Mephenytoin (marketed as Mesantoin by Novartis) is a hydantoin, used as an anticonvulsant. It was introduced approximately 10 years after phenytoin, in the late 1940s. The significant metabolite of mephenytoin is nirvanol (5-ethyl-5-phenylhydantoin), which was the first hydantoin (briefly used as a hypnotic). However, nirvanol is quite toxic and mephenytoin was only considered after other less toxic anticonvulsants had failed. It can cause potentially fatal blood dyscrasia in 1% of patients.

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Mephenytoin

Clinical data
AHFS/Drugs.com	Micromedex Detailed Consumer Information
MedlinePlus	a611020
Routes of administration	Oral
ATC code	N03AB04 (WHO)
Pharmacokinetic data
Metabolism	CYP2C19
Elimination half-life	7 hours
Identifiers
IUPAC name 5-Ethyl-3-methyl-5-phenyl-imidazolidine-2,4-dione
CAS Number	50-12-4 Y
PubChem CID	4060
IUPHAR/BPS	7223
DrugBank	DB00532 Y
ChemSpider	3920 Y
UNII	R420KW629U
KEGG	D00375 Y
ChEMBL	ChEMBL861 Y
CompTox Dashboard (EPA)	DTXSID9023257
ECHA InfoCard	100.000.012
Chemical and physical data
Formula	C12H14N2O2
Molar mass	218.256 g·mol−1
3D model (JSmol)	Interactive image
SMILES O=C2N(C(=O)NC2(c1ccccc1)CC)C
InChI InChI=1S/C12H14N2O2/c1-3-12(9-7-5-4-6-8-9)10(15)14(2)11(16)13-12/h4-8H,3H2,1-2H3,(H,13,16) Y Key:GMHKMTDVRCWUDX-UHFFFAOYSA-N Y
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Mephenytoin is no longer available in the US or the UK. It is still studied largely because of its interesting hydroxylation polymorphism.