Bactrim

**Trimethoprim/sulfamethoxazole** (**TMP/SMX**), also known as **co-trimoxazole** among other names, is an antibiotic used to treat a variety of bacterial infections. It consists of one part trimethoprim to five parts sulfamethoxazole. It is used for urinary tract infections, methicillin-resistant Staphylococcus aureus (MRSA) skin infections, travelers' diarrhea, respiratory tract infections, and cholera, among others. It may be used both to treat and prevent pneumocystis pneumonia and toxoplasmosis in people with HIV/AIDS. It can be given by mouth or intravenously.

Common side effects include nausea, vomiting, rash, and diarrhea. Severe allergic reactions and *Clostridium difficile* diarrhea may occasionally occur. Its use in pregnancy is not recommended. It appears to be safe for use during breastfeeding as long as the baby is healthy. TMP/SMX generally results in bacterial death. It works by blocking the making and use of folate by the microorganisms.

TMP/SMX was first sold in 1974. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. It is available as a generic medication. In the United States, it is about US$0.40 per dose. In 2016, it was the 106th most prescribed medication in the United States, with more than 6 million prescriptions.

## Medical uses

Co-trimoxazole was claimed to be more effective than either of its components individually in treating bacterial infections, although this was later disputed. Because it has a higher incidence of adverse effects, including allergic responses, its use has been restricted in many countries to very specific circumstances where its improved efficacy has been demonstrated.

It may be effective in a variety of upper and lower respiratory tract infections, kidney and urinary tract infections, gastrointestinal tract infections, skin and wound infections, sepsis, and other infections caused by sensitive organisms. Co-trimoxazole decreases the risk of recurrence of retinochoroiditis. The global problem of advancing antimicrobial resistance has led to a renewed interest in the use of co-trimoxazole more recently.

### Susceptibility

Organisms against which co-trimoxazole can be effective include:

* *Acinetobacter* spp.
* *Aeromonas hydrophila*
* *Alcaligenes*/*Achromobacter* spp.
* *Bartonella henselae*
* *Bordetella pertussis* (pertussis)
* *Brucella* spp.
* *Burkholderia cepacia*
* *Burkholderia mallei* (glanders)
* *Burkholderia pseudomallei* (melioidosis)
* *Chlamydia trachomatis* (chlamydia)
* *Chryseobacterium meningosepticum*
* *Citrobacter* spp.
* *Enterobacter* spp.
* *Escherichia coli*
* *Haemophilus influenzae*
* *Hafnia alvei*
* *Kingella* spp.
* *Klebsiella granulomatis*
* *Klebsiella pneumoniae*
* *Legionella* spp.
* *Listeria monocytogenes* (listeriosis)
* *Moraxella catarrhalis*
* *Morganella morganii*
* *Mycobacterium tuberculosis* (tuberculosis)
* *Neisseria gonorrhoeae* (gonorrhoea)
* *Neisseria meningitidis* (meningococcal disease)
* *Nocardia* spp.
* *Plesiomonas shigelloides*
* *Pneumocystis jirovecii*
* *Proteus mirabilis*
* *Proteus vulgaris*
* *Providencia rettgeri*
* *Providencia stuartii*
* *Salmonella typhi* (typhoid fever)
* Non-typhi *Salmonella*
* *Serratia spp.*
* *Shigella* spp.
* *Staphylococcus aureus*
* *Staphylococcus epidermidis*
* *Staphylococcus saprophyticus*
* *Stenotrophomonas maltophilia*
* *Streptococcus agalactiae*
* *Streptococcus faecalis*
* *Streptococcus pneumoniae*
* *Streptococcus pyogenes*
* *Streptococcus viridans*
* *Toxoplasma gondii* (toxoplasmosis)
* *Tropheryma whippelii* (Whipple's disease)
* *Vibrio cholerae* (cholera)
* *Yersinia enterocolitica*
* *Yersinia pestis* (bubonic plague)
* *Yersinia pseudotuberculosis*

The only notable nonsusceptible organisms are *Pseudomonas aeruginosa*, the mycoplasmae and *Francisella tularensis* (the causative organism of tularaemia).

### Pregnancy and breast feeding

Its use during pregnancy is contraindicated, although it has been placed in Australian pregnancy category C and American pregnancy category D. Its use during the first trimester (during organogenesis) and 12 weeks prior to pregnancy has been associated with an increased risk of congenital malformations, especially malformations associated with maternal folic acid deficiency (which is most likely related to the mechanism of action of co-trimoxazole) such as neural tube defects such as spina bifida, cardiovascular malformations (e.g. Ebstein's anomaly), urinary tract defects, oral clefts, and club foot in epidemiological studies. Its use later on during pregnancy also increases the risk of preterm labour (odds ratio: 1.51) and low birth weight (odds ratio: 1.67). Animal studies have yielded similarly discouraging results.

It appears to be safe for use during breastfeeding as long as the baby is healthy.

### Babies

Its use in those less than 2 months of age is not recommended due to the risk of adverse side effects.

## Adverse effects

See also: List of side effects of trimethoprim/sulfamethoxazole
### Contraindications

Contraindications include the following:

* Known hypersensitivity to trimethoprim, sulphonamides or any other ingredients in the formulations
* Pregnancy – especially in the period prior to birth
* Severe hepatic failure, marked liver parenchymal damage or jaundice.
* Serious haematological disorders and porphyria (due to the sulfonamide component of the preparation).
* Severe chronic kidney disease (CrCl <15 ml/min) where repeated measurements of the plasma concentration cannot be performed
* Co-trimoxazole should not be given to neonates during the first 6 weeks, except for the treatment/prophylaxis of pneumocytosis jiroveci (*P. carinii*) in infants of four weeks of age or greater.

### Interactions

Its use is advised against in patients being concomitantly treated with:

* ACE inhibitors like captopril, enalapril, lisinopril, perindopril, and ramipril due to the potential for additive hyperkalaemic effects
* Prilocaine — additive risk of methaemoglobinaemia
* Antiarrhythmics like amiodarone (increased risk of ventricular arrhythmias) and dofetilide (increased risk of QT interval prolongation)
* Antibacterials like dapsone (increases plasma levels of both drugs), methenamine (increased risk of crystalluria) and rifampicin (as it may lead to an increased plasma level of rifampicin and lower plasma levels of trimethoprim)
* Anticoagulants like warfarin and acenocoumarol — anticoagulant effects of either drug is potentiated by this combination
* Sulfonylureas — effects enhanced
* Phenytoin, half-life of phenytoin is increased
* Antifolates like pyrimethamine, proguanil and methotrexate increase the risk of associated side effects like bone marrow toxicity, folic acid supplementation should be considered. A significant risk of megaloblastic anaemia exists with doses of pyrimethamine in excess of 25 mg/wk.
* Antivirals, more specifically, lamivudine (increased plasma concentrations of lamivudine), zalcitabine (increased plasma concentrations of zalcitabine) and zidovudine (increased risk of haematological reactions)
* Procainamide and/or amantadine may have their plasma concentrations increased bilaterally or unilaterally.
* Clozapine and other antipsychotics — increased risk of haematological side effects
* Nucleoside analogue antineoplastics like azathioprine and mercaptopurine — increased risk of haematological toxicity
* Digoxin — increase in digoxin levels in a proportion of elderly patients
* Diuretics — elderly patients receiving thiazide diuretics are at a heightened risk for developing thrombocytopaenia while on co-trimoxazole
* Ciclosporin — patients who have received a kidney transplant and are receiving co-trimoxazole and ciclosporin concomitantly are at an increased risk of having a reversible deterioration in their kidney function.
* Spironolactone — concurrent use can increase the likelihood of hyperkalemia, especially in the elderly. The trimethoprim portion acts to prevent potassium excretion in the distal tubule of the nephron.
* Potassium aminobenzoate — effects of sulfonamides (like sulfamethoxazole) inhibited.
* Laboratory tests; trimethoprim and sulfonamides have been reported to interfere with diagnostic tests, including serum-methotrexate and serum-plasma creatinine levels, also urea, urinary glucose and urobilinogen tests.

### Overdose

Likely signs of toxicity include:

* Nausea
* Vomiting
* Dizziness
* Headache
* Mental depression
* Confusion
* Thrombocytopenia
* Uremia
* Bone marrow depression
* Loss of appetite
* Colic
* Drowsiness
* Unconsciousness

The recommended treatment for overdose includes:

* Administration of activated charcoal
* Stomach pumping
* General supportive measures
* Haemodialysis, which is moderately effective in clearing co-trimoxazole from the plasma.
* Calcium folinate treatment in cases of blood dyscrasias
* Forcing oral fluids

Alkalinisation of the urine may reduce the toxicity of sulfamethoxazole, but it may increase the toxic effects of trimethoprim.

## Pharmacology

The synergy between trimethoprim and sulfamethoxazole was first described in the late 1960s. Trimethoprim and sulfamethoxazole have a greater effect when given together than when given separately, because they inhibit successive steps in the folate synthesis pathway. They are given in a one-to-five ratio in their tablet formulations so that when they enter the body their concentration in the blood and tissues is roughly one-to-twenty — the exact ratio required for a peak synergistic effect between the two.

Sulfamethoxazole, a sulfonamide, induces its therapeutic effects by interfering with the *de novo* (that is, from within the cell) synthesis of folate inside microbial organisms such as protozoa, fungi and bacteria. It does this by competing with *p*-aminobenzoic acid (PABA) in the biosynthesis of dihydrofolate.

Trimethoprim serves as a competitive inhibitor of dihydrofolate reductase (DHFR), hence inhibiting the *de novo* synthesis of tetrahydrofolate, the biologically active form of folate.

Tetrahydrofolate is crucial in the synthesis of purines, thymidine, and methionine which are needed for the production of DNA and proteins during bacterial replication. Thus the net effect of each of these drugs is a bacteriostatic halt in replication. When combined, TMP and SMX are bactericidal.

The effects of trimethoprim causes a backlog of dihydrofolate (DHF) and this backlog can work against the inhibitory effect the drug has on tetrahydrofolate biosynthesis; this is where the sulfamethoxazole comes in, its role is in depleting the excess DHF by preventing it from being synthesised in the first place.

Pharmacokinetics of co-trimoxazole

## Society and culture

### Route

It can be given by mouth, as a tablet or suspension, or intravenously.

### Trade names

Trimethoprim/sulfamethoxazole may be abbreviated as SXT, TMP-SMX, TMP-SMZ, or TMP-sulfa.

Co-trimoxazole (BAN) is manufactured and sold by many different companies. The following list of brand names is incomplete:

* Bactrim, Bactrimel (manufactured by Roche and distributed in Europe)
* Bactrom (Venezuela)
* Bibactin (manufactured by PPM and distributed in Cambodia and some African countries)
* Biseptol
* Co-trimoxazole (Sandoz)
* Cotrim
* Deprim (AFT Pharmaceuticals)
* Diseptyl (Israel)
* Graprima Forte Kaplet (manufactured by PT Graha Farma and distributed in Indonesia)
* Infectrin, Bactrim (Brazil)
* Novo-Trimel
* Primotren (Lek in Slovenia and other countries)
* Polytrim
* Resprim
* Sanprima (manufactured by PT Sanbe Farma and distributed in Indonesia)
* Septra (Aspen Pharmacare and formerly GlaxoSmithKline)
* Septram (Panama)
* Septran (GlaxoSmithKline)
* Septrin (Spain)
* Sulfatrim
* Trisul
* Vactrim (manufactured and distributed in Laos)