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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The following links are for the short video clips demonstrating comparative operations of the paddle and the crescent-shape spindles.

Using a disintegrating tablet: Paddle and Crescent-shape spindle

Using a non-disintegrating tablet: Paddle and Crescent-shape spindle

Note: Dimensions may appear slightly distorted

In an earlier article (link) the mechanism of drug absorption was described considering ionization characteristics of drugs and the differences between the surface areas of the stomach and intestine. The purpose of the article was to explain and highlight how ionization of both an acidic and basic drug will impact in providing undissociated drug molecules both in the stomach and intestine. It is important to note that it is the undissociated drug molecules in the solution form which are responsible, and required, for drug absorption.

The low (acidic) pH of the stomach would favour high undissociated concentrations of acidic drugs compared to high (neutral or basic) pH of the intestine which will favour higher dissociated (ionized) concentrations. The opposite is true for basic drugs where low (acidic) pH of the stomach will result in higher concentrations of ionized or protonated basic drugs in the stomach compared to higher concentrations undissociated drugs in the intestine. The pH of the environments (stomach and intestine) explains only the ionization of drugs (acidic or basic) i.e. comparative availability of undissociated drug molecules in solution form but NOT the EXPECTED absorption of the drugs from these sites. The absorption of drugs, however, can only be explained based on the available surface areas of the stomach and intestine. As the intestine provides a much larger and efficient (permeable) surface, compared to stomach, thus it provides far superior and efficient drug absorption, as explained in the previous article. Continue reading →

This article provides an overview of mechanism of the drug absorption from the GI tract based on solubility/dissolution and dissociation/pH characteristics of a drug. It is argued that although pH values of the environment (stomach and intestine) may play a role, it is the availability of the large surface area of the intestine which predominantly is responsible for the drug absorption for both acidic and basic drugs. Furthermore, in the GI tract drugs exist in three forms i.e. solid (outside solvent/solution) and solid and ions in solution which are in equilibrium with one another. However, it is only the drug in solution form which is relevant for the absorption purpose. The roles of the interactions between drug (solid), drug/ions in solution and the surface areas are discussed in providing efficient drug absorption. Considering the absorption mechanism, the role of in vitro drug dissolution testing is also highlighted.Please click here for complete article