Discovery and development of antisecretory drugs for treating diarrheal diseases

Abstract

Diarrheal diseases constitute a significant global health burden and are a major cause of childhood mortality and morbidity. Treatment of diarrheal disease has centered on the replacement of fluid and electrolyte losses using oral rehydration solutions. Although oral rehydration solutions have been highly successful, significant mortality and morbidity due to diarrheal disease remains. Secretory diarrheas, such as those caused by bacterial and viral enterotoxins, result from activation of cyclic nucleotide and/or Ca(2+) signaling pathways in intestinal epithelial cells, enterocytes, which increase the permeability of Cl(-) channels at the lumen-facing membrane. Additionally, there is often a parallel reduction in intestinal Na(+) absorption. Inhibition of enterocyte Cl(-) channels, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, represents an attractive strategy for antisecretory drug therapy. High-throughput screening of synthetic small-molecule collections has identified several classes of Cl(-) channel inhibitors that show efficacy in animal models of diarrhea but remain to be tested clinically. In addition, several natural product extracts with Cl(-) channel inhibition activity have shown efficacy in diarrhea models. However, a number of challenges remain to translate the promising bench science into clinically useful therapeutics, including efficiently targeting orally administered drugs to enterocytes during diarrhea, funding development costs, and carrying out informative clinical trials. Nonetheless, Cl(-) channel inhibitors may prove to be effective adjunctive therapy in a broad spectrum of clinical diarrheas, including acute infectious and drug-related diarrheas, short bowel syndrome, and congenital enteropathies.

Keywords: CFTR; Ca(2+)-activated Cl(-) channels; CaCC; Chloride Channels; Diarrhea; HIV; ORS; Rotavirus; Small Molecules; cystic fibrosis transmembrane conductance regulator; human immunodeficiency virus; oral rehydration solution.

Figure 1. Cl⁻ channels as targets for therapy of secretory diarrheas

Diagram of fluid secretory mechanism in enterocytes lining intestinal crypts and villi, showing active Cl⁻ transport from the blood/sub-mucosa to the intestinal lumen facilitated by luminal membrane CFTR and CaCC channels. Top inset. CFTR channel pore showing proposed site of action of CFTR_inh-172 (arginine 347) and external pore blocking action of GlyH-101. N- NBD binding domain, r- regulatory domain.

Figure 2. Efficacy of Cl⁻ channel inhibitors in animal models of secretory diarrheas

A. CFTR inhibition prevents cholera toxin-induced fluid secretion. CFTR_inh-172 structure (left, top) and photographs of intestinal loops at 6 hours after injection with saline or cholera toxin (left, bottom). Dose-response for inhibition of loop fluid accumulation (right). Mice were given single dose of CFTR_inh-172 by intraperitoneal injection and loop weight measured at 6 hours. From ref. . B. Improved survival of suckling mice following gavage with cholera toxin without vs. with the lectin conjugate MalH-ConA showing chemical structure (left) and survival plot (right). From ref. . C. CaCC inhibition by a red wine extract prevents rotavirus-induced fluid secretion in neonatal mice. Schematic showing expulsion of a 1.9 mm-diameter, 1.5 mm-thick cylindrical volume of stool onto absorbent tissue paper in which stool water content is quantified by wetted area (left). (center) Mice were inoculated with rotavirus at day 0 and gavaged with 1-kDa wine extract twice a day. Photographs of absorbent tissue at 1 min after contacting stool specimen, just before (photos at left) and after (photos at right) removal of stool mass. Wetted area demarcated by thin line. (right) Wetted area at indicated days. From ref. .