Cytotoxic activities of some benzothiazole-piperazine derivatives

Abstract

Synthesis, characterization and cytotoxic activities of ten benzothiazole-piperazine derivatives were reported. In vitro cytotoxic activities of compounds were screened against hepatocellular (HUH-7), breast (MCF-7) and colorectal (HCT-116) cancer cell lines by sulphorhodamine B assay. Based on the GI₅₀ values of the compounds, most of the benzothiazole-piperazine derivatives are active against HUH-7, MCF-7 and HCT-116 cancer cell lines. Compound 1d is highly cytotoxic against all tested cancer cell lines. Further investigation of compound 1d by Hoechst Staining and Fluorescence-Activated Cell Sorting Analysis (FACS) revealed that this compound causes apoptosis by cell cycle arrest at subG₁ phase.

• Anticancer
• benzothiazole
• cytotoxicity
• piperazine
• sulphorodamine B

Introduction

Cancer, remaining to be a major health problem today, is caused by abnormal cell division without control. Cancer cells are able to spread into other parts of the body through blood and lymph system. Although there are many advances in treatment of cancer, most of anticancer drugs still need to be improved because of various limitations like emergence of drug resistance, low therapeutic index and lack of selectivity. Thus, cancer studies continue to develop new anticancer medicines, targeted therapies, monoclonal antibodies, etc.

Benzothiazole derivatives were reported to be highly cytotoxic against HCT-116 (colorectal cancer cell line)1, MCF-7 (breast cancer cell line)2, U937 and THP-1 (leukemia cancer cell lines)3. Isatin Mannich bases with benzothiazole moiety were reported as potential anti-breast cancer drugs. Flow cell cytometry showed that most active compound in series resulted in cell cycle arrest at G2/M phase4. Thiourea derivatives bearing benzothiazole moiety were reported to be cytotoxic against MCF-7 and HeLa breast cancer cell lines. Most active derivatives of the series were proven to act by damaging DNA with alkaline Comet assay5. 2-(4-Aminophenyl)benzothiazoles with cytotoxic activity against MCF-7 (breast) and IGROV-1 (ovarian) cancer cell lines caused generation of DNA adducts which induce apoptosis6.

Piperazine ring is another value for anticancer drug candidates. Piperazinobenzopyranones were reported as breast cancer resistance protein inhibitors which are usually responsible with demolished effect of anticancer medication7. Piperazine ring carrying nucleoside analogues were evaluated for their cytotoxic activities on various cancer cell lines and most active compounds were further analyzed to evaluate mechanism of action. Agents caused induction of senescence-associated cell death through the inhibition of some kinase proteins8. In addition, Yarim and her co-workers previously reported various piperazine derivatives with high cytotoxic activity against liver (HUH-7, FOCUS, MAHLAVU, HepG-2, Hep-3B), breast (MCF-7, BT20 and T47D), colon (HCT-116), gastric (KATO-3), cervix (HeLa) and endometrial (MFE-296) cancer cell lines9–11.

According to anticancer activity studies of benzothiazole-piperazine backbone, arylsulphonamides and arylthiol derivatives have potent cytotoxicity against a large scale of cancer cell lines such as breast (MCF-7), hepatocellular (HepG-2), prostate (DU-145) cancers and CD4⁺ human acute T-lymphoblastic leukaemia (CCRF-CEM)12^,13.

In this study, with the aid of aforementioned studies, we reported the synthesis, purification and characterization of novel compounds which contain benzothiazole-piperazine backbone in their molecular structure. These compounds were tested for their cytotoxic activities against hepatocellular (HUH-7), breast (MCF-7) and colorectal (HCT-116) cancer cell lines with sulphorhodamine B assay. The advanced analysis, Hoechst staining and fluorescence-activated cell sorting analysis were also done for compound 1d to understand the mechanism of cytotoxicity.

Methods and materials

Chemistry

All chemicals and reagents used in current study were of analytical grade. The reactions were monitored by thin layer chromatography (TLC) on Merck pre-coated silica GF254 plates. Melting points (°C) of the compounds were determined by using a Mettler Toledo FP62 capillary melting point apparatus (Mettler-Toledo, Greifensee, Switzerland) and are uncorrected. Infrared spectra were recorded on a Perkin-Elmer Spectrum One series FT-IR apparatus Version 5.0.1 (Perkin Elmer, Norwalk, CT), using potassium bromide pellets, the frequencies were expressed in cm⁻¹. Elemental analyses were performed on LECO 932 CHNS (LECO-932, St. Joseph, MI) instrument. UV spectra were recorded on Evolution™ 201/220 UV-visible spectrophotometer and ethanol was used as solvent. The ¹H-NMR spectra were recorded with a Varian Mercury-400 FT-NMR spectrometer (Varian Inc., Palo Alto, CA), using tetramethylsilane (TMS) as the internal reference, with chloroform (CDCl₃) as solvent, the chemical shifts were reported in parts per million (ppm). Coupling constants were recorded in Hertz (Hz).

General procedure for synthesis of 2-chloro-N-(6-methylbenzothiazol-2-yl)acetamide (1)

2-Amino-6-methylbenzothiazole (0.012 mol (2 g)) was dissolved in 42 ml benzene: triethlyamine mixture (20:1)14. Later, acetylation was performed with 0.011 mol (0.87 ml) chloroacetyl chloride in room temperature. Because of the highly reactant nature of chloroacetyl chloride, it was added slowly in small amounts. Reaction was monitored by TLC with silica gel plate and benzene:methanol (9:1) mobile phase mixture. Reaction completed in four days at room temperature. Precipitated crude product was filtered, washed with benzene and dried. Ethanol crystallization gave pure product.

General procedure for synthesis of N-(6-methylbenzothiazol-2-yl)-2-(4-substitued piperazinyl)acetamide derivatives (1a–f)

N-(6-methylbenzothiazol-2-yl)-2-(4-substituedpiperazinyl)acetamides were synthesized in acetone by the reaction of 0.0025 mol (0.602 g) 2-chloro-N-(6-methylbenzothiazol-2-yl)acetamide and 0.0025 mol of suitable piperazine, in presence of 0.0025 mol (0.345 g) anhydrous K₂CO₃. Reactions were monitored by TLC with silica gel plate and benzene:methanol (9:1) mobile phase mixture. Reactions completed in two days at room temperature. Potassium carbonate was removed by filtration. After evaporation of acetone, precipitated products were recrystallized from absolute ethanol or acetone:distilled water mixture.

General procedure for synthesis of 2-chloro-N-(6-ethoxybenzothiazol-2-yl)acetamide (2)

2-Amino-6-ethoxybenzothiazole (0.012 mol (2.22 g)) was dissolved in 42 ml benzene: triethylamine mixture (20:1)14. Later, acetylation was performed with 0.013 mol (0.78 ml) chloroacetyl chloride at room temperature. Because of the highly reactant nature of chloroacetyl chloride, it was added slowly in small amounts. Reaction was monitored by TLC with silica gel plate and benzene:methanol (9:1) mobile phase mixture. Reaction completed in four days at room temperature. Precipitated crude product was filtered, washed with benzene and dried. Ethanol crystallization gave pure product.

General procedure for synthesis of N-(6-ethoxybenzothiazol-2-yl)-2-(4-substitued-piperazinyl)acetamides (2a–d)

N-(6-ethoxybenzothiazol-2-yl)-2-(4-substituedpiperazinyl)acetamides were synthesized in acetone by the reaction of 0.0025 mol (0.677 g) 2-chloro-N-(6-ethoxybenzothiazol-2-yl)acetamide and 0.0025 mol of suitable piperazine, in presence of 0.0025 mol (0.345 g) anhydrous K₂CO₃. Reactions were monitored by TLC with silica gel plate and benzene:methanol (9:1) mobile phase mixture. Reactions completed in two days at room temperature. Potassium carbonate was removed by filtration. After evaporation of acetone, precipitated products were recrystallized from absolute ethanol or acetone:distilled water mixture.

Cytotoxicity studies

The cytotoxic activity of the synthesized compounds was investigated on liver (HUH-7), breast (MCF-7) and colon (HCT-116) cancer cell lines, by means of sulphorhodamine B (SRB) assays in triplicate. Serial dilutions from 100 µM to 2.5 µM were used, 5-fluorouracil (5-FU) was the reference compound.

Cell culture

The human cancer cell lines were grown in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin. Each cell line was maintained in an incubator at 37 °C supplied with 5% CO₂ and 95% air.

NCI-60 sulphorhodamine B (SRB) assay

Cancer cells (range of 2000 cell/well to 5000 cell/well) were inoculated into 96-well plates in 200 µl of media and incubated in 37 °C incubators containing 5% CO₂ and 95% air. After a 24 h incubation period, one plate for each cell line was fixed with 100 µl 10% ice-cold trichloroacetic acid (TCA). This plate represents the behaviour of the cells just prior to drug treatment and is accepted as the time-zero plate. The compounds to be tested were solubilized in DMSO to a final concentration of 40 mM and stored at +4 °C. While treating the cells with the compounds, the corresponding volume of the compound was applied to the cell to achieve the desired drug concentration and diluted through serial dilution. After drug treatment, the cells were incubated in 37 °C incubators containing 5% CO₂ and 95% air for 72 h. Following the termination of the incubation period after drug treatment, the cells were fixed with 100 µl 10% ice-cold TCA and incubated in the dark at +4 °C for 1 h. Then the TCA was washed away with ddH₂O five times and the plates were left to air dry. For the final step, the plates were stained with 100 µl of 0.4% sulphorhodamine B (SRB) solution in 1% acetic acid solution. Following staining, the plates were incubated in dark for 10 min at room temperature. The unbound dye was washed away using 1% acetic acid and the plates were left to air dry. To measure the absorbance results, the bound stain was then solubilized using 200 µl of 10 mM Tris-Base. The OD values were obtained at 515 nm.

Hoechst staining analysis

Cells were seeded on coverslips in 6-well plates. After overnight culture, cells were exposed to compounds at a concentration of their GI₅₀ values for 72 h. To determine nuclear condensation by Hoechst 33258 (Sigma-Aldrich) staining, coverslips were washed twice with icecold PBS, fixed in 1 ml of cold methanol for 10 min, and then incubated with 3 Ig/ml of Hoechst 33258 for 5 min in darkness. The coverslips were then rinsed with distilled water, mounted on glass microscopic slides using 50% glycerol, and examined under fluorescent microscopy (40×).

Fluorescence-activated cell sorting analysis

Human cancer cell line (HUH-7) of interest were inoculated into 100-mm culture dishes (300 000 cells/dish). Twenty-four hours later, cells were then treated with the desired compounds according to their GI₅₀ values and incubated for 72 h. Cells were then collected by trypsinization and the pellets were fixed in ice-cold 70% ethanol and stored at −20 °C. Before the analysis, the samples were stained with MUSE cell cycle reagent (contains propidium iodide solution) according to the manufacturer’s protocol. Cell cycle analysis was conducted with MUSE cell cycle analyzer.

Results

Chemistry

N-(6-methylbenzothiazol-2-yl)-2-[4-(2-methoxyphenyl)piperazinyl]acetamide (1a)

White powder, 48% (0.417 g), m.p. 121.9 °C (reported: 122–124 °C). UV (MeOH, λ_max, nm); 291 (log ɛ: 4.21). FT-IR (KBr, cm⁻¹); 3333 (N–H), 3058 (C–H, aromatic), 2908 (C–H, aliphatic), 1707 (C=O, amide), 1605 (C=C, aromatic), 1242 (C–N), 1056 (C–O)15. ¹H-NMR (CDCl₃, ppm); 2.48 (s, 3H, Ar–CH₃), 2.86 (t, 4H, piperazine H_2,6, J = 4.8 Hz), 3.18 (bs, 4H, H_3,5), 3.36 (s, 2H, –COCH₂N–), 3.88 (s, 3H, –OCH₃), 6.89 (d, 1H, phenyl H₂, J = 9.2 Hz), 6.94–6.98 (m, 2H, phenyl H_3,5), 7.02–7.06 (m, 1H, phenyl H₄), 7.25–7.28 (m, 1H, benzothiazole H₅), 7.62 (s, 1H, benzothiazole H₇), 7.68 (d, 1H, benzothiazole H₄, J = 8 Hz), 10.47 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₁H₂₄N₄O₂S (396.506): C, 63.61; H, 6.10; N, 14.13; S, 8.09. Found: C, 63.52; H, 6.14; N, 14.25; S, 8.04.

N-(6-methylbenzothiazol-2-yl)-2-[4-(2-methoxyethyl)piperazinyl]acetamide (1b)

Cream colored, shiny powder. 12% (0.099 g), m.p. 81.1 °C. UV (MeOH, λ_max, nm); 292 (log ɛ: 4.25). FT-IR (KBr, cm⁻¹); 3120 (N–H), 3050 (C–H, aromatic), 2946 (C–H, aliphatic), 1697 (C=O, amide) 1607 (C=C, aromatic), 1259 (C–N), 1067 (C–O). ¹H-NMR (CDCl₃, ppm); 2.46 (s, 3H, Ar-CH₃), 2.62 (t, 2H, –NCH₂CH₂–, J = 5.2 Hz), 2.69 (bs, 8H, piperazine), 3.27 (s, 2H, –COCH₂N–), 3.36 (s, 3H, –OCH₃), 3.51 (t, 2H, –CH₂OCH₃, J = 5.2 Hz), 7.23–7.26 (m, 1H, benzothiazole H₅), 7.61 (s, 1H, benzothiazole H₇), 7.67 (d, 1H, benzothiazole H₄, J = 8 Hz), 10.39 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₁₇H₂₄N₄O₂S (348.463): C, 58.26; H, 7.48; N, 15.99; S, 9.15. Found: C, 58.22; H, 7.44; N, 15.97; S, 9.18.

N-(6-methylbenzothiazol-2-yl)-2-(4-cyclohexylpiperazinyl)acetamide (1c)

White powder. 2.8% (0.026 g), m.p. 181.3 °C. UV (MeOH, λ_max, nm); 289 (log ɛ: 4.25). FT-IR (KBr, cm⁻¹); 3261 (N–H), 2932 (C–H, aromatic), 2853 (C–H, aliphatic), 1703 (C=O, amide), 1605 (C=C, aromatic), 1262 (C=N). ¹H-NMR (CDCl₃, ppm); 1.09–1.27 (m, 6H, cyclohexyl), 1.63–1.66 (m, 1H, cyclohexyl), 1.80–1.88 (m, 4H, cyclohexyl), 2.48 (s, 4H, piperazine H_2,6), 2.48 (s, 3H, Ar–CH₃), 2.67 (s, 4H, piperazine H_3,5), 3.26 (s, 2H, –COCH₂N–), 7.25–7.27 (m, 1H, benzothiazole H₅), 7.62 (s, 1H, benzothiazole H₇), 7.68 (d, 1H, benzothiazole H₄, J = 8 Hz), 10.41 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₀H₂₈N₄OS (372.528): C, 64.14; H, 8.07; N, 14.96; S, 8.56. Found: C, 64.12; H, 8.08; N, 14.97; S, 8.58.

N-(6-methylbenzothiazol-2-yl)-2-[4-(pyridin-4-yl)piperazinyl]acetamide (1d)

White powder. 22% (0.195 g), m.p. 100.8 °C. UV (MeOH, λ_max, nm); 293 (log ɛ: 4.13). FT-IR (KBr, cm⁻¹); 3192 (N–H), 3050 (C–H, aromatic), 2842 (C–H, aliphatic), 1703 (C=O, amide), 1603 (C=C, aromatic), 1212 (C–N). ¹H-NMR (CDCl₃, ppm); 2.48 (s, 3H, Ar-CH₃), 2.78 (t, 4H, piperazine H_2,6, J = 5.4 Hz), 3.45 (t, 4H, piperazine H_3,5, J = 4.8 Hz), 3.60 (s, 2H, –COCH₂N–), 6.68–6.70 (dd, 2H, pyridine H_3,5, J₁ = 1.6 Hz, J₂ = 5 Hz), 7.26–7.28 (m, 1H, benzothiazole H₅), 7.63 (s, 1H, benzothiazole H₇), 7.67 (d, 1H, benzothiazole H₄, J = 8.4 Hz), 8.30–8.32 (dd, 2H, pyridine H_2,6, J₁ = 1.6 Hz, J₂ = 5.2 Hz), 10.37 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₁₉H₂₁N₅OS (367.468): C, 61.76; H, 6.27; N, 18.95; S, 8.68. Found: C, 61.73; H, 6.29; N, 18.96; S, 8.66.

N-(6-methylbenzothiazol-2-yl)-2-[4-(3,4-dichlorophenyl)piperazinyl]acetamide (1e)

White powder. 16% (0.196 g), m.p. 243.8 °C. UV (MeOH, λ_max, nm); 293 (log ɛ: 4.25). FT-IR (KBr, cm⁻¹); 3278 (N–H), 3017 (C–H, aromatic), 2893 (C–H, aliphatic), 1707 (C=O, amide), 1605 (C=C, aromatic), 1240 (C–N). ¹H-NMR (CDCl₃, ppm); 2.48 (s, 3H, Ar–CH₃), 2.80 (t, 4H, piperazine H_2,6, J = 4.8 Hz), 3.27 (t, 4H, piperazine H_3,5, J = 5.2 Hz), 3.35 (s, 2H, –COCH₂N–), 6.74–6.77 (dd, 1H, phenyl H₂, J₁ = 3.2 Hz, J₂ = 8.6 Hz), 6.98 (d, 1H, phenyl H₆, J = 3.2 Hz), 7.25–7.26 (m, 1H, benzothiazole H₅), 7.30 (d, 1H, phenyl H₅, J = 9.2 Hz), 7.62 (s, 1H, benzothiazole H₇), 7.68 (d, 1H, benzothiazole H₄, J = 8.4 Hz), 10.33 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₀H₂₀Cl₂N₄OS (435.37): C, 54.92; H, 5.07; N, 12.81; S, 7.33. Found: C, 54.91; H, 5.04; N, 12.83; S, 7.31.

N-(6-methylbenzothiazol-2-yl)-2-[4-(4-chlorobenzyl)piperazinyl]acetamide (1f)

Cream-colored powder. 12% (0.115 g), m.p. 189.6 °C. UV (MeOH, λ_max, nm); 288 (log ɛ: 4.27). FT-IR (KBr, cm⁻¹); 3100 (N–H), 3038 (C–H, aromatic), 2929 (C–H, aliphatic), 1692 (C=O, amide), 1607 (C=C, aromatic), 1271 (C–N). ¹H-NMR (CDCl₃, ppm); 2.47 (s, 3H, Ar–CH₃), 2.54 (bs, 4H, piperazine H₃,₅), 2.66 (bs, 4H, piperazine H₂,₆), 3.27 (s, 2H, –N–CH₂–), 3.51 (s, 2H, –COCH₂N–), 7.22–7.28 (m, 4H, phenyl), 7.29–7.31 (m, 1H, benzothiazole H₅), 7.61 (s, 1H, benzothiazole H₇), 7.68 (d, 1H, benzothiazole H₄, J = 8.4 Hz), 10.4 (bs, 1H, –NHCOCH₂–). Anal. calcd. for C₂₁H₂₃ClN₄OS (414.952): C, 60.49; H, 6.04; N, 13.44; S, 7.69. Found: C, 60.41; H, 6.08; N, 13.41; S, 7.70.

N-(6-ethoxybenzothiazol-2-yl)-2-[4-(p-toluyl)piperazinyl]acetamide (2a)

Honey colored irregular crystals. 36.5% (0.375 g), m.p. 146.7 °C. UV (MeOH, λ_max, nm); 294 (log ɛ: 4.18). FT-IR (KBr, cm⁻¹); 3315 (N–H), 3006 (C–H; aromatic), 2980 (C–H; aliphatic), 1698 (C=O; amide), 1608 (C=C; aromatic) and 1219 (C–N). ¹H-NMR (DMSO, ppm); 1.46 (t, 3H, –OCH₂CH₃, J = 7.2 Hz), 2.29 (s, 3H, –Ph–CH₃), 2.81 (t, 4H, piperazine H_2,6, J = 5.2 Hz), 3.23 (t, 4H, piperazine H_3,5, J = 5.2 Hz), 2.29 (s, 2H, –COCH₂N–), 4.06–4.12 (m, 2H, –OCH₂CH₃), 6.86 (d, 2H, phenyl H_2,6, J = 8.8 Hz), 7.03–7.06 (dd, 2H, phenyl H_3,5, J₁ = 2.4 Hz, J₂ = 8.8 Hz), 7.09–7.11 (dd, 1H, benzothiazole H₅, J = 8.8 Hz), 7.26–7.29 (m, 1H, benzothiazole H₇), 7.67 (d, 1H, benzothiazol H₄, J = 8.8  Hz), 10.36 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₂H₂₆N₄O₂S (410.532): C, 64.05; H, 6.84; N, 13.58; S, 7.77. Found: C, 64.04; H, 6.82; N, 13.55; S, 7.80.

N-(6-ethoxybenzothiazol-2-yl)-2-[4-(o-chlorophenyl)piperazinyl]acetamide (2b)

Light brown colored powder. 47.5% (0.512 g), m.p. above 300 °C. UV (MeOH, λ_max, nm); 293 (log ɛ: 4.16). FT-IR (KBr, cm⁻¹); 3334 (N–H), 3065 (C–H; aromatic), 2971 (C–H; aliphatic), 1702 (C=O; amide), 1605 (C=C; aromatic), 1226 (C–N). ¹H-NMR (DMSO, ppm); 1.47 (t, 3H, –OCH₂CH₃, J = 7.2 Hz), 2.85 (t, 4H, piperazine H_2,6, J = 4.4 Hz), 3.17 (bs, 4H, piperazine H_3,5), 3.36 (s, 2H, –COCH₂N–), 4.07–4.12 (m, 2H, –OCH₂CH₃), 6.99–7.09 (m, 3H, phenyl H_4,5,6), 7.25 (d, 1H, phenyl H₃, J = 1.2 Hz), 7.26–7.29 (m, 1H, benzothiazole H₇), 7.38–7.39 (dd, 1H, benzothiazole H₅, J₁ = 1.2 Hz, J₂ = 8 Hz), 7.68 (d, 1H, benzothiazol H₄, J = 8.8 Hz), 10.41 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₁H₂₃ClN₄O₂S (430.951): C, 58.25; H, 5.82; N, 12.94; O, 7.39; S, 7.41. Found: C, 58.22; H, 5.82; N, 12.94; S, 7.40.

N-(6-ethoxybenzothiazol-2-yl)-2-[4-(p-cyanophenyl)piperazinyl]acetamide (2c)

Beige colored powder. 42.7% (0.45 g), m.p. above 300 °C. UV (MeOH, λ_max, nm); 292 (log ɛ: 4.21). FT-IR (KBr, cm⁻¹); 3194 (N–H), 3060 (C–H; aromatic), 2972 (C–H; aliphatic), 2215 (C≡N), 1699 (C=O; amide), 1605 (C=C; aromatic), 1225 (C–N). ¹H-NMR (DMSO, ppm); 1.46 (t, 3H, –OCH₂CH₃, J = 7.2 Hz), 2.80 (t, 4H, piperazine H_2,6, J = 5.2 Hz), 3.42 (t, 4H, piperazine H_3,5, J = 5.2 Hz), 3.35 (s, 2H, –COCH₂N–), 4.06–4.12 (m, 2H, –OCH₂CH₃), 6.88 (d, 2H, phenyl H_2,6, J = 8.8 Hz), 7.53 (d, 2H, phenyl H_3,5, J₁ = 8.8 Hz), 7.03–7.06 (dd, 1H, benzothiazole H₅, J₁ = 2.4 Hz, J₂ = 8.6 Hz), 7.26–7.29 (m, 1H, benzothiazole H₇), 7.67 (d, 1H, benzothiazole H₄, J = 8.8 Hz), 10.28 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₂H₂₃N₅O₂S (421.515): C, 62.39; H, 5.95; N, 16.54; S, 7.57. Found: C, 62.35; H, 5.96; N, 16.56; S, 7.58.

N-(6-ethoxybenzothiazol-2-yl)-2-[4-(o-cyanophenyl)piperazinyl]acetamide (2d)

Beige colored powder. 25.6% (0.27 g), m.p. above 300 °C. UV (MeOH, λ_max, nm); 290 (log ɛ: 4.23). FT-IR (KBr, cm⁻¹); 3290 (N–H), 3068 (C–H; aromatic), 2825 (C–H; aliphatic), 2221 (C≡N), 1703 (C=O; amide), 1605 (C=C; aromatic), 1225 (C–N). ¹H-NMR (DMSO, ppm); 1.45 (t, 3H, –OCH₂CH₃, J = 7.2 Hz), 2.88 (t, 4H, piperazine H_2,6, J = 4.4 Hz), 3.31 (t, 4H, piperazine H_3,5, J = 4.4 Hz), 3.38 (s, 2H, –COCH₂N–), 4.07–4.12 (m, 2H, –OCH₂CH₃), 7.03–7.09 (m, 3H, phenyl H_4,5,6), 7.51–7.56 (m, 1H, phenyl H₃), 7.26–7.29 (m, 1H, benzothiazole H₇), 7.58–7.61 (dd, 1H, benzothiazole H₅, J₁ = 1.6 Hz, J₂ = 8 Hz), 7.68 (d, 1H, benzothiazol H₄, J = 9.2 Hz), 10.36 (bs, 1H, –NHCOCH₂–). Anal Calcd for C₂₂H₂₃N₅O₂S (421.515): C, 62.39; H, 5.95; N, 16.54; S, 7.57. Found: C, 62.37; H, 5.96; N, 16.55; S, 7.58.

Biological activity

The cytotoxic activity of the synthesized compounds 1a–f and 2a–d was investigated on liver (HUH-7), breast (MCF-7) and colon (HCT-116) cancer cell lines, by means of sulphorhodamine B (SRB) assays in triplicate. As shown in Table 2, all tested compounds were screened with mean 50% growth inhibition concentration (GI₅₀) in micromolar concentration range. The cytotoxicity results show that most of the substituted benzothiazole-piperazine derivatives are active against tested cancer cell lines. Further investigation of compound 1d by Hoechst staining and FACS revealed that this compound causes apoptosis by cell cycle arrest at subG₁ phase.

Discussion

The synthesis of benzothiazole-piperazine derivatives is outlined in Figure 1. The final compounds are obtained by two stages which are N-acetylation of the primary amine and N-alkylation of the secondary amine. In order to obtain target compounds, firstly it is necessary to synthesize 2-chloro-N-(6-methylbenzothiazole-2-yl)acetamide (1) and 2-chloro-N-(6-ethoxybenzothiazole-2-yl)acetamide (2) starting from 2-amino-6-methylbenzothiazole and 2-amino-6-ethoxybenzothiazole, respectively. Finally, compounds 1 and 2 are reacted with various piperazine derivatives under basic conditions in which piperazines are N-alkylated to yield the final products. Structural and physical properties of the synthesized benzothiazole-piperazine derivatives are summarized in Table 1.

Figure 1. Synthesis of compounds 1a–f and 2a–d.

Table 1. Structural and physical properties of compounds 1a–f and 2a–d.

Compounds	R1	R2	M.P. (°C)	Yield %	Log P*
1a	Methyl	2-Methoxyphenyl	121.9†	48	4.15
1b	Methyl	2-Methoxyethyl	81.1	12	2.04
1c	Methyl	Cyclohexyl	181.3	18	3.74
1d	Methyl	4-Pyridyl	100.8	22	2.94
1e	Methyl	3,4-Dichlorophenyl	243.8	16	5.39
1f	Methyl	4-Chlorobenzyl	189.6	12	4.49
2a	Ethoxy	4-Methylphenyl	146.7	36.5	4.76
2b	Ethoxy	2-Chlorophenyl	>300	47.5	4.83
2c	Ethoxy	4-Cyanophenyl	>300	42.7	4.31
2d	Ethoxy	2-Cyanophenyl	>300	25.6	4.31

*Log P were calculated using ChemDraw Ultra V.9.0 (ChembridgeSoft Corp., Cambridge, MA).

†Melting point of compound 1a in the original reference15 is reported as 122–124 °C.

Table 2. Cytotoxic activity data for compounds 1a–f and 2a–d.

			Cancer cell line (GI50, µM)
Compounds	HCT-116	R^2	MCF-7	R^2	HUH-7	R^2
1a	4.8	1.0	15.1	1.0	5.7	1.0
1b	7.7	0.8	no inh	–	10.7	0.8
1c	15.1	0.9	26.9	0.8	7.9	0.9
1d	7.9	0.9	9.2	0.9	3.1	0.9
1e	8.0	0.9	16.6	0.9	9.7	1.0
1f	5.0	0.8	no inh	–	7.0	0.9
2a	4.5	1.0	61.4	0.9	9.4	1.0
2b	5.8	0.9	20.4	0.9	6.5	0.9
2c	6.3	0.8	29.8	0.9	7.4	0.8
2d	25.3	0.9	19.8	0.9	6.7	0.9
5-FU	30.66		3.51		18.67

Synthesized compounds were identified with UV, IR and ¹H-NMR spectra. In UV spectrum of compounds there is one significant band at 290 nm which represents n → π* transition of the series. In IR spectrum of benzothiazole-piperazine derivatives, characteristic N–H stretching band was observed nearly at 3330 cm⁻¹. Other stretching bands were observed approximately at 3050 cm⁻¹ (C–H; aromatic), 2980 cm⁻¹ (C–H; aliphatic), 1700 cm⁻¹ (C=O; amide), 1605 cm⁻¹ (C=C; aromatic) and 1240 cm⁻¹ (C–N). In ¹H-NMR spectra of benzothiazole-piperazine derivatives, the protons of piperazine were seen approximately at 2.70 and 3.33 ppm as triplets (J = 8.2 Hz). Amide N–H gave broad singlet nearly at 10.33 ppm. Methylene protons of –COCH₂N– gave singlet nearly at 3.35 ppm. The protons of benzothiazole were seen approximately at 7.67 ppm as doublet (J = 8 Hz) (H₄) at 7.25–7.27 ppm as multiplets (H₅) and 7.62 ppm as singlets (H₇). Methyl protons of Ar-CH₃ (1a–f) gave singlet nearly at 2.48 ppm. Methyl protons of Ar-OCH₂CH₃ (2a–d) gave triplets nearly at 1.45 (J = 7.2 Hz) ppm and methylene protons of Ar-OCH₂CH₃ (2a–d) gave multiplets nearly at 4.06–4.12 ppm.

In HCT-116 colorectal cancer cell line, the GI₅₀ values for benzothiazole-piperazine derivatives were in range of 4–25 µM. Among series, 4-methylphenyl substituted compound 2a (GI₅₀: 4.5 µM) had the highest cytotoxicity. Cyclohexyl ring (1c, GI₅₀: 15.1 µM) lowered the activity of 6-methylbenzothiazole derivatives. In addition, compounds 2c (GI₅₀: 6.3 µM) and 2d (GI₅₀: 25.3 µM) showed that electron withdrawing cyano group should be on para position rather than ortho position of phenyl ring for higher cytotoxicity.

In MCF-7 breast cancer cell line, the GI₅₀ values for benzothiazole-piperazine derivatives were in range of 9–60 µM. Among series, 4-pyridyl substituted compound 1d (GI₅₀: 9.2 µM) and 2-methoxyphenyl substituted compound 1a (GI₅₀: 15.1 µM) had highest cytotoxicities. Also compounds 1b (R₂: 2-methoxyethyl) and 1f (R₂: 4-chlorobenzyl) showed no inhibition against MCF-7 cell line.

The activity data showed that all synthesized compounds have good cytotoxic activity against HUH-7 liver cancer cell line in range of 3–10 µM. The most active compound against this cell line was 1d (R₂: 4-pyridyl, GI₅₀: 3.1 µM). Compounds 1a (R₂: 2-methoxyphenyl, GI₅₀: 5.7 µM) and 2b (R₂: 2-chlorophenyl, GI₅₀: 6.5 µM) were other highly active derivatives against HUH-7 cell line.

We did not find any significant correlation between GI₅₀ values and log P values of the compounds 1a–f and 2a–d (Table 1). Therefore, the difference in lipophilicity may not be a significant factor for the difference in cytotoxicity of the series in our study.

Compound 1d (R₁ = methyl, R₂ = 4-pyridyl) was found highly active against all tested cancer cell lines. Therefore, we carried out Hoechst staining (Figure 2) and Fluorescence-Activated Cell Sorting Analysis (FACS, Figure 3) for compound 1d to gain insight into its mechanism of action.

Figure 2. Fluorescence images of liver cancer (HUH7) cells^a,b. ^aCells were placed on cover slips and treated with GI₅₀ concentration of compound 1d or DMSO control for 72 h. ^bNuclear Hoechst 33258 stain was used to stain the cells.

Figure 3. Cell cycle distribution analysis of compound 1d.

To examine the type of cell death, induction of apoptosis was investigated by Hoechst staining. Human liver cancer cell line HUH-7 was treated with compound 1d. Hoechst staining showed condensed nuclei that indicated apoptotic cells in treated samples. Whereas in HUH-7 cell group treated with DMSO, no apoptotic cells were present. This result indicated that apoptosis is most likely to be the cell death type induced by compound 1d.

The effect of the compound 1d on the cell cycle was further characterized by FACS analysis, using a propidium iodide stain. This analysis revealed elevated subG₁ phased cells indicating the subG₁ cell cycle arrest compared to control cells treated with DMSO. This result that indicates the presence of cells arrested at subG₁ phase supports the induction of apoptotic cell death in those cells treated with compound 1d.

Conclusions

In this study, ten benzothiazole-piperazine derivatives were synthesized, purified and characterized by various analysis methods. In vitro cytotoxic activities were screened against colorectal (HCT-116), breast (MCF-7), and hepatocellular (HUH-7) cancer cell lines by sulphorhodamine B assay. Most of the compounds showed high cytotoxic activity against all tested cell lines. Hoechst staining and Fluorescence-activated cell sorting analysis, were also performed with compound 1d to understand the mechanism of cytotoxicity which revealed that this compound causes apoptosis by cell cycle arrest at subG₁ phase.

Declaration of interest

The authors have declared no conflict of interest. This work is supported by The Scientific & Technological Research Council of Turkey (TUBITAK) (Project Number: 114S115).