Hiprex - Methenamine hippurate

Synonym. Methenamine Hippurate Proprietary Names. Hiprex Urex. 

Methenamine Hippurate, USP. Mcthenamine hippur-aie (Hiprex) is the hippuric acid. salt of mcthenamine. It is readily absorbed after oral administration and is concentrated in the urinary bladder, where it exerts its antibacterial activity. Its activity is increased in acid urine. 

Methenamine (hexamethylenetetramine) is an aromatic acid that is hydrolyzed at an acid pH (<6) to liberate ammonia and the active alkylating agent formaldehyde, which denatures protein and is bactericidal. Methenamine is usually administered as a salt of either man-delic (Mandelamine) or hippuric Hiprex, Urex) acid. Not only do these acids acidify the urine, which is necessary to generate formaldehyde, but also, the resulting low urine pH is by itself bacteriostatic for some organisms. 

Methenamine is absorbed orally, but 10 to 30% decomposes in the gastric juice unless the drug is protected by an enteric coating. Because of the ammonia produced, methenamine is contraindicated in hepatic insufficiency. Excretion in the urine is nearly quantitative. When the urine pH is 6 and the daily urine volume is 1000 to 1500 mL, a daily dose of 2 g will yield a concennation of 18 to 60 pg/mL of formaldehyde this is more than the MIC for most urinary tract pathogens. Various poorly metabolized acids can be nsed to acidify the urine. Low pH alone is bacteriostatic, so acidification serves a double function. The acids commonly used are mandelic acid and hippuric acid (Uerx, Hiprex). 

Methenamine Hippnrate. Hexamethylenetetramine hippurate Haiprex Hippramine Hiprex Urex [Riker], Urotractan Viapta. C,jH21Ns03 mol wt 319.37. C 56.41%, H 6.63%, N 21.93%, O 15.03%. C jN CjiysiOj. Prepd by refluxing one mole hexamethylenetetramine with one mole hippuric acid in methanol Galat, UJS. pat. 3,004,026 (1959). 

Methenamine (Hiprex) At low urinary pH, is hydrolyzed to formaldehyde which is bactericidal. Formaldehyde kills Gram-negatives, Gram-positives, and funai. Precipitates in presence of sulfonamides. Ammonia produced is somewhat toxic, particularly to the liver. Decrease dose if hepatotoxicity develODS. PO. Conversion to formaldehyde in urine is enhanced by low pH. The acid salts, hippurate or mandelate, help maintain acidic urine. Is not converted to formaldehvde in serum. 

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