Adenosine kinase inhibitors: Polar 7-substitutent of pyridopyrimidine derivatives improving their locomotor selectivity

Article abstract of DOI:10.1016/S0960-894X(03)00642-5

We have discovered that polar 7-substituents of pyridopyrimidine derivatives affect not only whole cell AK inhibitory potency, but also selectivity in causing locomotor side effects in vivo animal models. We have identified compound, 1o, which has potent whole cell AK inhibitory potency, analgesic activity and minimal reduction of locomotor activity.

Full text of DOI:10.1016/S0960-894X(03)00642-5

Bioorganic & Medicinal Chemistry Letters 13 (2003) 3041–3044
Adenosine Kinase Inhibitors: Polar 7-Substitutent of
Pyridopyrimidine Derivatives Improving Their
Locomotor Selectivity
Guo Zhu Zheng,* Yue Mao, Chih-Hung Lee, John K. Pratt, John R. Koenig,
Richard J. Perner, Marlon D. Cowart, Gregory A. Gfesser, Steve McGaraughty,
Katharine L. Chu, Chang Zhu, Haixia Yu, Kathy Kohlhaas, Karen M. Alexander,
Carol T. Wismer, Joseph Mikusa, Michael F. Jarvis,
Elizabeth A. Kowaluk and Andrew O. Stewart
Abbott Laboratories, Neuroscience Research, Bldg. AP9A LL, 100 Abbott Park Road, Abbott Park, IL 60064-6115, USA
Received 14 March 2003; accepted 2 April 2003
Abstract—We have discovered that polar 7-substituents of pyridopyrimidine derivatives affect not only whole cell AK inhibitory
potency, but also selectivity in causing locomotor side effects in vivo animal models. We have identified compound, 1o, which has
potent whole cell AK inhibitory potency, analgesic activity and minimal reduction of locomotor activity.
# 2003 Elsevier Ltd. All rights reserved.
Adenosine (ADO) is a key homeostatic inhibitory neuro-
modulator that contributes to endogenous anti-noci-
ceptive and anti-inflammatory responses following
tissue injury. Cellular stress increases the local tissue
levels of extracellular ADO.1 Extracellular ADO acts on
cated that AK inhibitors potentially reduce thermal
hyperalgesia primarily through interactions with spinal
sites, while its ability to depress locomotor activity is
predominantly mediated by supraspinal sites.7 The
challenge was to modify our novel type of pyridopyr-
imidine derivatives to maintain sufficient efficacies in
animal pain models, but without significant locomotor
side effects.
specific cell-surface P1 purinergic receptors (A1, A2A
,
A2B and A3) proximal to the site of release to exert its
homeostatic effects. Adenosine has an extremely short
half-life in extracellular fluids. The endogenous actions
of ADO are limited to the tissue and cellular site where
it is released. By blocking the intracellular adenosine
kinase, which converts ADO to adenosine monopho-
sphate, selective adenosine kinase inhibitors increase the
extracellular concentration of endogenously released
adenosine,2 and therefore enhance its antinociceptive
and antiinflammatory action.3À5
Compounds with structures shown in Figure 1 are typi-
cal pyridopyrimidine adenosine kinase inhibitors. The
SAR clearly indicated that R1 and R2 groups played a
significant role in terms of in vitro/in vivo potencies,8
and their physicochemical properties. We now report
that polarity of these pyridopyrimidine derivatives,
caused by incorporating polar R1 and R2 moieties,
affects not only analgesic efficacy, but also locomotor
selectivity.
We have pursued pyridopyrimidine derivatives as a
novel class of adenosine kinase inhibitors.6 These deri-
vatives are efficacious in animal pain models. One con-
cern was to reduce the degree of locomotor side effects
caused by these new derivatives. Previous studies indi-
7-Substituted pyridopyrimidine derivatives, both
X=CH and N, can be made via several different
approaches. Atypical procedure to make these deriva-
tives is shown in Scheme 1. Dinitrile intermediate 3 was
generated by condensation of malononitrle with 3-ben-
zaldehyde in a mixture of ethanol and water (1:1) in the
presence of catalytic amounts of glycine.9 The reaction
*Corresponding author. Tel.: +1-847-935-7513; fax: +1-847-938-
0072; e-mail: guozhu.zheng@abbott.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00642-5

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G. Z. Zheng et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3041–3044
Figure 1. Novel pyridopyrimidine adenosine kinase inhibitors.
logues listed in Table 1. All these analogues, including
those polar analogues from 1j to 1o are within values
from 3 to 5, which is near the optimal range for CNS
penetration.11 While these analogues 1i–1o worked
very well in different animal pain models, such as car-
rageenan-induced thermal hyperalgesia (CARR),
reduced locomotor activity was observed (Table 1).
Switching X=CH to X=N (1c to 1d, entries 2 and 4)
did not make much difference in terms of in vitro/in
vivo potencies or locomotor selectivity. We then focused
on the variation of R1 and R2 moieties to alter the
selectivity. Increasing lipophilicity of the R1/R2 moiety,
from compound 1f to compound 1j (entries 6–10), led to
a significant decrease of locomotor selectivity. More-
over, compound 1j is more potent in the locomotor
assay [locomotor ED50 (po): 9 mmol/kg] than in analge-
sic assay [CARR ED50 (po): 30 mmol/kg], indicating
that this compound may distribute into brain more
effectively as compared to its closest derivative 1i, which
showed equal potencies for both CARR and locomotor
(ED50: 5 mmol/kg, po). The trend of decreased loco-
motor selectivity with increased lipophilicity of 7-sub-
stituents seems to indicate that incorporating more
polar R1/R2 moieties may help to enhance the desired
selectivity. It was also noticed that poor whole cell
inhibition of AK did not translate to improved loco-
motor selectivity, even though this property could also
be attributed to poor membrane/CNS permeability.
Compound 1b has a relatively weak whole cell value
[whole cell AK IC50=378.1 (Æ130.7) nM], yet this
compound is still very potent in the locomotor assay
(ED50=6 mmol/kg, po entry 2, Table 1). After introdu-
cing some polar moieties on the 7-substitutent, we
found the compounds indeed had weaker whole cell AK
inhibitory potency (entries 12–14, compounds 1l, 1m,
and 1n), with whole cell AK IC50 values ranging from
130 to 495 nM. The locomotor selectivity, however,
improved significantly. As mentioned above, the clogP
values (3–5) of these compounds and in vivo efficacy
(CARR) suggest that these new compounds with polar
7-substitutuent penetrate into the CNS. Therefore, the
reduced of locomotor side effects of these polar deriva-
tives are not likely to be caused by insufficient CNS
penetration. It may be the result of preferential parti-
tioning of these AKIs in the CNS. Efforts were made to
adjust the polarity of R1/R2 moiety in these derivatives,
so that a slightly less polar R1/R2 moiety might retain
good locomotor selectivity with improved whole cell
inhibitory potency. The hydroxybutyrolactone-piperidyl
moiety was found to have suitable polarity for reduced
locomotor side effects. Compound 1o (entry 15) has a
Scheme 1. (a) Malononitrile, glycine (cat); (b) (1) dimethyl malonate,
MgCl2, Et3N; (2) DMSO/H2O; (c) tributyl(1-ethoxyviny)tin, Pd2(dba)3,
P(furyl)3, DMF, 75–80 ꢀC, 4 h; (d) ammonium acetate, dichloro-
ethane, 80–85 ꢀC; (e) trisformaminomethane, formamide, 110–120 ꢀC;
(f) amine, DMSO, 100 ꢀC.
went very quickly, and was completed in a couple of
min at room temperature with very good yield. Acetyl
intermediates, such as 6, were prepared via two different
routes: when X=CH, 6-chloronicotinyl chloride (4) was
treated with dimethyl malonate under soft enolisation
conditions, MgCl2 and triethylamine, followed by
decarboxylation in 9:1 DMSO (methyl sulfoxide)/water
to give the desired ketone intermediate 6a (X=CH).10
The pyridazine ketone intermediate 6b (X=N) was
obtained, with high chemical yield, by palladium
mediated coupling reaction [using tris(di-benzylidene-
acetone)dipalladium and P(furyl)3] between dichloro-
pyridazine 5 and vinyl stannane in DMF (N,N-
dimethylformamide), followed by acidic hydrolysis.
Condensation of ketone intermediates like 6 with
dinitrile intermediate 3 in the presence of ammonium
acetate in dichloroethane gave aminocyanopyridine
intermediates 7. These latter intermediates (7) were then
cyclized to form the pyridopyrimidine ring intermediate
8 with trisformaminomethane in formamide at 110 ꢀC.
The final products (1) can be obtained by reacting
intermediate 8 with the desired amine in DMSO at
100 ꢀC (Scheme 1).
From our earlier SAR studies of these pyridopyrimidine
derivatives, R1/R2 moieties with less polar groups
favored the in vivo efficacies in various pain models.8
This is an indication that distribution into CNS may be
necessary for these analogues to achieve desired in vivo
efficacies.7 We calculated clogP values for all these ana-

G. Z. Zheng et al. / Bioorg. Med. Chem. Lett. 13 (2003) 3041–3044
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Table 1. Analgesic and locomotor activities of 7-substituted pyridopyrimidine adenosine kinase inhibitors
Entry
1
7-Substitutent
AK IC50 (nM)
Cell IC50 (nM)
CARRa ED50 (mmol/kg) (po)
Locomotor ED50 (mmol/kg) (po)
1a
7.5 (Æ0.3)
19.5 (Æ0.7)
2
8
2
1b
1c
1d
1e
1f
8.3 (Æ5.1)
10.0 (Æ2.0)
7.1 (Æ3.4)
18.8 (Æ9.7)
7.2 (Æ0.5)
10.4 (Æ6.0)
15.1 (Æ3.4)
2.8 (Æ0.1)
22.8 (Æ11.1)
31.7 (Æ7.3)
16.2 (Æ7.8)
6.5 (Æ3.4)
7.5 (Æ2.8)
8.1 (Æ2.4)
378.1 (Æ130.7)
71.0 (Æ23.7)
52.0 (Æ13.8)
74.4 (Æ2.8)
2
2
6
3
10
4
2
7
5
13
3
6
6
40.7 (Æ26.2)
51.7 (Æ14.8)
81.9 (Æ11.9)
17.3 (Æ4.5)
25
7
1g
1h
1i
7
20
8
3
10
9
5
5
10
11
12
13
14
15
1j
89.5 (Æ31.6)
43.7 (Æ14.1)
153.8 (Æ53.8)
130.1 (Æ17.7)
495.6 (Æ158.6)
50.2 (Æ17.5)
30
5
9
1k
1l
30
10
2
98% @ 30b 75% @ 100b
73% @ 30b
1m
1n
1o
4
91% @ 30b
4
97% @ 30b 87% @ 100b
aCarrageenan-induced thermal hyperalgesia.
b% control @ dose indicated (mmol/kg, po).
References and Notes
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In conclusion, we have discovered that a polar 7-sub-
stituent on these pyridopyrimidine derivatives enhance
the locomotor selectivity, while maintaining a high
degree of CNS penetration sufficient to allow com-
pounds to be potent analgesics.

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