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Methyldopa and Hydrochlorothiazide (Methyldopa / Hydrochlorothiazide) - Description and Clinical Pharmacology

 
 



DESCRIPTION

This fixed combination product combines two antihypertensives: methyldopa and hydrochlorothiazide.

Methyldopa

Methyldopa is an antihypertensive and is the L-isomer of alpha-methyldopa. It is levo-3-(3,4-Dihydroxy-phenyl)-2-methylalanine sesquihydrate. Its molecular formula is C10H13NO4•1 ½ H2O with a molecular weight of 238.24, and its structural formula is:

Methyldopa is a white to yellowish white, odorless fine powder and is sparingly soluble in water.

Hydrochlorothiazide

Hydrochlorothiazide is a diuretic and antihypertensive. It is the 3,4-dihydro derivative of chlorothiazide. Its chemical name is 6-Chloro-3,4-dihydro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide. Its molecular formula is C7H8CIN3O4S2 and its structural formula is:

Hydrochlorothiazide is a white, or practically white, crystalline powder with a molecular weight of 297.75 which is slightly soluble in water, but freely soluble in sodium hydroxide solution.

Each tablet for oral administration contains:

250 mg methyldopa and 15 mg
hydrochlorothiazide,
or
250 mg methyldopa and 25 mg
hydrochlorothiazide.

Potency for methyldopa is calculated on anhydrous basis. Each tablet also contains the following inactive ingredients: colloidal silicon dioxide, croscarmellose sodium, D&C Yellow #10 Aluminum Lake, FD&C Blue #1 Aluminum Lake, FD&C Yellow #6 Aluminum Lake, hypromellose, magnesium stearate, microcrystalline cellulose, polydextrose, polyethylene glycol, sodium lauryl sulfate, titanium dioxide and triacetin.

CLINICAL PHARMACOLOGY

Methyldopa

Methyldopa is an aromatic-amino-acid decarboxylase inhibitor in animals and in man. Although the mechanism of action has yet to be conclusively demonstrated, the antihypertensive effect of methyldopa probably is due to its metabolism to alpha-methylnorepinephrine, which then lowers arterial pressure by stimulation of central inhibitory alpha-adrenergic receptors, false neurotransmission, and/or reduction of plasma renin activity. Methyldopa has been shown to cause a net reduction in the tissue concentration of serotonin, dopamine, norepinephrine, and epinephrine.

Only methyldopa, the L-isomer of alpha-methyldopa, has the ability to inhibit dopa decarboxylase and to deplete animal tissues of norepinephrine. In man, the antihypertensive activity appears to be due solely to the L-isomer. About twice the dose of the racemate (DL-alpha-methyldopa) is required for equal antihypertensive effect.

Methyldopa has no direct effect on cardiac function and usually does not reduce glomerular filtration rate, renal blood flow, or filtration fraction. Cardiac output usually is maintained without cardiac acceleration. In some patients the heart rate is slowed.

Normal or elevated plasma renin activity may decrease in the course of methyldopa therapy.

Methyldopa reduces both supine and standing blood pressure. It usually produces highly effective lowering of the supine pressure with infrequent symptomatic postural hypotension. Exercise hypotension and diurnal blood pressure variations rarely occur.

Hydrochlorothiazide

The mechanism of the antihypertensive effect of thiazides is unknown. Hydrochlorothiazide does not usually affect normal blood pressure.

Hydrochlorothiazide affects the distal renal tubular mechanism of electrolyte reabsorption. At maximal therapeutic dosage all thiazides are approximately equal in their diuretic efficacy.

Hydrochlorothiazide increases excretion of sodium and chloride in approximately equivalent amounts. Natriuresis may be accompanied by some loss of potassium and bicarbonate.

After oral use diuresis begins within 2 hours, peaks in about 4 hours and lasts about 6 to 12 hours.

Pharmacokinetics and Metabolism

Methyldopa

The maximum decrease in blood pressure occurs four to six hours after oral dosage. Once an effective dosage level is attained, a smooth blood pressure response occurs in most patients in 12 to 24 hours. After withdrawal, blood pressure usually returns to pretreatment levels within 24 to 48 hours.

Methyldopa is extensively metabolized. The known urinary metabolites are: α- methyldopa mono-O-sulfate; 3-O-methyl-α-methyldopa; 3,4-dihydroxyphenyl acetone; α-methyldopamine; 3-O-methyl-α–methyldopamine and their conjugates.

Approximately 70 percent of the drug which is absorbed is excreted in the urine as methyldopa and its mono-O-sulfate conjugate. The renal clearance is about 130 mL/min in normal subjects and is diminished in renal insufficiency. The plasma half-life of methyldopa is 105 minutes. After oral doses, excretion is essentially complete in 36 hours.

Methyldopa crosses the placental barrier, appears in cord blood, and appears in breast milk.

Hydrochlorothiazide

Hydrochlorothiazide is not metabolized but is eliminated rapidly by the kidney. When plasma levels have been followed for at least 24 hours, the plasma half-life has been observed to vary between 5.6 and 14.8 hours. At least 61 percent of the oral dose is eliminated unchanged within 24 hours. Hydrochlorothiazide crosses the placental but not the blood-brain barrier and is excreted in breast milk.

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