Synthesis, Molecular Docking and Preliminary in-Vitro Cytotoxic Evaluation of Some Substituted Tetrahydro-naphthalene (2',3',4',6'-Tetra-O-Acetyl-β-D-Gluco/-Galactopyranosyl) Derivatives

A facile, convenient and high yielding synthesis of novel S-glycosides and N-glycosides incorporating 1,2,3,4-tetrahydronaphthalene and or 1,2-dihydropyridines moieties has been described. The aglycons 2, 4, and 7 were coupled with different activated halosugars in the presence of basic and acidic medium. The preliminary in-vitro cytotoxic evaluation revealed that compounds 3c, 3f, 5c and 7b show promising activity. A molecular docking study was performed against tyrosine kinase (TK) (PDB code: 1t46) by Autodock Vina. The docking output was analyzed and some compounds have shown hydrogen bond (H-B) formation with reasonable distances ranged from 2.06 A° to 3.06 A° with Thr 670 and Cys 673 residues found in the specified pocket. No hydrogen bond was observed with either Glu 640 nor Asp 810 residues, as was expected from pdbsum.

On the other hand recent advances have implicated the role of tyrosine kinases (TK) in the physiology of cancer. Though their activity is tightly regulated in normal cells, they may acquire transforming functions due to mutation(s), over expression leading to malignancy [18,19]. The activation of cancer cells can be blocked by selective tyrosine kinase inhibitors and thus considered to be a promising target for anticancer drug discovery. Tyrosine kinases are important mediators of the signal transduction process, leading to cell proliferation, differentiation, migration, metabolism and apoptosis. Tyrosine kinases are a family of enzymes, which catalyzes phosphorylation of select tyrosine residues in target proteins, using ATP. They are implicated in several steps of neoplastic development and progression. Tyrosine kinase signaling pathways normally prevent deregulated proliferation or contribute to sensitivity towards apoptotic stimuli. These signaling pathways are often genetically or epigenetically altered in cancer cells to impart a selection advantage to the cancer cells [20][21][22].
Regarding the above mentioned findings and our previous reports [23] and [24], the main goal of the present work was to design, synthesize and investigate the anticancer activity of some novel pyridine-tetrahydronaphthalene derivatives carrying carbohydrate residues attached through S-glycosidic or N-glycosidic bond formation. A molecular docking study was performed in order to interpret the activity expressed by compounds 3c, 3f, 5b, 5c, 7a and 7b.

Chemistry
A series of S-glycosides and N-glycosides was designed in such a way that they incorporate a tetrahydronaphthalene derivative attached to a glycosyl moiety through a S-linkage or N-linkage. Synthesis of our desired compounds was achieved by allowing the acetyl derivative 1-(1,2,3,4tetrahydronaphthaline-6-yl)ethanone (1) to react directly with the appropriate aromatic aldehyde and cyanothioacetamide in the presence of ammonium acetate, to afford the corresponding 2-thioxo(1H)pyridines 2a-c, respectively (Scheme 1, Table 1). The coupling between the aglycon and the activated cyclic sugars (2',3',4',6'-O-acetyl--D-gluco-(or galato-) pyranosyl bromide was achieved in a basic medium to give the corresponding thioglycoside derivatives 3a-f in a good yields, respectively (Scheme 1, Table 1). The structures of thioglycosides 3a-f were established and confirmed on the basis of their elemental analysis and spectral data (IR, 1 H-NMR, 13   Also, one-step synthesis of 2-imino (1H)pyridines 4a,b was accomplished by heating the acetyl derivative 1 with malononitrile and the appropriate aldehyde in the presence of ammonium acetate (Scheme 1, Table 1). The authors used these intermediate aglycones for the preparation of the new glycosides 5a-d (Scheme 1, Table 1). The formation of N-glycosides 5a-d was proven using spectral data (IR, 1 H-NMR, 13 C-NMR and MS).
On the other hand, the one pot reaction of compound 1 with the appropriate aromatic aldehyde and ethyl cyanoacetate in the presence of ammonium acetate, afforded the corresponding pyridones 6a,b, respectively (Scheme 2, Table 1). Hydrolysis of 6a,b in the presence of conc. sulphuric acid afforded the amide derivatives 7a,b in good yield. Finally the coupling between the aglycons 7a,b and the activated cyclic sugar 2',3',4',6'-tetra-O--D-galactopyranosyl bromide gave the corresponding N-glycosides 8a,b in a good yield respectively. The structures of N-glycosides 8a,b were established and confirmed for the reaction products on the basis of their spectral data.

Molecular Modeling Study
Docking Using Autodock Vina The tyrosine kinase (TK) crystal structure (PBD code = 1t46) was downloaded ( Figure 1). In order to find the important residues that form interactions with that inhibitor, the pdbsum server was used. It was found that Cys 673, Thr 670, Glu 640 and Asp 810 formed some interactions with different moieties of the inhibitor (Figure 2).   The affinity of any small molecule can be considered to be a unique tool in the field of drug design, as there is a relationship between the affinity of organic molecules and the free binding energy and this can contribute in prediction and interpretation of the activity of organic compounds toward a specific target protein. Here we used the Autodock Vina [25,26] program for docking and to obtain the affinities and RMSD (root mean square deviation) of all compounds.
The most visible feature that was observed in the docking output that all compounds fitted well inside the same docking site and hence, the next step was to compare their affinities and try to rank them as observed in the previous table and then to measure the ligands' interaction to find out which ones interact well and correlate that to the activity.
The six compounds with the highest calculated affinities were found to be compounds 7b, 3f, 5c, 5b, 3c and 7a, respectively (−11.6, −11.4, −11.2, −11.0, −10.7 and −10.3 Kcal/mol). Compounds 3c and 7b were found to have low RMSD values that reflect their better fit as they were expected to have a conformation with a low deviation from the expected best binding mode. No clear correlation between the biological activity and the calculated affinity was observed. The only observation that was found that the active compounds have the best affinities regardless of their ranking in activity.
Regarding the best mode of interaction for each compound of these six compounds, the docking output was visualized to determine these interactions. It was important to concentrate on the previously mentioned residues (Cys 673, Thr 670, Glu 640 and Asp 810) whether they were involved in these interactions or not. For compound 3c THR 670 ( Figure 3) was the major residue for making hydrogen bond with one of the acetyl groups found in the side chains of this compound.

In Vitro Cytotoxic Screening
Some of the newly synthesized compounds were tested at the Department of Tumor Pathology, National Cancer Institute, Cairo, Egypt. Ehrlich Ascites Carcinoma (EAC) cells were used for cytotoxic activity screening. The selected synthesized analogues 2b, 3a-f, 5b, 5c, 7a and 7b were tested for in vitro cytotoxic activity at two different concentrations. The results ( Table 2) are expressed in the form of the percentage of non-viable cells [27]. The in vitro evaluation revealed that 3c, 3f, 5c and 7b show promising activity. This provides a new class of antitumor agents for further biological evaluation.

General
Melting points (°C, uncorrected) were determined in open glass capillaries using a Barnstead 9001 Electrothermal melting point apparatus. Infrared (IR) spectra were recorded on a Perkin Elmer FT-IR Spectrum BX Spectrometer at cm −1 scale using KBr discs. 1 H-NMR and 13 C-NMR were recorded on a JEOL 300 MHz Spectrometer; chemical shifts are expressed in δ (ppm) with reference to TMS. The mass spectra were run at 70 ev with Packard 5890 Thewlett GC/MS Spectrometer, using the Electron Ionization (EI) technique. Elemental analyses (C, H, N) were carried out at the Microanalytical Data Center, Faculty of Science, Cairo, Egypt, and were in full agreement with the proposed structures within ±0.4% of the theoretical values. Thin layer chromatography was performed on precoated (0.75 mm) silica gel GF254 plates (E. Merck, Germany). Visualization was performed by illumination with UV light source (254 nm). Compounds 2b, 2c, 4a, 4b, 6a and 6b were prepared using the previously published procedures [10] and [11] as described below (Schemes 1 and 2).

Molecular Modelling
Autodock vina allows the flexible docking of ligands into their site of action. It has the ability to use all the rotable bonds of the ligands to give a number of conformations from which the best mode could be deduced. The crystal structure of tyrosine kinase was downloaded from protein data bank [28]. Pdb code = 1t46 was written in search area of the pdbsum by clicking on find tap the complexed inhibitor will be viewed showing all possible interactions with the binding site [29]. All compounds were built and saved as pdbqt format and docking with Autodock Vina was performed according to the specified condition in which the grid box was adjusted to have center x = 24.15, center_y = 22.8, and center_z = 12.9 by these centres the pocket with the main residues were involved inside the box.

Tumor
Ehrlich Ascites Carcinoma (EAC) cells was kindly provided by the National Cancer Institute (Cairo University, Cairo, Egypt). The tumor line was maintained in female Swiss albino mice by weekly intra-peritoneal injection of 2.5 × 10 5 cells/mouse according to the method recommended by the Egyptian National Cancer Institute, Cairo University. Such developed tumor is characterized by its moderate rapid growth which could not kill the animal due to the accumulation of ascites before about 14 days after transplantation. Cells were counted before injection using the bright line haemocytometer and dilutions made by physiological saline. The desired number of cells was injected in a volume of 0.5 mL/mouse. Solid Ehrlich carcinoma was induced by inoculation of 2.5 × 10 5 cells in the back between the thighs of each animal [30].

Animals
Adult female Swiss albino mice, obtained from the animal house of Cairo Cancer Institute (Cairo University, Egypt) weighing 18-22 g were used. The local committee approved the design of the experiments, and the protocol conforms to the guidelines of the National Institutes of Health (NIH). The animals were maintained on a standard pellet diet and tap water ad libitum.

In Vitro Test for Cytotoxic Effect
A set of sterile test tubes were used, where 2.5 × 10 5 tumor cells per ml were suspended in phosphate buffer saline, then 25, 50, 100 µg/mL of the test compound were added to the suspension, kept at 37 °C for 2 h. Trypan blue dye exclusion test was then carried out to calculate the percentage of non-viable cells [27].

Conclusions
Some substituted tetrahydronaphthalene (2,3,4,6-tetra-O-acetyl-β-D-gluco/-galactopyranosyl) derivatives were synthesized and tested for their anticancer activity. The preliminary in vitro cytotoxicity evaluation revealed that five compounds show promised activity. This encouraged us to do a molecular docking study to find out a suitable binding mode for these compounds. The docking was done against TK and the docking output was analyzed. The compounds have shown hydrogen bond formation with a reasonable distance ranging from 2.06 A° to 3.06 A° with Thr 670 and Cys 673 residues. No hydrogen bond was observed with neither Glu 640 nor Asp 810 residues as was expected from pdbsum. From that we can conclude that substituted tetrahydronaphthalene (2,3,4,6-tetra-Oacetyl-β-D-gluco / -galactopyranosyl) compounds may provide more activity by further study trying to modify their substitutions to give new derivatives with expected improved anticancer activity and that may be the subject of our future work.