Lecture 47 - Pharmacology of Anticonvulsant Drugs Flashcards

1
Q

Mechanism of action of anticonvulsant drugs:

A

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)

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2
Q

Decrease sodium influx, prolong inactivation of Na+ channels examples

A

carbamazepine oxcarbazepine
phenytoin
lacosamide
lamotrigine
valproate

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3
Q

Reduction of calcium influx (this is critical for absence seizures)

A

ethosuximide
lamotrigine
valproate

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4
Q

Enhance GABA-mediated neuronal inhibition

A

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)

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5
Q

Antagonism of excitatory transmitters (i.e., glutamate)

A

felbamate (antagonist of NMDA receptors)
topiramate (antagonist of kainate/AMPA receptors)

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6
Q

Why there are few drugs targeting K channels to treat seizures?

A

because HERG is also targeted –> heart problems

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7
Q

Molecular targets at the excitatory (glutamatergic) synapse

A

Presynaptic targets
§ Na+ channels
§ Ca2+ channels
Post-synaptic targets
§ NMDA receptors
§ AMPA receptors

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8
Q

Molecular targets at the inhibitory (GABAergic) synapse

A

Presynaptic targets
§ GABA transporter (GAT-1)
§ GABA transaminase (GABA-T)
Post-synaptic targets
§ GABAA receptors
§ GABAB receptors (?)

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9
Q

Drugs used to treat focal seizures and generalized tonic-clonic seizures

A

phenytoin, carbamazepine, oxcarbazapine, lacosamide, phenobarbital, primidone, diazepam, clonazepam, gabapentin, pregabalin, vigabatrin, tiagabine, felbamate, topiramate

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10
Q

A number of antiseizure drugs have a common

A

heterocyclic ring structure.
X group:
-N- hydantoin derivatives (phenytoin)
-C-N- barbiturates (phenobarbital)
-C- succinimides (ethosuximide)

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11
Q

Hydantoins

A

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)

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12
Q

Mechanism of sodium channel activation

A

Phenytoin and other anticonvulsants (e.g., carbamazepine, valproate) act by binding and stabilizing the inactivated state of Na+ channels.

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13
Q

Hydantoins: phenytoin (Dilantin): pharmacokinetics

A
  • 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).
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14
Q

Hydantoins: phenytoin (Dilantin): drug interactions

A
  • 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).
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15
Q

Hydantoins: phenytoin (Dilantin): toxicity

A
  • 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)
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16
Q

Iminostilbenes

A

carbamazepine (Tegretol) and oxcarbazepine (Trileptal)

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17
Q

Carbamazepine

A

– 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

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18
Q

Oxcarbazepine

A

– reduced toxicity compared to carbamazepine

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19
Q

Lacosamide (Vimpat)

A

– mechanism of action: enhances inactivation of voltage-gated Na+ channels
– toxicity: dermatological reactions, cardiac risks (PR interval prolongation), visual disturbances

20
Q

Barbiturates and benzodiazepines bind

A

an allosteric regulatory site on the GABAA receptor.

21
Q

Barbiturates

A

phenobarbital (Luminal) and primidone (Mysoline)

22
Q

Phenobarbital

A

drug of choice in infants up to 2 months of age
– structure: 3D structure similar to that of phenytoin
– oldest anti-seizure drug other than the bromides
– mechanisms of action:
* binds to an allosteric regulatory site on the GABAA receptor, increases duration of Cl- channel-opening events (and thus enhances GABA inhibitory signaling).
– drug interactions: induces liver cytochrome P450 enzymes
– toxicity: sedation, physical dependence (potential of abuse)

23
Q

Primidone

A

– mechanism of action: may be more similar to that of phenytoin than phenobarbital

24
Q

Benzodiazepines

A

diazepam (Valium) and clonazepam (Klonopin)

25
Q

Diazepam

A

especially useful for tonic-clonic status epilepticus; often administered as a rectal gel for acute control of seizure activity
– mechanisms of action:
* binds to an allosteric regulatory site on the GABAA receptor, increases frequency of Cl- channel-opening events (and thus enhances GABA inhibitory signaling).
– toxicity: sedation, physical dependence (tolerance); therefore, not useful for chronic treatment.

26
Q

Clonazepam

A

useful for acute treatment of epilepsy and absence seizures
– similar properties as for diazepam

27
Q

Gabapentin

A

used as an adjunct anti-seizure therapy (also used for neuropathic pain and migraine)
– structure: analog of GABA
– mechanisms of action:
* increases GABA release
* decreases presynaptic Ca2+ influx, thereby reducing glutamate release
– toxicity: sedation, ataxia, behavioral changes

28
Q

Pregabalin

A

– similar properties as for gabapentin

29
Q

Vigabatrin

A

used as an adjunct therapy for refractory patients
– structure: analog of GABA (γ-vinyl-GABA)
– mechanism of action:
* irreversible inhibitor of GABA transaminase (GABA-T), the enzyme responsible for degrading GABA
– toxicity: sedation, weight gain, agitation, psychosis, depression, visual field defects

30
Q

Tiagabine

A

also used as an adjunct therapy
– mechanism of action: inhibits GABA transporter (GAT-1)
– toxicity: nervousness, depression, tremor, sedation, ataxia

31
Q

Drugs targets at the inhibitory (GABAergic) synapse

A

Presynaptic targets
§ GABA transporter (GAT-1) (tiagabine)
§ GABA transaminase (GABA-T) (vigabatrin)
Post-synaptic targets
§ GABAA receptors (phenobarbital, benzodiazepines)

32
Q

Molecular targets at the excitatory, glutamatergic synapse

A

NMDA and AMPA (and kainate) receptors

33
Q

NMDA receptor

A

glutamate binding triggers an influx of Na+ and Ca2+ and an efflux of K+.

34
Q

AMPA receptor

A

glutamate binding triggers an influx of Na+ and an efflux of K+. This is also true of a 3rd type of ionotropic glutamate receptor, the kainate receptor.

35
Q

Felbamate

A

used as a 3rd line drug for refractory cases (especially for focal seizures)
– mechanism of action:
* NMDA receptor antagonist
– toxicity: severe hepatitis (which is why it’s a 3rd line drug)

36
Q

Topiramate

A

used as a monotherapy or an adjunct therapy
– structure: substituted monosaccharide (unique structure compared
to that of other anticonvulsants)
– mechanism of action:
* AMPA and kainate receptor antagonist
– toxicity: nervousness, confusion, cognitive dysfunction, sedation, vision loss

37
Q

Drugs used to treat absence seizures

A

ethosuximide

38
Q

Succinimides

A

ethosuximide (Zarontin)
– introduced in 1960 as a ‘pure petit-mal’ drug
– mechanism of action: blocks T-type Ca2+ channels (low-threshold
current) in thalamic neurons
– T-type Ca2+ channels are thought to be involved in generating the rhythmic discharge of an absence attack. (Remember from Lecture 47 that generalization involves thalamocortical signaling).
– Toxicity: GI distress, sedation, psychiatric disturbances

39
Q

T/F Gabapentin increases Cl- influx in postsynaptic neurons

A

True

40
Q

Drugs used to treat focal seizures, generalized tonic-clonic seizures, and absence seizures

A

clonazepam, lamotrigine, valproate, levetiracetam,

41
Q

Lamotrigine (Lamictal)

A

– uses: primary or adjunct therapy for focal and primary generalized seizures, including absence; also used for bipolar disorder
– structure: phenyltriazine
– mechanisms of action:
* inhibits Na+ and voltage-gated Ca2+ channels
* disrupts synaptic glutamate release
– toxicity: sedation, ataxia, serious skin rash (Stevens-Johnson Syndrome/toxic epidermal necrolysis)

42
Q

Valproate (Depaken)

A

– uses: focal and generalized seizures, including absence; bipolar disorder, migraine headache
– structure: fatty acid (note: ionized at physiological pH)
– mechanisms of action:
* inhibits Na+ and Ca2+ channels
* increases GABA levels (by stimulating glutamic acid decarboxylase or inhibiting GAT-1 or GABA-T)
– drug interactions:
* displaces phenytoin from plasma proteins
* inhibits the metabolism of phenytoin, carbamazepine, phenobarbital, lamotrigine
– toxicity: GI distress, hyperammonemia, hepatotoxicity (can be fatal – careful monitoring necessary), sedation, weight gain; tremor (at high doses)

43
Q

Levetiracetam (Keppra)

A

– uses: focal and generalized seizures, myoclonic seizures, status epilepticus
– mechanisms of action:
* binds the synaptic vesicular protein SV2A, and thus interferes with synaptic vesicle release and neurotransmission.
* also appears to interfere with calcium entry through Ca2+ channels and with intraneuronal calcium signaling.
* because of its unique mechanism of action, it is a candidate for treatment of status epilepticus cases that are refractory to other therapies.

44
Q

Brivaracetam (briviact)

A

analog of levetiracetam that acts via a similar mechanism, but with a higher affinity for SV2A.

45
Q

Drug targets at excitatory (glutamatergic) synapse

A

Presynaptic targets:
Na+ channels (phenytoin, carbamazepine, lacosamide, lamotrigine, valproate)
Ca2+ channels (ehtosuximide, lamotrigine, levetiracetam, valproate)
Post-synaptic targets:
NMDA receptors (felbamate)
AMPA receptors (topiramate)