PDZ-domain-mediated PPIs represents a promising strategy for
specific therapeutic intervention of signaling events rather than
targeting entire signaling cascades, as typically achieved by receptor
antagonists. PDZ domains are attractive therapeutical targets as
they are associated with a wide range of diseases and disorders
ranging from neurodegenerative diseases and mental disorders to
cancer [6, 7]. However, drugging PDZ domains have been shown
to be challenging as they are highly promiscuous and peptide drug
affinities tend to range in the low micromolar range [8, 9].
A successful story on targeting PDZ mediated interactions is
the development of NA-1 (Tat-NR2B9c, nerinetide), which published their phase III clinical trials in February 2020 [10], as well as
AVLX-144 (UCCB01–144), both targeting the PDZ domains of
the postsynaptic density protein of 95 (PSD-95) as potential stroke
treatment. During cerebral ischemia, an excessive amount of glutamate is released from the presynapse into the synaptic cleft. The
released glutamate hyper-activates the N-methyl-D-aspartate
(NMDA) receptors in the postsynaptic membrane, which leads to
increased influx of Ca
2+ ions into the postsynapse. Ca
2+ ions are
required to activate the calmodulin-dependent enzyme neuronal
nitric oxide synthase (nNOS), which forms a ternary complex with
the NMDA receptor and PSD-95, and generates nitric oxide
(NO) in excess that ultimately leads to excitotoxicity. The proximity
provided by the complex is crucial for NO production, and it is
mediated by the PSD-95 PDZ domains 1 and 2 [11–13].
Several attempts to inhibit generation of NO in cerebral ischemia have been made by targeting upstream and downstream proteins of PSD-95. For example, by competitive or noncompetitive
NMDA receptor antagonists [14–17], which blocks glutamatemediated activation of NMDA receptors and therefore inhibit
influx of Ca
2+ [18]. However, blocking the NMDA receptor has
severe side effects such as induced impairment of key brain functions, including sedation and psychotomimetic side effects. Furthermore, NMDA receptor agonists have a short therapeutic
window for drug administration, as they are effective only when
administrated before or shortly after a stroke [19]. Therefore, to
block the downstream interaction between the PDZ domains of
PSD-95 and their interaction partners nNOS and the NMDA
receptor has proven to prevent ischemic brain damage in both
rodents and nonhuman primates [20].
Targeting the PDZ domains of PSD-95 by small moleculebased inhibitors has been unsuccessful. Binding affinities did not
exceed the micromolar range, which is probably due more distant
key interaction points along the binding pocket compared to traditional small molecule targets [1]. PDZ domain binding typically
happens when an antiparallel β-strand ligand interacts with the βB
strand and the αB helix of the PDZ domain, making the PDZ
domain binding pocket especially large. Moreover, there are some
158
Dominik J. Essig et al.
specific therapeutic intervention of signaling events rather than
targeting entire signaling cascades, as typically achieved by receptor
antagonists. PDZ domains are attractive therapeutical targets as
they are associated with a wide range of diseases and disorders
ranging from neurodegenerative diseases and mental disorders to
cancer [6, 7]. However, drugging PDZ domains have been shown
to be challenging as they are highly promiscuous and peptide drug
affinities tend to range in the low micromolar range [8, 9].
A successful story on targeting PDZ mediated interactions is
the development of NA-1 (Tat-NR2B9c, nerinetide), which published their phase III clinical trials in February 2020 [10], as well as
AVLX-144 (UCCB01–144), both targeting the PDZ domains of
the postsynaptic density protein of 95 (PSD-95) as potential stroke
treatment. During cerebral ischemia, an excessive amount of glutamate is released from the presynapse into the synaptic cleft. The
released glutamate hyper-activates the N-methyl-D-aspartate
(NMDA) receptors in the postsynaptic membrane, which leads to
increased influx of Ca
2+ ions into the postsynapse. Ca
2+ ions are
required to activate the calmodulin-dependent enzyme neuronal
nitric oxide synthase (nNOS), which forms a ternary complex with
the NMDA receptor and PSD-95, and generates nitric oxide
(NO) in excess that ultimately leads to excitotoxicity. The proximity
provided by the complex is crucial for NO production, and it is
mediated by the PSD-95 PDZ domains 1 and 2 [11–13].
Several attempts to inhibit generation of NO in cerebral ischemia have been made by targeting upstream and downstream proteins of PSD-95. For example, by competitive or noncompetitive
NMDA receptor antagonists [14–17], which blocks glutamatemediated activation of NMDA receptors and therefore inhibit
influx of Ca
2+ [18]. However, blocking the NMDA receptor has
severe side effects such as induced impairment of key brain functions, including sedation and psychotomimetic side effects. Furthermore, NMDA receptor agonists have a short therapeutic
window for drug administration, as they are effective only when
administrated before or shortly after a stroke [19]. Therefore, to
block the downstream interaction between the PDZ domains of
PSD-95 and their interaction partners nNOS and the NMDA
receptor has proven to prevent ischemic brain damage in both
rodents and nonhuman primates [20].
Targeting the PDZ domains of PSD-95 by small moleculebased inhibitors has been unsuccessful. Binding affinities did not
exceed the micromolar range, which is probably due more distant
key interaction points along the binding pocket compared to traditional small molecule targets [1]. PDZ domain binding typically
happens when an antiparallel β-strand ligand interacts with the βB
strand and the αB helix of the PDZ domain, making the PDZ
domain binding pocket especially large. Moreover, there are some
158
Dominik J. Essig et al.