This section will illustrate how the pH at either side of a barrier can affect transfer of ionizable drugs between compartments.
In many cases, transfer of a drug through a transcellular pathway only occurs efficiently for the fraction of the drug that is non-charged, since the compound needs to cross the lipid barrier. Charged forms of the drug (whether positively or negatively, do not significantly partake in the distribution equilibrium across the membrane). This will affect drug uptake, distribution, and elimination.
Clearly, protonation can change the character of a compound either from charged to non-charged, or vice versa. In the former case, the drug will be designated here as "acid" (since this situation is exemplified by carboxyl-groups). In the latter, as "basic" (amines are the most common instance of this situation).
ACID:
A¯
+ H+  HA
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BASE:
B + H +
 BH
+ |
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