Metolazone is a quinazoline sulphonamide diuretic related to the thiazide class. The Food and Drug Administration (FDA) has approved metolazone to treat salt and water retention, causing edema accompanying congestive heart failure or chronic kidney disease. It is commonly used as an adjuvant therapy along with loop diuretics for the treatment of severe CHF to produce diuresis in patients refractory to loop diuretics monotherapy.[1] The synergy of these diuretics may help overcome the resistance that can develop with continued loop diuretic use. It can be used to treat edema associated with nephrotic syndrome, alone or in combination with spironolactone.[2]  Also, metolazone is indicated to treat hypertension alone or in combination with other antihypertensive drugs of a different class.[2] 

Metolazone works by inhibiting sodium transport across the epithelium of the renal tubules (mostly in the distal tubules), resulting in a decrease in sodium reabsorption and an increase in sodium, chloride, and water excretion. The shift in water and electrolytes results in hypovolemia, keeping the peripheral vascular resistance low and returning the cardiac output to normal. This production of diuresis resolves edema and contributes to the antihypertensive effect. While metolazone is similar to thiazide diuretics in its mechanism of action, it does differ in its use in patients with impaired renal function. Thiazide diuretics decrease glomerular filtration rate (GFR) and are therefore less effective in patients with renal impairment. Because metolazone works primarily in the distal convoluted tubule, rather than the proximal convoluted tubule, it has little effect on GFR and can be used in patients with a reduced GFR.[3] Moreover, dissimilar to loop diuretics, metolazone does not prompt renin secretion at the macula densa due to the stimulation of the renin-angiotensin-aldosterone system. Because of these actions, metolazone, even with no randomized trials, is recommended as decongestant therapy in the heart failure guidelines.

Metolazone is an oral agent, available in 2.5 mg, 5 mg, and 10 mg tablets. A dose of 5-20 mg once daily is recommended as the initial to treat edema associated with congestive heart failure or kidney disease. The dose should be titrated to the patient's response, then continued at the lowest dose needed to maintain diuresis. For the treatment of hypertension, 2.5-10 mg once daily is recommended initially with titration to the response.[4] 

The safety and efficacy of specific pediatric dosing have not been established. However, one study suggests 0.2 to 0.4 mg/kg/day by mouth every 12 to 24 hours.[5] No dose adjustment is required for renal or hepatic impairment or with concurrent use of hemodialysis. Metolazone is recommended to be taken in the morning due to its prolonged absorption and duration of action. In patients resistant to loop diuretics, metolazone should be given one hour before the regular morning dose of furosemide.

Hyperuricemia is a common adverse effect and is due to competitive inhibition of uric acid secretion and a decrease in extracellular fluid from diuretic effects. Other fluid and electrolyte imbalances, especially hyponatremia, hypokalemia, and hypomagnesemia may occur. Any electrolyte imbalances must be corrected before starting treatment.[6][7] Metolazone may impair glucose tolerance, causing hyperglycemia and glycosuria. Diabetic patients should have their blood and urine glucose levels monitored while on this therapy.[8] Diuretic therapy may be associated with increased serum cholesterol and triglycerides; however, long-term studies have found that after approximately one year of therapy, serum cholesterol levels return to normal or trend below the baseline. These temporary effects do not significantly increase the risk of coronary heart disease.[9] 

The patient may experience orthostatic hypotension, which can be worsened by other antihypertensive agents, alcohol, or narcotic medications. A weak association has been made between diuretic use and falls in the elderly, so caution should be used in patients 60 years and older.[10]  Because metolazone exerts its main effect in the distal tubule of the nephron, a change in renal function can be expected and lead to occasional instances of acute kidney injury.  The likelihood of these events has not been established, and metolazone may be the only option indicated with reduced renal function.[6] [7]  

Other rare adverse effects include agranulocytosis, photosensitization, aplastic anemia, Stevens-Johnson syndrome, and toxic epidermal necrolysis.[11]  

As metolazone is also excreted in breast-milk, the potential for adverse reactions in nursing infants exists. The risk of exposure to the infant should be weighed against the benefit of treatment for the mother. Other safer alternatives are available for diuretic therapy in pregnant individuals, including hydrochlorothiazide and chlorthalidone.[12] 

Therapy is contraindicated in those with known metolazone hypersensitivity. Although this medication is a thiazide-like diuretic, cross-reactivity is rarely seen between metolazone and other diuretics.[13] However, there is limited evidence of cross-reactivity between sulfonamide agents and metolazone; nevertheless, caution should be used in those with a sulfonamide allergy because metolazone possesses the chemical side chain.[14] Metolazone should not be used in patients experiencing a hepatic coma or hepatic encephalopathy because electrolyte disturbances can occur and exacerbate these disease processes.[15]

Anuric patients should have metolazone withheld. As it is excreted by the kidneys, renal dysfunction can cause a dangerous accumulation of the drug. Metalozone is safe to use in mild to moderate renal impairment, unlike other thiazide diuretics, but extreme caution should be used in those with severe renal impairment or anuria. Concurrent electrolyte imbalance is a contraindication to metolazone therapy, as these should be corrected prior to initiating metolazone.[6][7] Metolazone should be avoided in patients on lithium and digitalis as it can aggravate lithium and digitalis toxicity.