Discovery of MK-0952, a selective PDE4 inhibitor for the treatment of long-term memory loss and mild cognitive impairment

Article abstract of DOI:10.1016/j.bmcl.2010.09.087

The structure-activity relationship of a novel series of 8-biarylnaphthyridinones acting as type 4 phosphodiesterase (PDE4) inhibitors for the treatment of long-term memory loss and mild cognitive impairment is described herein. The manuscript describes a new paradigm for the development of PDE4 inhibitor targeting CNS indications. This effort led to the discovery of the clinical candidate MK-0952, an intrinsically potent inhibitor (IC ₅₀ = 0.6 nM) displaying limited whole blood activity (IC₅₀ = 555 nM). Supporting in vivo results in two preclinical efficacy tests and one test assessing adverse effects are also reported. The comparative profiles of MK-0952 and two other Merck compounds are described to validate the proposed hypothesis.

Full text of DOI:10.1016/j.bmcl.2010.09.087

Bioorganic & Medicinal Chemistry Letters 20 (2010) 6387–6393
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of MK-0952, a selective PDE4 inhibitor for the treatment
of long-term memory loss and mild cognitive impairment
Michel Gallant a, , Renee Aspiotis a, Stephen Day a, Rebecca Dias b, Daniel Dubé a, Laurence Dubé a,
⇑
Richard W. Friesen a, Mario Girard a, Daniel Guay a, Pierre Hamel a, Zheng Huang a, Patrick Lacombe a,
Sebastien Laliberté a, Jean-François Lévesque a, Susana Liu a, Dwight Macdonald a, Joseph Mancini a,
Donald W. Nicholson a, Angela Styhler a, Karen Townson b, Kerry Waters b, Robert N. Young a, Yves Girard a
a Merck Frosst Centre for Therapeutic Research, Kirkland, Québec, Canada H9H3L1
b Department of In Vivo Neuroscience, The Neuroscience Research Centre, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure–activity relationship of a novel series of 8-biarylnaphthyridinones acting as type 4 phos-
phodiesterase (PDE4) inhibitors for the treatment of long-term memory loss and mild cognitive impair-
ment is described herein. The manuscript describes a new paradigm for the development of PDE4
inhibitor targeting CNS indications. This effort led to the discovery of the clinical candidate MK-0952,
an intrinsically potent inhibitor (IC50 = 0.6 nM) displaying limited whole blood activity (IC50 = 555 nM).
Supporting in vivo results in two preclinical efficacy tests and one test assessing adverse effects are also
reported. The comparative profiles of MK-0952 and two other Merck compounds are described to
validate the proposed hypothesis.
Received 30 August 2010
Revised 14 September 2010
Accepted 15 September 2010
Available online 21 September 2010
Keywords:
MK-0952
PDE4 inhibitors
Mild cognitive impairment
Ó 2010 Elsevier Ltd. All rights reserved.
There are three distinct steps involved in the generation of
memory. The first step, acquisition, requires a certain level of
attention and alertness. The second step, called short-term mem-
ory is thought to reside within the hippocampus and acts as a
switchboard to determine which memory should be stored long-
term and what type of information is required for retrieval. The
third step, termed the consolidation step, generates long-term
memories and is a process that requires new protein synthesis.
Biochemically, short-term memory (or early phase long-term
potentiation) is primarily driven by calcium as the second messen-
ger. Repeated stimulation of calcium-mediated events leads to the
activation of cAMP which mediates the late phase long-term
potentiation (L-LTP) cascade, which is thought to be associated
with long-term memory.1 Within the L-LTP cascade, stimulation
of cAMP results in the activation of protein kinase A, which results
in the subsequent phosphorylation of the transcription factor
cAMP response element binding protein (CREB). Multiple target
genes are rapidly transcribed following CREB activation2 (some of
which are involved in L-LTP) resulting in new protein synthesis,
strengthening of existing synaptic connections, and de novo estab-
lishment of synaptic connections. Physiologically, hydrolysis of
cAMP is catalyzed by a multigene super-family of cyclic nucleotide
phosphodiesterases (PDEs). To date, 11 individual families of PDEs
have been described; among them is the type 4 cAMP-specific
phosphodiesterase (PDE4) which is particularly important for the
control of intracellular cAMP.3 Consequently, it is hypothesized
that inhibition of PDE4 will lead to the accumulation of cAMP
which will facilitate the L-LTP cascade and cognition enhance-
ment.4 The most convincing evidence to date that supports the
involvement of PDE4 in cognition comes from animal tests. Roli-
pram, a first-generation selective PDE4 inhibitor has been shown
to exert a reversal of the L-LTP deficit in aged mice,5 and to reverse
recognition memory impairment in rats.6
Adverse effects (AEs) associated with PDE4 inhibitors: First gener-
ation PDE4 inhibitors are known to produce dose-limiting AEs7
including emesis, nausea, colitis and vasculitis rendering their clin-
ical development challenging. AEs linked to PDE4 inhibition can be
triggered both centrally and/or peripherally based on known tissue
distribution. Numerous strategies to identify PDE4 inhibitors dis-
playing acceptable therapeutic window of efficacy over AEs have
been proposed. For example, efforts have been made to identify
isozyme-selective inhibitors,8 to pinpoint the molecular target of
emesis and efficacy by using highly emetic photoaffinity probes9
or finally to focus on peripherally restricted inhibitors.10 However,
to target a centrally-mediated disease such as cognitive impair-
ment, a new paradigm is needed to establish the necessary thera-
peutic window.
Hypothesis: Inhibition of the centrally located PDE4 enzyme,
due to low protein content in CSF, should correlate best with the
⇑
Corresponding author. Tel.: +1 514 428 3054; fax: +1 514 428 4939.
E-mail address: michel_gallant@merck.com (M. Gallant).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.09.087

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M. Gallant et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6387–6393
intrinsic activity of the inhibitor rather than its potency in whole
blood. On the other hand, AEs may be restricted predominately
to its peripheral whole blood activity. Therefore, a brain penetrat-
ing PDE4 inhibitor exhibiting a high plasma protein shift (i.e., low
whole blood potency) but retaining intrinsic potency on the
enzyme could demonstrate an improved therapeutic window.
Moreover, a compound with high oral bioavailability and rapid
absorption would minimize gastrointestinal (GI) related AEs by
reducing local GI exposure. The Condition Taste Aversion11 (CTA)
test (vide infra) was employed to evaluate malaise in rats while
in vivo efficacy was measured using the Novel Object Recogni-
tion12 (NOR) and the Water Maze Delayed Matching to Position
(WM) tests.13
third generation inhibitors,22 in which a carboxylic acid moiety was
introduced (Compounds 1 and 2), displayed a significant reduction
of the whole blood potency but maintained intrinsic activity on
the PDE4 enzyme. This high plasma protein shift is believed to be
mediated by their high affinity for serum albumin, that is, commonly
observed for lipophilic carboxylic acids.23
SAR studies were initiated by combining the naphthyridinone
core with the carboxylic acid biaryl pharmacophore. Oral bioavail-
ability in rat was also measured for most compounds to validate
our initial hypothesis regarding reduction in GI occupancy. The
position of the cyclopropanecarboxylic acid residue, relative to
the biphenyl tether, was evaluated first (Table 2). Unsubstituted
phenyl propionic acids were not explored as they were previously
found to display low oral bioavailability in rats due to extensive
glucuronidation and b-oxidative mediated degradation.22 Meta
and para-substituted analogs 3 and 4 displayed superior intrinsic
enzyme activities over their ortho analog 5. Moreover, the para
analog 3 exhibited the highest high whole blood to intrinsic activ-
ity ratio. The gem-dimethyl analogs 6 and 7, which were designed
to lower glucuronidation rate (steric hindrance) and alleviate
b-oxidation (removal of benzylic hydrogen), respectively, dis-
played lower oral bioavailability but superior half-life compared
to the cyclopropane analog 3. Neither the benzylic dimethyl 7
nor the cis-cyclopropane analog 8 exhibited the preferred whole
blood potency. Finally, the tetrazole 9, a carboxylic acid isostere
of 3, presented the optimum in vitro profile, but unfortunately
did not meet our bioavailability criteria. Consequently, the para-
substituted cyclopropane carboxylic acid 3 was determined to pos-
sess the best overall profile for further optimization. Although the
amide moiety present in compound 3 has previously been opti-
mized21 with regard to both metabolic stability and enzyme activ-
ity, we suspected that the introduction of the acidic moiety may
have rendered earlier SAR studies obsolete. Consequently, this area
of the naphthyridinone scaffold was optimized next.
PDE4 enzymatic activities: Inhibition of PDE4 leads to an increase
in intracellular cAMP which in turns inhibits the release of inflam-
matory mediators such as cytokines (TNF-
IFN- ) as well as the recruitment and activation of inflammatory
cells.14 This observation can be quantified in vitro by measuring
the ability of PDE4 inhibitors to block the LPS-stimulated TNF-
a, IL-2, IL-12, LTB4,
c
a
release in human (HWB) or rat (RWB) whole blood thereby allow-
ing a direct measurement of their relative potency.15 Intrinsic
activity of inhibitors against the four PDE4 isozymes A, B, C, and
D was assessed on their catalytic domain using a PDE4-GST con-
structs, expressed in SF9 cells via the baculovirus expression sys-
tem.16 PDE4 inhibitors described in this manuscript displayed
little or no isozyme specificity. Consequently, the PDE4 inhibitory
enzyme potencies described in this manuscript are averages of
the IC50’s of the relevant isozymes A, B, and D. These three iso-
zymes are widely distributed in the human brain, with the latter
two being more abundant. In contrast, PDE4C exhibit a more
restricted distribution in the human brain and is found only in
the cortex, some thalamic nuclei and the cerebellum). In the rat
brain, PDE4C is found exclusively in olfactory bulb.17
Our initial approach consisted in combining previously opti-
mized scaffolds of PDE4 inhibitors discovered in our laboratories
exhibiting in vitro activities in-line with our working hypothesis
(Table 1). Designed for the treatment of respiratory diseases,
clinical candidates MK-035918,19 and MK-087320,21 were optimized
for whole blood potency and exhibited desirable low nanomolar
intrinsic activity on the PDE4 enzyme. Additionally, second genera-
tion inhibitors, as exemplified by MK-0873, were found to address
the issue of undesirable CYP2C9 competitive inhibition primarily
by the introduction of the naphthyridinone pharmacophore. Finally,
Although numerous amide analogs were prepared and profiled,
none were found to be superior to the cyclopropyl amide 3 (Table
3). Noteworthy was the loss of inhibitory of activity upon increas-
ing the size of the nitrogen substituent, exemplified by 11, or by
the introduction of polarity, as shown by the tertiary alcohol 12.
Equipotent analogs (13, 14, and 16) were identified but they dis-
played lower oral bioavailability. The cyclopropylmethyl 15 offered
no real advantages over 3.
Table 2
SAR studies on the carboxylic acid residue
Table 1
PDE4 inhibitors developed at Merck
O
N
O
O
N
O
SO2Me
SO2Me
N
H
COOH
COOH
7
COOH
6
COOH
N
H
N
3: para
4: meta
5: ortho
H
N
N
N
N
Enantiomer A (1)
Enantiomer B (2)
MK-0359
MK-0873
N
8
9
N
N
O
OH
para-substituted
N
N
O
O
N
Entry
IC50
Ratio
Rat PKb
F (%) t1/2 (h)
a
MeO2S
PDE4QT (nM)
HWB (
lM)
HWB/PDE4QT
a
Entry
IC50
Ratio
3
4
5
6
7
8
9
0.57
0.54
4.6
0.88
0.85
1.6
0.30
0.11
1.3
0.61
0.075
0.16
>5
526
203
282
693
88
78
100
35
30
62
30
5
1.7
1
PDE4QTb (nM)
HWBc
(l
M)
CYP2C9d
(
lM)
HWB/PDE4QT
4.7
6.1
5.1
3.9
2.4
MK-0359
MK-0873
1
2
1.0
5.1
0.54
0.32
0.16
0.19
1.4
<1.0
40
1.7
3.2
160
37
2600
1900
100
>9090
0.6
0.55
a
IC50 determinations are averages of at least two experiments.
a
PDE4QT is the average of the IC50’s on isozymes PDE4A, B and D.
b
c
IC50 determinations are averages of at least two experiments.
Rat PK studies were conducted in Sprague–Dawley rats at 10 mg/kg PO in 0.5%
methocel (n = 2) and 5 mg/kg iv in PEG200 60% (n = 2).
b
Human whole blood activity as measured by inhibition of TNF-
Competitive CYP2C9 inhibition.
a release.
d

M. Gallant et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6387–6393
6389
Table 3
tion of a fluorine atom at the ortho position in relation to the
cyclopropane carboxylic acid addressed this problem and ulti-
mately led to the discovery of the clinical candidate MK-0952.24,25
The whole blood activity of MK-0952 in rats and rhesus monkey
was found to be similar to that in human with IC50 = 450 80 nM
(n = 9) and 940 270 nM (n = 4), respectively. Surprisingly, the
squirrel monkey whole blood potency (IC50 = 52 11 nM; n = 6)
was significantly higher. Pharmacokinetic studies (Table 5) in pre-
clinical species and tissue distribution studies in rats (i.e., brain
penetration, Table 6) were performed to complete the character-
ization of MK-0952. Overall, MK-0952 is orally bioavailable in all
preclinical species examined, and exhibits moderate clearance
and half-life. MK-0952 was not found to be highly brain penetrant
in rats (2–3%), but reaches brain concentrations 20-fold over its
intrinsic enzyme activity (0.65 nM) at a dose of 0.1 mg/kg. At this
dose, the corresponding plasma level is equivalent to its whole
SAR studies on the amide residue
O
N
O
N
H
N
H
N
H
N
H
OH
N
12
3
11
13
N
H
CF3
N
H
N
H
COOH
14
15
16
Rat PKb
a
Entry
IC50
Ratio
PDE4QT (nM)
HWB (
lM)
HWB/PDE4QT
F (%)
t1/2 (h)
3
11
12
13
14
15
16
0.57
32
17
0.58
0.51
0.35
0.52
0.30
>40
6.4
0.28
0.51
0.20
0.22
526
>1142
376
482
1085
571
78
1.7
58
25
4.1
3.2
blood potency (0.55 lM).
The metabolic profile of MK-0952 was determined in vitro by
performing hepatocyte and microsomal incubations and in vivo
using bile duct cannulated rats. A low extent of metabolism was
observed in hepatocytes: 2–10% in human (n = 3); 5–8% in male
rat (n = 2); 20% in female rat (n = 1); 10% in rhesus monkey
(n = 1); and 30% in dog (n = 1) and all species exhibited the same
major metabolites. The metabolic fate of MK-0952 is summarized
in Figure 2. The major metabolites were 21, the acyl glucuronide
and 22 the mono hydroxylated analog. Metabolite structures were
confirmed with synthetic standards.
432
20
2.4
a
IC50 determinations are averages of at least two experiments.
Rat PK studies were conducted in Sprague–Dawley rats at 10 mg/kg PO in 0.5%
b
methocel (n = 2) and 5 mg/kg iv in PEG200 60% (n = 2).
Having completed SAR studies on both the amide and acid por-
tion of the naphthyridinone scaffold, we next focused on the reso-
lution of compound 3. Initially, the pair of enantiomers 17 and 18
(Table 4) were separated using chiral chromatography and their
respective absolute configuration was established by X-ray crystal-
lography. Despite the fact that the pair of enantiomers displayed
similar intrinsic potency on the PDE4 enzymes, they differed sig-
nificantly in whole blood potency, with the R,R enantiomer 17 dis-
playing a three-fold greater whole blood shift. Unfortunately, while
compound 17 did meet our selection criteria of intrinsic versus
whole blood activity, it was found to elicit an unacceptable in-
crease in rat liver protein covalent binding as a function of time
(Fig. 1). Based on previous SAR of naphthyridinone containing ana-
logs, we speculated that the bioactivation event leading to this
phenomenon resulted from the formation of a short lived and
highly reactive metabolite derived from the cyclopropane carbox-
ylic acid. A follow-up SAR study demonstrated that the introduc-
(20mpk, 100-200uCi/kg, 60% PEG)
50
Compound 17
40
Compound 19
30
20
10
0
0
5
10
15
20
25
30
Time (hrs)
Table 4
Discovery of MK-0952
Figure 1. Rat liver covalent binding following oral dosing.
O
N
O
O
N
O
Table 5
Pharmacokinetic profile of MK-0952 in preclinical species
N
H
N
H
N
N
Doses (mg/kg) (PO/iv) F (%) Cl (mL/min/kg) t1/2 (h)
Rat
10/3
38
41
77
1.0
1.7
1.3
3.7
3.5
4.3
6.5
6.6
Squirrel monkey 10/1
Rhesus monkey
Dog
10/1
3/1
17
18
19
COOH
COOH
COOH
COOH
100
MK-0952
F
F
20
Table 6
Brain versus plasma exposure of MK-0952 as a function of dose
Dosea (mg/kg)
Brainb
(l
M)
Plasmac
(lM)
Brain/plasma (%)
a
Entry
IC50
HWBb
1.0 0.26 (n = 12)
Ratio
0.1
0.5
1
5
10
0.012
0.047
0.11
0.29
0.59
0.59
1.34
3.41
8.19
18.84
2.1
3.5
3.2
3.5
3.1
PDE4QT (nM)
(l
M)
HWB/PDE4QT
17
18
MK-0952
1.0
1000
350
1040
330
0.66
0.53
0.46
0.23 0.049 (n = 21)
0.55 0.097 (n = 17)
0.15 0.041 (n = 11)
20
a
b
c
MK-0952 was dosed orally as a sodium salt in 0.5% methocel.
Whole brain. Contamination by residual blood <0.5%. LOQ = 1 nM.
Plasma and brain level measured 90 min post-dosing at Cmax
a
IC50 determinations are averages of at least two experiments.
Standard error (mean/sq. root of the n value).
b
.

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M. Gallant et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6387–6393
O
N
O
N
H
MK-0952
(Asterisk indicates 3H position)
N
*
CO2H
UGT 1A1, 1A3, 1A6, 1A8, 1A9
(human, rat, rhesus monkey &dog)
CYP 1A1/2, 3A, 2C19
F
(human, rat, rhesus monkey &dog)
O
O
O
N
O
N
H
N
H
OH
N
N
N
OH
O
HO
O
OH
CO2H
CO2H
22
21
F
F
O
Figure 2. Structures of MK-0952’s metabolites.
In human hepatocytes, 21 was the major metabolite. Incuba-
tions with rat uridine 50-diphospho-glucuronosyl transferase
(UGT’s) showed that UGT 1A1, 1A3, 1A6, 1A8, and 1A9 contributed
to the formation of 21. Incubations with rat CYP’s indicated that
formation of 22 was mediated by CYP 1A, 3A and 2C19. Analysis
of bile from permanently bile-cannulated male rats dosed iv with
orthoformate in the presence of acetic anhydride to afford 2-chlo-
ronicotinoyl acrylate. Upon addition of 3-bromo aniline in CH2Cl2
at room temperature the amino acrylate I was formed. Cyclization
of the latter to the 1-bromophenyl-1,4-dihydro[1,8]naphthyridin-
4-one II was achieved by treatment of I with NaH at room temper-
ature. The naphthyridinone ester II was subsequently converted in
one step to the corresponding amide III by condensation with an
appropriately substituted and commercially available amine fol-
lowing dropwise addition of AlMe3. Two parallel strategies were
used to couple the naphthyridinone core to the acidic moiety. In
one the bromoaryl III was first converted to the pinacole boronate
IV by a palladium catalyzed coupling with pinacolato diboron.
Subsequent Suzuki coupling of the resulting boronate ester IV
with an appropriately substituted aryl bromide V afforded the
desired PDE4 inhibitor VI. Alternatively, the bromo aryl III was
coupled by Suzuki coupling with the boronate ester VII, prepared
as previously described. The chiral cyclopropane esters X were
prepared from the corresponding styrenes IX using an enantiose-
lective cyclopropanation described by Evans and co-workers.26
The required styrenes were synthesized via Wittig olefination
reaction of the corresponding benzaldehydes VIII. Racemic trans
cyclopropane were prepared via Horner–Wadsworth–Emmons
reaction followed by Simmons–Smith cyclopropanation. Finally,
3H-L-001037116 (3 mg/kg, 100
lCi/kg) showed that the major
component excreted in bile was the parent, accounting for >35%
of total radioactivity (0–8 h). The metabolites 21 and 22 and two
additional unknown metabolites (possibly a taurine conjugate)
were also observed. Analysis of plasma samples showed mainly
parent, with minor levels of 21 and 22. MK-0952 was neither a
CYP inhibitor (IC50 >100
inducer (23% increase CYP3A4 protein levels was observed at
10 M in human primary cultured hepatocytes relative to rifampi-
cin at 10 M) using Western blot). Based on the overall metabolic
lM on CYP2C9, 3A, 2D6 and 1A2) nor an
l
l
and CYP profile of MK-0952, we believe that the risk for this com-
pound to be either the victim or perpetrator of drug–drug interac-
tions is low.
In general, the PDE4 inhibitors described herein were prepared
according to the synthetic sequence described in Scheme 1. The
core naphthyridinone III was obtained in four steps starting from
2-chloronicotinoyl chloride. The latter was condensed with triethyl
O
N
O
O
O
N
O
O
O
R1
N
R2
b
Cl
a
OEt
OEt
c
N
N
Cl
N
N
Cl
N
H
I
II
III
Br
Br
O
O
B
Br
O
d
e
Br
O
d
R3
R3
O
N
O
N
R4
Br
R4
R1
R1
VII
V
N
R2
N
R2
Br
X
VIII
CHO
N
N
e
VI
IV
*
COOEt
R4
R4
*
Br
O
B
f
R
3
g
O
R4
R4
IX
Scheme 1. Synthesis of Naphthyridinone type PDE4 inhibitors. Reagents and conditions: (a) (i) Triethyl orthoformate/Ac2O/130 °C; (ii) 3-bromo aniline/CH2Cl2/rt; (b) NaH/
THF/0 °C–rt; (c) HNR1R2/1,2-dichloroethane/then AlMe3 dropwise, or (i) LiOH/MeOH/THF; (ii) NEt3/isobutyl chloroformate/CHCl3/rt; (iii) HNR1R2; (c) 5% Pd(OAc)2/PPh3 (1/3),
Na2CO3, n-propanol, 80 °C; (d) 5% PdCl2(dppf)2, KOAc, pinacolato diboron, DMF, 70 °C; (e) 5% Pd(OAc)2/PPh3 (1/3), Na2CO3, n-propanol, 80 °C; (f) (i) (Ph)3PCH3Br/THF/ꢀ78 °C/
add n-BuLi (ii) IX/ꢀ78 °C–rt; (g) (i) Cu(OTf)/(+ or ꢀ) 2,20-isopropylidene-bis(4-tert-butyl-2-oxazoline)/CHCl3/rt; (ii) add IX then N2CHCOOEt dropwise.

M. Gallant et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6387–6393
6391
the b-phenylisovaleric ester required for compound 7 was pre-
pared according to the procedure described by Corse and Rohr-
mann.27 A detailed description of all the experimental conditions
can be found in referenced patents
In vivo efficacy of MK-0952: In the Novel Object Recognition test,
rats were initially presented with two identical objects (sample
phase). After 24 h, they were presented with a triplicate copy of
the original object and a novel object (choice phase). The time
spent exploring the familiar object versus the novel object was re-
corded. If a rat recognizes the original object, it will spend more
time with the novel object. When rats were pretreated with MK-
0952 prior to the sample phase, they spent more time exploring
the novel object at both doses of 0.01 and 0.1 mg/kg compared to
vehicle-treated controls (Fig. 3). These results reflect an improve-
ment in long-term recognition memory. In these studies, MK-
0359 was included as a positive control, and showed similar results
to MK-0952 at a dose of 0.1 mg/kg.
In the Water Maze Delayed Matching to Position test, rats were
placed into the water maze at one of four randomly determined
start locations and were given four trials per day (maximum length
60 s, with an inter trial interval (ITI) 30 s) for eight days to find a
hidden platform submerged below the water surface using extra
maze cues (training phase). The platform was placed in a different
location each day, and the mean escape latency and swim speed
100
75
50
25
0
0.5
0.4
0.3
0.2
0.1
0.0
veh
0.01
0.1
0.1
veh
0.01
0.1
0.1
-0.1
-0.2
Dose (mg/kg)
Dose (mg/kg)
Figure 3. Assessment of efficacy of MK-0952 and MK-0359 in the Novel Object Recognition test. The data shown are the mean standard error of mean (SEM) of the d2 score
(time spent exploring the novel object–time spent exploring the familiar object/total exploration time) and the percentage score. MK-0952 and MK-0359 were dosed orally
each day (0.5% methocel) 90 min and 30 min, respectively, before the sample trial corresponding to their individual Cmax. The statistically significant differences between
groups (n = 21–24) were determined using Newman Keuls tests. Any animal that lay two SDs away from the group mean on two or more performance measures were
*
excluded from the analysis: p <0.05 versus vehicle-treated animals. See Ref. 28 for full experimental details.
Exp #1
28
Exp # 2
28
**
*
24
20
16
12
8
**
24
20
16
12
8
p = 0.08
p = 0.14
*
4
4
0
0
Vehicle
0.03
0.1
0.3
Vehicle
0.1
0.3
0.3
Dose (mg/kg)
Dose (mg/kg)
Exp # 3
Exp # 4
28
24
20
16
12
8
28
24
20
16
12
8
**
*
*
*
4
4
0
0
Vehicle
0.1
0.3
1.0
Vehicle
0.01
0.3
3.0
Dose (mg/kg)
Dose (mg/kg)
Figure 4. Assessment of efficacy of MK-0952 and MK-035919a in the Water Maze Delayed Matching to Position test. The data shown are the mean SEM savings score. MK-
0952 and MK-0359 were dosed orally each day (0.5% methocel) 90 min and 30 min, respectively, before the sample trial. The statistically significant differences between
*
**
groups (n = 10) were determined using Newman Keuls tests: p <0.05 versus vehicle-treated animals, p <0.01 versus vehicle-treated animals. See Ref. 29 for full
experimental details.

6392
M. Gallant et al. / Bioorg. Med. Chem. Lett. 20 (2010) 6387–6393
veh
LiCl
3.0
10.0
30.0
veh LiCl
0.1
0.3
1.0
3.0
100
50
MK-0952 (mg/kg)
MK-0952 (mg/kg)
100
50
0
0
**
-50
-100
-50
-100
**
**
**
Figure 5. Assessment of AEs induced by MK-0952 in the Conditioned Test Aversion test. The data shown are the mean SEM percentage change in liquid consumption from
baseline scores. MK-0952 was administered orally each day in 0.5% methocel immediately following the conditioning sessions. The statistically significant differences
**
between groups (n = 6–8) were determined using Dunnett’s tests: p <0.01 versus vehicle-treated animals. See Ref. 30 for full experimental details.
Table 7
Comparative in vivo profile of three distinct PDE4 inhibitors
Water maze Delayed Matching to Position test
PDE4QT (nM)
MEDa (mg/kg)
Plasmab (nM)b
Brainb (nM)b
B/P (%)
B/PDE4c
MK-0359
MK-0873
MK-0952
1.0 0.3
5.1 0.9
0.5 0.1
0.3
0.3
0.1
27
258
390
5.1
15
9.2
19
5.8
2.4
5.1
2.9
15
Conditioned Taste Aversion test
Plasma (nM)
RWB (nM)
23
MED (mg/kg)
Brain (nM)
P/RWBd
MK-0359
MK-0873
MK-0952
5
1.0
1.0
113
828
21
48
4.9
13
64 24
450 80
10
12,050
290
27
a
b
c
Minimum efficacious dose.
Plasma and brain exposure were measured on the tested animal as previously describe in Table 6.
Whole brain concentration to inhibitory enzyme potency ratio.
d
Plasma concentration to rat whole blood activity ratio.
measures were recorded on each trial. The ability of the test com-
pound to improve cognition is evaluated over a five days testing
phase. Rats may receive vehicle or the test compound. After a 4 h
delay between trials one and two, the difference in escape latencies
across these trials (termed the savings score) was recorded. The
ability of MK-0952 to improve cognition in this test was evaluated
over a series of four experiments so that a comprehensive dose
range, namely 0.01–3.0 mg/kg can be tested (Fig. 4). Previously dis-
closed results for MK-035919a were included as a comparator. In all
experiments, MK-0952 was shown to improve performance in the
water maze test with a minimum efficacious dose (MED) of
approximately 0.1 mg/kg.
In vivo assessment of MK-0952 induced AEs: The potential of MK-
0952 to induce malaise (emesis) in rats was assessed in the Condi-
tioned Taste Aversion test in two experiments. For two condition-
ing days, rats were given 20 min access to a solution of 0.1%
saccharin and the amount consumed was recorded. Immediately
after, rats received vehicle, MK-0952, or the positive control LiCl.
On test day, rats were presented with the saccharin solution again
and the amount of liquid consumed was recorded. The premise
was that if the compound administered induced malaise in the
rat, this ill-feeling would be associated with the novel saccharin
solution, and would be reflected by reduced consumption on the
test day. As a result, MK-0952 was shown to induce malaise in
the rat at a MED of 10 mg/kg (Fig. 5).
test was observed at whole brain concentrations slightly above the
intrinsic enzymatic activity. These results do not take into account
the possibility that distribution within different brain section
might vary among the inhibitor tested. Nevertheless, the ratio of
whole brain inhibitor concentration to intrinsic enzymatic activity
is a good tool to assess and compare different series of PDE4 inhib-
itors. In the CTA test, MK-0952 was better tolerated than MK-0359
and MK-0873 despite its significantly higher brain and plasma
exposure over its counterparts. The ratio of plasma level to whole
blood activity seems to be a useful measurement of tolerance
towards AEs induced by the inhibition of the PDE4 enzymes as
exemplified by the comparative ratios of the three studied com-
pounds. The therapeutic widow of MK-0952, determined by the
WM MED to CTA MED ratio, is 100. It is significantly superior to
the therapeutic widow (i.e., three) of the two other compounds
studied. In conclusion, the results presented in this manuscript val-
idate our initial hypothesis. Following successful safety studies in
rats and rhesus monkeys, MK-0952 completed Phase I clinical tri-
als to assess its human pharmacokinetic profile. Results from these
efforts will be disclosed in a separate manuscript.
Acknowledgments
The authors would like to thank Dr Austin Chen and Dr David
Powell for proofreading this manuscript.
Comparative profile of MK-0952, MK-0359 and MK-0873: All
three compounds were evaluated in the WM and CTA tests and
the overall data are summarized in Table 7. Despite its low brain
penetration, MK-0952 compares favorably to MK-0359 and MK-
0873 in the WM test as a result of its superior intrinsic enzymatic
activity over its brain concentration. In general, efficacy in the WM
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28. Habituation: male hooded Lister rats (Harlan, UK) were habituated over two
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constant during the day but was changed from day to day, and the movements
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Image Ltd, UK). Each animal received four trials per day with each trial lasting
60 s. If an animal failed to find the platform within this time, it was guided to
the platform by the experimenter. The animal spent 30 s on the platform
before being removed prior to its next trial.
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(n = 8 per group). Subsequently, on the test day the rats were presented with
the flavor again and the amount of liquid consumed was recorded.
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