It is known that guanylyl cyclase-C (GC-C) agonists increase cyclic guanosine monophosphate (cGMP) levels in the intestinal epithelium to promote secretion. This process is the basis for the use of exogenous GC-C agonists such as linaclotide for the treatment of chronic idiopathic constipation (CIC) and irritable bowel syndrome with constipation (IBS-C). Because GC-C agonists have limited use in pediatric patients, there is a need for alternative cGMP-elevating agents that are effective in the intestine.
In this study, researchers, from Augusta University in Augusta, GA, assessed whether the PDE-5 inhibitor sildenafil has similar effects as linaclotide on preclinical models of constipation. They were able to determine that sildenafil can have similar effects as linaclotide on the intestine, and may have therapeutic benefits for patients with CIC, IBS-C, and post-infectious IBS, according to a study published in the April 27, 2017 issue of the journal PLOS ONE.
Study authors, led by Sarah K. Sharman of the Department of Biochemistry and Molecular Biology, Cancer Research Center at Augusta University in Augusta, GA, noted that IBS is a functional gastrointestinal disorder characterized by altered bowel habits and abdominal pain that adversely affect quality of life. IBS is sub-classified as constipation-predominant--IBS-C, diarrhea-predominant--IBS-D, or mixed symptom--IBS-M. There is no cure for IBS, and current treatment strategies often require patients to take multiple medications to control their symptoms. To help normalize alterations in bowel habits, bulking agents, laxatives, and anti-diarrheals are usually prescribed, while tricyclic antidepressants and antispasmodics are prescribed to minimize visceral pain associated with the disease. The variable effectiveness of this symptom-targeted approach to IBS treatment underscores the need for alternative treatments.
In the study, six- to eight-week-old CD1 mice and C57/BL6 mice were used for testing. Male mice were used for immunohistochemistry (IHC), cGMP assays, and for the inflammation-recovery model. Both male and female mice were used for intestinal transit assays.
Based upon mean water intake per animal, stock sildenafil was added to the drinking water at concentrations that approximated doses of 0.36 to 17 mg/kg daily per mouse. Unless otherwise stated, the dose of sildenafil given in the drinking water was equivalent to 5.7 mg/kg per day. Study authors documented that for oral gavage and intraperitoneal injection, mice were administered 100 μl of 1.4 mg/ml of sildenafil. For intraperitoneal injections of vardenafil, pharmaceutical grade vardenafil was stored frozen in DMSO at 1.5 mg/ml and diluted 1/10 in phosphate buffered saline (PBS) prior to injection (100 μl twice daily).
Linaclotide was prepared by grinding the contents of the capsules in di-H2O using a tissue homogenizer and stored in aliquots at -80°C. Mice were gavaged daily with 100 μl of the linaclotide suspension at a concentration of 2.07 μg/ml. Loperamide HCl was dissolved in di-H2O at a concentration of 2.5 mg/ml. Mice were gavaged with 100 μl of loperamide one hour prior to the intestinal transit assay.
Researchers observed that oral administration of sildenafil caused increased cGMP levels in mouse intestinal epithelium showing that blocking cGMP-breakdown is an alternative approach to increasing cGMP in the gut. Both linaclotide and sildenafil reduced proliferation and increased differentiation in the colon mucosa, indicating common target pathways. The homeostatic effects of cGMP required gut turnover since maximal effects were observed after 3 days of treatment. Neither linaclotide nor sildenafil treatment affected intestinal transit or water content of fecal pellets in healthy mice.
To test the effectiveness of cGMP elevation in a functional motility disorder model, mice were treated with dextran sulfate sodium (DSS) to induce colitis and were allowed to recover for several weeks. The recovered animals exhibited slower transit, but increased fecal water content. An acute dose of sildenafil was used to normalize transit and fecal water content in the DSS-recovery animal model, and also in loperamide-induced constipation. The higher fecal water content in the recovered animals was due to a compromised epithelial barrier, which was normalized by sildenafil treatment.
“We demonstrated that sildenafil can normalize bowel transit in both a post-inflammatory and opioid-induced constipation models, suggesting possible benefit for human patients afflicted with IBS-C, CIC, and OIC,” researchers noted. “Indeed, sildenafil is well-tolerated even long term in pediatric patients where it is used to treat pulmonary hypertension. Results described here suggest that sildenafil might also benefit pediatric patients suffering from CIC and IBS-C.”
Other authors affiliated with this study include Bianca N. Islam, Yali Hou, Margaux Usry, Allison Bridges, Nagendra Singh, Subbaramiah Sridhar, Satish Rao, and Darren D. Browning, of Augusta University in Augusta, GA.