ALTAF SIR

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2. Bioavailability is defined as the rate and
extent(amount) of absorption of
unchanged drug from its dosage form.
2

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The parameters that are useful in determining the
bioavailability of a drug product include:
1. Plasma Data: (tmax), (Cmax), (AUC)
2. Urine date (Du
), (dDu/dt), (t)
3. Acute pharmacologic effect
4. Clinical Observation
5. In-vitro studies (drug dissolution)
Since the free or therapeutically active drug can be
accurately quantitated in biological fluids, plasma and
urine data give the most objective information on
bioavailability

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1. Plasma data
MSC
MEC
Cmax
tmax
AUC
Therapeutic
range
(window)
Absorption
phase
Elimination
phase
Plasma
Concentration
Time
onset
Duration
When a single dose of a drug is administered orally to a
patient, serial blood samples are withdrawn and the
plasma assayed for drug concentration at specificperiods
of time after administration, a plasma concentration-time
curve can be constructed.

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Important parameters used to assess bioavailability:
1. tmax: time of peak plasma concentration.
•tmax corresponds to the time required to reach maximum
drug administration.
•tmax reflects differences in absorption rates, the faster
the drug is absorbed the lower will be the time of the
peak concentration.

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2. Cmax:
• represents the maximum plasma drug concentration
obtained after oral administration of drug.
• Cmax provides an indication that the drug is
sufficiently systemically absorbed to provide a
therapeutic response.
• Cmax also provides warning of possibly toxic levels of
drug.

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3. AUC:
•The area under the curve is a measurement of the extent
of drug bioavailability.
•The AUC reflects the total amount of active drug which
reaches the systemic circulation.
•Determined by planimeter, trapezoidal rule or cut &
weigh method
po
iv

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In order to illustrate the usefulness of plasma
concentration-time curves in bioavailability studies to
assess the rate and extent of absorption consider the
administration :
• of single equal doses
• of three different formulations A, B and C
• of the same drug
• to the same healthy individual
• by the same route of administration
• on three separate occasions.
From the clinical viewpoint, which is the best formula?

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C
B
A
AUC
tmax
onset
Duration
Cmax
Formulation C

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Measurement of the concentration of intact drug and/or its
metabolite (s) in the urine can also be used to assess
bioavailability.
The assessment of bioavailability by urinary excretion is
based on the assumption that the appearance of the drug
and/or its metabolites in the urine is a function of the rate
and extent of absorption. (e.g. not influenced by urine pH)
2. Urine data

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Measurements involving metabolite levels in the urine are
only valid when the drug in question is not subject to
metabolism prior to reaching the systemic circulation
(intestinal or first pass)
Remember:
Definition of bioavailability is in terms of the extent and
the rate at which intact drug appears in the systemic
circulation after the administration of a known dose.

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Urine samples must be collected until all drug and/or
its metabolites has been excreted (this is indicated
by the cumulative urinary excretion curve becoming
parallel to the abscissa).
A cumulative urinary excretion curve is obtained by
collecting urine samples (resulting from total emptying of
the bladder) at known intervals after a single dose of the
drug has been administered.

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Important parameters used to assess bioavailability:
1. Du
: Maximum amount of the drug excreted in the urine
The cumulative amount of drug excreted in urine is
directly related to the total amount of drug absorbed.
When Du
 is obtained the plasma concentration is …….
2. dDu/dt : The rate of drug excretion
It is dependent on the first order elimination rate
constant K and the concentration of drug in plasma Cp.
A graph comparing the rate of drug excretion with respect
to time should be identical with the plasma leveltime
curve for that drug.

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3. t :time for maximum urinary
excretion
Is a useful parameter in bioequivalence
studies comparing several drug
products.

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X Y Z
The initial segments (X- Y) of the
curves reflect the 'absorption
phase‘ and the slope of this
segment of the urinary excretion
curve is related to the rate of
absorption of the drug into the
blood.
The total amount of intact drug
(and/or its metabolite) excreted in
the urine at point Z corresponds to
the time at which the plasma
concentration of intact drug is zero
and essentially all the drug has
been eliminated from the body.
Corresponding plots showing
the plasma concentration-time
curve (upper curve) and the
cumulative urinary excretion
curve (lower curve) obtained
following the administration of
a single dose of a drug by the
peroral route

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The total amount of drug excreted
at point Z may be quite different
from the total amount of drug
administered (i.e. the dose), WHY?
either because of incomplete
absorption on because of the drug
being eliminated by processes
other than urinary excretion.
X Y Z
Corresponding plots showing
the plasma concentration-time
curve (upper curve) and the
cumulative urinary excretion
curve (lower curve) obtained
following the administration of
a single dose of a drug by the
peroral route

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In order to illustrate how cumulative urinary excretion
curves can be used to compare the bioavailability of a
given drug consider the administration:
of single equal doses
of the three different formulations, A, B and C,
of the same drug
to the same healthy individual
by the same route of administration
on three different occasions
Cumulative urinary excretion curves may be used to
compare the rate and extent of absorption of a given drug
presented in different formulations?

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Formulation B
Formulation C
C
B
A
Rate of appearance of
the drug in urine
Total amount of drug
excreted

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In some cases the quantitative measurement of a drug is
not available or lacks sufficient accuracy and/or
reproducibility.
An acute pharmacologic effect such as effect on pupil
diameter, heart rate, or blood pressure…..etc. can be
useful as an index of drug bioavailability.
In this case an acute pharmacologic effect-time curve is
constructed.
The use of an acute pharmacologic effect to determine
bioavailability may require demonstration of a dose-
related response.
3. Acute Pharmacologic effect

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The clinical trials approach is the least sensitive, least
accurate, and least reproducible of the general
approaches for determining in-vivo bioavailability.
The FDA considers this approach only when analytical
methods and pharmacodynamics methods are not
available.
4. Clinical Response

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For many years medical practitioners have observed:
- either lack of response [therapeutic failure] (un-
available),
- good therapeutic response (reasonably available) or
- toxicity (highly available)
in patients receiving similar drug products.

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Variable clinical responses among individuals which are
unrelated to bioavailability might be due to difference in
the pharmacodynamics of the drug.
Differences in pharmacodynamics, which is the
relationship between the drug and the receptor site,
may be due to differences in receptor sensitivity to the
drug.
Various factors affecting pharmacodynamic drug
behavior may include age, drug tolerance, drug
interaction, and unknown pathophysiologic factors.

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• A- Release of drug from its dosage form into solution
• Drug dissolution studies may under certain
conditions give an indication of drug bioavailability.
• Ideally, the in-vitro drug dissolution rate should
correlate with in-vivo drug bioavailability.
5. In vitro studies

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Many factors may affect the rate of drug dissolution
(release)
1. Stirring rate:
Low stirring rates (50 – 100 rpm) are more
discriminating of formulation factors affecting
dissolution than high stirring rates.
2. Temperature:
Must be controlled, variations should be avoided. Most
dissolution tests are performed at 37°C.

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3. Quantity of dissolution medium:
• It should not be saturated with the drug, as no
further net drug dissolution will take place. i.e. sink
conditions should be maintained.
• According to USP the quantity of the medium should
be not less than 3 times that required to form a
saturated solution of the drug.

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4. Nature of dissolution medium:
• Normally buffered aqueous solutions are used. The pH
may differ according to intended site of drug release.
5. Dissolution apparatus:
• The most common official methods (according to USP)
used are:
• Rotating basket Method
• Paddle Method

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B. Permeability studies
• Partition coefficient
• Drug permeation
a. Cell culture
b. isolated intestinal mucosa

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Bioavailability studies are designed to
determine either:
absolute bioavailability (relative to an IV
formulation) or relative bioavailability
(with an alternate reference dosage form
with good absorption characteristics).
Bioavailability studies

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1. Absolute Bioavailability
Absolute bioavailability compares the bioavailability of
the active drug in systemic circulation following non-
intravenous administration (i.e., after oral, rectal,
transdermal, subcutaneous administration), with the
bioavailability of the same drug following IV
administration.
WHY intravenous?
because it is totally bioavailable.

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The comparison must be dose normalized if different
doses are used; consequently, each AUC is corrected
by dividing the corresponding dose administered.
Often the dose administered intravenously is:
lower to avoid toxic side-effects
Absolute Bioavailability =
AUC EV
AUC iv
Absolute Bioavailability =
AUC EV/dose EV
AUC iv/dose iv

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Absolute bioavailability using urinary excretion data may
be determined by comparing the total cumulative
amounts of drug excreted in the urine following
administration of drug via an absorption site and the
intravenous route, respectively, on different occasions to
the same subject.
For equivalent doses of administered drug:
Absolute Bioavailability =
Du EV
Du iv
If different doses are administered:
Absolute Bioavailability =
Du EV/dose EV
Du iv/dose iv

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2. Relative bioavailability
In the case of drugs that cannot be administered by
intravenous injection, the relative bioavailability is
determined rather than the absolute bioavailability
In this case the fraction (or percentage) of a given drug
that is absorbed intact into the systemic circulation of a
given drug from a 'test' dosage form is compared to that
of the same drug administered in ‘standard' dosage form,
which is either:
 an orally administered solution (from which the drug is
known to be well absorbed) or
an established commercial preparation of proven
clinical effectiveness.

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Relative Bioavailability =
Du test
Du std
Relative Bioavailability =
AUC test
AUC std
From plasma concentration-time curve:
From urinary excretion data:
In case of different doses test and standard must be
divided by the corresponding doses.

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Relative bioavailability, is thus used to determine:
whether test and standard dosage forms containing
equal doses of the same drug are equivalent or not
Equivalent in what?
in terms of their rates and extents of absorption
i.e. equivalent bioavailability. This is called
bioequivalence.
Bioequivalence

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Bioequivalence studies are therefore important
in determining whether chemically equivalent
drug products manufactured by different
companies are therapeutically equivalent, i.e.
produce identical therapeutic responses in
patients.

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 If two chemically equivalent drug products are
absolutely bioequivalent, their plasma
concentration time and/or cumulative urinary
excretion curve would be super-imposable
 In such a case there would be no problem in
concluding that these products were
bioequivalent.

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Nor would there be problem in concluding bioinequivalence
if the parameters associated with the plasma concentration
time and/or cumulative urinary excretion profile for the test
differed from the standard product by for instance, 50%.

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However, a problem arises in deciding whether the test and
standard drug products are bioequivalent when such
products show relatively small differences in their plasma
concentration-time curves and/or cumulative, urinary
excretion curves.

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This will depend on such factors as:
the toxicity
the therapeutic range
the therapeutic use of the drug.
A value of 20% for the tolerated difference used to be
regarded as suitable as a general criterion for determining
bioequivalence.
Thus if all the major parameters in either the plasma
concentration-time or cumulative urinary excretion curves
for two or more chemically equivalent drug products
differed from each other by less than 20%, these products
would have been judged to be bioequivalent.

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Bioequivalence study
Bioequivalence studies are performed to compare the
bioavailability of the generic drug product to the brand-
name drug product.
1. Medication:
test and reference drug formulations contain:
• the pharmaceutical equivalent drug
• in the same dose & strength
• in similar dosage forms
• given by the same route of administration

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2. Volunteers
• normal, healthy, adult, male and female
• 20 - 50 years old, 54 - 91 kg body weight
• Smokers are normally excluded
• No medications are taken prior to study (often 1 week)
• Subjects are generally fasted 10 – 12 hrs (overnight) before
drug administration and 2-4 hrs after dosing.

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3. Latin square crossover design
The Latin square crossover design plans the clinical trial
so that each subject receives each drug product only
once, with adequate time between medications for the
elimination of the drug from the body.
In this case each subject is his own control, and subject-
to-subject variation is reduced.
Possible carry-over effects from any particular drug
product are minimized by changing the sequence or
order in which the drug products are given to the
subject.

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Drug Product
Week 3
Week 2
Week 1
Subject
C
B
A
1
A
C
B
2
B
A
C
3
B
C
A
4
A
B
C
5
C
A
B
6

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4. Sampling
After each patient receives a drug product, blood
samples are collected at appropriate time intervals so
that a valid blood drug level-time curve may be
obtained.
The time intervals should be spaced so that one can
clearly describe:
the peak blood concentration,
the total area under the curve, and
the absorption and elimination phases of the
curve.
In some cases the measurement of drug in urine
samples may be necessary.

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5. Evaluation of the Data:
•The analytical method for measurement of the drug must
be validated for accuracy, precision, and sensitivity.
•The use of more than one analytical method during a
bioequivalence study may not be valid, because different
method may deliver different values.
•Pharmacokinetic parameters such as AUC, tmax, Cmax, k,
and t½ are determined.
•Data should be presented in both tabulated and
graphical form for evaluation.

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•Proper statistical evolution should be performed on the
estimated pharmacokinetic parameters.
• Statistics should be used, in bioavailability testing, as a
tool to determine if sufficient subject have been
included to minimize the effect of patient - to patient
variability in the data analysis.

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Definitions

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Pharmaceutical equivalents
Drug products that have identical:
 active drug ingredient (same salt, ester, or chemical
form)
 strength or concentration,
 dosage form,
 route of administration.
but may differ in:
color, flavor, shape, packaging, excipients, preservatives,
expiration time, and labeling.

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Bioequivalent drug products:
Bioequivalent drug products are pharmaceutical
equivalents that have similar bioavailability when given
in the same dose and studied under similar
experimental conditions.
Some drugs may be considered bioequivalent that are
equal in the extent of absorption but not in the rate of
absorption.
This is possible if the difference in the rate of
absorption is considered clinically insignificant, for
example, for drugs for chronic use.

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Pharmaceutical alternatives
Drug products that contain the same therapeutic moiety
but as different salts, esters or complexes.
e.g. tetracycline phosphate or tetracycline hydrochloride
equivalent to 250-mg tetracycline base are considered
pharmaceutical alternatives.
Different dosage forms and strengths within a product
line by a single manufacturer are pharmaceutical
alternatives
e.g. an extended-release dosage form and a standard
immediate-release dosage form of the same active
ingredient.
e.g. ampicillin suspension and ampicillin capsule

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Therapeutic alternatives
Drug products containing different active ingredients
that are indicated for the same therapeutic or clinical
objectives.
Active ingredients in therapeutic alternatives are from
the same pharmacologic class and are expected to have
the same therapeutic effect when administered to
patients for such condition of use.
For example, ibuprofen is given instead of aspirin;
cimetidine may be given instead of ranitidine.