chapter 7 molecular d s i potent p 3-k (pi3k...
TRANSCRIPT
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CHAPTER 7 MOLECULAR DOCKING STUDIES FOR IDENTIFICATION OF
POTENT PHOSPHOINOSITIDE 3-KINASES (PI3KS) INHIBITORS
7.1 Introduction
Phosphoinositide 3‐kinases (PI3Ks) constitute a class of enzymes that catalyse
phosphorylation of the 3‐hydroxyl position of phosphoinositides (PIs) at the inositol
ring. PI3Ks are classified into three major classes on the basis of substrate specificity
and sequence homology. The class IA PI3Ks, comprised of a regulatory subunit (p85)
and three different catalytic subunits (p110α, p110β, p110δ), are activated by
receptor tyrosine kinases (Katso et al., 2001). They have a vital role in a variety of
physiological processes such as metabolism, regulation, cell survival, mitogenic
signaling, cytoskeletal remodeling and vesicular trafikking (Cantley, 2002; Wymann
et al., 2003).
There is a great deal of interest in PI3Ks as cancer targets, particularly for the
p110α isoform which is mutated and/or over‐expressed in more than 30% of
tumours. The recent discoveries that the p110α isoform undergoes gene
amplification (Stephens et al., 2005) and is frequently mutated in primary tumors
(Gymnopoulos et al., 2007; Samuels et al., 2004; Vogt et al., 2007), together with
evidence that PTEN, a lipid phosphatase which acts as a negative regulator, is a
commonly inactivated tunor suppressor (Cantley et al., 1999), have genetically
validated p110α as an attractive target for cancer therapy. Thus, inhibitors of these
enzymes are expected to be useful in cancer treatment and hence have been
extensively explored as an attractive therapeutic candidates (Marone et al., 2008).
In this chapter we focus on design of specific inhibitors for class I PI3K,
particularly PI3Kα using computational approach. Since, the 3D crystal structure of
Pi3K‐α is available, we largely focus on structure based design.
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7.2 Material and methods
The crystal structures of Pi3K available in PDB were downloaded for the present
study. The compounds belonging to the series viz. Imidazoquinoline, liphagal and
tetrazolyl quinazolinone were used for generating virtual library for docking studies.
For customization of the exsiting software for our target a quantitative relationship
between the various docking scoring functions and Pi3K inhibitory activity was
established using ligand based studies.
7.2.1 Docking studies of PI3K isoforms with the ligands
All the computational studies were carried out in the Schrodinger suite 2012
molecular modeling software. The 2D structures of all the molecules were built in
the maestro window. All the molecules were then converted to their respective 3D
structure, with various conformers, tautomers and ionization states using the
Ligprep and Confgen modules (Watts et al., 2010; Chen et al., 2010). The molecules
were then minimized using the OPLS_2005 force field. The 3D crystal structures of
PI3Kα (PDB ID: 3HHM) (Mandelkar et al, 2009) and PI3Kδ (PDB Id: 2WXF) (Berndt et
al., 2010) reported in Protein Data Bank (PDB) were used as receptors for docking
studies. The proteins were downloaded from the PDB and were prepared for docking
using the Protein Preparation wizard. Hydrogens were added to the proteins and the
missing loops were built. Bond length and bond order correction was also carried out
for preparing the proteins for docking studies. The active site grid was generated
based on the already co‐crystallized ligand of the receptor using receptor grid
generation module. The co‐crystallized ligand was extracted and docked again in
order to establish a docking protocol for the selected target. Further the ligand
dataset was docked on to the receptor through the identified grid using Glide
module and flexible docking was carried out for all the conformers in order to find
out the binding mode of these ligands. The standard precision (SP) scoring function
of Glide was used for carrying out these studies (Friesner et al., 2004; Halgren et al.,
2004). The detailed methodology is described in the Methodology chapter
(Section3.3, Chapter 3).
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7.2.2 Selection of dataset (ligand compounds)
The following class of compounds were selected for the docking studies:‐
i. Imidazoquinoline analogs (NVP‐BEZ235) – The parent compound being from
Novartis, and the first PI3K inhibitor to enter the clinical trials. Presently it is
in Phase I clinical trials. BEZ235 is an imidazoquinoline derivative and a known
PI3K inhibitor. A virtual library of compounds was created based on this
compound and docked onto the crystal structure of PI3K. Grid was generated
based on the co‐crystallized structure and all the molecules of BEZ library
were docked on to that grid.
Preliminary docking studies for the following series of compounds have also
been initiated:‐
ii. Liphagal analogs – Liphagal was first selective inhibitor of PI3Kα.
iii. Tetrazolyl Quinazolinone analogs (CAL‐101) – The parent compound showed
promising Phase I results and presently is in early Phase – II clinical trials.
CAL101 is a selective PI3K class I inhibitor of Pi3Kδ. Hence, docking studies of
its analogs were carried out on both PI3Kα and Pi3Kδ in order to compare the
ligand‐receptor interaction and dock score of these two receptors with
tetrazolyl quinazoline analogs.
7.2.3 Quantitative energy value scoring function activity relationship
In order to customize the generic software Schrodinger for the target Pi3K‐α, a
quantitative regression analysis was carried out between the experimental IC50
values of the known inhibitors (Hayakawa et al., 2006; Hayakawa et al., 2007(a‐c))
and the scoring functions used for their docking analyses with the selected target
(PI3Kα). A data set of 65 molecules was taken from literature for this study which
was suitable divided into training set data and test set data. A highly statistical model
was prepared for the theoretical evaluation of the inhibitory activity of a new
molecule towards PI3Kα. The model along with dock scores and interaction studies
(of ligand with target) will also help in lead optimisation and would contribute
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towards the identification of a novel PI3Kα inhibitor. The detailed QSAR
methodology is described in the methodology chapter (Section 3.5, Chapter 3).
7.3 Results and Discussion
In order to understand the constitution of PI3Kα, the 3D crystal structure of Pi3K was
analyzed with respect to various domains present in it. PI3Kα comprises of Adaptor
Binding Domain (ABD), RAS Binding Domain (RBD), C2, Helical and the Kinase
domain. Further, the kinase domain is divided into N‐Lobe and C‐Lobe, and contains
some specific interesting regions for the ligand recognition and binding such as
Activation loop, Hinge region, Gatekeeper residues etc. (as shown in Figure 7.1). The
3D crystal structure was analyzed using the software Schrodinger, in order to identify
these regions within the structure. Also, the respective location of the co‐crystallized
and other docked ligands was determined within the structure, with respect to the
identified regions of the kinase.
Table 7.1: Reported 3D structures for human PI3Kα
S.NO PDB ID LENGTH LIGAND CHAINS SPECIES
1. 3HHM* 1091,373 PRESENT A,B Homo sapiens
2. 3HIZ* 1096,373 ABSENT A,B Homo sapiens 3. 2ENQ 158 ABSENT A Homo sapiens 4. 2RD0 1096,279 ABSENT A,B Homo sapiens
The above given table (Table 7.1) shows the reported 3D structure for human PI3K‐α
of which 3HHM and 3HIZ are mutants (His1047Arg) where as 2ENQ and 2RD0 are the
wild types. 3HHM was co‐crystallized with ligand (wortmanin). Since more than
1,500 PIK3CA mutations in diverse tumor types have been discovered (Liu et al.,
2009). The most common mutant is His1047Arg (Mandelkera et al., 2009), and
mutations at this residue in breast and uterine cancers are associated with clinical
prognosis (Mandelkera et al., 2009; Janku et al., 2011; Flavia et al., 2012), so we
took 3HHM 3D structure for the docking analysis.
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Figure 7.1. Structural analysis and identification of ligand recognition regions of Pi3K-α.
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The co‐crystallized ligand wortmanin was extracted from the downloaded 3D
crystal structure and was again docked using several docking protocols in Glide. The
best conformation match was observed using the standard precision (SP) scoring
function of Glide. The co‐crystallized and docked pose of wortmanin within the
binding pocket of PI3K‐α is shown in Figures 7.2(a) and 7.2(b) respectively.
Figure 7.2 Orientation of wortmanin within the binding pocket of PI3K-α in (a) 3D crystal structure 3HHM in PDB and (b) obtained using docking studies.
(a)
(b)
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7.3.1 Molecular docking studies and interaction analysis of Imidazoquinoline
analogs with PI3Kα
BEZ235 is an imidazoquinoline derivative and a known PI3K inhibitor. A virtual library
of about 500 compounds was created based on this compound and docked onto the
crystal structure of PI3K. Grid was generated based on the co‐crystallized structure
and all the molecules of BEZ library were docked on to that grid. The top 20
molecules (ranked better than the co‐crystallized ligand) on the basis of their dock
scores are shown in table 7.2.
From a library of about 500 molecules designed on the basis of SAR studies
reported in literature, 20 molecules showed better predicted binding affinity than
the standard (BEZ235), based on the dock score. The interaction of the best analog
i.e., IZQ23 showed the involvement of hinge region residue Val851 in H‐bonding with
the ligand, apart from Ser774 and Ser854. The extra stability to the ligand is
proposed to be provided by the hydrophobic cleft formed by the residues Trp780,
Leu807, Tyr836, Ile848,Val850, Val851, Met922, Phe930, Ile932 and Phe934 with
Trp780 and Tyr836 also involved in π‐π interaction with the ligand as shown in Figure
7.3.
It was observed that the sulphonate group increases the binding affinity of
the ligand within the binding pocket of PI3K‐α by orienting it in such a fashion that
the oxygen of the sulphonate is involved in H‐bonding with Ser854 and the hinge
residue Val851 is involved in H‐bonding with hydrogen of the amide group. From the
dock scores of other ligands in the library it was observed that the presence of
halogen increases the affinity towards the receptor. The structures with dock scores
of the molecules possessing better affinity than the standard are given in Table 7.2.
Besides this, docking studies were also initiated with the other two series of
compounds viz., liphagal analogs and tetrazolyl quinazolinone series. The validation
(as well as feedback for modification) of the in silico studies was carried out by wet
lab experiments carried out at Pharmacology division, IIIM Jammu.
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Figure 7.3. I
nteraction of tthe in silico beest Imidazoquinoline analog within the bin
ding pocket of
158
f Pi3Kα
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Table 7.2: Dockscores of BEZ library having scores better than the co-crystallized ligand (wortmanin). S.No Molecule Name Structure Dockscore 1 IZQ23
N
NN
O
CH3
CH3
N
CH3
N
NHS
CH3
OO
‐11.40
2 IZQ45
N
NN
N
CH3
CH3
N
O2N
N
O
NHCH3
‐10.71
3 IZQ11
N
NN
O
CH3
CH3
N
CH3
S
N
O
‐10.61
4 IZQ8
N
NNH
O
CH3
CH3
N
OCH3
O
‐9.31
5 IZQ15
N
NN
O
CH3
CH3
N
CH3
N
N
‐9.22
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6 IZQ10
N
NN
O
CH3
CH3
N
O
CH3
‐9.12
7 IZQ4
N
NNH
O
CH3
CH3
N
Cl
‐9.08
8 IZQ5
N
NNH
O
CH3
CH3
N
NO2
‐8.98
9 IZQ13
N
NN
O
CH3
CH3
N
CH3
N
N CH3
‐8.89
10 IZQ18
N
NN
O
CH3
CH3
N
CH3 NH
NH CH3
O
‐8.89
11 IZQ19
N
NN
O
CH3
CH3
N
CH3O
NH CH3
‐8.43
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12 IZQ16
N
NN
O
CH3
CH3
N
CH3N
OCH3
‐8.40
13 IZQ43
N
NN
N
CH3
CH3
N
O2N
CH3
‐8.34
14 IZQ20
N
NN
O
CH3
CH3
N
CH3
N
‐8.29
15 IZQ39
N
NN
N
CH3
CH3
N
O2NNH
N
‐8.08
16 IZQ33
N
NN
N
CH3
CH3
N
O2N
O
‐8.06
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17 IZQ22
N
NN
O
CH3
CH3
N
CH3
NH
OCH3
‐8.05
18 IZQ2
N
NN
O
CH3
CH3
N
CH3
F
‐7.89
19 IZQ1
N
NN
O
CH3
CH3
N
F
F
CH3
‐7.88
20 IZQ3
N
NN
O
CH3
CH3
N
CH3
Cl
‐7.83
21 Wortmanin ‐7.79 22 BEZ235
(standard)
O
N
N
N
N
N
CH3
CH3
CH3
‐7.57
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7.3.2 Molecular docking studies of Liphagal derivatives and Tetrazolyl
Quinazolinone series with PI3Kα
Docking studies were also initiated on Liphagal derivatives and tetrazolyl
quinazolinone series. Liphagal is the selective inhibitor of PI3kα. A series of liphagal
analogs were generated based on the available patent space and the constituent of
the binding pocket of PI3kα. Whereas, the purinyl‐quinazolinone scaffold (CAL‐101)
showed some promising Phase‐I results and presently is in Phase‐II studies. CAL101 is
a selective PI3K class I inhibitor of PI3Kδ. Based on the inputs from our medicinal
chemistry lab with respect to the available patent space, a series of molecules were
designed and docked onto the 3D structure of PI3Kα and PI3Kδ.
Majorly, all the molecules showed comparable dock scores with liphagal,
except for a boronic acid derivative of liphagal that showed exceptionally good
scores. From the interaction figure of liphagal (Figure 7.4), it was observed that
Val851, the hinge residue is involved in H‐bonding with one of the hydroxyl group in
the phenyl moiety of the molecule. Also, the formyl group is involved in H‐bonding
with Gln859. The phenyl is involved in π‐ π interaction with Trp780. From the
docking studies of the liphagal derivatives, it was observed that the boronic acid
derivatives shows significantly enhanced binding affinity towards the target. The
introduction of boronic acid strengthens the H‐bonding with the pivotal residues
such as Val851 and Gln859 and further the orientation is quite favourable for π‐ π
interactions with Trp780. A small hydrophobic cleft formed by the residues Ile800,
Tyr836, Ile848 and Val850 also may help in the strong interaction between the ligand
and the receptor (as shown in figure 7.4). The validation (as well as feedback for
modification) of the in silico studies was carried out by wet lab experiments carried
out at Pharmacology division, IIIM Jammu.
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Figure 7.4. Iof PI3K-α
nteraction of (a) liphagal annd (b) boronic acid derivative
e of liphagal wwithin the bindi
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ing pocket
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From the docking studies of tetrazolyl quinazolinone series, it was observed
that the series has better affinity towards PI3Kδ than PI3K‐α. The dock scores of
PI3Kα with all the molecules was much higher (less binding affinity) than the
standard. The binding affinity of all the molecules docked on PI3Kδ was also
comparatively lesser than the standard, except for a boronic acid derivative of
tetrazolyl quinazolinone (Figure 7.5) where it was well comparable with the
standard. Once again, the role of boronic acid in ligand binding was surfaced out. It
was observed that boronic acid plays an important role in the ligand binding. Based
on the interaction figure, no specific residue could be identified for PI3Kδ that tends
to play a crucial role in ligand binding, unlike the case of its other isoform PI3Kα.
Though, Glu826 & Val828 and Lys779 & Asp911 are involved in H‐bonding with the
standard (CAL101) and the boronic acid derivative respectively (Figure 7.5). Further
studies on this would be taken up on the form of a project. The validation (as well as
feedback for modification) of the in silico studies was carried out by wet lab
experiments carried out at Pharmacology division, IIIM Jammu.
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(a)
(c)
Figure molecuPI3Kα, the bindacid dethe bind
7.5 Interactiole i.e., CAL101(b) standard ding pocket o
erivative of teding pocket of
on figures o1 within the bimolecule i.e.,
of PI3Kδ and etrazolyl quinaf PI3Kδ
166
(b)
of (a) standainding pocket CAL101 with (c) the boronazolinone with
)
ard of hin nic hin
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Further, in order to customize the generic software Schrodinger for the target
PI3Kα, a quantitative regression analysis was carried out between the experimental
IC50 values of the known inhibitors (from literature) and the scoring functions used
for their docking analyses with the selected target (PI3Kα). A data set of 65
molecules was taken from literature (Hayakawa et al., 2006; Hayakawa et al.,
2007(a‐c)) for this study which was suitably divided into training set data and test set
data. A highly statistical model was prepared for the theoretical evaluation of the
inhibitory activity of a new molecule towards PI3Kα. The model along with dock
scores and interaction studies (of ligand with target) will also help in lead
optimisation and would contribute towards the identification of a novel PI3K‐α
inhibitor.
Final Model:
• Activity =11.06 ‐1.96 *r_glide_res:A836_dist + 1.45 *r_glide_XP_lipophilic
Evd W‐ 1.22 * r_glide_res:A856_Vdw ‐ 0.46 * r_glide_res: A854_Eint ‐0.13 *
r_i_glide_ecoul +0.82 * r_glide_res: A932_dist ‐2.37 * r_glide_res:A
930_vdw+3.33 * r_glide_res:A 836_h bond
• N=50; LOF=0.459; r2 =0.882; r2adj=0.858
• Ftest=38.152; LSE=0.212; r=0.939; q2=0.824
7.4 Conclusion
From the available 3D crystal structure of PI3K‐α and its docking studies with several
classes of compounds, it was observed that the hinge region (Val851) plays a major
role for the identification and binding of ATP competitor inhibitor. Besides this,
Tyr836, Gln859 also plays an important role. The role of aldehyde moiety in liphagal
was also identified from its interaction, where Gln859 is directly involved in H‐
bonding with the formyl group of liphagal. Allosteric site is proposed to be nearby
the ATP binding site, but the effective residues are still not clear. The role of boronic
acid moiety in the ligand significantly improves the binding for PI3Kα as well as
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PI3Kδ. It was also observed that the tetrazolyl quinazolinone derivatives has better
affinity towards PI3Kδ than PI3Kα. In order to customize the existing generalized
docking software, a QEvAR study was carried out based on the known PI3Kα
inhibitors where activity was made a dependent variable and the scoring functions
used in Glide as the dependent variables.