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Metopirone

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Metopirone

CLINICAL PHARMACOLOGY

Pharmacodynamics

The pharmacological effect of Metopirone (metyrapone) is to reduce cortisol and corticosterone production by inhibiting the 11-β-hydroxylation reaction in the adrenal cortex. Removal of the strong inhibitory feedback mechanism exerted by cortisol results in an increase in adrenocorticotropic hormone (ACTH) production by the pituitary. With continued blockade of the enzymatic steps leading to production of cortisol and corticosterone, there is a marked increase in adrenocortical secretion of their immediate precursors, 11-desoxycortisol and desoxycorticosterone, which are weak suppressors of ACTH release, and a corresponding elevation of these steroids in the plasma and of their metabolites in the urine. These metabolites are readily determined by measuring urinary 17-hydroxycorticosteroids (17-OHCS) or 17-ketogenic steroids (17-KGS).

Because of these actions, Metopirone (metyrapone) is used as a diagnostic test, with urinary 17-OHCS measured as an index of pituitary ACTH responsiveness. Metopirone (metyrapone) may also suppress biosynthesis of aldosterone, resulting in a mild natriuresis.

Pharmacokinetics

The response to Metopirone (metyrapone) does not occur immediately. Following oral administration, peak steroid excretion occurs during the subsequent 24-hour period.

Absorption

Metopirone (metyrapone) is absorbed rapidly and well when administered orally as prescribed. Peak plasma concentrations are usually reached 1 hour after administration. After administration of 750 mg, mean peak plasma concentrations are 3.7 µg/mL, falling to 0.5 µg/mL 4 hours after administration. Following a single 2000-mg dose, mean peak plasma concentrations of metyrapone in plasma are 7.3 µg/mL.

Metabolism

The major biotransformation is reduction of the ketone to metyrapol, an active alcohol metabolite. Eight hours after a single oral dose, the ratio of metyrapone to metyrapol in the plasma is 1:1.5. Metyrapone and metyrapol are both conjugated with glucuronide.

Excretion

Metyrapone is rapidly eliminated from the plasma. The mean ± SD terminal elimination half-life is 1.9 ± 0.7 hours. Metyrapol takes about twice as long as metyrapone to be eliminated from the plasma. After administration of 4.5 g metyrapone (750 mg every 4 hours), an average of 5.3% of the dose was excreted in the urine in the form of metyrapone (9.2% free and 90.8% as glucuronide) and 38.5% in the form of metyrapol (8.1% free and 91.9% as glucuronide) within 72 hours after the first dose was given.

Last reviewed on RxList: 12/14/2010
This monograph has been modified to include the generic and brand name in many instances.

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