the stomach at which point it is subjected to a pH of 1.8–2.2, as well as to a variety of
pepsin enzymes. Under such acidic conditions, certain functional groups, such as esters,
are vulnerable to hydrolysis—an important point of consideration during drug design.
From the stomach, the drug molecule sequentially enters the three portions of the small
intestine: duodenum, jejunum, and ileum. Within the small intestine the pH is alkalin-
ized to 7.8–8.4, and the drug molecule is subjected to a complex array of intestinal and
pancreatic enzymes including peptidases, elastase, lipases, amylase, lactase, sucrase,
12 MEDICINAL CHEMISTRY
Figure 1.2 The three phases of drug processing. The journey from the point of administration to
the microenvironment of the receptor is a complex and arduous journey for the drug molecule.
(Adapted from D. G. Grahame-Smith, J. K. Aronson (2002). Clinical Pharmacology and Drug
Therapy, 3rd Edn. New York: Oxford University Press. With permission.)