6-Amino-3-methylpyrimidinones as Potent, Selective, and Orally Efficacious SHP2 Inhibitors

Article abstract of DOI:10.1021/acs.jmedchem.8b01726

Protein tyrosine phosphatase SHP2 is an oncoprotein associated with cancer as well as a potential immune modulator because of its role in the programmed cell death PD-L1/PD-1 pathway. In the preceding manuscript, we described the optimization of a fused, bicyclic screening hit for potency, selectivity, and physicochemical properties in order to further expand the chemical diversity of allosteric SHP2 inhibitors. In this manuscript, we describe the further expansion of our approach, morphing the fused, bicyclic system into a novel monocyclic pyrimidinone scaffold through our understanding of SAR and use of structure-based design. These studies led to the identification of SHP394 (1), an orally efficacious inhibitor of SHP2, with high lipophilic efficiency, improved potency, and enhanced pharmacokinetic properties. We also report other pyrimidinone analogues with favorable pharmacokinetic and potency profiles. Overall, this work improves upon our previously described allosteric inhibitors and exemplifies and extends the range of permissible chemical templates that inhibit SHP2 via the allosteric mechanism.

Full text of DOI:10.1021/acs.jmedchem.8b01726

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Article
6-Amino-3-Methylpyrimidinones as Potent,
Selective, and Orally Efficacious SHP2 Inhibitors
Patrick Sarver, Michael G Acker, Jeffrey Bagdanoff, Zhuoliang Chen, Ying-Nan P Chen, Homan
Chan, Brant Firestone, Michelle Fodor, Jorge Fortanet, Huaixiang Hao, Murphy Hentemann, Mitsunori
Kato, Robert Koenig, Laura R. LaBonte, Gang Liu, Shumei Liu, Chen Liu, Eric McNeill, Morvarid
Mohseni, Martin Sendzik, Travis Stams, Stan Spence, Victoriano Tamez, Ritesh Tichkule, Christopher
S. Towler, Hongyun Wang, Ping Wang, Sarah L. Williams, Bing Yu, and Matthew J. LaMarche
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.8b01726 • Publication Date (Web): 28 Jan 2019
Downloaded from http://pubs.acs.org on January 28, 2019
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LaMarche, Matthew; Novartis Institutes for BioMedical Research Inc,
Oncology Medicinal Chemistry
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6-Amino-3-Methylpyrimidinones as Potent, Selective,
and Orally Efficacious SHP2 Inhibitors
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Patrick Sarver,a Michael Acker,b Jeffrey T. Bagdanoff,a Zhouliang Chen,a Ying-Nan
Chen,b Homan Chan,b Brant Firestone,b Michelle Fodor,c Jorge Fortanet,a Huaixiang
Hao,b Murphy Hentemann,b Mitsunori Kato,a Robert Koenig,a Laura R. LaBonte,d Gang
Liu,a Shumei Liu,b Chen Liu,b Eric McNeill,a Morvarid Mohseni,b Martin Sendzik,a Travis
Stams,c Stan Spence,b Victoriano Tamez,a Ritesh Tichkule,a Christopher Towler,e
Hongyun Wang,b Ping Wang,b Sarah L. Williams,a Bing Yu,a and Matthew J. LaMarche a*
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aGlobal Discovery Chemistry, bOncology Disease Area, cChemical Biology &
d
Therapeutics, Pharmacokinetic Sciences, Novartis Institutes for Biomedical Research,
and eChemical and Pharmaceutical Profiling, Novartis Pharmaceuticals, 250
Massachusetts Avenue, Cambridge, Massachusetts, 02139, United States
ABSTRACT
Protein tyrosine phosphatase SHP2 is an oncoprotein associated with cancer as well as a
potential immune modulator due to its role in the programmed cell death PD-L1/PD-1 pathway.
In the preceding manuscript, we described the optimization of a fused, bicyclic screening hit for
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potency, selectivity, and physicochemical properties in order to further expand the chemical
diversity of allosteric SHP2 inhibitors. In this second manuscript, we describe the further
expansion of our approach, morphing the fused, bicyclic system into a novel monocyclic
pyrimidinone scaffold through our understanding of SAR and use of structure-based design. These
studies led to the identification of SHP394 (1), an orally efficacious inhibitor of SHP2, with high
lipophilic efficiency, improved potency and enhanced pharmacokinetic properties. We also report
other pyrimidinone analogs with favorable pharmacokinetic and potency profiles. Overall, this
work improves upon our previously described allosteric inhibitors, and exemplifies and extends
the range of permissible chemical templates that inhibit SHP2 via the allosteric mechanism.
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CF3
NH2
N
S
N
NH2
O
O
N
N
SHP394
INTRODUCTION
Mutations in the Ptpn11 gene, which encodes the non-receptor protein tyrosine
phosphatase (PTP) SHP2, are associated with several human diseases. Hyperactivating mutations
of SHP2 has been observed in Noonan syndrome (50%),1 juvenile myelomonocytic leukemia
(JMML, 35%), myelodysplastic syndrome (10%), B-cell accute lymphoblastic leukemia (7%),
acute myeloid leukemia (AML, 4%),2 and a number of solid tumors.3 Although its precise role is
not fully understood, SHP2 functions in the cytoplasm downstream of various receptor-tyrosine
kinases and is involved in oncogenic cell signaling cascades including RAS-ERK, PI3K-AKT, and
JAK-STAT.4 SHP2 may also play a role in the PD-L1/PD-1 pathway, suggesting possible
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applications of SHP2 inhibition in immuno-oncology.5, Given the demonstrated importance of
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SHP2 to human diseases, small molecule inhibition of SHP2 has attracted significant attention in
the scientific community.7 Over the last two decades, a number of small molecule active site
inhibitors have been described, but have not progressed to the clinic.8 Due to the challenges
inherent to targeting the active site of phosphatases,9 for example the inherent polarity and
difficulty in achieving selectivity across the phosphatome, the general concept of allosteric
phosphatase inhibition is gaining momentum.10
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Previously, we reported a screening method that led to the identification of a new site for
allosteric SHP2 inhibition.11 Initial medicinal chemistry efforts identified a moderately potent,
selective, and orally bioavailable inhibitor, SHP099 (2, Figure 1a).12 By concurrently binding to
the interface of the N-terminal SH2, C-terminal SH2, and protein tyrosine phosphatase domains,
2 stabilizes an inactive conformation in which the active site is inaccessible to substrates. This
binding modality resembles the published inactive apo structure (PDB code 2SHP), 13 in which the
N-terminal SH2 domain blocks the active site, resulting in autoinhibition of PTP activity.
During the course of the discovery and characterization of 2 (Figure 1A), we sought to
investigate other chemical matter which would provide structural and property diversification.
Part of this effort to generate new chemotypes focused on alternative screening strategies. We14
and others15 employed oncogenic mutants of SHP2. We also engineered inhibitor-resistant
mutants to identify an additional allosteric site for SHP2 inhibition,16 and then demonstrated that
dual allosteric inhibition of SHP2 was possible. In the preceding paper,17 multiple, unique, fused-
bicyclic screening hits were optimized for potency, selectivity and structural diversity. These
studies identified potent and selective inhibitors that are structurally distinct from 2. Additionally,
this work further explored the allosteric pocket, identifying important elements of the
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pharmacophore such as a thioether functionality, and the conformationally restricted spirocyclic
amine motif.
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Despite the successful optimization for potency and selectivity of this fused-bicyclic
template, it remained difficult to identify pyrazolo-pyrimidinones, or related fused systems, which
coupled these properties with acceptable oral exposure and ADME properties. In order to further
broaden our approach to SHP2 allosteric inhibition and access more desireable ADME and
property space, we employed a scaffold-morphing strategy using the pyrazolo-pyrimidinones as a
starting point. This exploited our working knowledge of the SAR, a compendium of HTS data,
and structure-based design. In this, the second of two papers, we report the discovery of 6-amino-
3-methylpyrimidinones as potent, selective, and orally efficacious SHP2 inhibitors with favorable
physicochemical and ADME properties.
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RESULTS AND DISCUSSION
As in our previous studies, we utilized a well-precedented, fluoresence-based phosphatase
biochemical assay measuring dephosphorylation of 6,8-difluoro-4-methylumbelliferyl phosphate
(0.5 µM 2P-IRS-1, DIFMUP assay18). We also evaluated modulation of p-ERK and DUSP6 as
downstream markers of MAPK pathway activity, and antiproliferation activity in the EGFR
amplified human esophageal squamous cell carcinoma cell line KYSE520 and in Detroit-562, a
pharyngeal carcinoma cell line. Early investigations of the pyrazolo-pyrimidinone scaffold (see
preceding paper) for SHP2 inhibition led to the identification of 3 (Figure 1A), a 5,6-fused system
bearing the same aryl and aminopiperidine present in 2. Comparison of the X-ray binding poses
of 3 and aminopyrazine 2 (PDB codes 6MDB, 5EHR respectively; overlay in Figure 1B) indicated
significant overlap of the six-membered pyrimidinone ring and the aminopyrazine. Several
interactions of 3 with the protein are quite similar to those previously described for 2:12 the
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pyrimidinone carbonyl of 3 makes a crucial hydrogen bond with R111, positioning the residue for
Pi-cation stacking interaction with the dichlorophenyl motif; the dichlorophenyl occupies a
hydrophobic area of the pharmacophore (e.g., L254, Q257, P491); the pyrazole forms a hydrogen
bond with E250; the primary amine makes contacts with the backbone carbonyl of F113. Given
the challenges in identifying compounds from the pyrazolo-pyrimidinones with an acceptable
balance of potency, hERG selectivity, and oral exposure (see preceding paper), we redirected our
efforts towards monocyclic central ring systems in an effort to ease multiparameter optimization.
Thus, breaking the 5-6 ring fusion by removal of the pyrazole ring in 3 and joining the
dichlorophenyl directly to the pyrimidinone resulted in 4 (Figure 1A). Compound 4 is equipotent
to 2 and 3 in the biochemical assay (IC50 = 0.105 M). Importantly, as reduced lipophilicity is
strongly correlated with improved in vivo properties,19 the monocyclic pyrimidinone has lower
lipophilicity (cLogP = 1.9) and increased lipophilic efficiency (5.1).
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A.
Cl
Cl
Cl
Cl
N
NH2
N
NH
N
N
N
O
N
N
NH2
NH2
2
3
SHP099 ( )
SHP2 IC50 = 0.071 µM
p-ERK IC50 = 0.250 µM
antiprolif. IC50 = 1.4 µM
cLogP = 3.0, LipE = 4.5
SHP2 IC50 = 0.067 µM
p-ERK IC50 = 0.746 µM
antiprolif. IC50 = 4.8 µM
cLogP = 2.4, LipE = 4.9
Cl
Cl
NH2
N
O
N
N
NH2
4
SHP2 IC50 = 0.105 µM
p-ERK IC50 = ND
antiprolif. IC50 = ND
cLogP = 1.9, LipE = 5.1
B.
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C.
Figure 1. A. Design of pyrimidinones 4
from pyrazine (2) and pyrazolo-
pyrimidinone (3). B. Overlay of X-ray of 2-
SHP2 (PDB code 5EHR) and 3-SHP2
(PDB code 6MDB) in the allosteric binding
pocket of SHP2. C. Amine region overlay:
X-ray of 2-SHP2 and 3-SHP2.
The fused bicycle of 3 is also reminiscent of another screening hit, azabenzimidazole 5
(Figure 2A), a weak inhibitor of SHP2 (IC50 = 46 M) that also interacts with R111 via the pyridine
N, thereby preorganizing the dichlorophenyl ring into the hydrophobic cleft and towards a Pi-
cation interaction with R111 in a similar fashion to 2 and 3. Notably, 5 and 3 both join the dihalo-
arene with the central ring by the same number of atoms, yet with different functional groups: a
thioether vs. a pyrazole ring, respectively. With the removal of the pyrazole ring in 4, we
hypothesized that the thioether linker from 5 would successfully replace and mimic the pyrazole
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and also balance the substantially increased polarity of the central pyrimidinone ring. In addition,
our SAR understanding of the allosteric binding pocket allowed us to also consider amine
extension. In previous studies (see preceding paper), homologation of the amine displaced a
structural water allowing for binding interactions directly to the protein (Figure 1C and vide infra).
We thus incorporated the thioether linker and amine extension into our monocyclic pyrimidinone
design, which resulted in 6. We evaluated 6 in the biochemical and cellular assays and found
improved biochemical potency (IC50 = 0.012 M) relative to the aminopyrazine, pyrazolo-
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pyrimidinone, and aminopyrimidinone starting points (2: IC50= 0.071 M; 3: IC50= 0.064 M; 4:
IC50= 0.105 M, respectively), as well as improved cellular potency (6: p-ERK IC50 = 0.049 M,
antiproliferation IC50 = 0.273 M). Compound 6 was also evaluated for solubility in aqueous
buffer (0.25 mM at pH 6.8), permeability (Caco-2: 2.5 x 10-6 cm/s),20 and hERG activity (IC50 = 6
M). Taken together, these results indicated that the pyrimidinone template was an attractive
starting point for further optimization (e.g., potency, permeability, hERG selectivity). We were
further encouraged by the presence of the 4-pyrimidinone moiety in a range of marketed and
developmental drugs.21
A.
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Cl
H
N
S
Cl
N
NH2
6
7
8
9
N
5
SHP2 IC50 = 46 µM
cLogP = 4.8
LipE = 2.7
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Cl
NH2
N
Cl
S
O
N
N
6
H2N
SHP2 IC50 = 0.012 µM
p-ERK IC50 = 0.049 µM
antiprolif. IC50 = 0.273 µM
cLogP = 3.2, LipE = 4.9
CF3
S
N
N
O
N
N
H
7
H2N
SHP2 IC50 = 0.181 µM
p-ERK IC50 > 2.0 µM
antiprolif. IC50 > 2.0 µM
cLogP = 3.2, LipE = 3.6
B.
Figure
2.
A.
Thioether-
azabenzimidazole HTS hit 5 and
thioether-pyrimidinones 6 and 7. B.
X-ray of 7 and SHP2 (PDB code
6MD7).
Our optimization was facilitated by X-ray crystallography of a less potent analog (Figure
2A: 7, IC50 = 0.181 M) bound to SHP2 (PDB code 6MD7) in a conformation resembling the
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bound aminopyrazines and pyrazolopyrimidinones (Figure 2B). As anticipated, the central
pyrimidinone ring of 7 occupies a position nearly identical to that of the 5,6-fused pyrimidinone
in 3 and the aminopyrazine in 2. Further comparison of the binding pose of 7 and 2 indicated that
the aminopyrazine of 2, which was previously shown to engage in hydrogen bonding with
backbone carbonyl of E250 and substantially improve potency, could likely be reintroduced at the
6-position of the pyrimidinone (vide infra). The crystal structure of 7-SHP2 also revealed that the
pyrimidinone oxygen facilitates a Pi-cation interaction with R111 and the trifluoromethyl-pyridine,
through a hydrogen bond interaction. This resembles a similar Pi-cation interaction preorganized
by the pyrazine nitrogen of 2, despite a distance of one bond length between the pyrimidinone
oxygen and pyrazine nitrogen. In addition, the extended amine in 7 displaced a structural water
found in the X-ray of 2-SHP2 (PDB code 5EHR), making direct interactions with several residues
(e.g., T253, S109, E110). Importantly, 7 was selective over the hERG channel (IC50 > 30 M),
indicating that increasing the polarity of the central scaffold could result in higher hERG selectivity.
The overlay of 7 and other scaffolds (i.e., 2, 3: not shown) also showed excellent overlap of the
aryl and amine regions, suggesting that previously developed SAR could be transferrable from the
aminopyrazines and pyrazolopyrimidinones to the monocyclic pyrimidinones. This enabled the
application of a range of previously investigated fragments in our continued efforts to optimize
this novel template.
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Given the polar core present in the aminopyrimidinones, we hypothesized that re-
optimization of the aryl thioether and spirocyclic amine could yield significant improvements in
balancing potency, permeability, and oral exposure. First, we utilized a spiro[4.5]-furanyl-amine
moiety (e.g., 8, Table 1) which had proven to improve potency and permeability, and reduce the
pKa of the basic amine in other scaffolds (see preceding paper). Compound 8 exhibited 4 to 10-
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fold improvement compared to 6 in the cellular assays (p-ERK IC50 = 0.005 M, antiproliferation
IC50 = 0.069 M) and acceptable oral exposure (dose-normalized to 1 mg/kg PO, 0.95 µM·h) and
bioavailability (38 %F) in rat.
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NH2
N
O
N
compound
Ar-
Cl
amine
cLogP/LipE
3.2/4.9
hERG
IC50 (µM)
SHP2
IC50 (M)
p-ERK
IC50 (M) IC50 (M)
antiproliferation
NH2
NH2
N
N
Cl
Cl
S
6
0.012
0.005
0.049
0.005
0.273
0.069
6.0
4.0
Cl
S
S
S
S
8
3.0/5.8
O
Cl
NH2
N
N
N
9
0.048
0.014
0.023
0.028
0.010
0.018
0.220
0.295
0.297
1.5/6.0
2.0/6.0
1.2/6.6
>30
9.8
N
O
Cl
NH2
10
1
N
N
O
CF3
NH2
>30
O
Table 1. Optimization of the pyrimidinone scaffold for improved potency, hERG selectivity, and
oral exposure.
In addition, 8 was membrane permeable (Caco-2 Papp A-B = 6.8 x 10-6 cm/s), moderately
stable in rat liver microsomes (Clint = 39.3, L/min/mg, T1/2 = 37 min), and had no CYP inhibition
(IC50 = > 50 µM for CYP3A4, CYP2C9 and CYP2D6). Compound 8 was next evaluated in vivo
at 10, 30 and 100 mg/kg in mice bearing subcutaneously implanted KYSE520 xenografts. Free
concentration of 8 (92 % mouse plasma protein bound) correlated well with the MAPK pathway
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pharmacodynamic markers p-ERK and DUSP6. Maximal PD effects were observed at 100 mg/kg
and as early as 3 hours post-dose.
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Figure 3. PK-PD of 8 in KYSE520 tumor implanted immunocompromised mice at 10, 30, and
100 mg/kg. A. p-ERK modulation in KYSE520 model. B. DUSP6 modulation in KYSE520
model.
Although robust p-ERK modulation was observed in vivo, significant hERG inhibition (6,
8: IC50 = 4.0-6.0 M) still plagued the series despite varying the amine pKa, presumably due to
the lipophilic arene present. Redesign of the aryl group to include a pyridyl N in proximity to
K492, while satisfying the hydrophobic demands of the pocket formed by L254, Q257 and P491,
resulted in the more polar 2-methyl-3-chloropyridyl analog, 9. Compound 9 retained cellular
potency (antiproliferative IC50 = 0.220 M), had excellent aqueous solubility (0.88 mM in pH 6.8
buffer), and no accompanying hERG inhibition (IC50 >30 µM). Gratifyingly, pharmacokinetic
analysis in rats showed an improvement in rodent oral exposure (dose-normalized to 1 mg/kg PO,
2.01 µM·h) and bioavailability (56 %F) compared to 8 and 10 other tested pyrimidinones.
Attempts to further improve 9 by introducing additional lipophilic functional groups in the aryl
region generally resulted in no potency improvement, but increased hERG inhibition, as
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exemplified by 10. Further evolution of the aryl motif included transposition of the pyridyl
nitrogen meta to the thioether, and introduction of an ortho-trifluoromethyl group resulting in 1.
Compound 1 was approximately equipotent to 9 and 10 in biochemical and cellular assays,
selective over the hERG ion channel, and displayed high lipophilic efficiency (6.6).
10
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Though the unique nature of each of these heterocycles sometimes required various
synthetic approaches, many of the analogues were prepared following the synthetic route outlined
for 1 (Scheme 1). Beginning with the corresponding pyrimidine-dione (11), the spirocyclic amine
1217,22 was coupled using BOP;23 other well-precedented methods, including the synthesis of the
corresponding 2-chloro-4-pyrimidones with POCl3 and more modern coupling reagents, failed to
provide for a number of modified core analogs in acceptable yields. The coupled product was then
reacted with di-tert-butyl dicarbonate to afford the corresponding Boc-protected amine (13), which
substantially improved the handling of these otherwise water soluble compounds. Selective
iodination at the 2-position of the pyrimidinone ring afforded 5-iodopyrimidinone 14, which was
converted to the corresponding biaryl thioether through a copper-catalyzed Ullman reaction with
15. A final Boc deprotection with TFA followed by HPLC purification afforded the desired
product (1) in approximately 10% overall yield. This synthetic route was amenable to scale-up
and was successfully utilized to produce 1 on multi-gram scale.
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2
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6
NH2
O
HN
NH2
N
NH2
NH
12
NHBoc
O
O
N
N
7
8
9
a, b
O
N
O
11
13
10
11
12
13
14
15
16
17
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20
21
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CF3
NH2
N
SH
I
N
c
NHBoc
O
O
N
N
15
d, e
14
CF3
NH2
S
N
N
NH2
O
O
N
N
1
Scheme 1. Synthesis of N-methylamino pyrimidinone 1.
(a) Amine, BOP, DBU, DMF, RT, 3 days, 59% yield
(b) Boc2O, DIPEA, DMF, RT, 1 h, (c) NIS, DMF, RT,
10 min, 39% yield (2 steps) (d) ArSH, CuI, TMEDA,
K3PO4, 1,4-dioxane, 100 °C, 1.5 h, (e) TFA, DCM, 3 h,
51% yield (2 steps).
Due to the encouraging initial results with peripheral fragment optimization, and the
promising designs enabled by our crystal structures, we sought to further explore the minimum
pharmacophore requirements around the central pyrimidinone ring. Our central ring studies began
by maintaining the spiro[4.5]-amine and trifluoromethyl-pyridine in order to maintain potency and
minimize hERG activity (Table 2). Removal of the 6-position NH2 resulted in a loss in potency
(16: p-ERK IC50 = 0.096 M), and removal of the 3-Me further eroded cellular potency (17: p-
ERK IC50 > 1 M) which was reminiscent of 7. Removal of nitrogen at the 1-position of the N-
methylpyrimidinone resulted in pyridone 18 and aminopyridone 19, which substantially decreased
the biochemical activity. The 1-position N likely affects the R111 H-bond interaction via inductive
effects on the 4-position carbonyl, and also allows for piperidine conformations preferred for
binding. Taken together, the 6-amino, 3-methyl pyrimidinone, present in 1, was optimal.
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CF3
S
N
het
NH2
O
5
6
7
N
8
9
compound
core
cLogP/LipE
hERG
IC50 (µM)
SHP2
p-ERK
antiproliferation
IC50 (M) IC50 (M) IC50 (M)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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27
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29
30
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NH2
6
1
N
1
0.023
0.026
0.018
0.096
0.297
1.5
1.2/6.6
1.5/5.9
>30
>30
O
O
N
N
16
N
N
17
18
0.031
0.177
1.06
N.D.
N.D.
N.D.
1.3/6.2
2.1/4.7
>30
ND
O
O
N
H
N
NH2
N
19
0.429
N.D.
N.D.
1.1/5.3
ND
O
Table 2. Systematic removal of functional
groups from the core to define the
minimum pharmacophore.
Our efforts to further optimize 1 via the spirocyclic amine region similarly began with
efforts to improve permeability and potency by matching lipophilic fragments to the polar
pyrimidinone core. Holding the trifluoromethyl-pyridine fragment constant, we prepared 20,
which bears a γ-methylamino spiro[4.5]-amine lacking the cyclic ether common to 1 and 8-10.
This compound showed similar gains in cellular potency to those observed for 8 while crucially
maintaining selectivity against hERG (IC50 >30 µM). Disappointingly, rat pharmacokinetic data
indicated that this compound had very poor oral exposure (6 mg/kg PO, 0.03 µM·h) and
bioavailability (1 %F). In vitro microsomal clearance (Clint = 3.4 L/min/mg, T1/2 = 6.6 h in rat
liver microsomes) and permeability (Caco-2 Papp A-B < 0.5 x 10-6 cm/s) data suggested these
inadequate pharmacokinetic properties resulted from poor permeability and absorption, which we
attributed to the more basic and less sterically encumbered primary amine in 20. In order to
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improve the properties of 20, we introduced a fluorine β to the amine (e.g., 21), thus reducing the
pKa of the amine via an inductive effect.24 Although 21 showed promising potency (less than two-
fold reduction in antiproliferation potency compared to 20), the synthetic challenge of a molecule
bearing three contiguous stereocenters adjacent to a spirocyclic center on a cyclopentyl ring
reduced the attractiveness of this analogue when compared to other analogs (e.g., 1, 9). The
increased lipophilicity of 21 (cLogP = 2.3) compared to 1 and 9 (cLogP = 1.2, 1.5, respectively)
further reduced the desire to pursue this synthetically challenging target.
10
11
12
13
14
15
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17
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19
20
21
22
23
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NH2
N
O
N
compound
Ar-
amine
cLogP/LipE
hERG
SHP2
p-ERK
antiproliferation
IC50 (µM)
IC50 (M)
IC50 (M) IC50 (M)
CF3
NH2
N
N
S
1
0.023
0.022
0.018
0.004
0.297
0.098
1.2/6.6
2.3/6.2
>30
>30
N
N
O
NH2
CF3
CF3
S
S
20
NH2
N
21
0.018
0.005
0.160
2.1/5.9
>30
F
N
Table 3. Select spirocyclic amine SAR
Given the acceptable in vitro potency and hERG selectivity of pyrimidinone 1, we next
investigated its selectivity, physicochemical and pharmacokinetic properties. Initial evaluation
against a standard kinase and safety pharmacology panel (consisting of ion channels, kinases,
GPCRs) showed no evidence of off-target activity, suggesting that the pyrimidinone series
possessed the same overall selectivity observed for the aminopyrazines. The overall polarity of 1
enabled excellent aqueous solubility (0.98 mM in pH 6.8 buffer), but low permeability (Papp(A-
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B) 1.02 x 10-6 cm/s), and apparent efflux (B-A/A-B = 15) in Caco-2 cells. The metabolic stability
of 1 in both rat and mouse liver microsomes indicated low clearance (Clint = 7.9-11.1 L/min/mg;
T1/2 > 2 h) for both species. Subsequent pharmacokinetic studies of 1 in rat confirmed low
clearance (Clint = 21 L/min/mg; T1/2 = 10.9 h), moderate oral exposure (dose-normalized to 1
mg/kg PO, 0.45 µM·h) and oral bioavailability (29 %F). We speculate that low membrane
permeability and efflux contributed to this moderate oral exposure.
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With the acceptable disposition of 1 in rat, and the previous demonstration of p-ERK and
DUSP6 modulation in the KYSE520 model by 8, we next sought to broaden our characterization
of the series across cancer cell lineages. We evaluated 1 in the Detroit-562 pharyngeal carcinoma
cell line in vitro (antiproliferative IC50 = 1.38 M), and compared it to other inhibitors (8: IC50 =
0.35 M; 2 (SHP099): IC50 = 6.47 M; SHP38917: IC50 = 4.13 M). Although the absolute IC50
was perturbed as compared to the KYSE520 cells, possibly due to the differences in cell lines and
p-ERK assays, the rank order of compound potency across the two models remained constant. We
also evaluated 1 in the Detroit-562 mouse model, with tumors subcutaneously implanted into
immunocompromised mice (Figure 4). Single rising doses of 20, 40, and 80 mg/kg dosed twice
daily, led to dose-dependent exposure and reduction in expression of pharmacodynamic marker
DUSP6 mRNA (Figure 4A). An efficacy study employing 20, 40, and 80 mg/kg BID dosing of 1
demonstrated a clear dose-dependent reduction in tumor volume. Approximate tumor stasis was
observed by 21 days at the well-tolerated dose of 40 mg/kg twice daily (11% T/C), and tumor
regression was achieved at 80 mg/kg twice daily (34 % regression) after dosing for 14 days, with
a mean reduction on mouse host bodyweight (-2.2 % from first dose).
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Figure 4. PK/PD and efficacy of 1 in pharyngeal carcinoma Detroit-562 xenograft model.
DISCUSSION AND CONCLUSIONS
The development of potent, selective, and orally efficacious SHP2 inhibitors presents an
important challenge due to the increasingly appreciated biological significance of this oncogenic
protein tyrosine phosphatase in cancer, stromal, and immune cells. Following our discovery of an
allosteric pocket that enables stabilization of an autoinhibited conformation, we have pursued
several classes of SHP2 inhibitors and clinical investigation is underway.25 In this manuscript, we
apply SAR across aminopyrazine (e.g., 2), 5,6-fused bicyclic (e.g., 3), and 6,5-fused bicyclic (e.g.,
5) chemical templates in conjunction with X-ray crystallographic data to originate a monocyclic
pyrimidinone scaffold with high lipophilic efficiency.
The minimum pharmacophore
requirements of the central scaffold were determined, and the potency and ADME properties of
the pyrimidinones were further improved. These studies culminated in the identification of 1,
which showed correlative and dose-dependent PK, PD and efficacy when dosed orally to
immunocompromised mice bearing subcutaneously implanted Detroit-562 tumor cells. Further
perturbation of the aryl thioether and spiro[4.5]-amine moieties resulted in the synthesis of a
number of related analogues with similar antiproliferation potency to 1. Notably, 8 improved upon
the potency, and 9 improved the oral exposure and hERG selectivity as compared to 1.
Aminopyrimidinones 1, 8, 9, and the additional analogues described here provide compelling
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potent, selective, and orally bioavailable alternatives to previously disclosed SHP2 inhibitors for
future investigation of the multifaceted roles of SHP2 in signal transduction and human disease.
7
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EXPERIMENTAL SECTION
Compound synthesis and characterization. Compound purity was assessed by HPLC to confirm
> 95% purity. All solvents employed were commercially available anhydrous grade, and reagents
used as received unless otherwise noted. Purification of intermediates and final products was
carried out on a normal phase using an ISCO CombiFlash system and prepacked SiO2 cartridges
eluted with optimized gradients of either ethyl acetate/heptane mixture or
methanol/dichloromethane as described. Preparative high pressure liquid chromatography (HPLC)
was performed using a Waters 2525 pump with 2487 dual wavelength detector and 2767 sample
manager. Columns were Waters C18 OBD 5 μm, either 50x100 mm Xbridge or 30x100 mm
Sunfire. NMR spectra were recorded on a Bruker AV400 (Avance 400 MHz) or AV600 (Avance
600 MHz) instruments. Analytical LC-MS was conducted using an Agilent 1100 series with UV
detection at 214 nm and 254 nm, and an electrospray mode (ESI) coupled with a Waters ZQ single
quad mass detector. One of two methods was used: Method A) 5-95% acetonitrile/H2O with 5 mM
ammonium formate with a 2 min run, 3 μL injection through an inertisil C8 3 cm x 5 mm x 3μm;
Method B) 20-95% acetonitrile/H2O with 10 mM ammonium formate with a 2 min run, 3 μL
injection through an inertisil C8 3 cm x 5 mm x 3 μm. Purity of all tested compounds was
determined by LC/ESI-MS, data recorded using an Agilent 6220 mass spectrometer with
electrospray ionization source and Agilent 1200 liquid chromatography. The mass accuracy of the
system has been found to be < 5 ppm. HPLC separation was performed at 75 mL/min flow rate
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with the indicated gradient within 3.5 min with an initial hold of 10 seconds. 10 mM ammonia
hydroxide or 0.1 M TFA was used as the modifier additive in the aqueous phase.
7
8
9
Preparation of 6-Amino-2-((3S,4S)-4-amino-3-methyl-2-oxa-8-azaspiro[4.5]decan-8-yl)-3-
methyl-5-((2-(trifluoromethyl)pyridin-3-yl)thio)pyrimidin-4(3H)-one (1). A mixture of 6-
amino-3-methylpyrimidine-2,4(1H,3H)-dione (1 g, 7.09 mmol), (3S,4S)-3-methyl-2-oxa-8-
azaspiro[4.5]decan-4-amine (1.81 g, 7.44 mmol), BOP (6.27 g, 14.17 mmol), and DBU (7.48 mL,
49.6 mmol) in DMF (15 mL) was stirred for 60 h at RT. The resulting mixture was purified by
HPLC (gradient elution 2-12% MeCN in water, 5 mM NH4OH modifier) to give 6-amino-2-
((3S,4S)-4-amino-3-methyl-2-oxa-8-azaspiro[4.5]decan-8-yl)-3-methylpyrimidin-4(3H)-one
(2.08 g, 7.09 mmol). MS m/z 294.3 (M+H)+.
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A mixture 6-amino-2-((3S,4S)-4-amino-3-methyl-2-oxa-8-azaspiro[4.5]decan-8-yl)-3-
methylpyrimidin-4(3H)-one (2.08 g, 7.09 mmol), Boc2O (1.55 g, 7.09 mmol), and DIPEA (2.5
mL, 14.18 mmol) in DMF (14 mL) was stirred for 1 h at RT. The resulting mixture was poured
into a separation funnel containing sat. aq. NH4Cl (75 mL) and it was extracted with DCM (3 x 15
mL). The combined organic phases were dried over MgSO4, filtered, and the volatiles were
removed under reduced pressure to give tert-butyl ((3S,4S)-8-(4-amino-1-methyl-6-oxo-1,6-
dihydropyrimidin-2-yl)-3-methyl-2-oxa-8-azaspiro[4.5]decan-4-yl)carbamate (2.79 g, 7.09
mmol). MS m/z 394.4 (M+H)+. This compound was used in next step without futher purification.
A mixture of tert-butyl ((3S,4S)-8-(4-amino-1-methyl-6-oxo-1,6-dihydropyrimidin-2-yl)-
3-methyl-2-oxa-8-azaspiro[4.5]decan-4-yl)carbamate (2.79 g, 7.09 mmol) and NIS (1.76 g, 7.80
mmol) in DMF (14 mL) was stirred for 1 h at RT. The resulting mixture was poured into a
separation funnel containing sat. aq. Na2S2O3 (25 mL), sat. aq. NH4Cl (25 mL), and water (25 mL)
and was extracted with DCM (3 x 20 mL). The combined organic phases were dried over MgSO4,
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filtered, the volatiles were removed under reduced pressure, and the residue was purified by silica
chromatography (0 to 5% gradient of MeOH/DCM) to give tert-butyl ((3S,4S)-8-(4-amino-5-iodo-
1-methyl-6-oxo-1,6-dihydropyrimidin-2-yl)-3-methyl-2-oxa-8-azaspiro[4.5]decan-4-
yl)carbamate (1.51 g, 2.91 mmol). MS m/z 520.1 (M+H)+.
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A
mixture
of
tert-butyl
((3S,4S)-8-(4-amino-5-iodo-1-methyl-6-oxo-1,6-
dihydropyrimidin-2-yl)-3-methyl-2-oxa-8-azaspiro[4.5]decan-4-yl)carbamate (42 mg, 0.081
mmol), 2-(trifluoromethyl)pyridine-3-thiol (22 mg, 0.121 mmol), CuI (3.1 mg, 0.016
mmol),TMEDA (5 μL, 0.032 mmol), and K3PO4 (51 mg, 0.243 mmol) in dioxane (0.5 mL) was
stirred for 90 min at 100 °C. After cooling to RT, the reaction mixture was poured into a
separationfunnel containing aq. K2CO3 (2 M, 2 mL) and extracted with DCM (3 x 5 mL). The
combined organic phases were dried over MgSO4, filtered, and the volatiles were removed under
reduced pressure. The residue was dissolved in DCM (5 mL) and TFA (1 mL) was added. After
stirring for 20 min at RT, the volatiles were removed under reduced pressure and the residue was
purified by HPLC (gradient elution 15-40% MeCN in water, 5 mM NH4OH modifier) to give 6-
amino-2-((3S,4S)-4-amino-3-methyl-2-oxa-8-azaspiro[4.5]decan-8-yl)-3-methyl-5-((2-
(trifluoromethyl)pyridin-3-yl)thio)pyrimidin-4(3H)-one (1) as a white solid (20.0 mg, 0.043
1
mmol). H NMR (400 MHz, methanol-d4) δ ppm 8.24-8.41 (m, 1 H), 7.51 (d, J=7.58 Hz, 1 H),
7.41 (dd, J=8.34, 4.55 Hz, 1 H), 4.22 (dd, J=6.32, 5.05 Hz, 1 H), 3.84 (d, J=8.84 Hz, 1 H), 3.69 (d,
J=8.59 Hz, 1 H), 3.47-3.61 (m, 2 H), 3.36-3.47 (m, 3 H), 3.04-3.26 (m, 2 H), 3.03 (d, J=5.05 Hz,
1 H), 1.79-2.02 (m, 2 H), 1.60-1.78 (m, 2 H), 1.13-1.28 (m, 3 H). 19F NMR (376 MHz, methanol-
d4) δ ppm -66.36. HRMS calcd for C20H26F3N6O2S (M+H)+
471.1790, found 471.1809.
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Protein Expression and Purification. The gene encoding human SHP2 from residues
Met1−Leu525 was inserted into a pET30 vector. A coding sequence for a 6× histidine tag followed
by a TEV protease consensus sequence was added 5′ to the SHP2 gene sequence. The
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construct was transformed into BL21 Star (DE3) cells and grown at 37 °C in Terrific Broth
containing 100 μg/mL kanamycin. At an OD600 of 4.0, SHP2 expression was induced using 1
mM IPTG. Cells were harvested following overnight growth at 18 °C.
Cell pellets were resuspended in lysis buffer containing 50 mM Tris-HCl, pH 8.5, 25 mM
imidazole, 500 mM NaCl, 2.5 mM MgCl2, 1 mM TCEP, 1 μg/mL DNase1, and complete EDTA-
free protease inhibitor and lysed using a microfluidizer, followed by ultracentrifugation. The
supernatant was loaded onto a HisTrap HP chelating column in 50 mM Tris-HCl, 25 mM imidazole,
500 mM NaCl, 1 mM TCEP and protein was eluted with the addition of 250 mM imidazole. The
Nterminal histidine tag was removed with an overnight incubation using TEV protease at 4 °C.
The protein was subsequently diluted to 50 mM NaCl with 20 mM Tris-HCl, pH 8.5, 1 mM TCEP,
then applied to a HiTrap Q FastFlow column equilibrated with 20 mM Tris, pH 8.5, 50 mM NaCl,
1 mM TCEP. The protein was eluted with a 10 column volume gradient from 50 to 500 mM NaCl.
Fractions containing SHP2 were pooled and concentrated then loaded onto a HiLoad Superdex200
PG 16/100 column, exchanging the protein into the crystallization buffer, 20 mM Tris-HCl, pH
8.5, 150 mM NaCl, and 3 mM TCEP. The protein was concentrated to 15 mg/mL for use in
crystallization. Crystallization, DSF, and high throughput screening assays used the 1−525
construct of SHP2, while biochemical assays used the 2−593 construct.
Biochemical Assay. SHP2 is allosterically activated through binding of bis-tyrosyl-
phorphorylated peptides to its Src Homology 2 (SH2) domains. The latter activation step leads to
the release of the autoinhibitory interface of SHP2, which in turn renders the SHP2 PTP active and
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available for substrate recognition and reaction catalysis. The catalytic activity of SHP2 was
monitored using the surrogate substrate DiFMUP in a prompt fluorescence assay format. More
specifically, the phosphatase reactions were performed at room temperature in 384-well black
polystyrene plate, flat bottom, low flange, nonbinding surface (Corning, cat. no. 3575) using a
final reaction volume of 25 μL and the following assay buffer conditions: 60 mM HEPES, pH 7.2,
75 mM NaCl, 75 mM KCl, 1 mM EDTA, 0.05 % P-20, 5 mM DTT.
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The inhibition of SHP2 from the tested compounds (concentrations varying from 0.003 to 100 μM)
was monitored using an assay in which 0.5 nM of SHP2 was incubated with of 0.5 μM of peptide
IRS1_pY1172(dPEG8)pY1222(sequence H2N-LN(pY)IDLDLV-(dPEG8)LST(pY)ASINFQK-
amide). After 30−60 min incubation at 25 °C, the surrogate substrate DiFMUP (Invitrogen, cat.
no. D6567, 200 μM) was added to the reaction and incubated at 25 °C for 30 min (200 μM for
residue 2−593, 100 μM for residue 1−525 construct). The reaction was then quenched by the
addition of 5 μL of a 160 μM solution of bpV(Phen) (Enzo Life Sciences cat. no. ALX-270−204).
The fluorescence signal was monitored using a microplate reader (Envision, Perki-Elmer) using
excitation and emission wavelengths of 340 and 450 nm, respectively. The inhibitor dose−response
curves were analyzed using normalized IC50 regression curve fitting with control based
normalization.
Cellular Assay. p-ERK cellular assay using the AlphaScreen SureFire Phospho-ERK 1/2 Kit
(PerkinElmer) was performed as follows: KYSE520 cells (30,000 cells/well) were grown in 96-
well plate culture overnight and treated with SHP2 inhibitors at concentrations of 20, 6.6, 2.2, 0.74,
0.24, 0.08, and 0.027 μM for 2 h at 37 °C. Incubations were terminated by addition of 30 μL of
lysis buffer (PerkinElmer) supplied with the SureFire phospho-extracellular signal-regulated
kinase (p-ERK) assay kit (PerkinElmer). Samples were processed according to the manufacturer’s
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directions. The fluorescence signal from p-ERK was measured in duplicate using a 2101
multilabel reader (PerkinElmer Envision). The percentage of inhibition was normalized by the
total ERK signal and compared with the DMSO vehicle control.
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Cell Proliferation Assay. Cells (1500-cells/well) were plated onto 96-well plates in 100 μL
medium (RPMI-1640 containing 10% FBS, Lonza). Compounds with various concentrations (1.25,
2.5, 5, 10, and 20 μM) were added 24 h after cell plating. At day 5, 50 μL of Celltiter-Glo reagent
(Promega) was added, and the luminescent signal was determined according to the supplier’s
instruction (Promega).
Crystallization and Structure Determination. Sitting drop vapor diffusion method was used for
crystallization, with the crystallization well containing 17% PEG 3350 and 200 mM ammonium
phosphate and a drop with a 1:1 volume of SHP2 protein and crystallization solution. Crystals
were formed within 5 days and subsequently soaked in the crystallization solution with 2.5 mM of
2. This was followed by cryoprotection using the crystallization solution with the addition of 20%
glycerol and 1 mM compound 7, followed by flash freezing directly into liquid nitrogen.
Diffraction data for the SHP2/compound 2 and SHP2/compound 3 complexes are reported
elsewhere,12,17 and those for SHP2/compound 7 complex (PDB accession 6MD7) were collected
on a Dectris Pilatus 6M detector at beamline 17ID (IMCA-CAT) at the Advanced Photon Source
at Argonne National Laboratories. The data were measured from a single crystal maintained at
100 K at a wavelength of 1 Å, and the reflections were indexed, integrated, and scaled using
XDS.26 The spacegroup of the complex was P21 with two molecules in the asymmetric unit. The
structure was determined with Fourier methods, using the SHP2 apo structure13 (PDB accession
2SHP) with all waters removed. Structure determination was achieved through iterative rounds of
positional and simulated annealing refinement using BUSTER,27 with model building using
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COOT.28 Individual B-factors were refined using an overall anisotropic B-factor refinement along
with bulk solvent correction. The solvent, phosphate ions, and inhibitor were built into the density
in later rounds of the refinement. Data collection and refinement statistics are shown in Table 1 of
the Supporting Information.
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Pharmacokinetics. All animal related procedures were conducted under a Novartis IACUC
approved protocol in compliance with Animal Welfare Act regulations and the Guide for the Care
and Use of Laboratory Animals. Male C57BL/6 mice were obtained from Harlan Laboratories.
Following iv administration (via tail vein) at 1 mg/kg, approximately 50 μL of whole blood was
collected via tail transection, at 0.083, 0.5, 1, 2, 4, and 7 h postdose and transferred to an Eppendorf
microcentrifuge tube containing EDTA. Oral administration (at 5 mg/kg) and collection
procedures were similar to IV, except with whole blood collection at 0.25, 0.5, 1, 2, 4, and 7 h.
The blood was centrifuged at 5000 rpm, and plasma was transferred to a Matrix 96 well plate,
capped, and stored frozen (−20 °C) for parent compound analysis. Samples were precipitated and
diluted with acetonitrile containing internal standard and prepared for LC/MS/MS. An aliquot (20
μL) of each sample was injected into an API4000 LC/MS/MS system for analysis, and transitions
of 352.05 amu (Q1) and 267.10 amu (Q3) were monitored. All pharmacokinetic (PK) parameters
were derived from concentration−time data by noncompartmental analyses. All PK parameters
were calculated with the computer program WinNonlin (Version 6.4) purchased from Certara
Company (St. Louis, MO). For the intravenous dose, the concentration of unchanged compound
at time 0 was calculated based on a log−linear regression of first two data points to back-
extrapolate C(0). The area under the concentration−time curve (AUClast) was calculated using the
linear trapezoidal rule.
The bioavailability was estimated as the following equation:
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AUCinf, p.o.
Dosei.v
.
%F 
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AUCinf,i.v. Dosep.o.
.
7
8
Results are expressed as mean. No further statistical analysis was performed.
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Tumor Xenograft Experiments. All animal studies were carried out according to the Novartis
Guide for the Care and Use of Laboratory Animals. Six-week old female athymic NU/NU mice
(Charles River Labs, MA), were inoculated subcutaneously with either KYSE520 esophageal
carcinoma cells (ATCC) at a concentration of 2 x 106, or Detroit-562 pharyngeal carcinoma cells
(ATTC) at a concentration of 5 x 105 , in a suspension containing 50% phenol red-free matrigel
(BD Biosciences) in Hank’s balanced salt solution. For all PK/PD studies, mice were administered
a single dose of vehicle control (0.5% Methylcellulose, 0.1% Tween 80), 1 or 8, by oral gavage
once tumors reached roughly 300 mm3. Mice were subsequently euthanized at predetermined time
points following a single dose of compound at which point plasma and xenograft fragments were
harvested for determination of 1 or 8 plasma concentrations and PD modulation. Phospho-ERK
PD assessment was carried on lysed frozen tumor tissue fragments following the protocol provided
by the Mesoscale Discovery assay whole cell lysis kits (Catalog#: Total ERK1/2: K151DXD,
phospho-ERK1/2: K151DWD). Protein concentration was assessed by BCA (Pierce, Catalog #
23225) and 2.5mg, for total-ERK, or 20 mg for phospho-ERK, were loaded onto mesoscale plates.
Samples were analyzed by a Mesoscale plate reader. Data was analyzed and fold change in
phospho-ERK1/2 levels were calculated by normalizing to total ERK concentrations and the ratio
of phospho-ERK/total-ERK in the vehicle treated group. DUSP6 PD assessment was carried out
by quantitative real-time PCR. Frozen tumor fragments were processed to extract mRNA (Qiagen
#74106). RNA quantification was performed using Nanodrop 8000. One-step qPCR (Qiagen,
#204645) was performed on the 7900 HT Fast Real-Time PCR system (Themo Fisher Scientific)
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using a human DUSP6 primer set (Hs00737962, Life Technologies) multiplexed with a
housekeeping gene control primer, HPO (#4326314E, Life Technologies). Data was analyzed and
normalized to the expression of a housekeeping gene, human ribosomal protein lateral stalk
subunit P0, HPO, in order to calculate the fold change in mRNA expression with and without
compound treatment. For in vivo efficacy studies, subcutaneously implanted Detroit-562 tumor
xenografts were measured in the two dimensions length and width (l and w, respectively) with
digital calipers once to twice weekly from the start of dosing. Tumor volume was calculated using
a modified ellipsoid formula, where tumor volume (TV) (mm3) = [((l x w2) x 3.14159)) / 6], where
l is the longest axis of the tumor and w is perpendicular to l. Detroit-562 tumor bearing mice were
randomized into treatment groups based on tumor volume mean of 300 mm3 and were dosed by
oral gavage twice daily.
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ANCILLARY INFORMATION
Supporting information. The Supporting Information is available free of charge on the ACS
Publications website (insert DOI here), which includes X-ray data table, additional synthetic
procedures, and Molecular Formula Strings.
Accession Codes. 6MD7 for SHP2 in complex with compound 7. Authors will release the atomic
coordinates and experimental data upon article publication.
Corresponding author. Email: matthew.lamarche@novartis.com; telephone: 617-871-7729.
Novartis Institutes for Biomedical Research, Inc. 250 Massachusetts Avenue, Cambridge, MA
02139.
Author Contributions. The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manuscript.
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Notes. The authors declare no competing financial interest.
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Acknowledgements. Use of the IMCA-CAT beamline 17-ID at the Advanced Photon Source was
supported by the companies of the Industrial Macromolecular Crystallography Association
through a contract with Hauptman-Woodward Medical Research Institute. Use of the Advanced
Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors thank Yue Pan,
Dan Palacios, Joe Wzorek, Jonathan Hempel for editing suggestions, and the entire SHP2 team.
Abbreviations used. PTP, protein tyrosine phosphatase; RAS, rat sarcoma protein; AKT, protein
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kinase B; JAK, Janus kinase; STAT, Signal Transducer and Activator of Transcription proteins.
ERK, extracellular signal-regulated kinase; PI3K, phosphoinositide 3-kinase; MAPK, mitogen-activated
protein kinase; TCEP, tris(2-carboxyethyl)phosphine; IPTG, isopropyl β-D-1-thiogalactopyranoside;
EDTA, ethylenediaminetetraacetic acid; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid;
DTT, dithiothreitol.
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