Basic Science

Basic Science – GC-C Agonists

GC-C Agonists to Treat Gastrointestinal Disorders & Diseases

SP-304 and SP-333 are members of a novel class of non-systemic drugs for treatment of chronic constipation (CC), constipation-predominant irritable bowel syndrome (IBS-C) and other GI diseases. SP-304 is a synthetic analog of a natural peptide hormone called uroguanylin that regulates ion and fluid transport in the GI tract and is presently in clinical development to treat gastrointestinal disorders. SP-333 is a GC-C receptor agonist that is currently in preclinical development to treat GI diseases.

Uroguanylin and the Guanylate Cyclase C Receptor

Uroguanylin, a peptide hormone predominantly secreted in the GI tract, was discovered in the early 1990’s (1, 2). It is secreted predominantly in the intestinal tract where it functions by binding to a unique receptor found on epithelial cells of the intestine. This receptor, called guanylate cyclase C (GC-C), mediates synthesis of intracellular cyclic GMP (cGMP), a key regulator of cellular function. One key biochemical response mediated by uroguanylin is the activation of a chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR), which leads to augmented flow of chloride and water into the lumen of the gut which facilitate the natural digestive process.

Uroguanylin is a member of a class of natriuretic peptide hormones which are capable of binding to and activating (i.e. an agonist) the GC-C receptor. In addition, certain bacteria, including E. coli, produce an enterotoxin called ST-peptide that is structurally related to uroguanylin. This peptide is also an agonist of GC-C, capable of over-activating the GC-C receptor, promoting unregulated fluid transport into the intestine, and producing what is commonly called “travelers’ diarrhea”.

The Development of SP-304

Uroguanylin is a linear peptide comprised of 16 amino acids in length that is folded into a three-dimensional structure (or conformation) containing two intra-molecular disulfide linkages. This spatial structure and rigidity gives the hormone considerable stability in the intestinal tract. Although uroguanylin displays considerable temperature and protease resistance, it exists in both active and inactive conformers in aqueous solution.

Starting in 2000, Synergy scientists, using structure-function studies, developed a series of analogs of uroguanylin, leading to the identification of SP-304. A key amino-acid substitution enabled the Synergy team to create a drug, SP-304, with superior pharmacological properties (3). Unlike uroguanylin, SP-304 exists primarily in a single bioactive conformation. Orally administered SP-304 binds to and activates guanylate cyclase C (GC-C) expressed on the epithelial cells lining the GI mucosa, resulting in activation of CFTR, and leading to an augmented flow of chloride and water into the lumen of the gut to facilitate bowel movement. In animal models, oral administration of SP-304 promotes intestinal secretion and ameliorates gastrointestinal inflammation.

GC-C agonists for treatment of Chronic Constipation and IBS-C

Water plays a vital physiological role in the intestine. Proper water content of the intestinal lumen ensures normal transport of the content through the bowel. Water also facilitates maintenance of the intestinal mucus layer, which protects the GI mucosa from mechanical damage and the harmful effects of stomach acid and bacterial or viral pathogens. Insufficient water content can promote constipation, whereas excessive water content, as occurs in diseases like cholera and in incidents of food poisoning (e.g. E. coli -mediated GI infections), typically leads to diarrhea. One of the major functions of uroguanylin is to regulate fluid and ion transport into the lumen of the intestine.

In the GI tract, uroguanylin is produced locally by a class of intestinal cells (goblet and enterochromaffin cells) and secreted into the lumen of the intestine where it targets GC-C receptors on epithelial cells.

Uroguanylin Functions as Fluid Regulator in Intestine

Upon activation, the GC-C receptor promotes intracellular synthesis of cGMP, an intra-cellular mediator of cell function, leading to activation of CFTR and secretion of chloride and bicarbonate, along with water, into the intestinal lumen. Hence, uroguanylin, in concert with the GC-C receptor, plays a key role in regulating fluid and electrolyte balance in the intestine.

The hallmark of CC and IBS-C, is a chronic constipative condition that can be difficult to treat. Agonists of the GC-C receptor provide a novel approach to the treatment of CC and IBS-C. Recent clinical data reported on GC-C agonist have validated the use of GC-C receptor agonists to treat CC and IBS- C (4, 5, 6).

GC-C Receptor Agonists to treat Ulcerative Colitis

The GC-C pathway also has been found to regulate the anti-inflammatory effects of mediators such as nitric oxide and hemeoxygenase-1. Therapies that induce cGMP levels (e.g. phosphodiesterase-4 inhibitors) have previously been shown to demonstrate efficacy in murine (mouse) models of inflammatory bowel disease (IBD). SP-304 was recently shown to produce anti-inflammatory effects in animal models of ulcerative colitis, including TNBS- and DSS-induced colitis in mice (7). Importantly, the reduced inflammation was associated with down-regulation of key pro-inflammatory cytokines including IL-4, IL-5, IL-23, and TNF (8, 9). SP-304 treatment also was shown to down-regulate cyclooxygenase-2 (COX-2) mediated production of prostaglandin E2 (PGE2), which is an important inflammatory second messenger. Synergy presently is exploring a second GC-C receptor agonist SP-333 in preclinical studies to treat ulcerative colitis.

GC-C Agonist Mechanism-of-Action as Anti-Inflammatory Agent

SP-304 for prevention and control of colon cancer

Recent studies demonstrate that uroguanylin and a similar human intestinal hormone called guanylin are key regulators of growth and apoptosis in the epithelial layer of the intestinal mucosa (10). Disruption and/or irregularities in the turnover of cells, as is the case with individuals displaying reduced levels of endogenous uroguanylin, can lead to the development of polyps, colon cancer and inflammatory bowel diseases. Treatment of colon carcinoma cells with uroguanylin or with ST peptide was shown to inhibit cell proliferation and to induce apoptosis in a dose-dependent manner (11,12).