Figure 716

Central (^-adrenergic agonists. a-Methyldopa is a methyl-substituted amino acid that is active only after decarboxylation and conversion to ( -methyl-norepinephrine. The antihypertensive effect results from accumulation of ( 2-adrenergic receptors, displacing and competing with endogenous catecholamines. Methyldopa is absorbed poorly (<50%); peak plasma concentrations occur in 2 to 4 hours. It is metabolized in the liver and excreted in the urine mainly as the inactive O-sulfate conjugate. The plasma half-life of methyldopa (1 to 2 hours) and its metabolites is prolonged in patients with renal insufficiency; dose reduction is required.

Clonidine, an imidazoline derivative, acts by stimulating either central ( 2-adrenergic receptors or imidazole receptors. Clonidine may be administered orally or by a transdermal delivery system (TTS). When given orally, it is absorbed well (>75%); peak plasma concentrations occur in 3 to 5 hours. Clonidine is metabolized mainly in the liver; fecal excretion ranges from 15% to 30%, and 40% to 60% is excreted unchanged in the urine. In patients with renal insufficiency, the plasma half-life (12 to 16 hours) may be extended to more than 40 hours; dose reduction is required. When clonidine is administered transdermally, therapeutic plasma levels are achieved within 2 to 3 days.

Guanabenz, a guanidine derivative, is highly selective for central (^-adrenergic receptors. It is absorbed well (>75%); peak plasma levels are reached in 2 to 5 hours. Guanabenz undergoes extensive hepatic metabolism; less than 2% is excreted unchanged in the urine. The plasma half-life (approximately 6 hours) is not prolonged in patients with renal insufficiency.

Guanfacine is a phenylacetyl-guanidine derivative with a longer plasma half-life than guanabenz. It is absorbed well (>90%); peak plasma concentrations are reached in 1 to 4 hours. The drug is primarily metabolized in the liver. Guanfacine and its metabolites are excreted primarily by the kidneys; 24% to 37% is excreted as unchanged drug in the urine. The plasma half-life (15 to 17 hours) is not prolonged in patients with renal insufficiency [6,9].

THE SIDE EFFECT PROFILE OF CENTRAL «¡»-ADRENERGIC AGONISTS

The side effect profile of central (^-adrenergic agonists. The side effect profile of these agents is diverse [6,9].

Side effects

Mechanisms

Sedation/drowsiness

Stimulation of a2-adrenergic receptors in

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