Novel phenylamino acetamide derivatives as potent and selective κ opioid receptor agonists

Article abstract of DOI:10.1016/j.bmcl.2005.10.034

A novel series of phenylamino acetamide derivatives was synthesized. These amides were shown to be potent and selective κ opioid receptor agonists.

Full text of DOI:10.1016/j.bmcl.2005.10.034

Bioorganic & Medicinal Chemistry Letters 16 (2006) 645–648
Novel phenylamino acetamide derivatives as potent and selective
j opioid receptor agonists
Guo-Hua Chu,a,* Minghua Gu,a Joel A. Cassel,b Serge Belanger,b Gabriel. J. Stabley,b
Robert N. DeHaven,b Nathalie Conway-James,b Mike Koblish,b Patrick J. Little,b
Diane L. DeHaven-Hudkinsb and Roland E. Dollea
aDepartment of Chemistry, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA 19341, USA
bDepartment of Pharmacology, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA 19341, USA
Received 14 September 2005; revised 10 October 2005; accepted 12 October 2005
Available online 2 November 2005
Abstract—A novel series of phenylamino acetamide derivatives was synthesized. These amides were shown to be potent and selective
j opioid receptor agonists.
Ó 2005 Elsevier Ltd. All rights reserved.
It is well established that the pharmacological effects of
opioid-type drugs are mediated by three opioid receptor
types, l, j, and d.1 The most prominent opiate, mor-
phine, has had a long history of analgesia in clinical
practice. Its analgesic potency, mediated through
agonism of the l-opioid receptor, is associated with
undesired side effects, such as respiratory depression,
constipation, tolerance, and dependence. Receptor
selective j-agonists as potent and efficacious analgesics
are of particular interest because they produce analgesia
without the undesirable side effects of the l-opioids.2–4
However, sedation, dysphoria, and diuresis usually
accompany j-agonist applications, and thus prevented
their further development and commercialization as
analgesics.2–4 To avoid the side effects associated with
the CNS, in recent years attention has been focused
on the development of peripherally acting j-agonists
as potential analgesic therapeutics.2,5–10
acetamide derivatives with the general structure I, which
are bioisosteres of aryloxyacetamides with the replace-
ment of oxygen with nitrogen. The replacement of oxy-
gen with nitrogen will increase the polarity and
hydrophilicity of the compounds, and may limit the
CNS penetration and display peripheral selectivity. This
is expected to be beneficial to minimize or eliminate the
CNS side effects of the centrally active j opioid receptor
ligands while retaining antihyperalgesic activity.2 These
novel phenylamino acetamides possessed high and selec-
tive j- opioid receptor binding affinity. Compound 4,
O
O
OH
OH
O
N
O
N
N
N
CH3
CH3
Cl
Cl
111
Ki (κ) = 2 nM
2 11
During our screening of a combinatorial library, we
identified aryloxyacetamides (e.g., 1 and 2) as highly po-
tent j-agonists.11 Based on these lead compounds, we
recently reported the design and synthesis of two novel
series of constrained aryloxyacetamides 3.12 In this com-
munication, we report a novel series of phenylamino
Ki (κ) = 0.125 nM
µ/κ: 480; δ/κ: 40
µ/κ: > 2500; δ/κ: 370
R
N
O
O
OH
OH
N
O
N
N
N
X
X
CH3
CH3
n
Keywords: j Opioid-receptor agonists; Analgesic.
Corresponding author. Tel.: +1 484 595 1083; fax: +1 484 595
1551; e-mail: ghchu@adolor.com
312
I
*
n = 0: 2,3-Dihydrobenzofuran class
n = 1: Chromane class
(this work)
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.10.034

646
G.-H. Chu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 645–648
the most potent and selective j agonist in this novel
series of phenylamino acetamides, exhibited potent anal-
gesic effects in the in vivo formalin-induced nociception
assay and acetic acid-induced writhing assay.
to the target compounds 14–16 using the above-men-
tioned method.
The synthesis of the sulfonamides 17 and 18 was started
from N-4-nitrophenylglycine. Conversion to its methyl
ester under standard condition, protection of the anilinic
amino group as trifluoroacetamide, and hydrogenation
of the nitro group, followed by reaction of the resulting
aniline with methanesulfonyl chloride or propanesulfo-
nyl chloride using triethylamine as a base, afforded the
disulfonylated products. These intermediates were then
treated with LiOH to simultaneously cleave the trifluo-
roacetamide, the methyl ester, and one sulfonyl group
in the molecules, yielding the desired N-4-methanesulfo-
nylamino- and N-4-propanesulfonylamino-phenylgly-
cines. The resulting acids were transformed to the
target compounds 17 and 18 in the same manner de-
scribed above.
The synthesis of the target phenylamino acetamide deriv-
atives 4–18 is summarized in Scheme 1. Various known
substituted glycines, either commercially available or pre-
pared by literature procedures: N-phenylglycine, N-4-tri-
fluoromethyl, 3,4-dichloro,13 4-nitro,14 4-cyano,14,15
3-cyano,16 2-cyano17 phenylglycines, and N-methyl-N-
phenylglycine,18 N-acetyl-N-phenylglycine,19 and N-
methyl-N-4-cyanophenylglycine15 were coupled with the
diamine 1-(2-methylamino-(S)-2-phenyl-ethyl)-pyrrolidin-
(S)-3-ol20 using TBTU [O-(benzotriazol-1-yl)-N,N,N0,
N0-tetramethyluronium tetrafluoroborate] as the acylat-
ing reagent to yield the target compounds 4–13.
In our previous studies of the constrained chroman-2-
carboxamides and 2,3-dihydrobenzofuran-2-carboxa-
mides, compounds 3,12 as potent j-agonists, we found
that the sulfonylamino groups are the most preferred
substituents at the phenyl ring for obtaining both high
j affinity and low inhibitory activity at CYP2D6.12,21
We incorporated this SAR information into the struc-
ture of our new chemical series of phenylamino acetam-
ide derivatives I. Thus, compounds 14–18, which
contain sulfonylamino groups, were prepared. Hydroge-
nation of the N-para-, meta-, and ortho-cyanophenyl-
glycines gave the amino acids, which were converted
to their methyl esters under standard reaction condi-
tions. Reaction of these benzylamines with metha-
nesulfonyl chloride, followed by treatment with LiOH,
afforded the corresponding acids, which were converted
Phenylamino acetamide derivatives 4–18 were evaluated
in the in vitro opioid receptor binding assays and the
results are shown in Table 1.22 Compounds 4 and 6,
the bioisosteric analogs of the aryloxyacetamides 1 and
2 with the replacement of oxygen with nitrogen, exhibit-
ed sub-nanomolar j binding affinity and high selectivity
over l and d receptors. The substituents at the anilinic
nitrogen affected j binding. Compounds 11 and 13 with
the replacement of hydrogen with methyl group had a
slightly decreased j affinity and also a 2- to 3-fold
decrease in receptor selectivity compared to compounds
4 and 8, while the acylated analog 12 showed more than
a 20-fold loss of j affinity compared to compound 4,
indicating an unsubstituted amino group (NH) is impor-
tant for obtaining high j binding affinity. Substitution
O
H
O
H
d, e
N
NH
S
N
OH
H2N
O
O
O
a
b, c
R
O
R
O
OH
N
N
N
OH
a
N
X
X
4: X = H, R = H; 5: X = X = 4-CF3, R =H
6: 3,4-Cl, R = H; 7: X = 4-NO2, R = H
8: X = 4-CN, R = H; 9: X = 3-CN, R = H
10: X = 2-CN, R = H; 11: X = H, R = CH3
12: X = H, R = COCH3; 13: X = 4-CN, R = CH3
14: X = 4-CH3SO2NHCH2, R = H
X = H, 4-CF3, 3,4-Cl, 4-NO2,
4-CN, 3-CN, 2-CN;
R = H, CH3, COCH3
a
c, f, g, d
15: X =3-CH3SO2NHCH2, R = H
16: X = 2-CH3SO2NHCH2, R = H
17: X = 4-CH3SO2NH, R = H
18: X = 4-CH3CH2CH2SO2NH, R = H
R
S
R
S
O
O
N
O
O
O
H
N
OH
O
O
e
S
R
N
H
NCH2COOMe
COCF3
R = CH3, CH3CH2CH2
Scheme 1. Synthesis of phenylamino acetamide derivatives 4–18. Reagents: (a) 1-(2-methylamino-(S)-2-phenyl-ethyl)-pyrrolidin-(S)-3-ol dihydro-
chloride, TBTU, i-Pr2NEt, MeCN; (b) H2, Pd/C, MeOH, concd HCl; (c) MeOH, HCl (2.0 M in Et2O); (d) MeSO2Cl or n-PrSO2Cl, Et3N, DCM; (e)
LiOH, MeOH/THF/H2O (1:1:1); (f) (CF3CO)2O, Et3N, DCM; (g) H2, Pd/C.

G.-H. Chu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 645–648
647
Table 1. Opioid receptor binding results
R
N
O
OH
N
5
X
N
CH3
2
4
3
Compound
X
R
j Ki (nM)
j EC50 (nM)
l/j
d/j
CYP2D6 IC50 (nM)
4
5
H
4-CF3
H
0.17
0.39
0.11
0.21
0.66
0.5
0.05
0.8
5882
385
512
256
91
4100
190
H
6
3,4-Cl
4-NO2
H
0.27
0.23
0.49
0.58
0.42
0.23
7.4
427
905
760
7
H
119
409
66
3400
2500
3000
510
8
4-CN
3-CN
H
833
9
H
1140
667
10
11
12
13
14
15
16
17
18
2-CN
H
H
0.39
0.32
3.8
49
CH3
CH3CO
CH3
H
1656
>1315a
250
169
342
180
119
38
530
300
H
4-CN
1
16
2.3
670
4-CH3SO2NHCH2
3-CH3SO2NHCH2
2-CH3SO2NHCH2
4-CH3SO2NH
4-CH3CH2CH2SO2NH
24
81
38
8000
8700
1500
H
3.9
4.8
H
0.78
5.7
0.52
14
8.3
85
>877a
1000
10
47
H
<50% inhibition at 10 lM
9000
H
1.5
87
a l receptor Ki is estimated to be >5 lM.
groups at the phenyl ring were well tolerated by the j
receptor. Fourteen compounds out of fifteen with vari-
ous substituents displayed low nM j binding affinity.
The position of the substitution groups on the phenyl
ring impacted j binding affinity and selectivity. The j
affinity increased from para-substitution (compounds
8, 14) to meta-substitution (compounds 9, 15), and to
ortho-substitution (compounds 10, 16), the latter being
the most preferred pattern for sterically demanding sub-
stituents. The trend toward j selectivity was reversed,
with para-substitution having highest selectivity and
the ortho-substitution having lowest selectivity versus
the d receptor. The phenylamino acetamides containing
sulfonylamino groups, compounds 14–16 and 17, 18
showed decreased j binding affinity compared to the
non-substituted compound 4, but most of the com-
pounds still remained low nM j affinity.
effects, producing 96.2% antinociception at 300 lg giv-
en intrapaw (sc injection in dorsal surface of paw), in
the late phase formalin-induced flinching assay, and
inhibited acetic acid-induced writhing when adminis-
tered subcutaneously with an ED50 value of
0.017 mg/kg.
In summary, a novel series of phenylamino acetamide
derivatives, bioisosteres of aryloxyacetamides, was syn-
thesized and found to be highly potent j receptor ago-
nists. This discovery expands the SAR of j-agonists
beyond classic arylacetamides. Most of the compounds
displayed low nM j binding affinity, excellent selectivity
against l and d, and low CYP2D6 activity. Compound 4
with high
j affinity and potency (Ki = 0.17 nM,
EC50 = 0.05 nM) and >500-fold selectivity versus both
l and d receptors demonstrated potent analgesic effects
in the in vivo formalin-induced nociception and acetic
acid-induced writhing assays.
Agonists 4–18 were also evaluated against the cyto-
chrome P450 2D6 enzyme using a fluorescent-based as-
say.23 The reference compound, ICI 199441, a highly
potent and selective arylacetamide j-agonist, is a potent
inhibitor (IC50 = 26 nM) of CYP2D6.23 This is an unde-
sirable property to have in a drug.21,23 In general, j ago-
nists 4–18 did not significantly inhibit CYP2D6.
Unsubstituted 4 displayed an IC50 = 4.1 lM. Inhibitory
activity tended to correlate with lipophilicity. Agonists
5, 6, 11, and 12 possessing hydrophobic substituents
showed the greatest level of enzyme inhibition. Consis-
tent with previous SAR studies,12,23 sulfonamide-bear-
ing agonists, for example, 14–18, were weak inhibitors
of CYP2D6.
References and notes
1. Dhawan, B. N.; Cesselin, F.; Raghubir, R.; Reisine, T.;
Bradely, P. B.; Portoghese, P. S.; Hamon, M. Pharmacol.
Rev. 1996, 48, 567.
2. DeHaven-Hudkins, D. L.; Dolle, R. E. Curr. Pharm. Des.
2004, 10, 743.
3. Holzgrabe, U.; Brand, W. J. Med. Chem. 2003, 46, 1383.
4. Soukara, S.; Maier, C. A.; Predoiu, U.; Ehret, A.;
Jackisch, R.; Wunsch, B. J. Med. Chem. 2001, 44, 2814.
¨
5. Kumar, V.; Guo, D.; Cassel, J. A.; Daubert, J. D.;
DeHaven, R. N.; DeHaven-Hudkins, D. L.; Gauntner, E.
K.; Gottshall, S. L.; Greiner, S. L.; Koblish, M.; Little, P.
J.; Mansson, E.; Maycock, A. L. Bioorg. Med. Chem. Lett.
2005, 15, 1091.
Compound 4, the most potent and selective j agonist
and quite a weak CYP2D6 inhibitor in the new phe-
nylamino acetamide series, was tested in the in vivo
nociceptive assays.24 It displayed potent analgesic
6. Tuthill, P. A.; Seida, P. R.; Barker, W.; Cassel, J. A.;
Belanger, S.; DeHaven, R. N.; Koblish, M.; Gottshall, S.

648
G.-H. Chu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 645–648
L.; Little, P. J.; DeHaven-Hudkins, D. L.; Dolle, R. E.
Bioorg. Med. Chem. Lett. 2004, 14, 5693.
7. Giardina, G.; Clarke, G. D.; Grugni, M.; Sbacchi, M.;
Vecchietti, V. Farmaco 1995, 50, 405.
8. Barber, A.; Gottschlich, R. Expert Opin. Investig. Drugs
1997, 6, 1351.
9. Stein, C.; Machelska, H.; Binder, W.; Schafer, M. Curr.
Opin. Pharmacol. 2001, 1, 62.
18. Brady, W. T.; Gu, Y. Q. J. Org. Chem. 1989, 54, 2834.
19. DeRuiter, J.; Swearingen, B. E.; Wandrekar, V.; Mayfield,
C. A. J. Med. Chem. 1989, 32, 1033.
20. Zhang, W. Y.; Maycock, A. L.; Kumar, V.; Gaul, F.;
Chang, A.-C.; Guo, D. WO00014065, 2000.
21. Nelson, D. R.; Koymans, L.; Kamataki, T.; Stegeman, J.
J.; Feyereisen, R.; Waxman, D. J.; Waterman, M. R.;
Gotoh, O.; Coon, M. J.; Estabrook, R. W.; Gunsalus, I.
C.; Nebert, D. W. Pharmacogenetics 1996, 6, 1.
22. Opioid receptor binding assay: (a) DeHaven, R. N.;
DeHaven-Hudkins, D. L. In Current Protocols in Phar-
macology; Enna, S. J., Williams, M., Ferkany, J. W.,
Kenakin, T., Porsolt, R. D., Sullivan, J. P., Eds.; John
Wiley & Sons Inc.: New York, 1998; pp 1.4.1–1.4.12; (b)
Raynor, K.; Kong, H.; Chen, Y.; Yasuda, K.; Yu, L.; Bell,
G. I.; Reisine, T. Mol. Pharmacol. 1994, 45, 330; (c) All the
compounds listed in Table1 were full agonists as deter-
mined in a [35S]GTPcS functional binding assay.
`
10. Riviere, P. J.-M. Br. J. Pharmacol. 2004, 141, 1331.
11. Unpublished data.
12. Chu, G.-H.; Gu, M. H.; Cassel, J. A.; Belanger, S.;
Graczyk, T. M.; DeHaven, R. N.; Conway-James, N.;
Koblish, M.; Little, P. J.; DeHaven-Hudkins, D. L.; Dolle,
R. E. Bioorg. Med. Chem. Lett. 2005, 15, 5114.
13. Anderson, W. K.; Jones, A. N. J. Med. Chem. 1984, 27,
1559.
14. Dyer, E.; McCarthy, F. L.; Johnson, R. L.; Nagle, E. V.
J. Org. Chem. 1957, 22, 78.
15. Kaji, M.; Kojima, Y.; Katogi, S.; Nunomura, M. Eur. Pat.
Appl. EP 636939, 1995.
16. Bryson, A.; Davies, N. R.; Serjeant, E. P. J. Am. Chem.
Soc. 1963, 85, 1933.
17. Peet, N. P.; Sunder, S. J. Heterocycl. Chem. 1977, 14, 561.
23. Le Bourdonnec, B.; Ajello, C. W.; Seida, P. R.; Susnow,
R. G.; Cassel, J. A.; Belanger, S.; Stabley, G. J.; DeHaven,
R. N.; DeHaven-Hudkins, D. L.; Dolle, R. E. Bioorg.
Med. Chem. Lett. 2005, 15, 2647.
24. For a description of the in vivo assays, see Ref. 5.