Co-Trimoxazole (trimethoprim + sulfamethoxazole)

Brand/Trade Names: Bactrim, Septra, Septrin

Description: Co-trimoxazole (abbreviated SXT, TMP-SMX, or TMP-sulfa) is an antibiotic combination of trimethoprim and sulfamethoxazole, in the ratio of 1 to 5, used in the treatment of a variety of bacterial infections.
Co-trimoxazole exhibits a synergistic antibacterial effect when compared to each of its components administered singly. This is because trimethoprim and sulfamethoxazole inhibit successive steps in the folate synthesis pathway.

Tetrahydrofolate Synthesis Pathway

Sulfamethoxazole acts as a false-substrate inhibitor of dihydropteroate synthetase. Sulfonamides such as sulfamethoxazole are analogues of p-aminobenzoic acid (PABA) and are competitive inhibitors of the enzyme; inhibiting the production of dihydropteroic acid. Trimethoprim acts by interfering with the action of bacterial dihydrofolate reductase, inhibiting synthesis of tetrahydrofolic acid. Folic acid is an essential precursor in the de novo synthesis of the DNA nucleosides thymidine and uridine. Bacteria are unable to take up folic acid from the environment (i.e. the infection host) thus are dependent on their own de novo synthesis - inhibition of the enzyme starves the bacteria of two bases necessary for DNA replication and transcription.

Uses: Co-trimoxazole is more effective than either of its components individually in treating bacterial infections, though it is associated with a greater incidence of adverse effects including allergic responses. Its widespread use has been restricted in many countries to very specific circumstances where its improved efficacy is demonstrated. It may be effective in a variety of upper and lower respiratory tract infections, renal and urinary tract infections, gastrointestinal tract infections, skin and wound infections, septicaemias and other infections caused by sensitive organisms.

Specific indications for its use include:
- treatment and prophylaxis of pneumonia caused by Pneumocystis jirovecii (formerly
  identified as P. carinii) (Commmonly seen in immunocompromised patients including
  those suffering from HIV/AIDS)
- infections caused by Listeria monocytogenes, Nocardia spp.,
  Stenotrophomonas maltophilia (Zanthomonas maltophilia)
- melioidosis
- shigellosis
- traveller’s diarrhoea
- prophylaxis of cerebral toxoplasmosis in HIV patients
- Whipple’s disease


Additional Information Data And Resources:
Bactrim Prescribing Information
Septra - Product Monograph
Septra Prescribing Information