3-Pyrroline containing arylacetamides: a novel series of remarkably selective kappa-agonists.

Article abstract of DOI:10.1016/S0960-894X(02)00429-8

A series of 2-(substituted phenyl)-N-methyl-N-[(1S)-1-(substituted alkyl)-2-(1-(3-pyrrolinyl))ethyl]acetamides were synthesized and evaluated as highly selective kappa-agonists with K(i) values in low nanomolar range. 3-Pyrroline incorporated into the basic amino functionality in combination with 2-(methylthio)ethyl substituent on the carbon adjacent to the amide nitrogen remarkably enhanced the kappa-selectivity. 3,4-Dichlorophenyl derivative 1e was found the most potent and selective analgesic in this series with ED(50) value of 0.023 mg/kg.

Full text of DOI:10.1016/S0960-894X(02)00429-8

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2287–2290
3-Pyrroline Containing Arylacetamides:
A Novel Series of Remarkably Selective ꢀ-Agonists
Qi-Yong Mou, Jie Chen, You-Cheng Zhu, De-He Zhou,
Zhi-Qiang Chi and Ya-Qiu Long*
Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Sciences,
Chinese Academy of Sciences, 294 Taiyuan Road, Shanghai 200031, China
Received 19 April 2002; accepted 7 June 2002
Abstract—A series of 2-(substituted phenyl)-N-methyl-N-[(1S)-1-(substituted alkyl)-2-(1-(3-pyrrolinyl))ethyl]acetamides were syn-
thesized and evaluated as highly selective kappa-agonists with Ki values in low nanomolar range. 3-Pyrroline incorporated into the
basic amino functionality in combination with 2-(methylthio)ethyl substituent on the carbon adjacent to the amide nitrogen
remarkably enhanced the k-selectivity. 3,4-Dichlorophenyl derivative 1e was found the most potent and selective analgesic in this
series with ED50 value of 0.023 mg/kg. # 2002 Elsevier Science Ltd. All rights reserved.
Introduction
instead of conventional pyrrolidine substituent into
the k-pharmacophoric ethylenediamine framework.
Amongst the classical k-agonist series (Chart 1),10ꢀ14 the
basic amino functionality incorporated into a pyrroli-
dine is an important determinant for receptor affinity.
But the inclusion of a double bond in the pyrrolidine
ring has not been explored well in the context of
k-selectivity, only one case has been reported to display
equal potency relative to its pyrrolidinyl counterpart.12
So we were intrigued to investigate the effect of further
variations on the antinociceptive activity and k-selectiv-
ity with respect to the arylacetamide having 3-pyrroline
moiety. Our initial effort was focused on modifications
of the substituted phenyl acetic acid to give a new series
of compounds with general structure 1.
It is well established that opioid analgesics mediate their
effects through three opioid receptor subtypes, namely,
m, k and d.1 More specifically, highly selective k-opioid
agonists may provide a strong analgesic free from the
abuse potential and the adverse side effects of m-agonists
like morphine.2,3 In addition to their analgesic effects in
in vivo models, k-agonists have been shown to be potent
neuroprotective and antihyperalgesic agents.4 Further-
more, it was reported that k-agonists can downregulate
the expression of the human immunodeficiency virus
(HIV-1) in human microglial cells and CD4+lympho-
cytes.5,6 However, central-mediated side effects such as
sedation, dysphoria, and strong diuresis usually accom-
pany k-agonist applications.4
Peripherally restricted k-agonists7ꢀ9 were believed to
reduce or eliminate the side effects associated with the
CNS, yet no selective k-agonist compound has reached
the market. Our strategy is to produce a new class of
highly selective k-agonists, which could be eventually
modified to selectively act in the periphery as a potential
treatment for visceral pain or HIV-1 infection. To
achieve this goal, we chose the most flexible k-agonist,
ICI-199441 as a template and introduced 3-pyrroline
Chemistry
All compounds were prepared via the route depicted in
Scheme 1. Coupling of CBZ-protected (S)-(+)-amino
acid, that is (S)-(+)-valine 2a or (S)-(+)-methionine 2b
with 3-pyrroline gave 3a–b. The product 3a–b was
reduced with LiAlH4 to afford the diamine 4a–b. Con-
densation of the diamine with various arylacetyl chlor-
ides furnished a series of compounds of general structure
1 (see Table 1).15 3-Pyrroline was prepared following the
literature procedure16 which always contained a little bit
of pyrrolidine.17 However, by recrystallization we
obtained analytically pure products for biological assay.
*Corresponding author. Tel.: +86-21-6431-1833x512; fax: +86-21-
6437-0269; e-mail: yqlong@mail.shcnc.ac.cn
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00429-8

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Q.-Y. Mou et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2287–2290
Chart 1. Selected known k-agonists and designed new series.
Scheme 1.
Table 1. Mouse antinociceptive activity and opioid receptor binding data of 3-pyrroline containing arylacetamides 1a–1l
Compd
R
X
k-binding affinitya
m-binding affinityb
ED50 (mg/kg) ipc
1a
1b
1c
1d
1e
1f
1g
1h
1i
(CH3)2CH–
(CH3)2CH–
(CH3)2CH–
(CH3)2CH–
(CH3)2CH–
(CH3)2CH–
CH3SCH2CH2–
CH3SCH2CH2–
CH3SCH2CH2–
CH3SCH2CH2–
CH3SCH2CH2–
CH3SCH2CH2–
4-F
4-Cl
4-Br
37.0% (1)d
60.0% (1)
90.0% (1)
46.5% (10)d
42.0% (10)
0 (10)
13.5% (10)
0 (10)
10.5% (10)
0 (10)
0 (10)
0 (10)
0 (10)
1.16
0.25
0.18
2.57
0.023
0.17
ꢂ10
2.13
2.36
>10
0.18
2.0
4-OMe
3,4-Dichloro
4-NO2
4-F
4-Cl
4-Br
4-OMe
3,4-Dichloro
4-NO2
70.0% (1)
Ki=5.72ꢁ1.32 nM
80.0% (1)
70.0% (1)
64.0% (1)
80.0% (1)
80.0% (1)
90.0% (1)
1j
1k
1l
11.5% (10)
0(10)
Ki=3.06ꢁ1.14 nM
aInhibition of [3H]diprenorphine binding in cloned rat k-opioid receptor expressed in Sf9 insect cells, was presented as inhibition constants Ki or
percent inhibition of the radioligand binding.
bInhibition of [3H]diprenorphine binding in cloned human m-opioid receptor expressed in Sf9 insect cells, was presented as Inhibition Constants Ki
or percent inhibition of the radioligand binding.
cThe compounds were administered intraperitoneally.
dThe respective test concentration is indicated in parentheses as 10ꢀ6 M for receptor binding assay.

Q.-Y. Mou et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2287–2290
2289
Biological Results and Discussion
less lipophilic or more electron-donating. And both
compounds fall in the category of N-{(1S)-1-[2-
(methylthio)ethyl]-2-[1-(3-pyrrolinyl)]ethyl}acetamides.
Some structural and metabolite factors might be
involved in the in vivo potency. Presumably, the
analgesic activity of 2-(methylthio)ethyl derivatives was
compromised by their lower lipophilicity or greater lia-
bility to oxidation relative to the isopropyl ones. There-
fore, the nitro group and the 2-(methylthio)ethyl
substitution of 1l which showed the best k-activity
(Ki=3.06 nM) caused a marked drop in the potency
(ED50=2.0mg/kg); however, the dichloro substitution
of the isopropyl derivatives still maintained consider-
able analgesic activity (1e, ED50=0.023 mg/kg; 1k,
ED50=0.18 mg/kg) which produced the best com-
pounds in this series.
The structure/k-receptor affinity relationship studies
aided with molecular modeling18 have defined the fol-
lowing pharmacophoric elements: the basic amino
functionality, the substituted benzene ring and the
amide carbonyl group in the arylacetamides. In the
current study with the incorporation of unsaturated
pyrroline into the basic amino functionality, the effect
of the substituent R at the carbon a to the amide and
the substituent X on the phenyl ring was explored to
ensure an optimal combination for the most appro-
priate structural motif.
In vitro.19a,b Kappa (k) opioid receptor affinity was
determined by the displacement of bound [3H]diprenor-
phine in cloned rat k-opioid receptor expressed in Sf9
insect cells and mu (m) opioid receptor affinity was
obtained by the displacement of bound [3H]diprenor-
phine using membranes prepared from cells expressing
the cloned human m-opioid receptor (Table 1). The Ki
values were calculated by Cheng–Prusoff equation as
meansꢁstandard errors from three independent experi-
ments. The affinity for the k-receptor ranged from
nanomolar to micromolar concentrations. However, the
affinity of some compounds was found to be too low to
perform a full dose–effect curve. In these cases, the per-
centage of inhibition of the radioligand binding was
given. Thus, qualitative SAR will be discussed in the
following section.
Conclusions
The present work demonstrated that the introduction of
3-pyrroline in the basic amino functionality of arylace-
tamides dramatically improved the k/m selectivity, and
the 2-(methylthio)ethyl substituent on the carbon adja-
cent to the amide nitrogen was greatly beneficial for the
k-receptor binding. The SAR study on this new series of
3-pyrroline containing arylacetamides revealed some
potent and highly selective k-agonists, such as 1c, 1e,
1k, 1l, which could provide promising candidates for the
development of the safer analgesics.
As shown in Table 1, the 3-pyrroline containing aryl-
acetamides displayed significant k/m selectivity. Espe-
cially, 2-(methylthio)ethyl substituent at the carbon a to
the amide is more favorable than isopropyl for the
k-selectivity. Most of the N-{(1S)-1-[2-(methylthio)-
ethyl]-2- [1-(3-pyrrolinyl)]ethyl}acetamides (e.g., 1g, 1h,
1i, 1j, 1l) showed little binding to the m-opioid receptor
even at the concentration of 1ꢃ10ꢀ5 M while the com-
pound 1l displayed the highest binding affinity to the
k-receptor with Ki value of 3.06 nM.
Acknowledgements
The financial supports from National Natural Science
Foundation of China (No. 29790123), Chinese Acad-
emy of Sciences (Hundred Talent Program) and Minis-
try of Personnel of China are greatly appreciated.
References and Notes
Comparing the substituents at the 4-position on the
phenyl ring, electron-donating group such as methoxy
(1d and 1j) was disadvantageous to the affinity for the
k-receptor, whereas electron-withdrawing group such as
nitro (1f and 1l) greatly improved the k-binding. With
respect to the halogenated compounds, the increase in
both atom size and lipophilicity may cause parallel
increase in the k binding affinity. The excellent k-recep-
tor binding was derived from the 4-bromo derivatives
(1c, 1i). Furthermore, the 3,4-dichloro substitution (1e,
1k) provided potent affinity to k-receptor with Ki values
in nanomolar range.
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