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In August 2012, the US FDA approved linaclotide (also referred to as MD-1100), a first-in-class, orally administered 14-amino acid peptide as a therapy for patients suffering from chronic idiopathic constipation (CIC) and irritable bowel syndrome with constipation (IBS-C). Linaclotide and its active metabolite MM-419447, which results from the cleavage of the C-terminal tyrosine residue by carboxypeptidase A, mimic the actions of the endogenous intestinal peptides guanylin (15 amino acids) and uroguanylin (16 amino acids) by activating guanylyl cyclase C (GC-C) on the intestinal epithelium. Activation of GC-C leads to increased intra- and extracellular levels of cGMP and activation of the CFTR ion channel, resulting in increased levels of HCO₃^-, Cl^-, and water in the intestinal lumen and accelerated gastrointestinal transit. Based on an in vitro assaymeasuring the accumulation of cGMP in T84 cell exposed to an agonist, the EC₅₀ of linaclotide at pH 7.0 was 99±17.5 nM. In preclinical studies in mice using the transit of activated charcoal as ameasure of efficacy, linaclotide at 100 μg/kg significantly accelerated transit compared to wild-type mice treated with charcoal only or GC-Cnullmice treatedwith and without linaclotide.118 Efficacy was also seen in rats treatedwith linaclotide atdoses of 5, 10, and 20 μg/kg. Linaclotide has been synthesized using conventional solid-phase peptide technology.