Overview
DESCRIPTION Metyrosine is (–)-α-methyl-L-tyrosine or (α-MPT). It has the following structural formula: Metyrosine, USP is a white to off-white powder of molecular weight 195.22 g/mol and molecular formula C 10 H 13 NO 3 . It is very slightly soluble in methanol and insoluble in chloroform. Metyrosine, USP is supplied as capsules for oral administration.
Mechanism of Action
CLINICAL PHARMACOLOGY Metyrosine inhibits tyrosine hydroxylase, which catalyzes the first transformation in catecholamine biosynthesis, i.e. the conversion of tyrosine to dihydroxyphenylalanine (DOPA). Because the first step is also the rate-limiting step, blockade of tyrosine hydroxylase activity results in decreased endogenous levels of catecholamines, usually measured as decreased urinary excretion of catecholamines and their metabolites. In patients with pheochromocytoma, who produce excessive amounts of norepinephrine and epinephrine, administration of one gram to four grams of metyrosine per day has reduced catecholamine biosynthesis from about 35% to 80% as measured by the total excretion of catecholamines and their metabolites (metanephrine and vanillylmandelic acid).
Indications
- INDICATIONS AND USAGE Metyrosine capsules are indicated in the treatment of patients with pheochromocytoma for: 1. Preoperative preparation of patients for surgery. 2. Management of patients when surgery is contraindicated. 3. Chronic treatment of patients with malignant pheochromocytoma. Metyrosine capsules are not recommended for the control of essential hypertension.
Common Doses
- 250 mg
- 500 mg
- 4 g
- 2 g
Dosage
DOSAGE AND ADMINISTRATION The recommended initial dosage of metyrosine capsules for adults and children 12 years of age and older is 250 mg orally four times daily. This may be increased by 250 mg to 500 mg every day to a maximum of 4 g/day in divided doses. When used for preoperative preparation, the optimally effective dosage of metyrosine capsules should be given for at least five to seven days. Optimally effective dosages of metyrosine capsules usually are between 2 g/day and 3 g/day, and the dose should be titrated by monitoring clinical symptoms and catecholamine excretion.
Contraindications
- CONTRAINDICATIONS Metyrosine capsules are contraindicated in persons known to be hypersensitive to this compound.
Side Effects
- ADVERSE REACTIONS Central Nervous System Sedation The most common adverse reaction to metyrosine is moderate to severe sedation, which has been observed in almost all patients. It occurs at both low and high dosages. Sedative effects begin within the first 24 hours of therapy, are maximal after two to three days, and tend to wane during the next few days. Sedation usually is not obvious after one week unless the dosage is increased, but at dosages greater than 2000 mg/day some degree of sedation or fatigue may persist.
Interactions
- Drug Interactions Caution should be observed in administering metyrosine to patients receiving phenothiazines or haloperidol because the extrapyramidal effects of these drugs can be expected to be potentiated by inhibition of catecholamine synthesis. Concurrent use of metyrosine with alcohol or other CNS depressants can increase their sedative effects (see WARNINGS and PRECAUTIONS, Information for Patients ).