Design, synthesis and biological evaluation of hydroxy- or methoxy-substituted 5-benzylidene (thio) barbiturates as novel tyrosinase inhibitors Zhiyong Chen a , Dachuan Cai a , Dehai Mou a , Qin Yan b , Yifeng Sun a , Wenlong Pan a , Yiqian Wan b , Huacan Song b,⇑ , Wei Yi b,⇑ a Guangdong Provincial Public Laboratory of Analysis and Testing Technology, China National Analytical Center (Guangzhou), Building 34, 100 Xianlie Middle Road, Guangzhou 510070, PR China b School of Chemistry and Chemical Engineering, Sun Yat-sen University, 135 Xin Gang West Road, Guangzhou 510275, PR China article info Article history: Received 20 March 2014 Revised 26 April 2014 Accepted 28 April 2014 Available online 15 May 2014 Keywords: 5-Benzylidene(thio)barbiturate Tyrosinase inhibitor SARs Inhibition mechanism abstract Here a new class of hydroxy- or methoxy-substituted 5-benzylidene(thio)barbiturates were designed, synthesized and their inhibitory effects on the diphenolase activity of mushroom tyrosinase were evalu- ated. The results showed that several compounds had more potent tyrosinase inhibitory activities than the widely used tyrosinase inhibitor kojic acid (IC 50 = 18.25 lM). In particular, 3 0 ,4 0 -dihydroxylated 1e was found to be the most potent inhibitor with IC 50 value of 1.52 lM. The inhibition mechanism analysis revealed that the potential compounds 1e and 2e exhibited such inhibitory effects on tyrosinase by acting as the irreversible inhibitors. Structure–activity relationships’ (SARs) analysis also suggested that further development of such compounds might be of interest. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Tyrosinase (EC 1.14.18.1), also known as polyphenoloxidase (PPO), is a copper-containing bifunctional enzyme widely distrib- uted in nature. It catalyzes two distinct reactions of melanin syn- thesis, the hydroxylation of L-tyrosine by monophenolase action and the oxidation of L-DOPA to the corresponding o-dopaquinone by diphenolase action. 1 Melanin synthesis was responsible for the undesired enzymatic browning of fruits and vegetables and resulted in a loss of nutritional and market values. 2 The production of abnormal melanin pigmentation (melasma, freckles, ephelide, sneile lentigines, etc.) was a serious esthetic problem in human beings. 3 Moreover, tyrosinase was found to be involved the molt- ing process of insects 4 and contributed to the neurodegeneration associated with Parkinson’s disease. 5 Therefore, tyrosinase inhibi- tors have become increasingly important in food industry as well as in the medicinal and cosmetic products due to decreasing the excessive accumulation of pigmentation resulting from the enzyme action. To date, many efforts have been spent in the search for effective and safe tyrosinase inhibitors, and a large number of naturally occurring and synthetic tyrosinase inhibitors have been exten- sively reported. 6,7 However, only few of them are put into practical use due to their weak individual activities or safety concerns. Undoubtedly, it is still necessary to search and develop novel tyros- inase inhibitors with better activity and lower side effect. Among all the known tyrosinase inhibitors, polyphenol derivatives have attracted considerable attention due to their favorable interaction with the hydrophobic protein pocket sur- rounding the binuclear copper active site of tyrosinase, 8 such as N-hydroxycinnamoylphenalkyl amides, 9 hydroxysubstituted ben- zaldoximes, 10 aurones, 11 chalcones, 12 flavanones, 13 resveratrols and their analogs, 14 benzoate ester derivatives, 15 and pyrazole derivatives. 16 Recently, our groups also demonstrated a series of 4-o-substituted 5-benzylidene barbiturate and thiobarbiturate derivatives as potential tyrosinase inhibitors. 17 Inhibition mecha- nism analysis revealed that such type of compounds, similar to N-hydroxy-N 0 -phenylthiourea, N-hydroxy-N 0 -phenylurea 18 and thiosemicarbazide derivatives, 19 were able to efficiently complex the two copper ions in the active site of tyrosinase. Taking advantage of above information, we speculated that cou- pling products of proper hydroxy-substituted benzaldehydes with barbituric or thiobarbituric acid could not only complex the binu- clear copper active site of tyrosinase but also interact with the hydrophobic enzyme pocket, and thus, leading to increasing http://dx.doi.org/10.1016/j.bmc.2014.04.060 0968-0896/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. Tel.: +86 20 84110918; fax: +86 20 84112245. E-mail addresses: yjhxhc@mail.sysu.edu.cn (H. Song), yiwei2@mail2.sysu.edu.cn (W. Yi). Bioorganic & Medicinal Chemistry 22 (2014) 3279–3284 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry journal homepage: www.elsevier.com/locate/bmc