Activation loop targeting strategy for design of receptor-interacting protein kinase 2 (RIPK2) inhibitors

Article abstract of DOI:10.1016/j.bmcl.2018.01.044

Development of selective kinase inhibitors remains a challenge due to considerable amino acid sequence similarity among family members particularly in the ATP binding site. Targeting the activation loop might offer improved inhibitor selectivity since this region of kinases is less conserved. However, the strategy presents difficulties due to activation loop flexibility. Herein, we report the design of receptor-interacting protein kinase 2 (RIPK2) inhibitors based on pan-kinase inhibitor regorafenib that aim to engage basic activation loop residues Lys169 or Arg171. We report development of CSR35 that displayed >10-fold selective inhibition of RIPK2 versus VEGFR2, the target of regorafenib. A co-crystal structure of CSR35 with RIPK2 revealed a resolved activation loop with an ionic interaction between the carboxylic acid installed in the inhibitor and the side-chain of Lys169. Our data provides principle feasibility of developing activation loop targeting type II inhibitors as a complementary strategy for achieving improved selectivity.

Full text of DOI:10.1016/j.bmcl.2018.01.044

Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Activation loop targeting strategy for design of receptor-interacting
protein kinase 2 (RIPK2) inhibitors
Chalada Suebsuwong a, Daniel M. Pinkas b, Soumya S. Ray c, Joshua C. Bufton b,f, Bing Dai d, Alex N. Bullock b,
Alexei Degterev d, , Gregory D. Cuny e,
⇑
⇑
a Department of Chemistry, University of Houston, Science and Research Building 2, Houston, TX 77204, USA
b Structural Genomics Consortium, University of Oxford, Old Road Campus, Roosevelt Drive, Oxford OX3 7DQ, UK
c Stemetix Inc., 604 Webster St., Needham, MA 02494, USA
d Department of Developmental, Molecular & Chemical Biology, Tufts University School of Medicine, 136 Harrison Avenue, Boston, MA 02111, USA
e Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Science and Research Building 2, Houston, TX 77204, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Development of selective kinase inhibitors remains a challenge due to considerable amino acid sequence
similarity among family members particularly in the ATP binding site. Targeting the activation loop
might offer improved inhibitor selectivity since this region of kinases is less conserved. However, the
strategy presents difficulties due to activation loop flexibility. Herein, we report the design of recep-
tor-interacting protein kinase 2 (RIPK2) inhibitors based on pan-kinase inhibitor regorafenib that aim
to engage basic activation loop residues Lys169 or Arg171. We report development of CSR35 that dis-
played >10-fold selective inhibition of RIPK2 versus VEGFR2, the target of regorafenib. A co-crystal struc-
ture of CSR35 with RIPK2 revealed a resolved activation loop with an ionic interaction between the
carboxylic acid installed in the inhibitor and the side-chain of Lys169. Our data provides principle feasi-
bility of developing activation loop targeting type II inhibitors as a complementary strategy for achieving
improved selectivity.
Received 17 November 2017
Revised 19 January 2018
Accepted 23 January 2018
Available online xxxx
Keywords:
Kinase
Inhibitor
Activation loop
Receptor-interacting protein kinase 2
RIPK2
Ó 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Protein kinases mediate many diverse cellular functions, includ-
ing gene transcription, metabolism, cell-cycle progression,
cytoskeletal rearrangement, movement, proliferation, differentia-
tion and death (e.g. apoptosis and regulated necrosis) by catalyzing
phosphate transfer from adenosine triphosphate (ATP) to protein
substrates.1,2 Dysfunction of many protein kinases is associated
with a broad array of human diseases.3 Consequently, this class
of enzymes has become a key focus for therapeutic development.
The majority of protein kinase inhibitors currently approved for
clinical use bind to the catalytically active enzyme at the ATP site,
which is located near the hinge region between the N- and C-ter-
minal lobes and are referred to as type I inhibitors.4 However,
many protein kinases are capable of undergoing conformational
changes to catalytically inactive states, which have been targeted
with the goal of developing more selective allosteric inhibitors.
nal lobe, can rotate to an inactive a
C-helix-out conformation.5,6
Compounds that bind this state of the enzyme are referred to as
type I½ inhibitors.7 In another commonly used approach, inhibi-
tors are designed to stabilize kinase conformations in which the
three highly conserved amino acids at the beginning of the activa-
tion segment (e.g. Asp-Phe-Gly, known as the DFG motif) rotate to
the inactive DFG-out state disrupting catalysis. This change also
provides access to an adjacent hydrophobic pocket, which inhibi-
tors can bind. Compounds that engage only this hydrophobic
pocket are referred to as type III inhibitors, while those that also
maintain interactions with the hinge region are termed type II
inhibitors.7 Over the past two decades, exemplification of these
and additional classes of kinase inhibitors has steadily increased
with several compounds being approved for clinical use.
A kinase region that has not been widely exploited for inhibitor
design is the activation loop. This portion of the kinase is a flexible
sub-region of the activation segment and consists of 20–35 amino
acid residues that start after the DFG motif and end before the APE,
ALE or SPE sequence.8,9 The activation loop can mimic the protein
substrate by folding and interacting with the substrate binding
site. However, in the catalytically active kinase the activation loop
is displaced allowing the protein substrates and ATP to bind. The
For example, the aC-helix, the only helical segment in the N-termi-
⇑
Corresponding authors.
E-mail addresses: alexei.degterev@tufts.edu (A. Degterev), gdcuny@central.uh.
edu (G.D. Cuny).
f
Present address: Department of Biochemistry, University of Bristol, Biomedical
Sciences Building, University Walk, Bristol BS8 1TH, UK.
https://doi.org/10.1016/j.bmcl.2018.01.044
0960-894X/Ó 2018 The Author(s). Published by Elsevier Ltd.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article in press as: Suebsuwong C., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.01.044

2
C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
movement of the activation loop is linked to the rotation of the
DFG motif from inactive DFG-out to active DFG-in conformations.
As a result, the activation loop is ‘‘open” (e.g. disengaged) in the
active kinase state and oftentimes ‘‘closed” (e.g. engaged) in an
inactive kinase state.9 Designing inhibitors that can form interac-
tions with the activation loop in the ‘‘closed” catalytically inactive
state can be attractive as sequences and structures of activation
segments are poorly conserved. However, this strategy presents a
major challenge due to the flexibility of this loop and uncertainty
about its precise location and conformation, which is frequently
unresolved in reported protein kinase crystal structures. To date,
engagement of the activation loop by protein kinase inhibitors
has been demonstrated in a small number of cases. For example,
the side-chain of activation loop residue Y1230 (PDB ID: 2WGJ).10
The MEK1 inhibitor PD-184352 forms a series of hydrophobic
interactions with the activation loop in addition to a weak F-NH
dipole–dipole interaction with the backbone of Ser212 (PDB ID:
1S9J).11 Several structurally distinct type III receptor-interacting
protein kinase 1 (RIPK1) inhibitors engage activation loop residues
through hydrophobic interactions (PDB ID: 4ITI, 4ITJ and
5HX6).12,13 Finally, the type III RIPK1 inhibitor Nec-114,15 forms a
critical hydrogen bond with the side-chain of activation loop resi-
due Ser161 (PDB ID: 4ITH).13 Notably, Nec-1 displays exclusive
selectivity for RIPK1 and mutation of Ser161 to Ala was shown to
block inhibitor activity.14,16 However, in all of these cases it
appears that the inhibitor–activation loop interactions were
serendipitously discovered and were not part of the original
the c-Met type I½ inhibitor crizotinib forms a
p–p
interaction with
Arg171
:
H
N
H
N
H
N
H
N
CF3
Cl
N
H
N
N
O
O
O
N
O
O
F
O
Biaryl urea
Regorafenib
Fig. 1. Superimposed crystal structure of docked regorafenib (pink) in RIPK2 lacking a resolved activation loop (gray; PDB ID: 4C8B) and biaryl urea (yellow) in the RIPK2
(blue; PDB ID: 5AR7) structure with a resolved activation loop (highlighted in deep pink).
A
Arg171
vs Lys169
B
Ser161
Arg171
Fig. 2. (A) Sequence alignment of the kinase domains of human RIPK1 and RIPK2. The kinase domains were aligned based on sequence similarity (http://www.uniprot.org);
(B) Overlay structure showing positions of Ser161 (green) and Arg171 (deep pink) residues in the activation loops of RIPK1 (yellow) and RIPK2 (blue), respectively.
Please cite this article in press as: Suebsuwong C., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.01.044

C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
3
specific type II inhibitor regorafenib that we previously discovered
to inhibit RIPK2,17 leading to improved selectivity against VEGFR2,
the primary target of regorafenib. The unique binding mode of the
inhibitor engaging the side-chain of activation loop residue Lys169
via an ionic–ionic interaction was confirmed via a RIPK2Áinhibitor
co-crystal structure.
RIPK2 is a caspase recruitment domain (CARD) containing
kinase that has been implicated in nucleotide binding and
oligomerization domain 1 and 2 (NOD1 and NOD2) signaling.
Ile69
Arg171
4.4 Å
5.5 Å
NOD-RIPK2 interactions result in activation of nuclear factor
jB
Leu142
(NF-jB) and mitogen-activated protein (MAP) kinase pathways
to promote the transcription of pro-inflammatory cytokines.18 This
signaling pathway potentially plays a crucial role in an array of
pathological conditions including inflammatory bowel disease,
Crohn’s disease and multiple sclerosis.19
Leu135
RIPK2 was selected as the target kinase for several reasons.
Ponatinib and regorafenib, type II pan-kinase inhibitors that could
serve as initial templates for activation loop targeting, have previ-
ously been reported to inhibit recombinant RIPK2 with IC50 values
of 7 and 41 nM, repectively.17 A RIPK2 crystal structure in complex
with ponatinib (PDB ID: 4C8B) was also available, although the
activation loop was unresolved.17 Finally, a co-crystal structure of
RIPK2 in complex with a biaryl urea inhibitor structurally similar
Fig. 3. Docking of regorafenib (pink) in RIPK2 (purple; PDB ID: 5AR7) structure with
a resolved activation loop (highlighted in deep pink). Hydrophobic residues are
highlighted in yellow. Distances from meta- and para-positions of urea phenyl to
Arg171 shown.
design. Herein, we report our initial efforts at an activation loop
targeting design strategy for receptor- interacting protein kinase
2 (RIPK2) by introducing a specific functional group onto pan-
Table 1
Modifications to the urea benzene targeting the Arg171 residue in the activation loop of RIPK2.
Compound
R1
R2
Conc. (
l
M)
% Inhibition
RIPK2 WT
R171C RIPK2
CSR1
CSR2
CSR25
H
COOH
COOH
H
H
0.5
0.5
0.5
NI*
NI
43
ND*
ND
ND
CSR26
CH3
CH3
0.5
35
ND
CSR24
0.5
1
ND
CSR27
CSR28
H
H
0.5
0.5
12
32
ND
ND
CSR31
CSR30
CSR29
H
H
H
5.0
5.0
5.0
NI
69
47
NI
76
67
CSR32
H
5.0
25
ND
CSR33
CSR34
H
H
5.0
5.0
18
27
ND
17
CSR35
CSR36
F
F
5.0
1.0
70
94
64
92
*
ND: Not Determined; NI: No Inhibition.
Please cite this article in press as: Suebsuwong C., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.01.044

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C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
to regorafenib, where part of the activation loop in one of the
monomers (chain A) has been resolved, was recently reported
(PDB ID: 5AR7).20
Regorafenib was selected as the scaffold for activation loop tar-
geting since we previously reported that this molecule more selec-
tively inhibited NOD1/2 signaling in the cells compared to
ponatinib.17 Molecular docking of regorafenib was initially per-
formed using AutoDockTools-1.5.6 and a RIPK2ꢀponatinib co-crys-
tal structure lacking a resolved activation loop (PDB ID: 4C8B). The
results showed that regorafenib formed similar binding interac-
tions as ponatinib. The regorafenib docked structure was also over-
laid with the RIPK2Ábiaryl urea co-crystal structure that has the
activation loop resolved (PDB ID: 5AR7). Again the interactions
between the kinase and the two ligands were similar (Fig. 1).
Next, the kinase domain sequences of RIPK1 and RIPK2 were
aligned and compared for regions of divergence. The comparison
confirmed alignment of the DXG motifs, where X is Leu in RIPK1
and Phe in RIPK2, at the beginning of the activation segments.
Interestingly, RIPK1’s Ser161 residue in the activation loop, which
forms the critical hydrogen bond to the RIPK1 inhibitor Nec-1,
aligns with Lys169 in RIPK2’s activation loop (Fig. 2A). By contrast,
Ser161 in RIPK1 occupied a similar position to Arg171 in RIPK2
when the RIPK1ÁNec-1 co-crystal structure was superimposed on
the RIPK2Ábiaryl urea co-crystal structure and the resolved activa-
tion loops were compared (Fig. 2B). Since the residues (e.g. Lys169
and Arg171) identified in RIPK2 occupy a similar position as Ser161
in RIPK1 using two independent methods, are both hydrophilic and
basic, our design strategy for targeting RIPK2’s activation loop was
installation of hydrophilic/acidic functional groups on the urea
benzene to enable ionic–dipole or ionic–ionic interactions. Fur-
thermore, the functional groups were installed at the meta- and
para-positions, which were closest to Arg171 (5.5 and 4.4 Å,
respectively) based on the docking model of regorafenib with
RIPK2 (PDB ID: 5AR7), as shown in Fig. 3. This region of RIPK2 also
consists of hydrophobic residues such as Ile69, Leu135 and Leu142.
Therefore, more hydrophobic groups, e.g. a gem-dimethyl amine,
nitrile and ester, were installed for compensating unfavorable
interaction with hydrophobic residues in this allosteric pocket. Ini-
tially, a synthetically accessible virtual library was generated by
using the in silico combinatorial library algorithm CombiGlide
(Schrödinger LLC) based on regorafenib where the variations were
introduced in the urea benzene. According to the docking score and
synthetic feasibility, a small library of 15 regorafenib analogs (CSR
series) with various functional groups on the urea benzene was
synthesized in order to introduce ligand–activation loop interac-
tions (Table 1).
Scheme 1. Synthesis of intermediates 10a–d. Reagents and conditions: (a) CH3-
SCH2CH2OH, DIAD, PPh3, THF, 0 °C to rt, 24 h (76%); (b) CH3I, NaH, THF, 0 °C to rt, 16
h (30%); (c) H2SO4, reflux, 16 h (92%); (d) SOCl2, MeOH, DME, 0–40 °C, 18 h (78%);
(e) i) SOCl2, reflux, 16 h, ii) NH4OH, 0 °C, 1 h (87%); (f) (F3CCO2) 2PhI, H2O/MeCN, rt,
18 h (99%); (g) Boc2O, NaHCO3, THF, 0 °C to rt, 16 h (86%); (h) NH4Cl, Fe, EtOH/H2O,
reflux, 1 h (76–99%).
Scheme 2. Synthesis of 1,2,5-thiadiazolidin-3-one 1,1-dioxide intermediate 10e.
Reagents and conditions: (a) methyl 2-bromoacetate, Bu4NBr, NaHCO3, DMF, 90 °C,
18 h (62%); (b) 1) BocNHSO2Cl, Et3N, CH2Cl2, 0 °C, 4 h, 2) TFA, CH2Cl2, rt, 2 h (27%
over two steps); (c) NaH, THF, rt, 1 h (96%); (d) NH4Cl, Fe, EtOH/H2O, reflux, 1 h
(81%).
Phenyl urea intermediates with various hydrophilic moieties
(10) were synthesized by following the methods outlined in
Schemes 1–3. To synthesize intermediates 10a–d, a Mitsunobu
reaction between nitrophenol 1 and 2-(methylsulfanyl)ethan-1-ol
furnished 2. 2-(3-Nitrophenyl)acetonitrile (3) was methylated
using iodomethane to give 4. Hydrolysis of the nitrile under acidic
conditions gave carboxylic acid 5. Esterification of 5 delivered
intermediate 6. Alternatively, 5 was converted to amide 7 using
thionyl chloride and ammonium hydroxide. The rearrangement
of the primary amide to amine 8 was accomplished using [I,I-bis
(trifluoroacetoxy)iodo]benzene in a mildly acidic mixed of aque-
ous-organic solvents. The amino group of 8 was protected with
Boc to give 9. The nitrophenyl derivatives 2, 3, 6 and 9 underwent
iron-mediated nitro reduction to provide 10a–d (Scheme 1).
The 1,2,5-thiadiazolidin-3-one 1,1-dioxide intermediate was
prepared from commercially available 4-nitro-2-methylaniline
(11). Substitution of 11 with methyl bromoacetate provided 12,
which was then treated with tert-butyl chlorosulfonylcarbamate
followed by Boc removal to afford 13. Cyclization of 13 under basic
Scheme 3. Synthesis of intermediates 10f–h. Reagents and conditions: (a) methyl
chloroacetate, K2CO3, MeCN, rt, 3.5 h (83–99%); (b) SOCl2, MeOH, 0 °C to rt, 16 h
(93%).
condition delivered 14, which was reduced to give aniline 10e
(Scheme 2).
Methyl 2-(phenylthio)acetate intermediates were prepared by
either substitution or esterification. Nucleophilic substitution of
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C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
5
thiophenols with methyl chloroacetate furnished 10f and 10g,
while esterification of 16 delivered 10h (Scheme 3).
CSR35 lacked significant activity in the in vitro enzymatic assay
against the primary regorafenib target VEGFR2 (35.8 2.6% inhibi-
CSR analogs were synthesized from 10 according to the method
outlined in Scheme 4. Nucleophilic aromatic substitution between
17 and 4-amino-3-fluorophenol (18) under basic conditions fur-
nished diaryl ether 19. Intermediates 10a–h or commercially avail-
able 10i–l were treated with phenyl chloroformate under basic
conditions to provide carbamates 20. Condensation reactions
between 19 and 20 provided CSR24–25, 30, 36 and intermediates
21. Oxidation of 21a using mCPBA furnished CSR26. To remove
the Boc protecting group, 21d was treated with TFA to give
CSR28. Palladium-catalyzed hydrogenation of the nitrile present
in CSR25 delivered primary amine CSR27. Methyl ester intermedi-
ates were hydrolyzed with lithium hydroxide to yield carboxylic
acids CSR1–2, 29, and 31–35.
We initially hypothesized that the hydrophilic side-chain might
engage Arg171 residue resulting in favorable inhibition of wild-
type (WT) RIPK2 compared with R171C RIPK2, where the arginine
(from PDB 4C8B) was replaced with cysteine. Therefore, the 15 test
compounds were screened for their in vitro RIPK2 enzyme inhibi-
tion against RIPK2 WT and the R171C mutant of RIPK2 at a single
concentration. One of the carboxylic acid derivatives (e.g. CSR35)
demonstrated modest percent inhibition in this initial assessment
and was selected for further analyses. IC50 values of CSR35 were
determined that showed only a twofold preference in RIPK2 WT
inhibitory activity (RIPK2 WT IC50 = 2.26 0.11 mM versus R171C
RIPK2 IC50 = 4.87 0.96 mM). Since the carboxylic acid will be
deprotonated at pH 7.4, this functional group potentially forms
an ionic–ionic interaction with the activation loop. Notably,
tion at 20 lM), which does not have a basic residue in the same
position as established by a published structure of VEGFR2 in the
DFG-out conformation (Fig. 4C).
An X-ray structure of the RIPK2ÁCSR35 (PDB ID: 6ES0) was
determined at a resolution of 2.4 Å (Fig. 4). Two RIPK2 molecules
were found in the asymmetric unit. As expected, CSR35 was bound
to a DFG-out conformation and retained interactions characteristic
of most type II inhibitors. The pyridine nitrogen and hydrogen of
the amide formed hydrogen bonds with the hinge region
(Met98). The urea was engaged in hydrogen bonds with the cat-
alytic residue Glu66 and the backbone NH of Asp164 of the DXG
motif. But most interestingly, one of the monomers revealed that
Lys169 of the activation loop forms an ionic–ionic interaction with
the carboxylate of the ligand and occupied a similar position as
Arg171 in the previously reported structure. Engagement of
Lys169 was consistent with the predicted result from sequence
alignments (Fig. 2A) and likely accounts for modest inhibitory
activity differences between WT and R171C mutant RIPK2. In addi-
tion, this interaction may be more favorable since protonated ami-
nes provide stronger hydrophobic interactions compared to
guanidinium ions.21 Interestingly, the corresponding methyl ester
derivative (CSR36) demonstrated potent activity based on the ini-
tial screen, raising the possibility that an ionic–dipole interaction
with the activation loop might be advantageous (Fig. 5).
In order to further examine the contributions of the carboxylic
acid and urea moieties of CSR35 in the absence of the hinge bind-
ing group two additional compounds were prepared. The first
Scheme 4. Synthesis of CSR analogs with hydrophilic moieties on phenyl ring A. Reagents and conditions: (a) tBuOK, DMF, rt to 100 °C, 16 h (87%); (b) phenyl chloroformate,
Py, CH2Cl2, 0 °C to rt, 1.5 h (28–99%); (c) 19, Py, 90 °C, 16 h (28–61%); (d) mCPBA, CH2Cl2, rt, 1 h (31%); (e) TFA, CH2Cl2, rt, 16 h (84%); (f) H2, 10% Pd/C, MeOH, rt, 2 d (99%); (g)
LiOH, THF/H2O, 60 °C, 18 h (61–98%).
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C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
O
Glu66
H N
S
O
t9
Me
O
N
H
O
H
N
F
O
H
N
H
N
N
H
Lys169
N
O
O
O
S
O
F
O
H 3
N
O
N H
CSR35
O
Asp164
O
Fig. 4. (A and B) Co-crystal structure of CSR35 (pink) with RIPK2 (gray). Inhibitor CSR35 forms hydrogen bonds to the backbone of Met98, Glu66 in the
in the DFG motif. Lys169 forms an ionic interaction with CSR35’s carboxylate side-chain. In addition, a salt bridge between Glu66 on the C-helix and b3-Lys47 is evident in
the DFG-out/ C-helix-in conformation. Red dashed lines indicate hydrogen bonds and red solid lines displays the ionic–ionic interactions. (PDB ID: 6ES0); C) Superimposed
structure of RIPK2ÁCSR35 (gray and pink) and VEGFR2 (orange; PDB ID: 3WZE).
aC-helix, and Asp164
a
a
derivative was CSR35F1 that contained the carboxylic acid, but not
the urea. It showed only 10% inhibition at 10 M. The second
l
derivative (CSR35F2) contained both the urea and carboxylic acid.
Although it was less potent than CSR35, it retained RIPK2 inhibi-
tory activity (IC50 = 13.2 1.7 lM). This result raises the possibility
that type III inhibitors or type II/III hybrid inhibitors, as was previ-
ously generated for RIPK1,22 might be feasible for development of
RIPK2 inhibitors.
Fig. 5. Chemical structure of CSR35 fragments.
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C. Suebsuwong et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
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In conclusion, a proof-of-concept for a design strategy of target-
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CSR35 with RIPK2, including an ionic–ionic contact with the
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We thank Diamond Light Source for beamtime (proposal
mx15433), as well as the staff of beamline I04-1 for assistance with
data collection. The SGC is a registered charity (no. 1097737) that
receives funds from AbbVie, Bayer Pharma AG, Boehringer Ingel-
heim, Canada Foundation for Innovation, Eschelman Institute for
Innovation, Genome Canada, Innovative Medicines Initiative (EU/
EFPIA) [ULTRA-DD grant no. 115766], Janssen, Merck KGaA Darm-
stadt Germany, MSD, Novartis Pharma AG, Ontario Ministry of Eco-
nomic Development and Innovation, Pfizer, São Paulo Research
Foundation – FAPESP, Takeda, and Wellcome [106169/ZZ14/Z].
This work was also supported by NIH grants R01CA190542 and
R21AI124049 to A.D.
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Conflict of interest
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A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.bmcl.2018.01.044.
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Please cite this article in press as: Suebsuwong C., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.01.044