- Trimethoprim is a diaminopyrimidine antibiotic with a molecular formula of C14H18N4O3 and a molecular mass of 290.32 g/mol. Its structure features a pyrimidine ring with two amino groups and three methoxy substituents, which are critical for its antibacterial activity. This structural configuration classifies it as a member of the diaminopyrimidine family and distinguishes it from other classes of antibiotics.
- The antibiotic exerts its bacteriostatic action by inhibiting bacterial folate synthesis. It specifically binds to the enzyme dihydrofolate reductase (DHFR), preventing the reduction of dihydrofolic acid to tetrahydrofolic acid. This inhibition disrupts the synthesis of purines, thymidine, and amino acids, ultimately halting bacterial growth. Trimethoprim is often combined with sulfamethoxazole, which inhibits an earlier step in the folate pathway (dihydropteroate synthase), to create a synergistic bactericidal effect.
- Trimethoprim demonstrates broad-spectrum activity against a wide range of gram-positive and gram-negative bacteria, including Escherichia coli, Klebsiella species, Proteus mirabilis, and Staphylococcus aureus. It is also effective against some opportunistic pathogens, such as Pneumocystis jirovecii, making it a key component of prophylaxis and treatment for Pneumocystis pneumonia (PCP) in immunocompromised patients. However, its use is limited by widespread bacterial resistance due to mutations in the DHFR enzyme.
- The pharmacokinetic profile of trimethoprim includes good oral bioavailability, rapid absorption, and widespread tissue distribution. It achieves therapeutic concentrations in the kidneys, lungs, and urinary tract, making it effective for treating urinary tract infections, respiratory infections, and gastrointestinal infections. Trimethoprim undergoes hepatic metabolism and is excreted primarily in the urine, with a half-life of 8 to 12 hours. When combined with sulfamethoxazole, it is typically administered twice daily.
- Despite its efficacy, trimethoprim is associated with potential adverse effects, including gastrointestinal disturbances (e.g., nausea, vomiting), hematologic effects (e.g., megaloblastic anemia, leukopenia), and hypersensitivity reactions (e.g., rash). Its use requires careful monitoring in patients with folate deficiency or renal impairment.
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