Microbiology: HIPREX (meth-enamine 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, entero-cocci and staphylococci. Enter-obacter 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 a single 1-gram dose of HIPREX, 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.

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