Metolazone 2.5 mg, Spironolactone 50 mg Tablets

Spironolactone and its active metabolites are specific pharmacologic antagonists of aldosterone, acting primarily through competitive binding of receptors at the aldosterone-dependent sodium-potassium exchange site in the distal convoluted renal tubule. Spironolactone causes increased amounts of sodium and water to be excreted, while potassium is retained. Spironolactone acts both as a diuretic and as an antihypertensive drug by this mechanism. It may be given alone or with other diuretic agents that act more proximally in the renal tubule.

Absorption — The mean time to reach peak plasma concentration of spironolactone and the active metabolite, canrenone, in healthy volunteers is 2.6 and 4.3 hours, respectively.

Effect of food :Food increased the bioavailability of spironolactone (as measured by AUC) by approximately 95.4%. Patients should establish a routine pattern for taking spironolactone with regard to meals [see Dosage and Administration ( 2.1)] .

Distribution — Spironolactone and its metabolites are more than 90% bound to plasma proteins.

Elimination — The mean half-life of spironolactone is 1.4 hour. The mean half-life values of its metabolites including canrenone, 7-α-(thiomethyl) spirolactone (TMS), and 6-ß-hydroxy-7-α-(thiomethyl) spirolactone (HTMS) are 16.5, 13.8, and 15 hours, respectively.

Metabolism:Spironolactone is rapidly and extensively metabolized. Metabolites can be divided into two main categories: those in which sulfur of the parent molecule is removed (e.g., canrenone) and those in which the sulfur is retained (e.g., TMS and HTMS). In humans, the potencies of TMS and 7-α-thiospirolactone in reversing the effects of the synthetic mineralocorticoid, fludrocortisone, on urinary electrolyte composition were approximately a third relative to spironolactone. However, since the serum concentrations of these steroids were not determined, their incomplete absorption and/or first-pass metabolism could not be ruled out as a reason for their reduced in vivoactivities.

Excretion:The metabolites are excreted primarily in the urine and secondarily in bile.

Specific Populations — The impact of age, sex, race/ethnicity, and renal impairment on the pharmacokinetics of spironolactone have not been specifically studied.

Patients with Hepatic Impairment:The terminal half-life of spironolactone has been reported to be increased in patients with cirrhotic ascites [see Use in Specific Populations ( 8.7)] .

Drug Interaction Studies: — Drugs and Supplements Increasing Serum Potassium:Concomitant administration of spironolactone with potassium supplementation, salt substitutes containing potassium, a diet rich in potassium, or drugs that can increase potassium, including ACE inhibitors, angiotensin II antagonists, non-steroidal anti-inflammatory drugs (NSAIDs), heparin and low molecular weight heparin, may lead to severe hyperkalemia [see Warnings and Precautions ( 5.1) and Drug Interactions ( 7.1)] .

Lithium:Spironolactone reduces the renal clearance of lithium, inducing a high risk of lithium toxicity [see Drug Interactions ( 7.2)].

Nonsteroidal Anti-Inflammatory Drugs (NSAIDs):In some patients, the administration of an NSAID can reduce the diuretic, natriuretic, and antihypertensive effect of loop, potassium-sparing, and thiazide diuretics [see Drug Interactions ( 7.3)] .

Acetylsalicylic acid:A single dose of 600 mg of acetylsalicylic acid inhibited the natriuretic effect of spironolactone, which was hypothesized be due to inhibition of tubular secretion of canrenone, causing decreased effectiveness of spironolactone [see Drug Interactions ( 7.6)] .

Mitotane: The effect of concomitant spironolactone on the pharmacokinetics of mitotane has not been studied; however, patients exhibited significantly lower mitotane levels compared to those who did not receive concomitant spironolactone despite receiving higher mitotane doses [see Drug Interactions ( 7.8) ].

Metolazone is a quinazoline diuretic, with properties generally similar to the thiazide diuretics. The actions of metolazone result from interference with the renal tubular mechanism of electrolyte reabsorption. Metolazone acts primarily to inhibit sodium reabsorption at the cortical diluting site and to a lesser extent in the proximal convoluted tubule. Sodium and chloride ions are excreted in approximately equivalent amounts. The increased delivery of sodium to the distal tubular exchange site results in increased potassium excretion. Metolazone does not inhibit carbonic anhydrase. A proximal action of metolazone has been shown in humans by increased excretion of phosphate and magnesium ions and by a markedly increased fractional excretion of sodium in patients with severely compromised glomerular filtration. This action has been demonstrated in animals by micropuncture studies.

When metolazone tablets are given, diuresis and saluresis usually begin within one hour and may persist for 24 hours or more. For most patients, the duration of effect can be varied by adjusting the daily dose. High doses may prolong the effect. A single daily dose is recommended. When a desired therapeutic effect has been obtained, it may be possible to reduce dosage to a lower maintenance level.

The diuretic potency of metolazone at maximum therapeutic dosage is approximately equal to thiazide diuretics. However, unlike thiazides, metolazone may produce diuresis in patients with glomerular filtration rates below 20 mL/min.

Metolazone and furosemide administered concurrently have produced marked diuresis in some patients where edema or ascites was refractory to treatment with maximum recommended doses of these or other diuretics administered alone. The mechanism of this interaction is unknown (see WARNINGSand PRECAUTIONS: Drug Interactions).

Maximum blood levels of metolazone are found approximately eight hours after dosing. A small fraction of metolazone is metabolized. Most of the drug is excreted in the unconverted form in the urine.