Natural Tyrosinase Inhibitors: Role of Herbals in the Treatment of Hyperpigmentary Disorders

Page: [796 - 808] Pages: 13

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Abstract

Cutaneous pigmentation plays critical role in determining the color of skin along with photo protection of skin from dreadful effects of ultraviolet radiations. Conversely, abnormal accumulation of melanin is responsible for hyper pigmentary disorders such as melasma, senile lentigines and freckles. Because of the visible nature of dermatologic diseases, they have a considerable psychosomatic effect on affected patients. Tyrosinase inhibitors are molecules that interrelate in some way with the enzyme to prevent it from working in the normal manner. Past many decades witnessed the quest for the development of natural tyrosinase inhibitors due to imperative role played by tyrosinase in the process of melanogenesis and fungi or fruit enzymatic browning. Mechanism of pigmentation is characterized by the intact process of the synthesis of specialized black pigment within melanosomes. Melanin is synthesized by a cascade of enzymatic and chemical reactions. For this reason, melanin production is mainly controlled by the expression and activation of tyrosinase. In the current article, we discussed tyrosinase inhibitors from the natural sources, which can be an essential constituent of cosmetics products and depigmenting agents for the treatment of hyperpigmentory disorders.

Keywords: Alkaloids, tyrosinase, herbal, pigmentation, enzyme inhibitors, hyperpigmentary disorders.

Graphical Abstract

[1]
Gertrude, E.; Costin, M.; Vincent, J. Hearing human skin pigmentation: melanocytes modulate skin color in response to stress. Federat. Am. Soc. Exp. Biol, 2007, 6, 6649.
[2]
Ali, S.; Naaz, I. Current challenges in understanding the story of skin pigmentation: Bridging the morpho-anatomical and functional aspects of mammalian melanocytes in: Muscle cells and Tissue. In tech publications. 2015, , Europe, USA.
[3]
Lee, J.; Jung, E.; Park, J.; Jung, K.; Park, E.; Kim, J. Glycyrrhizin Induces Melanogenesis by Elevating a cAMP Level in B16 Melanoma Cells. J. Invest. Dermatol., 2005, 124, 405-411.
[4]
Ali, S.; Naaz, I. Comparative light and electron microscopic studies of dorsal skin melanophores of Indian toad, Bufo melanostictus. J. Microsc. Ultrastruc, 2014, 2(4), 230-235.
[5]
Kim, D.S.; Park, S.H.; Kwon, S.B.; Park, E.S.; Huh, C.H.; Youn, S.W.; Park, K.C. Sphingosyl phosphoryl choline-induced ERK activation inhibits melanin synthesis in human melanocytes. Pigment Cell Res., 2006, 19(2), 146-153.
[6]
Bruno, B.; Marco, C.; Laura, C. Moraceae Plants with Tyrosinase Inhibitory Activity: A Review. Mini Rev. Med. Chem., 2017, 17(2), 108-121.
[7]
Ali, S.A.; Coudhary, R.K.; Naaz, I. Understanding the challenges of melanogenesis, key role of bioactive compounds in the treatment of hyperpigmentary disorders. J. Pigment. Disord., 2015, 2(11), 4-21.
[8]
Li, Q.; Yang, H.; Mo, J.; Chen, Y.; Wu, Y.; Kang, C.; Sun, Y.; Sun, H. Identification by shape-based virtual screening and evaluation of new tyrosinase inhibitors. Peer J, 2018, 6e4206
[9]
Berg, J.M.; Tymoczko, J.L.; Stryer, L. New York: W H Freeman; 2002. Biochemistry. 5th edition.
[10]
Jiménez, M.; Chazarra, S.; Escribano, J.; Cabanes, J.; García-Carmona, F. Competitive inhibition of mushroom tyrosinase by 4-substituted benzaldehydes. J. Agric. Food Chem., 2001, 49(8), 4060-4063.
[11]
Strelow, J.; Dewe, W.; Iversen, P. Harold Brooks, Jeffrey Radding, James McGee and Jeffrey Weidner, "Mechanism of Action Assays for Enzymes, in G. S. Sittampalam, N. P. Coussens, H. Nelson. (editors), Assay Guidance Manual, Eli Lilly & Company and the National Center for Advancing Translational Sciences. 2004.
[12]
Cornish-Bowden, A. A simple graphical method for determining the inhibition constants of mixed, uncompetitive and non-competitive inhibitors. Biochem. J., 1974, 137(1), 143-144.
[13]
Parvez, S.; Kang, M.; Chung, H.S.; Cho, C.; Hong, M.C.; Shin, M.K.; Bae, H. Survey and mechanism of skin depigmenting and lightening agents. Phytother. Res., 2006, 20, 921-934.
[14]
Ando, H.; Kondoh, H.; Ichihashi, M.; Hearing, V.J. Approaches to identify inhibitors of melanin biosynthesis via the quality control of tyrosinase. J. Invest. Dermatol., 2007, 127, 751-761.
[15]
Vance, K.W.; Goding, C.R. The transcription network regulating melanocyte development and melanoma. Pigment Cell Res., 2004, 17, 318-325.
[16]
Goding, C.R. MITF from neural crest to melanoma: Signal transduction and transcription in the melanocyte lineage. Genes Dev., 2000, 14, 1712-1728.
[17]
Busca, R.; Ballotti, R. Cyclic AMP a key messenger in the regulation of skin pigmentation. Pigment Cell Res., 2000, 13, 60-69.
[18]
Hunt, G.; Todd, C.; Cresswell, J.E.; Thody, A.J. Alpha-melanocyte stimulating hormone and its analogue Nle4DPhe7 alpha-MSH affect morphology, tyrosinase activity and melanogenesis in cultured human melanocytes. J. Cell Sci., 1994, 107, 205-211.
[19]
Bellei, B.; Flori, E.; Izzo, E.; Maresca, V.; Picardo, M. GSK3beta inhibition promotes melanogenesis in mouse B16 melanoma cells and normal human melanocytes. Cell. Signal., 2008, 20, 1750-1761.
[20]
Bellei, B.; Pitisci, A.; Izzo, E.; Picardo, M. Inhibition of melanogenesis by the pyridinyl imidazole class of compounds: Possible involvement of the Wnt/beta-catenin signaling pathway. PLoS One, 2012, 7, 33021.
[21]
Wu, M.; Gong, L.; Haddad, M.M.; Bischof, O.; Campisi, J.; Yeh, E.T.; Medrano, E.E. Regulation of microphthalmia-associated transcription factor MITF protein levels by association with the ubiquitin-conjugating enzyme hUBC9. Exp. Cell Res., 2000, 255, 135-143.
[22]
Kim, D.S.; Hwang, E.S.; Lee, J.E.; Kim, S.Y.; Kwon, S.B.; Park, K.C. Sphingosine-1-phosphate decreases melanin synthesis via sustained ERK activation and subsequent MITF degradation. J. Cell Sci., 2003, 116, 1699-1706.
[23]
Wu, M.; Hemesath, T.J.; Takemoto, C.M.; Horstmann, M.A.; Wells, A.G.; Price, E.R.; Fisher, D.Z.; Fisher, D.E. c-Kit triggers dual phosphorylations, which couple activation and degradation of the essential melanocyte factor Mi. Genes Dev., 2000, 14, 301-312.
[24]
Sealy, R.C.; Hyde, J.S.; Felix, C.C.; Menon, I.A.; Prota, G. Eumelanins and pheomelanins: Characterization by electron spin resonance spectroscopy. Science, 1982, 217(4559), 545-547.
[25]
Yaar, M. Cutaneous pigmentation in health and disease: Novel and well-established players. J. Invest. Dermatol., 2013, 133(1), 11-13.
[26]
Sendoel, A.; Kohler, I.; Fellmann, C.; Lowe, S.W.; Hengartner, M.O. HIF-1 antagonizes p53-mediated apoptosis through a secreted neuronal tyrosinase. Nature, 2010, 465(7298), 577-583.
[27]
Bulengo-Ransby, S.M.; Griffiths, C.E.; Kimbrough-Green, C.K.; Finkel, L.J.; Hamilton, T.A.; Ellis, C.N.; Voorhees, J.J. Topical tretinoin (retinoic acid) therapy for hyperpigmented lesions caused by inflammation of the skin in black patients. N. Engl. J. Med., 1993, 328(20), 1438-1443.
[28]
Stern, R.S. Clinical practice. Treatment of photoaging. N. Engl. J. Med., 2004, 350(15), 1526-1534.
[29]
Bae-Harboe, Y.S.; Park, H.Y. Tyrosinase: A central regulatory protein for cutaneous pigmentation. J. Invest. Dermatol., 2012, 132(12), 2678-2680.
[30]
Jin, M.L.; Park, S.Y.; Kim, Y.H.; Park, G.; Son, H.J.; Lee, S.J. Suppression of α-MSH and IBMX-induced melanogenesis by cordycepin via inhibition of CREB and MITF, and activation of PI3K/Akt and ERK-dependent mechanisms. Int. J. Mol. Med., 2012, 29(1), 119-124.
[31]
Picardo, M.; Carrera, M. New and experimental treatments of cloasma and other hypermelanoses. Dermatol. Clin., 2007, 25, 353-362.
[32]
Hsu, C.K.; Chou, S.T.; Huang, P.J.; Mong, M.C.; Wang, C.K.; Hsueh, Y.P.; Jhan, J.K. Crude ethanol extracts from grape seeds and peels exhibit anti-tyrosinase activity. J. Cosmet. Sci., 2012, 63, 225-232.
[33]
Choo, S.J.; Ryoo, I.J.; Kim, K.C.; Na, M.; Jang, J.H. Hypo-pigmenting effect of sesquiterpenes from Inula britannica in B16 melanoma cells. Arch. Pharm. Res., 2014, 37, 567-574.
[34]
Chen, W.C.; Tseng, T.S.; Hsiao, N.W.; Lin, Y.L.; Wen, Z.H.; Tsai, C.C.; Lee, Y.C.; Lin, H.H.; Tsai, K.C. Discovery of Highly Potent Tyrosinase Inhibitor, T1, with significant anti-melanogenesis ability by zebrafish in vivo assay and computational molecular modeling. Scientif. Reports., 2015, 5, 7995.
[35]
Gillbro, J.M.; Olsson, M.J. The melanogenesis and mechanisms of skin-lightening agents--existing and new approaches. Int. J. Cosmet. Sci., 2011, 33, 210-221.
[36]
Lall, N.; Kishore, N. Are plants used for skin care in South Africa fully explored. J. Ethnopharmacol., 2014, 153, 61-84.
[37]
Ribeiro, A.S. Estanqueiro; Oliveira, M.B.; Lobo, J.M.S. Main benefits and applicability of plant extracts in skin care products. Cosmetics, 2015, 2, 48-65.
[38]
Hori, I.; Nihei, K.; Kubo, I. Structural criteria for depigmenting mechanism of arbutin. Phytother. Res., 2004, 18, 475-479.
[39]
No, J.K.; Soung, D.Y.; Kim, Y.J.; Shim, K.H.; Jun, Y.S. Inhibition of tyrosinase by green tea components. Life Sci., 1999, 65, 241-246.
[40]
Picardo, M.; Carrera, M. New and experimental treatments of cloasma and other hypermelanoses. Dermatol. Clin., 2007, 25, 353-362.
[41]
Tan, C.; Zhu, W.; Lu, Y. Aloin, cinnamic acid and sophorcarpidine are potent inhibitors of tyrosinase. Chin. Med. J., 2002, 115, 1859-1862.
[42]
Nerya, O.; Vaya, J.; Musa, R.; Izrael, S.; Ben-Arie, R. Glabrene and isoliquiritigenin as tyrosinase inhibitors from licorice roots. J. Agric. Food Chem., 2003, 51, 1201-1207.
[43]
Fu, B.; Li, H.; Wang, X.; Lee, F.S.; Cui, S. Isolation and identification of flavonoids in licorice and a study of their inhibitory effects on tyrosinase. J. Agric. Food Chem., 2005, 53, 7408-7414.
[44]
Lee, S.H.; Choi, S.Y.; Kim, H.; Hwang, J.S.; Lee, B.G. Mulberroside F isolated from the leaves of Morus alba inhibits melanin biosynthesis. Biol. Pharm. Bull., 2002, 25, 1045-1048.
[45]
Katsube, T.; Imawaka, N.; Kawano, Y.; Yamazakib, Y.; Shiwakuc, K. Antioxidant flavonol glycosides in mulberry (Morus alba L) leaves isolated based on LDL antioxidant activity. Food Chem., 2006, 97, 25-31.
[46]
Yamakoshi, J.; Sano, A.; Tokutake, S.; Saito, M.; Kikuchi, M. Oral intake of proanthocyanidin-rich extract from grape seeds improves chloasma. Phytother. Res., 2004, 18, 895-899.
[47]
Yoshimura, M.; Watanabe, Y.; Kasai, K.; Yamakoshi, J.; Koga, T. Inhibitory effect of an ellagic acid-rich pomegranate extract on tyrosinase activity and ultraviolet-induced pigmentation. Biosci. Biotechnol. Biochem., 2005, 69, 2368-2373.
[48]
Zhang, R.Z.; Zhu, W.Y.; Xie, F. Effect of hesperidin on B16 and HaCaT cell lines irradiated by Narrowband-UVB light. J. Clin. Dermatol., 2008.
[49]
Mapunya, M.B.; Hussein, A.A.; Rodriguez, B.; Lall, N. Tyrosinase activity of Greyiaflanaganii (Bolus) constituents. Phytomedicine, 2011, 18, 1006-1012.
[50]
Süntar, I.; Akkol, E.K.; Senol, F.S.; Keles, H.; Orhan, I.E. Investigating wound healing, tyrosinase inhibitory and antioxidant activities of the ethanol extracts of Salvia cryptantha and Salvia cyanescens using in vivo and in vitro experimental models. J. Ethnopharmacol., 2011, 135, 71-77.
[51]
Muhammad, A.; Sirat, H.M. Potent microbial and tyrosinase inhibitors from stem bark of Bauhinia rufescens (Fabaceae). Nat. Prod. Commun., 2013, 8, 1435-1437.
[52]
Li, W.J.; Lin, Y.C.; Wu, P.F.; Wen, Z.H.; Liu, P.L. Biofunctional constituents from Liriodendron tulipifera with antioxidants and anti-melanogenic properties. Int. J. Mol. Sci., 2013, 14, 1698-1712.
[53]
Zengin, G.; Uysal, A.; Gunes, E.; Aktumsek, A. Survey of phytochemical composition and biological effects of three extracts from a wild plant (Cotoneaster nummularia Fisch. et Mey.): A potential source for functional food ingredients and drug formulations. PLoS One, 2014, 9e113527
[54]
Karim, A.A.; Azlan, A.; Ismail, A.; Hashim, P. AbdGani, S.S. Phenolic composition, antioxidant, anti-wrinkles and tyrosinase inhibitory activities of cocoa pod extract. BMC Complement. Altern. Med., 2014, 14, 381.
[55]
Suwannalert, P.; Kariya, R.; Suzu, I.; Okada, S. The effects of Salaciareticulata on anti-cellular oxidants and melanogenesis inhibition in alpha-MSH-stimulated and UV irradiated B16 melanoma cells. Nat. Prod. Commun., 2014, 9, 551-554.
[56]
Park, J.; Park, J.H.; Suh, H.J.; Lee, I.C.; Koh, J. Effects of resveratrol, oxyresveratrol, and their acetylated derivatives on cellular melanogenesis. Arch. Dermatol. Res., 2014, 306, 475-487.
[57]
Cho, J.G.; Huh, J.; Jeong, R.H.; Cha, B.J.; Shrestha, S. Inhibition effect of phenyl compounds from the Oryzasativa roots on melanin production in murine B16-F10 melanoma cells. Nat. Prod. Res., 2015, 29, 1052-1054.
[58]
Han, E.; Chang, B.; Kim, D.; Cho, H.; Kim, S. Melanogenesis inhibitory effect of aerial part of Puerariathunbergiana in vitro and in vivo. Arch. Dermatol. Res., 2015, 307, 57-72.
[59]
Hsieh, T.F.; Chang, Y.N.; Liu, B.L.; Hsieh, T.F.; Chang, Y.N.; Liu, B.L. Effect of extracts of traditional Chinese medicines on anti-tyrosinase and antioxidant activities. J. Med. Plants Res., 2015, 9(48), 1131-1138.
[60]
Pejin, B.; Iodice, C.; Tommonaro, G.; Bogdanovic, G.; Kojic, V.; Rosa, S.D. Further in vitro evaluation of cytotoxicity of the marine natural product derivative 4′-leucine-avarone. Nat. Prod. Res., 2014, 28(5), 347-350.
[61]
Tommonaro, G.; Pejin, B.; Iodice, C.; Tafuto, A.; Rosa, S.D. Further in vitro biological activity evaluation of amino-, thio- and ester-derivatives of avarol. J. Enzyme Inhib. Med. Chem., 2015, 30(2), 333-335.
[62]
Tommonaro, G.; García-Font, N.; Vitale, R.M.; Pejin, B.; Iodice, C.; Cañadas, S.; Marco-Contelles, J.; Oset-Gasque, M.J. Avarol derivatives as competitive AChE inhibitors, non hepatotoxic and neuroprotective agents for Alzheimer’s disease. Eur. J. Med. Chem., 2016, 122, 326-338.
[63]
Azhar-Ul-Haq. Malik, A.; Khan, M.T.; Anwar-Ul-Haq; Khan, S.B.; Ahmad, A.; Choudhary, M.I. Tyrosinase inhibitory lignans from the methanol extract of the roots of Vitex negundo Linn. and their structure-activity relationship. Phytomedicine, 2006, 13, 255-260.
[64]
Kang, H.S.; Kim, H.R.; Byun, D.S.; Son, B.W.; Nam, T.J.; Choi, J.S. Tyrosinase inhibitors isolated from the edible brown alga Ecklonia stolonifera. Arch. Pharm. Res., 2004, 27, 1226-1232.
[65]
Li, X.; Kim, M.K.; Lee, U.; Kim, S.K.; Kang, J.S.; Choi, H.D.; Son, B.W. Myrothenones A and B, cyclopentenone derivatives with tyrosinase inhibitory activity from the marine-derived fungus Myrothecium sp. Chem. Pharm. Bull., 2005, 53, 453-455.
[66]
Tsuchiya, T.; Yamada, K.; Minoura, K.; Miyamoto, K.; Usami, Y.; Kobayashi, T.; Hamada-Sato, N.; Imada, C.; Tsujibo, H. Purification and determination of the chemical structure of the tyrosinase inhibitor produced by Trichoderma viride strain H1-7 from a marine environment. Biol. Pharm. Bull., 2008, 31, 1618-1620.
[67]
Komatsu, N.C.; Yamauchi, R.; Shibayama, S.; Hachisuka, M.; Kiuchi, F. Two new lignans and melanogenesis inhibitors from Schisandra nigra. J. Nat. Med., 2016, 70(3), 460-466.
[68]
Harborne, J.B.; Williams, C.A. Advances in flavonoid research since 1992. Phytochemistry, 2000, 55, 481-504.
[69]
Orhan, I.E.; Khan, M.T. Flavonoid derivatives as potent tyrosinase inhibitors - a survey of recent findings between 2008-2013. Curr. Top. Med. Chem., 2014, 14(12), 1486-1493.
[70]
Nguyen, H.X.; Nguyen, N.T.; Nguyen, M.H.K.; Le, T.H.; Van, T.N.; Hung, T.M.; Nguyen Mai, T.T. Tyrosinase inhibitory activity of flavonoids from Artocarpus heterophyllous. Chem. Cent. J., 2016, 10, 2.
[71]
Kim, Y.J.; Uyama, H. Tyrosinase inhibitors from natural and synthetic sources: structure, inhibition mechanism and perspective for the future. Cell. Mol. Life Sci., 2005, 62, 1707-1723.
[72]
Likhitwitayawuid, K. Stilbenes with tyrosinase inhibitory activity. Curr. Sci., 2008, 94, 44-52.
[73]
Ismail, T.; Shafi, S.; Srinivas, J.; Sarkar, D.; Qurishi, Y.; Jabeena, K.; Alam, M.S.; Mahabalarao, H.; Kumar, S. Synthesis and tyrosinase inhibition activity of trans-stilbene derivatives. Bioorg. Chem., 2016, 64, 97-102.
[74]
Egan, D.; O’Kennedy, R.; Moran, E.; Cox, D.; Prosser, E.; Thornes, R.D. The pharmacology, metabolism, analysis, and applications of coumarin and coumarin-related compounds. Drug Metab. Rev., 1990, 22, 503.
[75]
Borges, F.; Roleira, F.; Milhazes, N.; Santana, L.; Uriarte, E. Simple coumarins and analogues in medicinal chemistry: occurrence, synthesis and biological activity. Curr. Med. Chem., 2005, 12, 887.
[76]
Liu, J.; Wu, F.; Chen, L.; Zhao, L.; Zhao, Z.; Wang, M.; Lei, S. Biological evaluation of coumarin derivatives as mushroom tyrosinase inhibitors. Food Chem., 2012, 135(4), 2872-2878.
[77]
Matos, M.J.; Varela, C.; Vilar, S.; Hripcsak, G.; Borges, F.; Santana, L.; Uriarte, E.; Fais, A.A.; Petrillo, D.; Pintus, F.; Era, B. Design and discovery of tyrosinase inhibitors based on a coumarin scaffold. RSC Adv., 2015, 5, 94227-94235.
[78]
Xiao, H.; Meng, W.; Gui-Rui, Y.; Mei-Hua, Y.; He-Yao, W.; Ai-Jun, H. 2-Arylbenzofuran and tyrosinase inhibitory constituents of Morus notabilis. J. Asian Nat. Prod. Res., 2012, 14(12), 1103-1108.
[79]
Burlando, B.; Clericuzio, M.; Cornara, L. Moraceae Plants with Tyrosinase Inhibitory Activity: A Review. Mini Rev. Med. Chem., 2016, 16, 18.
[80]
Rana, J.; Diwakar, G.; Saito, L.; Scholten, J.D.; Mulder, T. Inhibition of melanin content by Punica lagins in the super fruit pomegranate (Punica granatum). J. Cosmet. Sci., 2013, 64(6), 445-453.
[81]
Chen, X.X.; Shi, Y.; Chai, W.M.; Feng, H.L.; Zhuang, J.X.; Chen, Q.X. Condensed tannins from Ficus virens as tyrosinase inhibitors: Structure, inhibitory activity and molecular mechanism. PLoS One, 2014, 9(3)e91809
[82]
Shaheen, F.; Ahmad, M.; Khan, M.T.; Jalil, S.; Ejaz, A.; Sultankhodjaev, M.N.; Arfan, M.; Choudhary, M.I. Atta-ur-Rahman. Alkaloids of Aconitum laeve and their anti-inflammatory antioxidant and tyrosinase inhibition activities. Phytochemistry, 2005, 66, 935-940.
[83]
Morita, H.; Kayashita, T.; Kobata, H.; Gonda, A.; Takeya, K.; Itokawa, H. Pseudostellarins D-F, new tyrosinase inhibitory cyclic peptides from Pseudostellaria heterophylla. Tetrahedron, 1994, 50(33), 9975-9982.
[84]
Choudhary, M.I.; Sultan, S.; Khan, M.T.; Rahman, A.U. Microbial transformation of 17alpha-ethynyl- and 17alpha-ethylsteroids, and tyrosinase inhibitory activity of transformed products. Steroids, 2005, 70(12), 798-802.
[85]
Sabudak, T.; Khan, M.T.; Choudhary, M.I.; Oksuz, S. Potent tyrosinase inhibitors from Trifolium balansae. Nat. Prod. Res., 2006, 20, 665-670.
[86]
Khan, S.B. Azhar-Ul-Haq; Afza, N.; Malik, A.; Khan, M.T.; Shah, M.R.; Choudhary, M.I. Tyrosinase-inhibitory long-chain esters from Amberboa ramosa. Chem. Pharm. Bull., 2005, 53, 86-89.
[87]
Khan, M.T.; Khan, S.B.; Ather, A. Tyrosinase inhibitory cycloartane type triterpenoids from the methanol extract of the whole plant of Amberboa ramosa Jafri and their structure-activity relationship. Bioorg. Med. Chem., 2006, 14, 938-943.
[88]
Khan, M.T.; Choudhary, M.I. Atta-ur-Rahman; Mamedova, R.P.; Aqzamova, M.A.; Sultankhodzhaev, M.N.; Isaev, M.I. Tyrosinase inhibition studies of cycloartane and cucurbitane glycosides and their structure-activity relationships. Bioorg. Med. Chem., 2006, 14, 6085-6088.
[89]
Ullah, F.; Hussain, H.; Hussain, J.; Bukhari, I.A.; Khan, M.T.; Choudhary, M.I.; Gilani, A.H.; Ahmad, V.U. Tyrosinase inhibitory pentacyclic triterpenes and analgesic and spasmolytic activities of methanol extracts of Rhododendron collettianum. Phytother. Res., 2007, 21, 1076-1081.