Melanogenesis inhibitory activity of a 7-O-9'-linked neolignan from Alpinia galanga fruit

Expanding Chemical Frontiers: Approaches for Generating Diverse and Bioactive Natural Product-Like Compounds Libraries from Extracts

The realms of natural products and synthetic compounds exhibit distinct chemical spaces that not only differ but also complement each other. While the convergence of these two domains has been explored through semisynthesis and conventional pharmacomodulation endeavours applied to natural frameworks, a recent and innovative approach has emerged that involves the combinatorial generation of libraries of 'natural product-like compounds' (NPLCs) through the direct synthetic derivatization of natural extracts. This has led to the production of numerous NPLCs that incorporate structural elements from both their natural (multiple saturated rings, oxygen content, chiral centres) and synthetic (aromatic rings, nitrogen and halogen content, drug-like properties) precursors. Through careful selection of extracts and reagents, specific bioactivities have been achieved, and this strategy has been deployed in various ways, showing great promise without reaching its full potential to date. This review seeks to provide an overview of reported examples involving the chemical engineering of extracts, showcasing a spectrum of natural product alterations spanning from simple substitutions to complete scaffold remodelling. It also includes an analysis of the accomplishments, perspectives and technical challenges within this field.

Journey of Alpinia galanga from kitchen spice to nutraceutical to folk medicine to nanomedicine

ETHANOPHARMACOLOGICAL IMPORTANCE

Alpinia galanga (L.) Willd (AG), belonging to Zingiberaceae family is used as a spice and condiment in various culinary preparations of Indonesia, Thailand and Malaysia. It has been also used as a key ingredient in various traditional systems of medicine for the treatment of throat infection, asthma, urinary ailments, inflammation and rheumatism amongst other conditions. AG is widely used as a functional food and included in various preparations to obtain its nutraceutical and pharmacological benefits of its phytoconstituents such as phenyl propanoids, flavonoids and terpenoids. Over the past decades, several researchers have carried out systematic investigation on various parts of AG. Numerous studies on AG rhizomes have shown positive pharmacological effects such as anti-inflammatory, anticancer, antipsoriasis, antiallergic, neuroprotective and thermogenesis. Till date, no comprehensive review summarizing the exploitation of AG into nanomedicine has been published.

AIM OF THE REVIEW

This comprehensive review aims to briefly discuss cultivation methods, propagation techniques, extraction processes for AG. The ethnopharmacological uses and pharmacological activities of AG extracts and its isolates are discussed in detail which may contribute well in further development of novel drug delivery system (NDDS) i.e. future nanomedicine.

MATERIALS AND METHODS

Information about AG was collected using search engine tools such as Google, Google Scholar, PubMed, Google Patent, Web of Science and bibliographic databases of previously published peer-reviewed review articles and research works were explored. The obtained data sets were sequentially arranged for better understanding of AG's potential.

RESULTS

More advanced genetic engineering techniques have been utilized in cultivation and propagation of AG for obtaining better yield. Extraction, isolation and characterization techniques have reported numerous phytoconstituents which are chemically phenolic compounds (phenyl propanoids, flavonoids, chalcones, lignans) and terpenes. Ethnopharmacological uses and pharmacological activity of AG are explored in numerous ailments, their mechanism of action and its further potential to explore into novel drug delivery system are also highlighted.

CONCLUSIONS

The review highlights the importance of plant tissue culture in increasing the production of AG plantlets and rhizomes. It was understood from the review that AG and its phytoconstituents possess numerous pharmacological activities and have been explored for the treatment of cancer, microbial infection, gastrointestinal disorders, neuroprotective effects, obesity and skin disorders. However, the use of AG as alternative medicine is limited owing to poor solubility of its bioactive components and their instability. To overcome these challenges, novel drug delivery systems (NDDS) have been utilized and found good success in overcoming its aforementioned challenges. Furthermore, efforts are required towards development of scalable, non-toxic and stable NDDS of AG and/or its bioactives.

Antiallergic Properties of Biflavonoids Isolated from the Flowers of Mesua ferrea Linn

The methanolic extract from the flowers of Mesua ferrea Linn. (Calophyllaceae) showed significant hyaluronidase inhibitory activity. Following a bioassay-guided separation of the extract, two biflavonoids, viz., mesuaferrone-A (1) and mesuaferrone-B (2), were isolated, along with ten flavonoids (3–12), two xanthones (13 and 14), three triterpenes (15–17), a phenylpropanoid (18), and five aromatics (19–24). Among the isolates, 1 and 2 (IC50 = 51.1 µM and 54.7 µM, respectively) exhibited hyaluronidase inhibitory activity equivalent to that of the commercially available antiallergic agents disodium cromoglycate (64.8 μM) and ketotifen fumarate (76.5 μM). These biflavonoids (1 and 2) are 8-8″ linked dimers that are composed of naringenin (1a) or apigenin (3), with their corresponding monomers lacking inhibitory activity (IC50 > 300 μM). In addition, 1 and 2 (IC50 = 49.4 µM and 49.2 µM, respectively) inhibited the release of β-hexosaminidase, which is a marker of antigen-IgE-mediated degranulation, in rat basophilic leukemia (RBL-2H3) cells. These inhibitory activities were more potent than those of the antiallergic agents tranilast and ketotifen fumarate (IC50 = 282 μM and 158 μM, respectively), as well as one of the corresponding monomers (1a; IC50 > 100 μM). Nonetheless, these effects were weaker than those of the other monomer (3; IC50 = 6.1 μM).

A New Monoterpene from the Rhizomes of Alpinia galanga and Its Anti-Vpr Activity

A new menthane-type monoterpene, alpigalanol (1), together with four known terpenes (2-5) were isolated from the ethyl acetate soluble fraction of the 70 % ethanol extract of the Alpinia galanga rhizomes. The structure of 1 was determined by spectroscopic analyses, including 1D- and 2D-NMR. The extract of the A. galanga rhizomes and all isolated compounds (1-5) possessed Vpr inhibitory activities against the TREx-HeLa-Vpr cells at a concentration of 1.25 μM without showing any cytotoxicity.

Indole Glycosides from Calanthe discolor with Proliferative Activity on Human Hair Follicle Dermal Papilla Cells

A methanol extract from the underground part of Calanthe discolor Lindl. (Orchidaceae) demonstrated significant proliferative activity on human hair follicle dermal papilla cells (HFDPC, % of control: 120.8 ± 0.2%) at 100 µg/mL against HFDPC. Through bioassay-guided separation of the extract, a new indole glycoside named 6'-O-β-D-apiofuranosylindican (1) was isolated along with six known compounds (2-7) including three indole glycosides. The stereostructure of 1 was elucidated based on its spectroscopic properties and chemical characteristics. Among the isolates, 1 (110.0 ± 1.0%), glucoindican (3, 123.9 ± 6.8%), and calanthoside (4, 158.6 ± 7.1%) showed significant proliferative activity at 100 µM. Furthermore, the active indole glycosides (1, 3, and 4) upregulated the expression of vascular endothelial growth factor (VEGF) and fibroblast growth factor-7 (FGF-7) mRNA and protein in HFDPC, which could be the mechanism of their proliferative activity.

Six New Phenylpropanoid Derivatives from Chemically Converted Extract of Alpinia galanga (L.) and Their Antiparasitic Activities

Chemical conversion of the extract of natural resources is a very attractive way to expand the chemical space to discover bioactive compounds. In order to search for new medicines to treat parasitic diseases that cause high morbidity and mortality in affected countries in the world, the ethyl acetate extract from the rhizome of Alpinia galanga (L.) has been chemically converted by epoxidation using dioxirane generated in situ. The biological activity of chemically converted extract (CCE) of A. galanga (L.) significantly increased the activity against Leishmania major up to 82.6 ± 6.2 % at 25 μg/mL (whereas 2.7 ± 0.8% for the original extract). By bioassay-guided fractionation, new phenylpropanoids (1-6) and four known compounds, hydroquinone (7), 4-hydroxy(4-hydroxyphenyl)methoxy)benzaldehyde (8), isocoumarin cis 4-hydroxymelein (9), and (2S,3S,6R,7R,9S,10S)-humulene triepoxide (10) were isolated from CCE. The structures of isolated compounds were determined by spectroscopic analyses of 1D and 2D NMR, IR, and MS spectra. The most active compound was hydroquinone (7) with IC50 = 0.37 ± 1.37 μg/mL as a substantial active principle of CCE. In addition, the new phenylpropanoid 2 (IC50 = 27.8 ± 0.34 μg/mL) also showed significant activity against L. major compared to the positive control miltefosine (IC50 = 7.47 ± 0.3 μg/mL). The activities of the isolated compounds were also evaluated against Plasmodium falciparum, Trypanosoma brucei gambisense and Trypanosoma brucei rhodeisense. Interestingly, compound 2 was selectively active against trypanosomes with potent activity. To the best of our knowledge, this is the first report on the bioactive "unnatural" natural products from the crude extract of A. galanga (L.) by chemical conversion and on its activities against causal pathogens of leishmaniasis, trypanosomiasis, and malaria.

Ent-kaurane-type diterpenoids from Isodonis Herba activate human hair follicle dermal papilla cells proliferation via the Akt/GSK-3β/β-catenin transduction pathway

A methanol extract from Isodonis Herba demonstrated significant proliferative effect on human hair follicle dermal papilla cells (HFDPC, % of control: 150.0 ± 2.0% at 20 µg/mL, p < 0.01). From the extract, 14 ent-kaurane-type diterpenoids (1-14), two abietane-type diterpenoids (15 and 16) and four triterpenoids (17-20) were isolated. Among the isolates, enmein (1, 160.9 ± 3.0% at 20 µM, p < 0.01), isodocarpin (2, 169.3 ± 4.9% at 5 µM, p < 0.01), nodosin (4, 160.5 ± 12.4% at 20 µM, p < 0.01), and oridonin (8, 165.4 ± 10.6% at 10 µM, p < 0.01) showed the proliferative effects. The principal component enmein (1) activated the expression of vascular endothelial growth factor (VEGF) mRNA, upregulated the production of VEGF and increased levels of phospho-Akt, phospho-GSK-3β, and β-catenin accumulation in HFDPC, which could be the mechanism of these activate proliferation activity.

Antileishmanial Activity of Lignans, Neolignans, and Other Plant Phenols

Secondary metabolites (SM) from organisms have served medicinal chemists over the past two centuries as an almost inexhaustible pool of new drugs, drug-like skeletons, and chemical probes that have been used in the "hunt" for new biologically active molecules with a "beneficial effect on human mind and body." Several secondary metabolites, or their derivatives, have been found to be the answer in the quest to search for new approaches to treat or even eradicate many types of diseases that oppress humanity. A special place among SM is occupied by lignans and neolignans. These phenolic compounds are generated biosynthetically via radical coupling of two phenylpropanoid monomers, and are known for their multitarget activity and low toxicity. The disadvantage of the relatively low specificity of phenylpropanoid-based SM turns into an advantage when structural modifications of these skeletons are made. Indeed, phenylpropanoid-based SM previously have proven to offer great potential as a starting point in drug development. Compounds such as Warfarin^® (a coumarin-based anticoagulant) as well as etoposide and teniposide (podophyllotoxin-based anticancer drugs) are just a few examples. At the beginning of the third decade of the twenty-first century, the call for the treatment of more than a dozen rare or previously "neglected" diseases remains for various reasons unanswered. Leishmaniasis, a neglected disease that desperately needs new ways of treatment, is just one of these. This disease is caused by more than 20 leishmanial parasites that are pathogenic to humans and are spread by as many as 800 sandfly species across subtropical areas of the world. With continuing climate changes, the presence of Leishmania parasites and therefore leishmaniasis, the disease caused by these parasites, is spreading from previous locations to new areas. Thus, leishmaniasis is affecting each year a larger proportion of the world's population. The choice of appropriate leishmaniasis treatment depends on the severity of the disease and its form of manifestation. The success of current drug therapy is often limited, due in most cases to requiring long hospitalization periods (weeks to months) and the toxicity (side effects) of administered drugs, in addition to the increasing resistance of the parasites to treatment. It is thus important to develop new drugs and treatments that are less toxic, can overcome drug resistance, and require shorter periods of treatment. These aspects are especially important for the populations of developing countries. It was reported that several phenylpropanoid-based secondary metabolites manifest interesting antileishmanial activities and are used by various indigenous people to treat leishmaniasis. In this chapter, the authors shed some light on the various biological activities of phenylpropanoid natural products, with the main focus being on their possible applications in the context of antileishmanial treatment.

Essential Oils of Alpinia nantoensis Retard Forskolin-Induced Melanogenesis via ERK1/2-Mediated Proteasomal Degradation of MITF

The anti-melanogenic activity of essential oils of Alpinia nantoensis and their bioactive ingredients were investigated in vitro. Treatment with leaf (LEO) and rhizome (REO) essential oils of A. nantoensis, significantly reduced forskolin-induced melanin production followed by down-regulation of tyrosinase (TYR) and tyrosinase related protein-1 (TRP-1) expression at both transcriptional and translational levels. Further studies revealed that down-regulation TYR and TRP-1 were caused by LEO/REO-mediated suppression of Microphthalmia-associated transcription factor (MITF), as evidenced by reduced nuclear translocation of MITF. Also, we found that LEO/REO induce the sustained activation of ERK1/2, which facilitate subsequent proteasomal degradation of MITF, as confirmed by that LEO/REO failed to inhibits MITF activity in ERK1/2 inhibitor treated cells. In addition, a significant increase of ubiquitinated MITF was observed after treatment with LEO and REO. Furthermore, the chemical composition of LEO and REO were characterized by gas chromatography-mass spectrometry (GC-MS) resulted that camphor, camphene, α-pinene, β-pinene, isoborneol and _D-limonene were the major compounds in both LEO and REO. Further studies revealed that α-pinene and _D-limonene were the active components responsible for the anti-melanogenic properties of LEO and REO. Based on the results, this study provided a strong evidence that LEO and REO could be promising natural sources for the development of novel skin-whitening agents for the cosmetic purposes.

Anti-proliferative activities of coumarins from the Thai medicinal plant Mammea siamensis (Miq.) T. Anders. against human digestive tract carcinoma cell lines

Geranylated coumarins named mammeasins G-J (1-4) were isolated from the methanol extract of the flowers of Mammea siamensis (Miq.) T. Anders. (Calophyllaceae) originating in Thailand. Their structures were established based on detailed spectroscopic analyses. The isolates, including previously reported coumarin constituents (5-28), exhibited anti-proliferative activities against human carcinoma cell lines HSC-2, HSC-4, MKN-45, and MCF-7. Mammeasin A (7, IC₅₀ = 13.6 μM) and surangin B (15, 15.2 μM), both consisting of the geranyl group, were found to show relatively strong activities against HSC-4 cells and their mechanisms of action were found to involve apoptotic cell death.

Synthesis of N-arylindazole-3-carboxamide and N-benzoylindazole derivatives and their evaluation against α-MSH-stimulated melanogenesis

We have designed and synthesized twenty-six N-arylindazole-3-carboxamide (3a-p) and N-benzoylindazole (6a-j) derivatives to discover with excellent inhibition activities of α-MSH-stimulated melanogenesis. In the bio evaluation studies of these compounds, we discovered eighteen compounds, out of twenty-six exhibited more potent inhibition than the positive control arbutin. From the SAR studies, we identified 3k and 6g as lead compounds which displayed almost 5 and 9 times more potent inhibition of α-MSH-stimulated melanogenesis respectively than the reference arbutin. It is also evident the presence of electron withdrawing group at para position (R³) for the compounds (3a-p) and presence of +M group at ortho position (R⁵) for the compounds (6a-j) were crucial for their excellent inhibition activities of α-MSH-stimulated melanogenesis.

Lignans and Neolignans: Plant secondary metabolites as a reservoir of biologically active substances

Lignans and neolignans are plant secondary metabolites derived from the oxidative coupling of phenylpropanoids. Biological activity of these phenolic compounds ranges from antioxidant, antitumor (terminaloside P, IC₅₀ = 10 nM), anti-inflammatory, anti-neurodegenerative (schibitubin B, IC₅₀ = 3.2 nM) and antiviral (patentiflorin A, IC₅₀ = 14-23 nM) to antimicrobial. In addition, it was observed that several members of this group, namely enterolactone and its biochemical precursors also known as phytoestrogens, possess important protective properties. Most of these lignans and neolignans are presented in reasonable amounts in one's diet and thus the protection they provide against the colon and breast cancer, to name a few, is even more important to note. Similarly, neuroprotective properties were observed (schisanwilsonin G, IC₅₀ = 3.2 nM) These structural motives also serve as an important starting point in the development of anticancer drugs. Presumably the most famous members of this family, etoposide and teniposide, synthetic derivatives of podophyllotoxin, are used in the clinical treatment of lymphocytic leukemia, certain brain tumors, and lung tumors already for nearly 20 years. This review describes 413 lignans and neolignans which have been isolated between 2016 and mid-2018 being reported in more than 300 peer-reviewed articles. It covers their source, structure elucidation, and bioactivity. Within the review, the structure-based overview of compounds as well as the bioactivity-based overview of compounds are described.

Kinetic, spectroscopic and computational docking study of the inhibitory effect of the pesticides 2,4,5-T, 2,4-D and glyphosate on the diphenolase activity of mushroom tyrosinase

The inhibitory effect of 2,4,5-T, 2,4-D, glyphosate and paraquat on the diphenolase activity of mushroom tyrosinase for oxidation of L-DOPA has been investigated by kinetic measurements, fluorescence spectroscopy and computational docking analysis. 2,4,5-T and 2,4-D inhibit the diphenolase activity of the enzyme following a competitive mechanism, while glyphosate is a mixed inhibitor according to Lineweaver-Burk kinetic analysis. The inhibitory activity follows the order glyphosate >2,4,5-T > 2,4-D with IC50 values of 65, 90 and 106 μM, respectively. Intrinsic tyrosinase fluorescence quenching and computational docking analysis suggest that 2,4,5-T and 2,4-D interact with the active site of the enzyme through hydrophobic interactions, while glyphosate also interacts with external residues of the active site of the enzyme by hydrogen bonding and hydrophilic interactions inducing conformational changes in the protein structure.

Total Synthesis of γ-Alkylidenebutenolides, Potent Melanogenesis Inhibitors from Thai Medicinal Plant Melodorum fruticosum

A hitherto unreported member of γ-alkylidenebutenolides in Melodorum fruticosum (Annonaceae), (4 E)-6-benzoyloxy-7-hydroxy-2,4-heptadiene-4-olide, named as isofruticosinol (4) was isolated from the methanol extract of flowers, along with the known related butenolides, namely, the (4 Z)-isomer (3) of 4, melodrinol (1), and its (4 E)-isomer (2). To unambiguously determine the absolute configuration at the C-6 position in these butenolides, the first total syntheses of both enantiomers of 2-4 were achieved over 6-7 steps from commercially available D- or L-ribose (D- and L-5). Using the same protocol, both enantiomers of 1 were also synthesized. Based on chiral HPLC analysis of all synthetic compounds ( S- and R-1-4), all naturally occurring butenolides were assigned as partial racemic mixtures with respect to the chiral center at C-6 (enantiomeric ratio, 6 S/6 R = ∼83/17). Furthermore, the melanogenesis inhibitory activities of S- and R-1-4 were evaluated, with all shown to be potent inhibitors with IC₅₀ values in the range 0.29-2.9 μM, regardless of differences in the stereochemistry at C-6. In particular, S-4 (IC₅₀ = 0.29 μM) and R-4 (0.39 μM) showed potent inhibitory activities compared with that of reference standard arbutin (174 μM).

Chemical Constituents and Bioactivities of Several Indonesian Plants Typically Used in Jamu

This article reviews the chemical constituents and bioactivities of several Indonesian plants typically used in Jamu prescriptions in Indonesia. Jamu is Indonesia traditional medicine: it consists of either a single ingredient or a mixture of several medicinal plants. One plant family always used in Jamu is Zingiberaceae (ginger), such as Curcuma domestica/C. longa, C. xanthorrhizae, C. heyneana, C. zedoaria, C. aeruginosa, Zingiber aromaticum, Alpinia galanga. We also report other commonly used plant families such as Justicia gendarussa and Cassia siamea, whose activities have been extensively explored by our department.

Structural Requirements of Alkylglyceryl-l-Ascorbic Acid Derivatives for Melanogenesis Inhibitory Activity

l-Ascorbic acid has multifunctional benefits on skin aesthetics, including inhibition of melanin production, and is widely used in cosmetics. It, however, has low stability and poor skin penetration. We hypothesize that alkylglyceryl-l-ascorbic acid derivatives, highly stable vitamin C–alkylglycerol conjugates, would have similar anti-melanogenic activity with better stability and penetration. We test 28 alkylglyceryl-l-ascorbic acid derivatives (1–28) on theophylline-stimulated B16 melanoma 4A5 cells to determine if they inhibit melanogenesis and establish any structure–function relationships. Although not the most potent inhibitors, 3-O-(2,3-dihydroxypropyl)-2-O-hexyl-l-ascorbic acid (6, IC₅₀ = 81.4 µM) and 2-O-(2,3-dihydroxypropyl)-3-O-hexyl-l-ascorbic acid (20, IC₅₀ = 117 µM) are deemed the best candidate derivatives based on their inhibitory activities and low toxicities. These derivatives are also found to be more stable than l-ascorbic acid and to have favorable characteristics for skin penetration. The following structural requirements for inhibitory activity of alkylglyceryl-l-ascorbic acid derivatives are also determined: (i) alkylation of glyceryl-l-ascorbic acid is essential for inhibitory activity; (ii) the 3-O-alkyl-derivatives (2–14) exhibit stronger inhibitory activity than the corresponding 2-O-alkyl-derivatives (16–28); and (iii) derivatives with longer alkyl chains have stronger inhibitory activities. Mechanistically, our studies suggest that l-ascorbic acid derivatives exert their effects by suppressing the mRNA expression of tyrosinase and tyrosine-related protein-1.

Labdane-Type Diterpenes, Galangalditerpenes A-C, with Melanogenesis Inhibitory Activity from the Fruit of Alpinia galanga

In our continuing study of biologically active natural products from the fruit of Alpinia galanga (Zingiberaceae), we newly isolated three new labdane-type diterpenes, termed galangalditerpenes A–C (1–3), along with four known sesquiterpenes (4–7) and two diterpenes (8 and 9). The stereostructures of 1–3 were elucidated on the basis of their spectroscopic properties. The melanogenesis inhibitory activities in theophylline-stimulated murine B16 melanoma 4A5 cells of these isolates, including the new diterpenes (1–3, IC₅₀ = 4.4, 8.6, and 4.6 μM, respectively), were found to be more than 6–87-fold higher than that of arbutin (174 μM), a commercially available positive control.

Melanogenesis-Inhibitory and Cytotoxic Activities of Chemical Constituents from the Leaves of Sauropus androgynus L. Merr. (Euphorbiaceae)

A new steroid, 20-hydroxyisofucosterol (stigmasta-5,24(28)-diene-3β,20β-diol) (7), along with six known compounds 1 - 6 were isolated from the MeOH extract of the leaves of Sauropus androgynus L. Merr. (Euphorbiaceae). The structure of new steroid was determined by HR-APCI-MS and various NMR techniques in combination with literature data. Subsequently, their anti-inflammatory, cytotoxic activities against five human cell lines, as well as inhibitory activities against the α-MSH induced melanogenesis on the B16 cell line were evaluated. As the results, steroid compounds, 6 and 7 exhibited moderate cytotoxic to HL60, AZ521, SKBR3, and A549 tumor cell lines (IC₅₀ 26.9 - 45.1 μm) with high tumor selectivity for A549 relative to WI38 cell lines (SI 2.6 and 3.0, resp.). And, flavonoid compounds, 4 and 5 exhibited superior inhibitory activities against melanogenesis (67.0 - 94.7% melanin content), even with no or low toxicity to the cells (90.1 - 99.6% cell viability) at the concentrations from 10 to 100 μm. Furthermore, Western blot analysis suggested that compound 5 could inhibit melanogenesis by suppressing the protein expressions of MITF, TRP-1, TRP-2, and tyrosinase.

Recent development of signaling pathways inhibitors of melanogenesis

Human skin, eye and hair color rely on the production of melanin, depending on its quantity, quality, and distribution, Melanin plays a monumental role in protecting the skin against the harmful effect of ultraviolet radiation and oxidative stress from various environmental pollutants. However, an excessive production of melanin causes serious dermatological problems such as freckles, solar lentigo (age spots), melasma, as well as cancer. Hence, the regulation of melanin production is important for controlling the hyper-pigmentation. Melanogenesis, a biosynthetic pathway to produce melanin pigment in melanocyte, involves a series of intricate enzymatic and chemical catalyzed reactions. Several extrinsic factors include ultraviolet radiation and chemical drugs, and intrinsic factors include molecules secreted by surrounding keratinocytes or melanocytes, and fibroblasts, all of which regulate melanogenesis. This article reviews recent advances in the development of melanogenesis inhibitors that directly/indirectly target melanogenesis-related signaling pathways. Efforts have been made to provide a description of the mechanism of action of inhibitors on various melanogenesis signaling pathways.

Melanogenesis Inhibitory Activity of Diterpenoid and Triterpenoid Constituents from the Aerial Part of Isodon trichocarpus

A methanol extract from the aerial part of Isodon trichocarpus (Labiatae) demonstrated inhibitory effects on melanogenesis in theophylline-stimulated murine B16 melanoma 4A5 cells (IC50 = 1.6 μg/mL). From the extract, nine diterpenoids (1–9) and four triterpenoids (10–13) were isolated. Among the isolates, enmein (1, IC50 = 0.22 μM), isodocarpin (2, 0.19 μM), nodosin (4, 0.46 μM), and oridonin (6, 0.90 μM) showed an inhibitory effect without notable cytotoxicity at the effective concentrations. These diterpenoids (1, 2, 4, and 6) inhibited the expression of tyrosinase, tyrosine-related protein (TRP)-1, and TRP-2 mRNA, which could be the mechanism of melanogenesis inhibitory activity.

Synthesis and biological evaluation of caffeic acid derivatives as potent inhibitors of α-MSH-stimulated melanogenesis

We have disclosed our effort to develop caffeic acid derivatives as potent and non-toxic inhibitors of α-MSH-stimulated melanogenesis to treat pigmentation disorders and skin medication including a cosmetic skin-whitening agent. The SAR studies revealed that cyclohexyl ester and secondary amide derivatives of caffeic acid showed significant inhibitory activities.