New Flavonoid Derivatives from Melodorum fruticosum and Their α-Glucosidase Inhibitory and Cytotoxic Activities

Three new flavonoid derivatives, melodorones A–C (1–3), together with four known compounds, tectochrysin (4), chrysin (5), onysilin (6), and pinocembrin (7), were isolated from the stem bark of Melodorum fruticosum. Their structures were determined on the basis of extensive spectroscopic methods, including NMR and HRESIMS, and by comparison with the literature. Compounds 1–7 were evaluated for their in vitro α-glucosidase inhibition and cytotoxicity against KB, Hep G2, and MCF7 cell lines. Among them, compound 1 exhibited the best activity against α-glucosidase and was superior to the positive control with an IC50 value of 2.59 μM. On the other hand, compound 1 showed moderate cytotoxicity toward KB, Hep G2, and MCF7 cell lines with the IC50 values of 23.5, 19.8, and 23.7 μM, respectively. These findings provided new evidence that the stem bark of M. fruticosum is a source of bioactive flavonoid derivatives that are highly valuable for medicinal development.


Structural Elucidation of the Isolates
Compound 1 was isolated as a colorless gum. The molecular formula C 21  The characteristic resonances of a flavone core structure were evident from the 1 H NMR data at δ H 7.03 (s, H-3), and 13 C NMR data at δ C 163.4 (C-2), 104.9 (C-3), and 182.4 (C-4) [18]. The NMR data revealed the presence of one hydrogen-bonded hydroxy (δ H 12.77), one methoxy (δ H 3.94), and one prenyloxy (one oxymethylene protons at δ H 4.46 (2H, d, J = 7.5 Hz), one olefinic proton at δ H 5.45 (1H, m), two methyl protons at δ H 1.71 (3H, s) and 1.63 (3H, s)) substituents. A singlet proton at δ H 6.97 was assigned to aromatic proton H-8 of the ring A. The remaining proton resonances were typical of an unsubstituted ring B flavone at δ H 8.11 (H-2 /6 ) and 7.58-7.63 (H-3 /4 /5 ). The HMBC correlation of H-1 (δ H 4.46) with C-6 (δ C 130.6) allowed the placement of the prenyloxy moiety at C-6 ( Figure 2). The methoxy group at δ H 3.94 (3H, s) located at C-7 was proven by the HMBC correlation of the methoxy substituent protons with C-7 (δ C 159.3). In turn, the HMBC correlations of 5-OH (δ H 12.77) with C-5 (δ C 152.8), C-6 (δ C 130.6), and C-10 (δ C 105.3) supported the position of the hydroxyl group at C-5. A careful comparison of the 1 H and 13 C NMR spectral data of 1 with baicalein [19] identified similar signals, distinguished by two hydroxyl groups at C-6 and C-7 (ring A) were replaced by the prenyloxy and methoxy substituents, respectively, in 1. This deduction was strongly confirmed by the HSQC and HMBC correlations ( Figure 2). From the aforementioned results, structure 1 was assigned as melodorone A. . The characteristic resonances of a flavone core structure were evident from the 1 H NMR data at δH 7.03 (s, H-3), and 13 C NMR data at δC 163.4 (C-2), 104.9 (C-3), and 182.4 (C-4) [18]. The NMR data revealed the presence of one hydrogen-bonded hydroxy (δH 12.77), one methoxy (δH 3.94), and one prenyloxy (one oxymethylene protons at δH 4.46 (2H, d, J = 7.5 Hz), one olefinic proton at δH 5.45 (1H, m), two methyl protons at δH 1.71 (3H, s) and 1.63 (3H, s)) substituents. A singlet proton at δH 6.97 was assigned to aromatic proton H-8 of the ring A. The remaining proton resonances were typical of an unsubstituted ring B flavone at δH 8.11 (H-2′/6′) and 7.58-7.63 (H-3′/4′/5′). The HMBC correlation of H-1″ (δH 4.46) with C-6 (δC 130.6) allowed the placement of the prenyloxy moiety at C-6 ( Figure 2). The methoxy group at δH 3.94 (3H, s) located at C-7 was proven by the HMBC correlation of the methoxy substituent protons with C-7 (δC 159.3). In turn, the HMBC correlations of 5-OH (δH 12.77) with C-5 (δC 152.8), C-6 (δC 130.6), and C-10 (δC 105.3) supported the position of the hydroxyl group at C-5. A careful comparison of the 1 H and 13 C NMR spectral data of 1 with baicalein [19] identified similar signals, distinguished by two hydroxyl groups at C-6 and C-7 (ring A) were replaced by the prenyloxy and methoxy substituents, respectively, in 1. This deduction was strongly confirmed by the HSQC and HMBC correlations ( Figure 2). From the aforementioned results, structure 1 was assigned as melodorone A.   . This was further supported by the 1 H and 13 C NMR spectral data, which displayed one oxymethine, one methoxy, one olefinic methine, two methylene, two methyl, six aromatic methine, and eight quaternary carbons ( Table 1). The spectroscopic ( 1 H and 13 C NMR) patterns of 2 were very similar to those of 1 except for the presence of a single bond between C-2 and C-3 in the heterocyclic ring C. This flavanone moiety was proven by the 1 H NMR spectrum, which showed an ABX spin system at δ H 5.62 (1H, dd, J = 12.6, 3.0 Hz), 3.31 (1H, dd, J = 17.4, 13.2 Hz), and 2.82 (1H, dd, J = 16.8, 3.0 Hz), corresponding to H-2, H-3ax, and H-3eq, respectively [20]. The 13 C NMR spectrum also confirmed the presence of a flavanone core structure because of the signals resonating at δ C 42.2 (C-3), 78.7 (C-2), and 197.1 (C-4) of the flavanone ring C. The absolute configuration of C-2 was assigned as S-configuration through contrastive analysis of the optical rotation data of 2 26 D −39.9) [21]. Based on the above spectral evidence, compound 2 was identified as a new flavanone and was named melodorone B.  (Table 1) showed three carbon signals at δ C 198.7 (C-4), 78.4 (C-2), and 42.8 (C-3), which are frequently observed in the flavanone series [22]. The 1 H NMR spectral data also justified the assignment of a flavanone moiety of 3 ( Table 1). The presence of two hydroxyl substituents at C-5 (δ C 148.3) and C-6 (δ C 136.1) was identified by the HMBC correlations of 5-OH with C-5, C-6, and C-10 (δ C 106.2), and of 6-OH with C-5, C-6, and C-7 (δ C 139.5) ( Figure S1). The 1 H and 13 C NMR data of 3 were nearly identical to those of (2S)-dihydrobaicalein [22], differing only in the resonance of the hydroxyl group was replaced by the resonance of a prenyl moiety at C-7 of ring A. This prenyl group was substituted at C-7, which was proven by the HMBC correlation of H-1 (δ H 3.26) with C-7. On the basis of these data, the structure of 3 was unambiguously established and named melodorone C.

α-Glucosidase Inhibitory Activity
Compounds 1-7 were assessed for their α-glucosidase inhibition, with acarbose as a positive control. The resulting IC 50 values ( Table 2) showed that all isolates, except 4, 6, and 7, displayed stronger inhibitory effects on α-glucosidase than acarbose (IC 50 179 µM). Especially, compounds 1-3 and 5 exhibited α-glucosidase inhibition with IC 50 values in the range of 2.59 to 4.00 µM, which were more strongly than acarbose. Among isolates, compound 1 revealed the most highly potent α-glucosidase inhibition with an IC 50 value of 2.59 µM. Previous research reported that 5 inhibited α-glucosidase effectively with an IC 50 value of 5.7 µM, while 4 and 7 showed no α-glucosidase inhibition [23], which was consistent with the findings of this study. The results indicated that the compounds from the stem bark of M. fruticosum, at least for 1-3 and 5, were active α-glucosidase inhibitors that could be used for the treatment of diabetes mellitus.

Cytotoxic Activity against Human Cancer Cell Lines
All isolates were further assessed for their in vitro cytotoxicity against three human cancer cell lines (KB, Hep G2, and MCF7), using an MTT assay (  [27]. These results were similar and supported the findings of this study. Compounds 4 and 5 were previously reported to exhibit no cytotoxic effect on the normal human colon fibroblastic CCD-18co and normal human epidermal keratinocytes (NHEKs) cell lines [28,29]. Additionally, compounds 5 and 7 were also discovered to have no cytotoxicity against the normal Vero cell line [30]. However, the cytotoxicity of 1-3 and 6 against normal human cell lines is not available, but it is recommended to be determined in the future.

Plant Material
M. fruticosum was collected in July 2017 from Lam Dong province, Vietnam. The material was authenticated by Dr. Dang Van Son. A voucher specimen (No US-A012) was deposited at the Herbarium of the Department of Organic Chemistry, Faculty of Chemistry, University of Science, National University-Ho Chi Minh City, Vietnam.

Extraction and Isolation
The dried and powdered stem bark of M. fruticosum (45.0 kg) was extracted with 95% EtOH (90 L × 3) at room temperature and concentrated under reduced pressure to give an EtOH crude extract (1.3 kg). This crude extract was suspended in water and partitioned with n-hexane and then EtOAc to yield n-hexane (45.0 g) and EtOAc (161.0 g) extracts. The n-hexane extract was fractionated by silica gel column chromatography (CC) eluted with n-hexane-EtOAc (90:10-0:10 v/v) and EtOAc-MeOH (10:0-0:10 v/v). The eluted fractions were combined into six fractions (HEX.1-HEX.6) on the basis of their TLC behavior. Fraction HEX.3 (6.5 g) was further separated by silica gel CC eluted with n-hexane-EtOAc

α-Glucosidase Inhibitory Assay
The α-glucosidase inhibition of all isolates was carried out according to a method adapted from a previous report [31]. Serial concentrations (2.0-256.0 µg/mL) of 1-7 and acarbose were prepared by dissolving in DMSO (400 mg/mL). Sodium phosphate buffer (100 mM, pH 6.8) was used to dissolve the α-glucosidase (0.4 U/mL) and substrate (2.5 mM pNPG). The substrate (40 µL) was added to the reaction mixture after the inhibitor (50 µL) was preincubated with α-glucosidase in 96-well plates at 37 • C for 10 min. A mixture without enzyme, sample compound, and acarbose served as blank, while the control contained only DMSO, enzyme, and substrate. The enzymatic reaction was carried out at 37 • C for 30 min and stopped by adding 0.2 M Na 2 CO 3 (130 µL). Absorbance at 410 nm to measure enzyme activity was recorded on a BIOTEK reader. The assays were conducted in triplicates, with acarbose serving as a positive control. The IC 50 values were calculated graphically using inhibition curves.

Cytotoxicity Assay
The cytotoxic evaluation of 1-7 and ellipticine against the growth of human epidermoid carcinoma (KB), human hepatocellular carcinoma (Hep G2), and human breast adenocarcinoma (MCF-7) cell lines was performed according to a previous procedure [32]. Ellipticine, a potent anticancer agent exhibiting multiple mechanisms of action, was used as the positive control [33][34][35]. The cancer cells were cultured in Dulbecco's Modified Eagle's Medium with 10% Fetal Bovine Serum, 1% penicillin and streptomycin, and 1% L-glutamine at 37 • C in a 5% CO 2 environment. The tested compounds were added at concentrations ranging from 0.5 to 128 µg/mL by dissolving in DMSO (20 mg/mL) and incubated for a further 72 h in the identical condition. After the treatment, each well was filled with an MTT solution (10 µL, 5 mg/mL) of phosphate buffer, which was left to stand for 4 h until intracellular purple formazan crystals appeared. The MTT was removed and a 100 µL DMSO solution was added. The blank contained only a medium without any cells and MTT, as well as the solubilizing solution. The solution's optical density (OD) at 540 nm was recorded on a BIOTEK reader. All experiments were carried out in triplicate for three independent experiments. IC 50 values were computed graphically using inhibition curves.

Conclusions
The chemical investigation of the stem bark of M. fruticosum afforded the isolation of three unprecedented flavonoid derivatives (1-3), including one new flavone (1) and two new flavanones (2 and 3), along with four known compounds (4-7). All isolated were obtained from M. fruticosum for the first time. Compounds 1-3 and 5 exhibited highly potent inhibition against α-glucosidase and were superior to the positive agent. Furthermore, compounds 1-5 selectively showed in vitro cytotoxicity against KB, Hep G2, and MCF7 cell lines. The results of this study reveal that M. fruticosum stem bark is a highly useful source of bioactive flavonoid derivatives that should be explored further in the medical field.