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BIOAVAILABILITY STUDIES
By
A. S. Adebayo, Ph.D.
5/23/2017
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Objectives:
To define various terms
relating to bioavailability
studies
 To understand some of the
past problems with
bioavailability
 To evaluate the components
and results of a
bioavailability study
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Purposes of Bioavailability Studies
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Bioavailability or drug product
evaluation studies, may be designed
to compare:
 One type of dosage form with
another, e.g. regular tablet with
sustained release tablet. For this
type of tablet ka values should be
slower; but F values should be
similar
 Two (or more) dosage forms made
by two different manufacturers.
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Definition of Terms
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Bioavailability - indicates a measure of the rate
and the extent (amount)) of therapeutically active
drug reaching the general circulation.
Bioequivalent Drug Products - means
pharmaceutical equivalents or pharmaceutical
alternatives whose rate and extent of
absorption do not show a significant
difference when administered at the same molar
dose of the therapeutic moiety under similar
experimental conditions, either single dose or
multiple dose.
Some pharmaceutical equivalents or
pharmaceutical alternatives may be equivalent in
the extent of their absorption but not in their rate
of absorption.
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Definition of Terms (Cont.)
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Bioequivalence Requirement means a
requirement imposed by the Food and Drug
Administration for the in vitro and/or in vivo
testing of specified drug products which must be
satisfied as a condition for marketing.
Brand Name is the trade name of the drug.
Chemical Name is the name used by the organic
chemist to indicate the chemical structure of the
drug.
Drug Product means a finished dosage form,
e.g., tablet, capsule, or solution, which contains
the active drug ingredient, generally, but not
necessarily, in association with inactive
ingredients.
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Definition of Terms (Cont.)
Generic Name - the established,
non-proprietary or common name
of the active drug in a drug
product. (as listed in official
books/Pharmacopoeias)
 Pharmaceutical Alternatives drug products that contain the
identical therapeutic moiety, or its
precursor, but not necessarily in the
same amount or dosage form or as
the same salt or ester.
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Definition of Terms (Cont.)
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Pharmaceutical Equivalent - drug
products that contain identical amounts of
the identical active drug ingredient, i.e.,
the salt or ester of the same therapeutic
moiety, in identical dosage forms, but not
necessarily containing the same
inactive ingredients, and that meet the
identical compendia or other applicable
standard of identity, strength, quality, and
purity, including potency and where
applicable, content uniformity,
disintegration times and/or dissolution
rate.
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Reasons for Bioequivalence Testing
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Chlorpropamide Peak plasma
concentration after
one brand was less
than ½ the peak
after the other two
products.
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Reasons for Bioequivalence Testing
(Cont.)
 Digoxin
–
Reported
toxicity due
to change of
formulation
resulting in
a two-fold
increase in
bioavailability.
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Phenytoin.
Incidence of
phenytoin
toxicity in
Australia in 1968
and 1969
following change
of diluent from
Calcium sulphate
to lactose.
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Reasons for Bioequivalence
Requirements by Regulatory Authorities
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The FDA may require bioavailability
studies for a variety of reasons including:
 Results from clinical studies indicate
that different drug products produce
different therapeutic results.
 Results from bioavailability studies
indicate that different products are not
bioequivalent.
 Drug has a narrow therapeutic range.
 Low solubility and/or large dose.
 Absorption is considerably less than
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100%
Bioavailability study characteristics

Drug
 The drug substance in each product
must be the same. Compare “like with
like“
 In the case of pro-drug administration, we
may compare the delivery of a dosage form
containing the drug with another dosage form
containing a pro-drug.
 This testing is generally conducted to
evaluate the usefulness of the pro-drug,
rather than a strict comparison of the drug
products.
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Once the usefulness of the pro-drug is demonstrated
comparisons between dosage forms all containing the
pro-drug should be undertaken to evaluate the drug
product performance.
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Drug product
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Cp versus Time for A and B with B
having Slower Absorption
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Subjects
A
number of factors are of
concern:
 Health
 Age
 Weight
 Enzyme status
 Number of subjects.
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Health
of subjects
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A cross- over design is usually employed to
reduce/eliminate the effect of individual subject
Even though each subject will act as their own
control it is usually best to have subjects of similar
kinetic characteristic so that major variations are
not introduced.
Thus healthy volunteers are often preferred by
drug product evaluation studies.
Informed consent should be obtained from each
volunteer and some biochemical and medical
examination will be used to confirm their medical
state.
For some drugs there may be special disease
states which may cause the exclusion of some
volunteers.
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Age
***
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As you will see later, age can have a
significant effect on drug
pharmacokinetics.
Elderly patients and young children
can have quite different kinetics
compared with young adults.
For better matched group, subjects
between the ages of 18 to 35 years
are preferred. Kinetic changes usually
are not important until age greater
than 60.
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Weight
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The apparent volume of
distribution is usually
proportional to weight in
subjects of normal weight for
height. However, in
overweight (or underweight)
subjects the Vd in L/kg maybe
somewhat different.
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Enzyme status

Smokers or subjects taking
certain other drugs may have
altered kinetics for the drug of
interest.
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This can be caused by alteration of
enzyme activity or by drug-drug
interactions.
These effects add complications to
a study and an attempt is usually
made to minimize these factors.
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Number of subjects
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The number of subjects included in the
study should be sufficient to see any real
(maybe 20% variation) differences in
bioavailability.
Usually 10 - 20 subjects are used in
these studies.
In clinical studies where the end-point is
some clinical response, much larger
numbers are required because of the
greater variability in clinical response.
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Assay
 The
same assay method
should be used for all
phases of the study.
 The assay method should
be sensitive and specific.
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Design
Usually a complete crossover design is used.
 With this design each
subject receives all products
with a wash-out period
between each dose
administration
 A factorial design may be
used to study multiple
source of variation.
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Cross over design (2 products)
Week 1
Week 2
Group 1
A
B
Group 2
B
A
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for two
products
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Cross over design (3 products)
Week 1
Week 2
Week 3
Group 1
A
B
C
Group 2
B
C
A
Group 3
C
A
B
Group 4
A
C
B
Group 5
C
B
A
Group 6
B
A
C
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for three
products
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Sample collection
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Biological fluids are usually collected.
The commonest biological fluids collected are
blood, urine and, at times, the saliva.
Blood samples are collected using sterile needles
and syringes into heparinized tubes.
Two to 5 ml samples are collected at predetermined time intervals.
The frequency and duration of collection is usually
related to the t1/2 of the drug: blood samples
should not be less than 3 times the t1/2 of the
drug.
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Sample collection/Treatment
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Blood samples are more frequently collected
within the first 12 hr, such that the tmax of the
drug can be ascertained.
Thereafter, collection could be done every day or
every other day depending on the t1/2 of the
drug.
Whole blood, plasma or serum can be used.
When using whole blood, the white blood cells or
red blood cells should be lysed by keeping the
sample in the freezer so as to free the drug.
The whole blood should then be centrifuged at
2000 revolutions/min. to obtain the plasma.
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Sample collection/Treatment
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To use the serum, the blood should be
collected into an ordinary tube (with
no heparin).
The blood clots and the serum
fraction is separated.
The analyses of blood in most cases
are normally done in plasma because
analysis done on whole blood gives
wide intra- and inter-individual
variations as a result of difference in
white blood cell count, a problem not
encountered with plasma analysis.
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Urine/salivary analysis
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When performing urine analysis,
samples must be collected for at least
10 times the t1/2 of the drug.
The total urine voided should be
collected.
Saliva samples may also be collected
at predetermined time intervals.
Before using saliva, studies must
have shown that correlation exists
between plasma and saliva levels of
the drug.
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Analysis of the Samples
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Requirements of analytical
method:
 Specificity
 Sensitivity
 Reproducibility
 Precision
 Rapidity of rate of assay
 Accuracy
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The statistical determination of
precision

s
C.V  %
x
Where C.V = coefficient of variation
 s= sample standard deviation;
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x =
average value or mean.
Precision requires a C.V value below
12%.
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Reproducibility, Accuracy & Specificity
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Reproducibility – The possibility of
obtaining the same result if another
individual performs the same
experiment in another place using
similar set up.
Accuracy – Is a measure of deviation
between the known and the obtained
result. The lower the value, the more
accurate the method
Specificity – The ability of the test to
differentiate between the drug and
the metabolite.
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UV Assay Methods
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The sensitivity of UV
spectrophotometers are low (cannot
measure up to nanogram level).
The specificity is also poor, may not
differentiate between the parent drug
and the metabolite.
Fluorimetric Method
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Useful for compounds that can
fluoresce e.g. quinine. Its specificity
is still poor but better than in UV
spec. (about 10 X).
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TLC-UV Method
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The Thin layer chromatograph
separates the metabolite from
the parent drug, while the UV
quantifies the amount of
unchanged drug. The sensitivity
is poor although the specificity
is improved.
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TLC-Fluorimetry
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This gives a very good result.
TLC separates and Fluorimeter
analyzes. However, assay time
is very long.
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HPLC
This is most popular method for
analysis of drugs in biological
fluids.
 It can detect as low as 1ng/ml
or 0.1ng/ml.
 It combines both the separation
and the quantification
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GLC/HPLC
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Gas-liquid chromatography is also
highly sensitive, selective and
reproducible.
HPLC has some advantages over GLC
because:
HPLC leads to higher selectivity (better
separation of drug from metabolite)
because both the column and the solvent
can be changed
GLC is more expensive
GLC is not suitable for thermolabile (heat
sensitive) substances
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Data analysis & Statistics
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The rate of absorption can be characterized
by the ka value and the time of peak
concentration.
The extent of drug absorption is
characterized by the F value or the peak
concentration or total AUC values.
Any differences in the average values of
these parameters can then be analyzed
statistically to determine the significance of
the differences.
The 5 % confidence level is usually used as
the criteria for acceptance.
The analysis of variance is a technique for
separating the effect of product, subject,
and sequence of treatment/administration.
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Analysis of Variance Table for ThreeWay Cross-Over Study
Source of
Variation
d.f.
SS
MS
F
Significa
nce Level
Total
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44.6
-
-
-
Subject
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28.3
2.58
10.1
p < 0.001
Week
2
0.14
0.068
0.27
n.s.
Treatmen
t
2
11.0
5.55
21.8
P < 0.001
Residual
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5.09
0.255
-
-
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THE END
 THANK
5/23/2017
YOU
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