Indole acids as a novel PDE2 inhibitor chemotype that demonstrate pro-cognitive activity in multiple species

Article abstract of DOI:10.1016/j.bmcl.2018.01.039

An internal HTS effort identified a novel PDE2 inhibitor series that was subsequently optimized for improved PDE2 activity and off-target selectivity. The optimized lead, compound 4, improved cognitive performance in a rodent novel object recognition task as well as a non-human primate object retrieval task. In addition, co-crystallization studies of close analog of 4 in the PDE2 active site revealed unique binding interactions influencing the high PDE isoform selectivity.

Full text of DOI:10.1016/j.bmcl.2018.01.039

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Accepted Manuscript
Indole Acids as a Novel PDE2 Inhibitor Chemotype that Demonstrate Pro-Cog-
nitive Activity in Multiple Species
Shawn J. Stachel, Melissa S. Egbertson, Jenny Wai, Michelle Machacek, Dawn
M. Toolan, John Swestock, Donnie M. Eddins, Vanita Puri, Georgia
McGaughey, Hua-Poo Su, Debbie Perlow, Deping Wang, Lei Ma, Gopal
Parthasarathy, John C. Reid, Pravien D. Abeywickrema, Sean M. Smith, Jason
M. Uslaner
PII:
S0960-894X(18)30049-0
https://doi.org/10.1016/j.bmcl.2018.01.039
BMCL 25565
DOI:
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 November 2017
19 January 2018
20 January 2018
Please cite this article as: Stachel, S.J., Egbertson, M.S., Wai, J., Machacek, M., Toolan, D.M., Swestock, J., Eddins,
D.M., Puri, V., McGaughey, G., Su, H-P., Perlow, D., Wang, D., Ma, L., Parthasarathy, G., Reid, J.C.,
Abeywickrema, P.D., Smith, S.M., Uslaner, J.M., Indole Acids as a Novel PDE2 Inhibitor Chemotype that
Demonstrate Pro-Cognitive Activity in Multiple Species, Bioorganic & Medicinal Chemistry Letters (2018), doi:
https://doi.org/10.1016/j.bmcl.2018.01.039
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Indole Acids as a Novel PDE2 Inhibitor Chemotype that
Demonstrate Pro-Cognitive Activity in Multiple Species
Shawn J. Stachel,a Melissa S. Egbertson,a Jenny Wai,a Michelle Machacek,a Dawn M.
Toolan,d John Swestock,a Donnie M. Eddins,b Vanita Puri,b Georgia McGaughey,c Hua-
Poo Su,c Debbie Perlow,a Deping Wang,c Lei Ma,d Gopal Parthasarathy,c John C. Reid,c
Pravien D. Abeywickrema,c Sean M. Smith,d Jason M. Uslanerd
a Department of Medicinal Chemistry, Merck & Co. Inc., PO Box 4, West Point, PA 19486, USA
bDepartment of pharmacology Merck & Co. Inc., PO Box 4, West Point, PA 19486, USA
c Department of Structural Biology, Merck & Co. Inc, PO Box 4, West Point, PA 19486, USA
d Department of Neuroscience, Merck & Co. Inc., PO Box 4, West Point, PA 19486, USA
This is where the receipt/accepted dates will go; Received Month XX, 2000; Accepted Month XX, 2000 [BMCL RECEIPT]
Abstract— An internal HTS effort identified a novel PDE2 inhibitor series that was subsequently optimized for improved PDE2 activity
and off-target selectivity. The optimized lead, compound 4, improved cognitive performance in a rodent novel object recognition task as
well as a non-human primate object retrieval task. In addition, co-crystallization studies of close analog of 4 in the PDE2 active site
revealed unique binding interactions influencing the high PDE isoform selectivity.
©2000 Elsevier Science Ltd. All rights reserved.
Schizophrenia is a debilitating disorder typically
diagnosed in individuals in their early to mid-twenties.
There are multiple symptoms, which are typically
clustered into so-called positive, negative, and cognitive
symptoms. Currently available therapies improve the
positive symptoms, such as hallucinations and
aggression, but tend not to affect negative symptoms
such as anhedonia, or the cognitive symptoms such as
memory and executive function deficits. Several
investigations have shown that cognitive dysfunction is
most predictive of every day function,1 and for this
reason a significant amount of research has been aimed
at identifying new mechanisms to treat cognitive
In an effort to identify novel treatments for cognitive
impairment associated with schizophrenia, we focused
on targets that could modulate the second messengers
cyclic guanosine monophosphate (cGMP) and cyclic
adenosine monophosphate (cAMP). These ubiquitous
secondary messengers are responsible for altering the
function of many intracellular proteins, are involved in
synaptic plasticity such as long-term potentiation, and
are downstream signaling molecules of dopamine and
glutamate, two neurotransmitters believed to be altered
in the disease. One strategy for affecting the levels of
these cyclic nucleotides is to inhibit 3', 5'-cyclic
nucleotide specific phosphodiesterases (PDEs), which
are a group of enzymes that degrade these secondary
messengers. The PDE superfamily includes twenty-one
genes that encode for eleven families of PDEs. These
disturbances
Unfortunately, this is still an unmet medical need.
associated
with
schizophrenia.

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was found to reduce the potency for DP1 while having
negligible effect on PDE2 potency. Next it was
discovered that expansion of the the fused five-
membered ring system to a six-membered ring (4)
enhanced PDE2 potency ~ 3-fold and also decreased
DP1 activity 3-fold between the two systems thus
improving the PDE2/DP1 selectivity from 2-fold to 25-
fold. Compound 4 also retained its high selectivity
profile against the other PDEs, i.e. 3-11, but now
included the previous deficient PDE1 with >1000-fold
selectivity versus PDE isoforms 1-11.
families are further subdivided based on catalytic
domain homology and substrate specificity and include:
cAMP specific, PDE4A-D, 7A and 7B, and 8A and 8B;
cGMP specific, PDE 5A, 6A-C, and 9A; and dual
substrate, PDE 1A-C, 2A, 3A and 3B, 10A, and 11A.
The homology between the families, ranging from 20%
to 45%, suggests that it may be possible to develop
selective inhibitors for each of these families.
Inhibition of PDE2 has been shown to enhance
cognitive function in multiple rodent assays of cognitive
performance measuring recognition memory, social
interactions and working memory, which are all
deficient in schizophrenia.2 However, as far as we
know, the effect of PDE2 inhibitors on cognitive
function in non-human primate, which might be a better
model of human cognition, has not been demonstrated.
Our efforts described here were aimed at identifying a
compound with suitable properties to evaluate the
cognitive effects of PDE2 inhibition in non-human
primate.
As such, compound 4 was profiled in vivo in rats and
displayed pharmacokinetic properties supportive of in
vivo pharmacodynamic studies (Cl = 19 mL/min/kg; t1/2
= 4.7 h; Vdss = 3.6 L/kg; %F = 30%; rat PPB = 98%).
As part of our internal effort to identify novel PDE2
inhibitors we embarked on an HTS campaign as well as
a fragment screening effort. The HTS screen yielded a
variety of potent hits. In particular, compounds of the
indole class represented by 1 demonstrated good
potency and pharmacokinetic properties to serve as a
starting point for series optimization (Figure 1). This
class of compounds had been previously explored by us
as part of the prostanoid receptor antagonist program
culminating in the Phase II clinical candidate
Laropiprant, 2, a selective prostaglandin DP1 receptor
antagonist that reduces the vasodilation and flushing
induced by niacin therapy.3,4 Compound 1 displayed
good PDE2 inhibitory activity as a screening hit (PDE2,
IC50 = 95 nM) and high PDE isoform selectivity with
selectivities for PDE3-11 > 1000-fold with the lowest
selectivity being for PDE1 at 124-fold. Unfortunately 1
still retained significant intrinsic activity as a DP1
antagonist, with a DP1 IC50 = 4 nM, that would need to
be address to advance the series.
Figure 2. Elaboration of indole hit 1.
Possessing sufficient potency, isoform selectivity, and
pharmacokinetic properties, compound
4
was
subsequently evaluated for its ability to improve
recognition memory as assayed in the novel object
recognition test. This task consisted of exposing
vehicle- or scopolamine-treated rats to two identical
objects and then exposing the rats 1h later to one object
they had previously been exposed to and one object that
was novel. Animals not administered scopolamine will
spend more time exploring a novel object than a
familiar one, indicating that they recognize the object
they had been exposed to. Administration of
scopolamine, however, impairs recognition memory
such that animals will explore both the familiar and
novel object approximately equally. The red dashed
line in Figure 3 represents the 50% mark, or equivalent
time exploring the novel and familiar objects. As seen
in Figure 3, compound 4 dose-dependently increased
novel object exploration at the 1 and 3 mg/kg dose
levels by ~15%, and pharmacokinetic studies indicate
Figure 1. HTS lead and subsequent benchmark compound 1.
As observed from the historical prostanoid program the
R stereochemistry for the acetic acid functional group is
preferred for DP1 potency in compound 1. Inversion of
the indole acid side chain stereochemistry from the R to
the S configuration as present in compound 3, Figure 2,

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that these doses produce exposures corresponding to
unbound compound levels in plasma equivalent to 1 and
2-fold over the IC50 value observed in the biochemical
PDE2 assay. The effect seen at these doses was
comparable to the effect of donepezil which is used as
the gold standard comparator in the assay. An inverted
U-shaped response was observed with reduced efficacy
at the higher dose levels, an effect often observed with
pre-clinical cognitive function and memory assays.5
begins with conversion of methyl 2-bromoacetate (5) to
the corresponding azide (6). An aldol condensation of
compound
6
with 2-bromo-4-fluorobenzaldehyde
yielded the α,β-unsaturated azido ester 7 which upon
thermolysis afforded indole ester 8. Alkylation of 8
with 4-bromobutanoate followed by Dieckmann
condensation resulted in formation of the β-keto ester
that was decarboxylated under acidic conditions to
produce ketone 9. Ketone 9 was stereoselectively
reduced using Corey-Bakshi-Shibata conditions with the
chiral oxaborolidine R-CBS reagent to give the S-
alcohol.9 The chiral alcohol was then elaborated to the
homologated ester 10 by activation of the alcohol as the
phosphate which was displaced by dimethyl malonate
and after subsequent decarboxylation afforded
intermediate 10. The thioaryl group was then installed
by
treatment
of
10
with
(trifluoromethyl)phenyldisulfane after which hydrolysis
1,2-bis-(4-
of the ester afforded the desired acid 4.
Figure 3. Rat Novel Object Recognition study with compound 4
Compound 4 was also tested in a non-human primate
model of executive function, the rhesus object retrieval
assay.6,7 As was seen in the rodent novel object
recognition study, statistically significant reversal of the
scopolamine impairment was observed at the 1 and 3
mg/kg doses, equating to 2 and 10-fold unbound plasma
concentration as a measure of fold over the biochemical
IC50 value.
Scheme 1: Synthetic route for compound 4.
Although compound 4 displayed several favorable
attributes such as good PDE2 potency, high isoform
selectivity and a positive in vivo pharmacodynamic
effect we still sought to further reduce the inherent DP1
activity of the series. This was achieved by reversing
the position on the indole nitrogen from
tetrahydropyrido-indole scaffold in to
a
a
Figure 4. Effects on Object Retrieval in Scopolamine-Impaired Adult
Rhesus Monkeys with compound 4.
4
tetrahydrocarbazole scaffold as shown in compound 11.
The carbazole scaffold also allowed for facile
introduction of a chiral carbon atom to replace the sulfur
Compound 4 and similar analogs were conveniently
prepared as depicted in Scheme 1.3,8 The synthesis