Methenamine Hippurate

Drug Class: Other genito-urinary preparations

Indication

Urinary tract infection

Methenamine is indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary. This drug should only be used after eradication of the infection by other appropriate antimicrobial agents. To reduce the development of drug-resistant bacteria and maintain the effectiveness of Methenamine and other antibacterial drugs, Methenamine should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

Pharmacology

Methenamine Hippurate has antibacterial activity because the methenamine component is hydrolyzed to formaldehyde in acid urine. Hippuric acid, the other component, has some antibacterial activity and also acts to keep the urine acid. The drug is generally active against E. coli, enterococci and staphylococci. Enterobacter aerogenes is generally resistant. The urine must be kept sufficiently acid for urea-splitting organisms such as Proteus and Pseudomonas to be inhibited.

Pharmacology: Within 1/2 hour after ingestion of a single 1-gram dose of Methenamine, antibacterial activity is demonstrable in the urine. Urine has continuous antibacterial activity when HIPREX is administered at the recommended dosage schedule of 1 gram twice daily. Over 90% of methenamine moiety is excreted in the urine within 24 hours after administration of a single 1-gram dose. Similarly, the hippurate moiety is rapidly absorbed and excreted, and it reaches the urine by both tubular secretion and glomerular filtration. This action may be important in older patients or in those with some degree of renal impairment.

Dosage

For adults and pediatric patients over 12 years of age: 1 tablet twice daily (morning and night).
For pediatric patients 6 to 12 years of age: ½ to 1 tablet twice daily (morning and night).

Since the antibacterial activity of Methenamine is greater in acid urine, restriction of alkalinizing foods and medications is desirable. If necessary, as indicated by urinary pH and clinical response, supplemental acidification of the urine should be instituted. The efficacy of therapy should be monitored by repeated urine cultures.

Administration

Interaction

Contraindications

Methenamine Hippurate tablets USP is contraindicated in patients with renal insufficiency, severe hepatic insufficiency, or severe dehydration. Methenamine preparations should not be given to patients taking sulfonamides because some sulfonamides may form an insoluble precipitate with formaldehyde in the urine.

Side Effects

Pregnancy & Lactation

In early pregnancy the safe use of Methenamine is not established. In the last trimester, safety is suggested, but not definitely proved. No adverse effects on the fetus were seen in studies in pregnant rats and rabbits. Methenamine taken during pregnancy can interfere with laboratory tests of urine estriol (resulting in unmeasurably low values) when acid hydrolysis is used in the laboratory procedure. This interference is due to the presence in the urine of methenamine and/or formaldehyde. Enzymatic hydrolysis, in place of acid hydrolysis, will circumvent this problem.

Precautions & Warnings

Prescribing Methenamine in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.

Care should be taken to maintain an acid pH of the urine, especially when treating infections due to urea-splitting organisms such as Proteus and strains of Pseudomonas.

In a few instances in one study, the serum transaminase levels were slightly elevated during treatment but returned to normal while the patients were still taking Methenamine. Because of this report, it is recommended that liver function studies be performed periodically on patients taking the drug, especially those with liver dysfunction.

Use in Special Populations

Geriatric Use: Clinical studies of Methenamine did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience 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 end of the dosing range, reflecting the greater frequency of decreased hepatic, renal or cardiac function, and of concomitant disease or other drug therapy.

Renal insufficiency and severe hepatic insufficiency: Methenamine is contraindicated in patients with renal insufficiency and severe hepatic insufficiency.

Therapeutic Class

Other genito-urinary preparations

Overdose Effects

Duration of treatment

Reconstitution

Storage Conditions

Keep below 30°C temperature, away from light & moisture. Keep out of the reach of children.