Sulfasalazine exerts its therapeutic effect in ulcerative colitis only after conversion by colonic bacteria. The active moiety released and the mechanism of its anti-inflammatory action are:

• A Sulfapyridine, which inhibits bacterial folate synthesis in the gut lumen
• B Mesalamine dimers, which scavenge reactive oxygen species systemically
• C 5-aminosalicylic acid, which inhibits prostaglandin and cytokine generation in the colonic mucosa ✓
• D Salicylic acid, which irreversibly acetylates cyclooxygenase in epithelial cells

Explanation

Colonic bacterial azoreductases cleave sulfasalazine into sulfapyridine and 5-aminosalicylic acid (mesalamine). Mesalamine is the active anti-inflammatory moiety, acting locally on the colonic mucosa to inhibit prostaglandin and leukotriene generation and modulate cytokines such as TNF-alpha and IL-1. Sulfapyridine is largely responsible for adverse effects like hypersensitivity and haemolysis, not efficacy. Mesalamine does not irreversibly acetylate COX the way aspirin does.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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