N -Alkyl-octahydroisoquinolin-1-one-8-carboxamides: Selective and nonbasic ?-opioid receptor ligands

Article abstract of DOI:10.1021/ml100040t

Herein, we report that N-alkyl-octahydroisoquinolin-1-one-8-carboxamides are a novel class of readily synthesized, selective ?-opioid receptor (KOR) ligands. A striking feature of this class of compounds is the absence of any basic nitrogen atoms. Many of these compounds have demonstrated exclusive affinity for the KOR over not only the ?-opioid receptor and the ?-opioid receptor but also 38 other G protein-coupled receptor targets. The general binding affinity of this class of compounds for the KOR combined with a streamlined route for analogue synthesis provide strong motivation for pursuing this interesting new scaffold as a basis toward new probes targeting the KOR.

Full text of DOI:10.1021/ml100040t

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N-Alkyl-octahydroisoquinolin-1-one-8-carboxamides:
Selective and Nonbasic κ-Opioid Receptor Ligands
Kevin J. Frankowski,† Partha Ghosh,† Vincent Setola,‡ Thuy B. Tran,‡
‡
,†
ꢀ
Bryan L. Roth, and Jeffrey Aube*
†Department of Medicinal Chemistry, University of Kansas, 2121 Simons Drive, Lawrence, Kansas 66047, and
‡Department of Pharmacology School of Medicine and NIMH Psychoactive Drug Screening Program CB 7365,
University of North Carolina Chapel Hill, 4072 Genetic Medicine Building, Chapel Hill, North Carolina 27514
ABSTRACT Herein, we report that N-alkyl-octahydroisoquinolin-1-one-8-carbox-
amides are a novel class of readily synthesized, selective κ-opioid receptor (KOR)
ligands. A striking feature of this class of compounds is the absence of any basic
nitrogen atoms. Many of these compounds have demonstrated exclusive affinity
for the KOR over not only the δ-opioid receptor and the μ-opioid receptor but also
38 other G protein-coupled receptor targets. The general binding affinity of this
class of compounds for the KOR combined with a streamlined route for analogue
synthesis provide strong motivation for pursuing this interesting new scaffold as a
basis toward new probes targeting the KOR.
KEYWORDS κ-Opioid receptor, isoquinolones
he κ-οpioid receptor (KOR) plays a significant role in a
broad range of physiological functions1-3 that include,
inter alia, addiction,4,5 depression,6-8 and pain relief.9
selection of amines afforded an initial set of 72 octahydroiso-
quinolone carboxamides. The requisite amine-containing
dienes 2 are readily obtained18,19 bearing diverse R1 groups.
A tandem Diels-Alder/acylation reaction sequence between
these dienes and maleic anhydride 3 then affords the
carboxylic acid scaffolds 4 in good yields (68-80%). The
exclusive cis, cis relative configuration of the bicyclic frame-
work and pendant carboxylic acid group in the product 4 is
consistent with either a Diels-Alder reaction followed by an
intramolecular acylation or the inverse sequence.20 The acid
scaffolds 4 were further diversified by carbodiimide-mediated
coupling with a selection of 12 commercially available amines to
afford the octahydroisoquinolone carboxamides 1. The nature
of the carboxylic acid and amine components utilized in the
construction of this initial library of octahydroisoquinolone
carboxamides is detailed in Figure 2. This sequence provides
an efficient and high-yielding route to a potentially vast collection
of octahydroisoquinolone carboxamides, as demonstrated by
the synthesis of this initial 72 member compound set.
Our interest in the development of methods to enable the
synthesis of compound collections is partially motivated by a
curiosity in the biological profile of the final products of these
efforts. To this end, 50 library compounds were selected to
represent the diverse range of functional groups in the
collection and subsequently screened against 41 individual
G protein-coupled receptor (GPCR) assays using the re-
sources of the NIMH Psychoactive Drug Screening Program.
T
Additionally, the naturally occurring hallucinogen salvinorin
A is a potent and selective KOR agonist implicating the KOR
in diseases of human perception.10 Accordingly, there is
great interest in discovering agents that are able to positively
or negatively modulate KOR function. In addition to a robust
and still-evolving collection of peptide-derived and natural
product ligands that interact with the KOR, synthetic small
molecules are of particular interest as potential therapeutic
compounds.11 -13 Three notable recent examples of the
latter are the aryl acetamide class of KOR agonists, exem-
plified by U-50,488,14 first introduced by VonVoigtlander
and Szmuszkovicz, the guandine derivative GNTI15 devel-
oped by Portoghese and co-workers, and the tetrahydroiso-
quinoline JDTic16,17 developed by Carroll and co-workers
(Figure 1). Herein, we report that comparatively simple
and synthetically accessible octahydroisoquinolone carbox-
amides of the general structure 1 represent a new class of
selective KOR ligands. This new chemotype is distinct from
most known small molecule KOR ligands in that the nitrogen
atoms present are neutral by their involvement in amide
bond resonance. Eight of the compounds reported here bind
to the KOR at concentrations <1 μM with no measurable
affinity for any other tested neurotransmitter receptor. Func-
tional studies reveal individual compounds of this chemo-
type to be full agonists of varying efficacy.
Recently, we reported an efficient synthesis of octahydro-
isoquinolone carboxylic acids utilizing a tandem Diels-
Alder/acylation sequence (Scheme 1).18 Subsequent elaboration
of the carboxylic acids via carbodiimide coupling with a
Received Date: February 20, 2010
Accepted Date: May 6, 2010
Published on Web Date: May 17, 2010
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Figure 1. Examples of known synthetic KOR ligands and the general isoquinolone amide structure.
Scheme 1. Synthesis of Octahydroisoquinolone Carboxamides
screened were found to be both highly selective for the
KOR and remarkably potent, most notably compounds 1l,
1n, 1y, 1aa, 1bb, and 1tt. Furthermore, the majority of the
compounds screened (35 out of 50) possesses experimen-
tally significant KOR binding (Ki < 10 μM), a remarkable hit
ratio from a set of compounds not synthesized with the
target assay in mind.
Isolated members of this compound set display binding
affinity for other GPCR targets. For example, compounds 1ss
and 1uu show potent binding affinity for the 5HT2B receptor
in addition to the KOR, the activation of which has been linked
to pulmonary hypertension.22 Compound 1mm displayed
selective, submicromolar affinity for the serotonin transporter,
and compound 1g was marginally selective for the DOR over
the KOR, providing promising entry points for future optimiza-
tion studies. At this time, however, we were most interested in
the KOR binding activity that appears to be generally char-
acteristic for this structural class of compounds.
The substituents on the 12 most potent compounds and
the numerical secondary binding data (Ki values) for the
DOR, KOR, and MOR receptor assays are shown in Table 1.
The data confirm high selectivity for the KOR over the DOR
and to a lesser extent the MOR for most of these examples,
except for one case where R2 = phenyl (entry 6). Com-
pounds where R2 = 4-chloro-3-trifluoromethylphenyl were
shown to be >10-fold selective for the KOR over the MOR/
DOR and to have the highest potency among the four sets of
amide derivatives examined (entries 9-12). While com-
pound 1xx containing a benzyl group in the R1 position
was highly potent, not all compounds containing benzyl at
R1 had similarly high affinity (cf. entries 1, 3, 6, and 12 in
Table 1).
Figure 2. Components for the synthesis of the octahydroisoquino-
lone amide products 1.
Compounds were first screened at each GPCR target at
a constant concentration (10 μM) to identify active
compounds.21 Active compounds from initial binding
screens were selected for Ki determinations using radioli-
gand binding assays. A summary of the full results of the
secondary binding screen is depicted in heatmap format in
Figure 3. Most striking is a general selectivity trend of the
compounds for the KOR over not only the δ-opioid receptor
(DOR) and the μ-opioid receptor (MOR) but also against the
other GPCR targets screened. Several of the compounds
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Figure 3. Activity of the isoquinonlone carboxamide compounds in secondary binding assays. Key: white, Ki > 10 μM/primary screen
missed; blue, Ki = 5-10 μM; green, Ki = 1-5 μM; yellow, Ki = 0.5-1 μM; red, Ki < 0.5 μM; and black, no data available. For the complete
binding data (Ki values) on these compounds, see the Supporting Information.
To further investigate the effect of the substituents on
potency, four additional compounds were synthesized ac-
cording to the protocol in Scheme 1 and screened for DOR,
KOR, and MOR binding in the primary and secondary assays
described above. These compounds, while not more potent
than compound 1{50} add a further three examples of
selective and potent KOR ligands in this structural class of
compounds (Table 2).
Compounds 1xx, 1yy, and 1zz were further evaluated in
KOR functional binding assays to determine whether they
behave as agonists or antagonists (Table 3). Thus, the con-
centration-dependent (ranging from 0.01 to 10000 nM)
inhibition of isoproterenol-stimulated cAMP accumulation
in hKOR-expressing HEK293T cells was measured for both
the test compounds and U-69,593 (a known KOR full
agonist). The three test compounds 1xx, 1yy, and 1zz were
all found to be full agonists (Emax = 100%) as compared to
U-69,593. The EC50 for compound 1zz is over 2 orders of
magnitude greater than the EC50 for the similar analogue
1yy; thus, given the two analogues' similar binding affinities,
the functional data suggest that the efficacy of this new
chemotype can be modulated by subtle structural modifica-
tions such as exchanging a triflouoromethyl substituent for a
methyl group.
In summary, we have discovered an attractive new
scaffold for KOR modulator discovery, with a number of
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Table 1. Selected Binding Data for Four Sets of Potent Isoquino-
lone Carboxamide Compounds
to procure these compounds is robust and provides an
efficient and straightforward route to analogues. The synth-
esis, screening, and evaluation of additional analogues are
ongoing and will be reported in due course.
SUPPORTING INFORMATION AVAILABLE Experimental
details and characterization data for all new compounds, structures,
and Ki values for each compound in all active assays shown in
Figure 3, purity assessment for all compounds, and assay protocols.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Ki (μM)
entry compound scaffold
R1
DOR
>10
KOR MOR
1
2
1l
1a
1a
1b
1b
1b
1c
1c
1c
1d
1d
1d
1d
benzyl
0.30 >10
0.17 >10
0.20 >10
0.16 >10
0.36 >10
AUTHOR INFORMATION
Corresponding Author: *To whom correspondence should be
addressed. Tel: 785-864-4496. E-mail: jaube@ku.edu.
1n
cylcohexyl
benzyl
>10
>10
>10
>10
3
1y
4
1aa
1bb
1cc
1ee
1gg
1ss
1tt
cylcohexyl
n-butyl
Funding Sources: We thank the National Institute of General
Medical Sciences (GM-49093 and PO50-GM069663), the National
Institute of Mental Health's Psychoactive Drug Screening Program
[Contract #HHSN-271-2008-000025-C (NIMH-PDSP)], and RO1DA-
017204 for financial support.
5
6
benzyl
0.85 0.15
2.57
7
cylcohexyl
3,4-dichloro-benzyl
cylcopropyl
cylcohexyl
n-butyl
>10
0.07
8.94
2.70
8
1.11 0.26
>10
ACKNOWLEDGMENT We are grateful to Benjamin Neuens-
wander for HR-MS and compound purification.
9
0.49 >10
0.11 >10
10
11
12
>10
>10
>10
1uu
1xx
0.19
2.22
3.55
ABBREVIATIONS KOR, κ-opioid receptor; DOR, δ-opioid
receptor; MOR, μ-opioid receptor; GPCR, G protein-coupled
receptor.
benzyl
0.005
Table 2. Additional Isoquinolone Carboxamide Analogues and
Opioid Receptor Binding Profiles
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