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Pharmacokinetics

Absorption Sulfamethoxazole is well-absorbed when administered topically. It is rapidly absorbed when it is orally administered. (1) drugbank

Distribution Sulfamethoxazole distributes into most body tissues as well as into sputum, vaginal fluid, and middle ear fluid. (toxnet, UPSI) It also crosses the placenta. About 70% of the drug is bound to plasma proteins. Its Tmax occurs 1 to 4 hours after oral administration. The mean serum half-life of sulfamethoxazole is 10 hours. (USPI) However, the half-life of the drug noticeably increases in patients with creatinine clearance rates equal to or less than 30 mL/minute. A half-life of 22-50 hours has been reported for patients with creatinine clearances of less than 10 mL/minute. (toxnet)

Metabolism Sulfamethoxazole is metabolized in the human liver to at least 5 metabolites. These metabolites are the N4-acetyl-, N4-hydroxy-, 5-methylhydroxy-, N4-acetyl-5-methylhydroxy-sulfamethoxazole metabolites, and an N-glucuronide conjugate. The CYP2C9 enzyme is responsible for the formation of the N4-hydroxy metabolite. In vitro studies suggest, sulfamethoxazole is not a substrate of the P-glycoprotein transporter. (USPI)

Excretion Sulfamethoxazole is primarily renally excreted via glomerular filtration and tubular secretion. (4, USPI) About 20% of the sulfamethoxazole in urine is the unchanged drug, about 15-20% is the N-glucuronide conjugate, and about 50-70 % is the acetylated metabolite. (toxnet) Sulfamethoxazole is also excreted in human milk. (USPI)