| Indication | For the treatment of urinary tract infections, uncomplicated pyelonephritis (with sulfamethoxazole) and mild acute prostatitis. May be used as pericoital (with sulfamethoxazole) or continuous prophylaxis in females with recurrent cystitis. May be used as an alternative to treat asymptomatic bacteriuria during pregnancy (only before the last 6 weeks of pregnancy). Other uses include: alternative agent in respiratory tract infections (otitis, sinusitus, bronchitis and pneumonia), treatment of Pneumocystis jirovecii pneumonia (acute or prophylaxis), Nocardia infections, and traveller's diarrhea. |
| Pharmacodynamics | Trimethoprim is a pyrimidine analogue that disrupts folate synthesis, an essential part of the thymidine synthesis pathway. Inhibition of the enzyme starves the bacteria of nucleotides necessary for DNA replication.The drug, therefore, exhibits bactericidal activity. |
| Mechanism of action | Trimethoprim binds to dihydrofolate reductase and inhibits the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF). THF is an essential precursor in the thymidine synthesis pathway and interference with this pathway inhibits bacterial DNA synthesis. Trimethoprim's affinity for bacterial dihydrofolate reductase is several thousand times greater than its affinity for human dihydrofolate reductase. Sulfamethoxazole inhibits dihydrofolate synthetase (aka dihydropteroate synthetase), an enzyme involved further upstream in the same pathway. Trimethoprim and sulfamethoxazole are commonly used in combination due to their synergistic effects. This drug combination also reduces the development of resistance that is seen when either drug is used alone. |
| Absorption | Readily and almost completely absorbed in the GI tract with peak serum concentrations attained 1-4 hours after oral administration. Widely distributed to tissues and fluids including kidney, lung, seminal fluid, aqueous humour, middle ear fluid, sputum, vaginal secretions, bile, bone and CSF. |
| Volume of distribution | Not Available |
| Protein binding | 42-46% bound to plasma proteins |
| Metabolism | Hepatic metabolism to oxide and hydroxylated metabolites. |
| Route of elimination | Ten to twenty percent of trimethoprim is metabolized, primarily in the liver; the remainder is excreted unchanged in the urine. After oral administration, 50% to 60% of trimethoprim is excreted in the urine within 24 hours, approximately 80% of this being unmetabolized trimethoprim. Trimethoprim also passes the placental barrier and is excreted in human milk. |
| Half life | 8-11 hours in adults with normal renal function |
| Clearance | Not Available |
| Toxicity | LD50=4850 (orally in mice) |