Antibacterial folate antagonists
Blocking the pathway bacteria use to make folate, needed for nucleic acids.
Also called folate synthesis inhibitors.
Class card
- Drugs
- Sulfamethoxazole and Trimethoprim
- Used for
- Urinary tract infections
- Pneumocystis pneumonia: treatment and prevention
- Shigella enteritis and traveler's diarrhea
- Middle ear infections in children
- Known for
- Severe skin reactions: SJS, TEN, DRESS
- High potassium (hyperkalemia)
- Name stems
- sulfa-
3 cards: the class, and each drug in it.
Mechanism of action
How do antibacterial folate antagonists work?
Bacteria must build their own folate to make the nucleic acids and proteins they grow on. These drugs block that pathway at two consecutive steps: sulfamethoxazole stops dihydrofolic acid being made, and trimethoprim stops it being turned into tetrahydrofolic acid. Given together as co-trimoxazole they act in synergy, and resistance develops more slowly than to either alone.
The drugs
Antibacterial folate antagonists with a card
- SulfamethoxazoleAlmost always given with trimethoprim as co-trimoxazole (TMP-SMX), the combination studied for every listed use; the two drugs block consecutive steps of folate synthesis.
- TrimethoprimUsed on its own mainly for urinary infections; with sulfamethoxazole (co-trimoxazole) it also treats Pneumocystis pneumonia and gut infections.
Uses
What are antibacterial folate antagonists used for?
- Urinary tract infections
- Pneumocystis pneumonia: treatment and prevention
- Shigella enteritis and traveler's diarrhea
- Middle ear infections in children
Side effects
Side effects of antibacterial folate antagonists
- Severe skin reactions: SJS, TEN, DRESSKnown forSerious
- High potassium (hyperkalemia)Known forSerious
- Rash and itching
- Nausea and vomiting
- Low blood counts, including megaloblastic anemiaSerious
- Hemolysis in G6PD deficiencySerious
- Liver injurySerious
- Kidney injurySerious
- Low sodium (hyponatremia)Serious
- Possible birth defects after early pregnancy useSerious
What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.
Watch for
Cautions
- Pregnancy: avoided in the first 12 weeks where possible, because lowering folate may harm the developing baby.
- Check potassium in kidney disease and with ACE inhibitors or other drugs that raise it.
- They raise phenytoin levels, and co-trimoxazole also strengthens warfarin and raises free methotrexate.
- Avoid or take great care in folate deficiency, G6PD deficiency and porphyria.
Remember them
A way to remember them
Double lock on folate: SMX shuts step one, TMP shuts step two.
Sulfamethoxazole (SMX) blocks dihydropteroate synthase and trimethoprim (TMP) blocks dihydrofolate reductase, the next step in making folate. Two blocks in a row work in synergy and slow resistance, which is why they are given together as co-trimoxazole.
Written for biofideX
Name stems
sulfa- means sulfonamide anti-infective
Sulfamethoxazole and sulfadiazine share it. Sulfasalazine, the anti-inflammatory parent drug of mesalazine, carries it too.
- Every drug name stem
Don't mix them up
Classes easily confused with these
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Trimethoprim-Sulfamethoxazole (NCI Thesaurus)
- Sulfamethoxazole and trimethoprim tablets: label (DailyMed)
- Trimethoprim tablets: label (DailyMed)
- Trimethoprim (NHS)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Antibiotics: DNA, folate and tuberculosis
Practice antibacterial folate antagonists with the other classes in this group: what each drug is, how it works, what it treats and what it's known for.
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