Discovery, synthesis, and molecular pharmacology of selective positive allosteric modulators of the δ-opioid receptor

Article abstract of DOI:10.1021/acs.jmedchem.5b00007

Allosteric modulators of G protein-coupled receptors (GPCRs) have a number of potential advantages compared to agonists or antagonists that bind to the orthosteric site of the receptor. These include the potential for receptor selectivity, maintenance of the temporal and spatial fidelity of signaling in vivo, the ceiling effect of the allosteric cooperativity which may prevent overdose issues, and engendering bias by differentially modulating distinct signaling pathways. Here we describe the discovery, synthesis, and molecular pharmacology of δ-opioid receptor-selective positive allosteric modulators (δ PAMs). These δ PAMs increase the affinity and/or efficacy of the orthosteric agonists leu-enkephalin, SNC80 and TAN67, as measured by receptor binding, G protein activation, β-arrestin recruitment, adenylyl cyclase inhibition, and extracellular signal-regulated kinases (ERK) activation. As such, these compounds are useful pharmacological tools to probe the molecular pharmacology of the δ receptor and to explore the therapeutic potential of δ PAMs in diseases such as chronic pain and depression.

Full text of DOI:10.1021/acs.jmedchem.5b00007

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Article
Discovery, Synthesis and Molecular Pharmacology of Selective
Positive Allosteric Modulators of the #-Opioid Receptor
Neil T Burford, Kathryn Livingston, Meritxell Canals, Molly Ryan, Lauren Budenholzer, Ying Han,
Yi Shang, John J Herbst, Jonathan O'Connell, Martyn Banks, Litao Zhang, Marta Filizola, Daniel
Bassoni, Tom S Wehrman, Arthur Christopoulos, John R. Traynor, Samuel W Gerritz, and Andrew Alt
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.5b00007 • Publication Date (Web): 22 Apr 2015
Downloaded from http://pubs.acs.org on May 6, 2015
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Discovery, Synthesis and Molecular Pharmacology of Selective Positive
Allosteric Modulators of the δ-Opioid Receptor
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Neil T. Burford , Kathryn E. Livingston , Meritxell Canals , Molly Ryan , Lauren Budenholzer , Ying Han , Yi
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Shang , John J. Herbst , Jonathan O’Connell , Martyn Banks , Litao Zhang , Marta Filizola , Daniel Bassoni , Tom
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S. Wehrman , Arthur Christopoulos , John R. Traynor , Samuel W. Gerritz , Andrew Alt .
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Bristol-Myers Squibb Company
Research and Development / Discovery
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Research Parkway
Wallingford, CT 06492
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University of Michigan Medical School
Dept. of Pharmacology
Ann Arbor, MI 48109
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Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences &
Department of Pharmacology
Monash University
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99 Royal Parade
Parkville, VIC, 3052
Australia
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Departments of Molecular Biophysics and Biochemistry & Pharmacology
Yale University
New Haven, CT 06520
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Department of Structural and Chemical Biology
Icahn School of Medicine at Mount Sinai (ISMMS)
Icahn Medical Institute Building, Room 16-20F
1425 Madison Avenue, Box 1677
New York, NY 10029-6574
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FORMA Therapeutics
Arsenal St, Suite 100
Watertown, MA 02472
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DiscoveRx Corporation
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2501 Albrae Street, Suite 100
Freemont, CA 94538
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Primity Bio
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350 Scott Blvd, ste 6101,
Santa Clara, CA 95054
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Abstract
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Allosteric modulators of G protein-coupled receptors (GPCRs) have a number of potential advantages compared to
agonists or antagonists that bind to the orthosteric site of the receptor. These include the potential for receptor
selectivity, maintenance of the temporal and spatial fidelity of signaling in vivo, the ceiling effect of the allosteric
cooperativity which may prevent overdose issues, and engendering bias by differentially modulating distinct
signaling pathways. Here we describe the discovery, synthesis and molecular pharmacology of δ-opioid receptor-
selective positive allosteric modulators (δ PAMs). These δ PAMs increase the affinity and/or efficacy of the
orthosteric agonists leu-enkephalin, SNC80 and TAN67, as measured by receptor binding, G protein activation, β-
arrestin recruitment, adenylyl cyclase inhibition and extracellular signal-regulated kinases (ERK) activation. As
such, these compounds are useful pharmacological tools to probe the molecular pharmacology of the δ receptor
and to explore the therapeutic potential of δ PAMs in diseases such as chronic pain and depression.
Keywords
Allosteric modulator, opioid receptor, β−arrestin
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Introduction
The δ-opioid receptor is a seven transmembrane domain (7TMD) receptor that belongs to the Class A family of G
protein coupled receptors (GPCRs). δ Receptor agonists have been shown to be antinociceptive especially in
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chronic pain models , and to have potential as antidepressant agents . The possible dual effects of δ receptor
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agonists to alleviate chronic pain and mitigate emotional disorders provide a particularly attractive therapeutic
strategy because of the high level of comorbidity between chronic pain and depression. However, agonists acting
directly at the δ receptor can show pro-convulsant effects in animal models, including non-human primates.
Indeed, it has been proposed that these seizurogenic properties of δ receptor agonists may be responsible for
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their antidepressant-like activity analogous to electro-convulsive therapy . On the other hand, slowing the rate of
administration of the δ receptor agonist SNC80 reduces seizurogenic activity but has no effect on anti-depressant
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–
like effects . Also, some δ receptor agonists (e.g. ADL5859) show no seizures in rat or mouse models . These
and other findings suggest that the convulsive properties of δ receptor agonists can be separated from their anti-
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7 8
.
depressant-like effects
Allosteric modulators for GPCRs bind to a site on the receptor that is topographically distinct from the site that
binds the orthosteric (or endogenous) agonist. Positive allosteric modulators (PAMs) increase the affinity and/or
efficacy of bound orthosteric agonist ligands. The operational model of allosterism allows the quantification of
allosteric effects and as such, it can estimate the binding affinity of the allosteric ligand to the free receptor (pKB),
the allosteric cooperativity factor (αβ) as well as any intrinsic agonist efficacy (τ ) of the allosteric ligand. PAMs
B
that have little or no intrinsic efficacy (τ ) but modulate the orthosteric agonist response, have a number of
B
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10 11
advantages over orthosteric ligands
. In particular, these PAMs can theoretically maintain the temporal and
spatial fidelity of endogenous receptor activation in vivo. The allosteric modulator binds to the target receptor but
remains effectively silent until the endogenous orthosteric agonist is presented to the receptor. Therefore, PAMs
can amplify the effect of endogenous signaling molecules without disrupting normal physiological regulation of
receptor activation, and might therefore be expected to exhibit superior efficacy and side-effect profiles compared
to traditional orthosteric agonists. Studies with δ receptor selective ligands, or utilizing a genetic deletion of the δ
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receptor , suggest that native opioid peptide signaling at the δ receptor mediates an increase in pain threshold in
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models of chronic pain and modulates mood states in rodent models . Therefore, positive allosteric modulation
of the δ receptor should enhance responses to the endogenous agonist peptides and thereby be therapeutically
efficacious. In addition, the finite nature of the agonist potency shift (defined by the allosteric cooperativity
factor), which saturates when the allosteric site is fully occupied, may increase the safety margin between
therapeutic effect and possible side-effects associated with over-activation of the target receptor. Finally, and
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pertinent to the δ-receptor system which is known to exhibit ligand-biased signaling , PAMs can modulate the
signaling bias of receptor activation toward desired pathways or engender bias from previously unbiased ligands
14 15
.
Thus, δ PAMs may provide a greater therapeutic window between pain relieving and antidepressant-like
effects and proconvulsive activity, compared with traditional δ receptor orthosteric agonists.
In this study we report the synthesis and structure-activity relationship (SAR) of the first described δ PAMs. One
of the most potent compounds identified, 3,3,6,6-tetramethyl-9-(4-((2-methylbenzyl)oxy)phenyl)-3,4,5,6,7,9-
hexahydro-1H-xanthene-1,8(2H)-dione (2, BMS-986187), was further characterized in radioligand binding assays
and using a range of cellular functional assays. 2 was shown to positively modulate orthosteric agonist binding
affinity and functional potency at the δ receptor, and enhance the efficacy of the partial agonist TAN67.
Results
Discovery and Structure-Activity Relationship (SAR) of δ Receptor PAMs
The δ PAM chemotype was identified from a high throughput screen (HTS) using a β-arrestin recruitment assay in
16 17
a PathHunter U2OS cell line coexpressing ꢀ and δ receptors (U2OS-OPRM1D1) (DiscoveRx, Freemont, CA)
.
The screen was executed in PAM mode by measuring activity in the presence of an EC concentration of both
10
endomorphin-I (a ꢀ receptor-selective agonist), and leu-enkephalin which in this assay and cell line was a relatively
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selective agonist for the δ receptor . Typically, when using HTS approaches to identify PAMs, an EC
20-40
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concentration of orthosteric agonist is used . However, in this HTS the sum of the two EC concentrations of
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agonists offered a compromise between the detection of both ꢀ and δ receptor PAMs and the ability to maintain
the overall signal window so that lower efficacy partial agonists could also be detected. Follow-up in vitro testing
to determine structural features necessary for PAM activity was performed utilizing CHO-PathHunter cell-lines
(CHO-OPRD1 and CHO-OPRM1) obtained from DiscoveRx. Concentration response curves (CRCs) for HTS hits were
determined both in agonist mode (in the absence of orthosteric agonist) to determine agonist activity of the test
compounds, and in PAM mode (in the presence of an EC concentration of orthosteric agonist) to determine
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allosteric modulator activity using the β-arrestin recruitment assays. Compound 7 (Table 1) was identified as a δ
PAM, producing a robust potentiation of the response to an EC concentration of leu-enkephalin.
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As shown in Scheme 1, we synthesized a series of close analogs of 7 to optimize δ PAM potency and selectivity.
None of the compounds exhibited significant agonist activity in a β-arrestin recruitment assay, but all of the
compounds produced measurable PAM activity at the δ receptor. 1 with an unsubstituted benzyl ring acted as a δ
PAM with an EC value of 0.2 ꢀM and showed 30-fold selectivity in the β-arrestin recruitment assay compared
50
with PAM activity at the ꢀ receptor. Introduction of a methyl group in various positions around the phenyl ring (2-
) suggested that ortho substitution increased δ receptor PAM activity by an order of magnitude, with minimal
4
effect on ꢀ receptor PAM activity, while meta and para substitution did not significantly affect δ or ꢀ receptor
PAM activity. The corresponding ortho-F analog 5 was not significantly more active than 1, suggesting that the
increased δ receptor activity with the o-methyl was due to a steric rather than an electronic effect. Similarly, the
meta- and para-F analogs 6-7, or the ortho-Cl analog 8 did not afford an increase in δ receptor activity.
Introduction of a second Cl- group in the meta position (9) provided a modest improvement in δ receptor activity
while maintaining selectivity. A more pronounced effect was observed with the ortho-Br analog 10 which
produced equipotent PAM activity to 2 at the δ receptor, but no observable PAM activity at the ꢀ receptor,
suggesting that 9-(4-((2-bromobenzyl)oxy)phenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-
1
,8(2H)-dione (10, BMS-986188) is the most δ receptor-selective analog we have identified to date. The effect of
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ortho substitution on δ receptor PAM potency and selectivity appears to be restricted to small substituents. As
shown with analogs 11-15, larger ortho substituents did not improve δ PAM activity and had no effect on
selectivity. Similarly, more drastic changes to the chemotype, such as increasing the chain length between the
ether oxygen and the phenyl ring, or replacement of the benzyl ether with a phenyl amide, yielded a significant
loss in δ receptor PAM activity (data not shown). The most potent δ PAM identified was 2, which in the presence
of an EC of leu-enkephalin, produced a β-arrestin response with an average EC of 33 nM in CHO-OPRD1 cells
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(Table 1). Representative agonist and PAM mode CRCs for 2 at the ꢀ and δ receptor are shown in Figure 1. In this
example, 2 produced little or no activity in agonist mode, but in PAM mode (in the presence of an EC of leu-
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enkephalin (in CHO-OPRD1 cells) or endomorphin-1 (in CHO-OPRM1 cells)) produced a response with an EC of 48
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nM in CHO-OPRD1 cells and 2 ꢀM in CHO-OPRM1 cells.
Multivariate statistical analysis of a relatively large number of physicochemical properties calculated for
the 15 compounds listed in Table 1 (see Methods section for details) revealed specific properties of the 15
compounds that were most likely to predict EC values at either δ (Table S3) or µ (Table S4) opioid receptors in
50
line with experimental data. Specifically, the total solvent accessible surface area (SASA, in square angstroms using
a probe with a 1.4 Å radius), the π (carbon and attached hydrogen) component of the SASA (PISA), and the
Parameterized Model Number 3 (PM3)-calculated ionization potential (IP.ev.) allow fitting of the predicted EC50
values at the δ opioid receptor (Figure S1). Similarly, the molecular weight of the molecule (mol.MW), the Van der
Waals surface area of polar nitrogen and oxygen atoms and carbonyl carbon atoms (PSA), and the number of
nitrogen and oxygen atoms (X.N and O) were identified as the best predictors for EC values at the µ opioid
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receptor (Figure S2). An estimation of the δ/µ selectivity based on the ratio of the predicted EC values at the δ
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and µ opioid receptors revealed a definite separation between compounds with a selectivity above or below a 33-
fold cutoff (Figure S3).
Binding Characterization of 2
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2 (at concentrations up to 30 ꢀM) does not inhibit binding of the orthosteric antagonist H-diprenorphine (DPN) to
CHO-hDOPr cell membranes suggesting that 2 is acting at an allosteric site to produce agonist and PAM activity
(
Figure 2A). However, in competition binding experiments 10 ꢀM of 2 increased the affinity of the orthosteric
3
agonists, leu-enkephalin (Figure 2B), SNC80 (Figure 2C) and TAN67 (Figure 2D) to displace H-DPN. This suggests
that 2 is an affinity modulator (the α component of the cooperativity factor) in the system tested (Table 2). The
affinity shift with the partial agonist TAN67 is less than that seen with the full agonists leu-enkephalin and SNC80
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(
Table 2).
Functional Characterization of 2
The PAM activity of 2 was further characterized in four different functional assays. In the CHO-OPRD1 PathHunter
cells, 2 effects on leu-enkephalin potency and efficacy were studied in both β-arrestin recruitment assays and
inhibition of forskolin-stimulated cAMP accumulation assays. Unlike the U2OS cell lines used in the HTS, where
forskolin was relatively ineffective at stimulating adenylyl cyclase activity, the recombinant CHO PathHunter cell
lines allowed us to investigate both β-arrestin recruitment and inhibition of forskolin-stimulated cAMP
accumulation in the same cell line. In the β-arrestin recruitment assay, 2 alone (up to 10 ꢀM) produced only
marginal agonist activity (~10% of a maximal response to leu-enkephalin) but produced a robust 18-fold increase
in the potency of leu-enkephalin (Figure 3A). A small increase in the maximal response to leu-enkephalin with 2,
relative to leu-enkephalin alone, was also observed. This suggests that 2 is a PAM with little or no intrinsic efficacy
in this system. In contrast, in the inhibition of forskolin-stimulated cAMP assay, 2 alone produced robust activity
resulting in full inhibition of cAMP accumulation at concentrations above 3 ꢀM (Figure 3B). At lower
concentrations, 2 increased the potency of leu-enkephalin. At a 370 nM concentration of 2 (the highest
concentration at which a potency for leu-enkephalin could be determined) the potency of leu-enkephalin was
increased by 56-fold. Similar findings were observed using the small molecule orthosteric agonist SNC80 in these
two assays (Table 3).
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Similar to the findings in the cAMP functional assay, 2 was also shown to be a PAM in [ S]GTPγS binding (Figure
4
A) and in ERK1/2 phosphorylation (Figure 4B) in CHO-hDOPr cells, showing agonist activity at higher
concentrations and increases in the potency of orthosteric agonist at lower concentrations. No agonist activity to 2
was observed in the parental CHO cells (lacking the δ receptor) in ERK1/2 phosphorylation or in parental CHO cells
in inhibition of cAMP accumulation assays (data not shown). 2 increased the potency of leu-enkephalin by 16-fold
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in the [ S]GTPγS binding assay in CHO-hDOPr membranes, and by 8-fold in the ERK1/2 phosphorylation assay in
CHO-hDOPr cells. Similar experiments were performed replacing leu-enkephalin with the orthosteric agonists
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SNC80 and the partial agonist TAN67. Using an operational model of allosterism (see Methods and Materials),
composite cooperativity (αβ) values and pK values (denoting the equilibrium dissociation binding constant for 2
B
at the δ receptor in the absence of orthosteric agonist i.e. at the free receptor) were determined for 2 across these
different assays and with different orthosteric agonists (Table 3).
The mean ± SEM pK across all the assays for 2 was 6.02 ± 0.16 (~1 ꢀM). One would expect that the pK values
B
B
should be the same across all the cell-lines, functional assays, and orthosteric agonists used, since the pKB
represents the binding affinity of 2 to the free receptor. Two way ANOVA with multiple comparison test of the pKB
values in Table 3 showed no significant difference between the different orthosteric agonist ligands used in the
same functional assay. For SNC80 and TAN67 there were also no significant differences in pK values across the
B
different functional pathways tested. However, for leu-enkephalin there were significant differences in the pKB
35
values between β-arrestin recruitment and [ S]GTPγS binding (p<0.01), and between β-arrestin recruitment and
cAMP inhibition (p<0.001).
Discussion
Using a β-arrestin recruitment assay, the SAR of a δ PAM chemotype identified from HTS was explored, resulting in
identification of compounds (1-15) with little or no agonist activity, but which produced PAM activity at the δ and
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ꢀ receptor. To compare the allosteric activity of the compounds we used increasing concentrations with a single
EC ) concentration of orthosteric agonist and analyzed the EC and Y values of the functional curves
(
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max
produced. Although the compounds exhibited a range of Ymax values in PAM mode (Table 1) which can correlate
with the allosteric cooperativity, the large proportion of the analogs tested exhibited efficacy close to or above
100% limiting the usefulness of the Ymax parameter for selecting compounds for further study. Instead, potency of
the PAM response was used and selectivity was determined using potency ratios between the PAM responses at
the δ receptor compared to the ꢀ receptor. While this procedure is useful for selecting δ receptor selective PAM
candidates to pursue, one must bear in mind that different orthosteric agonist ligands were used in the PAM
mode assays; leu-enkephalin for the δ receptor, and endomorphin-I for the ꢀ receptor. Since we currently know
little about the possible probe dependence of these PAM compounds at the δ and ꢀ receptor we cannot
necessarily assume that the reported selectivity will be the same with different orthosteric probe ligands.
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The selected dataset used for multivariate statistical analysis does not allow for thorough cross-validation of the
presented linear models, but our results suggest initial physicochemical properties that can be used as searching
criteria for additional compounds with potential PAM activity at δ and µ opioid receptors. 2 was selected for
further characterization since it had the highest PAM mode potency at the δ receptor and showed 100-fold
selectivity compared to the ꢀ receptor.
The multi-variate statistical analysis initially suggested that the compounds may not be readily soluble in aqueous
buffer at concentrations in the micromolar range. Also, nephelometry data (not shown) suggest that 2 and 10
show particulate matter in phosphate-buffered saline solution at concentrations above 1 ꢀM. When
nephelometry was repeated using the specific buffer used for the β-arrestin recruitment assays (HBSS +25 mM
HEPES and 10% FBS) in Table 1, 2 and 10 produced particulate matter above 3 ꢀM. While the majority of
responses to 2 in cells expressing the δ receptor were maximal at 1 ꢀM (and therefore, within the solubility
window predicted for 2), the ꢀ receptor responses (e.g. see Figure 1) also showed sigmoidal responses (i.e. the
responses were not biphasic) up to 30 ꢀM of 2, suggesting that solubility was not an issue in these assays in the
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specific buffers used. However, compound solubility should be an important consideration in further studies and
optimization of this chemical series.
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From competition binding studies, 2 did not affect H-DPN binding to the δ receptor, but increased the affinity of
orthosteric agonists suggesting that 2 does not bind to the orthosteric site of the δ receptor but can increase the
affinity of orthosteric agonists binding to the receptor (α cooperativity). The precise mechanism for this
cooperativity remains unknown. However, in this context it is tempting to make comparisons to recently
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discovered PAMs of the μ opioid receptor . The μ receptor PAM 2-(3-bromo-4-methoxyphenyl)-3-((4-
chlorophenyl)sulfonyl)thiazolidine (16, BMS-986122) has been found to differentially increase the affinity of
various orthosteric agonists, and the magnitude of the affinity increase (α value) produced by 16 correlates with
22
the intrinsic activity of the orthosteric ligand used . The mechanism by which 16 induces this affinity modulation
+
is suggested to be via reducing the affinity of Na for its binding site on the ꢀ receptor. The precise binding site for
1
6 on the μ receptor has not been clearly established and it is unknown whether the δ receptor PAMs described
here bind to an analogous binding site on the δ receptor, or act via a similar mechanism. However, several
21
analogs of 16 were found to exhibit weak activity at δ receptors , and most of the δ receptor PAMs described
here also exhibit some degree of activity at μ receptors. Therefore, it is possible that these δ receptor PAMs may
be binding to a site on the δ receptor that is analogous to the 16 binding site on the μ receptor, and may work
through a similar mechanism. The reduced affinity shift observed with 2 for the partial agonist TAN67 compared
with the agonists with higher intrinsic activity, Leu-enkephalin and SNC80 (Figure 2, Table 2), is consistent with this
+
hypothesis. It will be interesting to determine whether these δ PAMs reduce the affinity of Na for its binding site
on the δ receptor. Sodium ions are known to stabilize a lower affinity state of the δ receptor, and the molecular
+
23 24
basis for allosteric Na control of opioid receptor signaling has been elucidated recently
.
35
While TAN67 was a partial agonist in the CHO-hDOPr cell line for [ S]GTPγS binding giving 84% of maximal SNC80
response, it had even less intrinsic activity in a C6-DOPr cell-line at 41% of maximal SNC80 response (data not
shown). In the presence of 2 (300 nM), the maximal stimulation by TAN67 was increased to 67% of maximal
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SNC80 response. This suggests that 2 has some allosteric efficacy cooperativity (β), as well as the affinity
cooperativity (α) observed above.
In all of the functional assays, 2 acted as a PAM, increasing the potency of the response to orthosteric agonists.
No activity was observed in functional assays when 2 was added alone in CHO-parental cells (lacking the
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recombinant δ opioid receptor) in either the ERK activation assay or cAMP assay (data not shown). However, in
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cells expressing the δ receptor, 2 (when added alone) produced significant activity in cAMP inhibition, [ S]GTPγS
binding and ERK activation assays, suggesting that this activity is due to intrinsic efficacy of 2 at the δ receptor.
25
Thus, 2 is a PAM-agonist in these systems . In cells expressing the δ receptor, 2 showed little to no agonist
activity in the β-arrestin recruitment assay, which measures an event proximal to receptor activation with limited
signal amplification. The difference in observed agonist activity for 2 between the β-arrestin recruitment assay
and the cAMP assay (Figure 3) is likely reflective of a higher level of signal amplification and thus a higher receptor
26 27
reserve in the cAMP assay compared to the β-arrestin recruitment assay in the same cell-line
.
Phosphorylation of the receptor by G protein-coupled receptor kinases (GRKs) is thought to be a prerequisite for
28
β-arrestin recruitment . It would be interesting to see how 2 impacts δ receptor phosphorylation by GRKs and
consequently, desensitization and internalization of the receptor.
Calculation of pK values for 2 across the various functional assays with leu-enkephalin, SNC80 and TAN67, using
B
the allosteric operational model, showed some variability. These differences in pK values may result from the
B
allosteric effect not reaching a plateau or ceiling. This could reflect that the allosteric effect was sub-maximal at
concentrations below those at which full agonism was observed with 2, or that the highest concentrations of 2
used did not cause the allosteric EC shift to reach its ceiling. This can make accurate assessment of the allosteric
50
parameters more difficult to estimate in the model. Other variables, including the use of different cell lines, or use
of a tagged receptor (in the case of the PathHunter CHO-OPRD1 cell line) may also contribute to the variability of
values obtained in the model.
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The fact that we observed PAM effects with 2 at concentrations lower than those which produced agonist effects
is entirely consistent with the allosteric ternary complex model, because the former (PAM effects) are observed in
the presence of orthosteric agonist, and hence the affinity of the modulator for the receptor is higher, whereas
the latter (agonist effects) reflect the actions of the modulator at the free receptor and thus require higher
concentrations to achieve the same level of fractional occupancy. Therefore, a PAM with a large cooperativity
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factor (αβ) can exhibit functional activity that is far more potent than its K value. This has potential implications
B
for PAM drug discovery programs, suggesting that it is important to track functional PAM activity, rather than KB
values, when designing assays to support SAR. Additionally, this suggests that assays assessing target engagement
may dramatically underestimate the relevant receptor occupancy of a PAM, since the affinity of the PAM (and
therefore its fractional receptor occupancy) will be much higher at sites where orthosteric agonist is present.
While such sites may represent only a small fraction of the receptor population in vivo, they nonetheless represent
the relevant receptor population, since positive allosteric modulation can only occur when and where orthosteric
agonist is bound.
Despite the complexities discussed above, all available data suggests that 2 is a δ PAM or a δ PAM-agonist. In
future studies, it will be important to confirm the activity of 2 and its analogs in cells or tissues natively expressing
δ receptors. Further, it will be of significant interest to determine whether compounds such as 2 also exhibit
direct agonist activity in native systems expressing endogenous levels of δ receptors. PAMs devoid of intrinsic
agonist activity could theoretically have therapeutic advantages over PAM-agonists, particularly in the
maintenance of the temporal and spatial fidelity of endogenous receptor activation in vivo, as they would
effectively be silent when bound to the receptor until orthosteric (endogenous) agonist is presented to the
receptor. A key issue will be the determination of these effects in vivo. We intend to evaluate the in vivo activity
29
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3
of 2 and its analogs in models of acute and chronic pain , migraine , depression , and convulsive activity
which is a known liability of δ opioid receptor agonists that has limited the pursuit of δ receptor agonists as
potential therapeutics.
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In summary, we have identified and characterized δ receptor-selective PAMs including our lead compound 2.
Further studies are planned to assess probe dependence and signaling bias for these PAMs using a variety of
orthosteric opioid receptor ligands and functional assays. Additional research is also ongoing to determine if this
new class of compounds could represent a viable approach to develop new medicines for chronic pain, depression
and other therapeutic indications.
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Methods and Materials:
Chemistry
Analogs were purchased from external vendors (1, 3-5, 7) or synthesized according to Scheme 1 (2, 6, 8-15). All
purchased and newly synthesized analogs provided analytical data consistent with their assigned structures and
were >95% pure based on LCMS.
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Synthesis of Intermediate A (Scheme 1):
To a solution of 4-hydroxybenzaldehyde (1.5 g, 12.28 mmol) in 2-propanol (35 mL) was added 5,5-
dimethylcyclohexane-1,3-dione (3.44 g, 24.57 mmol) and H SO (98%, 0.098 mL, 1.842 mmol). The reaction
2
4
mixture was refluxed for 1.5 hours in an oil bath and then cooled to room temperature, forming a white
precipitate. After filtration, 3 grams of 9-(4-hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-
1
xanthene-1,8(2H)-dione was obtained in 65% yield (98% purity by LCMS analysis). H NMR (400MHz, CD Cl) δ
3
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.09 (d, J=8.6 Hz, 2H), 6.56 (d, J=8.6 Hz, 2H), 4.67 (s, 1H), 2.46 (s, 4H), 2.23 (s, 2H), 2.21 (s, 2H), 1.10 (s, 6H), 1.00 (s,
6H); ESI-MS m/z = 367.08 [M + H]+.
Synthesis of Analogs 1-15:
General Procedure. To a solution of 9-(4-hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-
1
,8(2H)-dione (100 ꢀmol, 36.6 mg) in DMF (1.2 mL) was added ArCH Br (200 ꢀmol) and Cs CO (65.2 mg, 200
2
2
3
µmol). The reaction mixture was stirred at room temperature overnight. 10 ꢀL of the reaction solution was taken,
dissolved in MeOH (0.2 mL) and analyzed by LCMS. The LCMS showed that the reaction was complete and the
desired product as a major peak was found. The product was purified via preparative LC/MS with the following
conditions: Column: XBridge C18, 19 x 200 mm, 5-μm particles; Mobile Phase A: 5:95 acetonitrile: water with 10
mM ammonium acetate; Mobile Phase B: 95:5 acetonitrile: water with 10 mM ammonium acetate; Gradient: 70-
1
00% B over 15 minutes, then a 5-minute hold at 100% B; Flow: 20 mL/min. Fractions containing the desired
product were combined and dried via centrifugal evaporation.
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Two analytical LC/MS injections were used to determine the final purity: Injection 1 conditions: Column: Waters
BEH C18, 2.0 x 50 mm, 1.7-μm particles; Mobile Phase A: 5:95 acetonitrile: water with 10 mM ammonium acetate;
Mobile Phase B: 95:5 acetonitrile: water with 10 mM ammonium acetate; Temperature: 50 °C; Gradient: 0%B, 0-
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00% B over 3 minutes, then a 0.5-minute hold at 100% B; Flow: 1 mL/min; Detection: UV at 220 nm.
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Injection 2 conditions: Column: Waters BEH C18, 2.0 x 50 mm, 1.7-μm particles; Mobile Phase A: 5:95 methanol:
water with 10 mM ammonium acetate; Mobile Phase B: 95:5 methanol: water with 10 mM ammonium acetate;
Temperature: 50 °C; Gradient: 0%B, 0-100% B over 3 minutes, then a 0.5-minute hold at 100% B; Flow: 0.5
mL/min; Detection: UV at 220 nm.
Proton NMR was acquired in deuterated CDCl or DMSO.
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9
,3,6,6-tetramethyl-9-(4-((2-methylbenzyl)oxy)phenyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (2, BMS-
86187)
1
H NMR (400MHz, CHLOROFORM-d) d 7.51 - 7.35 (m, 2H), 7.26 - 7.18 (m, 4H), 6.89 (dd, J=14.2, 8.6 Hz, 2H), 5.05
(s, 2H), 4.72 (s, 1H), 2.49 (d, J=5.9 Hz, 4H), 2.38 (d, J=7.8 Hz, 4H), 2.27 - 2.21 (m, 3H), 1.16 - 1.10 (m, 6H), 1.07 - 1.00
(
m, 6H). HRMS: Cal. C31 H35 O4 = 471.2530, found: 471.2538
9
-(4-((2-bromobenzyl)oxy)phenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (10,
BMS-986188)
The yield of the product was 20.6 mg, and its estimated purity by LCMS analysis was 100%.
1
H NMR (500MHz, DMSO-d6) δ 7.67 (d, J=7.7 Hz, 1H), 7.56 (d, J=7.3 Hz, 1H), 7.42 (t, J=7.5 Hz, 1H), 7.31 (t, J=7.3 Hz,
1
H), 7.10 (d, J=8.1 Hz, 2H), 6.88 (d, J=8.4 Hz, 2H), 5.04 (s, 2H), 4.48 (s, 1H), 2.54 (d, J=11.4 Hz, 4H), 2.26 (d, J=16.1
Hz, 2H), 2.09 (d, J=16.1 Hz, 2H), 1.04 (s, 6H), 0.91 (s, 6H). HRMS: Cal. C30 H32 O4 Br = 535.1478, found: 535.1478
Calculation and Analysis of Physicochemical Properties
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The Schrodinger Suite 2014-4 was used throughout. The two-dimensional (2D) sketcher tool of Maestro 10.0 was
used to draw 2D structures of each compound listed in Table 1. These 15 2D structures were imported as 3D
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structures into LigPrep v3.2, and their protonation state was assigned at physiological pH using Epik v3.0 . Fifty-
two physicochemical properties were calculated using QikProp v4.2. Among them, 38 properties (see Tables S1
and S2) displayed non-zero variance across the 15 ligands of Table 1, and were therefore used for multivariate
statistical analysis.
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The bayesglm function in the ‘arm’ R package was used to build a Bayesian linear regression model for the
relationship between the calculated physicochemical properties of the 15 compounds in Table 1 and their
measured EC values at δ- or µ-opioid receptors. Specifically, each linear model was built so that ꢀ = ∑ ꢁꢂꢃꢂ +
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ꢄꢅ, where P is the EC value to be predicted, C is the interception, and X and C are the calculated property and
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associated regression coefficient, respectively. We arbitrarily assigned a numerical value of 100 to EC values
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listed as “>10” in Table 1. To better discriminate the most potent compounds we used a logarithmic scale for EC .
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All models containing combinations of up to three properties (without interaction terms) were estimated, and the
best linear models for δ- (see Table S3 and Fig. S1) and µ-opioid (see Table S4 and Fig. S2) receptors were selected
based on the Akaike Information Criterion (AIC). Following prediction of -logEC values at δ- or µ-opioid receptors,
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the δ/µ selectivity was estimated as the difference of –ꢁlogEC = –log EC (δ)/ EC (µ) (see Fig. S3).
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Cell lines:
Chinese Hamster Ovary (CHO) PathHunter cells expressing enzyme acceptor (EA)-tagged β-arrestin 2 and either
ProLink (PK)-tagged δ receptor(CHO-OPRD1), or PK-tagged ꢀ receptor (CHO-OPRM1) were from DiscoveRx
(
Freemont , CA). Cells were grown in F-12 media (Invitrogen 11765), containing Hyclone FBS 10%, Hygromycin 300
g/mL (Invitrogen 10687) G418 800 ꢀg/mL (Invitrogen 10131) and maintained at 37 °C in a humidified incubator
ꢀ
containing 5% CO . These cells were used for β-arrestin recruitment assays and inhibition of forskolin-stimulated
2
cAMP accumulation assays described below.
FlpIn CHO cells (Invitrogen, Carlsbad, USA) were grown in Dulbecco’s modified Eagle’s medium (DMEM)
supplemented with 10% fetal bovine serum and maintained at 37 °C in a humidified incubator containing 5% CO2.
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FlpIn CHO cells were transfected with the pOG44 vector encoding Flp recombinase and the pDEST vector encoding
the human δ receptor (hDOPr) at a ratio of 9:1 using polyethylenimine as transfection reagent. 24 h after
transfection the cells (CHO-hDOPr) were subcultured and the medium was supplemented with 700 μg/mL
HygroGold as selection agent. Cells were grown and maintained in DMEM containing 20 mM HEPES, 5% fetal
bovine serum and 200 μg/mL Hygromycin-B. Cells were maintained at 37 °C in a humidified incubator containing
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% CO . These cells were used for ERK phosphorylation assays and membranes derived from these cells were
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used for [ S]GTPγS binding and H DPN binding studies as described below.
Materials:
PathHunter detection reagents were from DiscoveRx (Freemont, CA). Cell culture media and supplements were
from Life Technologies (Carlsbad, CA). Lance-Ultra cAMP detection reagents, Surefire ERK assay reagents,
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[
H]Diprenorphine (DPN), and [ S]GTPγS (guanosine-5’-O(3-thio)triphosphate) were from PerkinElmer Life
Sciences (Cambridge, MA). Endomorphin-I and TAN-67 were obtained from Tocris. All other chemicals, unless
otherwise specified, were purchased from Sigma (St. Louis, MO).
PathHunter β-Arrestin Assay
Confluent flasks of CHO-OPRM1 and CHO-OPRD1 cells were harvested with TrypLE Express, and resuspended in F-
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2 media supplemented with 10 % FBS and 25 mM HEPES, at a density of 6.67e cells /mL and plated (3 ꢀL / well)
o
into white solid TC-treated 1536-well plates (Corning, NY). Plates were incubated overnight at 37 C in a 5% CO2
humidified incubator. The next day, compounds (40 nL of 100 x final concentration in 100% DMSO) were added to
cell plates by acoustic dispense using an Echo-550 (Labcyte, Sunnyvale, CA) from Echo-qualified 1536-well source
plates (Labcyte). Next, 1 ꢀL of assay buffer (agonist mode), or assay buffer containing a low concentration (~4 x
EC ) of orthosteric agonist (PAM mode), were added to assay plates. The orthosteric agonists used are described
20
in the Results & Discussion. Plates were covered with a lid and incubated at room temperature for 90 min.
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Incubations were terminated by the addition of 2 ꢀL PathHunter Reagent (DiscoveRx). One hour later
luminescence was detected using a Viewlux imaging plate reader (PerkinElmer).
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Inhibition of Forskolin-Stimulated cAMP Accumulation Assays.
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CHO-OPRD1 cells were grown to confluence (as described above). Cells were harvested and resuspended at 1e
cells / mL in assay buffer (HBSS + 25 mM HEPES, +0.05% BSA). Compounds (30 nL of 100 x final concentration in
00% DMSO) were added to 1536-well white solid NT plates by acoustic dispense using an Echo-550 (Labcyte, CA)
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followed by a 1 ꢀL addition of cells (2000 cells / well) to all wells. Next, 1 ꢀL of either assay buffer (for agonist
mode) or assay buffer containing a 3x EC concentration of orthosteric agonist (PAM mode) was added. Finally, 1
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ꢀL of 3 x Forskolin (2 ꢀM final) was added. Plates were lidded and incubated for 45 min at RT. Incubations were
terminated by the addition of Lance-Ultra cAMP detection reagent (Perkin Elmer) (1.5 ꢀL of Eu-cryptate-labelled
cAMP tracer in lysis buffer, followed by 1.5 ꢀL of U-light conjugated anti-cAMP antibody in lysis buffer). After a 1
hr incubation at room temperature, time-resolved fluorescence (TRF) was detected on a Viewlux or Envision plate
reader (PerkinElmer) with excitation at 337 nm and emission reads at 615 nm and 665 nm. The ratiometric data
(665 nm read/615 nm read)*10,000 were then converted to cAMP (nM) based on a standard curve for cAMP
(
replacing the cell addition step) run at the same time and under identical conditions to the assay.
Characterization of δ-opioid receptor-selective PAMs in the CHO-OPRD1 cAMP assay, using curve-shift assays,
were performed as described above using orthosteric agonists described in the Results & Discussion.
Membrane Preparation
Confluent cells were rinsed with phosphate-buffered saline and then detached using harvesting buffer (0.68 mM
EDTA, 0.15 M NaCl, 20 mM HEPES, pH 7.4). Cells were pelleted by centrifugation at 300 x g for 3 min, followed by
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resuspension in cold 50 mM Tris-HCl buffer, pH 7.4. Pellet was rehomogenized using a Tissue Tearor and then
centrifuged at 20,000 x g for 20 min at 4 °C. The supernatant was discarded and the process was repeated for an
additional rehomogenization and centrifugation. The supernatant was discarded and the final pellet was
resuspended in 50 mM Tris-HCl and flash frozen in aliquots using liquid nitrogen. Aliquots were kept -80 °C until
assays. Protein concentration was determined using BCA protein assay with bovine serum albumin as the
standard.
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Radioligand Binding Assay
Cell membranes (as prepared above, 10µg/well) were incubated in the following mixture for 90 minutes at 25 °C:
assay buffer (50 mM Tris-HCl pH 7.4, 100 mM NaCl, 1 mM EDTA, 5 mM MgCl , 10 µM GTPγS), various
2
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concentrations of orthosteric and allosteric ligand, and 0.35-0.45 nM [ H]DPN. Nonspecific binding was
determined in the presence of 10 µM naloxone. Reactions were terminated by rapid filtration through glass
microfiber GF/C filters (Whatman) using a Brandell harvester and washed three times using cold 50 mM Tris-HCl
buffer. Filters were dried in a 50 °C oven and radioactivity was measured by liquid scintillation counting with
EcoLume liquid scintillation cocktail (MP Biomedicals) in a Wallac 1450 MicroBeta counter (Perkin Elmer).
35
[
S]GTPγS Assay
CHO-hDOPr cell membranes (as prepared above, 10 μg/well) were incubated for one hour at 30°C in buffer
3
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comprised of: 50 mM Tris-HCl (pH 7.4), 1 mM EDTA, 5 mM MgCl , 100 mM NaCl, 0.1 nM [ S]GTPγS and 30 μM
2
GDP (guanosine 5’-diphosphate) in a final volume of 200 μL. Orthosteric and allosteric ligands were also included,
35
with SNC80 used the maximal standard and assay buffer used to assess basal [ S]GTPγS binding. The reaction
was terminated by filtration through glass microfiber GF/C filters (Whatman) using a Brandell harvester. The
filters were rinsed, dried, and radioactivity was counted by liquid scintillation counting using EcoLume liquid
scintillation cocktail (MP Biomedicals) in a Wallac 1450 MicroBeta counter (Perkin Elmer).
ERK1/2 Phosphorylation Assay
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hDOPr FlpIn CHO cells (CHO-hDOPr) were seeded into 96-well plates at a density of 50,000 cells/well. After 5-7 h,
cells were washed with phosphate-buffered saline (PBS) and incubated overnight in serum-free DMEM. Initially,
time-course experiments were conducted at least twice for each ligand to determine the time required to
maximally promote ERK1/2 phosphorylation via the δ-receptor. Concentration-response experiments were
performed for the orthosteric ligands in the absence or presence of increasing concentrations of the allosteric
modulator at 37 °C. Stimulation of the cells was terminated by removing the media and the addition of 100 μL of
SureFire lysis buffer (PerkinElmer) to each well. The plate was shaken for 5 min at RT before transferring 5 μL of
the lysates to a white 384-well Proxiplate (PerkinElmer). Then 8 μL of a 240:1440:7:7 mixture of Surefire activation
buffer : Surefire reaction buffer : Alphascreen acceptor beads : Alphascreen donor beads was added to the
samples and incubated in the dark at 37 °C for 1.5 h. Plates were read using a Fusion-TM plate reader
(PerkinElmer).
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Data Analysis
For all experiments data were analyzed and EC or Ki values determined using nonlinear regression analysis to fit
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a logistic equation using GraphPad Prism version 6 (GraphPad, San Diego, CA). pK and αβ values were
B
20
determined using the “Operational Model of Allosterism” (see (1) below), using Graphpad Prism version 6.
n
Em(τ [A](K + αβ[B]) + τ [B]K )
A
B
B
A
___________________________________________________________________
E =
(1)
n
n
([A]K + K K + K [B] + α[A][B]) + (τ [A](K + αβ[B]) + τ [B]K )
B
A
B
A
A
B
B
A
Within this model, E is the pharmacological effect, K and K denote the equilibrium binding constants for the
A
B
orthosteric ligand, A, and the allosteric ligand, B, at the receptor. The binding cooperativity factor, α represents
the effect of the allosteric ligand on orthosteric agonist binding affinity, and vice versa. An activation cooperativity
factor, β, denotes the effect the allosteric ligand has on orthosteric agonist efficacy. Agonism constants τ and τ ,
A
B
represent the intrinsic activity of the orthosteric agonist and any intrinsic activity of the allosteric ligand,
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respectively, which is dependent on the cell context and receptor expression level of the cell system, and intrinsic
efficacy of the ligands used. The remaining parameters, Em and n, denote the maximal response of the system,
and the slope, respectively.
Acknowledgments
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The authors would like to thank Dr. Davide Provasi for providing the scripts needed for the statistical analyses.
MF wishes to acknowledge support for this work from the National Institutes of Health (NIH) grants DA026434 and
DA034049. JRT also acknowledges support from NIH grant DA035316. Computations relied on the Extreme
Science and Engineering Discovery Environment (XSEDE) under MCB080109N, and on the computational resources
and staff expertise provided by the Scientific Computing Facility at the Icahn School of Medicine at Mount Sinai.
Supporting Information Available: Specific properties of the 15 compounds listed in Table 1 that were most likely
to predict the experimentally determined EC values at either δ or ꢀ opioid receptors are revealed from
50
multivariante statistical analysis. This material is available free of charge via the internet at http://pubs.acs.org.
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Figure 1
β-arrestin recruitment response to 2 in agonist mode (in the absence of orthosteric agonist) and in PAM mode (in
the presence of an EC of orthosteric agonist) in PathHunter cells expressing δ receptors (CHO-OPRD1) and ꢀ
20
receptors (CHO-OPRM1). For CHO-OPRD1 cells the orthosteric agonist was leu-enkephalin and for CHO-OPRM1
cells the orthosteric agonist was endomorphin-I. In PAM mode, The EC response of orthosteric agonist was
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normalized to 0 %. 100 % represents the response to a maximally effective concentration of orthosteric agonist.
Data are presented as the mean ± SEM, n=4.
Table 1
Structure activity relationship of the δ-PAM chemotype in PathHunter CHO-OPRD1 and CHO-OPRM1 cells in a β-
arrestin recruitment assay. No activity was observed in agonist mode (in the absence of orthosteric agonist (data
not shown)). In PAM mode (in the presence of an EC of leu-enkephalin for OPRD1 cells, or an EC of
20
20
endomorphin-I for OPRM1 cells), robust responses were observed. The mean EC s, Y values, and potency ratio
50
max
of δ receptor activity /ꢀ receptor activity, in PAM mode are reported in the table (n=3).
Figure 2
3
Effect of 2 on H-diprenorphine (DPN) binding (A); and the effect of 10 ꢀM 2 on leu-enkephalin (B), SNC80 (C), and
TAN67 (D) competition binding curves, in CHO-hDOPr membranes. Ki values are shown in Table 2. Data are
presented as the mean ± SEM of 3 experiments.
Table 2
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Effect of 2 (10 ꢀM) on orthosteric agonist competition binding Ki values in CHO-hDOPr cell membranes. 2 had no
3
effect on H-DPN binding (see Figure 2) but increased the affinity of orthosteric agonist competition binding
curves. Data are presented as the mean ± SEM of 3 experiments.
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Figure 3
Effect of increasing concentrations of 2 on leu-enkephalin concentration response curves in β-arrestin recruitment
(
A), and in inhibition of forskolin-stimulated cAMP accumulation (B), in CHO-OPRD1 cells. Data are presented as
the mean ± SEM of 4 experiments.. Data were fitted to the operational model of allosterism (see Table 3).
Figure 4
35
Effect of increasing concentrations of 2 on leu-enkephalin concentration response curves in [ S]GTPγS binding in
35
CHO-hDOPr membranes (A), and in pERK in CHO-hDOPr cells (B). In the [ S]GTPγS binding assay, 0 % and 100 %
represent the basal response and the maximal response produced, respectively. In the pERK assay, 0 % represents
basal activity in serum-free media and 100 % represents the pERK response in the presence of 10 % serum. Data
are presented as the mean ± SEM, n = 3 to 7. Data were fitted to the operational model of allosterism (see Table
3
).
Table 3
Allosteric parameters for 2 at the δ receptor. Values for affinity, efficacy, and allosteric cooperativity for
orthosteric ligands and 2 are derived from the operational model of allosterism. Three different orthosteric
agonists were used (Leu-enkephalin, SNC80, and TAN67), across up to four functional assays (β-arrestin
35
recruitment, [ S]GTPγS binding, cAMP inhibition and pERK). In the model τ and τ represent the efficacy of the
A
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orthosteric agonist and allosteric modulator, respectively; pK and pK represent the binding affinity of the
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A
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orthosteric agonist and the allosteric modulator, respectively, to the free receptor; and αβ represents the
composite allosteric cooperativity factor. Data are presented as the mean ± SEM of 3 to 7 expts.
*
*
pK is fixed to its equilibrium binding affinity as ligand is a full agonist in all endpoints tested
A
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* the pK of Leu-enkephalin and TAN67 in endpoints where they are partial agonists was obtained from fitting
A
their CRC to the operational model of agonism to obtain a functional affinity in each endpoint tested.
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*
** the pK for TAN67 in [ S]GTPγS binding had to be fixed to the average of the pK obtained from Leu-enk and
B
B
SNC80 as neither allosteric agonism or potentiation reached a limit.
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