indication

Used as a diagnostic drug for testing hypothalamic-pituitary ACTH function. Occasionally used in Cushing's syndrome.

pharmacology

Metopirone is an inhibitor of endogenous adrenal corticosteroid synthesis.

mechanism of action

The pharmacological effect of Metopirone is to reduce cortisol and corticosterone production by inhibiting the 11-ß-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 is used as a diagnostic test, with urinary 17-OHCS measured as an index of pituitary ACTH responsiveness. Metopirone may also suppress biosynthesis of aldosterone, resulting in a mild natriuresis.

toxicity

Oral LD₅₀ in rats is 521 mg/kg. One case has been recorded in which a 6-year-old girl died after two doses of Metopirone, 2 g. Symptoms of overdose include cardiac arrhythmias, hypotension, dehydration, anxiety, confusion, weakness, impairment of consciousness, nausea, vomiting, epigastric pain, and diarrhea.

biotransformation

Hepatic. The major biotransformation is reduction of the ketone to metyrapol, an active alcohol metabolite. Metyrapone and metyrapol are both conjugated with glucuronide.

absorption

Absorbed rapidly and well when administered orally. Peak plasma concentrations are usually reached 1 hour after administration.

half life

1.9 ±0.7 hours.

route of elimination

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.

drug interactions

Cyproheptadine: This combination renders test invalid

Fosphenytoin: The combination renders the test invalid

Phenytoin: The combination renders the test invalid