This document provides information about bioavailability and bioequivalence studies. It defines bioavailability as the rate and extent to which a drug enters systemic circulation from a dosage form. Factors influencing bioavailability include pharmaceutical, patient, and route of administration factors. The objectives of bioavailability studies are discussed, including determining the influence of excipients and possible drug interactions. Types of bioavailability study designs covered include absolute vs relative bioavailability, single vs multiple dose studies, and using healthy subjects vs patients. Methods for measuring bioavailability through pharmacokinetic studies of plasma drug levels and urinary excretion studies are also summarized.

1. 1
Syed Rashed Faizan Mehdi
BIOAVALABILITY
&
BIOEQUIVALENCE

2. 2
TABLE OF CONTENTS
S.NO CONTENTS PAGE NO
1 Need for BA and BE 3-4
2 When should BE conducted 5-6
3 Bioavailability 7
4 Factors influencing BA 8-10
5 Objectives 11
6 Types of BA
• Absolute BA
• Relative BA
12
13-16
17-20
7 Single Vs Multiple dose study 21
8 Human volunteer Vs Patient 24
9 Measurement of BA 26
10 Absolute BA by I.V route is
not always 100%
38

3. 3
TABLE OF CONTENTS
S.NO CONTENTS PAGE NO
11 Why may a BA higher than
100% be compared
39
12 Bioequivalence 40
13 Types of bioequivalence 41

4. 4
Which formulation has higher
bioavailability?

5. NEED FOR BIOAVAILABILITY – BIOEQUIVALENCE
STUDIES:
Bioavailability
 To evaluate the absolute systemic availability of oral, topical,
intramuscular, or any other dosage forms.
 To determine if bioavailability parameters are linear over the
proposed clinical dosage range.
 To estimate inter and intra subject variability
 To study food effects
Bioequivalence
 Surrogate for therapeutic equivalence to enable switch
ability
 An appropriate measure for the quality control of the
products in vivo

6. When should bioequivalence studies conducted:
 When a generic formulation is tested against an
innovator brand.
 When the proposed dosage form is different from that
used in clinical trails.
 When significant changes are made in the manufacture
of the marketed formulation

7. 7
Introduction to bioavailability
 The therapeutic effectiveness of a dosage form, intended to treat a
systemic condition, is the ability to deliver the active ingredient to
its site of action at a rate and amount sufficient to cause the
desired response.
 This property of a dosage form is referred as physiologic
availability, biologic availability or bioavailability
 Bioavailability captures two essential features, namely how fast
the drug enters the systemic circulation (rate of absorption) and
how much of the nominal strength enters the body (extent of
absorption)
 Bioavailability of a drug can be determined by testing the drug
in biological fluids drawn at different intervals.

8. 8
Bioavailability
 It is defined as the rate and extent at which the unchanged drug
is delivered to the systemic circulation from a dosage form.
 The rate or rapidity with which a drug is absorbed is an
important consideration when a rapid onset of action is desired
as in the treatment of acute conditions like asthma attack, pain.
 A slower absorption rate is desired when the aim is to prolong
the duration of action or to avoid the adverse effects.
 Extent of absorption is significance in treatment of chronic
conditions like hypertension, epilepsy etc.
 If the size of the dose to be administered is same, then bioavailability
of drug form its dosage form depends upon 3 major factors
1. Pharmaceutical factors.
2. Patient related factors.
3. Route of administration. 8

9. Three distinct factors are involved to influencing bioavailability.
These are:
1.Pharmaceutical factors:
 physicochemical properties of the drug.
1. Particle size
2. Crystalline structure
3. Salt form
 Formulation and manufacturing variables.
1.Disintegration and dissolution time
2.Pharmaceutical ingredients
3.Nature and type of dosage form
9

10. 2. Patient related factors
 Physiologic factors.
1.Variations in pH of GI fluids
2.Gastric emptying rate
3. Intestinal motility
4. Presystemic and first-pass metabolism
5. Age, sex
6. Disease states
 Interactions with other substances.
1. Food
2. Fluid volume
3. Other drugs
10

11. 3. Route of administration:
1.Parentral administration 2.Oral administration
3.Rectal administration 4.Topical administration
 The influence of route of administration on drug’s bioavailability
generally follows
 this order: Parenteral > oral > rectal > topical.
 Within parenteral route, i.v. injection of a drug results in 100%
bioavailability as the absorption process is bypassed.
 In case of oral route, the dose available to the patient is called Bio
available dose which is often less than the administered dose.
 Therefore, the bio-available fraction F, refers to the fraction of
administered dose that enters the systemic circulation. The value
ranges between 0 to 1.
F =
𝑩𝒊𝒐𝒂𝒗𝒂𝒊𝒍𝒂𝒃𝒍𝒆 𝒅𝒐𝒔𝒆
𝑨𝒅𝒎𝒊𝒏𝒊𝒔𝒕𝒆𝒓𝒆𝒅 𝒅𝒐𝒔𝒆11

12. OBJECTIVES OF BA STUDIES:
Bioavailability studies are important in the…
 Primary stages of development of a suitable dosage form
for a new drug entity.
 Determination of influence of excipients, patient related
factors & possible interaction with other drugs on the
efficiency of absorption.
 Development of new formulations of the existing drugs.
 Control of quality of a drug product during the early
stages of marketing in order to determine the influence of
processing factors, storage & stability on drug absorption.
12

13. Types of bioavailability study design
1.Absolute VS. Relative Bioavailability
2.Single dose VS. Multiple dose study
3.Human volunteers-healthy subjects VS. Patients
13

14. 14
1. Absolute bioavailability
 It is the comparison of bioavailabilities of drug upon oral
and IV (Intra Vascular) administration.
 Intravenous dose is selected as a standard because the
drug is administered directly into the systemic circulation
and avoids absorption (100% bioavailability)
 The oral solution cannot be taken used as standard dose
due to the following demerits.
A. All the pharmacokinetic parameters cannot be
determined
B. It is difficult to differentiate between the dose
unabsorbed and metabolized

15. 15
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
Area under concentration
curve (AUC)
Absolute bioavailability – IV bolus

16. 16
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
Area under concentration
curve (AUC)
Absolute bioavailability – oral dosage form (A)

17. 17
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
For the same dose(IV vs. Oral),
the bioavailability is given by:
Absolute bioavailability of IV and Oral dosage form
F =
𝐴𝑈𝐶 𝑜𝑟𝑎𝑙
∗
𝑑𝑜𝑠𝑒 𝐼
.
𝑉
𝐴𝑈𝐶 𝐼
.
𝑉
∗ [𝑑𝑜𝑠𝑒] 𝑜𝑟𝑎𝑙

18. 18
 The relative bioavailability is comparison of the
bioavailability of an orally administered drug with the
bioavailability of an oral standard (suspension or
solution) of the same drug.
1. Relative bioavailability

19. 19
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
Area under concentration
curve (AUC)
Relative bioavailability for oral dosage form standard

20. 20
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
Area under concentration
curve (AUC)
Relative bioavailability for oral dosage form test

21. 21
0 5 10 15 20 25 30
0
20
40
60
80
100
Time
Concentration
For the same dose (Oral vs. Oral)
The bioavailability is given by:
F =
𝐴𝑈𝐶 𝑇𝑒𝑠𝑡
∗
𝑑𝑜𝑠𝑒 𝑆𝑡𝑑
𝐴𝑈𝐶 𝑆𝑡𝑑
∗ [
𝑑𝑜𝑠𝑒] 𝑇𝑒𝑠𝑡
Relative bioavailability for oral dosage form standard
and test

22. 22
 Single dose bioavailability studies are very common, easy,
offer less exposure and less tedious.
 But, it’s difficult to predict the steady state characteristics
and intersubject variability by this method.
2. Single dose VS. Multiple dose.

23. 23
 Multiple dose study is difficult to control(poor subject
compliance), exposes the subject to more drug , highly
tedious and time consuming but has several advantages like:
 More accurately reflect the manner in which the drug
should be used.
 Better evaluation of performance of a controlled release
formulation is possible.
 Nonlinearity in pharmacokinetics, if present, can be easily
detected.
 Easy to predict the peak & valley characteristic of the drug
since the bioavailability is determined at steady – state.
2. Single dose VS. Multiple dose.

24. 24
 Requires collection of fewer blood samples.
 Can be ethically performed in patients because of the
therapeutic benefit to the patient.
Multiple dose continued.
 In multiple dose study,
one must ensure that
steady state level has
been reached.
 For this, the drug
should be administered
for 5-6 elimination
half lives before
collecting blood
sample.

25. 25
 Ideally, bioavailability studies should be carried out in
patients for whom the drug is intended to be used because of
the apparent advantages
1. The patient will be beneficial from the study.
2. Reflects better therapeutic efficacy of a drug.
3. Drug absorption pattern in disease states can be
evaluated.
4. Avoids the ethical requirements of administering drugs to
the healthy subjects.
 In multiple dose study, they prefer patients rather than
healthy humans
3. Human volunteers-healthy subject VS. Patient

26. 26
 But, the drawbacks of using patients as volunteers are –
disease, other drugs the patients may be taking,
physiological changes, etc. may modify drug absorption
pattern.
 studies should be performed in young – 20 to 40 years,
healthy, male, adult volunteers, body weight within a range ±
10%, under restricted dietary and fixed activity condition.
 The consent of volunteers must be obtained and they must be
informed about the conditions to be followed during the
course of studies – to abstain from any other medication
for at least 2 weeks and to fast overnight prior to and for a
minimum of 2-4 hours post dosing as well as possible
hazards if any.
3. Human volunteers-healthy subject VS. Patient

27. 1. Pharmacokinetics ( Indirect method) – the assumption that the
pharmacokinetics profile reflects the therapeutics effectiveness of
a drug
1. plasma level-time studies
2. urinary excretion studies
2 Pharmacodynamics ( Direct method) – Measurement of drug
effects on a physiological process as a function of time.
1. Acute pharmacological studies
2. Therapeutic response
Measurement of bioavailability

28. 28
 The methods is based on assumption that two dosage form that
exhibit superimposable plasma level-time profile in a group of
subjects should result in identical therapeutic activity with the
“single dose study”
 The method involves collection of serial blood samples for periods
of 2-3 biological half-lives after drug administration, their
analysis for drug concentration and making a plot of
concentration VS. corresponding time of sample collection to
obtain the plasma level-time profile.
 The three parameters of plasma level-time studies which are
considered important for determining bioavailability are-
1) C max :-
 The peak plasma concentration that gives an indication whether
the drug is sufficiently absorbed systematically to provide a
therapeutic response
Plasma level time studies:

29. 29
 C max is a function of both rate and extent of absorption
 C max will increase with an increase in the dose as well as with
an increase in the absorption rate
Plasma level time studies:

30. 30
 T max:-
The peak time that gives an indication of the rate of
absorption.
It decreases as the rate of absorption increase.
 AUC:-
The area under the plasma level time curve that gives a
measure of the extent of absorption or the amount of drug
that reaches the systemic circulation.
 The extent of bioavailability can be determined by the
following equations
Plasma level time studies:
F =
𝐴𝑈𝐶 𝑜𝑟𝑎𝑙 ∗
𝑑𝑜𝑠𝑒 𝐼. 𝑉
𝐴𝑈𝐶 𝐼. 𝑉
∗ [𝑑𝑜𝑠𝑒] 𝑜𝑟𝑎𝑙
F =
𝐴𝑈𝐶 𝑇𝑒𝑠𝑡 ∗
𝑑𝑜𝑠𝑒 𝑆𝑡𝑑
𝐴𝑈𝐶 𝑆𝑡𝑑 ∗ [
𝑑𝑜𝑠𝑒] 𝑇𝑒𝑠𝑡

31. 31
 With the multiple dose studies, drug administration for
at least 5 biological half lives.
 A blood sample should be taken at the end of previous
dosing interval and 8 to 10 sample after the administration
of next dose.
 The extent of bioavailability can be given as
Plasma level time studies:
F =
𝑨𝑼𝑪 𝑻𝒆𝒔𝒕 𝒅𝒐𝒔𝒆 𝑆𝑡𝑑 τ 𝑻𝒆𝒔𝑡
𝑨𝑼𝑪 𝑆𝑡𝑑 𝒅𝒐𝒔𝒆 𝑻𝒆𝒔𝒕 τ 𝑠𝑡𝑑
τ= Tau is a dosing interval
 Bioavailability can also be determined from the peak plasma
concentration at steady state Css, max according to eq
F =
Css, max 𝑻𝒆𝒔𝒕 𝒅𝒐𝒔𝒆 𝑆𝑡𝑑 τ 𝑻𝒆𝒔𝑡
Css, max 𝑆𝑡𝑑
𝒅𝒐𝒔𝒆 𝑻𝒆𝒔𝒕
τ 𝑠𝑡𝑑

32. 32
Plasma level time studies:

33. 33
 In this method of assessing bioavailability is based on the
principle that the urinary excretion of unchanged drug
is directly proportional to the plasma concentration of
the drug.
 A rule of thumb that at least 20% of administered dose
is excreted unchanged in the urine then only the urinary
excretion data should be conducted.
 The study is particularly useful for
I. Drug extensively excreted unchanged in the urine
eg:sulphonamides and thiazide diuretics
II. Drug that have urine as the site of action
eg:nitrofurantoin and hexamine
Urinary excretion studies:

34. 34
 Method involves
1. Collection of urine at regular intervals for time-span
equal to 7 biological half-lives.
2. Analysis of unchanged drug in the collected sample.
3. Determination of amount of drug excreted in a each
interval and cumulative amount excreted
Urinary excretion studies:
 The three major parameters examined in urinary excretion
data obtained with a single dose study are
 D xu/dt max:-
 The maximum urinary excretion rate
 It is obtained from the peak of plot between rate of
excretion versus midpoint time of urine collection period

35. 35
Urinary excretion studies:
 Tu max:-
 The time for maximum excretion rate
 It is analogous to the T max of the plasma level data
 Its value decreases as the absorption rate increases
 The extent of bioavailability can be determined by the
following equations
 X∞
u:-
 The cumulative amount of drug excreted in the urine
 It is related to the AUC of plasma level data and increases as
the extent of absorption increases
F =
X∞
u 𝑜𝑟𝑎𝑙 ∗
𝑑𝑜𝑠𝑒 𝐼. 𝑉
X∞
u 𝐼
.
𝑉 ∗ [𝑑𝑜𝑠𝑒] 𝑜𝑟𝑎𝑙
F =
X∞
u 𝑇𝑒𝑠𝑡 ∗
𝑑𝑜𝑠𝑒 𝑆𝑡𝑑
X∞
u 𝑆𝑡𝑑
∗ [
𝑑𝑜𝑠𝑒] 𝑇𝑒𝑠𝑡

36. 36
Urinary excretion studies:
 With the multiple dose study to steady state, the
equation for computing bioavailability is
F =
Xu,ss, 𝑻𝒆𝒔𝒕 𝒅𝒐𝒔𝒆 𝑆𝑡𝑑 τ 𝑻𝒆𝒔𝑡
Xu,ss, 𝑆𝑡𝑑 𝒅𝒐𝒔𝒆 𝑻𝒆𝒔𝒕 τ 𝑠𝑡𝑑
 Where Xu,ss is the amount of drug excreted unchanged
during a single dosing interval at steady state
Time of sample collection

37. 37
 When bioavailability measurement by pharmacokinetic
methods is difficult, inaccurate or non-reproducible.
 Acute pharmacological effect like
I. EEG reading
II. ECG
III. Pupil diameter
 The methods requires measurement of response for at
least 3 biological half-lives of the drug
 Disadvantages:
1. Accurate correlation between measured response and
drug available from the formulation is difficult
Pharmacodynamic methods
• Acute pharmacological response method

38. 38
 Theoretically the most definite, this method is based on
observing the clinical response to a drug formulation
given to patient suffering from disease for which it is
intended to be used.
1. Quantitation of observed response is to improper.
2. Many patient receive more than one drug.
Pharmacodynamic methods
• Therapeutic response method

39. 39
• By assumption, a drug administered by the i.v. route has
100%bioavailability.
• This is true only if the active substance reaches arterial blood
without loss.
• Drugs are generally administered by the i.v. route and have first to
cross the pulmonary circulation before gaining access to arterial
blood.
• Lungs can be the site of an extensive first-pass effect and reduce
drug availability.
• This is the case for prostaglandins or some amines.
Absolute bioavailability by the I.V. route is not always 100%

40. 40
• Bioavailabilities higher than 100% are regularly reported, which
is conceptually impossible.
• The reasons for this are numerous, including experimental
errors and nonfulfilment of the assumption for computation of
absolute bioavailability.
Why may a bioavailability higher than 100% be computed

41. 41
Bioequivalence
 Bioequivalence indicates that the drug from two or
more similar dosage forms gets absorbed at the same
rate and to the same extent into the systemic
circulation.
 In simpler terms, the dosage forms are said to be
bioequivalent when their plasma level time profile
are identical.
 Generally the rate and extent of absorption of
bioequivalent drug product differ by 20% or less.

42. 42
• Chemical equivalence: It indicates that two or more drug products that
contain the same labelled chemical substance as an active ingredient
in the same amount.
• Bioequivalence: It is a relative term which denotes that the drug
substance in two or more identical dosage forms, reaches the systemic
circulation at the same relative rates and to the same relative extent
i.e their plasma concentration time profiles will be identical without
the significant statistical differences.
• Any statistically significant difference observed in the bioavailability of
the drug product is an indication of bioequivalence.
• Therapeutic equivalence: This term indicates that two or more drug
products that contain the same therapeutically active ingredient elicit
identical pharmacological effects and can control the disease to the
same extent.
Types of Bioequivalence

43. 43
• Clinical equivalence: when the same drug from 2 or more dosage forms
gives identical in vivo effects as measured by pharmacological
response or by control over a symptom or a disease.
• Pharmaceutical equivalence: The term implies that two or more drug
products are identical in strength, quality, purity, content uniformity,
disintegration and dissolution characteristics.
They may however differ in containing different excipients.
Types of Bioequivalence

44. 44
References:
1) Biopharmaceutics and pharmacokinetics by Bhramankar.
2) Bioequivalence and statistics in clinical pharmacology
Series editors: N. Keiding, B. Morgan, T. Speed, P. van der Heijden
3)Biopharmaceutics and pharmacokinetics by venkateshwaluru