Biomedical Engineering Reference
In-Depth Information
Recently, it was further demonstrated by generation of transgenic mice expressing ORF-74 that it is
indeed the constitutively activated receptor, which alone is the cause of Kaposi's sarcoma. Clearly, it
would be highly desirable to develop selective inverse agonists of ORF-74, which would very likely
inhibit or even prevent the development of Kaposi's sarcoma in HHV8 infected humans.
Constitutive activity can also be caused by somatic mutations. The known examples include
constitutively activating mutations in the thyrotropin receptor and the luteinizing hormone receptor,
which leads to adenomas, and the rhodopsin receptor, which leads to night blindness. Moreover, in
this case it is conceptually possible to alleviate the diseases by the development of inverse agonists.
12.3.6 A
LLOSTERIC
M
ODULATORS
Allosteric modulators can both be stimulatory or inhibitory (noncompetitive antagonists) and typi-
cally these compounds bind outside the orthosteric binding site (binding site of the endogenous
agonist). Allosteric modulators have a number of potential therapeutic benei ts compared to agonists
and competitive antagonists, which has led to signii cant increased pharmaceutical interest in recent
years. This increased interest has also been fueled by the development of functional high-throughput
screening assays, which has made it possible to screen for allosteric modulators (see Section 12.3.2).
The allosteric modulators mentioned in the following text act through allosteric mechanisms as
evident from the fact that they do not displace radiolabeled orthosteric ligands. Furthermore, their
activity is dependent on the presence of agonists as they do not activate the receptors by themselves.
The fact that they bind outside of the orthosteric ligand-binding pocket often leads to increased
receptor subtype selectivity. Evolutionary pressure has led to conservation of the orthosteric binding
site at different subtypes, as radical mutations would severely impact the binding properties. Thus, it
is often seen that the orthosteric binding site is much more conserved than the remaining part of the
receptor and accordingly, ligands binding to an allosteric site have a higher chance of being selective.
Likewise, the allosteric ligands will have a different pharmacophore than the endogenous ligand,
which might improve, e.g. bioavailability. For example, ligands acting at the orthosteric site of the
GABA
A
receptor need a negatively charged acid function and a positively charged basic function,
which greatly impairs the transport through biomembranes, whereas allosteric ligands such as the
benzodiazepine Diazepam (Chapter 15) does not have any charged groups and show excellent bio-
availability. It is well known that many agonists, particularly full agonists, lead to desensitization and
internalization of receptors. Unlike agonists, the positive modulators should prevent the development
of tolerance (as seen for, e.g. morphine), because they avoid the prolonged receptor activation leading
to desensitization and internalization. The fact that the receptors are stimulated in a more natural way
by positive modulators rather than the prolonged receptor activation caused by agonists may also lead
to a difference in physiological effects, which may or may not be an advantage.
12.3.6.1 Negative Allosteric Modulators (Noncompetitive Antagonists)
As noted in the previous section, the Schild analysis is very useful to discriminate between com-
petitive and noncompetitive antagonists, and an example of the latter is shown in Figure 12.13A.
CPCCOEt is a selective antagonist at the mGluR1 receptor, and the Schild analysis clearly dem-
onstrates that the antagonism is noncompetitive due to the depression of the maximal response
(compare with Figure 12.11). As noted previously, glutamate binds to the large extracellular amino-
terminal domain whereas CPCCOEt has been shown to bind to the extracellular part of the 7TM
domain. CPCCOEt does not hinder binding of glutamate to the extracellular domain, but hinder the
conformational change leading to receptor activation.
12.3.6.2 Positive Allosteric Modulators
Positive allosteric modulation can be achieved through several mechanisms. For example, benzo-
diazepines positively modulate the GABA
A
receptor by increasing the frequency of channel open-
ing (Chapter 15). Positive modulation can also be obtained by blocking receptor desensitization as
exemplii ed by cyclothiazide (Chapter 15).
