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Cotrimaxolefpr 5 marks pharmacology

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I now have comprehensive information from Lippincott's and Katzung's to write a complete 5-mark pharmacology answer on Cotrimoxazole. Here is the full answer:

Cotrimoxazole - Pharmacology (5 Marks)

Definition & Composition

Co-trimoxazole (TMP-SMX) is a fixed-dose combination of:
  • Trimethoprim (TMP) + Sulfamethoxazole (SMX) in a 1:5 ratio
  • Single-strength tablet: TMP 80 mg + SMX 400 mg
  • Double-strength tablet: TMP 160 mg + SMX 800 mg
  • Brand names: Bactrim, Septra
The combination was chosen because both drugs have similar half-lives (~10-12 hours) and show synergistic bactericidal activity together.

Mechanism of Action (Sequential Blockade of Folate Synthesis)

Cotrimoxazole produces synergism by blocking two sequential steps in tetrahydrofolic acid (THF) synthesis:
StepDrugEnzyme BlockedEffect
Step 1SulfamethoxazoleDihydropteroate synthase (competes with PABA)Blocks incorporation of PABA into dihydrofolic acid
Step 2TrimethoprimDihydrofolate reductasePrevents conversion of dihydrofolate → tetrahydrofolate
Result: THF deprivation → no purines, no thymidylate, no DNA synthesis → bactericidal effect (SMX alone is only bacteriostatic)
Trimethoprim is 20-50 times more potent than sulfonamides alone and has much higher affinity for bacterial dihydrofolate reductase than human dihydrofolate reductase - this accounts for selective toxicity.
Synergism diagram (Lippincott, p. 1037):
Synergism of trimethoprim and sulfamethoxazole in cotrimoxazole - bacteria grow significantly less with both drugs combined vs. either alone

Antibacterial Spectrum

Cotrimoxazole has a broader spectrum than sulfonamides alone:
Gram-positive: S. aureus (including MRSA), Listeria monocytogenes
Gram-negative: E. coli, H. influenzae, Legionella pneumophila, Proteus mirabilis, Salmonella typhi, Shigella spp.
Others: Pneumocystis jirovecii, Toxoplasma gondii, Nocardia spp., Stenotrophomonas maltophilia
NOT active against: Anaerobes, Mycoplasma, Chlamydia, Pseudomonas aeruginosa (except Stenotrophomonas)

Pharmacokinetics

  • Route: Oral (well absorbed) or IV (for severe infections)
  • Distribution: Widely distributed throughout body; crosses the blood-brain barrier and placenta
  • Special property: Trimethoprim is a weak base - concentrates in acidic prostatic and vaginal fluids, making it highly useful for prostatitis
  • Protein binding: Both drugs bound to serum albumin
  • Elimination: 50-60% TMP and 30-50% SMX excreted unchanged in urine within 24 hours
  • Dose reduction needed when creatinine clearance < 30 mL/min
When 1 part TMP is given with 5 parts SMX, peak plasma concentrations reach a 1:20 ratio, which is optimal for synergistic activity.

Therapeutic Uses

Cotrimoxazole therapeutic applications chart
  1. Urinary tract infections - most common use; drug of choice for uncomplicated UTIs
  2. Pneumocystis jirovecii pneumonia (PCP) - drug of choice; both treatment and prophylaxis in HIV patients with CD4 < 200 cells/mL
  3. Prostatitis - TMP concentrates in prostatic fluid
  4. MRSA skin and soft tissue infections
  5. Shigellosis and Salmonellosis (including typhoid carriers)
  6. Nocardiosis - drug of choice
  7. Listeria meningitis (in penicillin-allergic patients)
  8. Toxoplasmosis prophylaxis in immunocompromised
  9. Stenotrophomonas maltophilia infections - drug of choice
  10. Traveler's diarrhea, respiratory infections (H. influenzae)

Adverse Effects

Adverse EffectMechanism
Nausea, vomitingMost common GI side effects
Skin rash, Stevens-Johnson syndromeHypersensitivity (sulfonamide component)
CrystalluriaSMX precipitates in acidic/concentrated urine; prevent with hydration + alkalinization
Megaloblastic anemia, leukopenia, granulocytopeniaTMP-induced folate deficiency (treat with folinic acid/leucovorin)
Hemolytic anemiaG6PD-deficient patients
KernicterusSMX displaces bilirubin from albumin in neonates
HyperkalemiaTMP has potassium-sparing effect (blocks ENaC - similar to amiloride)
PhotosensitivitySulfonamide component

Resistance

Resistance requires simultaneous mutations in both components - hence less frequent than resistance to either drug alone. Mechanisms include:
  • Altered dihydrofolate reductase (reduced TMP binding) - plasmid-encoded, spread via conjugative transposons
  • Overproduction of dihydrofolate reductase
  • Reduced cell permeability / efflux pumps
  • Significant resistance now documented in E. coli (up to 30%)

Contraindications

  1. Neonates/infants < 2 months and pregnant women at term - risk of kernicterus
  2. Severe renal or hepatic impairment (reduce dose)
  3. G6PD deficiency - risk of hemolytic anemia
  4. Avoid with methenamine (crystallization in acidic urine)

Drug Interactions

  • Warfarin - SMX inhibits CYP2C9 → increased anticoagulation; also displaces warfarin from albumin
  • Methotrexate - displacement from albumin → toxicity
  • Phenytoin - CYP2C9 inhibition → elevated phenytoin levels
  • ACE inhibitors / potassium-sparing diuretics - additive hyperkalemia risk
Sources: Lippincott Illustrated Reviews: Pharmacology, pp. 1035-1041; Katzung's Basic and Clinical Pharmacology, 16th Ed., pp. 1301-1303; Goodman & Gilman's Pharmacological Basis of Therapeutics
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