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Hydralazine (Hydralazine Hydrochloride) - Description and Clinical Pharmacology



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Hydralazine Hydrochloride Injection, USP is an antihypertensive available in a 2 mL vial for intravenous and intramuscular administration.  Each mL of the sterile, nonpyrogenic colorless solution contains hydralazine hydrochloride USP, 20 mg; methylparaben NF, 0.65 mg; propylparaben NF, 0.35 mg; propylene glycol USP, 103.6 mg, and Water for Injection USP q.s.  The pH of the solution is 3.4 to 4.4. pH may be adjusted with hydrochloric acid and/or sodium hydroxide.  Hydralazine hydrochloride is 1-hydrazinophthalazine monohydrochloride, and its structural formula is:



Hydralazine hydrochloride USP is a white to off-white, odorless crystalline powder.  It is soluble in water, slightly soluble in alcohol, and very slightly soluble in ether.  It melts at about 275°C, with decomposition.


Although the precise mechanism of action of hydralazine is not fully understood, the major effects are on the cardiovascular system.  Hydralazine apparently lowers blood pressure by exerting a peripheral vasodilating effect through a direct relaxation of vascular smooth muscle.  Hydralazine, by altering cellular calcium metabolism, interferes with the calcium movements within the vascular smooth muscle that are responsible for initiating or maintaining the contractile state.

The peripheral vasodilating effect of hydralazine results in decreased arterial blood pressure (diastolic more than systolic); decreased peripheral vascular resistance; and an increased heart rate, stroke volume and cardiac output.  The preferential dilatation of arterioles, as compared to veins, minimizes postural hypotension and promotes the increase in cardiac output.  Hydralazine usually increases renin activity in plasma presumably as a result of increased secretion of renin by the renal juxtaglomerular cells in response to reflex sympathetic discharge.  This increase in renin activity leads to the production of angiotensin II, which then causes stimulation of aldosterone and consequent sodium reabsorption.  Hydralazine also maintains or increases renal and cerebral blood flow.

The average maximal decrease in blood pressure usually occurs 10-80 minutes after administration of hydralazine hydrochloride injection.  No other pharmacokinetic data on hydralazine hydrochloride injection are available.

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