Sulfamethoxazole

Absorption

Sulfamethoxazole is rapidly absorbed following oral administration and has a bioavailability of 85-90%. The Tmax is approximately 1-4 hours following oral administration, and the Cmax at steady-state is 57.4 - 68.0 μg/mL.

Toxicity

The oral LD50 of sulfamethoxazole in mice and rats is 2300 mg/kg and 6200 mg/kg, respectively.
Signs or symptoms of sulfonamide overdose include anorexia, colic, nausea, vomiting, dizziness, headache, drowsiness, and unconsciousness. Less common symptoms may include pyrexia, hematuria, and crystalluria. Later manifestations of overdose may include blood dyscrasias and jaundice. Treatment should be symptomatic and supportive, and may include gastric lavage or forced emesis if applicable. Monitor patient lab work for evidence of blood dyscrasias or electrolyte imbalances.

Description

Like sulfisoxazole, this drug is effective in treating infections caused by streptococci, gonococci, pneumococci, staphylococci as well as colon bacillus. Unlike sulfisoxazole, only about 70% of it binds with proteins in the plasma after oral administration, and it diffuses mostly to tissues and tissue fluids. However, since it is removed much slower than sulfisoxazole, it does not require frequent administration and is also the drug of choice for many systemic infections. Moreover, it is an ingredient of a combined drug named bactrim, biseptol, and so on (which will be examined later on), which has a fixed correlation with trimethoprim. Synonyms of this drug are gantanol, sinomin, sulfisomezole, and others.

The Uses of Sulfamethoxazole

An antibacterial drug. Sulfonamide antibiotic that blocks the synthesis of dihydrofolic acid by inhibiting the enzyme dihydropteroate synthase.

Background

Sulfamethoxazole is a bacteriostatic sulfonamide antibiotic that interferes with folic acid synthesis in susceptible bacteria. It is generally given in combination with trimethoprim, which inhibits a sequential step in bacterial folic acid synthesis - these agents work synergistically to block two consecutive steps in the biosynthesis of nucleic acids and proteins which are necessary for bacterial growth and division, and using them in conjunction helps to slow the development of bacterial resistance. In this combination, sulfamethoxazole is useful for the treatment of a variety of bacterial infections, including those of the urinary, respiratory, and gastrointestinal tracts.

Indications

Sulfamethoxazole is indicated in combination with trimethoprim, in various formulations, for the following infections caused by bacteria with documented susceptibility: urinary tract infections, acute otitis media in pediatric patients (when clinically indicated), acute exacerbations of chronic bronchitis in adults, enteritis caused by susceptible Shigella, prophylaxis and treatment of Pneumocystis jiroveci pneumonia, and travelers' diarrhea caused by enterotoxigenic E. coli.
In Canada, additional indications include the adjunctive treatment of cholera, treatment of bacillary dysentery, nocardiosis, and second-line treatment of brucellosis in combination with gentamicin or rifampicin.

What are the applications of Application

Sulfamethoxazole is an inhibitor of the enzyme dihydropteroate synthase

Pharmacokinetics

Sulfamethoxazole is a bacteriostatic sulfonamide antibiotic that inhibits a critical step in bacterial folate synthesis. It is generally given in combination with trimethoprim, a dihydrofolate reductase inhibitor, which inhibits the reduction of dihydrofolic acid to tetrahydrofolic acid. Studies have shown that bacterial resistance develops more slowly with the combination of the two drugs than with either trimethoprim or sulfamethoxazole alone, as together they inhibit sequential steps in the bacterial folate synthesis pathway.
Sulfonamides, including sulfamethoxazole, have been implicated in hypersensitivity reactions - these agents should be discontinued at the first sign of a developing rash, as this may signal the start of a more severe reaction such as Stevens-Johnson syndrome or toxic epidermal necrolysis. Sulfamethoxazole treatment may contribute to folate deficiency and should therefore be used with caution in patients at a higher risk of developing a deficiency. Hemolysis has been observed in patients with glucose-6-phosphate dehydrogenase deficiency who are using sulfamethoxazole/trimethoprim.

Metabolism

Sulfamethoxazole metabolism is mediated primarily by arylamine N-acetyltransferase (NAT) enzymes, which are responsible for acetylation of sulfamethoxazole at its N4 position. Sulfamethoxazole may also undergo oxidation at its C5 and N4 atoms, the latter of which is catalyzed by CYP2C9. Glucuronidation of the N4 atom, likely mediated by unspecified UGT enzymes, is an additional minor route of metabolism. None of the identified metabolites of sulfamethoxazole appear to carry antimicrobial activity.
The hydroxylamine metabolite of sulfamethoxazole, generated via oxidation by CYP2C9, may be further converted to a more reactive nitroso- metabolite.