Role of ionizaton and lipid solubility in drug absorption

ROLE OF IONIZATION AND LIPID SOLUBILITY IN DRUG ABSORPTION

  • Many drugs are weak electrolytes ie. they can ionize to a greater or lesser extent, according to environmental pH. Usually most molecules are present partly in the ionized and partly in the un-ionised state.
  • The degree of ionization influences lipid solubility and, in turn, their absorption, distribution and elimination.
  • The degree to which these drugs are lipid soluble (nonionized, the form in which drugs are able to cross membranes) is determined by their pKa and the pH of the medium containing the drug.
  • pKa of a drug is the pH at which 50% of the drug is ionized and 50% is non-ionized.
  • In monogastric animals with a low stomach pH, weak acids such as aspirin with a pKa of 3.5 tend to be better absorbed from the stomach than the weak bases because of the acidic conditions.
  • Weak bases are poorly absorbed from the stomach because they exist mostly in the ionized state in the acidic environment of the stomach . Weak bases are better absorbed from the small intestine where the environmental pH is more alkaline.
Last modified: Wednesday, 25 April 2012, 5:45 AM