Methenamine Mandelate (methenamine mandelate 500 mg) Dailymed

Methenamine Mandelate Tablets, USP

Description

Methenamine mandelate, USP, a urinary antibacterial agent, is the chemical combination of mandelic acid with methenamine. Methenamine mandelate, USP is available for oral use as film-coated tablets.

Methenamine mandelate tablets, USP contain 500 mg methenamine mandelate and the following inactive ingredients: microcrystalline cellulose, croscarmellose sodium, silicon dioxide, magnesium stearate, FD&C Blue #2, polyethylene glycol, polyvinyl alcohol, talc, titanium dioxide.

Clinical Pharmacology

Methenamine mandelate, USP is readily absorbed but remains essentially inactive until it is excreted by the kidneys and concentrated in the urine. An acid urine is essential for antibacterial action, with maximum efficacy occurring at pH 5.5 or less. In an acid urine, mandelic acid exerts its antibacterial action and also contributes to the acidification of the urine. Mandelic acid is excreted both by glomerular filtration and tubular excretion. The methenamine component is hydrolyzed in acid urine to ammonia and to the bactericidal agent formaldehyde.

Proportionally less formaldehyde is released as urinary pH approaches 6.0 and insufficient quantities are released above this level for therapeutic response. There is equally effective antibacterial activity against both gram-positive and gram-negative organisms, since the antibacterial action of mandelic acid and formaldehyde is nonspecific. There are reports that methenamine mandelate, USP is ineffective in some infections with Proteus vulgaris and urea-splitting strains of Pseudomonas aeruginosa and A aerogenes. Since urea-splitting strains may raise the pH of the urine, particular attention to supplementary acidification is required. However, results in any single case will depend to a large extent on the underlying pathology and the overall management.

Indications And Usage

Methenamine mandelate, USP is indicated for the suppression or elimination of bacteriuria associated with pyelonephritis, cystitis, and other chronic urinary tract infections; also those neurologic diseases leading to an infected residual urine. When used as recommended, methenamine mandelate, USP is particularly suitable for long-term therapy because of its safety and because resistance to the nonspecific bactericidal action of formaldehyde does not develop. Pathogens resistant to other antibacterial agents may respond to methenamine mandelate, USP because of the nonspecific effect of formaldehyde formed in an acid urine.

To reduce the development of drug-resistant bacteria and maintain the effectiveness of methenamine mandelate, USP and other antibacterial drugs, methenamine mandelate, USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria.

Prophylactic Use Rationale: Urine is a good culture medium for many urinary pathogens. Inoculation by a few organisms (relapse or reinfection) may lead to bacteriuria in susceptible individuals. Thus, the rationale of management in recurring urinary tract infection (bacteriuria) is to change the urine from a growth-supporting to a growth-inhibiting medium. There is a growing body of evidence that long-term administration of methenamine mandelate, USP can prevent the recurrence of bacteriuria in patients with chronic pyelonephritis.

Therapeutic Use Rationale: Methenamine mandelate, USP helps to sterilize the urine, and in some situations in which underlying pathologic conditions prevent sterilization by any means, it can help to suppress the bacteriuria. Methenamine mandelate, USP should not be used alone for acute infections with parenchymal involvement causing systemic symptoms such as chills and fever. A thorough diagnostic investigation as a part of the overall management of the urinary tract infection should accompany the use of methenamine mandelate, USP.

Contraindications

Methenamine mandelate tablets, USP are contraindicated in patients with renal insufficiency, severe hepatic disease, severe dehydration, and in patients who have exhibited hypersensitivity to any components of this product.

Warnings

Methenamine mandelate, USP should be avoided in patients with gout because it may precipitate urate crystals in their urine. A similar situation may arise in patients with a predisposition to the formation of uric acid stones.

Methenamine preparations should not be given to patients taking sulfonamides because some sulfonamides may form an insoluble precipitate with formaldehyde in the urine.

Precautions

General

Dysuria may occur (usually at higher than recommended dosage). This can be controlled by reducing the dosage and the acidification. When urine acidification is contraindicated or unattainable (as with some urea-splitting bacteria), the drug is not recommended.

Large doses of methenamine (8 g daily for 3 to 4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria.

Information for Patients

To assure an acidic pH, patients should be instructed to restrict or avoid milk products and antacids containing sodium carbonate or bicarbonate.

Laboratory Tests

As with all urinary tract infections, the efficacy of therapy should be monitored by repeated urine cultures. Urinary pH monitoring is required to assure an acidic

urinary pH (below 5.5).

Drug Interactions

Formaldehyde and sulfamethizole form an insoluble precipitate in acid urine; therefore, methenamine mandelate, USP should not be administered concurrently with sulfamethizole or other sulfonamides. Concurrent use of salicylates may lead to increased serum salicylate levels since excretion of salicylates is reduced in acidified urine.

Drug/Laboratory Test Interactions

Formaldehyde interferes with fluorometric procedures for determination of urinary catecholamines and vanillylmandelic acid (VMA), causing erroneously high results. Formaldehyde also causes falsely decreased urine estriol levels by reacting with estriol when acid hydrolysis techniques are used; estriol determinations which use enzymatic hydrolysis are unaffected by formaldehyde.

Formaldehyde causes falsely elevated 17-hydroxycorticosteroid levels when the Porter-Silber method is used and falsely decreased 5-hydroxyindoleacetic acid (5HIAA) levels by inhibiting color development when nitrosonaphthol methods are used.

CARCINOGENESIS AND MUTAGENESIS

Methenamine was evaluated for mutagenicity in the Ames Salmonella/mammalian microsome test. Five strains of Salmonella typhimurium (TA98, TA100, TA1535, TA1537 and TA1538) and a strain of Escherichia coli (WP2uvrA) were used. At a dose of 10,000 ug/plate methenamine showed mutagenic activity in Salmonella typhimurium TA98 and TA100 by metabolic activation and also showed mutagenic activity in TA98 without microsomal activation. 

In one large study, no evidence of carcinogenicity was found following long-term oral administration of methenamine 1.25 g/kg/day to rats (104 weeks) and mice (60 weeks).

Pregnancy

Teratogenic Effects. Pregnancy Category C.

Animal reproduction studies have not been conducted with methenamine mandelate, USP. It is also not known whether methenamine mandelate, USP can cause fetal harm when administered to a pregnant woman or can affect reproduction capacity. Since methenamine is known to cross the placental barrier, methenamine mandelate, USP should be given to a pregnant woman only if the potential benefit outweighs the risk.

Nursing Mothers

Methenamine is excreted in breast milk. Because of the potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Adverse Reactions

Gastrointestinal disturbances (nausea, stomach upset), generalized skin rash, dysuria (painful or difficult urination) may occur occasionally with the use of methenamine preparations. Microscopic and rarely gross hematuria have been described.

Overdosage

Minimize absorption by inducing vomiting or by gastric lavage, followed by administration of activated charcoal. Administer orally fluids and alkalinize with sodium bicarbonate.

Dosage And Administration

The average adult dose is 4 g a day given as two 500 mg tablets after each meal and at bedtime. Children 6 to 12 years of age should receive half the adult dose; one tablet 4 times a day.

How Supplied

Methenamine Mandelate Tablets, USP 500 mg are blue, oval, film-coated, debossed with “M460”. Supplied in bottles of 100 (NDC 58657-460-01) 

Dispense in a tight, light-resistant container as defined in the USP.

Store at 25°C (77°F); excursions permitted between 15°-30°C (59°-86°F). [See USP Controlled Room Temperature.]

Rx only

Marketed by: Method Pharmaceuticals, LLC Fort Worth, TX 76118 1-877-250-3427

Rev. 06/18

Principal Display Panel

NDC 58657-460-01 Methenamine Mandelate Tablets USP URINARY ANTIBACTERIAL 500 mg Rx Only 100 Tablets