In vitro and in silico studies of neolignans from Magnolia Grandiflora L. seeds against human cannabinoid and opioid receptors

Authors: PANDEY, PANKAJ - University Of Mississippi; KUMARIHAMY, MALLIKA - University Of Mississippi; IBRAHIM, MOHAMED - University Of Mississippi; ILIAS, MUHAMMAD - University Of Mississippi; DOERKSEN, ROBERT - University Of Mississippi

Submitted to: Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2023
Publication Date: 1/27/2023
Citation: Pandey, P., Kumarihamy, M., Ibrahim, M.A., Ilias, M., Doerksen, R.J. 2023. In vitro and in silico studies of neolignans from Magnolia Grandiflora L. seeds against human cannabinoid and opioid receptors. Molecules. https://doi.org/10.3390/molecules28031253.
DOI: https://doi.org/10.3390/molecules28031253

Interpretive Summary: Medicinal plants and their constituents have been used extensively as supplements to treat various neurological disorders, including pain, anxiety, convulsions, epilepsy, hysteria, and inflammation. In the United States, Magnolia grandiflora L. (Magnoliaceae), commonly known as Southern magnolia, is a plant of medicinal significance that is often grown as an ornamental tree. Its flowers and seeds are used in various traditional herbal remedies, including fever, rheumatism, and inflammation. Also, the bark of M. officinalis plays an important role in traditional Chinese and Japanese herbal medicine for the treatment of anxiety, sleep-related disorders, and allergic diseases. In addition, magnolia-based products have been used for smoking and as aphrodisiacs, antidepressants, and sedatives due to their hypothesized cannabimimetic and GABA-ergic like effects. Extensive research has been carried out on magnolia extracts and its major constituents to study their anti-cancer, anti-anxiety, antidepressant, cardiovascular, and anti-inflammatory activities. M. officinalis extract, its constituents, and their synthetic analogs have been reported as CB2 agonists and inverse agonists and also as CB-related orphan receptor GPR55 inhibitors. Our current work involves bioassay-guided isolation of the hexane extract of Magnolia Grandiflora seed, which showed significant displacement of radioligand at cannabinoid and opioid receptors (64% CB1, 74% CB2, d 93%, k 61%, and µ 85%), yielded 4-O-methylhonokiol (1), magnolol (2), and honokiol (3) with selective CB2 activity (Ki 0.29, 1.4, and 1.94 µM, respectively). In addition to CB and opioid activity, we report the first systematic in silico molecular docking and binding free-energy studies of neolignans (1-4) and their structurally-related previously reported analogs (4a, 5, 5a, 6, 6a, 6b) using the active-state X-ray crystal structures of CB1R and CB2R to gain an understanding of the various protein–ligand interaction patterns, their putative binding mode, and the observed structure-activity relationships (SAR) of these ligands. The insight gained from this study could provide a platform for medicinal chemists working in this important area of cannabinoid research to utilize a new scaffold for designing new analogs from neolignans. This, in turn, can lead to identifying new synthetic compounds with improved affinity, functional activity, and/or selectivity for the CB receptors

Technical Abstract: Magnolia grandiflora L. (Magnoliaceae) is a plant of considerable medicinal significance; its flowers and seeds have been used in various traditional remedies. Radioligand binding assays with the n-hexane extract of the seeds showed displacement of radioligand for cannabinoid (CB1 and CB2) and opioid delta, kappa, and mu receptors. Bioactivity-guided fractionation afforded 4-O-methylhonokiol (1), magnolol (2), and honokiol (3), which showed higher binding to cannabinoid rather than opioid receptors in radioligand binding assays. Compounds 1-3, together with the dihydro analog of 2 (4), displayed selective affinity towards CB2R (Ki values of 0.29, 1.4, 1.94, and 0.99 µM, respectively), compared to CB1R (Ki 3.85, 17.82, 14.55, and 19.08 µM, respectively). Due to the unavailability of an NMR or X-ray crystal structure of bound neolignans with the CB1 and CB2 receptors, a docking study was performed to predict ligand–protein interactions at a molecular level and to delineate structure-activity relationships (SAR) of neolignan analogs with the CB1 and CB2 receptors. The putative binding modes of neolignans 1-3 and previously reported related analogs (4, 4a, 5, 5a, 6, 6a, and 6b) into the active site of CB1 and CB2 receptors were assessed via molecular docking and binding free-energy (delta G) calculations. The docking and delta G results revealed the importance of a hydroxyl moiety in the molecules that forms strong H-bonding with Ser383 and Ser285 within CB1R and CB2R, respectively. The impact of a shift from a hydroxy- to the methoxy- group on binding affinity to CB1R versus CB2R was explained through delta G data and the orientation of the alkyl chain within the CB1R. This comprehensive SAR, influenced by the computational study and the observed in vitro displacement binding affinities, has indicated the potential of magnolia neolignans for developing new CB agonists for potential use as an analgesic, anti-inflammatory agents, or anxiolytics.