In vitro anti-melanogenic effects of chimeric compounds, 2-(substituted benzylidene)-1,3-indanedione derivatives with a β-phenyl-α, β -unsaturated dicarbonyl scaffold
Graphical abstract
Introduction
Tyrosinases (also known as polyphenol oxidases) play key roles in mammalian melanogenesis and in the enzymatic browning of fruit or fungi [1]. The active site of tyrosinase contains two central copper(II) ions, which interact with histidine residues in the common mushroom (Agaricus bisporus) and human malignant melanoma tyrosinase [2], [3]. Tyrosinases exist in various forms such as immature, mature and active forms in animals, plants and fungi [4]. Melanogenesis can be defined as the process that results in the formation of the dark macromolecular pigment melanin via a series of enzymatic and chemical reactions. Tyrosinase catalyzes different reactions in the melanin biosynthetic pathway in melanocytes such as the hydroxylation of l-tyrosine to l-DOPA and the oxidation of the l-DOPA to dopaquinone, which in man, is converted by a series of complex reactions involving cyclization and oxidative polymerizations to melanin [5], [6].
Melanin is the pigment largely responsible for the color of skin and functionally acts as a barrier against ultraviolet radiation [7]. However, elevated levels of melanin in skin create aesthetic problems such as melasma, freckles, and age spots, and are also associated with the pathogenesis of melanoma [8], [9], [10], [11], [12], [13], [14], [15], especially in the middle-aged and elderly [16]. Thus, research studies have focused on the development of tyrosinase inhibitors that prevent the production of excess melanin, and to date, have identified a large number of potent natural and synthetic tyrosinase inhibitors [9], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Some of these inhibitors such as arbutin, kojic acid and hydroquinone are being used as topical whitening or anti-hyperpigmentation agents [28], [29]. However, although hydroquinone and kojic acid are used as whitening agents, they are also associated with an elevated risk of thyroid cancer [30] and with nephrotoxic [31], genotoxic [13], and cytotoxic (to melanocytes) effects [32]. Arbutin, another tyrosinase inhibitor, has fewer side effects but is hydrolyzed to d-glucose and hydroquinone by primary skin microflora (Staphylococcus epidermidis and Staphylococcus aureus) [33].
Natural tyrosinase inhibitors are generally considered to be free of harmful side effects, but they have low potencies, poor stabilities, and are expensive, due to the lack of rich natural sources. Accordingly, scientists have focused on the development of low cost, potent synthetic tyrosinase inhibitors [34]. We have synthesized many tyrosinase inhibitors over the last decade [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], and as shown by Fig. 1, our studies have demonstrated that compounds containing the β-phenyl-α,β-unsaturated carbonyl scaffold often exhibit potent tyrosinase inhibitory activity. Initially, we synthesized compounds with the (E) scaffold geometry because they could be synthesized more easily and because they tend to be more stable than corresponding compounds with the (Z)-scaffold geometry.
In a previous study, to determine whether compounds with the (Z) geometry also exhibit potent tyrosinase inhibitory activities, we prepared a series of these compounds by condensing an appropriate benzaldehyde with 3-phenylisoxazol-5(4H)-one, which was easily prepared by reacting ethyl benzoylacetate with hydroxylamine·HCl in the presence of DABCO (1,4-diazabicyclo[2.2.2]octane) (Scheme 1) [48]. Due to steric hindrance by the phenyl group of 3-phenylisoxazol-5(4H)-one, compounds with a (Z)-β-phenyl-α,β-unsaturated carbonyl scaffold were predominately obtained. A (Z)-derivative with a 2,4-dihydroxyl substituent on the β-phenyl ring of the scaffold more potently inhibited mushroom and B16F10 (a melanoma derived cell-line) tyrosinase than kojic acid, which suggested the β-phenyl-α,β-unsaturated carbonyl scaffold can confer potent tyrosinase inhibitory activity, regardless of scaffold geometry.
Based on our previous findings, we wondered whether chimeric compounds with a β-phenyl-α,β-unsaturated dicarbonyl scaffold (Fig. 2) also exhibit high tyrosinase inhibitory activity. In this study, we report the synthesis of 2-(substituted benzylidene)-1,3-indanedione derivatives and their abilities to inhibit mushroom tyrosinase and tyrosinase in B16F10 cells (a murine melanoma cell-line). In addition, we investigated their abilities to inhibit melanogenesis in B16F10 cells and their cytotoxic effects on this cell-line.
Section snippets
Chemistry
Indanedione derivatives have been synthesized previously by many research groups [49], [50], [51]. As depicted in Scheme 2, Scheme 2, 2-(substituted benzylidene)-1,3-indanedione derivatives with the β-phenyl-α,β-unsaturated dicarbonyl scaffold were synthesized. The derivatives 1 – 12 were easily prepared by condensation between 1H and indene-1,3(2H)-dione and an appropriate benzaldehyde in 1 M HCl acetic acid solution. Twelve benzaldehydes including 2,4- and 3,4-dihydroxybenzaldehyde were used
Conclusions
To examine whether chimeric compounds with the β-phenyl-α,β-unsaturated dicarbonyl scaffold plays an essential role in the inhibition of tyrosinase, twelve 2-(substituted benzylidene)-1,3-indanedione derivatives with the β-phenyl-α,β-unsaturated dicarbonyl scaffold were synthesized by Knoevenagel condensation. Mushroom tyrosinase inhibitory assays showed two 1,3-indanedione derivatives (compounds 2 and 3) with a 2,4- and 3,4- hydroxyls on the phenyl ring of the scaffold, respectively, inhibited
General methods
1H NMR and 13C NMR data were obtained using a Varian Unity INOVA 400 spectrometer or a Varian Unity AS500 spectrometer (Agilent Technologies, Santa Clara, CA, USA); DMSO‑d6 or CDCl3 were used as solvents. All chemical shifts were measured in parts per million (ppm) versus residual solvent or deuterated peaks (δH 7.24 and δC 77.0 for CDCl3, δH 2.50 and δC 39.7 for DMSO‑d6). Coupling constants are presented in hertz. The following abbreviations are used for 1H NMR: singlet (s), broad singlet
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1A2C1004198).
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These authors (I.Y. Ryu, I. Choi, H.J. Jung, and S. Ullah) contributed equally to this work.