2. CONTENTS
Definitions
Objectives of Bioavailability studies
Types of Bioavailability studies
Methods of Bioavailability measurement
Bioequivalence experimental study designs
IVIVC Correlation
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3. Drug Product
Drug in Blood
Distribution to
Tissue and Receptor sites
Metabolism
Excretion
Bioavailability
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4. 4
Bioavailability:
Bioavailability is defined as the rate and extent of
absorption of unchanged drug from its dosage form.
Rate- acute conditions- asthma, pain etc
Extent (amount) – chronic conditions- hypertension.
Absolute bioavailability: When systemic availability of a
drug administered orally is determined in comparison to
its I.V. administration, denoted by F.
Relative bioavailability:
When systemic availability of a drug after oral
administration is compared with that of oral standard of
the same drug ( Solution or suspension) denoted by Fr.
Definitions
5. Equivalence:
Equivalence is a comparative term used to indicate that 2
or more substances labeled as equivalents are similar to
one another with respect to a specific characteristic or
function.
Chemical equivalence:
When two or more drug products contain the same
chemical substance as an active ingredient in the same
amount it is called chemical equivalence.
Pharmaceutical equivalents:
These are the drug products containing same API and are
identical in strength, dosage form and route of
administration. 5
6. 6
Therapeutic equivalence:
These are the drug products that are pharmaceutic
equivalents that can be expected to have same clinical
effect in-vivo.
Bioequivalence:
It is relative term that denotes drug substance in two or
more identical dosage forms reaches the systemic
circulation at the same relative rate to the same relative
extent.
i.e. plasma concentration-time profiles will be identical
without significant statistical differences.
These are the pharmaceutic equivalents that exhibit
comparable Bioavailability under similar conditions.
7. Objectives of Bioavailability studies
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It is important in the
Primary stages of development of dosage form of new drug
entity to find its therapeutic utility.
Determination of influence of excipients on absorption.
Development of new formulations of existing drugs.
Control of quality of drug products and to study the influence
of processing factors , storage and stability on absorption.
Comparison of drug in different dosage forms or same
dosage form of different manufacturer.
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Types of Bioavailability studies
Two types of Bioavailability studies:
First type : Assessment of BA of new Drug product formulations
of new drugs.
To establish P’kinetic parameters.
To know the influence of:
Excipients,
Processing factors
Packaging components
Patient related factors on formulation.
Second type: Comparison of a test formulation with that of a
reference standard dosage form (existing drugs).
To develop a new dosage form.
To improve an existing dosage form.
To compare the P’kinetic profile comparison.
9. Single dose versus multiple dose studies:
• Single dose bioavailability studies are very common, easy,
less exposure to drugs, less tedious.
• However it is difficult to predict the
steady state characteristics and inter
subject variability with these studies.
Advantages of multiple dose study:
-Accurately reflects manner in which drug will be used
clinically.
-Requires collection of few blood samples.
-Drug levels are higher due to cumulative effect and
useful for less sensitive analytical methods
-Better evaluation of controlled released formulation.
-Nonlinearity if present , can be easily determined.
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10. Healthy subjects versus Patients:
• Generally bioavailability study should be carried out in
patients,
• as patient get benefited from the study,
• reflects better therapeutic efficacy,
• drug absorption pattern in disease state can be studied,
• avoids ethical quandary of administering
drug to healthy subjects.
But there are also various drawbacks like,
• diseases,
• other drugs,
• physiological changes,
• fasting state is difficult etc.
Hence usually these studies performed on young (20-40yrs)
healthy male adult volunteers (body weight ±10%) under
restricted dietary and fixed activity conditions.
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11. 11
Methods of Bioavailability measurement
1. Pharmacokinetic methods
A. Plasma level time studies
B.Urinary excretion studies
2. Pharmacodynamic methods
A. Acute pharmacological response
B. Therapeutic response
12. Methods of Bioavailability measurement
Pharmacokinetic methods
1. Plasma level time studies:
most reliable method of choice comparison to urine data
method
Single dose: serial blood samples collection – 2-3 half lives
Plot of concentration vs time
For I.V. Sampling started within 5 min and subsequent
samples at 15 min intervals
For oral dose at least 3 points taken on absorption curve
( ascending part).
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13. Parameters considered important in plasma level time
studies
1. Cmax: It is peak plasma concentration. It increases with
dose as well as increase in rate of absorption.
2. Tmax: The peak time at which Cmax attainded.
3. AUC: Area under curve explains about amount of drug.
Doseoral
Doseintravenous
AUCintravenous
AUCoral
F
A U C T E S T
D o s e S T D
r e l
F
TEST
CSSMAX Dose t
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std
rel
F
A U C S T D D o s e T E S T
CSSMAX
DoseSTD ttest
1. Plasma level time studies:
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2. Urinary excretion studies:
This method is based on the principle that the urinary excretion of
unchanged drug is directly proportional to the plasma
concentration of drug.
It can be performed if
-At least 20% of administered dose is excreted unchanged in
urine.
The study is useful for
- Drugs that extensively excreted unchanged in urine eg.
Thiazide diuetics
- Drugs that have urine as site of action eg. Urinary antiseptics
like nitrofurontoin.
Steps involved:
-collection of urine at regular intervals for 7 half lives.
- Analysis of unchanged drug in collected sample.
- Determination of amount of drug at each interval and
cumulative as well.
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2. Urinary excretion studies:
Criteria's must be followed
- At each sample collection total emptying of bladder is
necessary.
- Frequent sampling is essential in the beginning to
compute correct rate of absorption.
- The fraction excreted unchanged in urine must
remain constant.
Parameters considered important in Urinary excretion
studies
1. (Dx/dt)max: Maximun urinary excretion rate
2. (tu)max: Time for maximum excretion rate
3. Xu∞: Cumulative amount of drug excreted in the
urine.
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1. Acute pharmacological response:
• When bioavailability measurement by
pharmacokinetic methods is difficult, inaccurate or
non reproducible this method is used. Such as ECG,
EEG, Pupil diameter etc.
• It can be determined by dose response graphs.
Responses measure for at least 3 half lives.
• Disadvantages:
- Pharmacological response is variable and accurate
correlation drug and formulation is difficult.
-Observed response may be due to active
metabolite.
Pharmacodynamic methods
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2. Therapeutic response:
This method is based on observing clinical response
in patients.
Drawbacks:
- Quantitation of observed response is too improper.
- Theoretically best method, but practically not.
- Pharmacodynamics of drug changes with Age,
Tolerance and interactions etc.
- The patient s receiving more than one drug
treatment may be compromised due to drug-drug
interaction.
Pharmacodynamic methods
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Bioequivalence studies
If a new product is intended to be a substitute for
approved medicinal product, then Equivalence must be
shown.
To ensure the clinical performance of such products,
Bioequivalence studies are carried out.
To compare the Bioavailability of generic products
with the brand products.
Bioequivalence experimental study designs
1.Completely randomized design
2.Randomized block design
3.Repeated measures, cross over design
4.Latin square design
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1.Completely randomized design:
All treatments are randomly allocated among all
experimental subjects.
e.g. If there are 20 subjects, number the from 1 to 20.
randomly select non repeating numbers among these
labels for the first treatment. And then repeat for all other
treatments .
Advantages:
• Easy to construct.
• Can accommodate any number of treatment and
subjects, Simple to analyze.
Disadvantages:
• Suited for few treatments.
• All subjects must be homogenous other wise random
error will occur.
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2.Randomized block design:
First subjects are divided into homogenous groups, called
blocks and then treatments are assigned at random within
blocks.
Subjects are divided as blocks considering similar
background, characteristics.
Advantages:
• Effective and Systematic grouping gives more precise
results.
• No need of maintaining equal sample size,
• Any number of treatments can be followed.
Disadvantages:
• Missing observations in a block require more complex
analysis.
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3.Repeated measures, carry over design:
• It is a kind of randomized block design where same
subject serves as a block.
• Same subject utilized repeatedly for each study period so
called as repeated measure design.
• Carry over effects of drugs are measured.
Advantages:
• Precise, Economic, can be performed with few subjects.
• Useful in observing effects of treatment over time in the
• same subject at different points of time, and
• in different subjects over a specified period of time
Disadvantages:
• Cary over effect due to preceding treatment.
• Wash out period necessary – 10 elimination half lives.
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4.Latin square design:
All other above designs are continuous trial.
However in Latin square design each subject receives
each treatment during the experiment.
It is a two factor design ( Rows=Subjects and
Columns=Treatments ). Carry –over effects are balanced.
Advantages:
• Minimize variability of plasma profiles and carry-over
effects.
• Small scale experiments can be carried out.
• Possible to focus on formulation variables.
Disadvantages:
• Long time study.
• More formulations more complex study.
• Subject dropout rates are high.
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In vitro dissolution studies and bioavailability:
• The physicochemical property of most drugs that has
greatest influence on absorption from GIT is dissolution
rate.
• However in vitro dissolution is good substitute for in
vivo study in terms of saving cost and time.
• The best available tool today which can at least
quantitatively assure about the bioavailability of drug from
its formulation is in vitro dissolution test.
24. In vitro- in vivo correlation ( IVIVC):
It is defined as the predictive mathematical model that describes the
relationship between in vitro property ( rate & extent of dissolution)
and in vivo response ( plasma drug concentration).
The main objective of developing and evaluating IVIVC is to use
dissolution test to serve as alternate for in vivo study in human beings.
IVIVC Levels:
Level A: The highest category of correlation. It represents point to
point correlation between in vitro dissolution and in vivo rate of
absorption.
Advantages: serves as alternate for in vivo study, change in manf.
Procedure or formula can be justified without human studies.
Level B: The mean in vitro dissolution time is compare with mean in
vivo residence time. It is not point to point correlation . Data can be
used for quality control standards.
Level C: It is single point correlation. e.g. t50%, Tmax, Cmax. This level
is only useful as guide for formulation development or quality
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control.
