What is a defining characteristic of ion trapping?

Ion trapping is characterized by the fact that the degree of ionization of a drug is influenced by pH gradients across biological membranes. This concept is rooted in the principles of acid-base chemistry, particularly the Henderson-Hasselbalch equation, which describes how the ionization of weak acids and bases depends on the surrounding pH relative to their pKa values.

In environments where the pH differs on either side of a membrane, a drug can exist in both ionized and non-ionized forms. The non-ionized form typically has higher lipid solubility and can easily cross cell membranes, while the ionized form is more water-soluble and does not easily penetrate lipid membranes. For instance, in an acidic environment, a weak acid tends to remain in its non-ionized form and can diffuse across the membrane, whereas in a more basic environment, it becomes ionized and becomes "trapped."

This dynamic creates a situation where drugs may accumulate in compartments where the pH differs from where they were absorbed, illustrating the principle of ion trapping. This phenomenon is particularly significant in various physiological conditions, such as in different compartments of the body (stomach vs. blood vs. cells), and can play a crucial role in drug efficacy and toxicity.