the drug entering the bloodstream and being distributed throughout the entirety of the
body, or “locally,” which involves site-specific administration directly onto the region of
pathology. Systemic administration may be achieved by the following routes: (1) via the
gastrointestinal tract (usually orally, sometimes rectally); (2) parenterally, using intra-
venous, sub-cutaneous, intramuscular, or (rarely) intra-arterial injection; (3) topically,
in which the drug is applied to the skin and is absorbed transdermally into the body to
be widely distributed via the bloodstream; or (4) by direct inhalation into the lungs.
The most frequent route of administration is oral. From the perspective of a drug
designer who is endeavoring to engineer drug molecules, many factors must be taken
into consideration when designing a drug for oral administration. On its journey from
the mouth (the point of first administration) to the drug’s receptor deep within the organ
systems of the body, the drug molecule undergoes a variety of potential assaults to the
integrity of its chemical structure. This attack begins in the mouth where saliva contains
digestive enzymes such as ptyalin or salivary α-amylase. The drug molecule next enters
DRUG MOLECULES: STRUCTURE AND PROPERTIES 11Figure 1.1 Drug-like molecules and druggable targets. Certain properties permit a molecule to
become a drug-like molecule and certain properties permit a macromolecule to become a drug-
gable target. When a drug-like molecule interacts with a druggable target to give a biological
response, it becomes a drug molecule and the druggable target becomes a receptor. When a drug
molecule is successfully and beneficially distributed to people with a disease, it becomes a useful
drug molecule.