Overview of Pharmacology
Understanding the basics of pharmacolo- mechanisms of action. For example, the
gy is an essential nursing responsibility. therapeutic class antineoplastics can be
Pharmacology is the science that deals further classified as alkylating agents,
with the physical and chemical proper- antibiotic antineoplastics, antimetabo-
ties, and biochemical and physiologic lites, antimitotics, biological response
effects, of drugs. It includes the areas of
pharmacokinetics, pharmacodynamics,
pharmacotherapeutics, pharmacognosy, How drugs are named
and toxicodynamics. A drug’s chemical, generic, trade, and offi-
The 2011 Nurse’s Drug Handbook cial names are determined at different
phases of the drug development process
deals primarily with pharmacokinetics, and serve different functions. For exam-
pharmacodynamics, and pharmacothera- ple, the various names of the commonly
peutics—the information you need to prescribed anticonvulsant divalproex
administer safe and effective drug thera- sodium are:
py (discussed below). Pharmacognosy is • Chemical name: Pentanoic acid,
the branch of pharmacology that deals 2-propyl-, sodium salt (2:1) or
(C16H31O4Na)
with the biological, biochemical, and eco- • Generic name: divalproex sodium
nomic features of naturally occurring • Trade name: Depakote
drugs. Toxicodynamics is the study of the • Official name: Divalproex Sodium
harmful effects that excessive amounts of Delayed-Release Tablets, USP
a drug produce in the body; in a drug A drug’s chemical name describes its
overdose or drug poisoning, large drug atomic and molecular structure. The
chemical name of divalproex sodium—
doses may saturate or overwhelm normal pentanoic acid, 2-propyl-, sodium salt
mechanisms that control absorption, dis- (2:1), or C16H31O4Na (pronounced val-
tribution, metabolism, and excretion. proate semisodium)—indicates that the
drug is a combination of two valproic
Drug Nomenclature acid compounds with a sodium molecule
Most drugs are known by several attached to only one side.
names—chemical, generic, trade, and Once a drug successfully completes
official—each of which serves a specific several clinical trials, it receives a generic
function. (See How drugs are named.) name, also known as the nonproprietary
However, multiple drug names can also name. The generic name is usually
derived from but shorter than the chemi-
contribute to medication errors. You may cal name. The United States Adopted
find a familiar drug packaged with an Names Council is responsible for select-
unfamiliar name if your institution ing generic names, which are intended
changes suppliers or if a familiar drug is for unrestricted public use.
newly approved in a different dose or for Before submitting the drug for FDA
a new indication. approval, the manufacturer creates and
registers a trade name (or brand name)
Drug Classification when the drug appears ready to be mar-
Drugs can be classified in various ways. keted. Trade names are copyrighted and
Most pharmacology textbooks group followed by the symbol ® to indicate
that they’re registered and that their use
drugs by their functional classification, is restricted to the drug manufacturer.
such as psychotherapeutics, which is Once the original patent on a drug has
based on common characteristics. Drugs expired, any manufacturer may produce
can also be classified according to their the drug under its own trade name.
therapeutic use, such as antipanic or A drug’s official name is the name
antiobsessional drugs. Drugs within a under which it’s listed in the United
States Pharmacopoeia (USP) and the
certain therapeutic class may be further National Formulary (NF).
divided into subgroups based on their
,2 Overview of Parmacology
modifiers, antineoplastic enzymes, and large to readily diffuse may rely on active
hormonal antineoplastics. diffusion, in which special carriers on
molecules, including peptides, amino
Pharmacokinetics acids, and glucose, transport the drug
Pharmacokinetics is the study of a drug’s through the membranes. However, some
actions—or fate—as it passes through molecules with selective membrane carri-
the body during absorption, distribution, ers can expel foreign drug molecules; this
metabolism, and excretion. is why many drugs can’t cross the blood-
ABSORPTION brain barrier.
Before a drug can begin working, it must Drug absorption begins at the admin-
be transformed from its pharmaceutical istration route. The three main adminis-
dosage form to a biologically available tration route categories are enteral, par-
(bioavailable) substance that can pass enteral, and transcutaneous. Depending
through various biological cell mem- on its nature or chemical makeup, a drug
branes to reach its site of action. This may be better absorbed from one site
process is known as absorption. A drug’s than from another.
absorption rate depends on its route of Enteral Administration
administration, its circulation through Enteral administration consists of the
the tissue into which it’s administered, oral, nasogastric, and rectal routes.
and its solubility—that is, whether it’s Oral: Drugs administered orally are
more water-soluble (hydrophilic) or fat- absorbed in the GI tract and then pro-
soluble (lipophilic). ceed by the hepatic portal vein to the
Although drugs may penetrate cellular liver and into the systemic circulation.
membranes either actively or passively, Although generally considered the pre-
most drugs do so by passive diffusion, ferred route, oral drug administration
moving inertly from an area of higher has a number of disadvantages:
concentration to an area of lower con- • The oral route doesn’t always yield suffi-
centration. Passive diffusion may occur ciently high blood concentrations to be
through water or fat. Passive diffusion effective.
through water—aqueous diffusion— • Bioavailability may be less than optimal
occurs within large water-filled compart- because of incomplete absorption and
ments, such as interstitial spaces, and first-pass elimination (the part of
across epithelial membrane tight junc- metabolism that occurs during transit
tions and pores in the epithelial lining of through the liver before the drug reach-
blood vessels. Aqueous diffusion is driven es the general circulation).
by concentration gradients. Drug mole- • Drug absorption may be incomplete if
cules that are bound to large plasma pro- the drug is degraded by digestive
teins, such as albumin, are too large to enzymes or the acidic pH in the stom-
pass through aqueous pores in this way. ach or if it’s excreted from the liver into
Passive diffusion through fat—lipid diffu- the bile.
sion—plays an important role in drug • Food in the GI tract, gastric emptying
metabolism because of the large number time, and intestinal motility may also
of lipid barriers that separate the aqueous impede drug absorption.
compartments of the body. The tendency Nasogastric: Drugs administered
of a drug to move through lipid layers through a nasogastric tube enter the
between aqueous compartments often stomach directly and are absorbed in the
depends on the pH of the medium—that GI tract.
is, the ability of the water-soluble or fat- Rectal: Rectal drugs and suppositories
soluble drug to form weak acid or weak also enter the GI tract directly after being
base. inserted in the rectum and absorbed
Drugs with molecules that are too through the rectal mucosa. After being
, Overview of Pharmacology 3
absorbed into the lower GI tract, rectal allows drug absorption through the skin
drugs enter the circulation through the or soft-tissue surface. Drugs may be
inferior vena cava, bypassing the liver and inhaled, inserted sublingually, applied
thus avoiding first-pass metabolism. topically, or administered by the eyes,
Suppositories, however, tend to travel ears, nose, or vagina.
upward into the rectum, where veins, Inhalation: Inhaled drugs may be given
such as the superior hemorrhoidal vein, as a powder and aerosolized or mixed in
lead to the liver. As a result, drug absorp- solution and nebulized directly into the
tion by this route is often unreliable and respiratory tract, where they’re absorbed
difficult to predict. through the alveoli. Inhaled drugs are
usually absorbed quickly because of the
Parenteral Administration
abundant blood flow in the lungs.
Parenteral routes may be used whenever
Sublingual: Sublingual drug adminis-
enteral routes are contraindicated or
tration involves placing a tablet, troche,
inadequate. These routes include intra-
or lozenge under the tongue. The drug is
muscular (I.M.), intravenous (I.V.), sub-
absorbed across the epithelial lining of
cutaneous (SubQ) and intradermal (I.D.)
the mouth, usually quickly. This route
administration. Drug absorption is much
avoids first-pass metabolism.
faster and more predictable after par-
Topical: Topical drugs—creams, oint-
enteral administration than after enteral
ments, lotions, and patches—are placed
administration.
on the skin and then cross the epidermis
I.M.: Drugs administered by the I.M.
into the capillary circulation. They may
route are injected deep into the muscle,
also be absorbed through sweat glands,
where they’re absorbed relatively quickly.
hair follicles, and other skin structures.
The rate of drug absorption depends on
Absorption by the skin is enhanced if the
the vascularity of the injection site, the
drug is in a solution.
physiochemical properties of the drug,
Ophthalmic: Ophthalmic drugs include
and the solution in which the drug is
solutions and ointments that are instilled
contained.
or applied directly to the cornea or con-
I.V.: I.V. drug administration involves
junctiva as well as small, elliptical disks
injecting or infusing the drug directly
that are placed directly on the eyeball
into the blood circulation, allowing for
behind the lower eyelid. The movements
rapid distribution throughout the body.
of the eyeball promote distribution of
This route usually provides the greatest
these drugs over the surface of the eye.
bioavailability.
Although ophthalmic drugs produce a
SubQ: Drugs administered by the sub-
local effect on the conjunctiva or anterior
cutaneous route are injected into the
chamber, some preparations may be
alveolar connective tissue just below the
absorbed systemically and therefore pro-
skin and are absorbed by simple diffusion
duce systemic effects.
from the injection site. The factors that
Otic: Drops administered into the
affect I.M. absorption also affect subcuta-
external auditory canal, otic drugs are
neous absorption. Absorption by the sub-
used to treat infection or inflammation
cutaneous route may be slower than by
and to soften and remove ear wax. Otic
the I.M. route.
solutions exert a local effect and may
I.D.: Drugs administered intradermally,
result in minimal systemic absorption
such as purified protein derivative (PPD),
with no adverse effects.
are injected into the dermis, from which
Nasal: Nasal solutions and suspensions
they diffuse slowly into the local micro-
are applied directly to the nasal mucosa
capillary system.
by instillation or inhalation to produce
Transcutaneous Administration local effects, such as vasoconstriction to
Transcutaneous drug administration reduce nasal congestion. Some nasal
, 4 Overview of Pharmacology
solutions, such as vasopressin, are admin- metabolism and excretion. Drug metabo-
istered by this route specifically to pro- lites—and, in some cases, the active drug
duce systemic effects. itself—are eventually excreted from the
Vaginal: Vaginal drugs include creams, body, usually through bile, feces, and
suppositories, and troches that are insert- urine. The primary organ for drug elimi-
ed into the vagina, sometimes using a nation is the kidney. Impaired renal func-
special applicator. These drugs are tion may alter drug elimination, thereby
absorbed locally to treat such conditions altering the drug’s therapeutic effect.
as bacterial and fungal infections. Other excretion routes include evapora-
DISTRIBUTION tion through the skin, exhalation from
Distribution is the process by which a the lungs, and secretion into saliva and
drug is transported by the circulating flu- breast milk.
ids to various sites, including its sites of A drug’s elimination half-life is the
action. To ensure maximum therapeutic amount of time required for half of the
effectiveness, the drug must permeate all drug to be eliminated from the body. The
membranes that separate it from its half-life roughly correlates with the
intended site of action. Drug distribution drug’s duration of action and is based on
is influenced by blood flow, tissue avail- normal renal and hepatic function.
ability, and protein binding. Typically, the longer the half-life, the less
often the drug has to be given and the
METABOLISM
longer it remains in the body.
Drug metabolism is the enzymatic con-
version of a drug’s structure into sub- Pharmacodynamics
strate molecules or polar compounds Pharmacodynamics is the study of the
that are either less active or inactive and biochemical and physiologic effects of
are readily excreted. Drugs can also be drugs and their mechanisms of action. A
synthesized to larger molecules. drug’s actions may be structurally specific
Metabolism may also convert a drug to a or nonspecific. Structurally specific drugs
more toxic compound. Because the pri- combine with cell receptors, such as pro-
mary site of drug metabolism is the liver, teins or glycoproteins, to enhance or
children, the elderly, and patients with inhibit cellular enzyme actions. Drug
impaired hepatic function are at risk for receptors are the cellular components
altered therapeutic effects. affected at the site of action. Many drugs
Biotransformation is the process of form chemical bonds with drug recep-
changing a drug into its active metabo- tors, but a drug can bond with a receptor
lite. Compounds that require metabolic only if it has a similar shape—much the
biotransformation for activation are same way that a key fits into a lock.
known as prodrugs. During phase I of When a drug combines with a receptor,
biotransformation, the parent drug is channels are either opened or closed and
converted into an inactive or partially cellular biochemical messengers, such as
active metabolite. Much of the original cyclic adenosine monophosphate or cal-
drug may be eliminated during this cium ions, are activated. Once activated,
phase. During phase II, the inactive or cellular functions can be turned either on
partially active metabolite binds with or off by these messengers.
available substrates, such as acetic acid, Structurally nonspecific drugs, such as
glucuronic acid, sulfuric acid, or water, to biological response modifiers, don’t com-
form its active metabolite. When bio- bine with cell receptors; rather, they pro-
transformation leads to synthesis, larger duce changes within the cell membrane
molecules are produced to create a phar- or interior.
macologic effect. The mechanisms by which drugs inter-
EXCRETION act with the body are not always known.
The body eliminates drugs by both Drugs may work by physical action (such
Understanding the basics of pharmacolo- mechanisms of action. For example, the
gy is an essential nursing responsibility. therapeutic class antineoplastics can be
Pharmacology is the science that deals further classified as alkylating agents,
with the physical and chemical proper- antibiotic antineoplastics, antimetabo-
ties, and biochemical and physiologic lites, antimitotics, biological response
effects, of drugs. It includes the areas of
pharmacokinetics, pharmacodynamics,
pharmacotherapeutics, pharmacognosy, How drugs are named
and toxicodynamics. A drug’s chemical, generic, trade, and offi-
The 2011 Nurse’s Drug Handbook cial names are determined at different
phases of the drug development process
deals primarily with pharmacokinetics, and serve different functions. For exam-
pharmacodynamics, and pharmacothera- ple, the various names of the commonly
peutics—the information you need to prescribed anticonvulsant divalproex
administer safe and effective drug thera- sodium are:
py (discussed below). Pharmacognosy is • Chemical name: Pentanoic acid,
the branch of pharmacology that deals 2-propyl-, sodium salt (2:1) or
(C16H31O4Na)
with the biological, biochemical, and eco- • Generic name: divalproex sodium
nomic features of naturally occurring • Trade name: Depakote
drugs. Toxicodynamics is the study of the • Official name: Divalproex Sodium
harmful effects that excessive amounts of Delayed-Release Tablets, USP
a drug produce in the body; in a drug A drug’s chemical name describes its
overdose or drug poisoning, large drug atomic and molecular structure. The
chemical name of divalproex sodium—
doses may saturate or overwhelm normal pentanoic acid, 2-propyl-, sodium salt
mechanisms that control absorption, dis- (2:1), or C16H31O4Na (pronounced val-
tribution, metabolism, and excretion. proate semisodium)—indicates that the
drug is a combination of two valproic
Drug Nomenclature acid compounds with a sodium molecule
Most drugs are known by several attached to only one side.
names—chemical, generic, trade, and Once a drug successfully completes
official—each of which serves a specific several clinical trials, it receives a generic
function. (See How drugs are named.) name, also known as the nonproprietary
However, multiple drug names can also name. The generic name is usually
derived from but shorter than the chemi-
contribute to medication errors. You may cal name. The United States Adopted
find a familiar drug packaged with an Names Council is responsible for select-
unfamiliar name if your institution ing generic names, which are intended
changes suppliers or if a familiar drug is for unrestricted public use.
newly approved in a different dose or for Before submitting the drug for FDA
a new indication. approval, the manufacturer creates and
registers a trade name (or brand name)
Drug Classification when the drug appears ready to be mar-
Drugs can be classified in various ways. keted. Trade names are copyrighted and
Most pharmacology textbooks group followed by the symbol ® to indicate
that they’re registered and that their use
drugs by their functional classification, is restricted to the drug manufacturer.
such as psychotherapeutics, which is Once the original patent on a drug has
based on common characteristics. Drugs expired, any manufacturer may produce
can also be classified according to their the drug under its own trade name.
therapeutic use, such as antipanic or A drug’s official name is the name
antiobsessional drugs. Drugs within a under which it’s listed in the United
States Pharmacopoeia (USP) and the
certain therapeutic class may be further National Formulary (NF).
divided into subgroups based on their
,2 Overview of Parmacology
modifiers, antineoplastic enzymes, and large to readily diffuse may rely on active
hormonal antineoplastics. diffusion, in which special carriers on
molecules, including peptides, amino
Pharmacokinetics acids, and glucose, transport the drug
Pharmacokinetics is the study of a drug’s through the membranes. However, some
actions—or fate—as it passes through molecules with selective membrane carri-
the body during absorption, distribution, ers can expel foreign drug molecules; this
metabolism, and excretion. is why many drugs can’t cross the blood-
ABSORPTION brain barrier.
Before a drug can begin working, it must Drug absorption begins at the admin-
be transformed from its pharmaceutical istration route. The three main adminis-
dosage form to a biologically available tration route categories are enteral, par-
(bioavailable) substance that can pass enteral, and transcutaneous. Depending
through various biological cell mem- on its nature or chemical makeup, a drug
branes to reach its site of action. This may be better absorbed from one site
process is known as absorption. A drug’s than from another.
absorption rate depends on its route of Enteral Administration
administration, its circulation through Enteral administration consists of the
the tissue into which it’s administered, oral, nasogastric, and rectal routes.
and its solubility—that is, whether it’s Oral: Drugs administered orally are
more water-soluble (hydrophilic) or fat- absorbed in the GI tract and then pro-
soluble (lipophilic). ceed by the hepatic portal vein to the
Although drugs may penetrate cellular liver and into the systemic circulation.
membranes either actively or passively, Although generally considered the pre-
most drugs do so by passive diffusion, ferred route, oral drug administration
moving inertly from an area of higher has a number of disadvantages:
concentration to an area of lower con- • The oral route doesn’t always yield suffi-
centration. Passive diffusion may occur ciently high blood concentrations to be
through water or fat. Passive diffusion effective.
through water—aqueous diffusion— • Bioavailability may be less than optimal
occurs within large water-filled compart- because of incomplete absorption and
ments, such as interstitial spaces, and first-pass elimination (the part of
across epithelial membrane tight junc- metabolism that occurs during transit
tions and pores in the epithelial lining of through the liver before the drug reach-
blood vessels. Aqueous diffusion is driven es the general circulation).
by concentration gradients. Drug mole- • Drug absorption may be incomplete if
cules that are bound to large plasma pro- the drug is degraded by digestive
teins, such as albumin, are too large to enzymes or the acidic pH in the stom-
pass through aqueous pores in this way. ach or if it’s excreted from the liver into
Passive diffusion through fat—lipid diffu- the bile.
sion—plays an important role in drug • Food in the GI tract, gastric emptying
metabolism because of the large number time, and intestinal motility may also
of lipid barriers that separate the aqueous impede drug absorption.
compartments of the body. The tendency Nasogastric: Drugs administered
of a drug to move through lipid layers through a nasogastric tube enter the
between aqueous compartments often stomach directly and are absorbed in the
depends on the pH of the medium—that GI tract.
is, the ability of the water-soluble or fat- Rectal: Rectal drugs and suppositories
soluble drug to form weak acid or weak also enter the GI tract directly after being
base. inserted in the rectum and absorbed
Drugs with molecules that are too through the rectal mucosa. After being
, Overview of Pharmacology 3
absorbed into the lower GI tract, rectal allows drug absorption through the skin
drugs enter the circulation through the or soft-tissue surface. Drugs may be
inferior vena cava, bypassing the liver and inhaled, inserted sublingually, applied
thus avoiding first-pass metabolism. topically, or administered by the eyes,
Suppositories, however, tend to travel ears, nose, or vagina.
upward into the rectum, where veins, Inhalation: Inhaled drugs may be given
such as the superior hemorrhoidal vein, as a powder and aerosolized or mixed in
lead to the liver. As a result, drug absorp- solution and nebulized directly into the
tion by this route is often unreliable and respiratory tract, where they’re absorbed
difficult to predict. through the alveoli. Inhaled drugs are
usually absorbed quickly because of the
Parenteral Administration
abundant blood flow in the lungs.
Parenteral routes may be used whenever
Sublingual: Sublingual drug adminis-
enteral routes are contraindicated or
tration involves placing a tablet, troche,
inadequate. These routes include intra-
or lozenge under the tongue. The drug is
muscular (I.M.), intravenous (I.V.), sub-
absorbed across the epithelial lining of
cutaneous (SubQ) and intradermal (I.D.)
the mouth, usually quickly. This route
administration. Drug absorption is much
avoids first-pass metabolism.
faster and more predictable after par-
Topical: Topical drugs—creams, oint-
enteral administration than after enteral
ments, lotions, and patches—are placed
administration.
on the skin and then cross the epidermis
I.M.: Drugs administered by the I.M.
into the capillary circulation. They may
route are injected deep into the muscle,
also be absorbed through sweat glands,
where they’re absorbed relatively quickly.
hair follicles, and other skin structures.
The rate of drug absorption depends on
Absorption by the skin is enhanced if the
the vascularity of the injection site, the
drug is in a solution.
physiochemical properties of the drug,
Ophthalmic: Ophthalmic drugs include
and the solution in which the drug is
solutions and ointments that are instilled
contained.
or applied directly to the cornea or con-
I.V.: I.V. drug administration involves
junctiva as well as small, elliptical disks
injecting or infusing the drug directly
that are placed directly on the eyeball
into the blood circulation, allowing for
behind the lower eyelid. The movements
rapid distribution throughout the body.
of the eyeball promote distribution of
This route usually provides the greatest
these drugs over the surface of the eye.
bioavailability.
Although ophthalmic drugs produce a
SubQ: Drugs administered by the sub-
local effect on the conjunctiva or anterior
cutaneous route are injected into the
chamber, some preparations may be
alveolar connective tissue just below the
absorbed systemically and therefore pro-
skin and are absorbed by simple diffusion
duce systemic effects.
from the injection site. The factors that
Otic: Drops administered into the
affect I.M. absorption also affect subcuta-
external auditory canal, otic drugs are
neous absorption. Absorption by the sub-
used to treat infection or inflammation
cutaneous route may be slower than by
and to soften and remove ear wax. Otic
the I.M. route.
solutions exert a local effect and may
I.D.: Drugs administered intradermally,
result in minimal systemic absorption
such as purified protein derivative (PPD),
with no adverse effects.
are injected into the dermis, from which
Nasal: Nasal solutions and suspensions
they diffuse slowly into the local micro-
are applied directly to the nasal mucosa
capillary system.
by instillation or inhalation to produce
Transcutaneous Administration local effects, such as vasoconstriction to
Transcutaneous drug administration reduce nasal congestion. Some nasal
, 4 Overview of Pharmacology
solutions, such as vasopressin, are admin- metabolism and excretion. Drug metabo-
istered by this route specifically to pro- lites—and, in some cases, the active drug
duce systemic effects. itself—are eventually excreted from the
Vaginal: Vaginal drugs include creams, body, usually through bile, feces, and
suppositories, and troches that are insert- urine. The primary organ for drug elimi-
ed into the vagina, sometimes using a nation is the kidney. Impaired renal func-
special applicator. These drugs are tion may alter drug elimination, thereby
absorbed locally to treat such conditions altering the drug’s therapeutic effect.
as bacterial and fungal infections. Other excretion routes include evapora-
DISTRIBUTION tion through the skin, exhalation from
Distribution is the process by which a the lungs, and secretion into saliva and
drug is transported by the circulating flu- breast milk.
ids to various sites, including its sites of A drug’s elimination half-life is the
action. To ensure maximum therapeutic amount of time required for half of the
effectiveness, the drug must permeate all drug to be eliminated from the body. The
membranes that separate it from its half-life roughly correlates with the
intended site of action. Drug distribution drug’s duration of action and is based on
is influenced by blood flow, tissue avail- normal renal and hepatic function.
ability, and protein binding. Typically, the longer the half-life, the less
often the drug has to be given and the
METABOLISM
longer it remains in the body.
Drug metabolism is the enzymatic con-
version of a drug’s structure into sub- Pharmacodynamics
strate molecules or polar compounds Pharmacodynamics is the study of the
that are either less active or inactive and biochemical and physiologic effects of
are readily excreted. Drugs can also be drugs and their mechanisms of action. A
synthesized to larger molecules. drug’s actions may be structurally specific
Metabolism may also convert a drug to a or nonspecific. Structurally specific drugs
more toxic compound. Because the pri- combine with cell receptors, such as pro-
mary site of drug metabolism is the liver, teins or glycoproteins, to enhance or
children, the elderly, and patients with inhibit cellular enzyme actions. Drug
impaired hepatic function are at risk for receptors are the cellular components
altered therapeutic effects. affected at the site of action. Many drugs
Biotransformation is the process of form chemical bonds with drug recep-
changing a drug into its active metabo- tors, but a drug can bond with a receptor
lite. Compounds that require metabolic only if it has a similar shape—much the
biotransformation for activation are same way that a key fits into a lock.
known as prodrugs. During phase I of When a drug combines with a receptor,
biotransformation, the parent drug is channels are either opened or closed and
converted into an inactive or partially cellular biochemical messengers, such as
active metabolite. Much of the original cyclic adenosine monophosphate or cal-
drug may be eliminated during this cium ions, are activated. Once activated,
phase. During phase II, the inactive or cellular functions can be turned either on
partially active metabolite binds with or off by these messengers.
available substrates, such as acetic acid, Structurally nonspecific drugs, such as
glucuronic acid, sulfuric acid, or water, to biological response modifiers, don’t com-
form its active metabolite. When bio- bine with cell receptors; rather, they pro-
transformation leads to synthesis, larger duce changes within the cell membrane
molecules are produced to create a phar- or interior.
macologic effect. The mechanisms by which drugs inter-
EXCRETION act with the body are not always known.
The body eliminates drugs by both Drugs may work by physical action (such
