Methenamine hippurate
Methenamine hippurate (Hip-Rex )
Methenamine
(meh- THEEN-ah-meen)
Pregnancy Category: C Hip-Rex Hiprex Urex (Rx)
Methenamine mandelate
Methenamine mandelate (Mandelamine)
Methenamine
(meh- THEEN-ah-meen)
Pregnancy Category: C Mandelamine (Rx)

Classification: Urinary tract anti-infective

Action/Kinetics: Converted in an acid medium into ammonia and formaldehyde (the active principle), which denatures protein. Formaldehyde levels in the urine may be bacteriostatic or bactericidal, depending on the pH; it is most effective when the urine has a pH value of 5.5 or less, which is maintained by using the hippurate or mandelate salt. Readily absorbed from GI tract, but up to 60% may be hydrolyzed by gastric acid if tablets are not enteric-coated. To be effective, urinary formaldehyde concentration must be greater than 25 mcg/mL. Peak levels of formaldehyde: 2 hr if using hippurate and 3-8 hr if using mandelate (if urinary pH is 5.5 or less) t 1/2: 3-6 hr. Seventy to 90% of drug and metabolites excreted in urine within 24 hr.

Uses: Acute, chronic, and recurrent UTIs by susceptible organisms, especially gram-negative organisms including Escherichia coli. As a prophylactic before urinary tract instrumentation. Never used as sole agent in the treatment of acute infections.

Contraindications: Renal insufficiency, severe liver damage, severe dehydration. Concurrent use of sulfonamides as an insoluble precipitate may form with formaldehyde.

Special Concerns: Use with caution in gout (methenamine may cause urate crystals to precipitate in the urine).

Side Effects: GI: N&V, diarrhea, anorexia, cramps, stomatitis. GU: Hematuria, albuminuria, crystalluria, dysuria, urinary frequency or urgency, bladder irritation. Dermatologic: Skin rashes, urticaria, pruritus, erythematous eruptions. Other: Headache, dyspnea, edema, lipoid pneumonitis.

Laboratory Test Alterations: False + urinary glucose with Benedict's solution. Drug interferes with determination of urinary catecholamines and estriol levels by acid hydrolysis technique (enzymatic techniques not affected). False + catecholamines, hydroxycorticosteroids, vanillylmandelic acid; false - 5-hydroxyindoleacetic acid.

Overdose Management: Treatment: Absorption following overdose may be minimized by inducing vomiting or by gastric lavage, followed by activated charcoal. Fluids should be forced.

Drug Interactions: Acetazolamide / Methenamine effect R/T inhibition of conversion to formaldehyde Sodium bicarbonate / Methenamine effect R/T inhibition of conversion to formaldehyde Sulfonamides / Chance of sulfonamide crystalluria R/T acid urine produced by methenamine Thiazide diuretics / Methenamine effect R/T urine alkalinity produced by thiazides

How Supplied: Methenanamine hippurate: Tablet: 1 g Methenamine mandelate: Tablet: 0.5 g, 1 g; Suspension: 0.5 g/5 mL

Dosage
?Tablets
Hippurate: Adults and children over 12 years: 1 g b.i.d. in the morning and evening; children, 6-12 years: 0.5 g b.i.d.
?Oral Suspension, Enteric-Coated Tablets
Mandelate: Adults: 1 g q.i.d. after meals and at bedtime; children 6-12 years: 0.5 g q.i.d.; children under 6 years: 0.25 g/13.6 kg q.i.d.