Phenoxybenzamine

IUPAC nomenclature

(RS)-N-Benzyl-N-(2-chloroethyl)-1-phenoxypropan-2-amine.

Classification

Phenoxybenzamine is nonselective α-adrenergic antagonist.

Physiochemical Properties

Mechanism of Action

i. Phenoxybenzamine competitively blocks α-adrenergic receptors.

ii. It leads to relaxation of muscles and widening of blood vessels.

iii. Widening of blood vessels results in decrease in blood pressure.  [1]

Structure Activity Relationship

• Substitutions with haloalkylamine provides selectivity for binding with the α-adrenoceptors.
• Lacking substituents on both phenoxy and oxyamino carbon chain produces moderate α₁-adrenoceptor selectivity.
• 2-ethoxyphenoxy and a 2-i-propoxyphenoxy moiety bearing compounds will have highest potency as an α-adrenoceptor antagonist.

Method of synthesis

i. Phenol is reacted with propyleneoxide to form 1-phenoxy-2-propanol.

ii. Latter compound undergoes chlorination with thionyl chloride to give 1-phenoxy-2propylchloride

iii. Reacting with 2-aminomethanol will leads to the formation of 1-phenoxy -2-(2-hydroxyethyl)aminopropane.

iv. Alkylation of the secondary aminogroup gives N-(2-hydroxyethyl)-N-(1-methyl-2-phenoxyethyl)benzylamine.

v. Hydroxyl group of the later compound is chlorinated to give phenoxybenzmine.

Therapeutic Uses

Phenoxybenzamine is used for:

• Treatment of high blood pressure
• Heavy sweating due to tumor in adrenal gland
• Treatment of benign prostatic hyperplasia
• Treatment of Hypoplastic left heart syndrome
• Treatment of complex regional pain syndrome

 Side Effects

Side effects of Phenoxybenzamine are:

• Tired feeling
• Upset stomach
• Blurred vision
• Dizziness
• Stuffy nose

MCQs

Q.1 Match the following with correct SAR of the drug phenoxybenzamine

 a) i-B, ii-C, iii-A

b) i-B, ii-A, iii-C

c) i-C, ii-B, iii-A

d) i-C, ii-A, iii-B

Q.2 Correct sequence for the True/False for correct IUPAC names of the drug can be?

• Phenoxybenzamine: 3-[(1R,2S)-2-amino-1-hydroxypropyl]phenol
• Dobutamine: (RS)-N-Benzyl-N-(2-chloroethyl)-1-phenoxypropan-2-amine
• Naphazoline: (RS)-4-(2-{[4-(4-hydroxyphenyl)butan-2-yl]amino}ethyl)benzene-1,2-diol
• Metaraminol: 2-(naphthalen-1-ylmethyl)-4,5-dihydro-1H-imidazole

a) TFTF

b) FFTF

c) FFFF

d) TTFT

Q.3 Melting point of phenoxybenzamine is?

a) 129°C

b) 39°C

c) 250°C

d) 39K

Q.4 Complete the following sentence:

“Phenoxybenzamine competitively blocks………. Which results in ……… of muscles.”

a) α-adrenergic receptors, relaxation

b) ß-adrenergic receptors, contraction

c) α-adrenergic receptors, contraction

d) ß-adrenergic receptors, relaxation

Q.5 Which amongst the following is not a therapeutic use of drug Phenoxybenzamine?

a) Treatment of hypertension

b) Treatment of hypoplastic left heart syndrome

c) Treatment of complex regional pain syndrome

d) Prevention of cancers

Q.6 Which of the following drug and their classification are correct?

I. Isoproterenol: ß₂-adrenergic agonist

II. Dopamine: selective α₂-adrenergic antagonist

III. Amphetamine: Mixed acting sympathomimetics

IV. Doxazosin: ß₁-adrenergic agonist

a) I, II

b) I, III

c) II, IV

d) III, IV

Q.7 Correct molecular weight of phenoxybenzamine?

a) 250.24 gm/mol

b) 521.3 gm/mol

c) 303.8 gm/mol

d) 124.6 gm/mol

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ANSWERS

1-a

2-c

3-b

4-a

5-d

6-b

7-c

REFERENCES