure is very similar to that of phenytoin
– mechanism of action: binds and stabilizes the inactivated state of Na+ channels
– drug interactions: induces liver cytochrome P450 enzymes, thereby increasing the rate of metabolism of itself and other drugs (e.g., phenytoin, ethosuximide, valproate, clonazepam)
– toxicity: blurred vision, ataxia, GI disturbances; sedation at high doses, serious skin rash (Stevens-Johnson Syndrome/toxic epidermal necrolysis); Drug reaction with eosinophilia and systemic symptoms (DRESS) hypersensitivity reaction
Oxcarbazepine
– reduced toxicity compared to carbamazepine