Identification of novel amino acid derived CCK-2R antagonists as potential antiulcer agent: Homology modeling, design, synthesis, and pharmacology

Article abstract of DOI:10.1021/ci3003655

The present study revisited the three-dimensional (3D) homology model of CCK-2R using human A_2a adenosine receptor and the resolved NMR based structure of the third extracellular loop of the CCK-2R as templates. Further in order to identify novel antiulcer agents, rational designing have been performed utilizing the substructure of a well-known CCK-2R antagonist benzotript as a lead molecule and submitted to the combined docking and simulation studies. This led to the understanding of the essential structure requirement as well as variation of binding mode among conformational isomers of small molecule CCK-2R antagonists. In the next step, preparation of each configurational isomer of these molecules was carried out and submitted for their in vitro activity followed by in vivo screening into antiulcer rat model. The biological screening of these compounds has not only validated the developed homology model of CCK-2R but also led to the identification of highly potent CCK-2R antagonist 6a as an orally active and safe candidate molecule having better antiulcer properties than the well-known drug benzotript.

Full text of DOI:10.1021/ci3003655

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Article
Identification of novel amino acid derived CCK-2R antagonists as potential
Anti-Ulcer agent: Homology Modeling, Design, Synthesis, and Pharmacology
Anil K. Saxena, and Amit K Gupta
J. Chem. Inf. Model., Just Accepted Manuscript • DOI: 10.1021/ci3003655 • Publication Date (Web): 16 Dec 2012
Downloaded from http://pubs.acs.org on December 27, 2012
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Identification of novel amino acid derived CCK-
2R antagonists as potential Anti-Ulcer agent:
Homology Modeling, Design, Synthesis, and
Pharmacology
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Amit K. Gupta#§, Kanika Varshney#§, Neetu Singh‡, Vaibhav Mishra‡, Mridula Saxena§$, Gautam
Palit‡, and Anil K Saxena§*
§Medicinal and Process Chemistry Division, ‡ Pharmacology Division, CSIR.-Central Drug
Research Institute, Lucknow, 226001, India
$ Authors present address: Department of Chemistry, AMITY University, Lucknow campus,
Lucknow.
RECEIVED DATE (to be automatically inserted)
TITLE RUNNING HEAD:
*To whom correspondence should be addressed.
Mailing address: Medicinal and Process Chemistry Division, C.S.I.R.-Central Drug Research
Institute, Lucknow, 226001, India.
Tel: +9152226124112/ex 4386, Fax: +9152226123405. Email: anilsak@gmail.com
# These authors have contributed equally to this work.
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Abstract
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The present study revisited the three-dimensional (3D) homology model of CCK-2R using
human A2a adenosine receptor and the resolved NMR based structure of the third
extracellular loop of the CCK-2R as templates. Further in order to identify novel antiulcer
agents, rational designing have been performed utilizing the sub structure of a well known
CCK-2R antagonist benzotript as a lead molecule and submitted to the combined docking and
simulation studies. This led to the understanding of the essential structure requirement as well
as variation of binding mode among conformational isomers of small molecule CCK-2R
antagonists. In the next step preparation of each configurational isomer of these molecules
were carried out and submitted for their in vitro activity followed by in vivo screening into
antiulcer rat model. The biological screening of these compounds has not only validated the
developed homology model of CCK-2R but also led to the identification of highly potent
CCK-2R antagonist 6a as orally active and safe candidate molecule having better antiulcer
properties than the well known drug benzotript.
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Keywords: Homology model; Sub-structure based drug discovery; CCK-2 receptor
antagonists; Synthesis; Pharmacology.
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1. Introduction.
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The cholecystokinin-(CCK) receptor plays a major role in the pathophysiology of
gastrointestinal disorders such as acid reflux, gastroesphogeal reflux disease, peptic ulcers1 as
well as GI adenocarcinoma2 and tumors.3 The CCK-Rs belong to G-protein coupled receptor
(GPCR) superfamily characterized by the presence of seven transmembrane (TM) domains
separated by alternating three intracellular (N-terminal) and three extracellular (C-terminus)
loop regions.4 The CCK-Rs have been classified into two distinct classes designated as CCK-
1 and CCK-2 on the basis of their affinity for the peptide agonists CCK and gastrin. Although
both CCK and gastrin share an identical carboxyl-terminal pentapeptide sequence yet they
differs towards their selectivity for CCK-1 and CCK-2Rs.5-6 The CCK-1Rs are specific for
CCK, whereas CCK-2Rs recognize both CCK and gastrin with high affinity. The
gastrointestinal polypeptide hormone gastrin has been known to stimulate gastric acid
secretion and gastrointestinal cell growth in peripheral tissues. Hence, efforts have been made
to develop agents that inhibit gastrin secretion, by acting through CCK-2R antagonists.7
Since Merck discovered asperlicin8 as the first natural product based non-peptide CCK-R
antagonist, the quest for the drug that may block the actions of the peptide hormone gastrin
has been started which led to the discovery of a wide range of small-molecule ligands.9
Although numerous non-peptide CCK-2R antagonists have been forwarded as potential
therapeutic agents for gastrointestinal disease, to date their promise remains unrealized10 due
to their unfavorable and variable biological effects.11 Therefore, there is still a considerable
interest in the development of novel small molecule CCK-2R antagonists with improved
biological profile. The knowledge about the 3D structure of the target may significantly add
in the understanding of essential structural requirements as well as biological activity
variation among the different ligands acting on the same target. The exact co-crystallized
structure of this receptor is still unknown presenting a major bottleneck for structure based
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discovery of novel CCK-2R antagonists and therefore earlier attempts have been made to
predict its 3D structure. Till date all of the reported CCK-2R homology models have been
basically derived from a homology model described by the Jagerschmidt et al. built several
years ago.12 This CCK-2R model was itself obtained from an AT1a receptor homology
model13 and was constructed using the transmembrane (TM) helical positions found in
bacteriorhodopsin structure. This starting CCK-2R model was then successively modified to
take into account new data coming from improvements of inactive bovine rhodopsin three-
dimensional structures.14-15 The major drawbacks of using bovine rhodopsin as template
include the less similarity (29%) and higher gap (5%) with rat CCK-2R and the divergences
among ligand binding sites. Further, in the light of resolved NMR based structure of the third
extracellular loop of the CCK-2R (PDB ID: 1L4T) 16 there is a need to rebuilt and validate the
3D structure of CCK-2R.
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The present study revisited the three-dimensional (3D) homology model of CCK-2R using
human A2a adenosine receptor (PDB ID: 3EML)17 and the resolved NMR based structure of
the third extracellular loop of the CCK-2R (PDB ID: 1L4T) as templates. This is followed by
the designing of molecules utilizing the substructure of well known CCK-2R antagonist
benzotript. Rigorous docking and simulation studies have been carried out to understand the
essential structure requirements as well as variation of binding modes among conformational
isomers of these ligands and all the configurational isomer of these molecules were prepared.
The detailed biological screening of these compounds validated the plausible homology
model of CCK-2R and also led to the identification of compound 6a with better antiulcer
properties than the known drug benzotript.
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2. RESULT AND DISCUSSION
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2.1. Generation of Preliminary Homology model of CCK-2R. The BLAST analysis for the
rat CCK-2R (Swissprot entry code P30553)18 showed less identity (29%) and higher gap
(5%) for bovine rhodopsin (PDB ID: 1F88) whereas the human A2a adenosine receptor (PDB
ID: 3EML) showed the highest identity (32%) with least gap (1%) for among the GPCRs
class having PDB coordinates. This result suggests the human A2a adenosine receptor as a
better candidate of choice as template for rat CCK-2R. However since the third extracellular
loop structure of CCK-2R has been resolved (1L4T), it showed 96% identity with rat CCK-
2R and hence this loop has been also incorporated in building the homology model.
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In order to find out the homology between binding sites of rat CCK-2R with other PDBs a
comparison of sequence identity and similarity was made. The global sequence identity and
similarity between the rat CCK-2R and bovine rhodopsin was found as 16% and 29 %
respectively whereas the same was found as 17% and 32% respectively between the rat CCK-
2R and the human A2a adenosine receptor. However, these values further increased to 30%
and 57% if only transmembrane (TM) region was considered. This analysis further projected
the human A2a adenosine receptor as best candidate for template selection. Therefore, in view
of the drawbacks of the earlier studies as described above and in the introduction section, a
plausible homology model of the rat's CCK-2R was generated using Modeller9v919
considering a high-resolution crystal structure of human A2a Adenosine Receptor (PDB ID:
3EML) as main template and incorporating the NMR based resolved structure of the third
extracellular loop of the CCK-2R (PDB ID: 1L4T) as a template of this region. The amino
acid sequence of the CCK-2R was retrieved from Swissprot database (entry code: P30553)18
and initially ClustalW based multiple sequence alignment was performed using these two
templates using “align2d_mult” script implemented in the Modeller9v9 so that motifs
common to all GPCRs and conserved amino acids between these sequences precisely
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matched. However, manual alignment of second template (PDB ID: 1l4t) was performed to
place it at the third extracellular loop of the CCK-2R. Highly conserved residues in each TM
were anchored and the 10 plausible models for CCK-2R were generated using “model-mult”
script of the modeler v9.9 (Figure 1). Among the generated models, the best model was
selected on the basis Ramachandran plot, low restraint violations and low numbers of main-
and side-chain bad conformations.
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Figure 1. Multiple sequence alignment of human A2a adenosine receptor (PDB ID: 3EML) and third
extracellular loop of the CCK-2R (PDB ID: 1L4T) with rat CCK-2R (entry code: P30553). The 7
TMs (TM1-TM7) are highlighted by green rectangle and are separated by extracellular (ECL1-3)) and
intracellular (ICL1-3) loops. The identical residues are shown in red color while residues with
moderate and weak similar residues are shown in orange and light yellow colors.
Though the homology model reported by us belongs to the rat family of CCK-2R utilizing the
human AT1a receptor but in view of 88% sequence identity between human and rat CCK-2R a
comparison of sequence alignment has been performed between the alignment reported in the
present manuscript with the alignment reported by the surgand’s et al. 20 where the authors
have described the human CCK-2R under human peptides receptor cluster and performed the
alignment of 26 receptors in this category to obtain the nature of critical residues and in
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constructing the phylogenetic tree.20 The alignment comparison studies showed that our
alignment corresponds well with the earlier studies where all the specified residues in the
assigned TM helixes were also found in the same TM helixes. Alignment comparison in
terms of highly conserved, moderately conserved and less conserved residues was also in
agreement with the earlier studies where the F6.44 ( Phe347) and W6.48 ( Trp351) residues were
also found highly conserved in our study similar to the earlier study as shown in Figure 1
whereas among the moderately conserved Y1.39 (Tyr61), I1.42 (Ile64), and V3.36 (Val138 )
residues Ile64 was found highly conserved, Val138 was found less conserved but Ile61 did
not showed any similarity in our study. Among the less conserved C2.57 (Cys107) M2.58
(Met108), I4.56 (Leu185), and W5.38 (Trp208) residues reported by Surgand’s et al. Cys107
was also found less conserved, Met108 was found moderately conserved while Leu185 and
Trp218 were found highly conserved in our study.
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2.2. Model refinements.
2.2.1. Loop refinement and solvent based molecular dynamics simulations. This above
homology model was imported to the maestro panel of the Schrodinger software package21
where it was examined through the Ramachandran plot and the loops with outlier residues
were refined using the "refine loops" tool with the "extended medium-serial-loop sampling"
procedure of the prime module of Schrodinger software package. Further, it was backbone-
constrained energy minimized through OPLS 2005 force field using MacroModel.22 The
obtained CCK-2R model was further inspected for its stability through C-α constrained low-
temperature (300K) molecular dynamics simulation (MDS) for 1000ps where all possible
translational and rotational movements of individual TM helices were taken into account
using MacroModel. The graphical plot analysis of Cα backbone rmsd, potential energy and
solvation energy of 100 sampled structures of 1000ps MDS further substantiated the stability
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for the developed plausible CCK-2R model (Figure 2). However, among the 100 sampled
structures the plausible CCK-2R, the model with minimum potential energy state and
favorable backbone conformation was submitted to the various online servers based model
quality assessments.
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Figure 2. Graphical plot analysis of potential energy, Cα backbone rmsd and solvation energy from
100 sampled structures of loop refined CCK-2R model
.
2.2.2. Model Quality Assessment. In view of inability of one single method to consistently
and accurately predict the three-dimensional structure of a protein, the resulting CCK-2R
homology model was subjected to different QA methods namely Procheck,23 ProQ,24 ProSA-
web,25a,b and QMEAN.26 Based on the data from various MQA methods (provided in
Supporting Information), the best homology model, with no outlier in Procheck, LGscore of
2.52 in ProQ (≥ 2.5 for very good model), QMEAN score of 0.50 (estimated model reliability
lies between 0-1) and Z-score (ProsaWeb) of -3.40, was selected for further studies.
2.3. Application of modeled CCK-2R in the substructure based designing of novel CCK-
2R antagonists. The application of homology model for new lead discovery, and further lead
optimization has already been established.27 The binding domain of amino acid derived
category of CCK-R antagonists is supposed to be same as the natural ligand. Despite of their
low CCK-R binding the advantage of this class of molecules includes the inhibition of opioid
receptors resulting in the diminishing of ulcer pain.28 Earlier, the semi rigid (cyclised)
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racemic analogues of benzotript were synthesized by our and other research groups which
showed potential anti-ulcer activity.29, 30a-b, 31a-b The benzotript is a flexible molecule, and it
may have many possible conformations. It is a well known that among the many possible
conformations or rotameric forms of a molecule, only a certain discrete conformation(s) are
responsible for the specific biological response. Therefore to predict the biological active
conformation the benzotript was submitted to the combined docking and simulation protocol.
The receptor grid (active site of the protein) was generated using the residues namely His207
(EL2), Tyr189 (TM4), and Asn358 (TM6) as these residues were earlier reported to involve
in the high affinity binding of the CCK to the active site of CCK-2R through site directed
mutagenesis studies.32 The structure of benzotript was drawn in the maestro panel of the
Schrodinger software while the ligand preparation has been performed using the Ligprep
protocol. To ensure the correct binding, the ligand protein complexes were subjected to low-
temperature molecular dynamics simulation (MDS) for 1000 ps using macromodel22 where
the graphical plot analysis of Cα backbone rmsd, potential energy and solvation energy of
protein ligand complex of 100 sampled structures for each ligand substantiated the stability of
the bound ligand into the active site of CCK-2R ( Figure 3).
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Figure 3. Graphical plot analysis of Cα backbone rmsd, potential energy and solvation
energy from 100 sampled structures of docked pose of benzotript-CCK-2R complex.
The binding site analysis of benzotript clearly deciphers that the hydroxylic group and
ketonic oxygen of carboxylic acid showed the H bond interaction with Val214, Arg361 and
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Ser219 respectively. Similarly, the oxygen of benzoyl group of the molecule showed the H
bond interaction with Asn358 (Figure. 4A and B). The 4-chloro of the benzoyl group showed
hydrophobic interactions with Tyr132, Meth134, and Trp351 while the aromatic ring itself
occupied the hydrophobic cavity formed by the residues Tyr132, Trp351, and Val214.
Similarly, the indole ring of the benzotript occupied the cavity made by the residues namely
Pro376, Arg361, Ile377, Ala368, and Leu372 (Figure. 4B).
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Figure. 4. Binding pose analysis of benzotript at the binding site of CCK-2R; (A). comparative 3D
binding pose view of benzotript (pink color), (B). 2D binding pose view of benzotript.
29,
In our continued efforts to design the novel CCK-2R antagonists with improved activity
30a-b, 33 the same cyclized core of benzotript was utilized to rationally design the novel CCK-
2R antagonists with improved antiulcer activity (Figure 5). The chlorine atom at para position
on phenyl group was substitution with propyl and methyl groups (Figure 5) as the
incorporation of n-alkyl group (methyl, propyl) was considered to add more hydrophobic
volume at this position. Further, the carboxylic group present at position 3 of benzotript was
replaced by its methyl ester as methyl esters have shown better antiulcer properties than their
corresponding acids in our earlier studies.29
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O
O
O
H
O
3
2
1
NH
NH
N
H
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O
N
H
R1
Cl
Benzotript
Compound 5-8
Figure 5. Substructure based approach to design compound 5-8.
2.4. Docking and simulation studies on designed CCK-2R antagonists.
A rigorous docking and scoring studies at the same receptor grid have been performed in
order to compare and predict the binding mode among the configurational isomers of these
ligands with benzotript. All the reported molecules along with benzotript were drawn in the
maestro panel of the Schrodinger software where the specific chirality of these molecules
were assigned while the ligand preparation has been performed using the Ligprep protocol.
The induced fit docking results into many binding poses among which the best docking pose
was initially selected on the basis of visual inspection of important residue interaction (either
of His207, Tyr189 or Asn358) with this ligand and thereafter highest docking score. To
ensure the correct binding, the ligand protein complexes were subjected to low-temperature
molecular dynamics simulation (MDS) for 1000 ps using macromodel22 where the graphical
plot analysis of Cα backbone rmsd, potential energy and solvation energy of protein ligand
complex of 100 sampled structures for each ligand substantiated the stability of the bound
ligand into the active site of CCK-2R. The structures of rationally designed compounds along
with their Glide docking score has been listed in Table 1.
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Table 1. Summary of CCK-2R antagonist activity of the synthesized compounds along with
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6
their Glide docking scores.
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S.No.
R1
Compound
Name
Chirality
IC50
Glide
docking
score
-8.46
-6.83
-8.98
-8.46
-8.43
-6.60
-8.68
-7.52
-9.29
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(µM)
n-propyl
n-propyl
n-propyl
n-propyl
methyl
methyl
methyl
methyl
-
(1S, 3S)
(1R, 3S)
(1R, 3R)
(1S, 3R)
(1S ,3S)
(1R ,3S)
(1R, 3R)
(1S,3R)
-
139.70 8.19
278.80 10.45
31.93 4.68
171.00 7.39
180.12 9.60
267.66 9.81
64.60 8.93
261.12 9.12
39.63 6.32
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9
5a
5b
6a
6b
7a
7b
8a
8b
Benzotript
The docking study showed the crucial role of Asn358 in the modulation of small molecule
CCK-2R antagonists as benzotript and other active CCK-2R antagonists. The binding site
analysis of each configurational isomer of these compounds revealed that the β carboline ring
of these molecules occupied the same binding cavity as with the indole ring of the benzotript
(Figure 6A, 6B, and 6C).
Figure 6. Comparison of binding pose view of (A). Benzotript (cyan colored carbon) along
with compound 6a (pink colored carbon), (B). compound 6a (green colored carbon),
compound 6b ( pink colored carbon), 5a ( blue colored carbon), and 5b(grey colored carbon)
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at the active site, (C). compound 8a (green colored carbon), 8b (pink colored carbon), 7a
(blue colored carbon), and 7b (yellow colored carbon) at the active site.
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9
More interestingly, the docking and simulation study revealed that the pendant phenyl ring
attached at position 1 of the β-carboline nucleus in all of these molecules occupied the same
hydrophobic cavity as with the phenyl ring of the para-chlorobenzoyl group of the benzotript.
In order to validate the developed homology model and to identify novel CCK-2R antagonist
all the four configurational isomers of each of these two molecules were prepared and
evaluated for their CCK-2R antagonistic activity.
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2.5. Chemistry. The synthesis of the 1,3-disubstituted tetrahydro-β-carbolines is shown in
Scheme 1. Both D (3R) and L (3S)-tryptophan methyl esters were synthesized by our earlier
reported procedure.34 The D and L esters on Pictet-Spengler cyclisations with 4-
propylbenzaldehyde and 4-methylbenzaldehyde gave the corresponding methyl 1-(4-
substituted phenyl)-2,3,4,9-tetrahydo-1H-pyrido[3,4-b]indole-3-carboxylates as a mixture of
both the cis and trans isomers as expected diastereomers. The diastereomers were separated
by column chromatography using chloroform: n-hexane as an eluent. As the Pictet-Spengler
reaction was carried out at under non stereospecific conditions, the kinetics were in support
of the cis-isomer.35 Thus, the formation of compound 5a, 6a, 7a and 8a exceeded the
formation of their counterpart trans diastereomers 5b, 6b, 7b, and 8b with % diastereomeric
excess (de) of 33%, 38%, 32% and 34%, respectively. The assignment of cis/trans-
stereochemistry for the tetrahydro-β-carbolines (compound 5-8) was based on a detailed
study of 1H & 13C NMR spectroscopic data well established in previous literature.36 In the 1H
proton NMR of the cis-isomer the H-1 signal was more up field as compared to that of trans-
isomer may be because of deshielding of the H-1 in the latter by carboxylate group at 3-
position. The 13C NMR signals for C-1 and C-3 were shifted more upfield in the trans-isomer
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in relation to cis-isomer with ꢀδ 3-4 ppm. This may be due to the presence of 1,3-diaxial
spatial interactions in the trans-isomer, where tetrahydropyridine ring exists in half-chair
conformation with the ester at C-3 located equatorially. No similar difference was detected
with C-3 in compounds 5-8. Moreover, a 300 MHz 2D NOESY experiment was done to see
the correlation between the C-1 and C3-hydrogens for cis-5a and trans-5b. The spectrum
showed a clear correlation between the C-1 and C3-hydrogens as expected for a cis-
relationship (figures in supporting information). Mass spectrometry of all derivatives showed
molecular ion peaks at M++1. Most of the 1,3-disubstituted-tetrahydro-β-carboline
derivatives (compound 5-8) were stable as they showed the base peaks as molecular ion
peaks. The infrared spectra of compound 5-8 showed bands at a stretching frequency around
3400 cm-1 for the N-H stretching and a band around 1736 cm-1 for the ester carbonyl
stretching. The optical rotations of derivatives (compound 5-8) showed that they are optically
active, with those derived from d-tryptophan dextrorotatory and those derived from l-
tryptophan levorotatory. The optical rotation angle of a 0.1% methanolic solution of
derivative 5a relative to 6a and compound 5b relative to 6b showed almost equivalent values
but opposite signs, indicating their enantiomeric nature. Enantiomer purity and enantiomeric
excess were confirmed by chiral RP-HPLC analysis using an alternate isocratic analytical
method (Analytical Chiradex (250x4 mm, 5µm) column, with mobile phase (60:40,
ACN:MeOH) eluting at 0.5 mL/min flow rate); co-injections with each other confirmed
chiral purity.
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Scheme 1a: Synthetic scheme of compound 5-8.
5
6
7
O
O
O
O
O
OH
8
9
NH
NH2.HCl
NH2
b
a
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N
H
c
N
H
N
H
3
R1
2
1
Where, R1= methyl, n-propyl
aReagents and conditions (a) SOCl2, MeOH, 0ºC (b) Substituted Aldehydes, MeOH: H2O, conc. HCl, 70ºC (c)
10% Na2CO3.
2.6. In vitro CCK-2R antagonist assay. Parietal cells have been used extensively as a tool to
study the anti-secretory effect of any test compound in vitro. The anti-secretory substances
may act to inhibit gastric acid secretion by limiting the secretory responses to either histamine
or gastrin. A study led by Letari et al. showed that the functional responses of gastrin on acid
secretion from parietal cells can be measured by monitoring the variations in cytosolic Ca2+
levels as gastrin receptor/CCK-2R activation is directly coupled to cytosolic increase in Ca2+
ions.37 Therefore in the present study we utilized the measurement of cytosolic Ca2+ decrease
in gastric cells as an alternative to the functional studies for the screening of gastrin receptor/
CCK-2R antagonists. The alterations in the intracellular [Ca2+] ions has been examined by
employing the fura-2AM method. The basal value of [Ca2+]i in the resting parietal cells was
(n = 3). Incubating the cells with rat gastrin (50nM) showed an average 1.7 fold increase over
basal cytosolic [Ca2+]i with 67.08 3.75µM.
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Figure 7. Dose dependent study of compounds, (A) 5a, (B) 5b, (C) 6a, (D) 6b, (E) 7a, (F)
7b, (G) 8a and(H) 8b on intracellular [Ca2+]i.
The dose dependent study of compound 5a at a concentration of 37.5µM, 75µM and 150µM
showed that it reduced the [Ca2+]i with 58.2 3.24, 54.9 5.26 and 46.71 2.96µM respectively
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(Figure 7A) resulting its IC50 value 139.70µM whereas compound 5b at a dose of 37.5µM,
75µM and 150µM reduced the [Ca2+]i with 62.97 2.09, 55.89 2.28 and 57.45 1.62
respectively (Figure 7B) resulting its IC50 value of 278.80µM. Compound 6a at a dose of
18.5µM, 37.5µM and 75µM reduced the [Ca2+]i with 39.5 2.33, 27.7 1.67 and 25.5 3.73µM
respectively (Figure 7C) resulting its IC50 value 31.93µM whereas compound 6b at a
concentration of 37.5µM, 75µM and 150µM reduced the [Ca2+]i with 54.5 1.86µM,
52.84 3.55µM and 52.41 3.20µM respectively (Figure 7D) resulting its IC50 value
171.00µM. Similarly compound 7a at a concentration of 37.5µM, 75µM and 150µM reduced
the [Ca2+]i with 62.9 3.78, 55.89 1.27 and 52.45 2.04 µM respectively (Figure 7E) resulting
its IC50 value 180.12µM whereas compound 7b at a concentration of 37.5µM, 75µM and
150µM reduced the [Ca2+]i with 62.97 2.09, 55.89 2.28 and 57.45 1.62 µM respectively
(Figure 7F) resulting its IC50 value 267.66µM. Compound 8a at a concentration of 37.5µM,
75µM and 150µM reduced the [Ca2+]i with 56.2 2.78, 45.6 1.37 and 35.2 2.14 respectively
(Figure 7G) resulting its IC50 value 63.40µM while the compound 8b at a concentration of
37.5µM, 75µM and 150µM reduced the [Ca2+]i with 61.4 2.39, 53.6 2.58 and 54.4 1.32 µM
respectively(Figure 7H) resulting its IC50 value 261.12µM. Positive control used in this study
as benzotript (gastrin receptor antagonist) inhibited the effect of gastrin stimulated [Ca2+]i
with an IC50 value 39.63 µM.
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The above results showed that incubation of parietal cell with rat gastrin significantly
elevated the intracellular Ca2+ level over basal values. Based on the dose dependent study
CCK-2R antagonist activity (IC50) of the eight synthesized compounds along with standard
drugs benzotript has been listed in Table 1, where the gastrin augmented/ CCK-2R elicited
amplification of Ca2+ level was most significantly reversed by compound 6a. These results
suggested that compound 6a inhibits CCK-2R more efficiently than the well known CCK-2R
antagonist benzotript. It has been observed that the docking scores were consistent with the
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biological activity (docking score of 5a >5b, 6a > 6b, 7a > 7b and 8a > 8b) where the cis
isomers (1R, 3R and 1S, 3S) of a respective series were also more active than their
corresponding trans isomers (1S, 3R and 1R, 3S). Further, the docking scores also
corroborated with the biological activity where the compounds having R configuration at C-3
position were more active than their S congeners at the same position. These findings were
consistent with the stereoisomers having methyl (compound 8a) or n-propyl group
(compound 6a) at para position. Compound 6a (1R, 3R) having the propyl group at R
position and R, R configuration (cis-isomer) at both the chiral center of the molecule was the
most active compound. The binding site analysis comparison analysis revealed that the
binding mode of compound 6a (1R, 3R) followed the similar pattern as with the benzotript
(Figure 6B). However, due to formation of semi-rigid structure in this molecule the Asn358
and Arg361 residues now interact with hetrocyclic nitrogen of pyrido group and ketonic
oxygen of ester group respectively through H-bonding. The para n-propylphenyl group of
compound 6a more accurately occupied the hydrophobic cavity (n-propyl group was
surrounded by residues Tyr132, Met134 whereas phenyl group was surrounded by residues
Trp351 and His381, Val354, Val214, Met134, Tyr132) as compared to para chlorobenzoyl
group of benzotript resulting its increased CCK2R antagonist activity than the benzotript
(Figure 8A, and 8B).
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Figure 8. Binding pose analysis of compound 6a at the binding site of CCK-2R; (A).
comparative 3D binding pose view of 6a (pink colored carbon), (B). 2D binding pose view
of 6a.
Compound 6b with 1R, 3S configuration had showed far reduced activity in comparison to its
cis analogue 6a (1R,3R). The docking study of compound 6b (1S, 3R) revealed that the
change in the conformation of n-propyl phenyl group from R form to S form alter the binding
site occupancy of the molecule where the n-propyl phenyl group now shifted towards Tyr132,
but loosed its hydrophobic interaction with Trp351. Further, due to this conformation change
compound 6b also lost its H bond interaction with His361 explaining its lower antagonist
activity (IC50, 171.00). The binding pose analysis revealed that the n-propyl phenyl group of
compound 5a (1S, 3S) showed better hydrophobic interaction with Tyr132 than the
compound 6b resulting its better CCK-2R antagonist activity than compound 6b whereas the
compound 5b (1R, 3S) which is a diastereomeric form of the molecule 5a (1S, 3S) failed to
interact with any amino acid residue through H bonding resulting its least CCK-2R antagonist
activity among its four possible diastereomers. The CCK-2R antagonist activity among the
para methyl stereoisomers (compound 8a, 8b, 7a, and 7b of Table. 1) followed the similar
trend as with their n-propyl substituted counterpart which is also evident through the binding
pose study (Figure 6C). The methyl group at para position of phenyl ring of configurational
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stereoisomeric compounds 8a (1R,3R), 8b (1S,3R), 7a (1S,3S), and 7b (1R,3S) interact with
the hydrophobic residues viz Tyr 132, Met134 and Trp351 less efficiently than their
respective n- propyl analouge resulting the reduced activity of these compounds (Table 1.)
In summary the docking study not only explained the higher activity of cis isomers (1R, 3R
and 1S, 3S) than their corresponding trans isomers (1S, 3R and 1R, 3S) but it also well
explained the higher activity of compounds with R configuration at C-3 position than their S
congeners at the same position.
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2.7. MTT assay. In order to evaluate the toxicity of the most active compound the MTT
assay of compound 6a was performed on gastric cells using the protocol described by
manufacturer. The relative number of viable cells/well was determined by formation of blue
formazan color product by the mitochondrial dehydrogenase activity in viable cells. The
treatment with the most active compound 6a at concentrations of 5, 10, 50 and 100 µM had
shown no cytotoxic effect on gastric cells (Figure 9).
Figure 9. MTT assay of the most active compound 6a.
2.7. In vivo anti-ulcer studies in cold restraint induced gastric ulcer (CRU) model. The
in vitro investigations showed that these compounds significantly inhibit the CCK-2R and
docking study explained their biological variability. Thus we next moved to evaluate their
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antiulcer/gastroprotective effect in the cold-restraint ulcer (CRU) model. Table 2 showed the
in vivo % protection of synthesized compounds in various gastric ulcer models.
Table 2. In vivo % protection of synthesized compounds in various gastric ulcer models.
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Compound
Name
Dose
% ulcer protection in rat CRU
model
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mg/kg, p.o.
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The gastric ulcer % protection data shown in Table 2 clearly showed that, the orally given
synthetic compounds 5a, 5b, 6a, 6b, 7a, 7b, 8a and 8b inhibited the formation of gastric ulcer
in rats caused by cold and restraint stress, in a dose-related manner. Among the synthesized
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compounds the most active CCK-2R antagonist, 6a has shown more efficient in reducing the
gastric lesions (81.2% at a dose of 25 mg/kg, p.o.) than the reference drug benzotript (62.4 %
at a dose of 80 mg/kg, p.o.) in CRU model. Cold and restraint stress alters the regulation of
acid secretion by increasing the acidity of gastric juice through vagal activation.38 Thus, the
significant protection imparted by compound 6a in this model reflected its involvement in the
regulatory mechanism of gastric acid secretion.
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3. CONCLUSION
In the last decade there has been a revived interest in CCK-2R receptor antagonists as potent
anti ulcer agents due to the limitation/ drawbacks of the current chemotherapies. The present
study revisited the three-dimensional (3D) homology model of CCK-2R using human A2a
adenosine receptor and the resolved NMR based structure of the third extracellular loop of
the CCK-2R as templates. In our continued efforts to design the novel antiulcer agents with
improved activity the sub-structure of a well known CCK-2R antagonist benzotript has been
selected as lead molecule and rational designing was performed. A combined docking and
simulation studies of these rationally designed molecules has led to the understanding of the
essential structure requirement as well as variation of binding mode among conformational
isomers of small molecule CCK-2R antagonists. In addition the docking studies also revealed
the crucial role of Asn358 for the activity of small molecule CCK-2R antagonists among
three important residues involved in the interaction with CCK. In the next step preparation of
each configurationally isomer of these molecules was carried out and the compounds were
submitted for their in vitro activity. The detailed in vitro and in vivo screening of these
compounds has not only validated the plausible homology model of CCK-2R but also led to
the identification of orally active and safe compound 6a having better CCK-2R antagonistic
and antiulcer properties than the known drug benzotript and may further be utilized for the
development of potent antiulcer agent.
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EXPERIMENTAL SECTION
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Computational Study. All calculations were performed on a Linux workstation equipped
with four parallel Intel Xeon X5460 processors (2.8 GHZ) with 12 GB total RAM.
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Protein was prepared using Protein Preparation Wizard implemented in Schrodinger package
using default options: bond orders were assigned, hydrogen were added, metals were treated,
and water molecules 5 Å beyond hetero groups were deleted. Hydrogen were then optimized
using the exhaustive sampling option and the protein was minimized to an rmsd limit from
the starting structure of 0.3Å using the Impref module of Impact with the OPLS_2005 force
field. All ligands were drawn in the maestro panel of the Schrodinger where the specific
chirality of these molecules has been assigned. Further, these ligands were prepared using
LigPrep protocol with Epik to expand protonation and tautomeric states at 7.0 2.0 pH units.
The MDS was run using settings as OPLS-2005 as force field, water as solvent model,
constant dielectric as electrostatic treatment, Polak-Ribiere Conjugate Gradient (PRCG) as
minimization method, maximum iterations of 500, molecular dynamics as dynamics method,
simulation temperature of 300 K, time step of 1.5fs, equilibration time of 10ps, and
simulations time of 1000ps. Rest settings were used as the default in the MacroModel. The
key interactions between these antagonists and the binding site residues of CCK-2R were
presented in the 3D-Molecular graphics were produced using PyMol program.39
Chemistry. All reagents and solvents were purchased from commercial sources and used
without purification. Reactions were monitored by thin layer chromatography on self-made
plates of silica gel G (Merck, India) or 0.25mm ready-made plates of silica gel 60F254,
(E.Merck, Darmstadt, Germany). Column chromatography was performed on silica gel
(Merck, 60 to 120mesh). Melting points were determined on an electrical heated m. p.
apparatus /using silicon oil bath. Infrared spectra (IR) were recorded on Perkin- FTIR model
PC spectrophotometer with frequency of absorptions reported in wave numbers. MS were
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recorded on JEOL spectrometer with fragmentation pattern reported as values. NMR spectra
were recorded with a 300 MHz spectrometers for1H NMR and 75 MHz for13C NMR on
Bruker Supercon Magnet Avance DRX-300 spectrometers in deuterated solvents with TMS
as internal reference (chemical shifts δ in ppm, coupling constant J in Hz) The following
abbreviations are used for multiplicity of NMR signals: s=singlet, d=doublet, q = quartet,
m=multiplet, dd=doublet of doublet, br=broad. Optical rotations were measured on a Perkin-
Elmer model 241 digital polarimeter. Elemental analyses were done on an Elementar Vario
ELIII analyzer. Analytical Purity was assessed by chiral RP-HPLC analysis using an alternate
isocratic analytical method. The stationary phase was a Analytical Chiradex (250 mm x 4.0
mm, 5µm) column, the mobile phase employed 60:40, ACN:MeOH and the flow rate was
maintained at 0.5 mL/min.
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Preparation of L-tryptophan methyl ester hydrochloride (3)
Thionyl chloride (7ml, 0.295 mol) was added drop wise to a stirred suspension of L-
tryptophan 1a (10g, 0.245 mol) in dry methanol(45 ml) at -10°C during 30 min and stirring
continued for 6 hrs during which temperature was gradually raised from -10°C to room
temperature (25°C). The reaction mixture was filtered under suction and the cream colored
product washed with ether (10ml.) and dried.
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Yield 58.5 gm (92%); m.p: 220-223°C; [α]D +17.5°; H1NMR (200 MHz, CDCl3+ DMSO-
d6):
δ 3.45 (d, J=5.90, 2H), 3.75 (s, 3H), 4.17 (m, 1H), 7.07(m, 2H), 7.28(s, 1H), 7.41(d,
J=7.72, 1H), 7.52(d, J=7.54, 1H), 8.60 (brs, 1H), 0.76(s,1H); FTIR (KBr cm-1): 603, 731,
1034,1225, 1285, 1352, 1443, 1499, 1580, 1748, 2021, 3292; MS(FAB): m/z 218 (M+).
D-tryptophan methyl ester hydrochloride (4)
Yield (92%); m.p: 214-217°C; [α]D20 -17.4°; H1NMR (200 MHz, CDCl3+ DMSO-d6):
δ 3.40
(d, J=5.90, 2H), 3.72 (s, 3H), 4.14-4.21 (m, 1H), 6.98-7.13(m, 2H), 7.39(d, J=7.77, 2H),
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7.39(d, J=7.72, 1H), 7.50(d, J=7.52, 1H), 10.97 (brs, 1H); FTIR (KBr cm-1): 3267, 1747,
1441, 1225, 1107, 1074, 731; MS(FAB): m/z 218 (M+).
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Preparation of (1S,3S) & (1R,3S)-Methyl- 1-(4-propylphenyl)-2,3,4,9-tetrahydo-1H-
pyrido[3,4-b]indole-3-carboxylate (5)
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4-propylbenzaldehyde (2.22g, 15mmol) was added to a solution of L-tryptophan methyl ester
hydrochloride 2a (2.54g, 10 mmol) in aq. Methanol (22 mL; ratio 10:1) during 30 minutes.
The reaction mixture was stirred for 4-6 hours at room temperature and progress of the
reaction was monitored by TLC. Upon completion of the reaction the reaction mixture was
concentrated and cooled to give the mixture of cis-trans (1S,3S)-methyl 1-(4-propylphenyl)-
2,3,4,9-tetrahydo-1H-pyrido[3,4-b]indole-3-carboxylate which was basified with 10 % aq
solution of Na2CO3 to give 3. The product was extracted using ethyl acetate and the organic
layer was washed with water, brine solution and finally dried over anhydrous Na2SO4. It was
then concentrated in vacuo to afford a residue which was purified by column chromatography
(60-120 mesh) using hexane/CHCl3 as eluent to seprate respective cis isomer followed by
trans isomer.
Cis- isomer (5a) Yield: 60 %; mp=90oC, [α]D20-38.209° (c=0.1, methanol), Rf = 0.78 (0.2%
MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.55 (s,1H), 7.47 (s,1H), 7.31-7.27 (m,4H), 7.21-7.13 (m, 4H),
5.23 (s,1H), 4.02-3.97 (dd, J1 = 11.16 Hz, J2 = 4.17 Hz, 1H), 3.82 (s, 3H), 3.27-3.21 (m,1H),
2.07-2.97 (m,1H), 2.80 (s, 1H), 2.61(t, J = 5.30 Hz, 2H), 1.70-1.62 (m,2H), 0.97 (t, J = 7.26
Hz, 3H); 13C NMR (CDCl3, 75 MHz)
δ 173.09, 143.29, 137.78, 136.22, 134.65, 129.05,
128.65, 127.13, 121.90, 119.58, 118.17, 111.04, 108.72, 58.34, 56.93, 52.26, 37.81, 25.70,
24.60, 13.92; IR (KBr): 3391, 3336, 2928, 2854, 1736 1444; MS (ESI+): m/z = 349 (M+H) +;
Retention time (chiral RP-HPLC):6.45 min; Anal. Calcd for C22H24N2O2: C, 75.83; H, 6.94;
N, 8.04%. Found: C, 74.82; H, 7.05; N, 7.97%.
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o
Trans-isomer (5b) Yield: 30 %; mp=120 C, [α]D20 -46.111° (c=0.1, methanol), Rf = 0.52
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(0.2% MeOH:CHCl3)
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1H NMR (CDCl3, 300MHz):
δ 7.58 (s,1H), 7.56 (s,1H), 7.28-7.19 (m,4H), 7.19-7.14 (m, 4H),
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5.37 (s,1H), 3.98 (t,J = 6.3 Hz, 1H), 3.73 (s, 3H), 3.27-3.21 (m,1H), 3.32-3.25 (dd, J1 = 10.24
Hz, J2 = 5.1 Hz, 1H), 3.17-3.10 (m, 1H), 2.59 (t, J = 7.38 Hz, 2H), 1.99 (s, 1H), 1.68-1.61
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(m,2H), 0.96 (t, J =7.26 Hz, 3H); 13C NMR (CDCl3, 300 MHz)
δ 174.20, 142.75, 139.27,
133.51, 128.85, 128.28, 127.07, 121.91, 119.50, 118.25, 110.8, 108.42, 54.74, 52.61, 52.11,
37.74, 24.68, 24.56, 13.88; IR (KBr): 3371, 3330, 2922, 2848, 1736 1452; MS (ESI+): m/z =
+
349 (M+H) ; Retention time (chiral HPLC): 8.20 min; Anal. Calcd for C22H24N2O2: C,
75.83; H, 6.94; N, 8.04%. Found: C, 74.23; H, 7.15; N, 8.37%.
(1R,3R) &(1S,3R)-Methyl-1-(4-propylphenyl)-2,3,4,9-tetrahydo-1H-pyrido[3,4-b]indole-
3-carboxylate (6)
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Cis- isomer (6a)Yield: 58 %; mp=85 C, [α]D20 38.209° (c=0.1, methanol), Rf = 0.78 (0.2%
MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.54 (s,1H), 7.48 (s,1H), 7.31-7.24 (m,3H), 7.21-7.13 (m, 4H),
5.27 (s,1H), 4.02-3.96 (dd, J1 = 11.17 Hz, J2 = 4.23 Hz, 1H), 3.82 (s, 3H), 3.27-3.21 (m,1H),
2.97-3.07 (m,1H), 2.61(t, J = 7.35 Hz, 2H), 1.73(bs, 1H), 1.70-1.63 (m,2H), 0.97 (t, J = 7.29
Hz, 3H); 13C NMR (CDCl3, 300 MHz)
δ 173.24, 143.23, 137.94, 136.13, 134.91, 129.04,
128.48, 127.17, 121.89, 119.60, 118.18, 110.92, 108.84, 58.40, 56.96, 52.21, 37.77, 25.76,
24.55, 13.85; IR (KBr): 3391, 3334, 2925, 2853, 1738 1446; MS (ESI+): m/z = 349 (M+H) +;
Retention time (chiral HPLC):6.51 min; HRMS (ESI+) calcd for C22H24N2O2 +H 349.1838,
found 349.1963.
Trans-isomer (6b)Yield: 26 %; mp=120 oC, [α]D20 46.111° (c=0.1, methanol), Rf = .52 (0.2%
MeOH:CHCl3)
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1H NMR (CDCl3, 300MHz):
δ 7.58 (s,1H), 7.56 (s,1H), 7.28-7.19 (m,4H), 7.19-7.14 (m, 4H),
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Hz, J2 = 5.1 Hz, 1H), 3.17-3.10 (m, 1H), 2.59 (t, J = 7.38 Hz, 2H), 1.91 (s, 1H), 1.68-1.61
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(m,2H), 0.96 (t, J =7.26 Hz, 3H); 13C NMR (CDCl3, 300 MHz)
δ 174.16, 142.71, 139.25,
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136.16, 133.4, 128.81, 128.26, 127.03, 121.87, 119.46, 118.20, 110.88, 108.38, 54.70, 52.54,
52.07, 37.70, 24.69, 24.52, 13.84; IR (KBr): 3371, 3330, 2922, 2848, 1736 1452; MS (ESI+):
m/z = 349 (M+H) +; Retention time (chiral HPLC): 7.18 min; Anal. Calcd for C22H24N2O2: C,
75.83; H, 6.94; N, 8.04%. Found: C, 75.59; H, 7.19; N, 7.87%.
(1S,3S) & (1R,3S)-Methyl-1-(4-methylphenyl)-2,3,4,9-tetrahydo-1H-pyrido[3,4-b]indole-
3-carboxylate (7)
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Cis-isomer (7a) yield: 59 %; mp=140 C, [α]D20 -27.0° (c=0.1, methanol), Rf = .66 (0.2%
MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.42 (d, J = 5.04 Hz, 1H), 7.29 (s,1H), 7.24-7.00 (m,8H), 5.12
(s,1H), 3.89 (dd, J1 = 10.98 Hz, J2 = 2.91 Hz, 1H), 3.78 (s, 3H), 3.17-3.11 (m,1H), 2.98-2.89
(m, 1H), 2.35(s, 3H), 1.72 (s, 1H); 13C NMR (CDCl3, 75 MHz)
δ
173.24, 138.39, 137.76,
136.16, 134.95, 129.62, 128.52, 127.18, 121.89, 119.60, 118.18, 110.93, 108.80, 58.39,
56.96, 52.21, 25.76, 21.19; IR (KBr): 3396, 3334, 2943, 2850, 1739, 1448; MS (ESI+): m/z =
+
321 (M+H) ; Retention time (chiral HPLC): 5.45 min; Anal. Calcd for C20H20N2O2: C,
74.98; H, 6.29; N, 8.74%. Found: C, 73.64; H, 5.90; N, 8.45%.
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Trans-isomer (7b) Yield: 30 %; mp=196 C, [α]D -33.034° (c=0.1, methanol), Rf = .50
(0.2% MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.59 (s,1H), 7.54 (d, J = 3.39 Hz, 1H), 7.25-7.14 (m,8H), 5.35
(s,1H), 3.95 (t, J = 3.48 Hz, 1H), 3.71 (s, 3H), 3.297-3.22 (dd, J1 = 15.36 Hz, J2 = 4.56 Hz,
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1H), 3.15-3.07 (m, 1H), 2.34 (s, 3H), 2.04 (S,1H); C NMR (CDCl3, 75.5 MHz)
δ 174.19,
139.04, 137.93, 136.19, 133.47, 129.43, 128.47, 127.03, 121.91, 119.50, 118.25, 110.95,
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108.41, 54.69, 52.49, 52.15, 24.73, 21.16; IR (KBr): 3376, 3331, 2998, 2878, 1743, 1454;
+
MS (ESI+): m/z = 321 (M+H) ; Retention time (chiral HPLC): 5.87 min; Anal. Calcd for
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C20H20N2O2: C, 74.98; H, 6.29; N, 8.74%. Found: C, 75.11; H, 6.27; N, 8.57%.
(1R,3R)&(1S,3R)–Methyl-1-(4-methylphenyl)-2,3,4,9-tetrahydo-1H-pyrido[3,4-
b]indole-3-carboxylate (8)
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Cis-isomer (8a) Yield: 57 %; mp=91 C, [α]D 27.0° (c=0.1, methanol), Rf = .66 (0.2%
MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.46 (d, J = 5.7 Hz, 1H), 7.32 (s, 1H), 7.24-7.05 (m,8H), 5.16
(s,1H), 3.95-3.89 (dd, J1 = 11.04 Hz, J2 = 4.20 Hz, 1H), 3.79 (s, 3H), 3.20-3.14 (dd, J1 =
12.06 Hz, J2 = 2.40 Hz, 1H), 3.01-2.92 (m, 1H), 2.36 (s, 3H); 13C NMR (CDCl3, 300 MHz)
δ
173.20, 138.41, 137.68, 136.15, 134.88, 129.61, 128.51, 127.16, 121.90, 119.60, 118.18,
58.38, 56.90, 52.21, 25.72, 21.17; IR (KBr): 3396, 3333, 2941, 2849, 1740, 1448; MS
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(ESI+): m/z = 321 (M+H) ; Retention time (chiral HPLC): 5.49 min; Anal. Calcd for
C20H20N2O2: C, 74.98; H, 6.29; N, 8.74%. Found: C, 75.04; H, 5.76; N, 8.86%.
o
trans-isomer (8b) Yield: 28 %; mp=190 C, [α]D20 33.034° (c=0.1, methanol), Rf = .50 (0.2%
MeOH:CHCl3)
1H NMR (CDCl3, 300MHz):
δ 7.70 (s, 1H), 7.58 (m, 1H), 7.27-7.16 (m,7H), 5.35 (s,1H),
3.97 (t, J = 6.66 Hz, 1H), 3.73 (s, 3H), 3.32-3.25 (dd, J1 = 15.39 Hz, J2 = 5.22 Hz, 1H), 3.17-
3.09 (dd, J1 = 14.37 Hz, J2 = 6.0 Hz, 1H), 2.36 (s, 3H), 2.27 (s, 1H); 13C NMR (CDCl3, 300
MHz)
δ 174.12, 139.02, 137.89, 136.21, 133.44, 129.40, 128.34, 127.03, 121.89, 119.47,
118.21, 110.93, 108.38, 54.68, 52.47, 52.08, 24.72, 21.11; IR (KBr): 3400, 3330, 2951, 2846,
1742, 1450; MS (ESI+): m/z = 321 (M+H) +; Retention time (chiral HPLC): 5.75 min; Anal.
Calcd for C20H20N2O2: C, 74.98; H, 6.29; N, 8.74%. Found: C, 74.26; H, 6.54; N, 8.51%.
Biology.
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Experimental Animals. Experimental protocols were approved by the Institutional Ethical
and Usage Committee of Central Drug Research Institute, Lucknow, following the guidelines
of the Committee for the Purpose of Control and Supervision of Experiments on Animals
(CPCSEA). Adult Sprague Dawley rats, weighing 130-180g procured from National
Laboratory Animal Centre, CDRI, were used in the study. Rats were housed three to four per
cage, in a room with temperature regulated at 22 20C, with a 12h/12h light/dark cycle
(lights on 07:00 h, lights off 19:00 h). Standard chow pellets and water were given ad libitum,
except during the period when food deprivation was applied.
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Intracellular Ca2+ monitoring
Preparation of isolated parietal cells from rat stomach. Gastric cell isolation was
performed as described earlier40 with some modifications.
Ca2+ measurement. Cytosolic free Ca2+ concentration, [Ca2+]i, was monitored with the
fluorescent Ca2+ indicator fura-2.40,41 Cells were incubated with fura-2/AM (final
concentration 4 µM) for 20 min at 37°C in Earle’s medium. After loading, the cell suspension
was diluted with 10 vols. of Earle’s medium, centrifuged for 5 min at 200 X g, finally
resuspended in 10 ml of Hepes-buffered saline (HBS composition in mM: NaCl 145, KC1 5,
MgCl2, 1, CaCl2 1, Hepes 10, glucose 10, pH 7.4) at 5-10 X 106 cells/ml concentration, and
kept at room temperature in the dark until use. The suspension (0.8 X l06 cells/l.5 ml) was
added to a glass cuvette thermostated at 37°C under continuous stirring. Controls received
either Ca2+ free HBS or dimethyl sulfoxide as vehicle. Fluorescence was measured by a dual
excitation fluorimetry in a fluorescence spectrophotometer (Varian, Cary Eclipse).
Wavelengths were set at 340 nm and 380 nm for excitation, 505 nm for emission; data points
were collected at 0.2-0.4 s intervals. [Ca2+]i was estimated from the ratios of 340 and 380 nm
signals according to the following equation.41
[Ca2+]i =Kd x β x R-R min/Rmax – R
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