Classification of Sulfonamides, Mechanism of Action, Dosage, Adverse Effects, Contraindications, Drug Interactions

Introduction to Sulfonamides

Definition: Synthetic antimicrobial agents containing the sulfonamide group (–SO₂NH₂).
Discovery: First widely used systemic antibacterial agents (Prontosil, 1930s).
Spectrum: Broad spectrum against gram-positive and gram-negative bacteria.
Nature: Bacteriostatic (inhibit growth, don’t kill directly).

Classification of Sulfonamides

1. Short-acting (t½ < 12 hrs)

Sulfisoxazole, Sulfamethoxazole

2. Intermediate-acting (t½ 12–24 hrs)

Sulfadiazine

3. Long-acting (t½ > 24 hrs)

Sulfadoxine

4. Topical sulfonamides

Sulfacetamide (eye/skin use)
Silver sulfadiazine (burns)

5. Combination formulations

Cotrimoxazole (Sulfamethoxazole + Trimethoprim)

Mechanism of Action

Target: Folic acid synthesis in bacteria.
Step Blocked:

Sulfonamides inhibit dihydropteroate synthase (compete with PABA – para-aminobenzoic acid).
Trimethoprim (in Cotrimoxazole) inhibits dihydrofolate reductase.

Effect: Blocks production of tetrahydrofolic acid → no nucleic acid synthesis → growth inhibition.

Sulfacetamide

Form: Ophthalmic drops/ointment, topical skin cream.
Therapeutic Uses:

Bacterial conjunctivitis
Trachoma
Seborrheic dermatitis (topical)

Dosage:

Ophthalmic: 10% solution, 1–2 drops every 2–3 hrs initially.

Adverse Effects:

Local irritation, allergic reaction, Stevens–Johnson syndrome (rare).

Contraindications:

Hypersensitivity to sulfa drugs.

Drug Interactions:

Potentiates effect of warfarin, phenytoin.

Silver Sulfadiazine

Form: 1% topical cream.
Therapeutic Uses:

Prevention and treatment of infections in burn wounds.

Dosage:

Apply thin layer once or twice daily to cleaned burn area.

Adverse Effects:

Local irritation, delayed wound healing in some cases, leukopenia.

Contraindications:

Sulfa allergy, pregnancy (near term), premature neonates.

Drug Interactions:

Minimal systemic absorption → fewer interactions.

Cotrimoxazole

Composition: Sulfamethoxazole + Trimethoprim (5:1 ratio).
Therapeutic Uses:

UTIs
Respiratory tract infections
Pneumocystis jirovecii pneumonia
Gastrointestinal infections (e.g., Shigellosis, traveler’s diarrhea)

Dosage:

Adult: 800 mg SMX + 160 mg TMP twice daily.

Adverse Effects:

Rash, nausea, hematological toxicity (megaloblastic anemia, leukopenia), hyperkalemia.

Contraindications:

Sulfa allergy, folate deficiency, pregnancy, neonates (< 2 months).

Drug Interactions:

Warfarin (↑ bleeding risk), methotrexate (↑ toxicity), phenytoin (↑ effects).

General Adverse Effects of Sulfonamides

Hypersensitivity reactions (rash → SJS/TEN)
Hematologic effects (hemolytic anemia in G6PD deficiency)
Crystalluria (drink plenty of water)
Photosensitivity

Key Precautions & Contraindications

Avoid in pregnancy and neonates (risk of kernicterus).
Avoid in G6PD deficiency.
Ensure adequate hydration to prevent crystalluria.

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