SP-304, also called Guanilib, is an analog of uroguanylin, a natural gastro-intestinal hormone that is produced in the gastro-intestinal (GI) tract and there is a key regulator of intestinal function. Uroguanylin works by activating a unique receptor on intestinal epithelial cells. The receptor, called the guanylate cyclase C (GC-C) receptor, promotes fluid and ion transport in the GI tract. Under normal conditions, the receptor is activated by the natural hormones uroguanylin and guanylin. Activation of the receptor leads to the transport of chloride and bicarbonate into the intestine. Water is carried with these ions into the lumen of the intestine, thereby producing a looser stool.

SP-304 is a synthetic analog of uroguanylin, and acts in an identical manner as the natural hormone – as an agonist (i.e. activator) of the GC-C receptor. The drug is administered orally. Importantly, SP-304 remains in the gut, and is not absorbed systemically into the body, exerting its effect on GC-C receptors in the gut lining. The drug is therefore anticipated to be very safe in clinical use. Learn More

Compound class

GC-C receptor agonist are a new class of drugs for gastrointestinal diseases. There are only two compounds in the class, SP-304 and Linaclotide. Linaclotide is an analog of ST-peptide, an enterotoxin produced by E. coli bacteria that is a main cause of traveler’s diarrhea. There is strong clinical evidence of efficacy and safety for this class of compounds. Lincaclotide has demonstrated excellent safety and efficacy in phase 2 trials for chronic constipation and constipation-predominant irritable bowel syndrome.

Clinical development

Callisto is developing SP-304 for chronic constipation and constipation predominant irritable bowel syndrome (IBS-C). These are very common GI conditions with few treatments available. SP-304 is presently in late pre-clinical development and clinical trials are expected to start in early 2008. Callisto believes that SP-304 will prove to be the best compound in the class.

Intellectual property

Callisto has a very strong intellectual property estate for SP-304. Our key patent, US7041786, issued on May 9, 2006 provides composition of matter protection on SP-304.


The market for functional gastrointestinal disorders is one of the largest untapped pharmaceutical markets remaining. There are up to 35 million people in the Unites States that suffer from chronic constipation and IBS-C. Of those, about 25% have a severe condition. The prevalence is similar in other developed countries. In the US there is at present only one prescription product approved for these conditions. Learn More