954 ellagic acid and VEGFR-2 in silico. The identiﬁcation of VEGF with VEGFR-2 is tightly associated with the second/ third immunoglobulin-like regions within the VEGFR-2 extracellular domain. Besides, the downstream structure from the fourth to seventh immunoglobulin-like domains in VEGFR-2 plays a major role in dimerization and activation [39]. Disappointedly, it has been shown that the overall structure of VEGFR-2 extracellular domain is excessively ﬂexible and change too much before and after binding with VEGF. Therefore, this dynamic nature prevented extra- cellular domain to be a good target for small molecular inhibitors. As mentioned above, dimerization within the extracel- lular domain of VEGFR-2 could induce the autophospho- rylation on numerous tyrosine residues within its intracellular domain. The phosphorylation is an ATP- consuming process. The ATP-binding region lies between N-terminal lobe and C-terminal lobe within VEGFR-2 catalytic domain. Many kinase inhibitors could exert their inhibitory effects through purely or partially competing against the adenosine triphosphate (ATP) and subsequently suppressing the receptor autophosphorylation. They were acting as ATP minetics that bound to this site and com- peted with cellular ATP [9, 11–13]. In this study, ellagic acid could stably locate at the ATP-binding pocket near the hinge region. At least 16 active residues existed within the catalytic unit of VEGFR-2, including Val846, Phe1045, Cys1043, Leu1033, Asn1031, Arg1030, Asn921, Gly920, Lys918, Cys917, Phe916, Val914, Lys866, Ala864, Gly841, Glu883, and Leu838. Only three active residues, i.e., Lys866, Glu883, and Phe1045 at the ATP pocket were essential for the stable conformation of VEGFR-2/ellagic acid complex. The binding mode of ellagic acid with VEGFR-2 differed considerably from that of 2-anilio-5- aryoxazole (1Y6B) [31], although they shared similar aromatic interaction with the same residue Phe1045. In addition to the strong p–p interactions with Phe1045, ellagic acid could also form hydrogen bonds mainly with two residues, i.e., Glu883 and Lys866. All the unique binding modes largely promoted the conformational sta- bility of the ellagic acid/VEGFR-2 complex. Overall, our study indicated that ellagic acid at non- toxic dosages exerted potent anti-angiogenesis activities via speciﬁcally targeting VEGFR-2 and its signaling pathway in breast cancer. As a natural inhibitor against VEGFR-2 with limited toxicity, ellagic acid is a prom- ising candidate for development of anti-angiogenesis agents. Acknowledgments We sincerely appreciate Serlina Suen in the government of Hong Kong special administrative region education bureau for editorial assistance. This research was funded by the seed funding from the University of Hong Kong. 123 Breast Cancer Res Treat (2012) 134:943–955 Conﬂict of interest None. 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