Anticonvulsants/Antiepileptics – Medicinal Chemistry

Anti-Convulsants

INTENDED LEARNING OBJECTIVES

At the end of this lecture, student will be able to:

• Distinguish between convulsions and epilepsy

• Differentiate the types of epilepsy

• Define anti convulsants/antiepileptics

• Categorize the various anticonvulsants/antiepileptics according to their structure

• Outline the synthesis of some anticonvulsants

• Recognize the specific uses of the different anticonvulsants

Introduction of Anticonvulsants/Antiepileptics

• Convulsions are involuntary spasms or a series of jerking of the muscles

– It can be experimentally produced by certain chemicals

– Eg. PTZ (pentylene tetrazole)

• Epilepsy is a disease that occurs due to central nervous
system (CNS) disorder, which is characterized by seizures and convulsions or abnormal body movements with the loss of consciousness.

Three principle types of epilepsy are found.

They are as follows:

  Grandmal: In which the seizures last from 2 to 5 min, being characterized by a sudden loss of consciousness, tonic and clonic convulsions of all muscles associated with urinary incontinence.

• Petitmal: The seizures last from 5 to 30 sec, being characterized by brief attacks of unconsciousness, usually occur in children at the age of 4 to 8 years.

• Psychomotor seizures: Characterized by attacks without convulsions and lasts from 2 to 3 min.

Anticonvulsants

• Antiepileptic/anticonvulsants:

A medication used to control (prevent) seizures
(convulsions) or stop an ongoing series of seizures.

• They prevent, stop, or lessen the excessive electric
activity in the brain

Anticonvulsants Mechanism of action:

The anticonvulsant therapy mediated by the drugs is through
different aspects of neurotransmission inhibition in the brain:

• By inhibiting sodium channels (phenytoin).

• By inhibiting gamma amino butyric acid (GABA) transaminase enzyme (vigabatrin).

• By inhibition of T-type calcium currents (ethosuximide,
valproate).

• By GABA agonistic activity (benzodiazepine).

Classification of Anticonvulsants

• BARBITURATES

– PHENOBARBITONE

– METHABARBITONE

• HYDANTOINS

– PHENYTOIN

– MEPHENYTOIN

– ETHOTOIN

• OXAZOLIDINE DIONES

– TRIMETHADIONE

– PARAMETHADIONE

• SUCCINIMIDES

– PHENSUXIMIDE

– METHSUXIMIDE

– ETHOSUXIMIDE

• UREA AND MONOACYL UREAS

– PHENACETAMIDE

– CARBAMAZEPINE

• BENZODIAZEPINES

– CLONAZEPAM

• MICELLENEOUS

– PRIMIDONE

– VALPROIC ACID

– GABAPENTIN

– FELBAMATE

I. Barbiturates

• Most of the barbiturates are sedatives and hypnotics. Only
a few of them show anticonvulsant characters.

• Three important barbiturates that show anticonvulsant
properties are the following:

SAR of Barbiturates

1. Optimum activity is observed when the substitution at C-5
is phenyl.

2. The 5, 5’-diphenyl derivative have less activity than
phenobarbitone.

3. N1and N3 substituents in some cases resulted in an increase
in activity

BARBITURATES Classification

• PHENOBARBITONE

• METHABARBITONE

PHENOBARBITAL INN, USAN, PHENOBARBITONE BAN

Classification of Anticonvulsants

Chemically it is 5-ethyl-5-phenyl-1H, 3H, 5H-pyrimidine-2,
4, 6-trione

• Phenobarbitone sodium is hygroscopic, bitter taste, water
soluble, odorless, white crystalline powder.

Medicinal Uses

• It is used both as sedative and hypnotic.

• It is the drug of choice in the treatment of grandma and
petitmal epilepsy

• It is useful in nervous and related tension states.

• An overdose of it can result in coma, severe respiratory
depression,

Methabarbital

Chemically it is 5,5-diethyl-1-methyl-1,3-diazinane-2,4,6-trione/5,5-Diethyl-1-methyl-2,4,6(1H,3H,5H)-pyrimidinetrione

It is a barbiturate anticonvulsant, used in the treatment of
epilepsy

II. Hydantoins

• Hydantoins are cyclic monoacylureas.

• They possess imidazoline-2, 4-dione heterocyclic system

• The hydantoins are close structural relatives of
barbituric acid, differing due to the lack of C-6 oxo group.

• The lack of this carbonyl group decreases the acidity. So
it is a weaker acid than that of barbiturates

SAR OF HYDANTOIN

• 5-phenyl or other aromatic substitution is essential for activity

• Alkyl substituent at position 5 may contribute to sedation

• 1,3-disubstituted hydantoins, exhibit activity against
chemically induced convulsion, while it remains ineffective against electric shock induced convulsion

Phenytoin:

• Phenytoin occurs as phenytoin sodium, chemically phenytoin is 5,5′-diphenylhydantoin.

Properties:      

Phenytoin occurs as white, crystalline powder, slightly
hygroscopic, soluble in water and in alcohol, practically insoluble in ether and in methylene chloride.

Medicinal uses:

• Phenytoin is used alone or in combination with
phenobarbital in the treatment of grand mal and psychomotor epilepsy

• It is also used in the treatment of other types of
convulsions

Phenytoin Synthesis

Mephenytoin:

• Mephenytoin is a hydantoin derivative. Chemically it is
5-ethyl-3-methyl 5-phenylhydantoin.

Properties:

Mephenytoin is water insoluble, colorless crystalline solid.
It forms a water soluble sodium salt which has an alkaline reaction.

Medicinal uses:

• It was introduced as a sedative-hypnotic and
anticonvulsant under the name Nirvanol, but it was withdrawn because of toxicity.

Ethotoin INN, USAN, BAN,:

• Ethotoin is a hydantoin anticonvulsant with anti-epileptic
activity.

Chemically it is 3-ethyl-5-phenylimidazolidine-2,4-dione

Medicinal uses:

• An antiepileptic, it is less toxic than phenytoin but also
less effective.

• Used in the treatment of Grandmal epilepsy

• It has a role as an anticonvulsant

III. Oxazolidinediones

• Replacement of the -NH group at position 1 of the
hydantoin systems with oxygen atom yields the oxazolidine- 2,4-dione system

• Trimethadione is only clinically used

SAR OF OXAZOLIDINE DIONES

• Replacement of the -NH group at position 1 of the hydantoin system with an oxygen atom yields the oxazolidine-2,4-dione system

• 3,5,5-Trimethadione (tridione) was the first drug
introduced specifically for treating absence seizures.

• The nature of the substituent on C-5 is important,
example, lower alkyl substituents towards antipetitmal activity while acyl substituents towards antigrandmal activity

• The N-alkyl substituent does not alter or afford the
activity since all the clinically used agents from this class undergo N-dealkylation in metabolism.

Trimethadione:

Trimethadione is an oxazolidinedione derivative. Chemically
it is 3, 5,5-trimethyloxazolidine-2,4-dione

Properties:

Trimethadione is colourless or almost colourless crystals,
soluble in water,very soluble in alcohol and in ether. It should be protected from light.

Medicinal uses:

• Used in the treatment of absence seizures

Paramethadione

Chemically it is 5-ethyl-3, 5-dimethyl oxazolidine-2,4-dione.

Structurally it is very closely related to trimethadione

Properties:

Paramethadione is available as clear, colourless liquid. It
is partially miscible with water. It should be stored in a well-closed container.

Medicinal uses:

• Used in the treatment of absence seizures

IV. SUCCINIMIDES

• Due to the inherent high-level of toxicity attributed by the
oxazolidinediones in prolonged therapy as anticonvulsants

• A vigorous attempt was made to replace them with better
effective and less toxic drugs.

• Three members of this class of compounds were introduced between early fifties to late fifties, namely; Phensuximide, Methsuximide and Ethosuximide.

All of them gained with acceptance for the treatment of
petit mal seizures specifically

SAR of Succinimides

 

• The activity of antiepileptic agents, such as the oxazolidine
2,4-dione with substituted succinamides (CH2 replace O) was logical choice for synthesis and evaluation.

• N-demethylation occurs to yield the putative active
metabolite.

• Both phensuximide and the N-demethyl metabolite are
inactivated by p-hydroxylation and conjugation.

PHENSUXIMIDE (Milontin)

• Phensuximide is a 2,5-pyrrolidinedione derivative

• Chemically it is N-methyl- 2-phenylsuccinimide

Properties: It is a crystalline solid, soluble in water and freely soluble in ethanol.

Medicinal Uses:

It is used in the treatment of petitmal epilepsy

Methsuximide: (Celontin)

Chemically it is 2-ethyl-2-methylsuccinimide.

Methsuximide is a 2,5-pyrrolidinedione derivative

Properties: It is a crystalline solid, soluble in water and freely soluble in ethanol.

Medicinal Uses:

• More active than Phensuximide

• It is used in the treatment of petitmal epilepsy

ETHOSUXIMIDE: (Zarontin)

Chemically it is 2-ethyl-2-methylsuccinimide.

ethosuximide is a 2,5-pyrrolidinedione derivative

Properties: It is a white or an almost white powder or waxy solid.

Freely soluble in water, ethanol, and methylene chloride.

Medicinal Uses:

• It is used in the treatment of petitmal epilepsy

ETHOSUXIMIDE SYNTHESIS

V. UREA AND MONOACYL UREAS

   PHENACETAMIDE

   CARBAMAZEPINE

General chemical structure of an urea

General chemical structure of an acylurea

PHENACETAMIDE: (Phenurone)

Properties: It is available as crystalline, slightly water soluble solid

Medicinal uses: Phenacemide is used as an anticonvulsant drug

CARBAMAZEPINE: (Tegretol, Zen, Zeptol)

Properties: It is a white or almost white crystalline powder and it shows polymorphism, slightly soluble in water, freely soluble in methylene chloride, but sparingly soluble in acetone and ethanol

Medicinal Uses:

• Carbamazepine, a urea derivative, is a broad spectrum antiseizure agent, but is toxic

• Used to treat partial seizures and grandmal seizures

CARBAMAZEPINE Synthesis

VI. BENZODIAZEPINES

CLONAZEPAM:

Clonazepam is 5-(2-chlorophenyl)-3-dihydro-7-nitro-2H-1,4- benzodialzepin-2-one

Properties: Clonazepam is slightly yellowish, crystalline powder, practically insoluble in water, slightly soluble in alcohol and in methanol, very slightly soluble in ether. It melts at about 239°C.

Medicinal Uses:

Clonazepam is used in the treatment of grand mal epilepsy

VII. MICELLENEOUS:

• PRIMIDONE

• VALPROIC ACID

• GABAPENTIN

• FELBAMATE

PRIMIDONE:

• Primidone is an antiepileptic agent related to
barbiturates

• Primidone is a diketone derived from hexahydropyrimidine chemically it is 5-ethyl-2, 3-dihydro-5-phenyl-4, 6-(1H, 5H)-pyrimidine dione.

Properties: A white or almost white, crystalline powder, very slightly soluble in water, slightly soluble in alcohol, practically insoluble in ether.

Medicinal Uses:

Primidone is an antiepileptic agent used to control grandmal
and psychomotorseizures.

VALPROIC ACID:

• Valproic acid was discovered as an anticonvulsant in 1963,
and it was approved by the FDA for seizures in 1978. 

• Valporic acid is 2-propylpentanoic acid. It is available
as sodium valproate.

• Valproic acid is not chemically related to other
anticonvulsants.

Properties:
Sodium valproate is hygroscopic, white, crystalline powder.

• It is freely soluble in water and should be stored in an
airtight container.

Medicinal Uses:

• Used for different types of seizures

• Absence seizures (petit mal seizures), generalized tonic-clonic seizures, complex partial seizures, and myoclonic seizures.

• Sodium valproate may be effective against myoclonic and
atonic seizures in young children

• considered as the agent of choice.

• Used for treating bipolar mania and migraine headaches

Gabapentin (Gabantin, Gaba, Rejuron)

• It is a GABA analogues

Properties: It is a colourless, crystalline substance, soluble in water.

Medicinal uses:

• It has been endorsed as an effective drug for the management of neuropathic pain.

• Used to treat partial seizures

Felbamate: (MedPointe)

• Felbamate is a dicarbamate derivative anticonvulsant chemically it is (3-carbamoyloxy-2-phenylpropyl) carbamate

Medicinal uses:

• An anticonvulsant used in the treatment of epilepsy.

• It is used to treat partial seizures

SUMMARY

• Epilepsy is a disorder characterized by dysfunction due to
excessive neuronal discharge and usually associated with some alteration of consciousness

• Antiepileptic is a medication used to control (prevent)
seizures (convulsions) or stop an ongoing series of seizures.Antiepileptic is a medication used to control (prevent)

seizures (convulsions) or stop an ongoing series of seizures.

• They prevent, stop, or lessen the excessive electric
activity in the brain

• Classification of anticonvulsant agents

• Drug profiles of some anticonvulsant agents

• Synthesis of some antiepileptics have been outlined.

• Although the drugs are effective antiepileptics, they have
numerous side effects. Thus, there is a need for better anti-epileptic drugs.

Also, Visit:

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