Linaclotide is a medicine used to treat constipation and irritable bowel syndrome with constipation (IBS-C). It works by acting on the surface of the gut lining, where it increases the movement of salt and water into the bowel. This softens stools, makes them easier to pass, and can also reduce gut pain

One advantage of linaclotide is that, unlike some natural substances in the gut, it is stable and can act throughout the intestine. Studies in animals show that it has the strongest effect in the upper small intestine, but it may act in other parts of the bowel as well. In people, however, it is not yet clear whether linaclotide mainly works in the small intestine or in the large intestine (colon). Knowing this is important, because it could help the investigators understand whether linaclotide might also be useful in other conditions, such as cystic fibrosis, where the gut does not handle fluid properly.

Linaclotide is taken as a capsule, but less than 1% is absorbed into the bloodstream. Instead, it stays in the gut, where it is broken down into smaller active parts. This means both the small intestine and colon may be exposed to its effects.

Until now, it has been hard to study this because traditional methods only measure one part of the gut at a time. A team at the University of Nottingham has developed MRI scanning methods that can safely and non-invasively measure water content in the small intestine and colon.

The aim of this pilot study is to use MRI in healthy volunteers to see exactly where linaclotide acts. This knowledge will help optimise future studies in conditions such as cystic fibrosis.

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. In CF, defective CFTR may lead to various clinical effects on the gastrointestinal (GI) system; indeed, many CF patients report significant GI symptoms, with the most frequent being attributable to the lower GI tract, including bloating, flatulence, abdominal pain, and borborygmi. Moreover, constipation is another prevalent GI symptom in CF patients. It is estimated that 10-57% of CF patients report constipation symptoms, with an even higher prevalence (approximately 73%) in adults over the age of 30. Symptoms such as constipation are hypothesised to occur due to decreased luminal fluid content resulting from a reduction in anion flow, which ultimately leads to increased amounts of viscous mucus and slowed transit of intestinal contents. Presently, these GI symptoms in CF patients are often treated with laxatives and enemas, and in some cases surgical treatment, which can be uncomfortable and invasive, suggesting that alternative treatments are required.

Linaclotide, a drug approved for safe use across Europe and in the UK, is a synthetic analogue of uroguanylin that activates guanylate cyclase-C (GC-C) receptors located on the luminal surface of intestinal epithelial cells. Activation of GC-C increases intracellular cyclic guanosine monophosphate (cGMP), which stimulates cGMP-dependent protein kinase II (PKGII) and protein kinase A (PKA). Collectively, these kinases activate CFTR, leading to chloride and water secretion into the intestinal lumen. In parallel, elevated cGMP inhibits the sodium-hydrogen exchanger 3 (NHE3), thereby reducing sodium and water absorption. Together, these actions promote fluid secretion and accelerate intestinal transit, leading to linaclotide being commonly prescribed for the treatment of chronic idiopathic constipation and irritable bowel syndrome with chronic constipation (IBS-C).

Meta-analyses confirm linaclotide's clinical benefit in chronic idiopathic constipation and IBS-C; however, its role in CF remains uncertain, though anecdotal reports and animal models suggest therapeutic potential. Linaclotide has less than 1% systemic bioavailability and is degraded in the upper small bowel by carboxypeptidases to an active metabolite, with a small proportion of the administered dose recovered in stool, providing exposure throughout the small and large intestine. However, the primary site of action in humans remains undefined.

Traditional perfusion methods provide only segmental information, limiting assessment of the whole intestine. By contrast, magnetic resonance imaging (MRI) enables non-invasive evaluation of gastrointestinal water content across multiple regions. The Nottingham group has validated MRI techniques for quantifying small bowel water content and assessing colonic chyme hydration via T1 mapping. These methods have been successfully applied to study the effects of various pharmacological agents on gastrointestinal function.

Aim: To define the site of action of linaclotide in healthy volunteers using MRI. Identifying the regional effects of linaclotide will help optimise its future evaluation in CF patients. Based on clinical observations in constipation, stool effects emerge within seven days; however, the investigators hypothesise that small bowel changes occur within 1-2 hours of dosing. Therefore, a one-day pre-dosing regimen is expected to elicit a measurable effect on both the small bowel and colon while minimising participant burden.

Subjects will take 290ug linaclotide /placebo on the day prior to study day (day -1) and on the study day

Inclusion Criteria:

Participant is willing and able to give informed consent for participation in the study

Not currently taking any medications (except for selective serotonin reuptake inhibitors, low dose tricyclic antidepressants, antihistamines, and oral contraceptive pill).

Aged between 18-60 years.

Ability to conform to the study protocol, including overnight fasting, dietary and lifestyle restriction, administering linaclotide and placebo intervention, MRI scanning, consuming the rice pudding/blue dye meal, and rating stool frequency and appearance.

Exclusion Criteria:

Contraindication to MRI scanning (i.e. metallic implants, pacemakers, history of metallic foreign body in eye(s) and penetrating eye injury, unable to lie flat and relatively still for less than 5 minutes.)

Pregnancy, lactating, or planning pregnancy during the investigation declared by candidate.

History declared by the candidate of pre-existing gastrointestinal disorder that may affect bowel function.

Reported history of previous resection of the oesophagus, stomach, or intestine (excluding appendix).

Intestinal stoma.

Any medical condition that may potentially compromise participation in the study e.g., known food intolerance to rice pudding, known contraindication to the oral administration of linaclotide or placebo.

Has a body mass index (BMI) value less than 18.5 or greater than 35.

Will not agree to follow dietary and lifestyle restrictions required.

Unable to stop drugs known to alter GI motility including mebeverine, opiates, monoamine oxidase inhibitors, phenothiazines, benzodiazepines, calcium channel antagonists for the duration of the study.

Participants who are currently (or in the past 3 months) taking antibiotics or probiotics as these may impact GI function.

Participation in night shift work the week prior to the study day. Night work is defined as working between midnight and 6.00 AM.

Anyone who in the opinion of the investigator is unlikely to be able to comply with the protocol e.g., cognitive dysfunction, chaotic lifestyle related to substance abuse.

Having taken part in a research study in the last 3 months involving invasive procedures or an inconvenience allowance

-