(E)-1-(Furan-2-yl)-(substituted phenyl)prop-2-en-1-one Derivatives as Tyrosinase Inhibitors and Melanogenesis Inhibition: An In Vitro and In Silico Study

Nitrophenylpiperazine derivatives as novel tyrosinase inhibitors: design, synthesis, and in silico evaluations

A novel series of 4-nitrophenylpiperazine derivatives (4a-m) was designed and synthesized as potential tyrosinase inhibitors. Comprehensive characterization using 1H-NMR, 13C-NMR, CNH, and IR techniques was performed for all target compounds. Subsequently, the derivatives were evaluated for their inhibitory activity against tyrosinase. Among them, compound 4l, featuring an indole moiety at the N-1 position of the piperazine ring, exhibited a significant tyrosinase inhibitory effect with an IC50 value of 72.55 μM. Enzyme kinetics analysis revealed that 4l displayed mixed inhibition of the tyrosinase enzymatic reaction. Molecular docking was carried out in the enzyme's active site to further investigate the enzyme-inhibitor interactions. Based on the findings, compound 4l shows promise as a lead structure for the design of potent tyrosinase inhibitors. This study paves the way for the development of more effective tyrosinase inhibitors for potential applications in various fields.

Heterocyclic Compounds as Synthetic Tyrosinase Inhibitors: Recent Advances

Tyrosinase is a copper-containing enzyme which is widely distributed in nature (e.g., bacteria, mammals, fungi) and involved in two consecutive steps of melanin biosynthesis. In humans, an excessive production of melanin can determine hyperpigmentation disorders as well as neurodegenerative processes in Parkinson's disease. The development of molecules able to inhibit the high activity of the enzyme remain a current topic in medicinal chemistry, because the inhibitors reported so far present several side effects. Heterocycle-bearing molecules are largely diffuse in this sense. Due to their importance as biologically active compounds, we decided to report a comprehensive review of synthetic tyrosinase inhibitors possessing heterocyclic moieties reported within the last five years. For the reader's convenience, we classified them as inhibitors of mushroom tyrosinase (Agaricus bisporus) and human tyrosinase.

Quercetin 3-O-(6″-O-E-caffeoyl)-β-D-glucopyranoside, a Flavonoid Compound, Promotes Melanogenesis through the Upregulation of MAPKs and Akt/GSK3β/β-Catenin Signaling Pathways

Quercetin 3-O-(6″-O-E-caffeoyl)-β-D-glucopyranoside is a flavonoid compound produced by various plants with reported antiprotozoal potential against E. histolytica and G. lamblia; however, its effects on skin pigment regulation have not been studied in detail. In this investigation, we discovered that quercetin 3-O-(6″-O-E-caffeoyl)-D-glucopyranoside (coded as CC7) demonstrated a more increased melanogenesis effect in B16 cells. CC7 exhibited no cytotoxicity or effective stimulating melanin content or intracellular tyrosinase activity. This melanogenic-promoting effect was accompanied by activated expression levels of microphthalmia-associated transcription factor (MITF), a key melanogenic regulatory factor, melanogenic enzymes, and tyrosinase (TYR) and tyrosinase-related protein-1 (TRP-1) and 2 (TRP-2) in the CC7-treated cells. Mechanistically, we found that CC7 exerted melanogenic effects by upregulating the phosphorylation of stress-regulated protein kinase (p38) and c-Jun N-terminal kinase (JNK). Moreover, the CC7 upregulation of phosphor-protein kinase B (Akt) and Glycogen synthase kinase-3 beta (GSK-3β) increased the content of β-catenin in the cell cytoplasm, and subsequently, it translocated into the nucleus, resulting in melanogenesis. Specific inhibitors of P38, JNK, and Akt validated that CC7 promotes melanin synthesis and tyrosinase activity by regulating the GSK3β/β-catenin signaling pathways. Our results support that the CC7 regulation of melanogenesis involves MAPKs and Akt/GSK3β/β-catenin signaling pathways.

Design and Synthesis of (Z)-5-(Substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one Analogues as Anti-Tyrosinase and Antioxidant Compounds: In Vitro and In Silico Insights

Many compounds containing the β-phenyl-α,β-unsaturated carbonyl (PUSC) scaffold, including cinnamamide derivatives, have been shown to inhibit tyrosinase potently in vitro and in vivo. Structural changes to cinnamamide derivatives were produced by adding a dithionate functional group to provide eight (Z)-5-(substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one analogs with high log p values for skin. These analogs were synthesized using a two-step reaction, and their stereochemistry was confirmed using the ³J_C4-Hβ values of C4 measured in proton-coupled ¹³C mode. Analogs 2 (IC₅₀ = 5.21 ± 0.86 µM) and 3 (IC₅₀ = 1.03 ± 0.14 µM) more potently inhibited mushroom tyrosinase than kojic acid (IC₅₀ = 25.26 ± 1.10 µM). Docking results showed 2 binds strongly to the active site of tyrosinase, while 3 binds strongly to an allosteric site. Kinetic studies using l-tyrosine as substrate indicated 2 and 3 competitively and non-competitively inhibit tyrosinase, respectively, which was supported by our docking results. In B16F10 cells, 3 significantly and concentration-dependently reduced α–MSH plus IBMX induced increases in cellular tyrosinase activity and melanin production and the similarity between these inhibitory patterns implied that the anti-melanogenic effect of 3 might be due to its tyrosinase-inhibitory ability. In addition, 2 and 3 exhibited strong antioxidant effects; for example, they reduced ROS and ONOO^– levels and exhibited radical scavenging activities, suggesting that these effects might underlie their anti-melanogenic effects. Furthermore, 3 suppressed the expressions of melanogenesis-associated proteins and genes in B16F10 cells. These results suggest (Z)-5-(substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one analogs offer a means of producing novel anti-melanogenesis agents.

Ultrasonic-Assisted Synthesis of Benzofuran Appended Oxadiazole Molecules as Tyrosinase Inhibitors: Mechanistic Approach through Enzyme Inhibition, Molecular Docking, Chemoinformatics, ADMET and Drug-Likeness Studies

Furan-oxadiazole structural hybrids belong to the most promising and biologically active classes of oxygen and nitrogen containing five member heterocycles which have expanded therapeutic scope and potential in the fields of pharmacology, medicinal chemistry and pharmaceutics. A novel series 5a-j of benzofuran-oxadiazole molecules incorporating S-alkylated amide linkage have been synthesized using ultrasonic irradiation and screened for bacterial tyrosinase inhibition activity. Most of the synthesized furan-oxadiazole structural motifs exhibited significant tyrosinase inhibition activity in the micromolar range, with one of the derivatives being more potent than the standard drug ascorbic acid. Among the tested compounds, the scaffold 5a displayed more tyrosinase inhibition efficacy IC₅₀ (11 ± 0.25 μM) than the ascorbic acid IC₅₀ (11.5 ± 0.1 μM). Compounds 5b, 5c and 5d efficiently inhibited bacterial tyrosinase with IC₅₀ values in the range of 12.4 ± 0.0-15.5 ± 0.0 μM. The 2-fluorophenylacetamide containing furan-oxadiazole compound 5a may be considered as a potential lead for tyrosinase inhibition with lesser side effects as a skin whitening and malignant melanoma anticancer agent.

Silencing of Rab23 by siRNA inhibits ultraviolet B-induced melanogenesis via downregulation of PKA/CREB/MITF

Recent investigations have shown that the Rab family of GTPases is associated with all aspects of melanogenesis. However, the effect of Rab23, which localizes to the plasma membrane and regulates the endocytic pathway within eukaryotic cells, in melanogenesis has not been reported. To understand the role of Rab23 in UVB-induced melanogenesis, we evaluated changes in the level of melanin, activity of tyrosinase, and levels of melanogenesis-related proteins such as microphthalmia transcription factor and tyrosinase-related protein-1 (TRP-1) and the melanosome transport-related protein complex Rab27a-melanophilin-myosin Va after the downregulation of Rab23 in B16F10 and SK-MEL-2 cells with or without UVB irradiation. Our results showed that downregulating Rab23 reduced the melanin level and tyrosinase activity and inhibited the expression of proteins involved in UVB-induced melanogenesis. Rab23 colocalized with mature melanosomes marked with TRP-1. Furthermore, downregulating Rab23 induced the abnormal accumulation of melanosomes around the nucleus. We demonstrated that the downregulation of Rab23 inhibited melanin synthesis and melanosome transport by decreasing the PKA/CREB/MITF pathway, which is the key regulator of UVB-induced melanogenesis.

Novel Anti-Melanogenic Compounds, (Z)-5-(Substituted Benzylidene)-4-thioxothiazolidin-2-one Derivatives: In Vitro and In Silico Insights

To confirm that the β-phenyl-α,β-unsaturated thiocarbonyl (PUSTC) scaffold, similar to the β-phenyl-α,β-unsaturated carbonyl (PUSC) scaffold, acts as a core inhibitory structure for tyrosinase, twelve (Z)-5-(substituted benzylidene)-4-thioxothiazolidin-2-one ((Z)-BTTZ) derivatives were designed and synthesized. Seven of the twelve derivatives showed stronger inhibitory activity than kojic acid against mushroom tyrosinase. Compound 2b (IC50 = 0.47 ± 0.97 µM) exerted a 141-fold higher inhibitory potency than kojic acid. Kinetic studies' results confirmed that compounds 2b and 2f are competitive tyrosinase inhibitors, which was supported by high binding affinities with the active site of tyrosinase by docking simulation. Docking results using a human tyrosinase homology model indicated that 2b and 2f might potently inhibit human tyrosinase. In vitro assays of 2b and 2f were conducted using B16F10 melanoma cells. Compounds 2b and 2f significantly and concentration-dependently inhibited intracellular melanin contents, and the anti-melanogenic effects of 2b at 10 µM and 2f at 25 µM were considerably greater than the inhibitory effect of kojic acid at 25 µM. Compounds 2b and 2f similarly inhibited cellular tyrosinase activity and melanin contents, indicating that the anti-melanogenic effects of both were due to tyrosinase inhibition. A strong binding affinity with the active site of tyrosinase and potent inhibitions of mushroom tyrosinase, cellular tyrosinase activity, and melanin generation in B16F10 cells indicates the PUSTC scaffold offers an attractive platform for the development of novel tyrosinase inhibitors.

Recent advances in the design and discovery of synthetic tyrosinase inhibitors

Tyrosinase is a copper-containing metalloenzyme that is responsible for the rate-limiting catalytic step in the melanin biosynthesis and enzymatic browning. As a promising target, tyrosinase inhibitors can be used as skin whitening agents and food preservatives, thus having broad potential in the fields of food, cosmetics, agriculture and medicine. From 2015 to 2020, numerous synthetic inhibitors of tyrosinase have been developed to overcome the challenges of low efficacy and side effects. This review summarizes the enzyme structure and biological functions of tyrosinase and demonstrates the recent advances of synthetic tyrosinase inhibitors from the perspective of medicinal chemistry, providing a better understanding of the catalytic mechanisms and more effective tyrosinase inhibitors.