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Phenytoin: Structure , Synthesis , SAR , Mechanism , Uses


Structure –

  • it , also known as Dilantin, has a chemical formula of C15H12N2O2 and a molecular weight of 252.27 g/mol. It has a three-dimensional structure with a phenyl ring, a hydantoin ring, and an amino acid side chain. The hydantoin ring contains four carbon atoms, two nitrogen atoms, and one oxygen atom, while the phenyl ring is a six-carbon ring with alternating double bonds. The amino acid side chain contains a secondary amine group (-NH-) and an aromatic ring.
  • The structure of Phenytoin can be represented by a structural formula, which shows the arrangement of atoms and chemical bonds in the molecule. The structural formula of Phenytoin is:
phenytoin Phenytoin: Structure , Synthesis , SAR , Mechanism , Uses
  • In this formula, H represents a hydrogen atom, C represents a carbon atom, N represents a nitrogen atom, O represents an oxygen atom, and Na represents a sodium atom. The double lines between some of the atoms represent double bonds, while the single lines represent single bonds.

Synthesis –

it can be synthesized in several steps from benzil, urea, and ethylenediamine. The overall synthesis can be summarized as follows:

  1. Benzil is reacted with hydrazine to form 1,2-diphenylhydrazine.
  2. 1,2-diphenylhydrazine is then treated with ethyl chloroformate to form a diethyl carbamate derivative.
  3. The diethyl carbamate derivative is then reacted with urea to form a hydantoin derivative.
  4. The hydantoin derivative is treated with ethylenediamine to form phenytoin.

The synthesis can be represented by the following chemical equations:

Step 1:

Benzil + hydrazine → 1,2-diphenylhydrazine

Step 2:

1,2-diphenylhydrazine + ethyl chloroformate → diethyl carbamate derivative

Step 3:

Diethyl carbamate derivative + urea → hydantoin derivative

Step 4:

Hydantoin derivative + ethylenediamine → Phenytoin

The resulting product is then purified and typically converted into the sodium salt form for use in pharmaceutical formulations.


The Structure-Activity Relationship (SAR) of Phenytoin is as follows:

  1. Aromatic hydantoin ring: The aromatic hydantoin ring in the phenytoin molecule is essential for its anticonvulsant activity. The ring structure is important for binding to the target site in the brain and blocking the sodium channels.
  2. Substituents on the phenyl ring: The substituents on the phenyl ring affect the potency of phenytoin. Compounds with electron-donating groups on the phenyl ring tend to be less potent than those with electron-withdrawing groups.
  3. Aliphatic side chain: The aliphatic side chain in the phenytoin molecule is important for its pharmacokinetic properties. Modifying the length or branching of the side chain can affect the metabolism and clearance of the drug.
  4. Dipole moment: The dipole moment of the phenytoin molecule is important for its pharmacokinetics. The dipole moment influences the solubility and absorption of the drug.
  5. Stereochemistry: The stereochemistry of the phenytoin molecule can affect its activity and pharmacokinetics. The S-enantiomer of phenytoin is more potent than the R-enantiomer and has a longer half-life.

Overall, the SAR of phenytoin suggests that the aromatic hydantoin ring is essential for activity, while modifications to the phenyl ring and aliphatic side chain can affect potency and pharmacokinetics.

Mechanism –

The exact mechanism of action of Phenytoin is not fully understood, but it is believed to involve multiple targets in the central nervous system. Phenytoin is known to block voltage-gated sodium channels, which reduces the influx of sodium ions into neurons during an action potential, thereby decreasing neuronal excitability. Additionally, Phenytoin enhances the activity of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, by increasing the release of GABA and reducing its metabolism.

The mechanism of it action on voltage-gated sodium channels can be described in more detail as follows:

  1. it binds to the channel in its inactive state, stabilizing it and prolonging its refractory period.
  2. This reduces the probability that the channel will open in response to a depolarizing stimulus, thereby decreasing the influx of sodium ions into the neuron.
  3. As a result, the threshold for firing an action potential is increased, and neuronal excitability is reduced.

Phenytoin also appears to have effects on other ion channels, including calcium channels and potassium channels, which may contribute to its antiepileptic activity. However, the precise mechanisms involved in these actions are not well understood.

Uses –

Phenytoin is an anticonvulsant medication that is primarily used for the treatment and prevention of seizures. Here are some of the main uses of phenytoin:

  1. Epilepsy: it is commonly used as a first-line treatment for partial and generalized tonic-clonic seizures in patients with epilepsy. It works by stabilizing the neuronal membrane and reducing the abnormal electrical activity that causes seizures.
  2. Status epilepticus: it is also used in the treatment of status epilepticus, a medical emergency characterized by prolonged seizures that can lead to brain damage or death if not treated promptly. Phenytoin can be given intravenously in this setting to quickly stop the seizures.
  3. Cardiac arrhythmias: it has some anti-arrhythmic properties and may be used to treat certain types of cardiac arrhythmias, particularly those that are caused by abnormal electrical activity in the heart.
  4. Neuropathic pain: it is sometimes used off-label to treat neuropathic pain, particularly in patients who do not respond to other treatments. It may work by reducing abnormal neuronal firing and dampening pain signals.
  5. Wound healing: it has been shown to promote wound healing in certain types of skin ulcers, particularly those associated with diabetes. It may work by increasing blood flow to the wound and promoting cell proliferation.

It’s important to note that phenytoin should only be used under the supervision of a healthcare professional, as it can have significant side effects and can interact with other medications.