Cytotoxic Flavonoids from the Leaves and Twigs of Murraya tetramera

Cytotoxic flavonoids of Murraya tetramera were investigated in this study. A novel flavonoid and twelve known flavonoids, including seven flavones (1–7), three flavanones (8–10), and three chalcones (11–13) were isolated from the leaves and twigs of Murraya tetramera. Chemical structures were elucidated by NMR combined with MS spectral analysis, and the new compound (6) was confirmed as 3′,5′-dihydroxy-5,6,7,4′-tetramethoxyflavone. Furthermore, all the isolated flavonoids were evaluated for their cytotoxicities against murine melanoma cells (B16), and human breast cancer cells (MDA-MB-231) by CCK-8 assay. Among them, compounds 7, 13, and 5 exhibited potent cytotoxic activities against B16 cell lines (IC50 = 3.87, 7.00 and 8.66 μg/mL, respectively). Compounds 5, 13, and 12 displayed potent cytotoxicities against MDA-MB-231 cell lines (IC50 = 3.80, 5.95 and 7.89 μg/mL, respectively). According to the correlation of the structure and activity analysis, 5-hydroxyl and 8-methoxyl substituents of the flavone, 8-methoxyl substituent of the flavanone, and 3′,5′-methoxyl substituents of the chalcone could be critical factors of the high cytotoxicity. The results indicated that the active flavonoids have potential to be developed as leading compounds for treating cancers.


Introduction
Cancer is a leading cause of death among most countries in the 21st century, and the incidence and mortality are rapidly growing worldwide [1]. Melanoma of the skin among males and breast cancer among females are two of most prevalent cancers in 2019 [2]. Melanoma arises from epidermal melanocytes and induces 80% of the dermatological cancer-related deaths [3]. Breast cancer is the second major cause of cancer deaths in female worldwide [4]. Serious side effects and drug resistance caused by conventional cancer treatment of chemotherapy and radiotherapy remain the major problems during the treatment [5,6]. Accordingly, plant secondary metabolites have been attracting more attention in drug development owing to multiple factors, and the various compounds discovered in plants made them as a rich source of cancer drug candidates [7][8][9][10]. Thousands of flavonoids have been isolated from stems, flowers, fruits, roots, and barks of the plants, moreover, many effective cytotoxic flavonoids from various plants were considered as potential leading compounds for the development of anticancer drugs [8,[11][12][13].
To explore potent cytotoxic flavonoids from M. tetramera as potential leading compounds for treating cancers and make comprehensive utilization of its natural resources, a phytochemical investigation was carried out, and the cytotoxicities were evaluated against murine melanoma cells (B16) and human breast cancer cells (MDA-MB-231) by CCK-8 assay.
The diverse cytotoxicities might be attributed to the different substituents of the flavonoids. Among flavones 1-7, flavones 5, 7, and 3 exhibited higher cytotoxicities against B16 cells (IC 50 = 8.66, 3.87, and 11.18 µg/mL) and MDA-MB-231 cells (IC 50 = 3.80, 14.93, and 23.46 µg/mL) than others. Thus, 5-hydroxyl and 8-methoxyl substituents of the flavone were essential for high cytotoxicity, which corresponds to the state of literature [8,35]. In addition, compared with flavone 6, flavone 2 showed a higher cytotoxicity against B16 and MDA-MB-231 cells with IC 50 values of 14.74 and 34.19 µg/mL. Therefore, if the methoxy substituents in position 3 and 4 of flavone was substituted by hydroxyl substituents, as found in flavone 6, the cytotoxicity was significantly reduced. Among the flavanones 8-10, flavanone 9 exhibited the highest cytotoxicity against B16 and MDA-MB-231 cells (IC 50 = 12.76 and 16.02 µg/mL). Thus, 8-methoxyl substituent of the flavanone could be a critical factor of the cytotoxic activity, which corresponds to the state of literature [35]. Among chalcones 11-13, chalcones 12 and 13 exhibited higher cytotoxicities against B16 cells (IC 50 = 11.53 and 7.00 µg/mL) and MDA-MB-231 cells (IC 50 = 7.89 and 5.95 µg/mL) compared with chalcone 11. Hence, 3 and 5 -methoxyl substituents of the chalcone could be a major factor of the cytotoxic activity. Overall, according to the correlation of the structure and activity analysis, the position of methoxy and hydroxyl substituents in the flavonoids may be the major factors of the anticancer efficacy. Further investigation is essential to clarify the structure-active relationships.

General Information
The NMR spectrometer (Bruker Avance III, Bruker, Karlsruhe, Germany) was used to record the NMR spectra at 500 MHz ( 1 H) and at 125 MHz ( 13 C). The mass spectrometer (Bruker Q-TOF, Bruker, Karlsruhe, Germany) was used to measure the HR-ESI-MS. Preparative HPLC was carried out using a Rainbow Kromasil-C 18 column (10 × 250 mm, 10 µm) on a Waters Delta Prep 4000 instrument with a dual λ absorbance detector (Waters 2487, Waters, Milford, USA). MCI GEL CHP20P of 75-150 µm (Kaiteki Company, Tokyo, Japan) was selected for column chromatography. Silica gel G plates were used for TLC analysis (Qingdao Haiyang Chemical Co., Ltd., Qingdao, China). The deuterated DMSO-d 6

Plant Material
The leaves and twigs of M. tetramera were harvested at Xishuangbanna, Yunnan Province, China in May 2014 and were identified by Dr. Liu, Q.R. (College of Life Sciences, Beijing Normal University, Beijing, China). The certificate specimen (BNU-CMH-Dushushan-2014-05-025-001) was stored at the Herbarium of Faculty of Geographical Science, Beijing Normal University.

Extraction and Isolation
The methanol extract of leaves and twigs of M. tetramera was obtained from our previous study and 90 fractions were received from the methanol extract by eluting with a stepwise gradient of PE/EtOAc and CHCl 3 /CH 3 OH [36]. Fr

Cytotoxicity Assay
The cytotoxicities of flavonoids 1-13 were determined by the standard CCK-8 assay [20,37]. B16 (Number: GDC0039) were originally provided by China Center for Type Culture Collection (Wuhan, China) and MDA-MB-231 (Number: CL0208) were obtained from the Fenghui Biotechnology Co., Ltd. (Changsha, China). Doxorubicin hydrochloride (DOX), the positive control, was purchased from Dalian Meilun Biotechnology Co., Ltd. (Dalian, China). B16 cells were cultured in RPMI 1640 medium and MDA-MB-231 cells were cultured in DMEM medium. The medium supplemented with 10% fetal bovine serum (Gibco Inc.), 100 U/mL penicillin and 0.1 mg/mL streptomycin. The tested cell lines were incubated at 37 • C, 5% CO 2 and 90% humidity in the CO 2 incubator (Binder, Tuttlingen, Germany). Firstly, 100 µL of the cell suspension was seeded into each well of 96-well plates (6 × 10 3 per well), and then incubated for 12-24 h to allow cellular attachment. After removing the medium, fresh medium containing seven concentrations of test compounds was added into cultured cells of 100 µL per well and incubated for 48 h. Secondly, 10 µL CCK-8 reagent was added into each well and placed in a CO 2 incubator for 1 h. Finally, the absorbance was recorded using a microplate reader (Bio-Rad, Hercules, CA, USA) at 450 nm. The 50% inhibitory concentration (IC 50 ) values were calculated using Probit analysis (SPSS V20.0).

Conclusions
A novel flavonoid and twelve known flavonoids, including seven flavones (1-7), three flavanones (8)(9)(10), and three chalcones (11)(12)(13) were isolated from the leaves and twigs of M. tetramera. The novel one (compound 6) was identified as 3 ,5 -dihydroxy-5,6,7,4tetramethoxyflavone. Results of cytotoxicity assay indicated that flavones 5 and 7 with 5-hydroxyl substituent, flavones 3 and flavanone 9 with 8-methoxyl substituent, chalcone 12 with 3 -methoxyl substituent and chalcone 13 with 5 -methoxyl substituent exhibited significant cytotoxic activities against B16 and MDA-MB-231 cell lines. According to the correlation of the structure and activity analysis, the position of methoxy and hydroxyl substituents in the flavonoids were the major factors of the high anticancer efficacy. The results indicated that the active flavonoids have potential to be developed as leading compounds for treating cancers.