Linaclotide: what it is, how it works and what studies have found

Linaclotide is a small peptide with an unusual job: it is meant to act inside the gut and stay there. Taken orally, it survives the stomach, switches on a receptor on the intestinal lining, and is broken down before meaningful amounts reach the blood. That design, and the three disulfide bonds that hold its 14 residues in shape, make it a useful case study for researchers in both pharmacology and peptide chemistry.

This overview covers its structure, mechanism, the pivotal trials, safety findings and, in more depth than most sources, the synthesis problem of getting three disulfide bonds right. It reports published findings and labelling, not guidance on clinical use.

What is linaclotide?

Linaclotide is a 14-amino-acid peptide agonist of guanylate cyclase-C (GC-C) [1]. The US label describes it as “structurally related to human guanylin and uroguanylin”, the intestine’s own GC-C ligands [1]. In receptor binding studies it competed with radiolabelled STa, the heat-stable enterotoxin of Escherichia coli that acts on the same receptor [2].

It was described in early pharmacology papers as a “first-in-class, orally administered 14-amino acid peptide” [3], and FDA approved it on 30 August 2012 as a new molecular entity [4].

Linaclotide structure, sequence and disulfide bonds

Property	Value
INN	Linaclotide [1]
CAS number	851199-59-2 [5]
Sequence	CCEYCCNPACTGCY (Cys-Cys-Glu-Tyr-Cys-Cys-Asn-Pro-Ala-Cys-Thr-Gly-Cys-Tyr) [1]
Length	14 amino acids [1]
Disulfide bonds	Cys1-Cys6, Cys2-Cys10, Cys5-Cys13 [1]
Molecular formula	C59H79N15O21S6 [1] [5]
Molecular weight	1526.8 [1]
Appearance and solubility	Amorphous, white to off-white powder; slightly soluble in water and 0.9% sodium chloride [1]
Active metabolite	MM-419447, CCEYCCNPACTGC (des-Tyr14) [6]

Six of the 14 residues are cysteines, and all six are paired. Nuclear magnetic resonance analysis showed that the molecular structure “is stabilized by three intramolecular disulfide bridges” [3]. The peptide was stable in simulated gastric fluid and completely resistant to pepsin [3].

Mechanism of action: GC-C and cGMP

The US label sets out the mechanism [1]:

1. Linaclotide and its active metabolite bind GC-C and act locally on the luminal surface of the intestinal epithelium.
2. GC-C activation increases both intracellular and extracellular cyclic guanosine monophosphate (cGMP).
3. Intracellular cGMP stimulates chloride and bicarbonate secretion into the lumen, mainly by activating the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel.
4. The result is more intestinal fluid and faster transit.

In vitro, linaclotide bound GC-C receptors on human T84 colon carcinoma cells with high, pH-independent affinity (Ki 1.23-1.64 nM) and produced concentration-dependent cGMP accumulation (EC50 99 nM) [3]. In GC-C knockout mice, linaclotide had no effect on intestinal secretion or transit, confirming that the receptor is required [2].

The pain pathway: extracellular cGMP

GC-C is expressed by the colonic mucosa but not by sensory neurons, so the effect on abdominal pain needed a separate explanation. In mice, linaclotide inhibited colonic nociceptors, more strongly during chronic visceral hypersensitivity; the cGMP released from epithelial cells also inhibited nociceptors, and the effect was lost in Gucy2c knockout mice and when cGMP transporters were blocked or the mucosa removed [7]. The authors concluded that extracellular cGMP “acts on and inhibits nociceptors, thereby reducing nociception” [7]. Evidence level: animal and in vitro, with a supporting post hoc analysis of a phase 3 trial.

Absorption, metabolism and the half-life question

Linaclotide is designed to be minimally absorbed, and the data bear that out:

Finding	Evidence level	Ref
Stable for 3 hours in simulated gastric fluid (pH 1) and completely resistant to pepsin	In vitro	[3]
Completely degraded after 30 minutes in jejunal fluid	In vitro	[2]
Oral bioavailability about 0.1%	Animal (rat)	[3] [2]
Converted to MM-419447 in the small intestine; disulfides then reduced and peptides proteolysed	Animal and human	[6]
3-5% of the dose recovered in human faeces as active peptide, mostly MM-419447	Human	[6]
Plasma linaclotide and metabolite below the limit of quantitation; AUC, Cmax and half-life cannot be calculated	Human (label)	[1]

So there is no meaningful “linaclotide half-life” in plasma. The label also states that linaclotide does not interact with cytochrome P450 enzymes in vitro and that renal or hepatic impairment is not expected to affect its clearance [1].

Development history and timeline

Year	Event	Ref
2010	Preclinical pharmacology published: GC-C selectivity, NMR structure, minimal oral bioavailability (Ironwood Pharmaceuticals)	[3] [2]
2011	Fmoc solid-phase route for multigram synthesis published	[8]
2011	Two phase 3 chronic constipation trials published (funded by Ironwood and Forest Research Institute)	[9]
2012	Two phase 3 IBS-C trials published	[10] [11]
30 August 2012	FDA approval, NDA 202811	[4]
26 November 2012	EU marketing authorisation for moderate to severe IBS-C in adults only	[20] [12]
2013	Metabolite MM-419447 and extracellular cGMP pain mechanism described	[6] [7]
2014 to 2016	Phase 3 IBS-C programme in Japan (ASP0456, Astellas)	[13]
2018	12-week low-dose trial in chronic idiopathic constipation published	[14]

The current NDA holder listed on Drugs@FDA is AbbVie [4].

Linaclotide clinical trials: key studies and findings

Study	Design	Population	n	Duration	Primary endpoint	Main finding	Ref
Trials 303 and 01	Two phase 3, randomised, placebo-controlled, two dose groups each	Chronic constipation	1276	12 weeks	≥3 CSBMs/week and +1 from baseline for ≥9 of 12 weeks	16.0-21.3% vs 3.3-6.0% (P < 0.01 all comparisons)	[9]
Trial 302 (Chey)	Phase 3, randomised, placebo-controlled	IBS-C	804	26 weeks	FDA responder endpoint	33.7% vs 13.9% (P < 0.0001); NNT 5.1	[10]
Trial 31 (Rao)	Phase 3, randomised, placebo-controlled, 4-week randomised withdrawal	IBS-C	800	12 + 4 weeks	FDA responder endpoint	33.6% vs 21.0% (P < 0.0001); NNT 8.0	[11]
Quigley 2013	Prespecified analysis of Trials 31 and 302	IBS-C	803 and 805	12 and 26 weeks	EMA co-primary endpoints	Pain/discomfort responders 54.8% vs 41.8% and 54.1% vs 38.5%	[15]
Schoenfeld 2018	Phase 3, randomised, placebo-controlled, low-dose arm	Chronic idiopathic constipation	1223	12 weeks	CSBM overall responder	13.4% vs 4.7% (P < 0.0001)	[14]

Chronic constipation (NEJM 2011)

In two 12-week trials with 1276 patients, the primary endpoint of three or more complete spontaneous bowel movements (CSBMs) per week plus an increase of at least one from baseline, for at least 9 of 12 weeks, was met by 21.2% and 16.0% of patients in the lower-dose groups and 19.4% and 21.3% in the higher-dose groups, compared with 3.3% and 6.0% on placebo [9]. All secondary endpoints improved significantly, and diarrhoea led to discontinuation in 4.2% of patients in both linaclotide groups [9].

IBS with constipation (2012)

In the 26-week trial, 33.7% of linaclotide-treated patients met the FDA responder endpoint (at least 30% improvement in worst abdominal pain plus at least one extra CSBM, in the same week, for at least 6 of 12 weeks) versus 13.9% on placebo [10]. Diarrhoea caused discontinuation in 4.5% versus 0.2% [10]. In the 12-week trial with randomised withdrawal, the same endpoint was met by 136/405 (33.6%) versus 83/395 (21.0%) patients [11]. Patients switched to placebo in the withdrawal period saw symptoms return, but not worsen beyond baseline [11].

The European submission used different co-primary endpoints. On those, abdominal pain or discomfort responders were 54.8% versus 41.8% (Trial 31) and 54.1% versus 38.5% (Trial 302), and IBS degree-of-relief responders 37.0% versus 18.5% and 39.4% versus 16.6% [15]. A post hoc analysis of the 26-week trial found that 70% of linaclotide patients versus 50% on placebo had at least a 30% reduction in abdominal pain [7].

Low-dose arm in chronic idiopathic constipation (2018)

A 12-week trial in 1223 patients tested a lower dose and the original dose against placebo. The primary endpoint was met by 13.4% in the low-dose arm versus 4.7% with placebo (P < 0.0001), and diarrhoea led to discontinuation in 0%, 2.4% and 3.2% of the placebo, low-dose and original-dose groups [14].

Safety findings reported in studies and labelling

• Diarrhoea is the most common adverse reaction. In the pooled adult IBS-C trials it occurred in 20% of linaclotide patients versus 3% with placebo, and severe diarrhoea was reported in 2% of adults with IBS-C or chronic idiopathic constipation [1].
• Other common reactions (at least 2%) in adults are abdominal pain, flatulence and abdominal distension [1].
• Boxed warning. Linaclotide is contraindicated in children under 2 years, because in neonatal mice a single clinically relevant adult dose caused deaths from dehydration [1].
• Nonclinical toxicology. Two-year carcinogenicity studies in rats and mice were negative, linaclotide was not genotoxic in Ames and chromosomal aberration assays, and fertility in rats was unaffected [1].
• Class comparison. A 2018 meta-analysis of 15 trials (10,369 patients) of linaclotide and plecanatide, the other GC-C agonist, found no statistically significant differences between the two in efficacy, diarrhoea or diarrhoea-related withdrawals [16].

Linaclotide regulatory status by region

Region	Status	Ref
United States	Approved 30 August 2012 (NDA 202811). Current indications: IBS-C in adults and children 7 years and older; chronic idiopathic constipation in adults; functional constipation in children 2 years and older	[1] [4]
European Union	Authorised 26 November 2012 for symptomatic treatment of moderate to severe IBS-C in adults; chronic idiopathic constipation is not an authorised EU indication	[20] [12]
Japan	Phase 3 IBS-C programme completed by Astellas (ASP0456); approval status not reviewed here	[13]
Other regions	Not reviewed here; check the national regulator

Linaclotide in laboratory research

Linaclotide’s pharmacology papers give a ready-made assay set for GC-C research [3] [2] [7]:

• Receptor binding: competition with [125I]-STa on intestinal mucosal membranes or T84 cells.
• Functional readout: intracellular cGMP accumulation in T84 cells, and cGMP release into the medium.
• Stability: simulated gastric fluid, pepsin and jejunal fluid incubations.
• In vivo pharmacodynamics: fluid secretion into ligated intestinal loops, intraluminal cGMP and gastrointestinal transit in wild-type and GC-C knockout mice.
• Visceral pain: colonic afferent recordings and dorsal horn activation after colorectal distension.

For identity and purity, reversed-phase HPLC with UV detection and LC-MS are standard, but they are not sufficient on their own for a disulfide-rich peptide. Misfolded disulfide isomers have exactly the same mass as linaclotide, so identity testing also needs a method that confirms the connectivity, such as peptide mapping after partial reduction or comparison with a reference standard. Conformational heterogeneity has been resolved in a linaclotide manufacturing batch using cyclic ion mobility mass spectrometry, which revealed coexisting conformers even under native conditions [17].

Synthesis, purity and impurities: the disulfide problem

Linaclotide is made by chemical synthesis [8]. A 2011 paper from the Barcelona group of Fernando Albericio put the challenge plainly: it “consists of achieving the correct and clean folding of its three disulfide bridges” [8].

Why three disulfides are hard

With six cysteines, the number of ways to form three disulfide bonds is 5 × 3 × 1 = 15. Only one of those, Cys1-Cys6, Cys2-Cys10 and Cys5-Cys13 [1], is linaclotide. The other 14 are disulfide isomers with identical mass and composition but different shape and, in general, different activity. Four of the six cysteines sit in adjacent pairs (Cys1-Cys2 and Cys5-Cys6), each of which must end up bonded to a different partner.

Strategies

The published approaches fall into three groups [8]:

Strategy	How it works	Trade-off
Random (one-step oxidative folding)	Assemble the fully reduced peptide, then oxidise in a redox buffer and let it fold to its preferred isomer	Simple, but the yield of the correct isomer depends on folding conditions and isomers must be removed
Semi-regioselective	Protect some cysteines orthogonally so that one disulfide is fixed, then fold the rest	Fewer possible isomers; more steps
Fully regioselective	Use three orthogonal thiol protecting groups and form each disulfide in sequence	Defines connectivity, but adds deprotection and oxidation steps that can lower yield

Góngora-Benítez and colleagues compared regioselective, semi-regioselective and random strategies, studying the choice of thiol protecting groups, their position in the sequence, the influence of neighbouring protecting groups and the order of disulfide formation, and reported an optimised Fmoc solid-phase synthesis suitable for multigram amounts [8]. Newer chemistry continues to target the problem: a 2021 method combining a small molecule, ultraviolet light and palladium for chemo- and regioselective cysteine activation formed multiple disulfide bonds in one pot and was demonstrated on linaclotide [18]. Route choices for peptides of this size are discussed in SPPS vs LPPS.

Impurity classes to expect

Impurity class	Origin	Detection note
Disulfide (folding) isomers	Incorrect pairing during oxidation	Same mass; separated by HPLC or ion mobility, confirmed by connectivity mapping
Partially oxidised or reduced species	Incomplete folding	Mass differs by 2 Da per missing disulfide
Disulfide-linked dimers and oligomers	Intermolecular pairing during oxidation	Higher mass; size-exclusion or LC-MS
Des-Tyr14 (MM-419447)	Loss of C-terminal tyrosine	Also the in vivo active metabolite [6]
Deletion and truncated sequences	Incomplete coupling in SPPS	LC-MS
Conformational variants and thermal degradants	Stress, storage	Resolved by cyclic ion mobility MS under thermal stress [17]

Disulfide exchange and the other degradation routes that apply to cysteine-rich peptides are summarised in peptide API impurities and specifications.

Storage and stability

The marketed capsules contain linaclotide-coated beads, with calcium chloride dihydrate and either L-histidine or L-leucine among the inactive ingredients, and are stored at 25 °C, protected from moisture, with the desiccant kept in the container [1]. Those conditions are for the finished capsules; the label does not set storage conditions for the drug substance itself. Under accelerated thermal stress, a linaclotide batch showed notable degradation and structural reorganisation [17]. For lyophilized research material without stabilising excipients, frozen storage in a sealed, desiccated container and minimal exposure to humidity are sensible defaults. See lyophilised peptide stability and storage.

Linaclotide vs related GC-C peptides

	Linaclotide	Plecanatide	STa (E. coli enterotoxin)
Type	Synthetic 14-residue peptide, three disulfides [1]	GC-C agonist [16]	Bacterial toxin acting on GC-C [2]
Relation to guanylin and uroguanylin	Structurally related [1]	GC-C agonist class [16]	Binds the same receptor; linaclotide competes with it [2]
FDA approval	30 August 2012 [4]	19 January 2017 [19]	Not a medicine
Systemic exposure	Below quantitation in plasma [1]	Not reviewed here	Not applicable

How Winhycare Pharma supplies linaclotide

Winhycare Pharma supplies linaclotide as research-use-only material, at a purity of at least 99% as stated in our brochure, with a certificate of analysis that confirms identity and purity. Custom synthesis is available. Product details are on the linaclotide page; to discuss supply, contact our pharma team.

For laboratory research use only. Not for human or veterinary use.