Discovery of potent and selective bicyclic A2B adenosine receptor antagonists via bioisosteric amide replacement

Article abstract of DOI:10.1016/j.bmcl.2010.01.077

Several new potent and selective A_2B adenosine receptor antagonists have been prepared in which the aryl-amide moiety of the lead series, exemplified by 1a, has been replaced by bioisosteric bicyclic moieties. Although the majority of compounds

Full text of DOI:10.1016/j.bmcl.2010.01.077

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1634–1637
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of potent and selective bicyclic A2B adenosine receptor antagonists via
bioisosteric amide replacement
*
Paul Eastwood , Jacob Gonzalez, Sergio Paredes, Silvia Fonquerna, Arantxa Cardús, Juan Antonio Alonso,
Arsenio Nueda, Teresa Domenech, Raquel F. Reinoso, Bernat Vidal
Almirall Research Center, Almirall S.A., Ctra. Laureà Miró 408, E-08980 St. Feliu de Llobregat, Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new potent and selective A2B adenosine receptor antagonists have been prepared in which the
aryl–amide moiety of the lead series, exemplified by 1a, has been replaced by bioisosteric bicyclic moi-
eties. Although the majority of compounds had generally improved microsomal stability as compared to
1a, this was not translated into overall improvements in the pharmacokinetic profiles of a representative
set of compounds.
Received 22 December 2009
Revised 11 January 2010
Accepted 12 January 2010
Available online 20 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Adenosine
A2B
Adenosine receptor antagonists
Asthma
Amide bioisosteres
A2B receptors are responsible for the adenosine-mediated re-
lease of several inflammatory cytokines from mast cells, airway
and bronchial epithelial cells, fibroblasts, smooth muscle cells,
intestinal epithelial cells, monocytes and dendritic cells. Therefore
A2B adenosine receptor antagonists have the potential to become a
new pharmacological drug class for the treatment of inflammatory
conditions such as asthma.1–4
preliminary pharmacokinetic studies. It was reasoned that bicyclic
derivatives of type 3 (Fig. 2), which retain the crucial hydrogen-
bond donor capacity of the amide, would not be subject to the po-
tential hydrolytic liability of the amide series and could also lead to
new and novel series of A2B adenosine receptor antagonists.6
Thus, a selection of bicylic derivatives of type 3 were prepared
(Schemes 1–3) and tested (Table 1) as potential A2B adenosine
receptor antagonists.
Imidazolone 7 was prepared in good yield (Scheme 1) via a Cur-
tius rearrangement of the amino acid 6 (prepared in three steps from
pyridone 45). Amino acid 6 was also used in the preparation of the
quinazoline dione 19 by treatment with urea at 200 °C under micro-
wave conditions. All attempts to transform imidazolone 7 into
In a previous letter5 we revealed that compounds of type 1,
when functionalized with appropriate substituents, are potent
and selective A2B adenosine receptor antagonists. Although certain
compounds from this series have shown good bioavailability in
pharmacokinetic studies, in general all compounds tested had
short intravenous half lives. In vitro metabolism experiments per-
formed with compounds of type 1 in rat and human microsomes,
both in the presence and absence of the oxidative co-factor
NADPH, indicated that, in several cases, substantial non-oxidative
hydrolytic metabolism was occurring to give the corresponding
2-aminopyridine derivatives 2 (Fig. 1).
Ar2
Hepatic
Microsomes
R1
(-NADPH)
Ar2
O
Ar1
N
N
H
Ar1
N
NH2
Amino pyridines of type 2 were themselves found to be weak
adenosine receptor antagonists, typically losing over two orders
of magnitude of potency when compared to the corresponding
amide derivatives.
1
2
Figure 1. Non-oxidative metabolism of compounds of type 1.
The potential metabolic instability of derivatives of type 1 could
be a contributing factor to the short half lives as assessed in
Ar2
Ar1
Ar2
Ar1
O
R1
N
N
H
N
N
H
* Corresponding author. Tel.: +34 933128610; fax: +34 933128635.
E-mail addresses: paulroberteastwood@yahoo.co.uk, paul.eastwood@almirall.
com (P. Eastwood).
1
3
Figure 2. Proposed bicylic bioisosteres of 1.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.077

P. Eastwood et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1634–1637
1635
7
(iv)
N
N
N
N
N
O
N
N
O
N
N
(i)
CO2H
NH2
(ii-iii)
(v)
N
H
4
O
N
Cl
N
O
N
19
5
6
(xi)
(vi-vii)
NH2
N
N
N
N
N
O
N
CO2H
O
(viii-ix)
O
(x)
N
N
N
N
H
N
NH
10
O
O
11
O
(xii)
12
O
O
8
9
Scheme 1. Reagents and conditions: (i) POCl3, reflux, 63%; (ii) Satd NH3 in EtOH, 100 °C, 70%; (iii) NaOH (aq), EtOH, reflux, 99%; (iv) (PhO)2P(@O)N3, Et3N, 1,4-dioxane, reflux,
56%; (v) urea, microwave, 200 °C, 58%; (vi) 2,4-dimethoxybenzylamine, Et3N, EtOH, microwave, 175 °C, 100%; (vii) KOH, ethylene glycol, 150 °C, 93%; (viii) (PhO)2P(@O)N3,
Et3N, 1,4-dioxane, reflux, 87%; (ix) NaH, MeI, DMF, rt, 85%; (x) TFA, thioanisole, 65 °C, 32%; (xi) NH2NH2ꢀH2O, EtOH, reflux, 71%; (xii) NaNO2 (aq), HCl (aq), AcOH, 0 °C then
H3PO2 (aq), 0 °C, 41%.
N
N
O
N
N
O
N
N
O
(i-ii)
(iii)
(iv-vi)
(vii-viii)
CO2H
O
CO2Me
Cl
NH2
Cl
4
14
N
N
N
H
13
(ix)
15
Scheme 2. Reagents and conditions: (i) 6 M NaOH (aq), 30% H2O2 (aq), EtOH, H2O, 50 °C, 80%; (ii) 10% KOH (aq), 120 °C, 100%; (iii) POCl3, 120 °C then MeOH, 0 °C to rt, 66%;
(iv) 2 M NaOH (aq), EtOH, rt, 98%; (v) (PhO)2P(@O)N3, Et3N, t-BuOH, reflux, 86%; (vi) TFA, CH2Cl2, rt, 70%; (vii) CuCl, NH3 (aq), 120 °C, 21%; (viii) (EtO)3CH, AcOH, 140 °C, 43%;
(ix) NH2NH2ꢀH2O, EtOH, reflux, 92%.
found to be potent A2B adenosine receptor antagonists although in
N
N
O
N
N
O
many cases the selectivity with respect to A1 was found to be
diminished when compared with the amide derivative 1a.
The compounds were profiled to assess their ADME properties
(Table 2). It was pleasing to note that, in rat hepatic microsomes,
non-oxidative metabolism (i.e., in the absence of the co-factor
NADPH) was not observed in any case with the cyclized derivatives
and, in general, turnover was found to be low with the exception of
the indole derivative 18. CACO-2 flux data indicated that in all
cases, except for that of compound 15, intestinal permeability
should not be a major issue with these derivatives.
The iv pharmacokinetic profiles in the rat of selected com-
pounds are presented in Table 3. Although plasma clearance values
for the cyclized compounds were, in general, lower than that seen
with the amide derivative 1a, half lives in rat remained short for all
compounds tested as a result of moderate-to-high plasma clear-
ance and low-to-moderate volumes of distribution. The furan moi-
ety is a potential metabolic liability13 present in all of the
compounds; extra-hepatic p450-mediated metabolism of this moi-
ety with the concomitant formation of reactive metabolites could
be a contributing factor in the plasma clearance of these deriva-
tives. Indeed, the clearance of compound 11 is higher than rat liver
blood flow (even though turnover is low) which would indicate
that, at least in this case, additional non-hepatic clearance mecha-
nisms are operating.
Br
(i-ii)
(iii-iv)
(v)
NH2
6
18
N
NH2
N
16
17
Scheme 3. Reagents and conditions: (i) Cu powder, quinoline, 230 °C, 70%; (ii) NBS,
DMSO, H2O, rt, 35%; (iii) TMS–acetylene, PdCl2(PPh3)2, CuI, Et3N, THF, 90 °C, 47%;
(iv) K2CO3, MeOH, rt, 100%; (v) t-BuOK, NMP, 70 °C, 78%.
compound 10 via alkylation gave inseparable mixtures of isomeric
products. Thus, a protecting group strategy was adopted. Nucleo-
philic aromatic substitution of the chloro nitrile 5 with 2,4-dime-
thoxybenzylamine followed by hydrolysis gave the amino acid 8
which was subjected to Curtius rearrangement and then cleanly
alkylated to give 9. Deprotection using TFA with thioanisole as a
scavenger7 gave the desired product 10. Treatment of the chloro ni-
trile 5 with hydrazine gave the amino indazole 11, which was sub-
jected to
a
diazotization/reduction sequence8 to give the
unsubstituted indazole 12. The imidazole derivative 14 was pre-
pared in a multi-step sequence as shown in Scheme 2. Intermediate
chloro ester 13 was treated with hydrazine to give 15 in good yield.
Finally, the indole derivative 18 was accessed via a 5-endo-dig cyc-
lisation9 of the amino acetylene derivative 17 (Scheme 3), which was
in turn prepared via a metal-catalyzed Sonogashira reaction10 of the
bromo derivative 16 with trimethylsilylacetylene followed by
deprotection.
An oral pharmacokinetic study (10 mg/kg, rat) conducted with 7
demonstrated that the compound had low bioavailability (F = 20%).
Given that the CACO-2 and solubility data indicate that absorption
is likely not be a major problem, the poor bioavailability could be
explained by extra-hepatic and/or phase II metabolism and/or bil-
iary excretion.
Biological results are given in Table 1.11,12 Compound 1a, a rep-
resentative derivative of the afore-mentioned amide series, is in-
cluded in the table as a comparison. It was gratifying to find that
both the cyclic urea derivatives 7 and 10 and the quinazoline dione
19 gave comparable activity/selectivity profiles to that of the
amide 1a. Indeed, all of the bicyclic heterocycles synthesized were
In summary, replacement of the amide moiety in the lead com-
pound 1a by bioisosteric bicyclic moieties led to potent and selective

1636
P. Eastwood et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1634–1637
Table 1
Biological activities and structures
Compound
Structure
Ki (nM) or % inhibition of radioligand binding at a compound concentration of 1 l
M11
hA2B
hA2A
hA1
hA3
N
N
N
N
N
N
N
N
N
N
O
1a
7
4
1
1
3
9
4
2
1
1
0
239 46
931 121
3754 866
N
N
N
N
N
N
N
N
N
N
H
O
N
H
N
6
11
8
769 190
317 33
2444 958
357 131
478 25
18%
17%
15%
2%
5
6
5
1
0
1
4
O
O
N
H
O
N
N
10
11
12
14
15
18
19
2126
3
N
H
O
N
NH2
N
858 136
1656 534
3605 2322
177 54
N
H
O
N
23
21
3
N
N
H
O
N
N
717 302
0%
N
H
O
N
O
82 17
2%
NH
N
H
O
N
13
1
619
1
252 75
7%
N
H
O
N
O
NH
181 25
1727 617
6267 2322
N
H
O
O
Table 3
Pharmacokinetic profiles of derivatives in rat (1 mg/kg iv)
Table 2
In vitro metabolism, CACO-2 permeability and solubility data
Compound
t1/2 terminal
(h)
Cmax
(ng/mL)
Cl
AUC0–1
(ng h/mL)
Vss
(L/kg)
Compound % Turnovera Papp (ꢁ 10ꢂ6 cm/s) Thermodynamic solubility
b
(L/h/kg)
@ pH 7.4 (lg/mL)
1a
7
11
12
15
19
0.5
1.0
0.4
0.6
0.4
0.4a
538
762
397
1049
1327
1677
3.8
3.0
6.1
1.4
2.0
1.2
267
350
164
736
504
880
1.6
1.0
3.7
0.8
0.6
0.75
1a
7
27
15
8
45
40
32
24
39
23
3.5
40
20
48
10
11
12
14
15
18
19
6
204
296
17
10
7
a
Value refers to the half-life of the
compound is eliminated from the plasma compartment.
a-elimination phase where >90% of the
88
0c
61
a
% turnover after a 30 min incubation period at 37 °C of a 5
compound with rat hepatic microsomes (1 mg/mL).
lM solution of test
was not translated into overall improvements inthe pharmacokinetic
parameters of a representative set of compounds tested. Additional
efforts in this area will be the subject of further communications.
b
Passive permeability through a CACO-2 monolayer determined using 12.5
test compound.
lM
c
Poor % mass balance.
Acknowledgements
A2B adenosine receptor antagonists which were hydrolytically stable
towards liver microsomes, and that had, in general, low turnover.
However the enhanced in vitro stability of the bicyclic compounds
We thank Maria Isabel Loza and Maria Isabel Cadavid from the
Universidad de Santiago de Compostela for performing the radioli-
gand binding assays mentioned in this work.

P. Eastwood et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1634–1637
1637
Spalluto, G.; Bacilieri, M.; Moro, S. Mini-Rev. Med. Chem. 2005, 5, 1053; (h)
Jacobson, K. A.; Gao, Z.-G. Nat. Rev. Drug Disc. 2006, 247.
References and notes
5. Eastwood, P.; Gonzalez, J.; Paredes, S.; Nueda, A.; Domenech T.; Alberti, J.;
Vidal, B. Bioorg. Med. Chem. Lett., in press.
6. Vidal, B.; Eastwood, P.; Gonzalez, J. PCT Int. Appl. WO2005100353, 2005.
7. Chen, P.; Barrish, J. C.; Iwanowicz, E.; Lin, J.; Bednarz, M. S.; Chen, B.-C.
Tetrahedron Lett. 2001, 42, 4293.
8. Almansa, C.; Virgili, M. PCT Int. Appl. WO 2004076450, 2004.
9. Rodriguez, A. L.; Koradin, C.; Dohle, W.; Knochel, P. Angew. Chem., Int. Ed. 2000,
39, 2488.
1. Feoktistov, I.; Biaggioni, I. J. Clin. Invest. 1995, 96, 1979.
2. Linden, J. J. Clin. Invest. 2006, 116, 1835.
3. (a) Feoktistov, I.; Goldstein, A. E.; Ryzhov, S.; Zeng, D.; Belardinelli, L.; Voyno-
Yasenetskaya, T.; Biaggioni, I. Circ. Res. 2002, 90, 531; (b) Feoktistov, I.; Ryzhov,
S.; Goldstein, A. E.; Biaggioni, I. Circ. Res. 2003, 91, 485; (c) Feoktistov, I.;
Ryzhov, S.; Zhong, H.; Goldstein, A. E.; Matafonov, A.; Zeng, D.; Biaggioni, I.
Hypertension 2004, 44, 649; (d) Rees, D. A.; Lewis, B. M.; Lewis, M. D.; Francis,
K.; Scanlon, M. F.; Ham, J. Br. J. Pharmacol. 2003, 140, 764; (e) Zhong, H.; Wu, Y.;
Belardinelli, L.; Zeng, D. Am. J. Respir. Cell Mol. Biol. 2006, 35, 587; (f) Novitskiy,
S.; Ryzhov, S.; Zaynagetdinov, R.; Goldstein, A.; Huang, Y.; Tikhomirov, O.;
Blackburn, M.; Biaggioni, I.; Carbone, D.; Feoktistov, I.; Dikov, M. Blood 2008,
112, 1822; (g) Ryzhov, S.; Zaynagetdinov, R.; Goldstein, A.; Novitskiy, S.; Dikov,
M.; Blackburn, M.; Biaggioni, I.; Feoktistov, I. J. Immunol. 2008, 180, 7212; (h)
Ryzhov, S.; Zaynagetdinov, R.; Goldstein, A.; Novitskiy, S.; Blackburn, M.;
Biaggioni, I.; Feoktistov, I. J. Pharmacol. Exp. Ther. 2008, 324, 694.
4. For recent reviews, see: (a) Caruso, M.; Varani, K.; Tringali, G.; Polosa, R. Curr.
Med. Chem. 2009, 16, 3875; (b) Haskó, G.; Csóka, B.; Németh, Z. H.; Vizi, E. S.;
Pacher, P. Trends Immunol. 2009, 30, 263; (c) Haskó, G.; Linden, J.; Cronstein, B.;
Pacher, P. Nat. Rev. Drug Disc. 2008, 7, 759; (d) Ham, J.; Rees, D. Endocr. Metab.
Immune Disord. Drug Targets 2008, 8, 244; (e) Beukers, M. W.; Meurs, I.;
Ijzerman, A. P. Med. Res. Rev. 2006, 26, 667; (f) Zablocki, J.; Elzein, E.; Kalla, R.
Expert Opin. Ther. Pat. 2006, 16, 1347; (g) Cacciari, B.; Pastorin, G.; Bolcato, C.;
10. Chinchilla, R.; Najera, C. Chem. Rev. 2007, 107, 874.
11. For details of the binding assays, see: Vidal, B.; Nueda, A.; Esteve, C.;
Domenech, T.; Benito, S.; Reinoso, R. F.; Pont, M.; Calbet, M.; López, R.;
Cadavid, M. I.; Loza, M. I.; Cárdenas, A.; Godessart, N.; Beleta, J.; Warrellow, G.;
Ryder, H. J. Med. Chem. 2007, 50, 2732.
12. Results obtained with compounds 7, 11, 18 and other related compounds in
functional assays measuring cAMP levels in both human HEK 293 cells
(expressing the human A2B receptor) and CHO cells (transfected with the
mouse A2B receptor) indicated that these compounds are acting as antagonists.
Further details can be found in Ref. 11.
13. Kalgutkar, A. S.; Gardner, I.; Obach, R. S.; Shaffer, C. L.; Callegari, E.; Henne, K.
R.; Mutlib, A. E.; Dalvie, D. K.; Lee, J. S.; Nakai, Y.; O’Donnell, J. P.; Boer, J.;
Harriman, S. P. Curr. Drug Metab. 2005, 6, 161.