Trimethoprim

Trimethoprim is an antifolate antibacterial agent that inhibits bacterial dihydrofolate reductase (DHFR), a critical enzyme that catalyzes the formation of tetrahydrofolic acid (THF) - in doing so, it prevents the synthesis of bacterial DNA and ultimately continued bacterial survival. Trimethoprim is often used in combination with sulfamethoxazole due to their complementary and synergistic mechanisms but may be used as a monotherapy in the treatment and/or prophylaxis of urinary tract infections. It is structurally and chemically related to pyrimethamine, another antifolate antimicrobial used in the treatment of plasmodial infections.

As a monotherapy, trimethoprim is indicated for the treatment of acute episodes of uncomplicated urinary tract infections caused by susceptible bacteria, including E. coli., K. pneumoniae, Enterobacter spp., P. mirabilis, and coagulase-negative Staphylococcus species. In various formulations in combination with sulfamethoxazole, trimethoprim is indicated for the following infections caused by bacteria with documented susceptibility: urinary tract infections, acute otitis media in pediatric patients (when clinically indicated), acute exacerbations of chronic bronchitis in adults, enteritis caused by susceptible Shigella, prophylaxis and treatment of Pneumocystis jiroveci pneumonia, and travelers' diarrhea caused by enterotoxigenic E. coli.

Trimethoprim is available as an ophthalmic solution in combination with polymyxin B for the treatment of acute bacterial conjunctivitis, blepharitis, and blepharoconjunctivitis caused by susceptible bacteria.