Synthesis and Anti-Tumor Activity of Novel Aminomethylated Derivatives of Isoliquiritigenin

Abstract

A series of new aminomethylated derivatives of isoliquiritigenin was synthesized. The structures of the compounds were confirmed by IR, MS, NMR, ¹³C-NMR and elemental analyses. Cytotoxic activities of these derivatives towards the human prostatic cell line PC-3, human mammary cancer cell line MCF-7 and human oophoroma cell line HO-8910 in vitro were tested. The IC₅₀ values showed cytotoxic activities of some of these new derivatives were relatively strong. Furthermore, tumor growth inhibition in vivo of aminomethylated derivatives of isoliquiritigenin 15 was superior to that of isoliquritigenin and reached inhibition rates of 71.68%. The detailed synthesis, spectroscopic data, biological and pharmacologicalactivities of the synthesized compounds were provided.

1. Introduction

Natural products play a prominent role in oncology because of their potent anticancer activity and good tolerability in normal tissues. Therefore, it is very significant for scientists to screen active compounds from plants and synthesized new derivatives with similar molecular structures as potential anti-tumor drugs or pro-drugs [1,2,3]. As a common active ingredient of many Chinese herbal medicines [4,5], flavonoids reveal little side effects in clinical research [6,7]. Among numerous flavonoids, isoliquiritigenin (ISL) attracted the attention of many scientists. ISL is one of the most important chalcone compound from licorice (Figure 1) and it has a variety of biological activities, such as anti-tumor, anti-virus, anti-free radical, anti-lipid peroxidation and anti-HIV activity, among which the anti-tumor effect came into the focus in recent years [8,9,10,11,12].

Figure 1. Structure of isoliquiritigenin.

Figure 1. Structure of isoliquiritigenin.

The promising anti-tumor effect of ISL stimulates research to modify the chalcone structures, in order to obtain novel derivatives with even stronger anti-tumor activities. So far, ISL and its derivatives have been shown to exert anti-tumor activities by the following mechanism: inhibition of cell proliferation [13,14]; induction of apoptosis [15,16], inhibition of angiogenesis [17], anti-oxidative effects leading to enhanced expression of tumor suppressor genes and decreased expression of oncogenes [18].

In order to enhance water-solubility, bioavailability, and tumor inhibitory activities of ISL, we used ISL as lead compound while remaining its hydroxyl and modifying side chains in A ring of chalcone with nitrogen heterocyclic ring compounds and open-chain amino through Mannich reaction [19,20,21]. We chose the Mannich reaction in our study, because the reaction products Mannich alkaline compounds have a quite wide range of biological activity, such as antibacterial, anticonvulsive, anti-inflammatory, anti-tumor activities [22,23,24,25]. For this reason, Mannich reactions are widely used in medicinal chemistry [26]. To the best of our knowledge, the synthesis of ISL derivatives with different side chains in the A ring containing nitrogen atoms has not been reported yet.

In our present study, chalcone was first synthesized by a hydroxyl-protection method and a consequent condensation reaction was performed in a base environment [27,28]. Then, different nitrogen atoms side chains (heterocyclic azo and open chain amino) were attached to the A ring by Mannich reactions. To test the cytotoxic activities of these derivatives in vitro, human prostatic PC-3, human mammary cancer MCF-7 and human oophoroma HO-4980 cells were used. Additionally, the in vivo anti-tumor activity of the compound with the lowest IC₅₀ value was tested in the mice transplanted with murine S180 sarcoma cells. Our results provide potential ISL derivatives for anti-tumor therapy in the future.

2. Results and Discussion

2.1. Chemistry

ISL has several active spots, and the two hydroxyl groups in ring A partly attributed to its anti-tumor activity [29]. ISL reveals poor water-solubility. It may be possible to improve its water-solubility by attaching an amino-group to ring A. Meanwhile, by structure-activity-relationship analyses of a series of anti-cancer drugs, we found that many functional groups in drugs contained nitrogen. As a result, heterocyclic azo-groups and open chain amino-groups were chosen to modify ring A by the Mannich reaction.

Scheme 1. The route of synthetic isoliquiritigenin derivatives.

Scheme 1. The route of synthetic isoliquiritigenin derivatives.

Reagents and conditions: (a) pyridine-PPTS, 2,3-dihydropyran, CH₂Cl₂, RT, 4 h; (b) pyridine-PPTS, 2,3-dihydropyran, NaHCO₃, CH₂Cl₂, RT, 4.5 h; (c) Ba(OH)₂, CH₃OH_, 45 °C, 12 h; (d) PTSA, CH₃OH, RT, 6 h; (e) amine, CH₃CHO, CH₃COOH, 16–18 °C.

According to this idea, we achieved the synthesis of novel derivatives 7–16 (Scheme 1). In our experiment, Substrate 1 (p-hydroxybenzaldehyde) and 2 (2,4-dihydroxyacetophenone) were first separately protected by 2,3-dihydropyrane in the presence of pyridine-PPTS, and compounds 3 and 4 were obtained. Then, a condensation reaction of compounds 3 and 4 was conducted with Ba(OH)₂ as catalyst and CH₃OH as solvent at 40 °C. Intermediate product 5 was obtained, which was converted to chalcone 6 (ISL) in good yield by deprotection using PTSA. This method using hydroxyl-protection before condensation reaction led to obviously higher yields compared with classical synthesis ways. Next, compounds 7–16 were all synthesized by Mannich reactions using chalcone 6, formaldehyde and amine in one-pot syntheses.

Interestingly, in the Mannich reaction, extending reaction times or enlarging reactant amounts did not effect the final yields. However, byproducts considerably increased. When Mannich reaction was stopped immediately after adding reagents, the highest yields were achieved. Mannich reactions are often accompanied by many side reactions [30], and with several reaction loci, not only one but several aminomethyl groups attached to the benzene ring of ISL during the reaction process. Besides, we found that the addition of a little bit excessive aldehyde and amine also contributed to higher yields.

2.2. Evaluation of Biological Activity

2.2.1. Cytotoxicity Assay

We examined the cytotoxicity of ISL nitrogenous derivatives against three cancer cell lines (PC-3, MCF-7 and HO-8910) and normal human THP-1 cells using MTT growth inhibition assays [31]. ISL was used as a positive control. For THP-1, all derivatives and ISL showed almost no cytotoxicity against normal cells. The IC₅₀ values were all above 100 µM. The IC₅₀ values for the three cancer cell lines were listed in Table 1. Although not all of these compounds exhibited considerable cytotoxicities against these cell lines, some still showed good cytotoxicities in vitro experiment.

Table 1. Cytotoxicity of isoliquiritigenin (ISL) derivatives.

Compound	R1	R2	Cytotoxicity (IC50, μM)				mp, °C
			PC-3 a	MCF-7	HO-4980	THP-1
7	Me	Me	>100	>100	>100	>100	>176
8	CH2CH2OCH2CH2		48.64	>100	96.43	>100	>180
9	H	C6H11	28.32	73.58	>100	>100	>168
10	i-Pr	i-Pr	>100	84.56	98.14	>100	>180
11	Et	Et	66.02	>100	97.32	>100	>176
12	H	CHCH3C2H5	57.05	84.78	>100	>100	>175
13	H	t-Bu	>100	>100	>100	>100	>170
14	H	C2H4OH	87.72	66.01	89.01	>100	>180
15	C2H4OH	C2H4OH	35.14	42.94	37.85	>100	106.1–8
16	CH2CH2CH2CH2		>100	81.84	74.87	>100	116.5–7.4
ISL	-	-	36.90	44.11	58.43	>100

^a PC-3, human prostate cancer cells; MCF-7, human breast cancer cells; HO-4980, human ovarian cancer cells.

Table 1. Cytotoxicity of isoliquiritigenin (ISL) derivatives.

Compound	R1	R2	Cytotoxicity (IC50, μM)				mp, °C
			PC-3 a	MCF-7	HO-4980	THP-1
7	Me	Me	>100	>100	>100	>100	>176
8	CH2CH2OCH2CH2		48.64	>100	96.43	>100	>180
9	H	C6H11	28.32	73.58	>100	>100	>168
10	i-Pr	i-Pr	>100	84.56	98.14	>100	>180
11	Et	Et	66.02	>100	97.32	>100	>176
12	H	CHCH3C2H5	57.05	84.78	>100	>100	>175
13	H	t-Bu	>100	>100	>100	>100	>170
14	H	C2H4OH	87.72	66.01	89.01	>100	>180
15	C2H4OH	C2H4OH	35.14	42.94	37.85	>100	106.1–8
16	CH2CH2CH2CH2		>100	81.84	74.87	>100	116.5–7.4
ISL	-	-	36.90	44.11	58.43	>100

^a PC-3, human prostate cancer cells; MCF-7, human breast cancer cells; HO-4980, human ovarian cancer cells.

For PC-3, compounds 9 and 15 had stronger growth inhibition effect, and the IC₅₀ were 28.32 µM and 35.14 µM for 72 h, respectively. These two compounds showed stronger cytotoxicities than ISL. For HO-8910, 15 showed the strongest cytotoxicity with the IC₅₀ values of 37.85 µM. For MCF-7, only 15 resulted in strong growth inhibition. Compound 15 was more potent than ISL against HO-8910 and MCF-7. Besides, the monoethanolamine substituted compound 14 showed much improved activity compared with the other derivatives, while compound 15 containing two hydroxyl attached with the nitrogen atom showed the most potent cytotoxicity. We conclude that amines containing hydroxyl located in 3′-position of the A ring could be highly beneficial for the activity of the compounds. In addition, compound 15 attached with amines containing two hydroxyl groups resulted in more potent cytotoxicities than compound 14 modified with amines containing only one hydroxy. Therefore, ISL derivative 15 was used to evaluate anti-tumor activity in S180 tumor-bearing mice for further studies.

Table 2. Effect of ISL and ISL derivative 15 (40 mg/kg, 80 mg/kg) on tumor weight and inhibitory ratio in S180 tumor-bearing mice.

Group	Treatment	Dose	Mice Number		TW (g) x ± SD	BWC a (%)	TIR b (%)
		(mg/kg)	Begin	End
I	normal saline	-	10	10	1.23 ± 0.534 **	+4.4	-
II	ISL	40	10	10	0.85 ± 0.369 **	−3.0	32.44
III	ISL derivative 15	40	10	10	0.74 ± 0.618 *	+12.8	39.72
IV	ISL derivative 15	80	10	10	0.35 ± 0.442 **	+7.8	71.68

Student’s t-test was used to compare tumor volumes of treated mice. * p values < 0.05 were considered significant, ** p values < 0.01 were considered very significant. ^a Percentage of mice body-weight change (BWC) after drug treatment: BWC% = (mean BW final day/mean BW first day × 100) − 100; ‘‘+” means bodyweight increase; ‘‘−” means body-weight decrease. ^b Tumor inhibitory rate: TIR% = (1 − average tumor weight of experimental group/the average tumor weight of model control group).

Table 2. Effect of ISL and ISL derivative 15 (40 mg/kg, 80 mg/kg) on tumor weight and inhibitory ratio in S180 tumor-bearing mice.

Group	Treatment	Dose	Mice Number		TW (g) x ± SD	BWC a (%)	TIR b (%)
		(mg/kg)	Begin	End
I	normal saline	-	10	10	1.23 ± 0.534 **	+4.4	-
II	ISL	40	10	10	0.85 ± 0.369 **	−3.0	32.44
III	ISL derivative 15	40	10	10	0.74 ± 0.618 *	+12.8	39.72
IV	ISL derivative 15	80	10	10	0.35 ± 0.442 **	+7.8	71.68

Student’s t-test was used to compare tumor volumes of treated mice. * p values < 0.05 were considered significant, ** p values < 0.01 were considered very significant. ^a Percentage of mice body-weight change (BWC) after drug treatment: BWC% = (mean BW final day/mean BW first day × 100) − 100; ‘‘+” means bodyweight increase; ‘‘−” means body-weight decrease. ^b Tumor inhibitory rate: TIR% = (1 − average tumor weight of experimental group/the average tumor weight of model control group).

Figure 2. Effect of ISL and ISL derivative 15 on S180 tumor growth. ISL (40 mg/kg) was used as control drug. (A): Untreated negative control; (B): Treatment with low dose ISL derivative 15 (40 mg/kg); (C): Treatment with high dose ISL derivative 15 (80 mg/kg); (D): Treatment with ISL (40 mg/kg) as positive control.

Figure 2. Effect of ISL and ISL derivative 15 on S180 tumor growth. ISL (40 mg/kg) was used as control drug. (A): Untreated negative control; (B): Treatment with low dose ISL derivative 15 (40 mg/kg); (C): Treatment with high dose ISL derivative 15 (80 mg/kg); (D): Treatment with ISL (40 mg/kg) as positive control.

2.2.2. In Vivo Toxicity and Antitumor Studies

S180 tumor-bearing mice were treated with 80 or 40 mg/kg ISL derivative 15 and tumor inhibition rates of 71.68% and 39.72%, respectively, were obtained (Table 2). For comparison, ISL (40 mg/kg) was analyzed as positive control. Changes in body weight were used as parameter for toxicity. Loss of body weight was not found after exposure of animals to 80 or 40 mg/kg ISL derivative 15, whereas ISL caused a slight weight loss in animals. Tumors were surgically removed after treatment with ISL and derivative 15, and the results showed significant inhibition of tumor growth (Figure 2). The activity of ISL derivative 15 in S180 tumor-bearing mice indicated that ISL derivative 15 might be an attractive candidate for clinical use in oncology.

3. Experimental Section

3.1. Chemistry

¹H (500 MHz) and ¹³C (125 MHz) NMR spectra were recorded with a Bruker instrument, and reported with TMS as internal standard and DMSO. Chemical shifts (δ values) and coupling constants (J values) are given in ppm and Hz. MS analysis was carried out on an API-3000 LC-MS-MS instrument. The melting points were determined on a WRS-1B digital melting point apparatus (Shanghai Precision & Scientific Instruments Co., Shanghai, China). Elemental analyses were carried out in a EA3000 CHNS-O analyzer. TLC analysis was carried out on silica gel plates GF254. Flash chromatography (Lisure Science (Suzhou) Co., Ltd., Suzhou, China) was performed with silica gel 300–400 mesh using a glass column. Unless otherwise indicated, reagents and solvents were obtained from commercial suppliers.

Procedure for chalcone 6: p-hydroxybenzaldehyde (219.8 mg, 1.8 mmol) and pyridine-PPTS (45.2 mg, 0.18 mmol) were dissolved in dichloromethane (5 mL),2,3-dihydropyran (0.25 mL, 2.7 mmol) was dropwise added. The reaction was stirred at room temperature for 4 h, the mixture was treated with saturated NaCl and extracted with ether. Solvent was removed under reduced pressure to obtain oily crude product (Yield: 94.4%). Similarly, 2,4-dihydroxyacetophenone (152.2 mg, 1.0 mmol), pyridine-PPTS (25.1 mg, 0.1 mmol), and 2,3-dihydropyran (0.14 mL, 1.5 mmol) were added into a round bottom flask and stirred for 2 h, NaHCO₃ (80.0 mg, 1 mmol) was added, continued for another 2.5 h, the mixture was filtered. Oily crude 4 product was obtained after solvent was removed under reduced pressure (Yield: 91.2%). These two hydroxyl-proctected products were dissolved in CH₃OH (5 mL), Ba(OH)₂ (428.4 mg, 2.5 mmol) was added as catalyst. The reaction was stirred at 45 °C (TLC detected) and after 12 h it was diluted with CH₂Cl₂. Then, HCl was added to adjust the system to weak acid. The mixture was extracted with CH₂Cl₂. The condensation product 5 was obtained after solvent was removed in vacuo and was following dissolved in CH₃OH (Yield: 77.9%). PTSA (4.8 mg, 0.028 mmol) was added and the mixture was stirred at room temperature for 6 h, the reaction mixture was poured into ice water and yellow crystals were filtered, washed and then dried. Finally, target compound 6 was obtained (Yield: 95.8%).

General procedure for ISL derivatives 7–16: ISL 128 mg (0.5 mmol) were put into a round bottom flask, and substituted amines (0.6 mmol) were added. Temperature was kept at 16–18 °C. The mixture of 36% formaldehyde solution (0.046 ml, 0.6 mmol) and acetic acid (2 ml) was quickly and dropwise added under electromagnetic stirring. The reaction immediately stopped, when the agents has been dropwise added. The mixture was treated with water and extracted with EtOAc. The organic phase was dried over Na₂SO₄. The solvent was removed under reduced pressure, and the crude mixture was purified by column chromatography with EtOAc/MeOH (5:1 to 1:2 v/v) as eluent to afford the target compounds.

(E)-1-(3-(dimethylaminomethyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (7). According to the general procedure for ISL derivatives using dimethylamine as amine, the lead compound was obtained as yellow powder. Weight: 43.5 mg. Yield: 27.8%. Melting point: >176 °C; ¹H-NMR (500 MHz, DMSO): 2.41 (6H, s, (CH₃)₂N-), 3.84 (2H, s, -CH₂N), 6.22 (1H, d, J = 9 Hz, H3), 6.82~6.84 (2H, m, H5, H6), 7.67~7.75 (4H, m, H2′,3′, 5′, 6′), 8.03 (1H, d, J = 9 Hz, CH₂); ¹³C-NMR (125 MHz, DMSO): 190.65, 172.59, 170.20, 164.47, 160.58, 143.63, 132.05, 131.46, 126.36, 118.04, 116.29, 111.01, 110.15, 106.89, 54.17, 43.76. ESI-MS, m/z [M+H]⁺ 314.3. For C₁₈H₁₉NO₄: C, 68.99; H, 6.11; N, 4.47. Found: C, 68.90; H, 6.08; N, 4.42.

(E)-1-(2,4-dihydroxy-3-(morpholinomethyl)phenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (8). According to the general procedure for ISL derivatives using morpholine as amine, the lead compound was obtained as yellow powder. Weight: 62.84 mg. Yield: 35.4%. Melting point: >180 °C; ¹H-NMR (500 MHz, DMSO): 2.51 (4H, s, -(CH2)₂N), 3.61 (4H, s, -(CH₂)₂O), 3.74 (2H, s, -CH₂N), 6.38 (1H, d, J = 9 Hz, H3), 6.83~6.85 (2H, m, H5, H6), 7.75~7.77 (4H, m, H2′, 3′, 5′, 6′), 8.13 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 191.68, 165.27, 163.75, 160.27, 44.30, 131.47, 131.24, 125.75, 117.31, 115.82, 112.35, 107.99, 107.75, 66.00, 52.54, 51.99. ESI-MS, m/z [M+H]⁺ 356.3. Anal. Calcd. For C₂₀H₂₁NO₅: C, 67.59; H, 5.96; N, 3.94. Found: C, 67.54; H, 5.89; N, 3.87.

(E)-1-(3-(cyclohexylaminomethyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (9). According to the general procedure for ISL derivatives using cyclohexylamine as amine, the lead compound was obtained as yellow powder. Weight: 60.9 mg. Yield: 33.2%. Melting point: >168 °C; ¹H-NMR (500 MHz, DMSO): 1.14~1.99 (10H, m, -CH₂), 2.74 (1H, s, -CHN), 4.01 (2H, s, -CH₂N), 5.99 (1H, d, J = 9 Hz, H3), 6.80~6.82 (2H, m, H5, H6), 7.59~7.70 (4H, m, H2′, 3′, 5′, 6′), 7.88 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 188.20, 175.04, 164.28, 159.67, 141.63, 131.33, 130.63, 126.14, 117.91, 115.74, 111.99, 108.27, 105.14, 54.78, 29.88, 25.02, 23.93. ESI-MS, m/z [M+H]⁺ 368.4. Anal. Calcd. For C₂₂H₂₅NO₄: C, 71.91; H, 6.86; N, 3.81. Found: C, 71.89; H, 6.87; N, 3.83.

(E)-1-(3-(diisopropylamino)-methyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (10). According to the general procedure for ISL derivatives using diisopropylamine as amine, the lead compound was obtained as yellow powder. Weight: 57.56 mg. Yield: 31.2%. Melting point: >180 °C; ¹H-NMR (500 MHz, DMSO): 1.12 (12H, s, -CH₃), 3.23 (2h, s, -CHN), 3.96 (2H, s, -CH₂N), 6.16 (1H, d, J = 9 Hz, H3), 6.82~6.84 (2H, m, H5, H6), 7.70~7.74 (4H, m, H2′, 3′, 5′, 6′), 8.00 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 190.52, 170.27, 163.17, 160.02, 143.27, 131.01, 125.89, 117.50, 115.78, 110.58, 109.59, 106.60, 49.11, 30.66, 18.27. ESI-MS, m/z [M+H]⁺ 370.3. Anal. Calcd. For C₂₂H₂₇NO₄: C, 71.52; H, 7.37; N, 3.79. Found: C, 71.51; H, 7.35; N, 3.75.

(E)-1-(3-((diethylamino)methyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (11). According to the general procedure for ISL derivatives using diaethylamin as amine, the lead compound was obtained as yellow powder. Weight: 72.97 mg. Yield: 42.8%. Melting point: >176 °C; ¹H-NMR (500 MHz, DMSO): 1.09 (6H, t, J = 7 Hz, CH₃), 2.71~2.75 (4H, q, J = 7 Hz, (CH₂)₂N-), 3.92 (2H, s, -CH₂N), 6.19 (1H, d, J = 9 Hz, H3), 6.82~6.84 (2H, m, H5, H6), 7.67~7.75 (4H, m, H2′, 3′, 5′, 6′), 8.02 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 190.80, 170.48, 164.19, 160.52, 143.69, 131.85, 131.48, 126.38, 118.03, 116.27, 111.08, 110.13, 106.68, 49.28, 46.45, 10.79. ESI-MS, m/z [M+H]⁺ 342.1. Anal. Calcd. For C₂₀H₂₃NO₄: C, 70.36; H, 6.79; N, 4.10. Found: C, 70.43; H, 6.74; N, 4.20.

(E)-1-(3-((sec-butylamino)methyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (12). According to the general procedure for ISL derivatives using sec-butylamine as amine, the compound was obtained as yellow powder. Weight: 46.71 mg. Yield: 27.4%. Melting point: >175 °C; ¹H-NMR (500 MHz, DMSO): 0,92~0.93 (6H, d, J = 7 Hz, CH3), 1.82~1.88 (1H, m, -CHN), 2.56 (2H, d, J = 7 Hz, -CH₂-), 3.99 (2H, s, -CH₂N), 6.04 (1H, d, J = 9 Hz, H3), 6.81~6.83 (2H, m, H5, H6), 7.61~7.71 (4H, m, H2′, 3′, 5′, 6′), 7.91 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 189.17, 174.56, 164.68, 160.22, 142.45, 131.84, 131.19, 126.59, 118.34, 116.24, 111.95, 109.23, 105.78, 54.73, 44.17, 26.79, 20.58. ESI-MS, m/z [M+H]⁺ 342.3. Anal. Calcd. For C₂₀H₂₃NO₄: C, 70.36; H, 6.79; N, 4.10. Found: C, 70.33; H, 6.84; N, 4.07.

(E)-1-(3-((tert-butylamino)methyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (13). According to the general procedure for ISL derivatives using tert-butylamine as amine, the lead compound was obtained as yellow powder. Weight: 65.64 mg. Yield: 38.5%. Melting point: >170 °C; ¹H-NMR (500 MHz, DMSO): 1.27 (9H, s, CH3), 3.98 (2H, s, -CH₂N), 5.99 (1H, d, J = 9 Hz, H3), 6.81~6.83 (2H, m, H5, H6), 7.59~7.70 (4H, m, H2′, 3′, 5′, 6′), 7.88 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 188.59, 164.80, 160.18, 142.09, 131.89, 126.63, 118.39, 116.24, 112.66, 108.63, 105.64, 54.03, 37.54, 26.47. ESI-MS, m/z [M+H]⁺ 342.2. Anal. Calcd. For C₂₀H₂₃NO₄: C, 70.36; H, 6.79; N, 4.10.

1-(2,4-dihydroxy-3-((2-hydroxyethylamino)methyl)phenyl)-3-(4-hydroxyphenyl)propen-1-one (14). According to the general procedure for ISL derivatives using monoethanolamine as amine, the lead compound was obtained as yellow powder. Weight: 41.62 mg. Yield: 25.3%. Melting point: >180 °C; ¹H-NMR (500 MHz, DMSO): 2.82 (2H, s, -CH₂OH), 3.58 (2H, s, -CH₂N), 4.01 (2H, s, -CH₂N), 6.03 (1H, d, J = 8 Hz, H3), 6.82~6.83 (2H, m, H5, H6), 7.63~7.69 (4H, m, H2′, 3′, 5′, 6′), 7.90 (1H, d, J = 8 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 165.03, 160.16, 142.20, 131.89, 131.14, 126.64, 118.40, 116.23, 112.23, 108.95, 105.79, 58.14, 49.26, 43.30. ESI-MS, m/z [M+H]⁺ 330.5. Anal. Calcd. For C₁₈H₁₉NO₅: C, 65.64; H, 5.81; N, 4.25. Found: C, 65.66; H, 5.85; N, 4.21.

(E)-1-(3-((bis(2-hydroxyethyl)amino)methyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (15). According to the general procedure for ISL derivatives using diethanolamine as amine, the lead compound was obtained as yellow powder. Weight: 69.94 mg. Yield: 37.5%. Melting point: 106.1–106.8 °C; ¹H-NMR (500 MHz, DMSO): 2.74 (4H, s, -(CH₂)₂N), 3.58~3.60 (4H, t, J = 6 Hz, -CH₂OH), 3.98 (2H, s, -CH2N), 6.28 (1H, d, J = 9 Hz, H3), 6.83~6.85 (2H, m, H5, H6), 7.74~7.76 (4H, m, H2′, 3′, 5′, 6′), 8.08 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 191.61, 172.47, 168.05, 164.05, 160.66, 144.30, 131.88, 131.6, 212.63, 1117.93, 116.30, 112.05, 109.29, 108.12, 58.07, 55.86, 50.29, 21.53. ESI-MS, m/z [M+H]⁺ 374.1. Anal. Calcd. For C₂₀H₂₃NO₆: C, 64.33; H, 6.21; N, 3.75. Found: C, 64.32; H, 6.27; N, 3.69.

(E)-1-(2,4-dihydroxy-3-(pyrrolidin-1-ylmethyl)phenyl-3-(4-hydroxyphenyl)prop-2-en-1-one (16). According to the general procedure for ISL derivatives using pyrrolidine as amine, the lead compound was obtained as yellow powder. Weight: 60.0 mg. Yield: 35.4%. Melting point: 116.5–117.1 °C; ¹H-NMR (500 MHz, DMSO): 1.84 (4H, s, -CH2-CH2-), 2.82~2.84 (4H, t, J = 6 Hz, (CH₂)₂N-), 3.99 (2H, s, -CH₂N), 6.18 (1H, d, J = 9 Hz, H3), 6.82~6.84 (2H, m, H5, H6), 7.65~7.73 (4H, m, H2′, 3′, 5′, 6′), 8.00 (1H, d, J = 9 Hz, H2); ¹³C-NMR (125 MHz, DMSO): 190.20, 172.54, 171.03, 164.57, 160.48, 143.31, 132.02, 131.39, 126.43, 118.10, 116.28, 110.64, 110.51, 107.105, 334 , 50.31, 23.54. ESI-MS, m/z [M+H]⁺ 340.3. Anal. Calcd. For C₂₀H₂₁NO₄: C, 70.78; H, 6.24; N, 4.13. Found: C, 70.85; H, 6.20; N, 4.15.

3.2. Biological Assays

3.2.1. Cytotoxicity Assay

Cytotoxicity was evaluated against four human cell lines (MCF-7 breast cancer, PC-3 prostatic cancer, HO-8910 ovarian cancer, and THP1 normal cells). Briefly, MCF-7, PC-3 and HO-8910 cells were separately plated in 96-well culture plates (1 × 10⁵ cells/well). After 24 h incubation, cells were treated with compounds 7–16 (0, 3.13, 6.25, 12.5, 25, 50 and 100 µM) for 72 h. MTT solution (5 mg/mL) was then added to each well. After 4 h incubation, the formazan precipitate was dissolved in 100 mL dimethyl sulfoxide (DMSO), and then the absorbance was measured in an ELISA reader (Thermo Molecular Devices Co., Union City, CA, USA) at 570 nm. The cell viability ratio was calculated by the following formula: Inhibitory ratio (%) = (1 − OD_treated/OD_control) × 100%.

3.2.2. Animal Treatment

Female BALB/c mice (6–8 weeks old, 25 ± 2 g) were obtained from the Third Affiliated Hospital of Harbin Medical University. S180 sarcoma cells were purchased from the Experimental Animal Center of Peking University Health Science Center (Heilongjiang, China). The animals were housed in polypropylene cages and maintained under controlled conditions of 12 h light/12 h dark photoperiod and 55 ± 5% relative humidity at room temperature (28 ± 2 °C). After fed in our facility for 1 week, 40 mice were induced S180 tumor cells according to Wu et al. with some modification [32]. All mice were subcutaneously implanted with 1 × 10⁶ cells/mice on the right flank. After inoculation for 24 h, all animals were randomly divided into one untreated control group, one treated positive control group and two treated experimental groups (each group contained 10 mice).

Group I (untreated control) received orally the same volume of 0.9% normal saline once per day as the treated groups. Group II (treated positive control) received orally ISL at a dosage of 40 mg/kg body weight once per day. Group III (low dose of ISL derivative 15 treatment) received ISL derivative 15 orally (dissolved in 0.2 mL normal saline) at a dosage of 40 mg/kg body weight once per day. Group IV (high-dose of ISL derivative 15 treatment) received ISL derivative 15 orally (dissolved in 0.2 mL normal saline) at a dosage of 80 mg/kg body weight once per day.

4. Conclusions

A series of novel aminomethylated derivatives of ISL were synthesized and their anti-tumor activities in vitro were evaluated against three cancer cell lines. Through the results, we found that ISL derivatives contained two or more aminemethyl and nitrogen connected with methylene substituent exhibited considerable cytotoxicity. S180 tumor-bearing mice were treated with 80 or 40 mg/kg ISL derivative 15 and tumor inhibition rates were 71.68% and 39.72%, respectively. Our results indicated that ISL derivative 15 might be an attractive candidate for clinical use in oncology in the future and the scheme we designed to synthesize ISL derivatives might become a useful method for chemical synthesis and modification of flavone drugs.