Vasodilators

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Chapter: Medicinal Chemistry : Antihypertensive Drugs

Antihypertensive Drugs : Vasodilators - Synthesis and Drug Profile - i. Hydralazine (Apresoline) ii. Minoxidil (Mintop, Coverit, Multigain) iii. Diazoxide (Hyperstat) iv. Sodium Nitroprusside (Nipride)


SYNTHESIS AND DRUG PROFILE

Vasodilators


i. Hydralazine (Apresoline)


Synthesis


Properties and uses: It is a white or almost white crystalline powder, which is slightly soluble in methylene chloride, but soluble in water and slightly soluble in alcohol. It is one of the few drugs that cause substantial vasodilation in the kidney and it increases renal plasma flow even when the blood pressure drops considerably. The side effects of tachycardia and palpitations may precipitate attacks of angina pectoris. It is used in the treatment of moderate to severe hypertension.

Assay: Dissolve the substance in water and add 35 ml of hydrochloric acid. Titrate with 0.05 M potassium iodate and determine the end point potentiometrically.

Storage: It should be stored in well-closed airtight containers and protected from light.

Dose: The usual dose is 25 to 100 mg oral twice daily; 20 to 40 mg I.V. in emergency.

Dosage forms: Hydralazine HCl injection I.P., Hydralazine injection B.P., Hydralazine tablets B.P.


ii. Minoxidil (Mintop, Coverit, Multigain)


Synthesis


Properties and uses: It is a white or almost white crystalline powder, which is slightly soluble in water, soluble in methanol and in propylene glycol. Minoxidil, which is a piperidino pyrimidine derivative directly relaxes arteriolar smooth muscle, and is indicated for patients with severe hypertension, who do not respond to other drugs. It is used as the last line of therapy to treat moderate to severe essential hypertension. It often is effective in hypertension refractory as compared to all other therapies. A side effect is excessive hair growth. Consequently, the drug is used topically to restore hair growth in androgenic alopecia and alopecia areata.

Assay: Dissolve in anhydrous acetic acid and titrate with 0.1 M perchloric acid. Determine the end point potentiometrically.

Storage: It should be stored in well-closed airtight containers and protected from light.

Dose: The usual dose is initially 5 mg; can be increased to 40 mg/day.

Dosage forms: Minoxidil scalp application B.P.


iii. Diazoxide (Hyperstat)


Synthesis


Properties and uses: It is a white or almost white and fine crystalline powder that is soluble in dilute solutions of alkali hydroxides, in dimethylformamide, slightly soluble in alcohol, but insoluble in water. A potent vasodilator, especially in the intravenous route, it can be used intravenously as a hypotensive drug in acute hypertensive crisis. Vasodilation is primarily the result of arteriolar dilation, so that orthostatic hypotension is usually minimal. It is also used as antihyperglycemic agent.

Assay: Dissolve it by gentle heating in a mixture of water and dimethylformamide. Titrate with 0.1 M sodium hydroxide and determine the end-point potentiometrically.

Dose: The usual dose is 1 mg/kg every 10 min

Dosage forms: Diazoxide Injection B.P., Diazoxide Tablets B.P.


iv. Sodium Nitroprusside (Nipride) 

Na2[Fe(CN)5NO]. 2H2O

Synthesis


Properties and uses: It is a reddish-brown powder or crystals, which are freely soluble in water, but slightly soluble in alcohol. A potent directly acting peripheral vasodilator, this drug has an immediate onset of effect and short duration of action, therefore, it is effective in treating hypertensive emergencies, but must be given by continuous intravenous infusion. Side effects of this drug include significant hypotension and cyanide or thiocyanate toxicity.

Assay: Dissolve the substance in water and add dilute sulphuric acid. Titrate with 0.1 M silver nitrate and determine the end point potentiometrically.

Storage: It should be stored in well-closed airtight containers and protected from light.

Dose: The usual dose is 1 mg/kg/min by I.V. infusion.

Dosage forms: Sodium nitro prusside intravenous infusion B.P.


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