Figure 732

Direct-acting vasodilators. Hydralazine is the prototype of direct-acting vasodilators. Absorption is more than 90%. Peak plasma levels occur within 1 hour but may vary widely among individuals. This is because hydralazine is subject to polymorphic acetylation; slow acetylators have higher plasma levels and require lower drug doses to maintain blood pressure control compared with rapid acetylators. Bioavailability for slow acetylators ranges from 30% to 35%; bioavailability for rapid acetylators ranges from 10% to 16%. Hydralazine undergoes extensive hepatic metabolism; it is mainly excreted in the urine in the form of metabolites or as unchanged drug. The plasma half-life is 3 to 7 hours. Dose reduction may be required in the slow acetylator with renal insufficiency.

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