Proof of Principle Study for an Efficacy Trial of Linaclotide for 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