Active ingredient: Methyclothiazide - Basic Profile / Key Facts

Basic Profile / Key Facts

Drug Category

• Diuretics, Thiazide

Dosage Forms

• Tablet for oral administration (5 mg)

Indications

For use in the management of hypertension either as the sole therapeutic agent or to enhance the effect of other antihypertensive drugs in the more severe forms of hypertension. Also used as adjunctive therapy in edema associated with congestive heart failure, hepatic cirrhosis, and corticosteroid and estrogen therapy.

Pharmacology

Methyclothiazide, a diuretic-antihypertensive agent, is a member of the benzothiadiazine (thiazide) class of drugs. Methyclothiazide has a per mg natriuretic activity approximately 100 times that of the prototype thiazide, chlorothiazide. At maximal therapeutic dosages, all thiazides are approximately equal in their diuretic/natriuretic effects. Like other benzothiadiazines, methyclothiazide also has antihypertensive properties, and may be used for this purpose either alone or to enhance the antihypertensive action of other drugs.

Mechanism of Action

Methyclothiazide appears to block the active reabsorption of chloride and possibly sodium in the ascending loop of Henle, altering electrolyte transfer in the proximal tubule. This results in excretion of sodium, chloride, and water and, hence, diuresis. As a diuretic, methyclothiazide inhibits active chloride reabsorption at the early distal tubule via the Na-Cl cotransporter, resulting in an increase in the excretion of sodium, chloride, and water. Thiazides like methyclothiazide also inhibit sodium ion transport across the renal tubular epithelium through binding to the thiazide sensitive sodium-chloride transporter. This results in an increase in potassium excretion via the sodium-potassium exchange mechanism. The antihypertensive mechanism of methyclothiazide is less well understood although it may be mediated through its action on carbonic anhydrases in the smooth muscle or through its action on the large-conductance calcium-activated potassium (KCa) channel, also found in the smooth muscle.

Absorption

Rapidly absorbed following oral administration.

Toxicity

Acute oral toxicity (LD₅₀): >4000 mg/kg [Rat]. Symptoms of overdosage include electrolyte imbalance and signs of potassium deficiency such as confusion, dizziness, muscular weakness, and gastrointestinal disturbances.

Biotrnasformation / Drug Metabolism

Not Available

Contraindications

Methyclothiazide is contraindicated in patients with anuria and in patients with a history of hypersensitivity to this compound or other sulfonamide-derived drugs.

Drug Interactions

Hypokalemia can sensitize or exaggerate the response of the heart to the toxic effects of digitalis (e.g., increased ventricular irritability).

Hypokalemia may develop during concomitant use of steroids or ACTH.

Insulin requirements in diabetic patients may be increased, decreased, or unchanged.

Thiazides may decrease arterial responsiveness to norepinephrine. This diminution is not sufficient to preclude effectiveness of the pressor agent for therapeutic use.

Thiazide drugs may increase the responsiveness of tubocurarine.

Lithium renal clearance is reduced by thiazides, increasing the risk of lithium toxicity.

Thiazides may add to or potentiate the action of other antihypertensive drugs. Potentiation occurs with ganglionic or peripheral adrenergic blocking drugs.