Linaclotide

Linaclotide is a 14-amino-acid synthetic peptide containing three disulfide bonds. It is structurally related to the endogenous intestinal peptides guanylin and uroguanylin, and especially to the heat-stable enterotoxins produced by some strains of Escherichia coli that cause secretory diarrhoea. Developers at Microbia, later Ironwood Pharmaceuticals, recognised that this toxin family could be adapted into a molecule that activates the same intestinal receptor in a controlled way.

Linaclotide was approved by the FDA in 2012, marketed as Linzess, and in the EU as Constella, for chronic idiopathic constipation and irritable bowel syndrome with constipation. A defining feature is that it acts locally on the intestinal epithelium, with minimal systemic absorption reported in pharmacokinetic studies. This gut-restricted profile has made linaclotide a reference compound for research into luminally acting peptides. Its development also drew attention to the guanylate cyclase-C pathway as a regulator not only of fluid secretion but also of visceral sensation, a link explored in animal models of abdominal pain.

Linaclotide binds and activates guanylate cyclase-C on the luminal surface of intestinal epithelial cells. Receptor activation converts GTP to cyclic GMP. Rising intracellular cGMP activates protein kinase G II, which phosphorylates the CFTR chloride channel, promoting chloride and bicarbonate secretion into the lumen. Sodium and water follow, increasing intestinal fluid and accelerating transit. Some cGMP is also exported from epithelial cells, and in rodent studies this extracellular cGMP reduced the firing of colonic pain-sensing afferent nerves. This second action is the basis of research into linaclotide's effects on visceral hypersensitivity, separate from its secretory effects.