5-ASA / IBD Agents

Inflammatory bowel disease (IBD) — chiefly ulcerative colitis and Crohn’s disease — is chronic immune-driven inflammation of the bowel wall, causing bloody diarrhea, cramping, and urgency. The aminosalicylates (5-ASA) are the first-line drugs for mild-to-moderate ulcerative colitis, used both to induce and to maintain remission.

The active molecule is 5-aminosalicylic acid (5-ASA, mesalamine). Unlike most drugs, it is not meant to be absorbed — it works topically on the inflamed gut lining itself, dampening local inflammation by inhibiting prostaglandin and leukotriene production, scavenging reactive oxygen species, and blocking inflammatory signaling. Because the drug must reach the diseased segment, much of the pharmacology is about delivery: pH-dependent coatings and azo-bonds carry it to the colon, and enemas/suppositories treat distal (rectal) disease.

Sulfasalazine is the original agent: a prodrug in which 5-ASA is chemically bonded (an azo bond) to a sulfa carrier called sulfapyridine. The intact molecule is too big to absorb in the small intestine, so it travels to the colon, where bacteria cleave the azo bond — releasing the active 5-ASA locally exactly where it is needed.

The catch: the freed sulfapyridine carrier IS absorbed, and it causes most of sulfasalazine’s side effects — because it is a sulfonamide. This is a direct cross-link to the Sulfonamides class: a sulfa allergy (including risk of Stevens–Johnson syndrome), hemolysis in G6PD deficiency, folate malabsorption, and reversible male infertility (oligospermia). It also causes a harmless orange-yellow discoloration of urine, skin, and contact lenses. Pure mesalamine formulations were developed precisely to deliver 5-ASA without the sulfa carrier — and they are generally well tolerated.