Preparation of Sesquiterpene Lactone Derivatives: Cytotoxic Activity and Selectivity of Action

Cancer is one of the most important causes of death worldwide. Solid tumors represent the great majority of cancers (>90%) and the chemotherapeutic agents used for their treatment are still characterized by variable efficacy and toxicity. Sesquiterpene lactones are a group of naturally occurring compounds that have displayed a diverse range of biological activities including cytotoxic activity. A series of oxygenated and oxy-nitrogenated derivatives (4–15) from the sesquiterpene lactones cumanin (1), helenalin (2), and hymenin (3) were synthesized. The silylated derivatives of helenalin, compounds 13 and 14, were found to be the most active against tumor cell lines, with GI50 values ranging from 0.15 to 0.59 μM. The ditriazolyl cumanin derivative (11) proved to be more active and selective than cumanin in the tested breast, cervix, lung, and colon tumor cell lines. This compound was the least toxic against splenocytes (CC50 = 524.1 µM) and exhibited the greatest selectivity on tumor cell lines. This compound showed a GI50 of 2.3 µM and a SI of 227.9 on WiDr human colon tumor cell lines. Thus, compound 11 can be considered for further studies and is a candidate for the development of new antitumor agents.


Introduction
According to recent studies, 60% of newly identified chemical entities are natural products, semi-synthetic analogs, or synthetic compounds based on their pharmacophores [1]. This occurs together with the increased incidence of life-threatening diseases such as AIDS, cancer, hepatitis, etc. [2]. It is noteworthy that some semi-synthetic compounds derived from a natural product sometimes show higher bioactivity than the original natural compound. Considering the wide range of biological activities, such as antiparasitic, antiproliferative, anti-inflammatory, antiviral, antibacterial, and antifungal activity, sesquiterpene lactones have attracted scientific interest [3][4][5][6][7][8][9].
Analyzing the structure of molecules used in cancer therapy, most of them show nitrogenated functional groups like amide and carbamate moieties in their framework [10]. Recently, it a series of disubstituted 1,2,3-triazoles has been reported exhibiting potent cytotoxicity in the nanomolar range and tubulin inhibitory activity in the low micromolar range [11]. This kind of compound is considered an interesting unit in the design of anticancer drugs. Such heterocycles may act according to their dipolar character, rigidity, and ability to form hydrogen bridge bonds, or simply as connectors [12].
Synthetic organic chemists have shown great interest in 1,2,3-triazoles for developing new biologically active molecules. Triazole moieties do not occur naturally, but 1,2,3-triazole cores may form the basis of small-molecule pharmaceutical leads. Molecules containing this heterocyclic nucleus have being reported to have anti-HIV, antimicrobial, anti-allergic, antifungal, and antitumor activity [13].
In a previous work, we reported the cytotoxicity enhancement of some sesquiterpenes and iridoids when their lipophilicity was increased by adding alkyl and/or aryl-silyl functionalities on the natural framework [12].
In consideration of the abovementioned factors, we herein describe our findings aimed at the synthesis and cytotoxic evaluation of oxygenated and oxy-nitrogenated derivatives from sesquiterpene lactones (STLs) cumanin (1), helenalin (2) and hymenin (3) (Figure 1). It should be emphasized that these STLs can be isolated in significant quantities from the natural sources. Furthermore, these plants are widely distributed in Argentina [14][15][16].

Introduction
According to recent studies, 60% of newly identified chemical entities are natural products, semi-synthetic analogs, or synthetic compounds based on their pharmacophores [1]. This occurs together with the increased incidence of life-threatening diseases such as AIDS, cancer, hepatitis, etc. [2]. It is noteworthy that some semi-synthetic compounds derived from a natural product sometimes show higher bioactivity than the original natural compound. Considering the wide range of biological activities, such as antiparasitic, antiproliferative, anti-inflammatory, antiviral, antibacterial, and antifungal activity, sesquiterpene lactones have attracted scientific interest [3][4][5][6][7][8][9].
Analyzing the structure of molecules used in cancer therapy, most of them show nitrogenated functional groups like amide and carbamate moieties in their framework [10]. Recently, it a series of disubstituted 1,2,3-triazoles has been reported exhibiting potent cytotoxicity in the nanomolar range and tubulin inhibitory activity in the low micromolar range [11]. This kind of compound is considered an interesting unit in the design of anticancer drugs. Such heterocycles may act according to their dipolar character, rigidity, and ability to form hydrogen bridge bonds, or simply as connectors [12].
Synthetic organic chemists have shown great interest in 1,2,3-triazoles for developing new biologically active molecules. Triazole moieties do not occur naturally, but 1,2,3-triazole cores may form the basis of small-molecule pharmaceutical leads. Molecules containing this heterocyclic nucleus have being reported to have anti-HIV, antimicrobial, anti-allergic, antifungal, and antitumor activity [13].
In a previous work, we reported the cytotoxicity enhancement of some sesquiterpenes and iridoids when their lipophilicity was increased by adding alkyl and/or aryl-silyl functionalities on the natural framework [12].
In consideration of the abovementioned factors, we herein describe our findings aimed at the synthesis and cytotoxic evaluation of oxygenated and oxy-nitrogenated derivatives from sesquiterpene lactones (STLs) cumanin (1), helenalin (2) and hymenin (3) (Figure 1). It should be emphasized that these STLs can be isolated in significant quantities from the natural sources. Furthermore, these plants are widely distributed in Argentina [14][15][16].

Chemistry
The STLs cumanin (1), helenalin (2), and hymemin (3) were isolated from Ambrosia tenuifolia Spreng., Gaillardia megapotamica var. megapotamica Spreng., and Parthenium hysterophorus L., respectively ( Figure 1). In preliminary bioactivity tests, these STLs showed significant cytotoxic activity, with helenalin being the most active (2). Therefore, STLs have been used as starting materials for the preparation of a series of oxygenated and oxo-nitrogenous products; these modifications led to an improved activity of the obtained derivatives. In this way, derivatives 4 to 15 were prepared ( Figure 2). In preliminary bioactivity tests, these STLs showed significant cytotoxic activity, with helenalin being the most active (2). Therefore, STLs have been used as starting materials for the preparation of a series of oxygenated and oxo-nitrogenous products; these modifications led to an improved activity of the obtained derivatives. In this way, derivatives 4 to 15 were prepared ( Figure 2). The presence of a hydroxyl group in the structure of the natural compound allowed for the preparation of acetylated and silylated derivatives from these STLs. In this regard, the diacetylated derivative 4 was prepared under standard conditions from structure 1, and silylated derivatives 5 to 7 of were obtained under standard conditions and by varying the silylating agent (Scheme 1). The silylation reaction of cumanine with TBDPSiCl led to the monosilylated derivative at C3. The introduction of this voluminous group resulted in high steric hindrance. NMR, COSY, and HMBC analysis allowed for the coupling of H4 (δ = 3.45, dd, J = 8 Hz) with the hydrogen of the hydroxyl group of the position 3; HSQC analysis confirms the above since the signal corresponding to OH does not show C-H coupling. HRMS-ES (m/z) analysis: [M + Na] + : Calcd. for C31H40O4Si2Na: 527.2594; found 527.2586, determines the presence of monosilylated derivative 7.
Acetylated and silylated derivatives (12 to 15) were prepared from STLs 2 and 3 (Scheme 2). The presence of a hydroxyl group in the structure of the natural compound allowed for the preparation of acetylated and silylated derivatives from these STLs. In this regard, the diacetylated derivative 4 was prepared under standard conditions from structure 1, and silylated derivatives 5 to 7 of were obtained under standard conditions and by varying the silylating agent (Scheme 1). The silylation reaction of cumanine with TBDPSiCl led to the monosilylated derivative at C3. The introduction of this voluminous group resulted in high steric hindrance. NMR, COSY, and HMBC analysis allowed for the coupling of H4 (δ = 3.45, dd, J = 8 Hz) with the hydrogen of the hydroxyl group of the position 3; HSQC analysis confirms the above since the signal corresponding to OH does not show C-H coupling. HRMS-ES (m/z) analysis: [ Synthetic organic chemistry has encouraged great interest in 1,2,3-triazoles in the development of new biologically active molecules [12]. The triazole moiety does not occur in nature, but 1,2,3triazole cores may form the basis of small-molecule pharmaceutical leads. Molecules containing this heterocyclic nucleus have been reported to have anti-HIV, antimicrobial, anti-allergic, antifungal, and antitumor activities [12]. One of the most popular reactions within the click chemistry paradigm is the Cu (I)-catalyzed 1,3-dipolar Hüisgen cycloaddition of alkynes and azides. This reaction proceeds with great efficiency and selectivity in aqueous media and yields triazole moieties [17]. Our first objective was to obtain alkynes 8 and 9 using STL 1 as the starting material (see Scheme 3). Compound 1 was treated with propargyl bromide and sodium hydride in THF to obtain derivatives 8 and 9, and working conditions were optimized in order to increase the yield of both products (for the characterization of the propargyl group at C3 see the Supplementary Materials). Subsequently, under Hüisgen conditions, 8 and 9 were converted into the corresponding mono and di-triazole derivatives 10 and 11, respectively (Scheme 3). Synthetic organic chemistry has encouraged great interest in 1,2,3-triazoles in the development of new biologically active molecules [12]. The triazole moiety does not occur in nature, but 1,2,3-triazole cores may form the basis of small-molecule pharmaceutical leads. Molecules containing this heterocyclic nucleus have been reported to have anti-HIV, antimicrobial, anti-allergic, antifungal, and antitumor activities [12]. One of the most popular reactions within the click chemistry paradigm is the Cu (I)-catalyzed 1,3-dipolar Hüisgen cycloaddition of alkynes and azides. This reaction proceeds with great efficiency and selectivity in aqueous media and yields triazole moieties [17]. Our first objective was to obtain alkynes 8 and 9 using STL 1 as the starting material (see Scheme 3). Compound 1 was treated with propargyl bromide and sodium hydride in THF to obtain derivatives 8 and 9, and working conditions were optimized in order to increase the yield of both products (for the characterization of the propargyl group at C3 see the Supplementary Materials). Subsequently, under Hüisgen conditions, 8 and 9 were converted into the corresponding mono and di-triazole derivatives 10 and 11, respectively (Scheme 3).

Antiproliferative Activity
In vitro antiproliferative activity was evaluated using the protocol of the National Cancer Institute (NCI) after 48 h of drug exposure using the sulforhodamine B (SRB) assay. Results, expressed as GI50 values, are shown in Table 1. Data revealed that oxygenated derivatives (5-7, 13-15) are more active than natural products (1-3) with GI50 values ranging from 0.15 to 6.8 μM, in all cell lines. Helenalin silylated derivatives 13 and 14 were found to be the most active against all tested cell lines, with GI50 values ranging from 0.15 to 0.59 μM. We cannot discard that increasing lipophilicity may well result in an increase in cytotoxicity in vitro.
Most of the cumanin derivatives have shown higher cytotoxic activity than the natural parent compound 1. The presence of two 1,2,3-triazole groups in derivative 11 has increased the activity values by one order of magnitude compared to cumanin (1). Compounds 4, 8, 9, and 10 displayed moderate activity and similar results to 1, with the exception of compounds 8 and 9, which were more active than 1 against WiDr cells.
Hymenin derivative (15) has shown increased activity compared with natural compound 3.

Antiproliferative Activity
In vitro antiproliferative activity was evaluated using the protocol of the National Cancer Institute (NCI) after 48 h of drug exposure using the sulforhodamine B (SRB) assay. Results, expressed as GI 50 values, are shown in Table 1. Data revealed that oxygenated derivatives (5-7, 13-15) are more active than natural products (1-3) with GI 50 values ranging from 0.15 to 6.8 µM, in all cell lines. Helenalin silylated derivatives 13 and 14 were found to be the most active against all tested cell lines, with GI 50 values ranging from 0.15 to 0.59 µM. We cannot discard that increasing lipophilicity may well result in an increase in cytotoxicity in vitro.
Most of the cumanin derivatives have shown higher cytotoxic activity than the natural parent compound 1. The presence of two 1,2,3-triazole groups in derivative 11 has increased the activity values by one order of magnitude compared to cumanin (1). Compounds 4, 8, 9, and 10 displayed moderate activity and similar results to 1, with the exception of compounds 8 and 9, which were more active than 1 against WiDr cells.
Hymenin derivative (15) has shown increased activity compared with natural compound 3.  The GI 50 mean of each derivative was compared to the activity of its parent compound. For compounds 1, 2, and their respective derivatives, a one-way Anova + Dunnett's test was carried out. T-test was used for compound 3 and its derivative. Asterisks indicate significant differences. * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.

Cytotoxicity on Primary Cell Culture Activity
The cytotoxicity of the sesquiterpene lactones and its derivatives was evaluated using mouse splenocytes. Table 2 shows the results of the cytotoxicity assay, as CC 50 (the concentration of each compound that causes 50% cell death) and the selectivity indexes. Cumanin (1) was the least toxic natural compound against splenocytes (CC 50 = 29.4 µM) compared with the other sesquiterpene lactones (2 and 3). Cumanin derivatives (4-11) have shown CC 50 values and selectivity indexes higher than natural compound 1. Compounds 4-9 displayed moderate selectivity, while compound 10 showed low selectivity on the tested cell lines. The incorporation of two triazole groups in cumanin (1) reduced cytotoxicity on normal cells and improved selectivity against tumor cell lines. Compound 11 was the least toxic against splenocytes (CC 50 = 524.1 µM) and presented the greatest selectivity on the tested cell lines. This compound showed a GI 50 of 2.3 and a SI of 227.9 on human colon tumor cell line WiDr. The CC 50 mean of each derivative was compared to the activity of its parent compound. For compounds 1, 2, and their respective derivatives, a one-way Anova + Dunnett's test was carried out. T-test was used for compound 3 and its derivative. Asterisks indicate significant differences. * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001. Compounds 2 and 3, as well as their corresponding derivatives, displayed a low selectivity of action against tumor cell lines. Similar results have been obtained for compound 12 by Lee et al. [18].

General
Unless otherwise stated, all solvents were purified by standard techniques. Reactions requiring anhydrous conditions were performed under an argon atmosphere. Anhydrous magnesium sulfate was used for drying solutions. Reactions were monitored by thin-layer chromatography (TLC) on silica gel plates (60 F254) (Merck KGaA, Darmstadt, IN, USA), and visualized with UV light, 2.5% phosphomolybdic acid in ethanol, or vanillin with acetic and sulfuric acid in ethanol with heating. Purification was performed by column chromatography (CC) on silica gel (230-400 mesh) using n-hexane and ethyl acetate gradient as solvent. 1 H NMR spectra were recorded on a Bruker (Bruker Biospin GmbH, Silberstreifen, Rheinstetten, Germany) 200, 500, or 600 MHz, 13 C NMR spectra were recorded at 50 and 125 M Hz, and chemical shifts are reported relative to internal Me 4 Si (δ = 0). Melting points were determined by using an Electrothermal IA9000 melting point apparatus; results are reported in degrees Celsius and are uncorrected. Optical rotations were recorded on a 343 Perkin Elmer polarimeter (Waltham, MA, USA). High-resolution ESI mass spectra were obtained from a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, a RF-only hexapole ion guide, and an external electrospray ion source. HRMS spectra were obtained on a Micromass AutoSpec (Oakville, ON, Canada) mass spectrometer.

Plant Material
The species used in the present work were Ambrosia tenuifolia Spreng. In all cases the aerial parts were collected, dried at room temperature to a constant weight, manually fragmented or ground, as appropriate, and placed in plastic bags from which the oxygen was removed and replaced by argon inert gas for storage until the moment of processing.

Isolation of Cumanin from A. tenuifolia
Extraction conditions: 1.25 kg of aerial parts dried at room temperature to constant weight were processed. The plant material was extracted with acetone (3 × 48 h) at room temperature and the organic extracts were concentrated in vacuo. The residue (30 g) was collected and chromatographed in a 63-cm, 7 cm diameter column, using Sigel 60 G (70-230 mesh) (Merck KGaA, Darmstadt, IN, USA) and eluting with n-hexane-EtOAc mixtures of increasing polarity [16].
Under these conditions, 6 g of cumanin (1) were obtained in the fractions eluted with hexane: ethyl acetate 2:8. These values represent a yield of 4.8 g of cumanin per kg of dry plant.

Isolation of Helenalin from G. megapotámica var. megapotámica
The dried to constant weight aerial parts (1.50 kg) of G. megapotamica var megapotamica were extracted (3 × 48 h) with MeOH at room temperature. The organic extracts were combined and concentrated in a rotary evaporator. The dry extract was solubilized in 1 L of a mixture of MeOH: H 2 O (8:2) and stored in a refrigerator overnight. After this cooling time, the organic phase was separated and the aqueous phase was washed with MeOH (3 × 500 mL). The organic phases were combined, and solvent was removed by rotary evaporator [18]. Thirty-five grams of dry extract were recovered and chromatographed on a 65-cm length and 7 cm in diameter column, using as stationary phase of Sigel 60 G (70-230 mesh) and eluting with n-hexane:EtOAc mixtures of increasing polarity. After two separative processes in CC, 10.2 g of helenalin (2) were obtained, representing a yield of 6.8 g per kg of dry plant.

Extraction and Purification of Hymenin from P. hysterophorus
For preparing the crude extract of the dried to constant weight aerial parts of P. hysterophorus, 1.10 kg of plant material was used and extracted by maceration at room temperature with CHCl 3 for 24 h. The maceration was filtered and the extract was taken to dryness in a rotary evaporator. Subsequently, the dried extract was solubilized in hot EtOH and left overnight at room temperature. Then, it was filtered and the ethanolic solution was extracted with CHCl 3 (4 × 150 mL). The organic phase was dried over anhydrous Na 2 SO 4 and filtered, and the solvent was removed under reduced pressure at a temperature lower than 40 • C until 20 g of a gummy-like dry residue was obtained [18]. The fractionation was carried out by preparative CC (65 × 7 cm) using Sigel 60G (70-230 mesh) as stationary phase and eluting with mixtures of n-hexane and EtOAc of increasing polarities. After the chromatographic separation, 3.4 g of hymenin (3) were obtained, representing a yield of 3.1 g per kg of dry plant.

Chemistry
Preparation of derivative 4: compound 1 (50 mg) (1 eq., 0.188 mmol) of 1 was dissolved in 1 mL of pyridine (0.2 M) and 1 mL of acetic anhydride (50 eq., 9.4 mmol) was added. The reaction was monitored by TLC after extraction with EtOAc in acid medium. After 6 h, the reaction was complete and quenched by pouring in saturated copper sulfate solution and extracting with ethyl acetate (3 × 50 mL each)., The organic phase was washed with distilled water (3 × 25 mL each), dried with anhydrous sodium sulfate, and concentrated under reduced pressure. Under these conditions, 49 mg of the derivative 4 was recovered as a white solid, yield 75%.
Preparation of derivative 5: compound 1 (50 mg) (1 eq., 0.187 mmol) was dissolved in 1.5 mL of Cl 2 CH 2 (0.1 M), 45 mg (3.5 eq., 0.658 mmol) of imidazole, and 69 mg of TMSiCl (0.08 mL, 0.657 mmol) were added; reaction was carried out at room temperature and confirmed by TLC to be complete after 3 h. The reaction was quenched with a concentrated solution of (NH 4 ) 2 SO 4 (40 mL) and extracted with Et 2 O (3 × 20 mL). The organic phase was washed with water (30 mL) and dried with Na 2 SO 4 , filtered, concentrated, and purified by isocratic CC (30 × 0.5 cm) using Sigel 60 G (70-230 mesh), as stationary phase, and a mixture of n-hexane-EtOAc (60:40) as a mobile phase to give 48 mg of compound 5 as a yellow solid, yield 63%.
Preparation of derivative 6: compound 1 (50 mg) (1 eq., 0.187 mmol) was dissolved in 1.5 mL of Cl 2 CH 2 (0.1 M), 45 mg (3.5 eq., 0.657 mmol) of imidazole and 87 mg of DMISOPSiCl (0.1 mL, 0.657 mmol) were added; the reaction was carried out at room temperature and confirmed by TLC to be complete after 1 h. The reaction was quenched with a concentrated solution of (NH 4 ) 2 SO 4 (40 mL) and extracted with Et 2 O (3 × 20 mL). The organic phase was washed with water (30 mL) and dried with Na 2 SO 4 , filtered, concentrated, and purified by isocratic CC (30 × 0.5 cm) using Sigel 60 G (70-230 mesh), as stationary phase, and a mixture of n-hexane:EtOAc (50:50) as mobile phase to give 51 mg of compound 6 as a yellow solid, yield 58%.
Preparation of derivative 7: compound 1 (50 mg) (1 eq., 0.187 mmol) was dissolved in 1.5 mL of Cl 2 CH 2 (0.1 M), 45 mg (3.5 eq., 0.657 mmol) of imidazole and 211 mg TBDPSiCl (0.2 mL, 0.657 mmol) were added; reaction was carried out at room temperature. The reaction was confirmed to be complete by TLC after 4 h. The reaction was quenched with a concentrated solution of (NH 4 ) 2 SO 4 (40 mL) and extracted with Et 2 O (3 × 20 mL). The organic phase was washed with water (30 mL) and dried with Na 2 SO 4 , filtered, concentrated, and purified by CC (30 × 0.5 cm) using Sigel 60 G (70-230 mesh), as stationary phase and 160 mL of a mixture of n-hexane:EtOAc (80:20) and 320 mL of a mixture of n-hexane:EtOAc (70:30) as mobile phase to give 43 mg of compound 7 as a yellow solid, yield 45%.
Preparation of derivative 11: To obtain dithriazole 11, compound 9 (20 mg) (1eq., 0.03 mmol) was dissolved in 1 mL (0.1 M) of a 1:1 ethanol/water mixture at room temperature, and 15 mg of benzyl azide was added (1 eq., 0.03 mmol), then 1 mg of CuSO 4 .5H 2 O (0.1 eq., 0.003 mmol) and 3.29 mg of sodium ascorbate (0.3 eq., 0.009 mmol) were also added. The reaction was confirmed by TLC to be complete after 96 h. The reaction was quenched with 20 mL of water and extracted with ethyl ether (3 × 30 mL). The ether phase was dried with Na 2 SO 4 , filtered, concentrated under reduced pressure. Then, it was purified by CC of Sigel 60G (70-230 mesh) using a mixture of n-hexane:EtOAc: (40:60) as mobile phase to give 8.9 mg (yield 21.9%) of compound 11 as a white amorphous solid.
Preparation of derivative 12: 50 mg (1 eq., 0.190 mmol) of helenalin (2), 1 mL (50 eq., 9.5 mmol) of acetic anhydride, and 1 mL (0.2M) of pyridine were stirred at room temperature. The reaction was confirmed to be complete by TLC after 6 h. The reaction was quenched with saturated copper sulfate solution and extracted with ethyl acetate (3 × 50 mL). The organic phase was washed with water (3 × 25 mL), dried with Na 2 SO 4 , and concentrated under reduced pressure. Then, it was purified by CC of Sigel 60G (70-230 mesh) using n-hexane:EtOAc (40:60) as mobile phase. Forty-two milligrams of 12 were obtained (yield 70%) as an amorphous solid.
Preparation of derivative 15: 100 mg of hymenin (3) (1 eq., 0.351 mmol) and 84 mg (3.5 eq., 1.23 mmol) of imidazole in 1.5 mL of dichloromethane were dissolved at room temperature. After 15 min of stirring, 0.2 mL (3.5 eq., 1.23 mmol) TMSiCl was added. The reaction lasted 72 h. After this time, the reaction was poured into 40 mL of ammonium chloride solution and extracted with ethyl ether (3 × 20 mL). The organic phase was washed with water (30 mL) and dried with Na 2 SO 4 , filtered, concentrated, and purified by CC (30 × 0.5 cm) of silica gel 60G (70-230 mesh) using n-hexane:EtOAc (30:70) as eluent; 44.6 mg of 18 (yield 35%) were obtained as an amorphous solid. Then, the SRB assay was performed. Optical density (OD) of each well was measured at 492 nm, using BioTek's PowerWave XS Absorbance Microplate Reader (Highland Park, Winooski, VT, USA). Values were corrected for background OD from wells containing only culture medium. The percentage growth (PG) was calculated with respect to untreated control cells (C) at each level of drug concentrations based on the difference in OD at the start time (T0) and at the end of drug exposure (T), according to NCI formulas. Therefore, if T is greater than or equal to T0, the calculation is 100 × [(T-T0)/(C-T0)]. If T is lower than T0, denoting cell death, the calculation is 100 × [(T-T0)/(T0)]. The effect is defined as the growth percentage, where 50% growth inhibition (GI 50 ) represents the concentration at which PG is +50. Based on these calculations, a PG value of 0 corresponds to the number of cells present at the beginning of drug exposure, while negative PG values denote net cell death.

Cytotoxicity on Primary Cell Culture
In a 96-well plate, spleen cells from Balb/c mouse (1.5 × 10 5 ) were incubated with different drug dilutions (200, 100, 50, 10, and 5 µg/mL) in RPMI medium containing 10% fetal calf serum. After 48 h of incubation at 37 • C (5% CO 2 ), cells were harvested, washed once with PBS, and stained with 2.5 µg/mL propidium iodide (PI) for 5 min at room temperature. Subsequently, cell death was assessed by flow cytometry using a BD FACSaria II cytometer. Cells incubated only with a drug vehicle were used as a 100% viability control and death percentage was calculated according to the following formula: Then the concentration capable of causing cell death in 50% of splenocytes (CC 50 ) was determined using a non-linear regression approach.

Statistical Analysis
Results are presented as means ± SD. GraphPad Prism 5.0 software (GraphPad Software Inc., San Diego, CA, USA) was employed to carry out calculations. The results account for three to four independent experiments.

Conclusions
Given these results, using natural compounds can be a viable strategy to prepare new active molecules. Sesquiterpene lactones were the starting material for their transformation into several oxygenated and oxo-nitrogenated derivatives by chemical reactions aiming at the hydroxylated positions. Our strategy was to obtain new derivatives, including functionalities such as acetates, silyl ethers, and 1,2,3-triazoles. Although sesquiterpene lactones showed interesting antiproliferative activity values, a significant number of these synthetic derivatives showed greater activity than the naturally occurring parent product. Many of the synthesized analogs were more selective toward tumor cell lines than normal cells. Compound 11, the ditriazolyl cumanin derivative, proved to be more active and selective than cumanin in the tested breast, cervix, lung, and colon tumor cell lines. Thus, this compound can be considered for further studies and is a possible candidate for developing new antitumor agents.
Finally, this work aims at illustrating the possibility of obtaining new naturally-occurring anti-tumor leads from molecular frameworks, some of which exhibit significantly improved bioactivity just by common chemical transformations.