Mechanism of action
The drug blocks folic acid metabolism in microorganisms. Sulfamethoxazole competitively inhibits the synthesis of dihydrofolic acid, while trimethoprim blocks the enzyme dihydrofolate reductase, preventing the conversion of dihydrofolate into the active coenzyme form — tetrahydrofolate. The dual blockade of the metabolic pathway ensures synergism and significantly slows down the development of bacterial resistance.
Pharmacological effects
Unlike monotherapy with sulfonamides, the combination has a bactericidal effect. The drug is well absorbed in the gastrointestinal tract and distributed in tissues, creating high concentrations in bronchial secretions, bile, urine, and prostate tissue. It crosses the blood-brain barrier, which is important for CNS infections.
Indications
Co-trimoxazole is effective for infections caused by Haemophilus influenzae and is also the drug of choice in the treatment of Pneumocystis pneumonia (Pneumocystis carinii). It is used as an alternative regimen in toxoplasmosis.
Side effects and contraindications
The main risk is associated with antifolate activity: prolonged use can cause macrocytic hyperchromic anemia due to folic acid deficiency. The drug is contraindicated during pregnancy due to the risk of teratogenic effects.
Prescription details
The components are selected taking into account an identical elimination rate (about 12 hours). When treating anemias caused by folate deficiency, it must be remembered that in pernicious anemia, folic acid is prescribed strictly in combination with vitamin B12.
Briefly: answers to topical questions
Which enzyme is competitively inhibited by sulfamethoxazole?
Sulfamethoxazole, as a representative of sulfonamides, acts at the earliest stage of folic acid synthesis. Its target is the enzyme dihydropteroate synthase. The drug causes competitive antagonism with para-aminobenzoic acid, which leads to the blockade of dihydropteroate formation.
What microorganisms are included in the antibacterial spectrum of co-trimoxazole?
Co-trimoxazole has a wide spectrum of antibacterial activity and is effective against gram-positive and gram-negative bacteria, both aerobic and anaerobic. Haemophilus influenzae, Neisseria meningitidis, pneumococci, and certain strains of Bacteroides are the most sensitive.
Co-trimoxazole: "Double strike" — blocking two enzymes so the bacterium doesn't have time to mutate.
Class and exam questions
Why is co-trimoxazole more effective than individual sulfonamides?
The combination blocks two different enzymes in the folate synthesis chain. Bacterial survival would require simultaneous mutation in two loci, which is highly unlikely.
What type of anemia can develop when taking antifolate drugs?
Taking folic acid antagonists (including trimethoprim) can lead to the development of macrocytic hyperchromic anemia.
Can folic acid be prescribed alone in pernicious anemia?
No, this is unacceptable. Folic acid in pernicious anemia is prescribed only together with vitamin B12, otherwise neurological disorders will not be alleviated.
What else to study on this topic
- Comparative characteristics of short- and long-acting sulfonamides.
- Mechanisms of resistance development to antifolate drugs.
- Pharmacokinetic differences between sulfamethoxazole and sulfadoxine.
- The role of folic acid in erythrocyte metabolism.
- Features of toxoplasmosis therapy in immunocompromised patients.
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