Classification
Antiepileptic drugs are categorized based on their target in the pathogenic pathway. Valproic acid and its salts belong to agents with a combined mechanism of action (along with topiramate).
Due to their polyvalent effects, such drugs cannot be assigned to a single narrow category. For comparison, more targeted agents include:
- Voltage-gated sodium channel blockers (phenytoin, carbamazepine, lamotrigine).
- Agents enhancing GABAergic inhibition (barbiturates, benzodiazepines, vigabatrin).
- T-type calcium channel blockers (ethosuximide).
- Inhibitors of excitatory amino acids (felbamate).
Valproates combine several of these effects, justifying their placement in a distinct pharmacological subcategory.
Mechanism of Action (Pharmacodynamics)
The polyvalent effect of the drug consists of three main components (the so-called "triple effect"):
- Blockade of voltage-gated sodium ($Na^+$) channels. The drug acts on the presynaptic membrane, suppressing high-frequency neuronal firing directly within the epileptogenic focus. This stabilizes the membrane and prevents the spread of pathological excitation.
- Enhancement of GABAergic inhibition. Valproates increase the levels of the primary inhibitory neurotransmitter in the CNS — GABA (gamma-aminobutyric acid). This occurs via two complementary processes:
- Inhibition of catabolism: blockade of GABA transaminase reduces the breakdown of the neurotransmitter.
- Stimulation of synthesis: increased activity of glutamate decarboxylase accelerates the formation of GABA.
- Blockade of T-type calcium ($Ca^{2+}$) channels. The target is localized on the postsynaptic membrane (often associated with thalamic neurons). This mechanism is specifically important for suppressing absence seizures.
Pharmacokinetics
When administered orally, the drug is well absorbed in the gastrointestinal tract. Peak plasma concentration is reached on average in 2 hours (the exact time may vary depending on the specific formulation). It is metabolized in the body and excreted by the kidneys as oxidation products or glucuronide conjugates.
Indications
Due to its broad spectrum of activity, sodium valproate is effective in nearly all forms of epilepsy. Main indications include:
- Generalized seizures (both tonic-clonic/grand mal and absence/petit mal seizures).
- Partial (focal) seizures.
- Myoclonic epilepsy.
Adverse Effects and Contraindications
The safety profile of valproates requires careful patient monitoring. Potential adverse reactions include:
- GI tract: dyspeptic symptoms (nausea, vomiting, diarrhea, abdominal pain).
- CNS: somnolence, tremor, ataxia, nystagmus, and diplopia (double vision).
- Hematologic: impaired blood coagulation, thrombocytopenia, anemia.
- Allergic reactions: various skin rashes.
Contraindications: The drug is strictly contraindicated in patients with severe hepatic dysfunction or pancreatic disorders.
Formulation and Administration
Both valproic acid and its sodium salt are used in clinical practice. Known brand names include Depakote, Epilim, Convulex.
- Formulation: 0.3 g tablets.
- Dosing regimen: Oral, 0.3 g 2–3 times daily.
- Administration tips: Take during or immediately after meals. Tablets should be swallowed whole without chewing to minimize irritation of the gastrointestinal mucosa.