The response time is plotted on a time–response curve that shows the onset
time response, the peak time response, and the duration. All three parameters are
used when administering the drug in order to determine the therapeutic range—
when the drug will become effective, when it will be most effective, and when
the drug is no longer effective. It is also used to determine when a drug is
expected to reach a toxic level.
For example, the time–response curve of an analgesic is used for pain man-
agement. Once the peak response time is reached, the effectiveness of the drug
to block pain diminishes. The time–response curve indicates when the phar-
maceutical response is no longer present requiring that an additional dose be
administered to the patient.RECEPTOR THEORY
The pharmaceutical response is realized when a drug binds to a receptor on the
cell membrane. These are referred to as reactive cellular sites. The activity of
the drug is determined by the drug’s ability to bind to a specific receptor. The
better the fit, the more biologically active the drug. Receptors are proteins,
glycoproteins, proteolipids, or enzymes. Depending on the drug, binding
either initiates a physiological response by the cell or blocks a cell’s physio-
logical response.
Receptors are classified into four families.1.Rapid-Cell Membrane-Embedded Enzymes: A drug binds to the sur-
face of the cell causing an enzyme inside the cell to initiate a physio-
logical response.
2.Rapid-Ligand-Gated Ion Channels: The drug spans the cell membrane
causing ion channels within the membrane to open resulting in the flow
of primarily sodium and calcium ions into and out of the cell.
3.Rapid-G Protein-Couple Receptor Systems: The drug binds with the
receptor causing the G protein to bind with guanosine triphosphate (GTP).
This in turn causes an enzyme inside the cell to initiate a physiological
response or causes the opening of the ion channel.
4.Prolonged-Transcription Factors: The drug binds to the transcription fac-
tors on the DNA within the nucleus of the cell and causes the transcript
factor to undergo a physiological change.A drug that causes a physiological response is called an agonistand a drug
that blocks a physiological response is referred to as an antagonist. The effect of(^34) CHAPTER 2 Drug Action and Drug Interactions