Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and imidazo[2,1-f][1,2,4]triazin-8-ones as α2/α3 subtype selective GABAA agonists for the treatment of anxiety

Article abstract of DOI:10.1016/j.bmcl.2005.12.027

Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and imidazo[2,1-f][1,2,4]triazin-8-ones are high affinity GABA_A agonists. Compound 16d has good oral bioavailability in rat, functional selectivity for the GABA_Aα2 and

Full text of DOI:10.1016/j.bmcl.2005.12.027

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1582–1585
Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones
and imidazo[2,1-f][1,2,4]triazin-8-ones as a2/a3 subtype
selective GABAA agonists for the treatment of anxiety
*
´
Simon C. Goodacre, David J. Hallett, Robert W. Carling, Jose L. Castro,
David S. Reynolds, Andrew Pike, Keith A. Wafford, Robert Newman,
John R. Atack and Leslie J. Street
Neuroscience Research Centre, Merck, Sharp and Dohme Research Laboratories, Terlings Park,
Eastwick Road, Harlow, Essex CM20 2QR, UK
Received 30 November 2005; revised 7 December 2005; accepted 7 December 2005
Available online 27 December 2005
Abstract—Imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and imidazo[2,1-f][1,2,4]triazin-8-ones are high affinity
GABAA agonists. Compound 16d has good oral bioavailability in rat, functional selectivity for the GABAAa2 and a3-subtypes
and is anxiolytic in a conditioned animal model of anxiety with minimal sedation observed at full BZ binding site occupancy.
Ó 2006 Elsevier Ltd. All rights reserved.
c-Aminobutyric acid (GABA) is the major inhibitory
neurotransmitter in the brain.1,2 GABAA receptors are
ligand-gated chloride ion channels composed of five
transmembrane-spanning subunits.3 These five subunits
come from a family of 16 (a1–6, b1–3, c1–3, d, e, p, and h)4
most frequently in a combination of two a, two b and
one c subunits.5 These ion channels are also the site of
action of the benzodiazepine (BZ) class of anxiolytic
agents.6 The BZ binding site occurs at the interface of
a c2 and either an a1, a2, a3 or a5 subunit, with the a
subunit being a key determinant of the pharmacology
of this site.7 Clinically used BZ anxiolytics are high effi-
cacy agonists that do not discriminate between a1, a2,
a3 and a5 subtypes. These compounds, such as diaze-
pam, also show sedative and muscle-relaxant properties.
Studies using transgenic mice with point mutations in
the a subunit have shown that GABAA receptors con-
taining the a1 subunit mediate the sedative/muscle relax-
ant effects of BZs, whereas those containing an a2 and/
or an a3 subunit mediate anxiolytic and anticonvulsant
effects.8–10
The imidazo[1,2-a]pyrimidine 111 and the imidazo[1,2-
a][1,2,4]triazine 212 have previously been reported
as high affinity, GABAAa2/a3 agonists with functional
selectivity for the GABAAa2/a3 receptors over
GABAAa1. Here we report our investigations on
manipulation of the aza-bicyclic scaffold to afford
compounds with excellent functional selectivity and
pharmacokinetic properties (Fig. 1).
The nitrogen at the 8-position in the imidazo[1,2-a]
pyrimidine 1 results in a tenfold increase in BZ affinity
compared with the analogous imidazo[1,2-a]pyridine.11
Work towards a new series of GABAAa2/a3 agonists
focused on exploiting this interaction with the GABAA
receptor. Electron density mapping13 of the imidazopyr-
imidine 3 suggested that a carbonyl group at the 8-posi-
Keywords: Imidazo[1,2-a]pyrazin-8-ones; Imidazo[1,2-d][1,2,4]triazin-
8-ones; Imidazo[2,1-f][1,2,4]triazin-8-ones; GABAA agonists; Func-
tionally selective for GABAAa2 and a3; Anxiolytic.
*
Figure 1. Previously described clinical candidates, the imidazo[1,2-
a]pyrimidine and imidazo[1,2-a][1,2,4]triazine.
Corresponding author. Tel.: +44 (0) 1279440000; fax: +44 (0) 1279
440390; e-mail: simon_goodacre@merck.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.12.027

S. C. Goodacre et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1582–1585
1583
tion, as in compound 4, would have a very similar elec-
tron density to the nitrogen, Figure 2, and so may have
similar affinity-enhancing properties (Table 1).
To test the hypothesis, the compounds targeted were the
imidazo[1,2-d][1,2,4]triazin-8-one 10, the imidazo[2,1-
f][1,2,4]triazin-8-ones 13a–b and the imidazo[1,2-a]pyra-
zin-8-ones 16a–d and these compounds were prepared as
shown in Schemes 1 and 2.11,14,17 The aryl bromides and
boronates required for the Heck and Suzuki couplings
were prepared according to Scheme 1. Bromide 515
was converted to the boronate ester under Miyaura con-
ditions and then coupled with the desired aryl bromides
using a Fu protocol,16 before reduction with stannous
Scheme 1. Reagents and conditions: (i) Bis(pinacolato)diboron,
PdCl2(dppf), KOAc, 1,4-dioxane, 90 °C, 95%; (ii) Ar-Br, Pd2(dba)3,
PtBu3, KF, THF, rt to 50 °C, 90–95%; (iii) SnCl2Æ2H2O, EtOH, 75–
95%; (iv) NaNO2, 48% HBr(aq), CuBr, 0–50 °C, 64–92%.
chloride and conversion to the bromides 6a–c by a Sand-
meyer reaction. A Miyaura reaction with these bromides
gave the boronate esters 7a–c.
The imidazotriazinone 9 was prepared by cyclisation of
6-amino-4-methyl-3-(methylthio)-1,2,4-triazin-5(4H)-one
8
17 with bromoacetaldehyde followed by removal of the
thiomethyl group with Raney nickel. The imidazotriazi-
nones 12a–b were synthesized by cyclisation of imidaz-
ole 1118 with DMF-DMA at elevated temperatures
followed by alkylation using the Curran protocol.19
A
Heck reaction20 was used to couple the imidazotriazi-
nones 9 and 12a–b with bromide 6b to give 10 and
13a–b, respectively. The imidazopyrazinone analogues
16a–d were prepared from 2-amino-3-chloropyrazine
via cyclisation with bromoacetaldehyde to give chloro-
imidazopyrazine 15. Bromination of 15, followed by
selective Suzuki coupling with 7a–c, gave the biaryl-
chloroimidazopyrazine. Methoxide displacement of the
Figure 2. Electrostatic potential mapping of the imidazo[1,2-a]pyrim-
idine and imidazo[1,2-a]pyrazin-8-one. Red and blue colourations
indicate high and low electron densities, respectively.
Table 1. Binding affinity and efficacy for imidazo[1,2-a]pyrazin-8-ones, imidazo[1,2-d][1,2,4]triazin-8-ones and Imidazo[2,1-f][1,2,4]triazin-8-ones at
GABAA receptor subtypes
Compound
R1
X
Y
R2
Kia (nM)
a3
Efficacyb
a1
a5
a1 (%)
a3 (%)
a5 (%)
1
2
—
—
—
—
—
—
—
—
1.4
0.6
4.0
1.5
4.6
4.7
3.4
0.7
1.9
0.59
0.9
0.52
0.9
+6
+4
+57
+65
—
+20
+44
—
10
Me
Me
Et
CH
N
N
4-F
4-F
4-F
H
2.5
6.5
4.8
2.3
À46
À9
13a
13b
16a
16b
16c
16d
CH
CH
CH
CH
CH
CH
+1
—
—
N
2.4
0.95
0.1
À19
+27
À27
+3
À7
Me
Me
Me
Et
CH
CH
CH
CH
0.53
0.7
+51
+44
+28
+43
—
—
4-F
6-F
4-F
0.39
0.19
0.15
0.16
0.28
—
À8
À4
a Ki values for binding to the benzodiazepine sites of stably expressed human recombinant GABAA receptors with the composition axb3c2 (x = 1, 2,
3 or 5). Inhibition of the binding of 1.8 nM [3H]Ro15-1788 was measured and the concentration required to inhibit binding by 50% (IC50) was
converted to a Ki value according to the Cheng–Prusoff equation. Data shown are mean values for 3–6 determinations.
b Efficacy measured at GABAA receptors stably expressed in L(tkÀ) cells using whole cell patch clamp recording and represents the effect of the test
compound on the current produced by a sub-maximal concentration of GABA (EC20).21 Values are means of at least seven independent
experiments.

1584
S. C. Goodacre et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1582–1585
methyl at the 7-position were investigated. Increasing
the lipophilicity of the 7-substituent by replacing the
N-methyl with N-ethyl gave 16d, a compound with the
desired profile, similar to previously described develop-
ment candidates 1 and 2.11,12
Thus, 16d is a GABAAa1 antagonist and a partial ago-
nist on both the GABAAa2 (+27%) and the GABAAa3
subtypes. Compound 16d had good pharmacokinetics in
rat (F = 38%; Clp = 17 ml/min/kg; t1/2 = 5.0 h), with an
ID50 of 0.7 mg/kg po in a rat [3H]Ro15-1788 in vivo
binding assay.22 Compound 16d was active in the rat ele-
vated plus maze assay,23 a model of anxiety, at 3 mg/kg
po (86% BZ site occupancy) with no impairment seen in
the beam walking assay,24 a model of sedation, at
100 mg/kg po.
In summary, we have described the synthesis and bio-
logical activity for a series of imidazopyrazinones and
two series of imidazotriazinones as novel BZ receptor li-
gands. Compound 16d was identified as a functionally
selective GABAAa2/a3 agonist which is an orally bio-
available, non-sedating anxiolytic in animal models.
Acknowledgment
The authors thank Peter Hunt for help with calculating
the electron density mapping.
Scheme 2. Reagents and conditions: (i) BrCH2CH(OEt)2, 48%
HBr(aq) then NaHCO3, iPrOH, 47–65%; (ii) Ra-Ni, EtOH, 40 °C,
24%; (iii) 6a–c, Pd(OAc)2, PPh3, KOAc, DMA, 120 °C, 66–77%; (iv)
DMF-DMA, DowthermÒ, 210 °C, 83%; (v) NaH, LiBr, R1I, 60 °C,
73–83%; (vi) Br2, KBr, NaOAc, MeOH, 0 °C, 90-95%; (vii) 7a–c,
Cs2CO3, Pd(PPh3)4, THF, reflux, 64–74%; (viii) NaOMe, MeOH,
CH2Cl2, 40 °C, 90–95%; (ix) HBr (30% w/w in AcOH), 95 °C, 74–85%.
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