Mechanism of Action and Pharmacological Effect
The enzyme dihydrofolate reductase plays a critical role in microbial metabolism by reducing dihydrofolate (DHF) to tetrahydrofolate (THF). This process is essential for the subsequent synthesis of purines and pyrimidines—the core building blocks of DNA—as well as for the methylation of dUMP to dTMP.
Drugs in this class, such as trimethoprim and pyrimethamine, act as folic acid analogues, producing competitive inhibition of DHFR. The selectivity of these drugs relies on genetic and structural differences between human and pathogen enzyme isoforms.
Characteristics of Individual Agents
- Trimethoprim exhibits pronounced bacteriostatic activity. Used as monotherapy for uncomplicated urinary tract infections (UTIs) because it is excreted unchanged by the kidneys. However, it is most commonly used in combination with sulfonamides.
- Pyrimethamine selectively inhibits protozoal DHFR. In clinical practice, it is combined with sulfadiazine to treat toxoplasmosis (remaining a primary chemotherapeutic option), and was historically used for malaria.
Synergy with Sulfonamides and Cotrimoxazole
Combining DHFR inhibitors with sulfonamides (e.g., in cotrimoxazole) transforms two bacteriostatic agents into a potent bactericidal regimen.
The basis of this synergy lies in the sequential blockade of metabolic pathways:
- Sulfamethoxazole decreases the intracellular concentration of dihydrofolic acid.
- The drop in substrate concentration enhances the efficacy of trimethoprim by reducing competition for enzyme binding.
This combination helps overcome microbial resistance. Even if a bacterium produces an altered DHFR with low drug affinity, this mutant enzyme also exhibits reduced affinity for the natural substrate. The sulfonamide depletes dihydrofolic acid reserves to a critical level, rendering the mutated enzyme ineffective.
Pharmacokinetics and Safety of Cotrimoxazole
The success of the sulfamethoxazole + trimethoprim combination is due to the matched elimination rates of both components (a half-life of approximately 12 hours).
- Absorption and Distribution: The drug is well absorbed in the gastrointestinal tract and distributes widely into tissues and fluids, achieving high concentrations in bronchial secretions, bile, urine, the prostate, and brain tissue (crosses the blood-brain barrier).
- Indications: Used for respiratory tract infections (including Pneumocystis jirovecii pneumonia), ENT infections, genitourinary infections, brucellosis, malaria, and toxoplasmosis.
- Precautions: Contraindicated during pregnancy, severe hepatic and renal impairment, and blood dyscrasias.