Figure 1.
The ester prodrug concept. Carboxylate-containing drugs are poorly permeable due to Coulombic repulsion. Masking the negative charge makes the drug more readily permeable in a nonspecific fashion. The active drug is revealed only in cells that contain an enzyme capable of hydrolyzing the ester bond to reveal the active moiety. In a targeted prodrug strategy, only cells that contain such an esterase would incur the effects of the drug.