Epistemis

Sulfonamides and Cotrimoxazole

  • Synthetic Inhibitors of Metabolic Pathways

Cotrimoxazole is the synergistic combination of sulfamethoxazole and trimethoprim. Its sequential action effectively blocks the synthesis of folic acid in bacteria, offering broad coverage against nosocomial and opportunistic pathogens.

Mechanism

💊 Generic and Commercial Names

Cotrimoxazole / Sulfamethoxazole + Trimethoprim (Septrin, Soltrim, Bactrim).

🔬 Pharmacological Group

Sequential folic acid antagonists (Sulfonamides and Diaminopyrimidines).

Mechanism of Action

Its antibacterial action is based on a coordinated double enzymatic blockade:

  • Synergistic Effect of Double Sequential Blocking:
    1. Step 1: Inhibition of Dihydropteroate Synthase (DHPS): Sulfamethoxazole is a competitive structural analogue of para-aminobenzoic acid (PABA). It binds and irreversibly inhibits the bacterial enzyme DHPS, blocking the conversion step of PABA to dihydrofolic acid.
    2. Step 2: Inhibition of Dihydrofolate Reductase (DHFR): Trimethoprim is a highly selective and competitive inhibitor of the bacterial enzyme dihydrofolate reductase (DHFR), which has a binding affinity 100,000 times higher than that of the enzyme from animal cells. It blocks the conversion of dihydrofolate to its active metabolic cofactor, tetrahydrofolic acid, necessary for the de novo synthesis of purines and pyrimidines.

Pharmacokinetics

Key Pharmacokinetics

Parameter Quantitative Pharmacokinetic Profile (Fixed 5:1 ratio in tablets)
Routes of AdministrationOral and IV (in 60-90 minute infusion dissolved in abundant glucose serum).
Dural absorptionExtraordinary oral bioavailability (~90-100%) for both components on an empty stomach, with rapid dural absorption. Tmax from 1 to 4 hours.
Distribution and TissuesVery high (1.2 - 2.0 L/kg). Trimethoprim excellently penetrates the parenchyma of difficult tissues such as the prostate, bronchial secretions and inflamed CSF.
Protein BindingSulfamethoxazole ~70% (highly bound to albumin); Trimethoprim ~44%.
Hepatic MetabolismSuffers partial hepatic metabolism by N4-acetylation and inactive hydroxylation (60%). No major CYP450 inhibitors.
Excretion and Half-LifeRenal glomerular excretion and tubular secretion unchanged at 60-80% for both compounds. Sulfamethoxazole half-life: 10.0 hours; Trimethoprim: 8.0 to 10.0 hours (prolonged in severe anuria).

Antimicrobial Spectrum

  • Gram-Positive and Opportunistic High Potency: Excellent against Pneumocystis jirovecii (opportunistic fungus), Toxoplasma gondii, Nocardia spp., Listeria monocytogenes, MRSA of community origin and enteric Gram-negative bacteria (except P. aeruginosa).

Indicators and dose

Dosage and Adjustment

  • Common Dosage (Adults): 800/160 mg tablets every 12 hours.
  • Pneumocystis pneumonia: Intensive treatment at doses of 15 to 20 mg/kg/day (based on the trimethoprim component) divided every 6 to 8 hours by IV or oral route in 21-day treatments.
  • Adjustment in Kidney Failure:
    • Clcr 15-30 mL/min: Reduce the recommended daily cumulative dose by half (50%).
    • Clcr < 15 mL/min: Absolutely contraindicated.

Security

Contraindications and ADRs

  • Contraindications: Congenital glucose-6-phosphate dehydrogenase deficiency; first trimester of pregnancy (potent antifolate); pre-existing severe kidney or liver failure.
  • Adverse Effects (ADR):
    • Severe Immunoallergic Skin Reactions: Mild maculopapular rash, urticaria. High risk of inducing the dreaded Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN), severe reactions with systemic epidermal detachment and high mortality.
    • Hyperkalemia and Tubular Toxicity: Trimethoprim acts as an analogue of the diuretic amiloride, competitively blocking epithelial sodium channels (ENaC) of the renal distal collecting duct. This reduces renal excretion of potassium, significantly raising serum levels, which is favored in elderly or nephropathic patients.
    • Reversible myelotoxicity: Megaloblastic anemia and thrombocytopenia in malnourished people.

Clinical

Clinical Management of Prophylaxis in HIV and Immunosuppressed Patients

Cotrimoxazole is the first choice prophylaxis in immunosuppressed patients with CD4 lymphocyte cell count less than 200 cells/uL to prevent pneumonia due to Pneumocystis jirovecii and encephalitis due to Toxoplasma gondii. It is prescribed at a low daily dose of 800/160 mg orally 3 times a week or daily as tolerated, requiring periodic control of serum potassium and kidney function.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

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System
Antimicrobial and Infectious
Cluster
Synthetic Inhibitors of Metabolic Pathways
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