Apresoline ® hydrochloride
hydralazine hydrochloride USP
Apresoline, hydralazine hydrochloride USP, is an antihypertensive, available as 10-, 25-, 50-, and 100-mg tablets for oral administration. Its chemical name 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, and has a molecular weight of 196.64.
Inactive Ingredients. Acacia, D&C Yellow No. 10 (10-mg tablets), FD&C Blue No. 1 (25-mg and 50-mg tablets), FD&C Yellow No. 5 and FD&C Yellow No. 6 (100-mg tablets), lactose, magnesium stearate, mannitol, polyethylene glycol, sodium starch glycolate, starch, and stearic acid.
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.
Hydralazine is rapidly absorbed after oral administration, and peak plasma levels are reached at 1-2 hours. Plasma levels of apparent hydralazine decline with a half-life of 3-7 hours. Binding to human plasma protein is 87% Plasma levels of hydralazine vary widely among individuals. Hydralazine is subject to polymorphic acetylation; slow acetylators generally have higher plasma levels of hydralazine and require lower doses to maintain control of blood pressure. Hydralazine undergoes extensive hepatic metabolism; it is excreted mainly in the form of metabolites in the urine.