Pharmacokinetics
Pharmacokinetics examines the movement of a drug over time through the body.
Routes of drug administration
A. Enteral
Oral: the most common. Most drugs absorbed from the GI tract enter the portal circulation and
encounter the liver before they are distributed into the general circulation. First pass metabolism by the
intestine or liver limits the efficacy of many drugs when taken orally. For example, more than 90 % of
nitroglycerin is cleared during a single passage through the liver.
Sublingual: diffuse into the capillary network and, therefore, enters the systemic circulation
directly.
Rectal: 50 % of the drainage of the rectal region bypasses the portal circulation, thus, the
biotransformation of drug by the liver is minimized.
B. Parenteral
Parenteral administration is used for drugs that are poorly absorbed from the GI tract, and for
agents, such as insulin, that are unstable in the GI tract.
Parenteral administration is also used for treatment of unconscious patients, and under
circumstances that require a rapid onset of action.
The three major parenteral routes are intravascular, intramuscular and subcutaneous.
Intravascular: intravenous (IV) injection is the most common parenteral route. This route
permits a rapid effect and a maximal degree of control over the circulating levels of the drug.
Intramuscular: drugs administered IM can be aqueous solutions or specialized depot
preparations. Absorption of drugs in aqueous solution is fast, whereas that from depot preparations is slow.
Subcutaneous (SC): is somewhat slower than IV.
C. Other
Inhalation: provides the rapid delivery of a drug across the large surface area of the mucous
membranes of the respiratory tract and pulmonary epithelium, producing an effect almost as rapidly as with
IV injection.
Intranasal: desmopressin is administered intranasally in the treatment of diabetes insipidus,
salmon calcitonin, a peptide hormone used in the treatment of osteoporosis, is also available as a nasal
spray. The abused drug, cocaine, is generally taken by sniffing.
Intrathecal/intraventricular: it is sometimes necessary to introduce drugs directly into the CSF.
For example, amphotericin B is used in treating cryptococcal meningitis.
Topical: is used when a local effect of the drug is desired.
Transdermal: this route of administration achieves systemic effects by application of drugs to
the skin, usually via a transdermal patch. This route is most often used for the sustained delivery of drugs,
such as the antianginal drug nitroglycerin.
Absorption of drugs
Absorption is the transfer of a drug from its site of administration to the bloodstream.
A. Transport of a drug from the GI tract
Passive diffusion: from a region of high concentration to a region of low concentration.
Passive diffusion does not involve a carrier, the process is not saturable, and shows a low structural
specificity. The vast majority of drugs gain access to the body by this mechanism.
Active transport: involves specific carrier proteins that span the membrane. Energy dependent
and driven by the hydrolysis of ATP.
B. Effect of pH on drug absorption
Most drugs are either weak acids or weak bases.
A drug passes through membranes more readily if it is uncharged. Therefore, the effective
concentration of the permeable form of each drug at its absorption site is determined by the relative
concentrations of the charged and uncharged forms. The ratio between the two forms is, in turn, determined
by the pH at the site of absorption, and by the strength of the weak acid or base, which is represented by the
pKa. The lower the pKa, the stronger the acid.
The relationship of pKa and the ratio of acid-base concentrations to pH is expressed by the Henderson-
Hasselbalch equation
pH = pKa + log nonprotonated species/protonated species
, This equation is useful in determining how much drug will be found on either side of a membrane that
separates two compartments that differ in pH, for example, stomach (pH 1-1,5) and blood plasma (pH 7,4).
C. Physical factors influencing absorption
Blood flow to the absorption site: blood flow to the intestine is much greater than the flow to the
stomach, thus, absorption from the intestine is favored over that from the stomach.
Shock severely reduces blood flow to cutaneous tissues, thus minimizing the absorption from
subcutaneous administration.
Total surface area available for absorption
Contact time at the absorption surface: parasympathetic input increases the rate of gastric emptying,
whereas sympathetic input prolongs gastric emptying.
Bioavailability
Bioavailability is the fraction of administered drug that reaches the systemic circulation.
A. Determination of bioavailability
Bioavailability is determined by comparing plasma levels of a drug after a particular route of
administration, with plasma drug levels achieved by IV injection, in which all of the agent enters the
circulation.
B. Factors that influence bioavailability
First pass hepatic metabolism: many drugs, such as propranolol or lidocaine, undergo significant
biotransformation during a single passage through the liver.
Solubility of the drug: for a drug to be readily absorbed, it must be largely hydrophobic yet have some
solubility in aqueous solutions.
Chemical instability: some drugs, such as penicillin G, are unstable in the pH of the gastric contents.
Others, such as insulin, are destroyed in the GI tract by degradative enzymes.
Nature of the drug formulation: size, salt form, crystal polymorphism, and the presence of excipients
( such as binders and dispersing agents)
C. Bioequivalence
Two related drugs are bioequivalent if they show comparable bioavailability and similar times to
achieve peak blood concentrations.
D. Therapeutic equivalence
Two similar drugs are therapeutically equivalent if they have comparable efficacy and safety.
Drug distribution
Drug distribution is the process by which a drug reversibly leaves the bloodstream and enters the
interstitium and/or the cells of the tissues.
The delivery of a drug from the plasma to the interstitium primarily depends on blood flow, capillary
permeability, the degree of binding of the drug to plasma and tissue proteins, and the relative hydrophobicity
of the drug.
A. Blood flow
The rate of blood flow to the tissue capillaries varies widely as a result of the unequal
distribution of cardiac output to the various organs.
This differential blood flow partly explains the short duration of hypnosis produced by a bolus
IV injection of thiopental. The high blood flows to the brain together with the superior lipid solubility of
thiopental permit it to rapidly move into the CNS and produce anesthesia.
B. Capillary permeability
Determined by capillary structure and by the chemical nature of the drug.
Capillary structure varies widely in terms of the fraction of the basement membrane that is exposed by
slit junctions between endothelial cells.
The brain for example has no split junctions, and the tightly juxtaposed endothelial cells form tight
junctions that constitute the BBB.
Hydrophobic drugs, which have a uniform distribution of electrons and no net charge, readily move
across most biological membranes.
C. Binding of drugs to proteins
Reversible binding to plasma proteins sequesters drugs in a nondiffusible form, and slows their transfer
out of the vascular compartment.
Plasma albumin is the major drug-binding protein, and may act as a drug-reservoir.
Volume of distribution
The volume of distribution is a hypothetical volume of fluid into which a drug is disseminated.
A. Water compartments in the body
, Plasma compartment: if a drug has a very large molecular weight or binds extensively to
plasma proteins, it is too large to move out through the endothelial slit junctions of the capillaries and, thus,
is effectively trapped within the plasma (vascular) compartment. The plasma compartment constitutes about
6 % of the body weight or, about 4 L of body fluid. Heparin shows this type of distribution.
Extracellular fluid: if a drug has a low molecular weight but is hydrophilic, it can move
through the endothelial slit junctions of the capillaries into the interstitial fluid. However, it can not move
across the membranes of the cells to enter the water phase inside the cell. Therefore, these drugs distribute
into a volume that is the sum of the plasma water and the interstitial fluid, which together constitute the ec
fluid. This is about 20% of the body weight. Aminoglycoside antibiotics show this type of distribution.
Total body water: if a drug has a low molecular weight and is hydrophobic, not only can it
move into the interstitium through the slit junctions, but can also move through the cell membranes into the
ic fluid. The drug therefore distributes into a volume of about 60 % of body weight. Ethanol exhibits this
apparent volume of distribution.
Other sites: in pregnancy, the fetus may take up drugs and thus increase the volume of
distribution. Drugs which are extremely lipid soluble, such as thiopental, may also have unusually high
volumes of distribution.
B. Apparent volume of distribution
A drug rarely associates exclusively with only one of the water compartments of the body. Instead, the
vast majority of drugs distribute into several compartments. Therefore, the volume into which drugs
distribute is called the apparent volume of distribution, or Vd.
Distribution of drug in the absence of elimination: The concentration within the vascular compartment
is the total amount of drug administred divided by the volume into which is distributes. C = D/Vd or Vd =
D/C
Distribution of drug when elimination is present: the rate for most drugs is first order and shows a linear
relationship with time if lnC is plotted versus time. This is because the elimination processes are not
saturated.
Binding of drugs to plasma proteins
Drug molecules may bind to plasma proteins (usually albumin).
Bound drugs are pharmacologically inactive.
A. Binding capacity of albumin
The binding of drugs to albumin is reversible, and may show low capacity (one drug molecule
per albumin molecule) or high capacity (a number of drug molecules binding to a single albumin molecule).
Drugs can also bind with varying affinities. Albumin has the strongest affinity for anionic
drugs (weak acids) and hydrophobic drugs. Most hydrophilic and neutral drugs do not bind albumin.
B. Competition for binding between drugs
When two drugs are given, each with high affinity for albumin, they compete for the available binding
sites.
The drugs with high affinity for albumin can be divided into two classes, depending on whether the
dose of the drug is greater than or less than the binding capacity of albumin.
Class I drugs: if the dose is less than the binding capacity of albumin, then the dose/capacity ratio is
low.
Class II drugs: these drugs are given in doses that greatly exceed the number of albumin binding sites.
A relatively high proportion of the drug exists in the free state, not bound to albumin.
This assignment of drug classification assumes importance when a patient who is taking a class I drug,
such as tolbutamide, is given a class II drug, such as a sulfonamide antibiotic.
Drug metabolism
Drugs are most often eliminated by biotransformation and/or excretion into the urine or bile.
The liver is the major site for drug metabolism, but specific drugs may undergo biotransformation in
other tissues.
A. Kinetics of metabolism
First order kinetics: the metabolic transformation of drugs is catalyzed by enzymes, and most
of the reactions obey Michaelis-Menten kinetics. That is, the rate of drug metabolism is directly proportional
to the concentration of free drug. Constant FRACTION is eliminated per unit of time!
Zero-order kinetics: if the dose is very high, the rate of metabolism remains constant over time.
Enzymes are saturated, so constant AMOUNT of drug is eliminated per unit of time!
B. Reactions of drug metabolism
, The kidney cannot efficiently eliminate lipophilic drugs that readily cross cell membranes and are
reabsorbed in the distal tubules. Therefore, lipid-soluble agents must first be metabolized in the liver using
two general sets of reactions, called phase I and phase II.
Phase I: convert lipophilic molecules into more polar molecules. The phase I reactions most frequently
involved in drug metabolism are catalyzed by the cytochrome P450 system. Oxidation, reduction,
hydroxylation
Phase II: this phase consists of conjugation reactions. Many metabolites are too lipophilic to be retained
in the kidney tubules after phase I. a subsequent conjugation reaction results in a polar compound.
Glucuronidation is the most common and the most important conjugation reaction. Acetylation, sulfa-
conjugation and methylation
Not all drugs undergo phase I and II reactions in that order.
Drug elimination
Removal of drug from the body may occur via a number of routes, the most important being through the
kidney into the urine.
Other routes include the bile, intestine, lung, or milk in nursing mothers.
A. Renal eleimination of a drug
Glomerular filtration: free drug is filtered into bowman’s capsule
Proximal tubular secretion: by two energy-requiring active transports, one for anions and one
for cations. These transports have low specificity.
Distal tubular reabsorption: the process by which a drug gets more polar by addition of another
drug not to be reabsorbed is called ion trapping. If uncharged, is reabsorbed as its concentration increases.
B. Quantitative aspects of renal drug elimination
Plasma clearance is expressed as the volume of plasma from which all drug appears to be removed in a
given time, for example as ml/min.
Extraction ratio: this ratio is the decline of drug concentration in the plasma from the arterial to the
venous side of the kidney. The drugs enter the kidneys at concentration C1 and exit the kidneys at
concentration C2. the extraction ratio = C2/C1
Excretion rate = clearance * plasma concentration. This can be used to determine the half-life of a drug.
T1/2 = ln 0,5/ke = 0,693 Vd/CL ke – the first order constant for drug elimination
from the total body CL - clearance
CL total = CLhepatic + CLrenal + CLpulmonary + CLother or CLtotal = keVd
Kinetics of continuous administration
A. Kinetics of IV infusion
With continuous IV infusion, the rate of drug enter into the body is constant. The rate of drug
exit from the body increases proportionately as the plasma concentration increases, and at every point in
time, it is proportional to the plasma concentration of the drug.
Following the initiation of an IV infusion, the plasma concentration of drug rises until the rate
of drug eliminated from the body precisely balances the input rate. Thus a steady-state is achieved in which
the plasma concentration of drug remains constant.
With most drugs, the steady state concentration is directly proportional to the infusion rate.
Furthermore, the steady-state concentration is inversely proportional to the clearance of the drug.
One can assume that a drug will reach steady-state in about four half-lives.
B. Kinetics of fixed-dose/fixed-time-interval regimens
Single IV injection: the circulating drug decreases exponentially with time.
Multiple IV injections: a steady-state is achieved.
Pharmacokinetics examines the movement of a drug over time through the body.
Routes of drug administration
A. Enteral
Oral: the most common. Most drugs absorbed from the GI tract enter the portal circulation and
encounter the liver before they are distributed into the general circulation. First pass metabolism by the
intestine or liver limits the efficacy of many drugs when taken orally. For example, more than 90 % of
nitroglycerin is cleared during a single passage through the liver.
Sublingual: diffuse into the capillary network and, therefore, enters the systemic circulation
directly.
Rectal: 50 % of the drainage of the rectal region bypasses the portal circulation, thus, the
biotransformation of drug by the liver is minimized.
B. Parenteral
Parenteral administration is used for drugs that are poorly absorbed from the GI tract, and for
agents, such as insulin, that are unstable in the GI tract.
Parenteral administration is also used for treatment of unconscious patients, and under
circumstances that require a rapid onset of action.
The three major parenteral routes are intravascular, intramuscular and subcutaneous.
Intravascular: intravenous (IV) injection is the most common parenteral route. This route
permits a rapid effect and a maximal degree of control over the circulating levels of the drug.
Intramuscular: drugs administered IM can be aqueous solutions or specialized depot
preparations. Absorption of drugs in aqueous solution is fast, whereas that from depot preparations is slow.
Subcutaneous (SC): is somewhat slower than IV.
C. Other
Inhalation: provides the rapid delivery of a drug across the large surface area of the mucous
membranes of the respiratory tract and pulmonary epithelium, producing an effect almost as rapidly as with
IV injection.
Intranasal: desmopressin is administered intranasally in the treatment of diabetes insipidus,
salmon calcitonin, a peptide hormone used in the treatment of osteoporosis, is also available as a nasal
spray. The abused drug, cocaine, is generally taken by sniffing.
Intrathecal/intraventricular: it is sometimes necessary to introduce drugs directly into the CSF.
For example, amphotericin B is used in treating cryptococcal meningitis.
Topical: is used when a local effect of the drug is desired.
Transdermal: this route of administration achieves systemic effects by application of drugs to
the skin, usually via a transdermal patch. This route is most often used for the sustained delivery of drugs,
such as the antianginal drug nitroglycerin.
Absorption of drugs
Absorption is the transfer of a drug from its site of administration to the bloodstream.
A. Transport of a drug from the GI tract
Passive diffusion: from a region of high concentration to a region of low concentration.
Passive diffusion does not involve a carrier, the process is not saturable, and shows a low structural
specificity. The vast majority of drugs gain access to the body by this mechanism.
Active transport: involves specific carrier proteins that span the membrane. Energy dependent
and driven by the hydrolysis of ATP.
B. Effect of pH on drug absorption
Most drugs are either weak acids or weak bases.
A drug passes through membranes more readily if it is uncharged. Therefore, the effective
concentration of the permeable form of each drug at its absorption site is determined by the relative
concentrations of the charged and uncharged forms. The ratio between the two forms is, in turn, determined
by the pH at the site of absorption, and by the strength of the weak acid or base, which is represented by the
pKa. The lower the pKa, the stronger the acid.
The relationship of pKa and the ratio of acid-base concentrations to pH is expressed by the Henderson-
Hasselbalch equation
pH = pKa + log nonprotonated species/protonated species
, This equation is useful in determining how much drug will be found on either side of a membrane that
separates two compartments that differ in pH, for example, stomach (pH 1-1,5) and blood plasma (pH 7,4).
C. Physical factors influencing absorption
Blood flow to the absorption site: blood flow to the intestine is much greater than the flow to the
stomach, thus, absorption from the intestine is favored over that from the stomach.
Shock severely reduces blood flow to cutaneous tissues, thus minimizing the absorption from
subcutaneous administration.
Total surface area available for absorption
Contact time at the absorption surface: parasympathetic input increases the rate of gastric emptying,
whereas sympathetic input prolongs gastric emptying.
Bioavailability
Bioavailability is the fraction of administered drug that reaches the systemic circulation.
A. Determination of bioavailability
Bioavailability is determined by comparing plasma levels of a drug after a particular route of
administration, with plasma drug levels achieved by IV injection, in which all of the agent enters the
circulation.
B. Factors that influence bioavailability
First pass hepatic metabolism: many drugs, such as propranolol or lidocaine, undergo significant
biotransformation during a single passage through the liver.
Solubility of the drug: for a drug to be readily absorbed, it must be largely hydrophobic yet have some
solubility in aqueous solutions.
Chemical instability: some drugs, such as penicillin G, are unstable in the pH of the gastric contents.
Others, such as insulin, are destroyed in the GI tract by degradative enzymes.
Nature of the drug formulation: size, salt form, crystal polymorphism, and the presence of excipients
( such as binders and dispersing agents)
C. Bioequivalence
Two related drugs are bioequivalent if they show comparable bioavailability and similar times to
achieve peak blood concentrations.
D. Therapeutic equivalence
Two similar drugs are therapeutically equivalent if they have comparable efficacy and safety.
Drug distribution
Drug distribution is the process by which a drug reversibly leaves the bloodstream and enters the
interstitium and/or the cells of the tissues.
The delivery of a drug from the plasma to the interstitium primarily depends on blood flow, capillary
permeability, the degree of binding of the drug to plasma and tissue proteins, and the relative hydrophobicity
of the drug.
A. Blood flow
The rate of blood flow to the tissue capillaries varies widely as a result of the unequal
distribution of cardiac output to the various organs.
This differential blood flow partly explains the short duration of hypnosis produced by a bolus
IV injection of thiopental. The high blood flows to the brain together with the superior lipid solubility of
thiopental permit it to rapidly move into the CNS and produce anesthesia.
B. Capillary permeability
Determined by capillary structure and by the chemical nature of the drug.
Capillary structure varies widely in terms of the fraction of the basement membrane that is exposed by
slit junctions between endothelial cells.
The brain for example has no split junctions, and the tightly juxtaposed endothelial cells form tight
junctions that constitute the BBB.
Hydrophobic drugs, which have a uniform distribution of electrons and no net charge, readily move
across most biological membranes.
C. Binding of drugs to proteins
Reversible binding to plasma proteins sequesters drugs in a nondiffusible form, and slows their transfer
out of the vascular compartment.
Plasma albumin is the major drug-binding protein, and may act as a drug-reservoir.
Volume of distribution
The volume of distribution is a hypothetical volume of fluid into which a drug is disseminated.
A. Water compartments in the body
, Plasma compartment: if a drug has a very large molecular weight or binds extensively to
plasma proteins, it is too large to move out through the endothelial slit junctions of the capillaries and, thus,
is effectively trapped within the plasma (vascular) compartment. The plasma compartment constitutes about
6 % of the body weight or, about 4 L of body fluid. Heparin shows this type of distribution.
Extracellular fluid: if a drug has a low molecular weight but is hydrophilic, it can move
through the endothelial slit junctions of the capillaries into the interstitial fluid. However, it can not move
across the membranes of the cells to enter the water phase inside the cell. Therefore, these drugs distribute
into a volume that is the sum of the plasma water and the interstitial fluid, which together constitute the ec
fluid. This is about 20% of the body weight. Aminoglycoside antibiotics show this type of distribution.
Total body water: if a drug has a low molecular weight and is hydrophobic, not only can it
move into the interstitium through the slit junctions, but can also move through the cell membranes into the
ic fluid. The drug therefore distributes into a volume of about 60 % of body weight. Ethanol exhibits this
apparent volume of distribution.
Other sites: in pregnancy, the fetus may take up drugs and thus increase the volume of
distribution. Drugs which are extremely lipid soluble, such as thiopental, may also have unusually high
volumes of distribution.
B. Apparent volume of distribution
A drug rarely associates exclusively with only one of the water compartments of the body. Instead, the
vast majority of drugs distribute into several compartments. Therefore, the volume into which drugs
distribute is called the apparent volume of distribution, or Vd.
Distribution of drug in the absence of elimination: The concentration within the vascular compartment
is the total amount of drug administred divided by the volume into which is distributes. C = D/Vd or Vd =
D/C
Distribution of drug when elimination is present: the rate for most drugs is first order and shows a linear
relationship with time if lnC is plotted versus time. This is because the elimination processes are not
saturated.
Binding of drugs to plasma proteins
Drug molecules may bind to plasma proteins (usually albumin).
Bound drugs are pharmacologically inactive.
A. Binding capacity of albumin
The binding of drugs to albumin is reversible, and may show low capacity (one drug molecule
per albumin molecule) or high capacity (a number of drug molecules binding to a single albumin molecule).
Drugs can also bind with varying affinities. Albumin has the strongest affinity for anionic
drugs (weak acids) and hydrophobic drugs. Most hydrophilic and neutral drugs do not bind albumin.
B. Competition for binding between drugs
When two drugs are given, each with high affinity for albumin, they compete for the available binding
sites.
The drugs with high affinity for albumin can be divided into two classes, depending on whether the
dose of the drug is greater than or less than the binding capacity of albumin.
Class I drugs: if the dose is less than the binding capacity of albumin, then the dose/capacity ratio is
low.
Class II drugs: these drugs are given in doses that greatly exceed the number of albumin binding sites.
A relatively high proportion of the drug exists in the free state, not bound to albumin.
This assignment of drug classification assumes importance when a patient who is taking a class I drug,
such as tolbutamide, is given a class II drug, such as a sulfonamide antibiotic.
Drug metabolism
Drugs are most often eliminated by biotransformation and/or excretion into the urine or bile.
The liver is the major site for drug metabolism, but specific drugs may undergo biotransformation in
other tissues.
A. Kinetics of metabolism
First order kinetics: the metabolic transformation of drugs is catalyzed by enzymes, and most
of the reactions obey Michaelis-Menten kinetics. That is, the rate of drug metabolism is directly proportional
to the concentration of free drug. Constant FRACTION is eliminated per unit of time!
Zero-order kinetics: if the dose is very high, the rate of metabolism remains constant over time.
Enzymes are saturated, so constant AMOUNT of drug is eliminated per unit of time!
B. Reactions of drug metabolism
, The kidney cannot efficiently eliminate lipophilic drugs that readily cross cell membranes and are
reabsorbed in the distal tubules. Therefore, lipid-soluble agents must first be metabolized in the liver using
two general sets of reactions, called phase I and phase II.
Phase I: convert lipophilic molecules into more polar molecules. The phase I reactions most frequently
involved in drug metabolism are catalyzed by the cytochrome P450 system. Oxidation, reduction,
hydroxylation
Phase II: this phase consists of conjugation reactions. Many metabolites are too lipophilic to be retained
in the kidney tubules after phase I. a subsequent conjugation reaction results in a polar compound.
Glucuronidation is the most common and the most important conjugation reaction. Acetylation, sulfa-
conjugation and methylation
Not all drugs undergo phase I and II reactions in that order.
Drug elimination
Removal of drug from the body may occur via a number of routes, the most important being through the
kidney into the urine.
Other routes include the bile, intestine, lung, or milk in nursing mothers.
A. Renal eleimination of a drug
Glomerular filtration: free drug is filtered into bowman’s capsule
Proximal tubular secretion: by two energy-requiring active transports, one for anions and one
for cations. These transports have low specificity.
Distal tubular reabsorption: the process by which a drug gets more polar by addition of another
drug not to be reabsorbed is called ion trapping. If uncharged, is reabsorbed as its concentration increases.
B. Quantitative aspects of renal drug elimination
Plasma clearance is expressed as the volume of plasma from which all drug appears to be removed in a
given time, for example as ml/min.
Extraction ratio: this ratio is the decline of drug concentration in the plasma from the arterial to the
venous side of the kidney. The drugs enter the kidneys at concentration C1 and exit the kidneys at
concentration C2. the extraction ratio = C2/C1
Excretion rate = clearance * plasma concentration. This can be used to determine the half-life of a drug.
T1/2 = ln 0,5/ke = 0,693 Vd/CL ke – the first order constant for drug elimination
from the total body CL - clearance
CL total = CLhepatic + CLrenal + CLpulmonary + CLother or CLtotal = keVd
Kinetics of continuous administration
A. Kinetics of IV infusion
With continuous IV infusion, the rate of drug enter into the body is constant. The rate of drug
exit from the body increases proportionately as the plasma concentration increases, and at every point in
time, it is proportional to the plasma concentration of the drug.
Following the initiation of an IV infusion, the plasma concentration of drug rises until the rate
of drug eliminated from the body precisely balances the input rate. Thus a steady-state is achieved in which
the plasma concentration of drug remains constant.
With most drugs, the steady state concentration is directly proportional to the infusion rate.
Furthermore, the steady-state concentration is inversely proportional to the clearance of the drug.
One can assume that a drug will reach steady-state in about four half-lives.
B. Kinetics of fixed-dose/fixed-time-interval regimens
Single IV injection: the circulating drug decreases exponentially with time.
Multiple IV injections: a steady-state is achieved.
