Trimethoprim

Trimethoprim Full Name

Trimethoprim

Trimethoprim Introduction

Trimethoprim is a synthetic antibiotic with broad-spectrum antibacterial activity, chemically classified as a diaminopyrimidine derivative. It is rarely used alone. Its most well-known application is in a fixed-dose combination (typically at a 1:5 ratio) with the sulfonamide antibiotic sulfamethoxazole (SMX). This combination strategy is based on strong pharmacological synergy, which not only significantly enhances antibacterial activity but also helps delay the development of bacterial resistance. From a pharmacokinetic perspective, trimethoprim is well and rapidly absorbed after oral administration, with a bioavailability exceeding 90%. Its lipophilic nature allows for wide distribution into body tissues and fluids, including effective penetration across the blood-brain barrier and the prostate barrier. It achieves therapeutic concentrations in cerebrospinal fluid, sputum, bile, and prostatic fluid. The drug undergoes limited metabolism primarily in the liver, and approximately 80% is excreted unchanged via the kidneys through glomerular filtration and tubular secretion. Therefore, dose adjustment is necessary for patients with renal impairment to avoid drug accumulation and potential toxicity.

Figure 1. Folate synthesis pathway and sites of action of trimethoprim and sulfamethoxazole. (Source: Masters PA, et al. 2003)

The core function of trimethoprim lies in its role as a potent and specific inhibitor of dihydrofolate reductase (DHFR). By mimicking the structure of dihydrofolate (DHF), trimethoprim binds with very high affinity to bacterial DHFR, competitively inhibiting the enzyme's activity and blocking the conversion of DHF to the functional tetrahydrofolate (THF). This ultimately prevents bacteria from synthesizing essential nucleic acids and proteins, inhibiting their growth and reproduction, thus exerting a potent bacteriostatic effect. Its affinity for bacterial DHFR is approximately 50,000 to 100,000 times greater than for mammalian (including human) DHFR. This vast difference in affinity means that at therapeutic concentrations effective against bacterial growth, trimethoprim has a negligible impact on the human body's own folate metabolism. When combined with sulfamethoxazole, the mechanism of synergy is perfectly illustrated: sulfamethoxazole inhibits an upstream step in the folate synthesis pathway (inhibiting dihydropteroate synthase), while trimethoprim inhibits a downstream step. This creates a "sequential dual blockade" of the same metabolic pathway. This combination often converts the effect from bacteriostatic to bactericidal and makes it more difficult for bacteria to develop resistance through a single mutation.

Clinically, the therapeutic spectrum of trimethoprim covers a variety of infectious diseases caused by susceptible strains. Its most classic and important indication is the treatment and prevention of urinary tract infections (UTIs), particularly acute uncomplicated cystitis caused by Escherichia coli. It also plays a significant role in treating community-acquired respiratory infections (e.g., acute exacerbations of chronic bronchitis) and gastrointestinal infections such as traveler's diarrhea caused by enterotoxigenic E. coli. One of the most indispensable roles of trimethoprim is in immunocompromised patient populations. Especially for patients with AIDS, it serves as a first-line agent for both the prevention and treatment of Pneumocystis jirovecii pneumonia (PJP), significantly reducing the morbidity and mortality associated with this opportunistic infection.

Alternate Names for Trimethoprim

Trimethoprim

• 1. Masters PA, et al. Trimethoprim-sulfamethoxazole revisited. Arch Intern Med. 2003 Feb 24;163(4):402-10.
  2. Cohen Tervaert JW. Trimethoprim-sulfamethoxazole and antineutrophil cytoplasmic antibodies-associated vasculitis. Curr Opin Rheumatol. 2018 Jul;30(4):388-394.