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BIOASSAY OF DRUGS
By: underguidance of:
Shaikh ZohraMeena Dr. Anuradha Mishra
Bioassay is defined as the estimation of
the potency of an active principle in a unit
quantity of preparation.
Detection and measurement of the
concentration of the substance in a
preparation using biological methods.
Bioassays, as compared to other methods of assay
(e.g. chemical or physical assay) is very important.
Because it is the only method of assay if
1. Active principle of drug is unknown or cannot be
isolated, e.g. insulin, posterior pituitary extract etc.
2. Chemical method is either not available or if
available, it is too complex and insensitive or
requires higher dose e.g. insulin, acetylcholine.
3. Chemical composition is not known, e.g. long acting
4. Chemical composition of drug differs but have the
same pharmacological action and vice-versa, e.g.
cardiac glycosides, catecholamines etc.
The basic principle of bioassay is to
compare the test substance with the
International Standard preparation of the
same and to find out how much test
substance is required to produce the same
biological effect, as produced by the
Mainly two types are there.
Quantal response: the response is in the
form of ‘all or none’, i.e. either no
response or maximum response.
Drugs producing quantal effect can be
bioassayed by End-point method.
Graded response: response is proportional to the
dose and response may lie between no response
and the maximum response.
• Bracketing /direct matching
• Multiple point assays
Three point assay
Four point assay
Six point assay
• Cumulative dose response
1. End- point method
The threshold dose producing a predetermined effect is
Comparison between the results of standard and the test
drug is done.
e.g. bioassay of digitalis in cats.
The cat is anaesthetized with chloralose and its blood pressure
The drug is slowly infused into the animal.
The moment the heart stops beating and blood pressure falls to
zero, the volume of fluid infused is noted down.
Two series of such experiments-one using standard digitalis and the
other using test preparation of digitalis is done .
potency is calculated as follows:
Threshold dose of the Standard
Conc. of Unknown = X Conc. of Std.
Threshold dose of the Test
2. Matching and bracketing method
A constant dose of the standard is bracketed by varying dose of
test sample until an exact matching between the response of std
& that of the sample is achieved.
Initially, two responses of the standard are taken.
The doses are adjusted such that one is giving response of
approximately 20% and other 70% of the maximum.
The response of unknown which lies between two responses of
standard dose is taken.
The panel is repeated by increasing or decreasing the dose s of
standard till all three equal responses are obtained.
The dose of test sample is kept constant.
At the end, a response of the double dose of the standard and
test which match each other are taken.
These should give equal responses. Concentration of the test
sample can be determined as follows:
Dose of the Standard
Conc. of Unknown = X Conc. of Std.
Dose of the Test
Example: Bioassay of Histamine on guinea
3. Graphical method
This method is based on the assumption of the dose-response
Log-dose-response curve is plotted and the dose of standard
producing the same response as produced by the test sample is
directly read from the graph.
In simpler design, 5-6 responses of the graded doses of the
standard are taken and then two equiactive responses of the test
sample are taken.
The height of contraction is measured and plotted against the log-
The characteristic of log-dose response curve is that it is linear in
the middle (20-80%).
Thus, the comparison should be done within this range only. In
other words, the response of test sample must lie within this
Dose Log dose Response
0.1 -1 0.9 25.71
0.1 -1 0.9 25.71
0.2 -0.69 1.5 42.85
0.4 -0.39 2 57.14
0.6 -0.22 2.8 80
0.8 -0.09 3.5 100
1 0 2.3 65.71
0.1 -1 0.7 20
0.3 -0.52 1.3 37.14
Observations and calculation
Maximum response (3.5) is considered 100% response.
% response = 0.9 x 100 = 25.71
Other methods which are based on the dose-response
relationship include 3 point, 4 point and 6 point methods.
In these 3 methods, the responses are repeated several times and
the mean of each is taken.
Thus, chances of error are minimized in these methods.
In 3 point assay method 2 doses of the standard and one dose of
the test are used.
In 4 point method 2 doses of standard and 2 doses of the test are
used. In 6 point method 3 doses of standard and 3 doses of the
test are used.
Similarly one can design 8 point method also.
The sequence of responses is followed as per the Latin square method
of randomization in order to avoid any bias.
The mean responses are calculated and plotted against log-dose and
amount of standard producing the same response as produced by the
test is determined graphically as well as mathematically:
n1 T - S1 n2 Cs
Conc. of Unknown = t X antilog S2 - S1 X log n1
n1 = Lower Standard dose
n2 = Higher Standard dose
t = Test dose
S1 = Response of n1
S2 = Response of n2
T = Response of test (t)
Cs = Concentration of standard
Bioassay of histamine by three
Similarly, in 4 point method, amount of standard producing the
same response as produced by the test can be determined by
graphical method. It is determined mathematically as follows:
n1 (S1 + S2) – (T1 + T2) n2
Conc. Of unk= t1 X antilog (S2 + T2) - (S1 + T1) X log n1
t1 = lower dose of test
t2 = higher dose of test
T1 = response of t1
T2 = response of t2.
Commonly used method for the bioassay of
antagonist is simple graphical method.
The responses are determined in the form of
the % inhibition of the fixed dose of agonist.
These are then plotted against the log dose
of the antagonist and the concentration of
unknown is determined by finding out the
amount of standard producing the same
effect as produced by the test.
In this method, two responses of the same dose of agonist
(sub maximal giving approximately 80% of the maximum
response) are taken.
The minimum dose of standard antagonist is added in the
bath and then the response of the same dose of agonist is
taken in presence of antagonist.
The responses of agonist are repeated every 10 mins till
recovery is obtained.
The higher dose of standard antagonist is added and
responses are taken as before.
Three to four doses of the standard antagonist are used and
then one to two doses of test sample of the antagonist is
The percentage inhibition is calculated, plotted against log
dose of antagonist and the concentration of unknown is
determined as usual.
1. Rabbit Head-drop Method :
Principle: d-Tubocararine hydrochloride is
injected into the marginal vein of a rabbit’s ear
till the rabbit’s neck muscles are relaxed such
that the animal cannot hold its head up. The
total amount of test sample required to produce
the endpoint is compared with the total amount
of the standard sample required to produce
Selection of Rabbits: Rabbits weighing 2 kg.
are used. Animals should be free from disease,
obtained from a healthy colony and should be
accustomed with the experimental procedure.
o Rabbit is placed in a holder with its head protruding
o The head should be freely movable.
o Minimum 8 rabbits are used.
Rabbit head drop method for the bioassay of d-tubocurarine.
(i) : i.v. inj. of d-tubocurarine. (ii) : Head drop after injection.
o They are divided into two groups each
containing 4 rabbits.
o First group will receive standard sample and
the second group will receive the sample
o d-Tubocurarine solution is injected at a
constant speed by infusion apparatus through
the marginal vein.
o Injection should be given at a rate of 0.4
ml/min and should take about 10 min. Dose
0.012% w/v in saline.
o Infusion is continued till the rabbit will not be
in a position to hold its head erect or there
will be no response by focusing light on the
o Rabbits recover immediately from the effect
o During the experiment there is a possibility of
respiratory embarrassment which is treated
by injecting neostigmine, methyl sulphate
(0.05 mg.) and atropine sulphate immediately
through the marginal ear vein.
o Cross-over test is carried out to minimize
biological error due to animal variation.
o Those rabbits which received the standard
sample on the first day will be given test
sample on the second day of experiment and
o Mean dose which produces head drop of the
test sample is compared with the mean dose
of standard preparation.
Principle: Potency of the test sample is
compared with that of the standard preparation by
determining the action on the cardiac muscle.
Standard Preparation and Units: The standard
preparation is a mixture of dried and powdered
digitalis leaves (1 unit = 76 mg.)
Preparation of Extracts: Exact amount of the
powder is extracted with dehydrated alcohol in a
continuous extraction apparatus for six hours. The
final extract should contain 10 ml. (5 ml. alcohol +
5 ml. water) per 10 g. of digitalis powder. It should
be stored in between 5 oC and –5 oC.
o Minimum 6 pigeons are used for testing each
o The weight of the heaviest pigeon should not
exceed twice the weight of the lightest pigeon.
o Food is withheld 16-28 hours before the
o Pigeons are divided on the basis of their sex,
weight and breed, into two groups.
o They are anaesthetized with anesthetic ether.
o One side of the wing is dissected and the alar
vein is cannulated by means of a venous
cannula. Dilutions are made with normal
o The test sample and standard sample is
infused through cannula.
o In pigeons, stoppage of heart is associated with
a characteristic vomiting response called
o The milk from the crop sac of pigeons is being
ejected out. This may be taken as the end point
response of digitalis.
o The lethal dose per kg. of body weight is
determined for each pigeon.
o The potency of the test sample is determined
by dividing the mean lethal dose of standard by
the mean lethal dose of the test sample.
Principle: The potency of oxytocin is
determined by comparing its activity with that
of the Standard Preparation of oxytocin under
the conditions of a suitable method of assay.
Standard Preparation: The Standard
Preparation is the 4th International Standard
for Oxytocin, established in 1978, consisting
of freeze-dried synthetic oxytocin peptide with
human albumin and citric acid (supplied in
ampoules containing 12.5 Units)
o Anaesthetize a young healthy adult cockerel weighing
1.2 to 2.3 kg with an anesthetic that will maintain a
prolonged and constant high blood pressure.
o Expose the gluteus primus muscle in one thigh and cut
and retract it to reveal the popliteal artery and crural
o Cannulate the popliteal artery and record the blood
o Cannulate the crural or brachial vein.
o Prepare standard solution with saline. Inject
o Inject 2 doses of standard solution into
cannulated vein and record blood pressure.
o Dose should cause decrease in B.P.
o Interval between 2 injections is 3-10 mins or
depends on rate at which B.P. comes to
o Dilute test sample with saline same as
o Ratio between standard and test should be
o If animal become insensitive due to repetitive
doses so another animal is taken.
o Measure all responses and result is calculated
by standard statistical method.