VPT 311: General and Systemic Veterinary Pharmacology (2+1)

ABSORPTION OF DRUGS FROM THE GASTROINTESTINAL TRACT

• Before a drug can be absorbed, it must dissolve in the aqueous contents of the gut. Thus, the actual amount of the drug present in a dose is only one of the factors that will affect the amount of drug actually absorbed or available.
• Factors affecting absorption include
  • Molecular size of the drug and its concentration
  • Degree of ionization (depends on the pKa of the drug and pH of the medium)
  • Lipid solubility of the neutral or nonionized form of the drug.
  • Chemical or physical interaction with co-administered preparations and food constituents
  • Pharmaceutical preparation and dosage form, especially their disintegration rate and dissolution rate 
  • Gastric motility and secretion as well as gastric emptying
  • Intestinal motility and secretion as well as intestinal transit time
  • Fluid volume within the gastrointestinal tract
  • Osmolality of the intestinal contents
  • Intestinal blood and lymph flow
  • Disruption of the functional and structural integrity of the gastric and intestinal epithelium
  • Drug biotransformation within the intestinal lumen by microflora or within the mucosa by host enzymes
  • Volume and surface area of the absorbing surface. Stomach has a relatively small surface area compared to the duodenum. Hence absorption is more in the duodenum.
  • Presence of food in the stomach. Normally in full stomach there is a delay in absorption because the drug gets diluted in the stomach contents.