Linaclotide is a synthetic 14-amino-acid peptide that almost never leaves your gut. Sold as Linzess in the United States and Constella in Europe, it was the first drug in its class: an agonist of guanylate cyclase-C (GC-C), a receptor on the cells lining the intestine. The StatPearls monograph describes it as a potent, highly selective GC-C agonist structurally related to guanylin and uroguanylin, the body's own hormones that switch the same receptor on.
The molecule is a small cyclic peptide held together by three disulfide bonds (between Cys1-Cys6, Cys2-Cys10, and Cys5-Cys13), with the formula C59H79N15O21S6 and a molecular weight near 1527. CAS number 851199-59-2.
How it works
Linaclotide binds GC-C on the luminal side of intestinal epithelial cells. That raises intracellular cyclic GMP (cGMP), which activates the CFTR chloride channel and drives chloride and bicarbonate into the gut lumen while blocking sodium reabsorption. Water follows the electrolytes, so stool gets softer and transit speeds up. There is a second effect that matters for IBS: cGMP released into the surrounding tissue appears to dampen colonic pain-sensing nerves, which is the proposed basis for linaclotide's reduction in abdominal pain (StatPearls; preclinical work in Bryant et al., 2010, PubMed 20307554).
Because it is taken orally but barely absorbed, linaclotide acts locally. Systemic bioavailability is negligible, which is unusual for a drug and is central to its safety profile.
The clinical evidence
Linaclotide is one of the better-studied agents in functional GI disease. Approval rested on large randomized, double-blind, placebo-controlled Phase 3 trials in IBS-C, dosing 290 mcg once daily for 12 or 26 weeks. Both met their prespecified primary and secondary endpoints, including the FDA responder endpoint, which combined improvement in abdominal pain with an increase in complete spontaneous bowel movements. A prespecified analysis using EMA endpoints (Quigley et al., 2013, PubMed 23116208) reported that roughly 34% of linaclotide-treated patients met the FDA responder endpoint versus about 17% on placebo. Separate trial programs supported the chronic idiopathic constipation indication.
Diarrhea is the most common adverse effect and the main reason patients stop. It can occasionally be severe.
Regulatory status
- FDA: approved August 30, 2012 for adults with IBS-C and chronic idiopathic constipation. The label has since expanded, including a pediatric functional constipation indication and, more recently, use in younger children with IBS-C (FDA prescribing information).
- EMA: approved in the EU as Constella for IBS-C.
- It is contraindicated in very young children (risk of serious dehydration) and in patients with known or suspected mechanical GI obstruction.
Linaclotide is a prescription medicine, not a performance or physique compound, and it is not the kind of injectable peptide that the bodybuilding and anti-aging markets trade in. It carries the standard boxed warning around pediatric use rather than any doping concern.
The buyer and quality angle
Most people encounter linaclotide as a regulated, pharmacy-dispensed capsule, where identity and purity are handled by the manufacturer and regulators. The compound also shows up in the research-chemical supply chain, where vendors list synthetic linaclotide and its active metabolite (MM-419447) as "for research use only, not for human consumption." In that channel there is no regulatory backstop, so the usual aggregator questions apply: is there a recent third-party certificate of analysis, does HPLC-MS confirm both identity and purity (commonly 95-99% for peptides), and does the COA match the lot in hand. Linaclotide's three disulfide bonds make correct folding a real synthesis challenge, so an identity check matters more here than for a simple linear peptide.
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