SAR and biological evaluation of 3-azabicyclo[3.1.0]hexane derivatives as μ opioid ligands

Article abstract of DOI:10.1016/j.bmcl.2012.01.099

3-Azabicyclo[3.1.0]hexane compounds were designed as novel achiral μ opioid receptor ligands for the treatment of pruritus in dogs. In this paper, we describe the SAR of this class of opioid ligand, highlighting changes to the lead structure which led to

Full text of DOI:10.1016/j.bmcl.2012.01.099

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2200–2203
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
SAR and biological evaluation of 3-azabicyclo[3.1.0]hexane derivatives
as
l
opioid ligands
⇑
⇑
Graham Lunn , Lee R. Roberts , Stephane Content, Douglas J. Critcher, Sara Douglas, Ashley E. Fenwick,
David M. Gethin, Graham Goodwin, David Greenway, Sean Greenwood, Kim Hall, Martin Thomas,
Stephen Thompson, David Williams, Gavin Wood, Andrew Wylie
Pfizer Global Research and Development, Sandwich Laboratories, Ramsgate Road, Sandwich CT13 9NJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
3-Azabicyclo[3.1.0]hexane compounds were designed as novel achiral
treatment of pruritus in dogs. In this paper, we describe the SAR of this class of opioid ligand, highlighting
changes to the lead structure which led to compounds having picomolar binding affinity, selective for the
l opioid receptor ligands for the
Received 19 December 2011
Revised 24 January 2012
Accepted 25 January 2012
Available online 2 February 2012
l
receptor over d and
j
subtypes. Some subtleties of functional activity will also be described.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
l
Opioid
Opioid
Pruritus is intense itch, often associated with flea allergy
dermatitis and atopic dermatitis in dogs. Excessive scratching in
response to itch can result in wounds then liable to secondary bac-
terial or fungal infection, causing further inflammation and itch
sensation. Pruritus in dogs is a major cause for veterinary referral.
Current therapies include corticosteroids, antihistamines, essential
fatty acid dietary supplements and various emollients. However
there is a continuing need for alternative improved treatments of
pruritus. We desired a safe non steroidal agent which rapidly
blocked the itch sensation, suitable for once or twice a day oral
dosing.
thus allowing a symmetrical achiral series which demonstrated full
opioid antagonist pharmacology.
As the project developed, we wanted to identify an agent in the
series which had, among other desired properties, increased po-
tency and ideally subtype selectivity for the dog
l opioid receptor.
We still wanted an achiral compound with low potential cost of
goods. Several hundred library and singleton compounds were
prepared which varied in three key regions; ‘Head’, ‘Quaternary
H
O
S
N
Head
l
-Opiate antagonists have been reported to be antipruritic.1,2
Previously we have described the design and synthesis of a novel
achiral class of potent opioid receptor antagonists, such as 1
O
Quaternary
Centre
l
(Fig. 1) which produced rapid and dramatic reduction in pruritic
behaviour in dogs with flea allergy dermatitis.3
N
Compound 1 was shown to be an antagonist with a pA2 of 7.97
Tail
against
l in an isolated guinea-pig myenteric plexus-longitudinal
1
preparation. In the mouse tail flick model at 10 mg/kg s.c. 1 showed
antagonism of morphine induced anti-nociception.3
Figure 1. Example of 3-azabicyclo[3.1.0]hexane class of
l opioid receptor
The key element of the design in this class was the ‘trans’ rela-
tionship across the cyclopropyl ring between the phenyl and the
fused five-membered rings to lock the molecule with the phenyl
ring into the ‘equatorial’ position (Fig. 2). Unlike the phenylpiperi-
dine class of opioid ligands, there is no longer required further
piperidine substitution to favour the phenyl equatorial conformer,
antagonists.
CH3
CH3
MeO2SHN
MeO2SHN
N
N
R
R
CH3
CHIRAL
FUSED ACHIRAL
⇑
Corresponding author.
E-mail address: lee.roberts@pfizer.com (L.R. Roberts).
Figure 2. Design of locked geometry in 3-azabicyclo[3.1.0]hexane core.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.01.099

G. Lunn et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2200–2203
2201
centre’ and ‘Tail’ denoted in Figure 1. These included a range of
sulfonamides, heterocyclic bioisosteric replacements of the sulfon-
amide, various substituents at the quaternary centre and many
‘tail’ variations.4,5 The core template enabled significant SAR
variation with a large majority of compounds acting as antagonists.
In this communication, we will outline a small selection of some of
the opioid receptor binding structure–activity relationships in this
series leading to compounds with picomolar activities, and
highlight some functional subtleties that emerged during our
investigations.
NO2
NO2
NO2
(i)
(ii) a
(ii) b
R2
R2
R2
O
NNH2
N+
O
O
N
N
R3
H
NH2
N
R1
NO2
S
O O
(ii) c
(iii) a,b
(iv)
R2
R2
R2
Compounds were made according to the general reaction
Schemes 1 and 2. The chemistry utilised to vary the ‘quaternary
centre’ and the ‘head’ (Scheme 1) followed that previously
described.3–5 Further variation at the ‘tail’ was afforded via re-
moval of an allyl protecting group to reveal versatile intermediate
amines, which were elaborated with alkylation or acylation/reduc-
tion procedures (Scheme 2).
O
O
N
R3
N
R3
N
R3
Scheme 1. Reagents and conditions: (i) H2NNH2, industrial methylated spirit,
water, reflux; (ii) (a) MnO2, dioxane, 20 °C; (b) dioxane, 20 °C; (c) reflux; (iii) (a) Fe,
CaCl2, EtOH, water, reflux; (b) LiAlH4, THF, À15 °C; (iv) DCM, NEt3, R1SO2Cl.
Compounds were tested in dog l, j and d opioid receptor bind-
ing assays (Table1).6 In general, variation on the head group from
initial example 1 did little to improve binding potency, with the
methyl sulfonamide being overall optimum. On the whole at the
quaternary position, ethyl and then methyl were optimum, with
ethyl being in most cases superior as exemplified by pairs 1 versus
3 and 12 versus 13. Where the addition of extra lipophilic binding
made the greatest impact was with a methyleneindane substituent
as the tail, for example, examples 12, 13 and 15 with significant
increase in lipophilic efficiency (LipE), a measure of lipophilicity
per unit of in vitro potency.7 The increase in LipE seen in these
examples due to optimally placed lipophilicity could be explained
in part due to conformational constraint of the ‘tail group’ whereby
H
H
H
N
R1
N
R1
N
R1
S
S
S
O
O
O
O
O O
R2
R2
R2
(i)
(ii)
N
N
N
R3
H
Scheme 2. Reagents and conditions: (i) 1,3-dimethylbarbituric acid, Pd(PPh3)4,
DCM, reflux; (ii) (a) alkyl halide or (b) amidation then reduction.
Table 1
Dog l, j and d opioid receptor binding for compounds 1–15
Compound Structure
Dog
l
Dog
j
Dog d
% Decrease of
Human
l
l
antagonist
cLogP LEd
LipEe
binding
binding
binding
twitch tension @
binding Ki,
GTP-
c S Ki,
Ki, nMa
Ki, nMa
Ki, nMa
1
l
Mc
nMa,b
nMa
O O
S
N
H
H
H
1
4.1
94
183
<10
4.8
3.5
3.6
0.39 4.1
N
O O
S
N
H
H
H
2
37
219
2900
—
—
—
4.1
0.36 3.3
N
O
O
S
N
H
H
H
3
4
1.5
67
79
<10
<10
—
—
—
—
4.1
4.0
0.4
4.1
N
O O
S
N
H
H
H
14
270
869
0.44 3.8
N
(continued on next page)

2202
G. Lunn et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2200–2203
Table 1 (continued)
Compound Structure
Dog
l
Dog
j
Dog d
% Decrease of
Human
l
l
antagonist
cLogP LEd
LipEe
binding
binding
binding
twitch tension @
binding Ki,
GTP-
c S Ki,
Ki, nMa
Ki, nMa
Ki, nMa
1
l
Mc
nMa,b
nMa
O O
S
N
H
H
H
5
6
104
1330
1530
>10,000
>10,000
—
—
—
—
—
5.0
5.1
0.37 2.0
N
O O
S
N
N
H
H
H
71
—
—
0.34 2.0
N
O O
S
N
H
H
H
7
50
1340
>10,000
—
—
4.2
0.40 2.8
N
N
N
O O
S
N
H
H
H
H
H
8
9
3
48
308
32
—
—
—
—
—
—
—
—
4.9
5.0
4.8
4.1
0.45 3.6
0.42 3.0
0.41 3.1
0.41 3.9
O O
S
N
H
10
14
10
328
791
172
66
<10
O O
S
N
H
H
H
10
11
1580
464
18
N
O O
S
N
O
H
H
H
>10
N
O O
S
N
H
H
H
12
0.55
51
384
<10
0.81
0.3
3.6
0.44 5.4
N
O O
S
N
H
H
H
13
0.2
22
29
—
—
—
4.1
0.45 5.5
N

G. Lunn et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2200–2203
2203
Table 1 (continued)
Compound Structure
Dog
l
Dog
j
Dog d
% Decrease of
Human
l
l
antagonist
cLogP LEd
LipEe
binding
binding
binding
twitch tension @
binding Ki,
GTP-
c S Ki,
Ki, nMa
Ki, nMa
Ki, nMa
1
l
Mc
nMa,b
nMa
O O
S
N
H
H
H
14
N
2.6
70
595
—
6.6
2.5
3.0
0.40 5.6
OH
O O
S
N
H
H
H
15
0.07
4.5
19
—
0.1
0.08
2.5
0.46 7.7
N
OH
a
Values are means of two or more experiments.
See Ref. 9 for details.
b
c
Human binding Ki determined from [3H]DAMGO filter binding assay.
d
e
LE = Ligand efficiency (from dog
l
binding).
binding data (LipE = pKi À cLogP).
LipE calculated from cLogP and dog
l
the methyleneindane has fewer rotational bonds than the
n-propylbenzene.
Acknowledgments
As a strategy to minimize cLogP for the benefit of overall phys-
icochemical properties, it was pleasing that a hydroxyl on the
phenpropyl tail chain 14, and on the methyleneindane 15 were
well tolerated and indeed provided a compound 15 with picomolar
activity, improved LipE and slightly improved ligand efficiency
(LE).8 In terms of subtype selectivity, few significant improvements
were seen across the examples made, however the selectivity seen
in 1 was maintained.
The authors thank Adrian Foster, Steve Gibson, Bill Howson,
Richard Armer and Jinee Patel for their contributions to this work.
References and notes
1. Bergasa, N. V.; Talbot, T. L.; Alling, D. W.; Schmitt, J. M.; Walker, E. C.; Baker, B.
L.; Korenman, J. C.; Park, Y.; Hoofnagle, J. H.; Jones, E. A. Gastroenterology 1992,
102, 544.
2. Metze, D.; Reimann, S.; Beissert, S.; Luger, T. J. Am. Acad. Dermatol. 1999, 41,
533.
3. Lunn, G.; Banks, B. J.; Crook, R.; Feeder, N.; Pettman, A.; Sabnis, Y. Bioorg. Med.
Chem Lett. 2011, 21, 4608.
4. Banks, B. J.; Crook, R. J.; Gibson, S. P.; Lunn, G.; Pettman, A. J. PCT. Int. Appl.
WO2000039089.
To confirm functional activity, some of the compounds were
tested further in the isolated guinea-pig myenteric plexus-longitu-
dinal electrical field stimulation (EFS) preparation or in a
onist GTP-
binding assay.9 In the EFS functional assay,
compounds were administered first to the tissue, followed by a
cumulative full dose response with the receptor agonist DAMGO.
A decrease of greater than 10% of twitch tension, caused by the
effect of the compound alone at concentration of 1 M, was consid-
l antag-
c
S
5. Banks, B. J.; Critcher, D. J.; Fenwick, A. E.; Gethin, D. M.; Gibson, S. P.; Lunn, G.
PCT. Int. Appl. WO2001098267.
6. Dog receptor binding protocols for l, and described in Banks, B. J.; Crook, R. J.;
l
Gibson, S. P.; Lunn, G.; Pettman, A. J. PCT. Int. Appl. WO20039089, 1999.
7. Ryckmans, T.; Edwards, M. P.; Horne, V. A.; Correia, A. M.; Owen, D. R.;
Thompson, L. R.; Tran, I.; Tutt, M. F.; Young, T. Bioorg. Med. Chem. Lett. 2009, 19,
4406.
8. Hopkins, L. A.; Groome, C. R.; Alex, A. Drug Discovery Today 2004, 9, 430.
9. Guinea-pig myenteric plexus-longitudinal muscle strips were prepared
following the procedure from Henderson, G.; Hughes, J.; Kosterlitz, H. W. Br.
J. Pharmacol. 1975, 53, 505, attached to an isometric transducer and an
l
ered as ‘possible agonist like activity’ (full agonist DAMGO gave
100% reduction). The large majority of compounds tested in this
3-azabicyclo[3.1.0]hexane series showed no agonist activity
(<10%). However in the project there were a few rare examples
of where, as the molecules grew in size in particular locations, a
hint of ‘possible agonist like activity’ was observed, such as exam-
ples 8, 10 and 11. As such, because of concern of abuse potential
with full or even potential weak partial agonists, such compounds
progressed no further in our studies. Compounds 12, 14 and 15 be-
electrode. The tissues were then placed under
a 1 g tension and left to
equilibrate for 1 h, washed with Kreb’s solution every 15 min, maintained at
37 °C throughout with constant supply of 95% O2/5% CO2. After equilibration,
the tissues were rebalanced and then electrically stimulated using frequency of
0.1 Hz and 1 ms pulse width. When all the tissues were stabilised,
supramaximal voltage was determined. This voltage was used for the rest of
the experiment. The tissues were then washed and left for 20 min. 1
kappa antagonist nor-Binaltorphimine was then added and left for 10 min.
M of captopril, a peptidase inhibitor, was added to prevent the breakdown
lM of the
haved as antagonists in the human antagonist GTP
cS assay and
showed potency similar to that seen in the other binding assays.
1
l
of DAMGO. When the twitches plateaued, a dose range of test compound was
added. After a further 10 min, a cumulative dose response was constructed for
the mu agonist DAMGO (each concentration 3 min contact time with 1 min
recording). pA2 for 1 = 7.97 ( 0.26 SD) based on n = 3).
In summary the SAR of some analogues in an achiral 3-azabicy-
clo[3.1.0]hexane series of
l opioid receptor ligands has been
described. Substituents at the basic nitrogen in particular the
methyleneindane scaffold, as exemplified by compounds 13 and
15, provided binding improvements giving picomolar activity.
Significantly, this was achieved by a big increase in LipE for com-
pound 15. Further exploration of this SAR in this series was then
exploited to provide a compound for clinical evaluation.10
10. McHardy, S. F.; Heck, S. D.; Guediche, S.; Kalman, M.; Allen, M. P.; Tu, M.; Bryce,
D. K.; Schmidt, A. W.; Vanase-Frawley, M.; Callegari, E.; Doran, S.; Grahame, N.
J.; McLean, S.; Liras, S. Med. Chem. Commun. 2011, 2, 1001.