Design, Synthesis, and Anticancer Activities of Novel 2-Amino-4-phenylthiazole Scaffold Containing Amide Moieties

Appropriately substituted 2-amino-4-phenylthiazole derivatives were designed and synthesized according to the structural characteristics of crizotinib. .e obtained compounds were characterized using H NMR, C NMR, and HRMS. .e target compounds 5a–o were evaluated for their in vitro antiproliferative activity against A549, HeLa, HT29, and Karpas299 human cancer cell lines. Based on results of biological studies, some of these compounds exhibited significant antiproliferative activity. Compound 5b possessed outstanding growth inhibitory effects on the four cell lines, especially for HT29 cell with IC50 value of 2.01 μM. Along with the biological assay data, a molecular docking study suggests that the target compounds were a potential inhibitor.

Developed by Pfizer Co., crizotinib was a potent and selective small-molecule dual inhibitor of the mesenchymalepithelial transition factor (c-Met) kinase and anaplastic lymphoma kinase (ALK), possessing effective tumor growth inhibition and good pharmaceutical properties. In general, the cocrystal structure of crizotinib deposited in c-Met (PDB code 2WGJ, Figure 2(a)) revealed that a hydrophobic 2,6dichloro-3-fluorobenzyl moiety formed π-π stacking interaction with Tyr1230 of c-Met kinase. In addition, this aryl fragment was extended into the U-shaped binding site, which adopted an ideal conformation for involvement in a hydrophobic effect with Met1211 and Gly1163. Furthermore, the 2-aminopyridine moiety formed two hydrogenbonding interactions with the Met1160 and Pro1158 of the hinge region [18][19][20]. e 4-(1H-pyrazol-1-yl)piperidine moiety faced to the solvent accessible region, indicating that this hydrophobic fragment was relatively prone to bioisosteric replacement [19].
To design a novel scaffold of c-Met inhibitor (Figure 2(b)), a bioactive 2-amino-4-phenylthiazole scaffold was used as the skeleton. In the meanwhile, 2-aminopyridine moiety in crizotinib was replaced by the amide moiety, and the ability to efficiently form hydrogen bonds was retained. As the cocrystal structure of crizotinib has demonstrated that the halogen substitution at the phenyl ring is most favorable, we decided to modify group R 1 of the terminal phenyl ring 2 so as to investigate whether the cellular activity can be improved. Moreover, a flexible 2-morpholinoacetamido motif was introduced instead of the piperidinyl moiety in crizotinib. According to Figure 2(a), the possible additional hydrogen bond may be established in this way. In the proposed study, a novel 2-amino-4-phenylthiazole scaffold containing amide moieties was synthesized and evaluated for its antiproliferative activities.

General Synthesis of the Target Compounds.
e synthetic route of the target compounds in the current work is depicted in Scheme 1. 2-Bromo-1-(3-nitrophenyl)ethanone was warmed with thiourea in ethanol to obtain compound 1 in a good yield. e amino moiety of compound 1 was Nacylated with the excess chloroacetyl chloride in dichloromethane at room temperature to yield compound 2. In search of effective methods to prepare compound 2 by reacting compound 1 with 2-chloroacetic anhydride, we have found that 2-aminothiazole derivatives are quite inactive against a nucleophilic reaction. With abovementioned compounds, the reaction is not complete under reflux for 24 h. Compound 2 reacted with morpholine using Na 2 CO 3 as base in refluxing anhydrous ethanol to afford compound 3. Compound 4 was prepared by treatment of compound 3 with reducing reagent tin (II) chloride dihydrate in refluxing ethanol. Eventually, intermediate 4 reacted with a series of acyl chloride to give the target compounds 5a-o based on the published procedure [21]. Reagents    Journal of Chemistry

In Vitro Antiproliferative Activity.
e obtained compounds were evaluated for antiproliferative activities with the application of the MTT method using human nonsmall cell lung cancer cell (A549), human cervical cancer cell (HeLa), human colon cancer cell (HT29), and human lymphoma cell line (Karpas299). Crizotinib was employed as positive control. e results are shown in Table 1. Some of these compounds displayed better growth inhibitory effects especially for HT29 cell.
In the structure of compounds 5a, 5d, 5e, 5g, and 5j, the metasubstitution on the benzene ring 2 (Figure 2(b)) was modified to determine the contribution of this substitution to the antiproliferative activity. In vitro activity of these compounds demonstrated that the meta-halogen-(especially chloro-) substituted benzene ring 2 was favorable for anticancer activity. Compounds 5b and 5d compared with 5c have shown that 3,4-diCl-or 3-Cl-substituted benzene ring 2 ( Figure 2(b)) are better than 2,4-diCl substitution on the corresponding benzene ring in terms of inhibitory activity. is result suggested that the ortho substituent on benzene ring 2 are not conducive to antiproliferative activity. Compound 5k, with a furan ring at R (Scheme 1) instead of the benzene ring 2, was only low activity toward four tumor cells, suggesting that the substituted phenyl groups were more beneficial than a furan ring to the inhibitory activity. Compound 5l-o displayed no inhibitory activity toward test cell lines probably due to the lack of the side chains containing aromatic amide groups.

In Vitro Enzymatic
Assays. Finally, the inhibitory effect of compound 5b against c-Met kinase was further examined. According to Table 2, compound 5b exhibited a moderate c-Met enzymatic potency, suggesting that the inhibition of c-Met kinase may be a main mechanism for the antiproliferative activity of the selected compound 5b.

Western Blot Analysis.
e results from cell proliferation activities showed that some target compounds, especially for 5b, strongly inhibited the growth of HT29 cells better than Scheme 1: General synthesis of the target compounds. that of the Karpas299 cells (ALK overexpression). To further elucidate the molecular mechanisms of target compounds, compounds 5b and 5e were selected for further studies. HT29 cell was treated with HGF, and the activation of c-Met was subsequently monitored by western blot analysis. As shown in Figure 3, crizotinib (5 μM) obviously inhibited the HGF-induced c-Met phosphorylation in HT29 cells. When HT29 cell was treated with 5 μM of compounds 5b and 5e for 24 h, it could be observed that the compound 5b obviously blocked the MET signaling pathway. Compound 5e with a relatively low activity has not marked inhibition of the c-Met phosphorylation. ese data suggested that the target compounds, especially for compound 5b, might be adopted as a novel c-Met inhibitor.

Molecular Properties and Drug-likeness. Molecular
properties and drug-likeness model score (a combined effect of physicochemical properties, pharmacokinetics, and pharmacodynamics of a compound and is represented by a numerical value) were calculated online by MolSoft software (http://molsoft.com/mprop). Estimation of druglikeness properties are presented in Table 3.
For most target compounds, the parameters such as molecular weight (MW), number of hydrogen bond acceptors (HBA), number of hydrogen bond donors (HBD) and MolLogP were all in the range of Lipinski's "rule of five" [22]. e molecular weight of compound 5e and 5f was slightly beyond the range of acceptable limit. erefore, it was possible that these target compounds might have acceptable physicochemical properties of oral candidate drugs.
Additionally, the number of potential hydrogen bond donors and acceptors in the ligand must be appropriate. e difference existing in HBA between the target compounds and crizotinib is presented in Table 3. An excessive number of hydrogen bond acceptors in the target compounds may lead to redundant affinities and intramolecular hydrogen bonds. Besides, these analyses were consistent with the observation that the biological activity of the target compounds was worse than crizotinib.

Molecular Docking.
To elucidate the active compound to c-Met, we determined the c-Met kinase-binding domain (PDB code 2WGJ) in complex with the representative compound 5b. Figure 4 showed the superimposition of crizotinib and the docked compound 5b.
e two compounds were embedded in the "U shape" pocket of c-Met. e thiazole ring of compound 5b formed a π-π interaction with Tyr 1230 of c-Met protein.
e similar interaction between crizotinib and Tyr1230 has been described in Figure 2(a). Additionally, the compound 5b also formed two hydrogen bonds in the active site with Asp1164 and Met1160, respectively. However, the phenyl ring 2 ( Figure 2(b)) of compound 5b faced the solvent accessible region, similar to the hydrophobic 4-(1H-pyrazol-1-yl)piperidine moiety of crizotinib, which may explain the cause of slightly worse antiproliferative activity of the target compounds.

2-chloro-N-(4-(3-nitrophenyl)thiazol-2-yl)acetamide 2:
To a solution of compound 1 (2.22 g, 0.01 mol) and triethylamine (1.52 g, 0.015 mol) in anhydrous CH 2 Cl 2 (40 mL), chloroacetyl chloride (1.35 g, 0.012 mol) in CH 2 Cl 2 (20 mL) was added dropwise at 0°C. e reaction mixture was stirred at room temperature. When the reaction was nished, saturated Na 2 CO 3 was added to the solution until the pH became weakly basic. e mixture was separated with a separatory funnel, and the organic layer was washed with cold water, dried, and evaporated under vacuum to yield the product.

Primary Anticancer Assay.
e cells were seeded in 96well microplates at a density of 5 × 10 3 cells per well and incubated with 5% CO 2 at 37°C overnight. On the next day, the target compounds were added into the culture medium. e final concentration of DMSO in the medium was less than 0.5%. Triplicates of each concentration were used. After the cells were treated with compounds for 24 h, the supernatant was removed, and 5 mg/mL of a fresh prepared solution of MTT was added to each well and incubated with the cells at 37°C for another 4 h. e medium was removed, and 100 μL of DMSO was added to each well to dissolve formazan. e absorbances at 490 nm (for absorbance of MTT formazan) and 630 nm (for the reference wavelength) were measured by the microplate reader. Cell growth inhibition rates formula is (AC − AT)/AC × 100% (AC, absorbance value of the blank control group; AT, absorbance value of the experimental group). e IC 50 was calculated using GraphPad Prism version 6.00 software from the nonlinear curve.

Kinase Assay.
e c-Met activity was performed by enzyme-linked immunosorbent assay (ELISA), and 20 μg/mL poly (Glu, Tyr) 4:1 (Sigma) was precoated in 96well plates. en, 50 μL of 10 μmol/L ATP solution diluted in kinase reaction buffer (50 mmol/L HEPES, pH 7.4, 50 mmol/L MgCl 2 , 0.5 mmol/L MnCl 2 , 0.2 mmol/L Na 3 VO 4 , and 1 mmol/L DTT) was added to each well. Various concentrations of compound 5b diluted in 1% DMSO (v/v) were added to each reaction well. Blank DMSO (1% v/v) was used as the negative control. e kinase reaction was initiated after the addition of purified tyrosine kinase proteins. After incubation for 60 min at 37°C, the plate was washed three times with phosphate buffered saline (PBS) containing 0.1% Tween 20 (T-PBS). en antiphosphotyrosine (PY99) antibody (1 : 500 diluted in 5 mg/mL BSA TPBS) was then added. After 30 min incubation at 37°C, the plate was washed three times. Horseradish peroxidase-conjugated goat antimouse IgG (1 : 2000 diluted in 5 mg/mL BSA T-PBS) was added. e plate was then incubated at 37°C for 30 min and washed three times. Finally, a solution containing 0.03% H 2 O 2 and 2 mg/mL o-phenylenediamine in 0.1 mol/L citrate buffer (pH 5.5) was added. e reaction was terminated by the addition of 2 mol/L H 2 SO 4 , and the plate was read using a multiwell spectrophotometer ((VERSAmax ™ , Molecular Devices) at 490 nm. e inhibition rate (%) was calculated using the following equation: (1−A490/(A490 control)) × 100%. IC 50 values were calculated from the inhibition curve.

Western Blot.
Cells were harvested and washed twice with cold PBS, stimulated with HGF (40 ng/mL) for 15 min, then resuspended and lysed in RIPA buffer (150 mM NaCl, 1% NP-40, 1% SDS, 1 mM PMSF, 10 μg/ml leupeptin, 1 mM aprotinin, 50 mM Tris-Cl, pH 7.4) at 4°C for 30 min. e protein concentration was determined by the BCA assay. Cell lysate was separated by 10% SDS-PAGE, and then transferred to PVDF membranes. e membranes were blocked with 5% nonfat milk and incubated with primary antibodies at 4°C overnight. Horseradish peroxidaseconjugated secondary antibodies were used against corresponding primary antibody at room temperature. Bound antibodies were revealed by the enhanced chemiluminescence (ECL) technique.

Molecular
Docking. Molecular docking was performed using the CDOCKER protocol (the Discovery Studio 3.5 software package, Accelrys, Co. Ltd). e cocrystal structure of c-Met (PDB code 2WGJ) was downloaded from the RCSB Protein Data Bank (http://www.rcsb.org). Protein preparation was carried out using the Prepare Protein protocol, and all bound water was deleted from the protein. e ligand preparation was carried out using the Prepare Ligand protocol. e docking parameters were set as default. e lowest binding energy was taken as the best-docked conformation of the ligand for the protein.

Conclusion
A series of new substituted 2-amino-4-phenylthiazole derivatives were designed and synthesized according to the structural characteristics of crizotinib and screened for biological activity. Some of these novel compounds have mild to good anticancer activity. Among these compounds, compound 5b possessed outstanding growth inhibitory effects, especially for HT29 cells with an IC 50 value of 2.01 µM. Further experimental results support that the target compounds may be potential inhibitors of c-Met kinase.

Journal of Chemistry
Data Availability e data used to support the findings of this study are included within the article. e 1 H NMR and 13 C NMR of synthesized compounds used to support the findings of this study are included within the supplementary information file.

Conflicts of Interest
e authors declare that they have no conflicts of interest.