Design, synthesis and biological evaluation of hydroxy- or methoxy-substituted 5-benzylidene(thio) barbiturates as novel tyrosinase inhibitors

Tyrosinase Inhibition and Antimelanogenic Effects of Resorcinol-Containing Compounds

Tyrosinases (TYRs) are copper-containing metalloenzymes present in a large diversity of species. In human, hTYR is responsible for pivotal steps in melanogenesis, catalysing the oxidation of l-tyrosine to l-DOPA and further to dopaquinone. While numerous TYR inhibitors have been reported, polyphenolic compounds tend to dominate the literature. However, many of these compounds, particularly monophenols and catechols, have been identified as alternative substrates rather than true inhibitors, given their structural similarity to natural substrates. Resorcinol-containing compounds have emerged as promising candidates to address this challenge, as the meta-dihydroxy moiety in resorcinol demonstrates resistance to TYR-mediated oxidation, while retaining the favourable interactions with copper ions provided by the hydroxy groups. Although their precise mechanism of action remains debated, resorcinol derivatives have yielded some of the most active compounds against isolated mushroom and human TYRs, as well as clinically used dermocosmetic agents like rucinol and thiamidol, which exhibited very promising effects in patients with facial melasma. This review outlines the development of resorcinol-containing TYR inhibitors, categorized by scaffold type, ranging from simple alkyl analogues to intricate synthetic derivatives. Mechanistic insights about the resorcinol-TYR interaction are also presented and debated.

Kinetic studies, molecular docking, and antioxidant activity of novel 1,3-diphenyl pyrazole-thiosemicarbazone with anti-tyrosinase and anti-melanogenesis properties

This study reports the Design Hypothesis of a novel series of 1,3-diphenyl pyrazole-thiosemicarbazone as novel tyrosinase inhibitors (TYRI). The designed compounds were prepared and their TYRI activity and mechanisms were studied. The results showed that the selected compounds exhibited potent tyrosinase inhibitory activities greater than that of kojic acid (KA). Lead candidates, denoted as 6g and 6n, with a para-hydroxyphenyl group attached to the 3-position of the pyrazole ring demonstrated IC50 values of 2.09 and 3.18 µM, respectively. The potency of these compounds was approximately 5-8 times higher than that of KA. The in vitro melanin content of 6g or 6n-treated melanoma cells resulted in significant efficacy in melanin reduction. The DPPH assay result revealed that the tyrosinase inhibition mechanism for these derivatives was independent of a redox effect and corresponded to the interaction with tyrosinase. According to the Lineweaver-Burk plot, the most potent compounds, 6g and 6n, exhibit a mixed type of inhibition, primarily noncompetitive inhibition. In silico molecular docking studies were employed to determine the binding mode and explore the Design Hypothesis in detail. The results suggested that these compounds could be considered promising leads for the further development of novel inhibitors to treat disorders related to tyrosinase.

Barbiturate-sulfonate hybrids as potent cholinesterase inhibitors: design, synthesis and molecular modeling studies

Aim: Design and synthesis of a series of 5-benzylidene(thio)barbiturates 3a-r.Methodology: Evaluation of the inhibitory activity of the new chemical entities on acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) using Donepezil as the standard reference.Results & Conclusion: Compound 3r emerged as the most potent AChE inhibitor (IC50 = 9.12 μM), while compound 3q exhibited the highest inhibitory activity against BChE (IC50 = 19.43 μM). Toxicological bioassays confirmed the absence of cytotoxicity for the most potent compounds at the tested doses. Molecular docking analysis demonstrated that the tested derivatives effectively bind to the active sites of both enzymes. Overall, this study sheds light on the potential of barbiturate-sulfonate conjugates as promising drug candidates.

Tyrosinase Inhibitors: A Perspective

Due to its integral role in the biosynthesis of melanin in all kingdoms of life, tyrosinase has become an extremely important target for inhibition in several sectors of research including agricultural and cosmetic research. Inhibitors of tyrosinase have made it to the market in the cosmetics industry, but their use has been limited due to conflicting efficacy and potential toxicity, which has led to several small molecules being removed from the market. Undaunted, researchers have continued to pursue tyrosinase inhibitors with varying degrees of success. These pursuits have built an impressive and rich library of research. This review is intended to provide a perspective of the past twenty years (2003-2023) of research on tyrosinase inhibitors by highlighting exemplar molecules and developments.

An expedient metal-free cascade route to chromonyl diene scaffolds: thermodynamic vs. kinetic control

A piperidine-catalyzed reaction between 3-formylchromone, 1,3-dimethyl barbituric acid, and ylidenemalononitriles is developed that offers chromonyl diene products in good yields. This cascade reaction proceeds via the insertion of ylidenemalononitriles between the Knoevenagel adduct obtained from 3-formylchromone and 1,3-dimethylbarbituric acid, where the pyrimidine-based enaminone is integrated with the chromone through the central diene linker. Similarly, introducing pyrimidine-based enaminone into the terminal part of the chromonyl diene scaffold gave an equilibrium mixture of rotational isomers in DMSO, which could be separated and isolated by crystallization. The computational analysis confirmed the role of barbiturate in directing the type of final chromonyl diene via kinetic or thermodynamic control. Moreover, computations revealed that one of these species, observed in the NMR spectra, is produced by the bond cleavage in the spirocyclic intermediate.

Cephalosporin as Potent Urease and Tyrosinase Inhibitor: Exploration through Enzyme Inhibition, Kinetic Mechanism, and Molecular Docking Studies

In present study, eleven cephalosporin drugs were selected to explore their new medically important enzyme targets with inherited safety advantage. To this end, selected drugs with active ingredient, cefpodoxime proxetil, ceftazidime, cefepime, ceftriaxone sodium, cefaclor, cefotaxime sodium, cefixime trihydrate, cephalexin, cefadroxil, cephradine, and cefuroxime, were evaluated and found to have significant activity against urease (IC50 = 0.06 ± 0.004 to 0.37 ± 0.046 mM) and tyrosinase (IC50 = 0.01 ± 0.0005 to 0.12 ± 0.017 mM) enzymes. Urease activity was lower than standard thiourea; however, tyrosinase activity of all drugs outperforms (ranging 6 to 18 times) the positive control: hydroquinone (IC50 = 0.18 ± 0.02 mM). Moreover, the kinetic analysis of the most active drugs, ceftriaxone sodium and cefotaxime sodium, revealed that they bind irreversibly with both the enzymes; however, their mode of action was competitive for urease and mixed-type, preferentially competitive for tyrosinase enzyme. Like in vitro activity, ceftriaxone sodium and cefotaxime sodium docking analysis showed their considerable binding affinity and significant interactions with both urease and tyrosinase enzymes sufficient for downstream signaling responsible for observed enzyme inhibition in vitro, purposing them as potent candidates to control enzyme-rooted obstructions in future.

Fluoroquinolones as Tyrosinase Inhibitors; Enzyme Kinetics and Molecular Docking Studies to Explore Their Mechanism of Action

The binding of fluoroquinolones, the most commonly prescribed antibiotics, with melanin is well explored. However, their binding patterns and exact mechanism of interaction with tyrosinase, a key enzyme in melanogenesis, are not explored yet. Thus, in the present study, seven fluoroquinolone drugs were selected to characterize their interactions with the tyrosinase enzyme: ciprofloxacin, enoxacin sesquihydrate, ofloxacin, levofloxacin, sparfloxacin, moxifloxacin and gemifloxacin. The results confirmed that all the drugs execute excellent enzyme activity, with an inhibition range from IC50 = 28 ± 4 to 50 ± 1.9 μM, outperforming the standard hydroquinone (IC50 = 170 μM). Later, kinetic studies revealed that all the drugs showed irreversible, but mixed-type, tyrosinase inhibition, with a preferentially competitive mode of action. Further, 2D and 3D docked complexes and binding analyses confirmed their significant interactions in the active region of the target enzyme, sufficient for the downstream signaling responsible for the observed tyrosinase inhibition. Thus, this is the first report demonstrating their mechanism of tyrosinase inhibition, critical for melanin-dependent responses, including toxicity.

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.

SAR and QSAR research on tyrosinase inhibitors using machine learning methods

Tyrosinase is a key rate-limiting enzyme in the process of melanin synthesis, which is closely related to human pigmentation disorders. Tyrosinase inhibitors can down-regulate tyrosinase to effectively reduce melanin synthesis. In this work, we conducted structure-activity relationship (SAR) study on 1097 diverse mushroom tyrosinase inhibitors. We applied five kinds of machine learning methods to develop 15 classification models. Model 5B built by fully connected neural networks and ECFP4 fingerprints achieved the highest prediction accuracy of 91.36% and Matthews correlation coefficient (MCC) of 0.81 on the test set. The applicability domains (AD) of classification models were defined by d S T D - P R O method. Moreover, we clustered the 1097 inhibitors into eight subsets by K-Means to figure out inhibitors' structural features. In addition, 10 quantitative structure-activity relationship (QSAR) models were constructed by four machine learning methods based on 813 inhibitors. Model 6 J, the best QSAR model, was developed by fully connected neural networks with 50 RDKit descriptors. It resulted in a coefficient of determination (r ²) of 0.770 and a root mean squared error (RMSE) of 0.482 on the test set. The AD of Model 6 J was visualized by Williams plot. The models built in this study can be obtained from the authors.

A systematic review of synthetic tyrosinase inhibitors and their structure-activity relationship

Tyrosinase is a copper-containing oxidation enzyme, which is responsible for the production of melanin. This enzyme is widely distributed in microorganisms, animals and plants, and plays an essential role in undesirable browning of fruits and vegetables, antibiotic resistance, skin pigment formation, sclerotization of cuticle, neurodegeneration, etc. Hence, it has been recognized as a therapeutic target for the development of antibrowning agents, antibacterial agents, skin-whitening agents, insecticides, and other therapeutic agents. With great potential application in food, agricultural, cosmetic and pharmaceutical industries, a large number of synthetic tyrosinase inhibitors have been widely reported in recent years. In this review, we systematically summarized the advances of synthetic tyrosinase inhibitors in the literatures, including their inhibitory activity, cytotoxicity, structure-activity relationship (SAR), inhibition kinetics, and interaction mechanisms with the enzyme. The collected information is expected to provide a rational guidance and effective strategy to develop novel, potent and safe tyrosinase inhibitors for better practical applications in the future.

1,2,3-Triazole-linked 5-benzylidene (thio)barbiturates as novel tyrosinase inhibitors and free-radical scavengers

In this study, benzyl-1,2,3-triazole-linked 5-benzylidene (thio)barbiturate derivatives 7a-d and 8a-h were designed as potential tyrosinase inhibitors and free-radical scavengers. The twelve derivatives were synthesized via the [3+2] cycloaddition reaction of the corresponding benzyl azide as a dipole and the corresponding alkyne as a dipolarophile in the presence of copper(I) species, generated in situ from copper(II)/ascorbate. The thiobarbiturate derivative 8h and the barbiturate derivative 8b bearing 4-fluoro and 4-bromo groups on the benzyl-triazole moiety were found to be the most potent tyrosinase inhibitors with IC₅₀ values of 24.6 ± 0.9 and 26.8 ± 0.8 μM, respectively. Almost all the compounds showed a good radical scavenging activity with EC₅₀ values in the range of 29.9-324.9 μM. Derivatives 7a, 8f, and 8h were the most potent free-radical scavengers with EC₅₀ values of 29.9 ± 0.8, 36.8 ± 0.9, and 39.2 ± 1.1 μM, respectively. The kinetic analysis revealed that compound 8h was a mixed-type tyrosinase inhibitor. The molecular docking analysis indicated that 8b and 8h were well accommodated in the active site of the tyrosinase enzyme and possessed the most negative binding energy values of -8.55 and -8.81 kcal/mol, respectively. Moreover, it was found that the two residues, Asn81 and Glu322, played a significant role in forming stable enzyme-inhibitor complexes.

Uses of Cyclohexan-1,3-dione for the Synthesis of Thiazole, Pyrazole, Thiophene, Isoxazole and Pyran Derivatives with Antitumor Activities

Background:

A wide range of thiazole, pyrazole and pyran derivatives gained special attention due to pharmacological activities especially therapeutic activities. Many pharmacological drugs containing the thiazole and pyrazole nuclei are known in the market.

Methods:

The 2-arylidencyclohexan-1,3-dione 3a-c were the key starting compounds for many heterocyclic reactions to produce substituted heterocyclic derivatives.

Results:

Antiproliferative activities of the produced compounds against six cancer cell lines A549, HT-29, MKN-45, U87MG, SMMC-7721 and H460 were measured in which the compounds showed high inhibition. The most promising compounds were tested against tyrosine kinases (c-Kit, Flt-3, VEGFR-2, EGFR, and PDGFR). Structure-Activity Relationship (SAR) was rationalized by assessing the varying structural features of the molecules. In addition, the most active compounds were selected for Pim-1 inhibition.

Conclusion:

Thirty compounds were synthesized. Ten of them (3a, 3c, 5a, 5c, 7a, 10f, 11a, 13c, 16a and 16c) were the most active compounds for selected cancer cell lines. Compounds 3c, 5c, 7a, 10f, 13c and 16c showed high inhibition toward the tyrosine kinases while compounds 3c, 5c and 10f were the most potent to inhibit Pim-1.

Novel Amide Derivatives as Potent Tyrosinase Inhibitors; In-vitro, In-vivo Antimelanogenic Activity and Computational Studies

BACKGROUND

Tyrosinase is involved in the melanin biosynthesis and the abnormal accumulation of melanin pigments leading to hyperpigmentation disorders. Controlling the melanogenesis could be an important strategy for treating abnormal pigmentation.

METHODS

In the present study, a series of amide derivatives (3a-e and 5a-e) were synthesized aiming to inhibit tyrosinase activity and melanin production. All derivatives were screened for tyrosinase inhibition in a cell-free system. The possible interactions of amide derivatives with tyrosinase enzyme and effect of these interactions on tyrosinase structure were checked by molecular docking in silico and by Circular Dichroism (CD) studies, respectively. The most potent amide derivative (5c) based on cell-free experiments, was further tested for cellular ROS inhibition and for tyrosinase activity using mouse skin melanoma (B16F10) cells.

RESULTS

The tyrosinase inhibitory concentration (IC50) for tested compounds was observed between the range of 68 to 0.0029 µg/ml with a lowest IC50 value of compound 5c which outperforms the reference arbutin and kojic acid. The cellular tyrosinase activity and melanin quantification assay demonstrate that 15µg/ml of 5c attenuates 36% tyrosinase, 24% melanin content of B16F10 cells without significant cell toxicity. Moreover, the zebrafish in vivo assay reveals that 5c effectively reduces melanogenesis without perceptible toxicity. Furthermore, the molecular docking demonstrates that compound 5c interacts with copper ions and multiple amino acids in the active site of tyrosinase with best glide score (-5.387 kcal/mol), essential for mushroom tyrosinase inhibition and the ability to diminish the melanin synthesis in-vitro and in-vivo.

CONCLUSION

Thus, we propose compound 5c as a potential candidate to control tyrosinase rooted hyperpigmentation in the future.

Synthesis, computational studies, tyrosinase inhibitory kinetics and antimelanogenic activity of hydroxy substituted 2-[(4-acetylphenyl)amino]-2-oxoethyl derivatives

The over expression of melanogenic enzymes like tyrosinase caused many hyperpigmentaion disorders. The present work describes the synthesis of hydroxy substituted 2-[(4-acetylphenyl)amino]-2-oxoethyl derivatives 3a-e and 5a-e as antimelanogenic agents. The tyrosinase inhibitory activity of synthesized derivatives 3a-e and 5a-e was determined and it was found that derivative 5c possesses excellent activity with IC50 = 0.0089 µM compared to standard kojic acid (IC50 = 16.69 µM). The presence of hydroxyl groups at the ortho and the para position of cinnamic acid phenyl ring in compound 5c plays a vital role in tyrosinase inhibitory activity. The compound 5d also exhibited good activity (IC50 = 8.26 µM) compared to standard kojic acid. The enzyme inhibitory kinetics results showed that compound 5c is a competitive inhibitor while 5d is a mixed-type inhibitor. The mode of binding for compounds 5c and 5d with tyrosinase enzyme was also assessed and it was found that both derivatives irreversibly bind with target enzyme. The molecular docking and molecular dynamic simulation studies were also performed to find the position of attachment of synthesized compounds at tyrosinase enzyme (PDB ID 2Y9X). The results showed that all of the synthesized compounds bind well with the active binding sites and most potent derivative 5c formed stable complex with target protein. The cytotoxicity results showed that compound 5c is safe at a dose of 12 µg/mL against murine melanoma (B16F10) cells. The same dose of 5c was selected to determine antimelanogenic activity; the results showed that it produced antimelenogenic effects in murine melanoma (B16F10) cells. Based on our investigations, it was proposed that compound 5c may serve as a lead structure to design more potent antimelanogenic agents.

Synthesis, computational molecular docking analysis and effectiveness on tyrosinase inhibition of kojic acid derivatives

Tyrosinase inhibitors have become increasingly important as whitening agents and for the treatment of pigmentary disorders. In this study, the synthesis of kojic acid derivatives having 2-substituted-3-hydroxy-6-hyroxymethyl/chloromethyl/methyl/morpholinomethylpiperidinyl- methyl/pyrrolidinylmethyl-4H-pyran-4-one structure (compounds 1-30) with inhibitory effects on tyrosinase enzyme were described. One-pot Mannich reaction was carried out by using kojic acid/chlorokojic acid/allomaltol and substituted benzylpiperazine derivatives in presence of formaline. Subsequently, cyclic amine (morpholine, piperidine and pyrrolidine) derivatives of the 6th-position of chlorokojic acid were obtained with nucleophilic substitutions in basic medium. The structures of new compounds were identified by FT-IR, ¹H- and ¹³C NMR, ESI-MS and elemental analysis data. The potential mushroom tyrosinase inhibitory activity of the compounds were evaluated by the spectrophotometric method using l-DOPA as a substrate and kojic acid as the control agent. The potential inhibitory activity was also investigated in silico using molecular docking simulation method. Tyrosinase inhibitory action was significantly more efficacious for several compounds (IC₅₀: 86.2-362.1 µM) than kojic acid (IC₅₀: 418.2). Compound 3 bearing 3,4-dichlorobenzyl piperazine moiety was proven to have the highest inhibitory activity. The results of docking studies showed that according to the predicted conformation of compound 3 in the enzyme binding site, hydroxymethyl group provides a metal complex with copper ions and enzyme. Thus, this interaction explain the high inhibitory activities of the compounds 1, 3 and 4 possessing hydroxymethyl substituent supporting the mushroom assay results with docking studies. In accordance with the results, it is suggested that Mannich bases of kojic acid bearing substituted benzyl piperazine groups (compounds 1, 3, 4, 11, 13, 14, 23, 24, 28, and 29) could be promising antityrosinase agents. Additionally, considering the relationship between tyrosinase inhibitory activity results and molecular docking, a new tyrosinase inhibition mechanism can be proposed.

Crystal structure, spectroscopic studies and theoretical studies of thiobarbituric acid derivatives: understanding the hydrogen-bonding patterns

In addition to their wide-ranging applications in the pharmaceutical industry, thiobarbituric acid (TBA) derivatives are also known to possess applications in engineering and materials science. 20 TBA derivatives, with diversity at the N and C-5 positions through acylation, Schiff base formation, Knoevenagel condensation, thioamide and enamine formation, were studied. The absolute configurations for six derivatives, namely 5-acetyl-1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, C₁₀H₁₄N₂O₃S, A01, 1,3-diethyl-5-propionyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, C₁₁H₁₆N₂O₃S, A02, tert-butyl [1-(1,3-diethyl-4,6-dioxo-2-thioxohexahydropyrimidin-5-yl)-3-methyl-1-oxobutan-2-yl]carbamate, C₁₈H₂₉N₃O₅S, A06, 1,3-diethyl-4,6-dioxo-2-thioxo-N-(p-tolyl)hexahydropyrimidine-5-carbothioamide, C₁₆H₁₉N₃O₂S₂, A13, 5-(1-aminoethylidene)-1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, C₁₀H₁₅N₃O₂S, A17, and 5-(1-aminopropylidene)-1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione, C₁₁H₁₇N₃O₂S, A18, were confirmed by single-crystal X-ray crystallography, which indicates the formation of intramolecular hydrogen bonding in all six cases and intermolecular hydrogen bonding for A17. In A13, the presence of two intramolecular hydrogen bonds was observed. The stabilization of the enol form over the keto form was confirmed by computation. In order to convert the keto form to the enol form, an energy barrier of 55.05 kcal mol^-1 needs to be overcome, as confirmed by transition-state calculations.

Exploiting the Thiobarbituric Acid Scaffold for Antibacterial Activity

Thiobarbituric acid (TBA) has been considered a privileged structure for developing antimicrobial agents. Diversity was obtained at positions N and at C5 through acylation, Schiff base formation, Knoevenagel condensation, and thioamide and enamine formation. The present work describes the synthesis of small libraries based on the TBA moiety and above-mentioned reactions. Preliminary antimicrobial activity screening of the prepared compounds against selected bacteria (both Gram-positive and -negative) showed the best results for the Boc-Phe-TBA derivative. These results could be useful for designing and building libraries based on other amino acids with distinct protecting groups.

Chemical Constituents from Apios americana and Their Inhibitory Activity on Tyrosinase

The goal of this study was to identify phytochemicals with inhibitory activity against tyrosinase. Nine compounds 1–9 were isolated from the tubers of Apios americana. This is the first report of aromadendrin 5-methyl ether (1) being isolated from the Apios species. Among them, compounds 2 and 8 showed inhibitory activity toward tyrosinase. Based on a Dixon plot, the potential K_i values of competitive inhibitors 2 and 8 were calculated as 10.3 ± 0.8 µM and 44.2 ± 1.7 µM, respectively. An IC₅₀ value of 13.2 ± 1.0 µM was calculated for the slow-binding inhibitor 2 after preincubation with tyrosinase. Additionally, the predicted binding sites between the receptor and ligand, as well as secondary structure changes, in the presence of 2 were examined by molecular simulation.

Skin whitening agents: medicinal chemistry perspective of tyrosinase inhibitors

Melanogenesis is a process to synthesize melanin, which is a primary responsible for the pigmentation of human skin, eye and hair. Although numerous enzymatic catalyzed and chemical reactions are involved in melanogenesis process, the enzymes such as tyrosinase and tyrosinase-related protein-1 (TRP-1) and TRP-2 played a major role in melanin synthesis. Specifically, tyrosinase is a key enzyme, which catalyzes a rate-limiting step of the melanin synthesis, and the downregulation of tyrosinase is the most prominent approach for the development of melanogenesis inhibitors. Therefore, numerous inhibitors that target tyrosinase have been developed in recent years. The review focuses on the recent discovery of tyrosinase inhibitors that are directly involved in the inhibition of tyrosinase catalytic activity and functionality from all sources, including laboratory synthetic methods, natural products, virtual screening and structure-based molecular docking studies.

Study on the design, synthesis and structure-activity relationships of new thiosemicarbazone compounds as tyrosinase inhibitors

52 Structure-based thiosemicarbazone compounds bearing various substituted-lipophilic part, including substituted-benzaldehyde, substituted-phenylalkan-1-one and their biphenyl-type thiosemicarbazone analogs, were designed, synthesized and evaluated as new tyrosinase inhibitors. The results demonstrated that 22 compounds have potent inhibitory activities against tyrosinase with the IC₅₀ value of lower than 1.0 μM. On the basis of the obtained experimental data, the structure-activity relationships (SARs) were rationally derived. Besides, the inhibition mechanism and the inhibitory kinetics of selected compounds 3d and 6e were investigated, revealing that such type of compounds were belonged to the reversible and competitive tyrosinase inhibitors. To verify the safety of these developed thiosemicarbazone compounds, four randomly selected compounds 3d, 4e, 6a and 9a were also tested in 293T cell line for the evaluation of the cytotoxicity. Interestingly, all these compounds almost did not perform any toxicity to 293T cells even at a high concentration of 1000 μmol/L. Taken together, these results suggested that such compounds could serve as the highly efficient and more safe candidates for the treatment of tyrosinase-related disorders.

Development of highly potent melanogenesis inhibitor by in vitro, in vivo and computational studies

The present work describes the synthesis of few hydroxylated amide derivatives as melanogenesis inhibitors. In vitro, in vivo and computational studies proved that compound 6d is a highly potent melanogenesis inhibitor compared to standard kojic acid. The title amides 4a–e and 6a–e were synthesized following simple reaction routes with excellent yields. Most of the synthesized compounds exhibited good mushroom tyrosinase inhibitory activity, but compound 6d showed excellent activity (IC₅₀ 0.15 µM) compared to standard kojic acid (IC₅₀ 16.69 µM). Lineweaver–Burk plots were used for the determination of kinetic mechanism, and it was found that compounds 4c and 6d showed non-competitive inhibition while 6a and 6b showed mixed-type inhibition. The kinetic mechanism further revealed that compound 6d formed irreversible complex with the target enzyme tyrosinase. The Ki values determined for compounds 4c, 6a, 6b and 6d are 0.188, 0.84, 2.20 and 0.217 µM respectively. Results of human tyrosinase inhibitory activity in A375 human melanoma cells showed that compound 6d exhibited 91.9% inhibi-tory activity at a concentration of 50 µg/mL. In vivo cytotoxicity evaluation of compound 6d in zebrafish embryos showed that it is non-toxic to zebrafish. Melanin depigmentation assay performed in zebrafish indicated that compound 6d possessed greater potential in decreasing melanin contents compared to kojic acid at the same concentration. Computational studies also supported the wet lab findings as compound 6d showed a highest binding affinity with the target protein (PDBID: 2Y9X) with a binding energy value of −7.90 kcal/mol. Molecular dynamic simulation studies also proved that amide 6d formed the most stable complex with tyrosinase. Based upon our in vitro, in vivo and computational studies, we propose that compound 6d is a promising candidate for the development of safe cosmetic agent.

Carvacrol derivatives as mushroom tyrosinase inhibitors; synthesis, kinetics mechanism and molecular docking studies

The present work describesthe development of highly potent mushroom tyrosinase inhibitor better than the standard kojic acid. Carvacrol derivatives 4a-f and 6a-d having substituted benzoic acid and cinnamic acidresidues were synthesized with the aim to possess potent tyrosinase inhibitory activity.The structures of the synthesized compounds were ascertained by their spectroscopic data (FTIR, ¹HNMR, ¹³CNMR and Mass Spectroscopy).Mushroom tyrosinase inhibitory activity of synthesized compounds was determined and it was found that one of the derivative 6c possess higher activity (IC₅₀ 0.0167μM) than standard kojic acid (IC₅₀ 16.69μM). The derivatives 4c and 6b also showed good tyrosinase inhibitory activity with (IC₅₀ 16.69μM) and (IC₅₀ 16.69μM) respectively.Lineweaver—Burk and Dixon plots were used for the determination of kinetic mechanism of the compounds 4c and 6b and 6c. The kinetic analysis revealed that compounds 4c and 6b showed mixed-type inhibition while 6c is a non-competitive inhibitor having Ki values19 μM, 10 μM, and 0.05 μMrespectively. The enzyme inhibitory kinetics further showed thatcompounds 6b and 6c formed irreversible enzyme inhibitor complex while 4c bind reversibly with mushroom tyrosinase.The docking studies showed that compound 6c have maximum binding affinity against mushroom tyrosinase (PDBID: 2Y9X) with binding energy value (-7.90 kcal/mol) as compared to others.The 2-hydroxy group in compound 6c interacts with amino acid HIS85 which is present in active binding site. The wet lab results are in good agreement with the dry lab findings.Based upon our investigation we may propose that the compound 6c is promising candidate for the development of safe cosmetic agent.

Synthesis of chiral pyrazolo[4,3-e][1,2,4]triazine sulfonamides with tyrosinase and urease inhibitory activity

A new series of sulfonamide derivatives of pyrazolo[4,3-e][1,2,4]triazine with chiral amino group has been synthesized and characterized. The compounds were tested for their tyrosinase and urease inhibitory activity. Evaluation of prepared derivatives demonstrated that compounds (8b) and (8j) are most potent mushroom tyrosinase inhibitors whereas all of the obtained compounds showed higher urease inhibitory activity than the standard thiourea. The compounds (8a), (8f) and (8i) exhibited excellent enzyme inhibitory activity with IC₅₀ 0.037, 0.044 and 0.042 μM, respectively, while IC₅₀ of thiourea is 20.9 μM.

Barbituric acid–triphenylamine adduct as an AIEE-type molecule and optical probe for mercury(ii)

A barbituric acid–triphenylamine adduct displayed interesting aggregation-induced emission enhancement (AIEE) features in a THF–water co-solvent system and can act as a fluorescence turn-on probe for Hg²⁺.

Analogues of ethionamide, a drug used for multidrug-resistant tuberculosis, exhibit potent inhibition of tyrosinase

Tyrosinase catalyzes two distinct sequential reactions in melanin biosynthesis: the hydroxylation of tyrosine to DOPA followed by the oxidation of DOPA to dopaquinone. The central roles of melanin in living species have motivated researchers to maintain constant efforts to discover new agents that modulate tyrosinase activity. In this study, we report on the inhibition of tyrosinase by ethionamide and its analogues. Ethionamide, 2-ethylpyridine-4-carbothioamide, is a second-line antituberculosis drug used for the treatment of multidrug-resistant tuberculosis. The chemical similarity of ethionamide to phenylthiourea, a well-known tyrosinase inhibitor, led us to investigate its inhibitory effects on mushroom tyrosinase and the IC50 was calculated as 4 μM. Five analogues of ethionamide, including another antituberculosis drug, prothionamide, were also inhibitory, with values for IC50 in the range of 3-43 μM. Fluorescence quenching experiments supported a mechanism of direct binding. In contrast, isoniazid, a structural analogue and first-line antituberculosis drug, was a poor inhibitor of tyrosinase. We also tested the effects of ethionamide and its analogues on melanin content in B16F10 cells. At a concentration of 50 μM, the molecules, pyridine-2-carbothioamide and thiobenzamide substantially decreased the melanin content by 44% and 37%, respectively. In addition to identifying other interactions, docking simulations showed that the carbothioamide groups of the molecules make essential contacts with the catalytic di-copper atoms. Our results suggest that carbothioamide can be a central moiety for the development of new and potent tyrosinase inhibitors.

Design and synthesis of new barbituric- and thiobarbituric acid derivatives as potent urease inhibitors: Structure activity relationship and molecular modeling studies

In this study 36 new compounds were synthesized by condensing barbituric acid or thiobarbituric acid and respective anilines (bearing different substituents) in the presence of triethyl orthoformate in good yields. In vitro urease inhibition studies against jack bean urease revealed that barbituric acid derived compounds (1-9 and 19-27) were found to exhibit low to moderate activity however thiobarbituric acid derived compounds (10-18 and 28-36) showed significant inhibition activity at low micro-molar concentrations. Among the synthesized compounds, compounds (15), (12), (10), (36), (16) and (35) showed excellent urease inhibition with IC50 values 8.53 ± 0.027, 8.93 ± 0.027, 12.96 ± 0.13, 15 ± 0.098, 18.9 ± 0.027 and 19.7 ± 0.63 μM, respectively, even better than the reference compound thiourea (IC50 = 21 ± 0.011). The compound (11) exhibited comparable activity to the standard with IC50 value 21.83 ± 0.19 μM. In silico molecular docking studies for most active compounds (10), (12), (15), (16), (35) and (36) and two inactive compounds (3) and (6) were performed to predict the binding patterns.

Design and discovery of tyrosinase inhibitors based on a coumarin scaffold

A novel series of 3-aryl and 3-heteroarylcoumarins displaying tyrosinase inhibitory activity.