Cotrimoxazole (Co-Trimoxazole / TMP-SMX) - Therapeutic Status

1. What Is It?

• Trimethoprim (TMP) + Sulfamethoxazole (SMX) in a 1:5 ratio (by weight)
• Brand names: Bactrim, Septra
• Formulations: Single-strength (SS): TMP 80 mg + SMX 400 mg; Double-strength (DS): TMP 160 mg + SMX 800 mg; IV solution: 80 mg TMP + 400 mg SMX per 5 mL

In much of the world, the combination of TMP with SMX is known as cotrimoxazole. - Goodman & Gilman's Pharmacological Basis of Therapeutics, 14th Ed.

2. Mechanism of Action

• TMP is 100,000 times more selective for bacterial DHFR than mammalian DHFR, explaining its low toxicity in humans.
• Each drug alone is bacteriostatic; the combination is often bactericidal when the organism is sensitive to both.
• The formulation achieves a 20:1 plasma ratio of SMX:TMP (the optimal ratio for in vitro synergy).

3. Antimicrobial Spectrum

4. Therapeutic Uses (Clinical Applications)

A. Urinary Tract Infections (UTIs)

• TMP-SMX is effective for uncomplicated cystitis: DS tablet (160/800 mg) twice daily for 3 days
• For complicated UTI or pyelonephritis: 10-14 days
• Empiric use is NOT recommended if local E. coli resistance exceeds 20% or if the patient received TMP-SMX recently
• TMP concentrates in prostatic fluid (due to lipid solubility and the more acidic pH of prostatic secretions), making it effective for acute and chronic bacterial prostatitis
• Prophylaxis for recurrent UTIs: 1 SS tablet three times weekly

B. Pneumocystis jirovecii Pneumonia (PCP)

• Drug of choice for both treatment and prophylaxis
• Treatment: High-dose TMP 15-20 mg/kg/day + SMX 75-100 mg/kg/day IV or oral in 3-4 divided doses for 21 days; adjunctive corticosteroids for moderate-to-severe disease (PaO₂ <70 mmHg)
• Primary prophylaxis: CD4+ count <200 cells/μL in HIV patients - one DS tablet daily or three times weekly
• Secondary prophylaxis: Indefinitely in HIV until immune reconstitution
• IV route preferred for moderately severe to severe disease

C. Skin and Soft Tissue Infections

• Effective against MRSA skin and soft tissue infections (first-line alternative to doxycycline)
• Standard dose DS tablet twice daily; bone and joint infections due to S. aureus: 8-10 mg/kg/day of TMP component

D. Respiratory Tract Infections

• Acute exacerbations of chronic bronchitis (mild outpatient cases)
• Acute otitis media in children and acute maxillary sinusitis in adults (due to susceptible H. influenzae, S. pneumoniae)
• NOT recommended for streptococcal pharyngitis (does not eradicate organism)

E. Gastrointestinal Infections

• Shigellosis (alternative to fluoroquinolones; use only if susceptibility confirmed due to rising resistance)
• Typhoid / Salmonella (alternative therapy)
• Cyclospora cayetanensis infection - drug of choice
• Cystoisosporiasis (Isospora belli): 10-day course, followed by long-term suppression in immunosuppressed patients
• No longer routinely recommended for traveler's diarrhea due to widespread resistance

F. Nocardiosis

• TMP-SMX is the drug of choice for Nocardia asteroides infections (pulmonary, CNS, disseminated)
• High doses required, often combined with imipenem or amikacin for severe disease

G. Toxoplasmosis (Alternative therapy)

• Used as an alternative when pyrimethamine + sulfadiazine is not tolerated
• Also used for prophylaxis in HIV patients (same dose as PCP prophylaxis covers Toxoplasma as well)

H. Stenotrophomonas maltophilia

• Drug of choice for infections with this intrinsically multidrug-resistant organism
• High doses required (TMP 15-20 mg/kg/day IV)

I. Listeria infections

• Preferred alternative when patients cannot tolerate ampicillin (IV TMP-SMX)

J. Gram-negative Bacteremia

• Can treat bacteremia caused by susceptible Enterobacteriaceae; alternative for some MDR species (Enterobacter, Serratia)

K. Melioidosis (Burkholderia pseudomallei)

• TMP-SMX alone has been shown effective and better tolerated than TMP-SMX + doxycycline for eradication therapy

L. Pediatric Prophylaxis

• Standard prophylaxis agent in children with recurrent UTI, vesicoureteral reflux
• Children's dose: TMP 1-2 mg/kg/day (avoid in infants <2 months; risk of hyperbilirubinemia)
• Also used in pediatric HIV for OI prophylaxis (avoid <3 months)

5. Pharmacokinetics

• TMP concentrates in prostatic fluid, vaginal fluid (more acidic environments) and sputum - useful for site-specific infections
• Dose reduction required when CrCl is 15-30 mL/min (reduce by half); avoid if CrCl <15 mL/min

6. Bacterial Resistance

• Mechanisms:
  • Point mutations in DHFR (TMP resistance) or DHPS (SMX resistance)
  • Plasmid-encoded resistant DHFR (via transposons on conjugative plasmids - most common)
  • Reduced cell wall permeability
  • Overproduction of DHFR
• Resistance to the combination is lower than to either agent alone
• Growing resistance in E. coli (up to 30%+ in many regions) and S. pneumoniae limits empiric use

7. Adverse Effects

8. Drug Interactions

9. Contraindications

• Allergy to sulfonamides or trimethoprim
• Infants <2 months (risk of kernicterus - sulfonamides displace bilirubin from albumin)
• Pregnancy (especially first and third trimesters): folate antagonism risks neural tube defects; risk of neonatal hemolysis and hyperbilirubinemia at term
• Megaloblastic anemia due to folate deficiency
• Severe renal or hepatic impairment (relative contraindication)

10. Summary of Therapeutic Status

• Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th Ed., Chapter 57 (TMP-SMX section)
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Chapter 46
• Lippincott Illustrated Reviews: Pharmacology, Chapter 31