Biomedical Engineering Reference
In-Depth Information
100
120
5-HT
2B
100
10
80
luo-3
60
1
40
20
0.1
0
-12
1
10
100
[Ca
2+
] (nM)
1000
10000
-11
-10
-9
Log [Agonist] (M)
-8
-7
-6
-5
-4
-3
(A)
(B)
NH
2
NH
2
NH
Cl
N
HO
HO
N
H
H
N
5-HT ( )
MK-212 ( )
2-Me-5-HT ( )
FIGURE 12.10
(A) Relation between Ca
2+
concentration and relative l uorescence intensity of the l uores-
cent probe l uo-3. (B) The 5-HT
2B
receptor subtype belong to the superfamily of GPCRs and are coupled to
increase in inositol phosphates and intracellular Ca
2+
. Cells expressing 5-HT
2B
receptors were loaded with
l uo-3 and the l uorescence was determined upon exposure to the endogenous agonist 5-HT (
) and the partial
agonists MK-212 (
) and 2-Me-5-HT (
) on a FLIPR
™
. (Adapted from Jerman, J.C. et al.,
Eur. J. Pharmacol
.,
414, 23, 2001.)
12.3.3 P
ARTIAL
AND
F
ULL
A
GONISTS
Agonists are characterized by two pharmacological parameters: potency and maximal response.
The most common way of describing the potency is by measuring the agonist concentration, which
elicit 50% of the compound's own maximal response (the EC
50
value). The maximal response is
commonly described as percent of the maximal response of the endogenous agonist. The maximal
response is also often described as efi cacy or intrinsic activity, which was dei ned by Stephenson
and Ariëns, respectively. Compounds, such as 2-Me-5-HT and MK-212 in Figure 12.10, show a
lower maximal response than the endogenous agonist and are termed partial agonists. The parame-
ters potency and maximal response are independent of each other and on the same receptor it is thus
possible to have, e.g., a highly potent partial agonist and a low potent full agonist. Both parameters
are important for drug research, and it is thus desirable to have a pharmacological assay system that
is able to determine both the potency and the maximal response of the tested ligands.
12.3.4 A
NTAGONISTS
Antagonists do not activate the receptors, but block the activity elicited by agonists and accordingly
they are only characterized by the parameter afi nity. The most common way of characterization
of antagonists is by competition with an agonist (functional assay) or a radioactively labeled ligand
(binding assay). In both cases, the antagonist concentration is increased and displaces the agonist or
radioligand, which are held at a constant concentration. It is then possible to determine the concen-
tration of antagonist that inhibits the response/binding to 50% (the IC
50
value). The IC
50
value can
then be transformed to afi nity (
K
) by the Cheng-Prusoff equation:
