Figure 729

Peripheral adrenergic neuronal blocking agents. Guanethidine is the prototype peripheral adrenergic neuronal blocking agent. Absorption is incomplete and variable; only 3% to 30% is absorbed over 12 hours. Peak plasma levels are reached in 6 hours. The drug rapidly leaves the plasma for extravascular storage sites, including sympathetic neurons. Guanethidine is eliminated with a plasma half-life of 4 to 8 days, a time course that corresponds with its antihypertensive effect. Approximately 24% of the drug is excreted unchanged in the urine; the remainder is metabolized by the liver into more polar, less active, metabolites that are excreted in the urine and feces. When therapy is initiated or the dosage is changed, three half-lives (approximately 15 days) are required to accumulate

87.5% of a steady-state level. By administering loading doses of guanethidine at 6-hour intervals (the nearly maximal effect from a single oral dose), blood pressure can be lowered in 1 to 3 days. In patients with severe renal insufficiency, drug excretion is decreased; dose reduction is required.

Guanadrel is a guanethidine derivative with a short therapeutic half-life. Absorption is greater than 85%; peak plasma concentrations are reached in 1 to 2 hours. Guanadrel is metabolized by the liver. Elimination occurs through the kidney; approximately 40% of the drug is excreted unchanged in the urine. In patients with renal insufficiency, the plasma half-life (10 to 12 hours) is prolonged; dose reduction is required [6,9].

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