Bioavailability and bioequivalance notes for Pharm.D.Ch:7

1. S.RAVIKIRAN
IV-PHARM.D
J.K.K.NATTRAJA COLLEGE OF PHARMACY

2. Introduction
 Bioavailability is a measurement of the rate and extent of drug
that reaches the systemic circulation from a drug product or a
dosage form.
 There are two different types of bioavailability studies:
1.First type involves an assessment of the bioavailability of new
drug formulation ,pharmacokinetic parameters following
different routes of administration of the new drug are obtained
and are utilized in developing an optimum dosage regimen.
2.Second type is comparison of a test formulation with that of
reference standard dosage form that is proved to have
therapeutic efficacy and safety is known as bioequivalence
studies .

3. Bioavailability
Def: Bioavailability is a measurement of rate and extent of
active drug that reaches the systemic circulation and
available at the site of action.
(OR)
Def: Bioavailability is defined as the rate and the absorption
of drug that reaches the biological system in an active
form, capable of exerting the desired pharmacological
effect, including its onset, intensity and duration of its
action.

4.  The bioavailability or systemic availability of an orally
administered drug depends largely on the absorption and
the extent of hepatic metabolism ¾The bioavailability of
an oral dosage form is determined by comparing the Area
Under Curve (AUC) after oral administration of a single
dose with that obtained when given IV.
 Drug bioavailability = AUC (oral)/ AUC (IV)
= Bioavailable dose/Administered dose.

5. Bioequivalence
Def: It refers to a procedure that compares the bioavailability of
a drug from different formulations.
 The aim of bioavailability study is to find out the dosage form
influence on the biological performance of the drug.
 The bioavailability study protocol used to detect differences in
the rate and extent of absorption that are attributable only to
dosage form variability and should avoid variabilities due to
other factors.
 Bioequivalence studies are for determination o f the
therapeutic equivalence between the pharmaceutical
equivalence generic drug product and a corresponding
reference listed drug.

6. LIMITATION OF BIOAVAILABILITYAND
BIOEQUIVALENCE
 A cross over design may be difficult for drugs with a long
elimination half life.
 Highly variable drugs may require a far greater number of subjects
to meet the FDA bioequivalence characteristics.
 Certain characteristics in the biotransformation of drugs make it
difficult to evaluate the bioequivalence of such drugs.
 For e.g. for drugs that are stereoisomer with a different rate of
biotransformation and a different pharmacodynamic response, the
measurement of individual isomers may be difficult for analytical
reasons.
 Drugs that are administered by routes other than the oral route
drugs/dosage forms that are intended for local effects have minimal
systemic bioavailability. Eg. ophthalmic, dermal, intranasal and
inhalation drug products.

7. Bioavailability study protocol
 The aim of bioavailability study is to find out the dosage form
influence on the biological performance of the drug, sensitivity
to detect differences in the rate and extent of absorption.
 Bioavailability study protocol divided into:-
A. Study objective.
B. Study design.
1.Experimental design.
2.Wash out period.
3.Drug products.
I. Test product.
II.Recognized standard.

8. 4.Route of administration.
5.Dosage regimen.
6.Frequency and duration of sampling.
7.Randomization of drug administration.
8. Single-multiple-dose study design.
9.Subjects.
I. Healthy subjects versus patients.
II. Subject selection.
a.Medical history.
b.Physical examination.
c.Laboratory tests.
III. Study conditions.
10.Analysis of biological fluids.

9. C. Methods of Assessment of Bioavailability.
1.Plasma data.
2.Urine data.
3.Acute pharmacological effect.
4.Clinical response.
D. Analysis and Presentation of Data.
1.Statistical treatment of data-Analysis of variance.
2.Format of data.

10. Study objective
 Bioavailability studies are performed for new drugs to
establish pharmacokinetic parameters including ™Rate of
absorption,Rate of excretion,™Metabolism,Elimination,Half life
of single and multiple dose administration.
 Factors have to be considered in conducting a bioavailability
study are rate and extent of absorption of a drug in to the
systemic circulation.
 Distribution and elimination are influenced by variety of
factors.
 Subject factors such as age, sex, disease state, food habits,
body weight ,time of administration and sampling, analytical
method and compartment model used in P.K-parameters.

11. Study design
 Study design is two types
1.Parallel design.
2.Cross over design.
a.Latin square cross over design.
b.Balanced incomplete block design.
Parallel design
The aim of experimental design is to minimize the experimental
variables and to avoid a bias.In parallel design two
formulations are administered to two groups of volunteers.
Disadvantages:- The intersubject variation is not being corrected.

12. Cross over design
 Minimizes the effect of intersubject variability in the study.
 Two types of cross over design is are used in bioavailability
trials:
1.Latin square cross-over design
2.Balanced incomplete block design (BIBD).
 This design has several advantages:
1.It minimizes the effect of inter-subject variability in the
study.
2.It minimizes the carry over effects, minimizes the time
effect.
3.On bioavailability ,it requires less number of subjects to
get results.

13. Advantages
 Minimize the effect of intersubject variability.
 It minimizes the carry over effects.
 It minimizes the time effect on bioavailability.
Disadvantages
 It requires longer time to complete the study.
 Increase number of study periods leads to high subject
dropouts and the study becomes difficult.

14. Washout period
 The time interval between the two treatments is called
washout period.
 Washout period is required for the elimination of the
administered dose of a drug.
 Washout period is a function of the half and dose of the
drug administered.
Eg.Digitoxin which has a half life of 6-9 days
washout period.

15. Drug products
Test products: Test products are generally evaluated for
following reasons.
 To compare biological performance of a test product to
that of a recognized standard i.e. bioequivalence studies.
 To select best dosage form of a new drug or existing drug
among different dosage forms. E.g. tablet, capsule,
emulsion and suspension.
REFERENCE STANDARD: generic product has to be
compared with some standard dosage form to verify its in
vivo performance .FDA accepts any innovators drug
product as a reference standard.

16. Sampling
 The sampling scheme should be frequent enough to define
the absorption phase, the peak and elimination phase
during a drug’s time course in the body.
 To estimate the AUC from the data, sampling has to be
carried out till the concentration of the drug reaches the
linear elimination phase.
 Sampling should proceed until 1/10 or 1/20 of the peak
levels reached.
 The AUC from the data,sampling has to carried out till the
Concentration of the drug reaches the linear elimination
phase.

17. Route of administration
 Most of the times, orally administered dosage forms are subjected for
bioavailability studies.
 Dosage forms administered by other routes (buccal, transdermal and
intramuscular) should also be evaluated for their biological performance.
Single dose vs multiple dose study design
 Useful to know wheather a single dose studies are better or multiple dose
studies are better for the assessment of the bioavailability of a drug
product.
 If dosage forms are to be evaluated only for bioequivalence purposes,
single dose studies are usually sufficient.
 Eg. analgesics for the relief of headache need only single dose studies.
 However certain dosage forms (time release products, enteric coated
preparations) requires multiple dose studies.

18. Administration of drug products
 Administration of drug products to the subjects should be
based on randomization.
 After the administration of the drug products, blood
samples are withdrawn from the subjects at fixed time
points. It takes some time to take a sample from each
subject, and the total time difference between first subject
and last subject may range from 10 to 20 minutes.
 This 10 to 20 minutes difference would represent a
substantial change in the drug concentrations observed in
the blood.

19. Sampling
 The sampling scheme should be frequent enough to define the absorption
phase, the peak and elimination phase during a drug’s time course in the
body.
 To estimate the AUC from the data, sampling has to be carried out till the
concentration of the drug reaches the linear elimination phase.
Selection of subjects
Healthy subjects vs patients:
 Use of healthy volunteers avoids much of variations that are possible with
patients.
 Some of special problems associated with testing in patients are given
below:-
1.It is difficult to obtain many patients in a given place.
2.The severity of a disease varies from one patient to another.

20. Study conditions
 The selected subject should be
1.Maintained on a uniform diet.
2.None of them should have taken any drug atleastone
week prior to study.
 In general bioavailability trials are conducted on subjects that
has fasted overnight.
Analysis of biological samples
 The biological samples collected as per the sampling
procedure have to be analyzed immediately after the study.
 Analysis of biological samples are carried out by
1.Analytical method
2.Non specific analytical method.

21. Methods of assessment of bioavailability
 Methods of assessment of bioavailability are in two types:-
1. Pharmacokinetic methods (indirect methods)
2.Pharmacodynamic methods (direct methods).
Pharmacokinetic methods:-
The parameters that are useful in determining the bioavailability of drug from
a drug product based on indirect methods are
1.Plasma data
a.Time of peak plasma concentration (tp)
b.Peak plasma concentration (Cmax)
c.Area under the plasma concentration-time curve (AUC).
2.Urine data
a.The rate of drug excretion in the urine (dXu/dt)
b.The cumulative amount of drug excreted in the urine (Xu∞)
c.The time for maximum urinary excretion (tu∞).

23.  MINIMUM EFFECTIVE PLASMA CONCENTRATION:-
The minimum plasma concentration of the drug required to achieve a
given pharmacological or therapeutic response. This value varies from drug to drug
and from individual to individual as well as with the type and severity of the
disease.
 MAXIMUM SAFE CONCENTRATION:-
The plasma concentration of the drug beyond which adverse effects are
likely to happen.
 THERAPEUTIC RANGE:-
The range of plasma drug concentration in which the desired response is
achieved yet avoiding adverse effect. The aim is clinical practice is to maintain
plasma drug concentration within the therapeutic range.
 ONSET OF ACTION:-
On set of action is the time required to achieve the minimum effective
plasma concentration following administration of drug formulation.

24.  DURATION OF ACTION:-
 Duration of action of the therapeutic effect of the drug is defined as the
time period during which the plasma concentration of the drug exceeds the
minimum effective level.
 INTENSITY OF ACTION:-
In general, the difference between the peak plasma concentration and the
minimum effective plasma concentration provides a relative measure of the
intensity of the therapeutic response of the drug.
Peak Plasma concentration:-
Peak concentration (Cmax) represents the highest concentration attained
by the drug in the plasma. At this concentration, rate of drug input becomes equal
to rate of drug output.
It is clear that formulation A should produce pain relief than formulation
B, even though it seemed well absorbed, would not produce the desired
pharmacological effect and would be ineffective in producing analgesia.

25. URINARY EXCRETION
 This method can be based if urinary excretion of
unchanged drug is the main mechanism of elimination of
the drug.
 Bioavailability can be calculated as follows,
F = (Du∞)/ f o
F = Fraction of the dose absorbed
Du∞ = cumulative amount of drug excreted in the urine
o f = fraction of unchanged drug excreted in the urine.

26. FACTORS AFFECTING BIOAVAILABILITY
 The dose of drug administered, i.e. the blood level will
rise and fall in Proportion to the dose administered.
 The same as the first but brought about by a different
process, of drug absorbed from a given dosage form. The
effect of only one half of the drug absorbed from a dosage
form is equivalent to lowering the dose.
 The rate of absorption of the drug, if absorption from the
dosage form is more rapid than the rate of absorption then
toxic level can be exceeded. If absorption from the dosage
form is sufficiently slow minimum effective level cannot
be attained.

27. THANK YOU
By,
S.Ravikiran
IV-Pharm.D