Screening of Antidiarrhoeal Drugs

Screening of Antidiarrhoeal Drugs

1. Definition, scope & rationale of antidiarrhoeal screening

• Antidiarrhoeal screening = the battery of in vivo and in vitro preclinical assays used to detect and quantify a test compound's ability to normalise experimentally induced diarrhoea, by acting on one or both of the two limbs that generate diarrhoea — intestinal hypersecretion (fluid/electrolyte) and hypermotility (accelerated propulsion) (Vogel 4e V3 Part XI, pp.2415–2422).
• Diarrhoea is experimentally reproduced by two mechanistic families of inducers, and the model chosen must match the mechanism to be probed (Vogel 4e V3 Part XI, pp.2416–2421):
  • Secretory diarrhoea inducers — prostaglandins (PGE₂ and its stable analogue 16,16-dimethyl-PGE₂), castor oil / ricinoleic acid, magnesium sulphate, bile and bile salts (taurocholate, taurochenodeoxycholate), cholera toxin, carbachol.
  • Motility / propulsion accelerators — sennosides and other stimulant laxatives that accelerate large-intestinal transit.
• A valid antidiarrhoeal assay should ideally discriminate the antisecretory component from the antipropulsive (antimotility) component of a drug's action, because a compound can be antipropulsive without being antisecretory (e.g. some opioids) — the castor-oil test in rats was explicitly proposed as an in vivo method able to evaluate both antipropulsive and antisecretory activity (Niemegeers et al. 1984, in Vogel 4e V3 Part XI, p.2419).
• Reference (standard) drugs used as positive controls across these models are the μ-opioid antidiarrhoeals loperamide and diphenoxylate/difenoxine, the classical opiate morphine/codeine, the antimuscarinic atropine (and methylscopolamine), the α₂-adrenoceptor agonists clonidine and SK&F 35886, and chlorpromazine — each reducing faecal output and/or prolonging the diarrhoea-free period (Vogel 4e V3 Part XI, pp.2416–2422).
• ⚠ Scope note: this topic covers experimental evaluation and screening of antidiarrhoeal drugs only (models, apparatus, inducers, endpoints, reference standards); it does not cover clinical antidiarrhoeal pharmacotherapy (drug classes, doses, clinical uses, ADRs) — that is a separate clinical topic.

Two-limb screening framework (secretion vs motility)

• Assays are organised in Vogel Part XI under three functional headings that map onto the two limbs plus the shared readout (Vogel 4e V3 Part XI, pp.2415–2432):
  • Intestinal Secretion — Laxative Activity in Rats; Enteropooling Test; Inhibition of Chloride Secretion in Rabbit Colon (Ussing chamber).
  • Antidiarrhoeal Effect (composite in-vivo diarrhoea) — Castor Oil-Induced Diarrhoea; Antidiarrhoeal Effect in Cecectomised Rats; Antidiarrhoeal Effect in Cold-Restrained Rats.
  • Gut Motility / propulsion — Isolated Ileum (Magnus technique); Propulsive Gut Motility (charcoal-meal transit); Colon Motility; In-vivo spasmolytic assay.