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1,2,3,4,6-Penta-O-galloyl-β-D-glucose (pentagalloylglucose, PGG, 1) is a naturally occurring hydrolysable tannin that can be found in many medicinal plants, such as Paeoniae radix and Rhus typhina (Fig. 1). PGG has been found to exhibit a wide range of biological and pharmacological activities, such as anti-diabetic, anti-oxidant, anti-cancer, antiinflammatory, and anti-allergic effects in vitro and in vivo. Although promising in vivo data has been accumulated, detailed pharmacokinetic studies on PGG might be required to define its exact mechanism, toxicity, or effective dosage for use as a therapy or chemoprevention in several human diseases. On the other hand, cell permeability studies using human intestinal epithelial Caco-2 cells have suggested that PGG has poor bioavailability due to limited transport through the cell membrane, and in part by degradation into triand tetragalloyl glucose. Gallic acid (2, GA) and its methyl ester (3, MG1) are also common natural products and are known to have antiinflammatory, anti-oxidant, and anti-tumor activity. We thought that GA could be another possible metabolite of PGG which might be generated by the hydrolysis of PGG in the gastrointestinal tract when it taken orally.Accordingly, we wondered whether the in vivo activities of PGG are mediated by direct actions or through its metabolite GA. Breast cancer is a heterogeneous group of diseases. A large proportion of breast cancer (60-70%) is characterized by high expression of estrogen receptor (ER) or progesterone receptor (PR) or both, indicating that estrogen is required for tumor growth. 20 to 30 percent of breast cancers express a high level of HER-2. Other breast cancers (15-20%), known as triple-negative breast cancer, do not express ER, PR, and high levels of HER-2 protein. In the present work, the cyototoxic effects of PGG and GA against breast cancer cells, MCF-7 and MDA-MB231, were examined to compare their activities and to gain insight on the role of GA in the anticancer activity of PGG. We also included GA methyl ester, MG1 and its three semi-synthetic methyl ethers (4-6), which were modified by methylation at the phenolic part of GA. Methyl gallate (3, MG1) and its three methyl ethers 4-6 (MG2-4) were synthesized from GA (2), as shown in Scheme 1. GA was heated at reflux in methanol in the presence of a catalytic amount of c-H2SO4 to afford MG1 in 86% yield. MG1 was transformed into its three methyl ethers, MG2-4, by using excess MeI in DMF. To test the effect of ER on the growth inhibitory activities, PGG, GA, and its derivatives MG1-4 were treated with MCF-7 cells in the presence or absence of estrogen, and their cell growth inhibition effects were evaluated by MTT assay (Fig. 2). Among the compounds tested, GA exhibited cytotoxicity most potently in a dose-dependent manner, followed by MG1, PGG, and MG4. The cytotoxic effects of GA and MG1 showed 50% growth inhibition at concentrations of 15.7 ± 0.6 and 19.2 ± 0.8 μM, respectively, in the presence of estrogen. In the same assay system, PGG showed only 36.7 ± 1.2% inhibition of cell growth at

Comparison of Cytotoxic Effects of Pentagalloylglucose, Gallic Acid, and its Derivatives Against Human Cancer MCF-7 and MDA MB-231 Cells