GC-C Agonists to Treat Gastrointestinal Disorders & Diseases

Plecanatide 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. Plecanatide 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 Plecanatide

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 plecanatide. A key amino-acid substitution enabled the Synergy team to create a drug, plecanatide, with superior pharmacological properties (3). Unlike uroguanylin, plecanatide exists primarily in a single bioactive conformation. Orally administered plecanatide 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 plecanatide 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.

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 agonists 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). Plecanatide 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). Plecanatide 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, with an improved protein stability profile, to treat ulcerative colitis.

GC-C agonists 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).

A deficiency of uroguanylin is thought to be one of the primary reasons for the development of polyps in the colon, considerably increasing the risk of colon cancer. Oral treatment with uroguanylin in an animal model of colon cancer, the Apc +/min mouse, showed that uroguanylin inhibits polyp formation and also retards the progression of polyps to adenocarcinomas (12). These mice are genetically engineered such that they mimic the etiology of colon carcinogenesis in humans. Thus, GC-C agonists such as plecanatide have potential as agents to prevent colon and other GI cancers.

References

1. 1.Hamra FK, Fan X, Krause WJ, Freeman RH, Chin DT, Smith CE, Currie MG, Forte LR. Prouroguanylin and proguanylin: purification from colon, structure, and modulation of bioactivity by proteases. Endocrinology. 1996 Jan;137(1):257-65.
   

2. 2.Forte, L.R., Guanylin regulatory peptides: structures, biological activities mediated by cyclic GMP and pathobiology. Reg. Pept., 81:25-39, 1999.
   

3. 3.Shailubhai, K., G. Nikiforavich and Jacob, G.S. Guanylate Cyclase receptor agonists for the treatment of tissue inflammation and carcinogenesis. U.S. Patent 7,041,786, Issue may 9, 2006.
   

4. 4.Viola Andresen, Michael Camilleri, et al., Effect of 5 Days Linaclotide on Transit and Bowel Function in Females With Constipation-Predominant Irritable Bowel Syndrome. Gastroenterology 2007;133:761–768
   

5. 5.Johnston JM, Kurtz CB, Drossman DA, Lembo AJ, Jeglinski BI, MacDougall JE, Antonelli SM, Currie MG., Pilot study on the effect of linaclotide in patients with chronic constipation. Am J Gastroenterol. 2009 Jan;104(1):125-32.
   

6. 6.Shailubhai, K., Gerson, W.,Talluto, C., and Jacob, G.S. (2008) A Randomized, Double-blind, Placebo-Controlled, Single-, Ascending-, Oral-Dose Safety, Tolerability and Pharmacokinetic Study of Plecanatide in Healthy Adult Human Male and Female Volunteers. Poster presented at the Digestive Disease Week, May 29- June 4, 2008
   

7. 7.Shailubhai, K., Talluto, C., Comiskey, S., Plevy S., and Jacob, G.S. (2009) GuanylateCyclase-C Agonists as a New Class of Drug Candidates for GI Motility and Inflammatory Bowel Disease. Poster # 226, presented at the 6th Neurogastroenterology and motility meeting held from August 27 to 30 in Chicago.
   

8. 8.Refaat AF. Hegazi, Fengling Li, Gabriel Calilao, Antonia Sepulveda, Kunwar Shailubhai, Scott E. Plevy SP304, an analog of uroguanylin, ameliorates inflammation in a model of experimental colitis. Abstract, presented at Digestive Disease Week, 2005
   

9. 9.Kunwar Shailubhai, Gary S. Jacob and Scott Plevy. Plecanatide, an agonist of Guanylate Cyclase C, is a new class of oral drug candidate that ameliorates inflammation in models of experimental colitis. To be presented at the 2007 Crohn's and Colitis Foundation National Research and Clinical Conference – 6th Annual Advances in the Inflammatory Bowel Diseases conference. December 6-9, 2007. Florida, USA.
   

10. 10.Shailubhai. K. Therapeutic applications of guanylate cyclase receptor agonists. Curr. Opin. Drug Disc. Dev. 5(2): 2002.
    

11. 11.Pitari GM, Di Guglielmo MD, Park J, Schulz S, Waldman SA. Guanylyl cyclase C agonists regulate progression through the cell cycle of human colon carcinoma cells. Proc. Natl. Acad. Sci. U.S.A. 98: 7846-7851, 2001.
    

12. 12.Shailubhai, K., Yu, H. H., Karunanandaa, K., Wang, J. Y., Eber, S. L., Wang, Y., Joo, N. S., Kim, H. D., Miedema, B. W., Abbas, Z., Boddupalli, S. B., Mark G. Currie., and Forte, L. R. (2000) UG treatment suppresses polyps formation in APCmin/+ mouse and induces apoptosis in human colon adenocarcinoma cells by a cGMP dependent mechanism. Cancer Research 60, 5151-5157.