Absorption mechanism of drugs from the GI tract: Scientific and intuitive considerations

For assessing potential absorption behaviour of drugs from the GI tract, the following points may be useful:

  1. Drugs are preferentially absorbed as non-ionized (un-dissociated or un-protonated) drug species, therefore, solubility/concentration of the non-ionized species at the site of absorption is to be considered and not that of the ionized (protonated or salt) form. For example, drugs such as diltiazem, metoprolol and propranolol are considered highly water soluble, however, in reality, these are low solubility drugs. The reason for this discrepancy is that these drugs are available and administered as hydrochloride salts, which make them appear highly water soluble. However, following administration in humans, drugs are dissociated from the salt forms depending on the pH of the surrounding environment. They behave according to their native (intrinsic) basic forms, which usually have low aqueous solubilities. In vitro (e.g. for dissolution testing), these drugs may freely dissolve as a salt but in vivo these will behave as native (basic) low solubility drugs. Therefore, in reality in the terminology of BCS, such drugs should belong to the Class II and not Class I, as they are commonly referred to.
  2. Similarly, an acidic drug such as a propionic acid based NSAID e.g. naproxen as a sodium salt may provide high in vitro aqueous solubility, but would remain a low solubility drug as a native acid just like others such as ibuprofen and diclofenac.

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