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TABLETS
MIDAMOR®
(AMILORIDE HCl)
DESCRIPTION
Amiloride HCl, an antikaliuretic-diuretic agent, is a pyrazine-carbonyl-guanidine that is unrelated chemically to other known antikaliuretic or diuretic agents. It is the salt of a moderatelystrong base (pKa 8.7). It is designated chemically as 3,5-diamino-6-chloro-N-(diaminomethylene)pyrazinecarboxamide monohydrochloride, dihydrate and has a molecular weight of 302.12. Itsempirical formula is C6H8ClN7O•HCl•2H2O and its structural formula is: MIDAMOR* (Amiloride HCl) is available for oral use as tablets containing 5 mg of anhydrous amiloride HCl. Each tablet contains the following inactive ingredients: calcium phosphate, D&CYellow 10, iron oxide, lactose, magnesium stearate and starch.
CLINICAL PHARMACOLOGY
MIDAMOR is a potassium-conserving (antikaliuretic) drug that possesses weak (compared with thiazide diuretics) natriuretic, diuretic, and antihypertensive activity. These effects have beenpartially additive to the effects of thiazide diuretics in some clinical studies. When administeredwith a thiazide or loop diuretic, MIDAMOR has been shown to decrease the enhanced urinaryexcretion of magnesium which occurs when a thiazide or loop diuretic is used alone. MIDAMORhas potassium-conserving activity in patients receiving kaliuretic-diuretic agents.
MIDAMOR is not an aldosterone antagonist and its effects are seen even in the absence of MIDAMOR exerts its potassium sparing effect through the inhibition of sodium reabsorption at the distal convoluted tubule, cortical collecting tubule and collecting duct; this decreases the netnegative potential of the tubular lumen and reduces both potassium and hydrogen secretion andtheir subsequent excretion. This mechanism accounts in large part for the potassium sparingaction of amiloride.
MIDAMOR usually begins to act within 2 hours after an oral dose. Its effect on electrolyte excretion reaches a peak between 6 and 10 hours and lasts about 24 hours. Peak plasma levelsare obtained in 3 to 4 hours and the plasma half-life varies from 6 to 9 hours. Effects onelectrolytes increase with single doses of amiloride HCl up to approximately 15 mg.
Amiloride HCl is not metabolized by the liver but is excreted unchanged by the kidneys. About 50 percent of a 20 mg dose of MIDAMOR is excreted in the urine and 40 percent in the stoolwithin 72 hours. MIDAMOR has little effect on glomerular filtration rate or renal blood flow.
Because amiloride HCl is not metabolized by the liver, drug accumulation is not anticipated inpatients with hepatic dysfunction, but accumulation can occur if the hepatorenal syndromedevelops.
* Registered trademark of MERCK & CO., Inc.
COPYRIGHT MERCK & CO., Inc., 1985All rights reserved INDICATIONS AND USAGE
MIDAMOR is indicated as adjunctive treatment with thiazide diuretics or other kaliuretic- diuretic agents in congestive heart failure or hypertension to: a. help restore normal serum potassium levels in patients who develop hypokalemia on the b. prevent development of hypokalemia in patients who would be exposed to particular risk if hypokalemia were to develop, e.g., digitalized patients or patients with significant cardiacarrhythmias.
The use of potassium-conserving agents is often unnecessary in patients receiving diuretics for uncomplicated essential hypertension when such patients have a normal diet. MIDAMOR haslittle additive diuretic or antihypertensive effect when added to a thiazide diuretic.
MIDAMOR should rarely be used alone. It has weak (compared with thiazides) diuretic and antihypertensive effects. Used as single agents, potassium sparing diuretics, includingMIDAMOR, result in an increased risk of hyperkalemia (approximately 10% with amiloride).
MIDAMOR should be used alone only when persistent hypokalemia has been documented andonly with careful titration of the dose and close monitoring of serum electrolytes.
CONTRAINDICATIONS
MIDAMOR should not be used in the presence of elevated serum potassium levels (greater than 5.5 mEq per liter).
Antikaliuretic Therapy or Potassium Supplementation MIDAMOR should not be given to patients receiving other potassium-conserving agents, such as spironolactone or triamterene. Potassium supplementation in the form of medication,potassium-containing salt substitutes or a potassium-rich diet should not be used with MIDAMORexcept in severe and/or refractory cases of hypokalemia. Such concomitant therapy can beassociated with rapid increases in serum potassium levels. If potassium supplementation is used,careful monitoring of the serum potassium level is necessary.
Impaired Renal Function Anuria, acute or chronic renal insufficiency, and evidence of diabetic nephropathy are contraindications to the use of MIDAMOR. Patients with evidence of renal functional impairment(blood urea nitrogen [BUN] levels over 30 mg per 100 mL or serum creatinine levels over 1.5 mgper 100 mL) or diabetes mellitus should not receive the drug without careful, frequent andcontinuing monitoring of serum electrolytes, creatinine, and BUN levels. Potassium retentionassociated with the use of an antikaliuretic agent is accentuated in the presence of renalimpairment and may result in the rapid development of hyperkalemia.
Hypersensitivity MIDAMOR is contraindicated in patients who are hypersensitive to this product.
WARNINGS
Like other potassium-conserving agents, amiloride may cause hyperkalemia (serum potassium levels greater than 5.5 mEq per liter) which, if uncorrected, is potentially fatal. Hyperkalemiaoccurs commonly (about 10%) when amiloride is used without a kaliuretic diuretic. This incidenceis greater in patients with renal impairment, diabetes mellitus (with or without recognized renalinsufficiency), and in the elderly. When MIDAMOR is used concomitantly with a thiazide diuretic inpatients without these complications, the risk of hyperkalemia is reduced to about 1-2 percent. It isthus essential to monitor serum potassium levels carefully in any patient receiving amiloride,particularly when it is first introduced, at the time of diuretic dosage adjustments, and during anyillness that could affect renal function.
The risk of hyperkalemia may be increased when potassium-conserving agents, including MIDAMOR, are administered concomitantly with an angiotensin-converting enzyme inhibitor, anangiotensin II receptor antagonist, cyclosporine or tacrolimus. (See PRECAUTIONS, DrugInteractions.) Warning signs or symptoms of hyperkalemia include paresthesias, muscularweakness, fatigue, flaccid paralysis of the extremities, bradycardia, shock, and ECGabnormalities. Monitoring of the serum potassium level is essential because mild hyperkalemia isnot usually associated with an abnormal ECG.
When abnormal, the ECG in hyperkalemia is characterized primarily by tall, peaked T waves or elevations from previous tracings. There may also be lowering of the R wave and increased depthof the S wave, widening and even disappearance of the P wave, progressive widening of the QRScomplex, prolongation of the PR interval, and ST depression.
Treatment of hyperkalemia: If hyperkalemia occurs in patients taking MIDAMOR, the drug should be discontinued immediately. If the serum potassium level exceeds 6.5 mEq per liter,active measures should be taken to reduce it. Such measures include the intravenousadministration of sodium bicarbonate solution or oral or parenteral glucose with a rapid-actinginsulin preparation. If needed, a cation exchange resin such as sodium polystyrene sulfonate maybe given orally or by enema. Patients with persistent hyperkalemia may require dialysis.
Diabetes Mellitus In diabetic patients, hyperkalemia has been reported with the use of all potassium-conserving diuretics, including MIDAMOR, even in patients without evidence of diabetic nephropathy.
Therefore, MIDAMOR should be avoided, if possible, in diabetic patients and, if it is used, serumelectrolytes and renal function must be monitored frequently.
MIDAMOR should be discontinued at least three days before glucose tolerance testing.
Antikaliuretic therapy should be instituted only with caution in severely ill patients in whom respiratory or metabolic acidosis may occur, such as patients with cardiopulmonary disease orpoorly controlled diabetes. If MIDAMOR is given to these patients, frequent monitoring of acid-base balance is necessary. Shifts in acid-base balance alter the ratio of extracellular/intracellularpotassium, and the development of acidosis may be associated with rapid increases in serumpotassium levels.
PRECAUTIONS
GeneralElectrolyte Imbalance and BUN Increases Hyponatremia and hypochloremia may occur when MIDAMOR is used with other diuretics and increases in BUN levels have been reported. These increases usually have accompaniedvigorous fluid elimination, especially when diuretic therapy was used in seriously ill patients, suchas those who had hepatic cirrhosis with ascites and metabolic alkalosis, or those with resistantedema. Therefore, when MIDAMOR is given with other diuretics to such patients, carefulmonitoring of serum electrolytes and BUN levels is important. In patients with pre-existing severeliver disease, hepatic encephalopathy, manifested by tremors, confusion, and coma, andincreased jaundice, have been reported in association with diuretics, including amiloride HCl.
Drug Interactions When amiloride HCl is administered concomitantly with an angiotensin-converting enzyme inhibitor, an angiotensin II receptor antagonist, cyclosporine or tacrolimus, the risk ofhyperkalemia may be increased. Therefore, if concomitant use of these agents is indicatedbecause of demonstrated hypokalemia, they should be used with caution and with frequentmonitoring of serum potassium. (See WARNINGS.) Lithium generally should not be given with diuretics because they reduce its renal clearance and add a high risk of lithium toxicity. Read circulars for lithium preparations before use of suchconcomitant therapy.
In some patients, the administration of a non-steroidal anti-inflammatory agent can reduce the diuretic, natriuretic, and antihypertensive effects of loop, potassium-sparing and thiazide diuretics.
Therefore, when MIDAMOR and non-steroidal anti-inflammatory agents are used concomitantly, the patient should be observed closely to determine if the desired effect of the diuretic is obtained.
Since indomethacin and potassium-sparing diuretics, including MIDAMOR, may each beassociated with increased serum potassium levels, the potential effects on potassium kinetics andrenal function should be considered when these agents are administered concurrently.
Carcinogenicity, Mutagenicity, Impairment of Fertility There was no evidence of a tumorigenic effect when amiloride HCl was administered for 92 weeks to mice at doses up to 10 mg/kg/day (25 times the maximum daily human dose). AmilorideHCl has also been administered for 104 weeks to male and female rats at doses up to 6 and8 mg/kg/day (15 and 20 times the maximum daily dose for humans, respectively) and showed noevidence of carcinogenicity.
Amiloride HCl was devoid of mutagenic activity in various strains of Salmonella typhimurium with or without a mammalian liver microsomal activation system (Ames test).
Pregnancy Pregnancy Category B. Teratogenicity studies with amiloride HCl in rabbits and mice given 20 and 25 times the maximum human dose, respectively, revealed no evidence of harm to the fetus,although studies showed that the drug crossed the placenta in modest amounts. Reproductionstudies in rats at 20 times the expected maximum daily dose for humans showed no evidence ofimpaired fertility. At approximately 5 or more times the expected maximum daily dose for humans,some toxicity was seen in adult rats and rabbits and a decrease in rat pup growth and survivaloccurred.
There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, this drug should beused during pregnancy only if clearly needed.
Nursing Mothers Studies in rats have shown that amiloride is excreted in milk in concentrations higher than those found in blood, but it is not known whether MIDAMOR is excreted in human milk. Becausemany drugs are excreted in human milk and because of the potential for serious adversereactions in nursing infants from MIDAMOR, a decision should be made whether to discontinuenursing or to discontinue the drug, taking into account the importance of the drug to the mother.
Pediatric Use Safety and effectiveness in pediatric patients have not been established.
Clinical studies of MIDAMOR did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinicalexperience has not identified differences in responses between the elderly and younger patients.
In general, dose selection for an elderly patient should be cautious, usually starting at the low endof the dosing range, reflecting the greater frequency of decreased hepatic, renal or cardiacfunction, and of concomitant disease or other drug therapy.
This drug is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function. Because elderly patients aremore likely to have decreased renal function, care should be taken in dose selection, and it maybe useful to monitor renal function. (See CONTRAINDICATIONS, Impaired Renal Function.) ADVERSE REACTIONS
MIDAMOR is usually well tolerated and, except for hyperkalemia (serum potassium levels greater than 5.5 mEq per liter — see WARNINGS), significant adverse effects have been reportedinfrequently. Minor adverse reactions were reported relatively frequently (about 20%) but therelationship of many of the reports to amiloride HCl is uncertain and the overall frequency wassimilar in hydrochlorothiazide treated groups. Nausea/anorexia, abdominal pain, flatulence, andmild skin rash have been reported and probably are related to amiloride. Other adverseexperiences that have been reported with amiloride are generally those known to be associatedwith diuresis, or with the underlying disease being treated.
The adverse reactions for MIDAMOR listed in the following table have been arranged into two groups: (1) incidence greater than one percent; and (2) incidence one percent or less. The incidence for group (1) was determined from clinical studies conducted in the United States (837patients treated with MIDAMOR). The adverse effects listed in group (2) include reports from thesame clinical studies and voluntary reports since marketing. The probability of a causalrelationship exists between MIDAMOR and these adverse reactions, some of which have beenreported only rarely.
Angina pectorisOrthostatic hypotensionArrhythmiaPalpitation potassium levels(>5.5 mEq per liter)*** Skin rashItchingDryness of mouthPruritusAlopecia NervousnessMental confusionInsomniaDecreased libidoDepressionSomnolence Visual disturbancesNasal congestionTinnitusIncreased intraocular pressure PolyuriaDysuriaUrinary frequencyBladder spasmsGynecomastia ** Reactions occurring in 3% to 8% of patients treated with MIDAMOR. (Those reactions occurring in less than 3% of the patients are Other reactions have been reported but occurred under circumstances where a causal relationship could not be established. However, in these rarely reported events, that possibilitycannot be excluded. Therefore, these observations are listed to serve as alerting information tophysicians.
Activation of probable pre-existing peptic ulcerAplastic anemiaNeutropeniaAbnormal liver function OVERDOSAGE
No data are available in regard to overdosage in humans.
The oral LD50 of amiloride hydrochloride (calculated as the base) is 56 mg/kg in mice and 36 to 85 mg/kg in rats, depending on the strain.
It is not known whether the drug is dialyzable.
The most likely signs and symptoms to be expected with overdosage are dehydration and electrolyte imbalance. These can be treated by established procedures. Therapy with MIDAMORshould be discontinued and the patient observed closely. There is no specific antidote. Emesisshould be induced or gastric lavage performed. Treatment is symptomatic and supportive. Ifhyperkalemia occurs, active measures should be taken to reduce the serum potassium levels.
DOSAGE AND ADMINISTRATION
MIDAMOR should be administered with food.
MIDAMOR, one 5 mg tablet daily, should be added to the usual antihypertensive or diuretic dosage of a kaliuretic diuretic. The dosage may be increased to 10 mg per day, if necessary.
More than two 5 mg tablets of MIDAMOR daily usually are not needed, and there is little controlledexperience with such doses. If persistent hypokalemia is documented with 10 mg, the dose canbe increased to 15 mg, then 20 mg, with careful monitoring of electrolytes.
In treating patients with congestive heart failure after an initial diuresis has been achieved, potassium loss may also decrease and the need for MIDAMOR should be re-evaluated. Dosageadjustment may be necessary. Maintenance therapy may be on an intermittent basis.
If it is necessary to use MIDAMOR alone (see INDICATIONS), the starting dosage should be one 5 mg tablet daily. This dosage may be increased to 10 mg per day, if necessary. More thantwo 5 mg tablets usually are not needed, and there is little controlled experience with such doses.
If persistent hypokalemia is documented with 10 mg, the dose can be increased to 15 mg, then20 mg, with careful monitoring of electrolytes.
HOW SUPPLIED
No. 3381 — Tablets MIDAMOR, 5 mg, are yellow, diamond-shaped, compressed tablets, coded MSD 92 on one side and MIDAMOR on the other. They are supplied as follows: NDC 0006-0092-68 bottles of 100.
Protect from moisture, freezing and excessive heat.

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