CHAPTER
CHAPTER 1
Drug Action:
Pharmaceutic,
Pharmacokinetic, and
Pharmacodynamic Phases
Drug Action
OUTLINE
Tablets are not 100% drug
Pharmaceutic Phase
Excipients are used in drug
Pharmacokinetic Phase preparation to allow the drug to take
1. Absorption on a particular size and shape and to
2. Distribution enhance drug dissolution.
3. Protein Binding Drug additives such as the ions
4. Metabolism, or Biotransformation potassium (K) and sodium (Na) in
5. Excretion, or Elimination penicillin potassium and penicillin
Pharmacodynamic Phase sodium, increase the absorbability of
the drug.
1. Dose Response and Maximal Efficacy
2. Onset, Peak, and Duration of Action
3. Receptor Theory Note: penicillin is poorly absorbed by the GI
tract because of gastric acid. However, by
4. Therapeutic Index and Therapeutic
making the drug a potassium or sodium salt,
Range (Therapeutic
penicillin can then be absorbed.
5. Window)
3 Phases wherein a drug or capsule Disintegration - is the breakdown of a tablet
undergoes into smaller particles.
1. Pharmaceutic
2. Pharmacokinetic Dissolution - is the dissolving of the smaller
3. Pharmacodynamic particles in the GI fluid before absorption.
Rate of dissolution - is the time it takes the
Note: Not all drugs undergo all 3 phases. For drug to disintegrate and dissolve to become
example, When the drug is administered available for the body to absorb it.
parenterally by subcutaneous (subQ),
intramuscular (IM), or intravenous (IV) routes,
Note: Generally, drugs are both disintegrated
there is no pharmaceutic phase.
and absorbed faster in acidic fluids with a pH
PHARMACEUTIC PHASE of 1 or 2 rather than in alkaline fluids.
The pharmaceutic phase (dissolution) Alkaline drugs would become ionized and
is the first phase of drug action. have difficulty crossing cell membrane
Occurs in the GI tract barriers. Both the very young and older adults
Drugs need to be in solution have less gastric acidity; therefore, drug
so that they can be absorbed. absorption is generally slower for those drugs
drug in solid form (tablet or absorbed primarily in the stomach.
capsule) must disintegrate into small
particles to dissolve into a liquid, a
process known as dissolution.
Absorption is reduced if the villi are
decreased in number because of
Enteric-coated drugs – drugs that resist
disease, drug effect, or the removal of
disintegration in the gastric acid of the
small intestine.
stomach, so disintegration does not occur
Protein-based drugs such as insulin
until the drug reaches the alkaline
and growth hormones are destroyed
environment of the small intestine.
in the small intestine by digestive
Enteric-coated tablets can remain in the enzymes.
stomach for a long time; therefore, their
Processes of Drug Absorption
effect may be delayed in onset.
Passive Absorption
Enteric-coated tablets or capsules and
sustained-release(beaded) capsules should Absorption occurs mostly by diffusion
not be crushed. Crushing would alter the (movement from higher
place and time of absorption of the drug. concentration to lower concentration)
With diffusion, the drug does not
Food in the GI tract may interfere with the
require energy to move across the
dissolution of certain drugs. Some drugs
membrane
irritate the gastric mucosa, so fluids or food
may be necessary to dilute the drug Active Absorption
concentration and to act as protectants.
Requires a carrier such as an enzyme
PHARMACOKINETIC PHASE or protein to move the drug against a
concentration gradient
Pharmacokinetics is the process of
Energy is required for active
drug movement to achieve drug
absorption.
action.
The four processes are absorption,
distribution, metabolism (or
Pinocytosis
biotransformation), and excretion (or
elimination). Is a process by which cells carry a drug
The nurse applies knowledge of across their membrane by engulfing
pharmacokinetics when assessing the the drug particles
patient for possible adverse drug
Drug Action
effects.
The nurse communicates assessment The GI membrane is composed mostly of lipid
findings to members of the health (fat) and protein, so drugs that are lipid
care team in a timely manner to soluble pass rapidly through the GI
promote safe and effective drug membrane. Water-soluble drugs need a
therapy for the patient. carrier, either enzyme or protein, to pass
through the membrane.
Absorption
Large particles pass through the cell
It is the movement of drug particles
membrane if they are nonionized (have no
from the GI tract to body fluids by
positive or negative charge). Weak acid drugs
passive absorption, active absorption,
such as aspirin are less ionized in the
or pinocytosis.
stomach, and they pass through the stomach
Most oral drugs are absorbed into the
lining easily and rapidly.
surface area of the small intestine
through the action of the extensive
mucosal villi.
An infant’s gastric secretions have a Some drugs do not go directly into the
higher pH (alkaline) than those of systemic circulation following oral absorption
adults; therefore, infants can absorb but pass from the intestinal lumen to the liver
more penicillin. via the portal vein.
Certain drugs such as calcium
carbonate and many of the
antifungals need an acidic
environment to achieve greater drug
absorption; thus food can stimulate
the production of gastric acid.
Hydrochloric acid destroys some
drugs such as penicillin G; therefore, a
large oral dosage of penicillin is
needed to offset the partial dose loss.
Drugs administered by many routes
do not pass through the GI tract or
liver. These include parenteral drugs,
eyedrops, eardrops, nasal sprays,
respiratory inhalants, transdermal
drugs, and sublingual drugs.
Remember, drugs that are lipid
soluble and nonionized are absorbed
faster than water-soluble and ionized
drugs.
Poor circulation to the stomach as a
result of shock, vasoconstrictor drugs,
or disease hampers absorption.
Pain, stress, and foods that are solid,
hot, or high in fat can slow gastric
emptying time, so the drug remains in
the stomach longer.
Exercise can decrease blood flow by
causing more blood to flow to the
peripheral muscle, thereby decreasing
blood circulation to the GI tract.
Drugs given IM are absorbed faster in
muscles that have more blood vessels
(e.g., deltoids) than in those that have
fewer blood vessels (e.g., gluteals).
Subcutaneous tissue has fewer blood
vessels, so absorption is slower in
such tissue.
