2-(2-Furanyl)-7-phenyl[1,2,4]triazolo[1,5-c]pyrimidin-5-amine analogs as adenosine A2A antagonists: The successful reduction of hERG activity. Part 2

Article abstract of DOI:10.1016/j.bmcl.2005.05.043

The structure-activity relationship (SAR) exploration using 2-(2-furanyl)-7-phenyl[1,2,4]triazolo-[1,5-c]pyrimidin-5-amine (1) as a template led to the identification of a novel class of potent and selective adenosine A_2A receptor (AR) antagoni

Full text of DOI:10.1016/j.bmcl.2005.05.043

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3675–3678
2-(2-Furanyl)-7-phenyl[1,2,4]triazolo[1,5-c]pyrimidin-5-amine
analogs as adenosine A2A antagonists: The successful reduction
of hERG activity. Part 2
Julius J. Matasi, John P. Caldwell, Hongtao Zhang, Ahmad Fawzi, Guy A. Higgins,
Mary E. Cohen-Williams, Geoffrey B. Varty and Deen B. Tulshian*
Department of Medicinal Chemistry—CV&CNS, Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth,
NJ 07033, USA
Received 25 March 2005; revised 5 May 2005; accepted 11 May 2005
Available online 27 June 2005
Abstract—The structure–activity relationship (SAR) exploration using 2-(2-furanyl)-7-phenyl[1,2,4]triazolo-[1,5-c]pyrimidin-5-
amine (1) as a template led to the identification of a novel class of potent and selective adenosine A2A receptor (AR) antagonists.
However, these compounds were found to be associated with significant hERG activity. This report discusses the strategy and out-
come of an expanded SAR focused on addressing the hERG liability. As a result, compounds 21 and 24 possess excellent in vitro
profiles, highly promising in vivo profiles, and acceptable levels of hERG channel inhibition.
Ó 2005 Elsevier Ltd. All rights reserved.
Our previous communication1 discussing 2-(2-furanyl)-
7-phenyl[1,2,4]triazolo[1,5-c]pyrimidin-5-amine analogs,
reported several analogs (2–4) in this series which dem-
onstrated high binding affinity for adenosine A2A recep-
tor (AR) and very good selectivity over A1 AR. Several
of these compounds displayed moderate to good anti-
cataleptic activity (15–65% inhibition) when dosed oral-
ly at 3 mg/kg in a rat catalepsy assay. However, these
analogs were found to be associated with the blockade
of the hERG (human Ether-a-go-go Related Gene)
channel. The blockade of hERG K+ channels has been
cited as a major factor in the drug-induced alteration
of cardiac ventricular repolarization that is responsible
for prolongation of the heart rate-corrected QT interval
(QTc). Since QTc prolongation is believed to increase the
risk of cardiac arrhythmia in patients and can lead to
torsades de points or sudden death, it became critical
to monitor these compounds for the inhibition of the
hERG channel.2
efflux FLIPR assay at a concentration of 5 lg/mL.
Compounds 3 and 4 exhibited hERG inhibition of
81% and 68%, respectively. However, compound 2 pos-
sessed acceptable hERG activity of 15% inhibition
which indicates that basic nitrogen in the side chain
may be responsible for the inhibition of the hERG
channel. In addition, it has been reported that decreas-
ing the pKa of the basic amine results in lower hERG
activity.2
As a result, several compounds from this series were
screened for hERG activity using a standard rubidium
Keywords: Adenosine receptor; Antagonist; Arylindenopyrimidines;
hERG.
*
Corresponding author. Tel.: +1 908 740 3505; fax: +1 908 740 7164;
e-mail: deen.tulshian@spcorp.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.05.043

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J. J. Matasi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3675–3678
Moreover, in silico pharmacological models of the
hERG channel–ligand interactions suggest that proton-
ated amines participate in a favorable cation–p interac-
tion with the Tyr652 residue of the hERG channel.
These theories appear to support the observation that
compounds in this series containing basic nitrogens ex-
hibit higher hERG inhibitory activity than those with-
out basic nitrogens. Consequently, SAR development
was focused on reducing the basicity of the C-7 side
chain.
2-furoic hydrazide, followed by a dehydrative rearrange-
ment3 with N,O-bis(trimethylsilyl) acetamide (BSA) to
provide 9. Subsequent deprotection of 9 gave 10 as an
advanced intermediate. Compounds 16–20 were pre-
pared via subjecting 10 to various known reaction con-
ditions to afford targets in
a parallel fashion.
Compounds 21 and 22 were synthesized by first, prepar-
ing the fully elaborated piperazines which were then
coupled with 5, followed by steps b–g as shown in
Scheme 1. The piperidine analogs were prepared as
shown in Scheme 2. The Pd-catalyzed coupling of 5 with
1,4-dioxa-8-azaspiro[4.5]decane produced 11. Com-
pound 11 was converted to its corresponding boronic es-
ter 12 which was then converted to 14 via steps e–g.
Deprotection of 14 provided 15 as an advanced interme-
diate. Compound 15 was converted to 23 and 24 under
reductive amination conditions. All compounds re-
ported herein gave satisfactory analytical results.4
Piperazines 10 and 16–20 were prepared according to
the general procedure shown in Scheme 1. The palla-
dium (Pd) catalyzed coupling of (3-bromo-phenoxy)-tri-
isopropyl-silane (TIPS) (5) with 1-(tbutoxycarbonyl)-
piperazine afforded 6. Compound 6 was deprotected
with tetrabutylammonium fluoride (TBAF), followed
by conversion to the corresponding triflate and then
conversion to pinacol boronic ester 7. The palladium-
catalyzed coupling between 7 and 2-amino-4,6-dichloro-
pyrimidine produced 8. Chloride 8 was reacted with
The in vitro results of the A2A AR binding assays5 are
expressed as inhibition constants (Ki, nM) and A1/A2A
Scheme 1. Reagents and conditions: (a) 1-(tbutoxycarbonyl)-piperazine, Pd(OAc)2, P(tBu)3, NaOtBu, toluene (80–100%); (b) TBAF, THF
(90–100%); (c) Tf2O, Et3N, CH2Cl2 (80%); (d) Bis(pinacolato)diboron, PdCl2(dppf), dppf, KOAc, dioxane (82%); (e) 2-amino-4,6-dichloropyrim-
idine, Pd(PPh3)4, K2CO3, CH3CN/H2O (50–70%); (f) 2-furoic hydrazide, BuOH; (g) BSA; (h) HCl, dioxane (73%).
Scheme 2. Reagents and conditions: (a) 1,4-dioxa-8-azaspiro[4.5]decane, Pd(OAc)2, P(tBu)3, NaOtBu, toluene (83%); (b) TBAF, THF (89%); (c)
Tf2O, Et3N, CH2Cl2 (85%); (d) Bis(pinacolato)diboron, PdCl2(dppf), dppf, KOAc, dioxane (40%); (e) 2-amino-4,6-dichloropyrimidine, Pd(PPh3)4,
K2CO3, CH3CN/H2O (82%); (f) 2-furoic hydrazide, BuOH; (g) BSA; (h) TFA, CH2Cl2 (80%).

J. J. Matasi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3675–3678
3677
describes the selectivity over the A1 AR. All assays were
performed in duplicates and reported as mean values.
These compounds were not tested against the other
known adenosine receptor subtypes, A3 and A2B. The
hERG activity was measured using a rubidium efflux
FLIPR assay and is expressed as a percent inhibition
at a concentration of 5 lg/mL.6 The anti-cataleptic
activity in the rat catalepsy assay is expressed in percent
reduction of cataleptic effect relative to vehicle upon oral
dosing at 3 mg/kg and determined at 1 and 4 h after the
cataleptic effect was observed.
selectivity over A1 AR. Substitution of N-4 of 10 with
electron withdrawing groups produced compounds 9
and 16–19 with subnanomolar binding affinities for
A2A AR while retaining high selectivity over A1 AR.
Methoxyethoxyphenyl substituted analog 21 was found
to be more than 1000-fold selective over A1 AR. The
SAR of C-4 substituted piperidine analogs 14, 15, 23,
and 24 showed that a variety of substituents were toler-
ated at C-4 of the piperidine.
Derivatization of the basic nitrogen produced significant
reduction of hERG channel inhibition in the piperazine
series. This is clearly demonstrated by comparing the
hERG activity of compound 3 with that of compound
Results in Table 1 show that among the piperazine ana-
logs, unsubstituted piperazine 10 displays the lowest
Table 1. Receptor affinity, in vivo activity and hERG activity of C-7 arene analogs
Compound
R
A2A Ki (nM)
A1/A2A
% Reduction of
cataleptic activity (3 mg/kg) 1 h/4 h
% Inhibition of
hERG (5 lg/mL)
9
10
16
0.5
5.7
0.5
745
66
34/43
NT
À3
NT
10
525
13/13
17
18
1.0
0.5
269
366
0/23
8
5
23/47
19
20
0.8
1.3
511
229
28/0
À3
23/32
13
21
22
14
15
23
24
1.0
2.3
1059
154
470
133
122
116
55/40
19/46
45/20
NT
À4
3
0.4
8
1.3
NT
9
18.0
0.8
10/12
36/59
1

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21 (81% vs 13%, respectively). Slightly basic amines were
also tolerated in the side chains of both piperidine 24
and piperazine 20. The hERG activity is also reduced
when an amine is sterically hindered as in compound
23. From these limited results it is not clear if reduction
of the hERG liability is due to reduced basicity or struc-
tural modifications. Further work is required to under-
stand these results.
studies. Test compounds were administered orally and
catalepsy was retested at 1 and 4 h after administration.
Male CD rats (Charles River Laboratories) weighing
200–240 g were used for all studies. Upon arrival at
our holding facility, rats were housed four per cage, with
food and water available ad libitum. Rats were main-
tained on a 12 h light/dark cycle (light on 07:00; lights
off 19:00). All studies were carried out in accordance
with the National Institutes of Health Guide for the
Care and Use of Laboratory Animals.
Having identified compounds with high-binding affinity
for A2A AR, several compounds from Table 1 were as-
sessed for their pharmacokinetic profiles and anti-cata-
leptic activity in the rat at an oral dose of 3.0 mg/kg.
Among these compounds, 21 and 24 were found to have
acceptable pharmacokinetic profiles (AUC = 160 and
310 ng h/mL, respectively). Both compounds displayed
potent oral anti-cataleptic activity at 1 and 4 h after
dosing.
Acknowledgments
We wish to thank Dr. William Greenlee for his support
for this work. We would also like to thank Dr. Steve Sor-
ota and Xue-Song Zhang for performing hERG activity
assay. Finally, we would like to thank Lisa Silverman for
her help in the preparation of this manuscript.
Encouraged by this result, it was decided to investigate
compounds 21 and 24 in a secondary rodent model. In
this assay, hypolocomotion is induced by administration
of a 1 mg/kg dose of the A2A agonist, CGS-21680. The
antagonists are pretreated for 4 h at varying doses.
Compound 24 displayed activity at 3 mg/kg (100% inhi-
bition) and 1 mg/kg (53% inhibition) whereas 21 dis-
played moderate activity (61% and 59%, respectively).
The reversal of CGS-21680 agonist activity affected by
compounds 21 and 24 is mediated through A2A AR.
References and notes
1. Matasi, J. J.; Caldwell, J. P.; Zhang, H.; Fawzi, A.; Cohen-
Williams, M. E.; Varty, G. B.; Tulshian, D. B. Bioorg. Med.
Chem. Lett. 2005, 15, 3670.
2. De Ponti, F.; Poluzzi, E.; Cavalli, A.; Recanatini, M.;
Montanaro, N. Drug Safety 2002, 25, 263.
3. (a) Dimroth, O. Ann. 1909, 364, 183; (b) Dimroth, O. Ann.
1927, 459, 39.
4. Compound 21: 1H NMR (CDCl3, 400 MHz) d: 7.63 (m,
2H), 7.42 (m, 3H), 7.25 (dd, 1H), 7.07 (dd, 1H), 6.90 (m,
4H), 6.59 (dd, 1H), 6.05 (br s, 2H), 4.09 (t, 2H), 3.74 (t, 2H),
3.45 (s, 3H), 3.43 (m, 4H), 3.26 (m, 4H); Compound 24: 1H
NMR (CDCl3, 400 MHz) d: 7.62 (s, 1H), 7.58 (s, 1H), 7.39
(s, 2H), 7.34 (t, 1H), 7.24 (d, 1H), 7.02 (d, 1H), 6.59 (q, 1H),
6.15 (br s, 2H), 3.83 (d, 2H), 3.73 (m, 4H), 2.80 (t, 2H), 2.59
(m, 4H), 2.34 (m, 1H), 1.97 (m, 2H), 1.68 (m, 2H).
5. Adenosine A2A and A1 binding assays: [3H]SCH-58261 and
[3H]DPCPX binding assays for adenosine A2A and A1
receptors, respectively, were performed as described
previously7.
In summary, we have discovered a series of A2A AR
antagonists which displayed excellent affinity, selectivity
over the A1 AR, and oral anti-cataleptic activity.
Through modification of the side chain, the hERG
activity was significantly reduced. It was also demon-
strated that selected compounds 21 and 24, upon oral
administration, showed potent activity in a rodent cata-
leptic assay.
Catalepsy procedure: The rodent model for ParkinsonÕs
Disease is the rat catalepsy assay where the dopamine
D2 receptor antagonist, haloperidol (1 mg/kg, subcuta-
neous) is administered. After 30 min, rats were placed
facing upward on a wire screen inclined at 60°. The time
taken for the rat to move one limb was measured, with a
cut-off time of 120 s. Rats showing >75 s of immobility
(catalepsy) were used for subsequent A2A antagonist
6. Sorota, S.; Zhang, X.-S.; Margulis, M.; Tucker, K.;
Priestley, T. ASSAY and Drug Development Technologies
2005, 3, 47.
7. (a) Dionisotti, S.; Ongini, E.; Zocchi, C.; Kull, B.; Arslan,
G.; Fredholm, B. B. Br. J. Pharmacol. 1997, 121, 353; (b)
Klotz, K.-N.; Hessling, J.; Hegler, J.; Owman, C.; Kull,
B.; Fredholm, B. B.; Lohse, M. J. Arch. Pharmacol. 1998,
357, 1.