Sulfamethoxazole

Sulfamethoxazole is an antibiotic. The substance comes from the group of sulfonamides. Sulfamethoxazole inhibits the folic acid synthesis of bacteria and thus has a bacteriostatic effect. It is used in a fixed combination with trimethoprim under the name cotrimoxazole.

What is sulfamethoxazole?

Sulfamethoxazole is a substance that belongs to the group of sulfonamides. It is used as an antibiotic and is approved in fixed combination with trimethoprim for the treatment of bacterial urinary tract and respiratory infections.

This fixed combination of the two drugs is in a ratio of 5: 1 and is known under the name of co-trimoxazole. Special indications are infections with Pneumocystis jirovecii, nocardia asteroides, Stenotrophomonas maltophilia.

Pharmacological effect

Like all substances from the group of sulfonamides, sulfamethoxazole is a synthetically manufactured substance. The effect of sulfonamides is based on the fact that they prevent bacteria from producing folic acid. Folic acid is important for the production of nucleotides. As a result, the bacteria cannot multiply, since it is not possible to copy their genetic material without folic acid. Sulphonamides have a bacteriostatic effect because they do not kill the bacteria but prevent them from multiplying.

Sulfamethoxazole in particular is a competitive antagonist of dihydropteroate synthase. Aminobenzoic acid (PABA) is a natural substrate for this enzyme. The enzymatic reaction, which is inhibited by the sulfamethoxazole, is an important step in the synthesis of bacterial folic acid. This cannot be synthesized, which means that the DNA of the bacteria cannot be copied, as folic acid is required to replicate the DNA.

This inhibition is irrelevant for humans, as they do not synthesize folic acid themselves, but ingest it through food. The plasma half-life of sulfamethoxazole is about nine to eleven hours.

Medical application & use

In a fixed combination with trimethoprim, sulfamethoxazole is used under the name cotrimoxazole. Co-trimoxazole is indicated for infections of the upper and lower respiratory tract (excluding streptococcal angina), infections of the kidneys and the lower urinary tract, infections of the male and female genital organs and infections of the gastrointestinal tract.

It is also approved for the treatment of brucellosis, nocardiosis, non-genuinely mycotic mycetoma and South American blastomycosis. Accordingly, in addition to its antibacterial effect, cotrimoxazole is also effective against some fungi.

Treatment with co-trimoxazole is also possible as a therapeutic attempt in the initial stage of granulomatosis with polyangiitis. The mechanism of action for this indication is not known. As already mentioned, another special indication is Pneumocystis jirovecii pneumonia.

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Risks & side effects

The safety profile of the fixed combination of sulfamethoxazole with trimethoprim is considered to be good, and its use is therefore harmless. The side effects attributable to sulfamethoxazole are mainly skin problems, especially eczema.

In addition to undesirable effects on the skin, the sulfamethoxazole can also cause leukopenia, anemia, thrombocytopenia, ataxia, convulsions, psychoses, depression, nausea, vomiting, loss of appetite and diarrhea.

In addition to the undesirable effects caused by the sulfamethoxazole, side effects that can be traced back to the trimethoprim may occur. These include gastrointestinal problems, skin reactions, aseptic meningitis, an increase in transaminases, bilirubin, creatine and urea, slight changes in the blood count and fever. With prolonged use the sperm formation can be disturbed.

In combination with trimethoprim, sulfamethoxazole leads to a prolongation of the QT time. Co-trimoxazole must therefore not be used at the same time as other drugs that prolong the QT interval and should not be used in long QT syndrome.

Furthermore, cotrimoxazole must not be used in the case of known hypersensitivity to one of the two active ingredients, in erythema exudativum multiforme, in existing blood count changes, in glucose-6-phosphate dehydrogenase deficiency, in severe renal insufficiency, in severe liver damage, in porphyria and in premature infants and newborns with hyperbilirubinemia .

In milder kidney and liver disorders, as well as thyroid disorders and in newborns under 5 weeks of age, cotrimoxazole should only be used with special care and control.

Sulphamethoxazole increases the anticoagulant effect of 4-hydroxycoumarins and the blood sugar-lowering effect of sulphonylureas. In the case of co-trimoxazole, it should be noted that trimethoprim enhances the effect of phenytoin, cardiac glycosides and procainamide. In addition, the plasma concentration of methotrexate and the hormones in the birth control pill can be influenced.