CLINICAL PHARMACOLOGY SECTION.

ACTIONS Microbiology: Methenamine hippurate tablets, USP 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.Human Pharmacology: Within 1/2 hour after ingestion of single 1-gram dose of methenamine hippurate, USP, antibacterial activity is demonstrable in the urine. Urine has continuous antibacterial activity when methenamine hippurate tablets, USP is administered at the recommended dosage schedule of gram twice daily. Over 90% of methenamine moiety is excreted in the urine within 24 hours after administration of 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.

DESCRIPTION SECTION.

DESCRIPTION. Methenamine hippurate tablets, USP are available as g oval shaped, scored and peach colored tablets. Chemically, methenamine hippurate is Hexamethylenetetramine monohippurate. The molecular formula of methenamine hippurate is C15H21N5O3 and molecular weight is 319.36. Its structural formula is:Each methenamine hippurate tablet, USP intended for oral administration contains g of methenamine hippurate. In addition, it also contains the following inactive ingredients: colloidal silicon dioxide, magnesium stearate, povidone K29/32, saccharin sodium and FD&C Yellow Aluminum Lake as color additive.Meets USP Dissolution Test 2.. Chemical Structure Methenamine Hippurate Tablets gm.

DOSAGE & ADMINISTRATION SECTION.

DOSAGE AND ADMINISTRATION. tablet (1 g) twice daily (morning and night) for adults and pediatric patients over 12 years of age. 1/2 to tablet (0.5 to g) twice daily (morning and night) for pediatric patients to 12 years of age. Since the antibacterial activity of methenamine hippurate tablets, USP 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.

INDICATIONS & USAGE SECTION.

INDICATIONS. Methenamine hippurate tablets, USP are 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 hippurate tablets, USP and other antibacterial drugs, methenamine hippurate tablets, USP 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.

PRECAUTIONS SECTION.

PRECAUTIONS. Prescribing methenamine hippurate tablets, USP in the absence of proven or strongly suspected bacterial infection or prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.1. 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.2. In 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 hippurate tablets, USP. Because of this report, it is recommended that liver function studies be performed periodically on patients taking the drug, especially those with liver dysfunction.3. Use in Pregnancy: In early pregnancy the safe use of methenamine hippurate tablets, USP 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 hippurate tablets, USP 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.. Geriatric Use. Clinical studies of methenamine hippurate tablets, USP 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.Methenamine hippurate tablets, USP are contraindicated in patients with renal insufficiency and severe hepatic insufficiency (see CONTRAINDICATIONS).. Information for Patients. Patients should be counseled that antibacterial drugs including methenamine hippurate tablets, USP should only be used to treat bacterial infections. They do not treat viral infections (e.g., the common cold). When methenamine hippurate tablets, USP are prescribed to treat bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by methenamine hippurate tablets, USP or other antibacterial drugs in the future.