Many mechanisms, one catastrophic teratogen
2-propylpentanoic acid (valproic acid) · C8H16O2 · MW 144.21 g/mol · salts: sodium valproate, divalproex (Depakote) · CAS 99-66-1 · ChEMBL CHEMBL109
Valproate (Depakote, Epilim). A branched short-chain fatty acid that refuses to be one drug: use-dependent Nav block, enhanced GABAergic tone (GAD up, GABA-T down), and HDAC inhibition that is probably why the teratogenicity is so ugly. First-line for many epilepsies and a bipolar workhorse — and the anticonvulsant you should not start in anyone who could become pregnant unless nothing else works. The neural-tube and neurodevelopmental signal is not a footnote.
Valproate prolongs recovery of voltage-gated sodium channels from inactivation (use-dependence), increases GABA (stimulate GAD, inhibit GABA transaminase and succinic semialdehyde dehydrogenase), and inhibits histone deacetylases. No single PDB ligand pose explains the clinical profile — unlike lamotrigine #059, which is a cleaner Nav story with a real co-crystal. We show 8THH as the Nav pocket lamotrigine occupies, honestly labeled as a surrogate for one arm of valproate, not a valproate complex.
The HDAC/epigenetic arm is the leading explanation for the valproate embryopathy: neural-tube defects (especially spina bifida), cardiac and limb malformations, and a dose-dependent hit to IQ/neurodevelopment. This is why pregnancy-prevention programs exist around the drug in Europe and why lamotrigine/levetiracetam are preferred when pregnancy is possible.
Slows recovery from inactivation → less high-frequency firing. One anticonvulsant arm, not the whole molecule.
GAD stimulation + GABA-T inhibition. More inhibitory tone, also tremor and sedation.
Epigenetic; plausible driver of teratogenicity and some mood effects. Not a 'bonus feature.'
High and saturable — free fraction rises in overdose, hypoalbuminemia, and with displacers (aspirin, some NSAIDs).
Inhibits CYP2C9, UGT1A4 (raises lamotrigine — SJS risk if you don't cut lamotrigine), epoxide hydrolase (carbamazepine epoxide).
Hepatotoxicity (under 2 years, mitochondrial disease), pancreatitis, teratogenicity. Any one of these is a stop-the-line event.
Tmax depends on formulation (divalproex delayed). t½ ~9–16 h. Hepatic glucuronidation and β-oxidation (mitochondrial — hence the hepatotoxicity in kids with metabolic disease). Levels are interpreted as total; free fraction is what the brain sees.
Valproate inhibition of UGT1A4 roughly doubles lamotrigine levels. Start lamotrigine lower and slower, or you buy Stevens–Johnson. The two drugs are often paired in bipolar depression; the interaction is not optional knowledge. See #059.
Valproate is an eight-carbon acid. It does not look like a Nav pore drug in 8THH, and it isn't crystallized there. FlexAIDΔS would refuse a single-pose narrative: the clinical ΔG is distributed across enzymes (GABA-T, HDAC) and channels. The teratogenicity is an epigenetic occupancy problem more than a docking problem. We still show a Nav because that arm is real — and we label the absence of a valproate ligand in the map.
Four organ systems, four ways to stop the drug. Teratogenicity is the one that outlives the prescription.
| Target | Affinity | Rel. | Action |
|---|---|---|---|
|
Nav
use-dependent inactivation
|
µM class
one arm; no co-crystal
|
Inhibitor | |
|
GABA-T / GAD
GABA elevation
|
enzymatic
not a GABA-A PAM like benzos
|
Enzyme | |
|
HDAC
epigenetic
|
µM
teratogenicity hypothesis
|
Inhibitor | |
|
GABAA BZD site
diazepam pocket
|
no
not a benzo
|
None |