Antisecretory drugs

Drugs Acting on Digestive System

Antidiarrhoeals

Intestinal Antiseptics

These agents are used to treat severe diarrhoeal forms, which are due to microbial infection. They mainly comprise of certain members of the sulphonamides and antibiotics that are poorly absorbable in the GIT, and thus, reach in high concentrations to the small and large bowels. Examples are sulphasalazine, sulphaguanidine, phthalyl sulphathiazole, succinyl sulphathiazole. Various combinations of sulphonamides and antibiotics along with kaolin are available either in the form of cream or suspension. Streptomycin, neomycin, chloramphenicol, tetracyclines and nystatin are the examples of such antibiotics used for this purpose.

Antisecretory drugs

i. Sulphasalazine (salicylazosulphapyridine)

Synthesis

Metabolism: It undergoes reductive metabolism by gut bacteria, converting the drug into sulphapyridine and 5-amino salicylic acid, which are the active components.

Properties and uses: Sulphasalazine is a bright yellow or brownish-yellow fine powder, practically insoluble in water and methylene chloride and sparingly soluble in alcohol. It dissolves in dilute solutions of alkali hydroxides and used in the treatment of inflammatory bowel diseases such as ulcerative colitis.

Assay: Dissolve and dilute the sample in 0.1 M sodium hydroxide and add 0.1 M acetic acid and measure the absorbance at the maxima of 359 nm using ultraviolet spectrophotometer.

Dosage forms: Sulphasalazine tablets B.P.

ii. Bismuth subsalicylate

It acts by decreasing prostaglandin synthesis in the intestinal mucosa, thereby reducing Cl– secretion. It has some prophylatic value in traveller’s diarrhoea (probably due to weak antibacterial action also), but it is rather inconvenient to carry and take.

iii. Atropine: Atropinic drugs can reduce bowel motility and secretion, but have poor efficacy in secretory diarrhoeas. They may benefit nervous/drug (neostigmine, metaclopramide, reserpine) induced diarrhoeas and provide some symptomatic relief in dysenteries diverticulitis.

iv. Octreotide: This somatostatin analogue has a long plasma as well as potent antisecretory/antimotil ity action on the gut. It has been used to control diarrhoea in carcinoid and vasoactive intestinal peptide secreting tumours and for refractory diarrhoea in AIDS patients, but needs to be given by subcutaneous injections.

v. Racecadotril: This recently introduced prodrug is rapidly converted to thiorphan, an enkephalinase inhibitor. It prevents the degradation of endogenous enkephalins, which are mainly δ opioid receptor agonists. Racecadotril decreases intestinal hypersecretion, without affecting motility by lowering mucosal cAMP due to enhanced enkephalins action. It is indicated in the short-term treatment of acute secretory diarrhoeas.