the blood–brain barrier and most drugs designed to enter the brain will likewise traverse
the maternal–placental barrier.
1.1.2.3 Pharmacodynamic Phase
After a drug molecule has surmounted the barriers of the pharmacokinetic phase and
has been distributed throughout the body, it ultimately reaches the microenvironment of
the receptor where its biological effect will be exerted. Once the drug molecule has
entered the region of its receptor, it is in the pharmacodynamic phase. During this
phase, the molecule binds to its receptor through the complementarities of their mole-
cular geometries. The functional groups of the drug molecule interact with correspond-
ing functional groups of the receptor macromolecule via a variety of interactions,
including ion–ion, ion–dipole, dipole–dipole, aromatic–aromatic, and hydrogen bond-
ing interactions. The binding of the drug molecule to its receptor enables the desired
biological response to occur. The nature of the drug–receptor interaction is described in
detail in chapter 2.
DRUG MOLECULES: STRUCTURE AND PROPERTIES 17Figure 1.3 The three phases of drug processing. Different organ systems inflict varying degrees
of assault on the integrity of the drug molecule during its journey to the receptor. Stomach acid
initiates the assault. Liver enzymes may destroy the drug in a first pass effect. If the drug is too
polar, the kidney will rapidly excrete it.