Mesalazine is a non-steroidal anti-inflammatory drug (NSAID) structurally related to the salicylates. This molecule is used to treat mild to moderately active ulcerative colitis. Mesalazine's mechanism of action is still not understood, but it is believed that the molecule has a topical anti-inflammatory effect on colonic epithelial cells. It targets the prostaglandin G/H synthase-1 (COX-1) and prostaglandin G/H synthase-2 (COX-2) in the cyclooxygenase pathway. The cyclooxygenase pathway begins in the cytosol with phospholipids being converted into arachidonic acid by the action of phospholipase A2. The rest of the pathway occurs on the endoplasmic reticulum membrane, where prostaglandin G/H synthase 1 & 2 convert arachidonic acid into prostaglandin H2. Prostaglandin H2 can either be converted into thromboxane A2 via thromboxane A synthase, prostacyclin/prostaglandin I2 via prostacyclin synthase, or prostaglandin E2 via prostaglandin E synthase. COX-2 is an inducible enzyme, and during inflammation, it is responsible for prostaglandin synthesis. It leads to the formation of prostaglandin E2 which is responsible for contributing to the inflammatory response by activating immune cells and for increasing pain sensation by acting on pain fibers. Mesalazine inhibits the action of COX-1 and COX-2 on the endoplasmic reticulum membrane. This reduces the formation of prostaglandin H2 and therefore, prostaglandin E2 (PGE2). The low concentration of prostaglandin E2 attenuates the effect it has on stimulating immune cells and pain fibers, consequently reducing inflammation and pain. Furthermore, mesalazine also inhibits arachidonate 5-lipoxygenase, diminishing the inflammatory processes (leukotriene biosynthesis) and the peroxisome proliferator-activated receptor gamma (inhibiting transcription factors; NF-kappa-B-mediated proinflammatory responses). Mesalazine is administered as an oral tablet or as a rectal suppository.

Mesalazine Pathway References