Two new cytotoxic tetralin derivatives from Panicum turgidum

Abstract Two new tetralin compounds were isolated from the aerial parts of Panicum turgidum Forssk. Their structures were elucidated by comprehensive 1 D and 2 D NMR experiments as well as high resolution ESI mass spectrometry. In addition, these two compounds have been tested for in vitro cytotoxicity against SKOV3 (Ovarian Carcinoma) and BT-549 (Breast Carcinoma). Compound 1 showed good cytotoxic activities against SKOV3 and BT 549 with IC50 value of 5.65 ± 0.31 and 10.3 ± 0.56 µg/mL, respectively Graphical Abstract


Introduction
The Poaceae family is one of the largest families of higher plants, consists of more than 12000 species within 780 genera (Boulos 1983). Panicum is a very diverse genus, with its natural range of distribution in the tropics and warmer temperate regions. It is one of the largest genera within the Poaceae, consisting of about 450 annual and perennial species (Clayton and Renvoize 1986). Some Panicum plants have been used in folk medicines to eye infections (El-Ghazali et al. 2010), diuretic (Hozaien et al. 2019) and a folk remedy for tympanitis in cattle as the ingestion of juice from Panicum is reported to stimulate the movements of isolated intestines of goats and other animals; addition of a small quantity of the grass to lucerne feed responds to the inhibitory effects of the lucerne feed on intestines. As a result, it is proved to be a helpful therapy in preventing tympanitis in cattle (Duke and Wain 1981).
Panicum turgidum commonly known as Thummam, is one the important species belongs to genus Panicum, present as a perennial bunchgrass, growing in dense bushes. The stems are long-jointed, hard and polished, with few leaves, this is a drought and salt-tolerant species (Aganga and Tshwenyane 2003, Khan et al. 2009) and used for flour, fodder, thatch and erosion control (Anthelme and Michalet 2009). Panicum turgidum is the best forage and pasture grass in the tropics (Ajoku et al. 2015. Previous reports established that the most prevalent bioactive compounds in the genus of Panicum are steroids especially saponintype which are reported to possess anti-diabetic and antibacterial activities (Ajoku et al. 2015, Doss et al. 2011. Moreover, it was reported that this plant growing in Nigeria exhibited antimalaria, anti-inflammatory, anti-rheumatic (Ajoku et al. 2015, Okokon et al. 2011, Zaki et al. 2016b). Antiplasmodial and analgesic activities have also been reported (Ajoku et al. 2015). To the best of our knowledge, there are few reports on the phytoconstituents from the Panicum turgidum growing in Egypt (Zaki et al. 2016a, Zaki et al. 2017, Zaki et al. 2022. Therefore, in this paper we investigated the air-dried aerial parts of this plant phytochemically and report the isolation and identification of two new tetralins. Their structures were revealed on the basis of extensive spectroscopic analysis of their 1 D-and 2 D-NMR and HRESIMS spectra.

Structural elucidation of the new compounds
In continuation to our previous work for the isolation of cytotoxic compounds from P. turgidum (Zaki et al. 2016a, Zaki et al. 2017, Zaki et al. 2016b, the unstudied methylene chloride fraction showed a promising activity against the cancer cell lines SKOV3 (Ovarian Carcinoma) and BT-549 (Breast Carcinoma) with IC 50 of 43 and 20 mg/mL, respectively, in MTT colorimetric assay. The phytochemical investigation of the fraction led to the isolation of two new compounds 1 and 2 ( Figure 1).
Compound 2 was isolated as an amorphous powder with a molecular ion peak at m/z 403.2095 [M þ Na] þ in HRESIMS (Supplementary material Figures S15) consistent with the molecular formula of C 21 H 32 O 6 (calcd. 403.2097, C 21 H 32 O 6 Na þ ). The careful analysis of NMR spectra including the 1 D and 2 D (Supplementary material  Table S1 & Figures; S1 to S13) revealed that compound 2 has the same nucleus of compound 1, tetralin, with additional 6 carbons attributed to a glucose moiety. The correlation between H-1 0 (d 4.57) and C-3 (d 74.7) in HMBC spectrum, confirmed the linkage of the sugar to C-3. The resulted chemical structure of 2 is determined to be 1,1,5,6,7-pentamethyl-tetralin-3-b-O-glucoside. Compounds 1 and 2 are new and it's the first time to report the tetralin-type (tetrahydronaphthalene derivatives) compounds in genus Panicum. Several examples of tetrahydronaphthalene derivatives were isolated from plants. Ligukangtinol is a tetralin norsesquiterpenoid compound isolated from Ligularia kangtingensis belonging to the family Compositae (Xie et al. 2014). Meerungrueang and Panichayupakaranant, reported the isolation of a tetralin lignanamide derivative, known as foveolatamide from Ficus foveolata (Meerungrueang and Panichayupakaranant 2016). Foveolatamide showed promising antibacterial activity against Streptococcus pyogenes. Wang et al. reported the isolation of three aryl-tetralin-type lignans, including two new derivatives from the roots of a plant belonging to the family Rosaceae, Sanguisorba officinalis, and the known derivative exhibited a moderate cytotoxic effect on MV4-11 and MDA-MB-231 cell lines (Wang et al. 2019). Another tetrahydronaphthalene lignan, Lyoniresinol, has been reported as a major component of Lyonia ovalifolia var. elliptica wood, it demonstrated effective inhibition of tyrosinase enzyme (Rahman et al. 2007). Furthermore, Hara et al. has described anti-HIV-1 tetrahydronaphthalene compounds in the extract of Schizandra chinensis fruit through a screening study of traditional herbal medicine in East Asia (Hara et al. 1997). Therefore, the tetralin compounds and their derivatives have a wide range of biological activities and are distributed in different families such as; Compositae, Roseace, Moraceae, Schisandraceae, and Ericaceae.

Cytotoxic activity
The cytotoxic activity of the two new compounds was evaluated against SKOV3 (Ovarian Carcinoma) and BT-549 (Breast Carcinoma) cell lines in comparison with staurosporine as positive control. Potent cytotoxicity was exhibited by compound 1 on SKOV3 and BT 549 with IC50 value of 5.65 ± 0.31 and 10.3 ± 0.56 mg/mL, respectively, using staurosporine as a standard cytotoxic drug (Kumar et al. 2018). Compound 2 showed a moderate activity with IC 50 of 54.7 ± 2.97 and 81.7 ± 4.44 mg/mL against SKOV3 and BT 549, respectively, compared to staurosporine with IC 50 value of 4.1 ± 0.22 and 13.8 ± 0.75 against SKOV3 and BT 549, respectively.

General experimental procedures
The NMR spectra including one-and two-dimension were acquired on Bruker BioSpin GmbH 400.18 and 500.09 MHz spectrometer. The deuterated solvents used are CD 3 OD and CDCl 3 . The solvent peaks at d 7.260 and 3.310 were used as internal reference for 1 H-NMR and at d 77.160 and 49.0 for 13 C-NMR. The exact mass of isolated compounds was obtained from mass spectrometer of Agilent Technologies 6200 series. The glass column chromatography was performed by using either silica gel (32-63m, Dynamic adsorbents Inc.) or RP-18 silica (38-83 m, Polar bond, J. T. Baker). The column monitoring was conducted using TLC of silica gel F 254 aluminum sheet (Fluka) or Reversed Silica RP-18 F 254 S aluminum sheet (Merck). The spots detection was performed under ultra violet lamp of wavelength of 254 and 366 nm. Spots characterization was enhanced by spraying with 1% vanillin sulfuric, and then heating for 2 min. the solvents used throughout the process of isolation and purification were of analytical grade.

Plant material
The shoots of Panicum turgidum Forssk. were obtained from arid area in the eastern desert of Egypt in April, 2017 and authenticated by the staff members, Department of Botany, Faculty of Science, Mansoura University, Egypt. A representative sample of the plant material (PT-3-PD) was kept in Department of Pharmacognosy, Faculty of Pharmacy, Mansoura University.

Cytotoxicity assay
The cytotoxicity of compounds 1 and 2 against cell lines SKOV3 and BT549 was performed using MTT based assay following the previously reported protocol (Zaki et al. 2017).

Conclusion
In summary, the phytochemical investigation of methylene chloride fraction of Panicum turgidum resulted in the identification of two new tetralin derivatives. The in vitro cytotoxicity activity of these two compounds was evaluated against different cancer cell lines. Compound 1 showed good cytotoxic activities against SKOV3 and BT 549 with IC 50 value of 5.65 ± 0.31 and 10.3 ± 0.56 mg/mL, respectively.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The author(s) reported there is no funding associated with the work featured in this article.