ASIAN JOURNAL OF CHEMISTRY ASIAN JOURNAL OF CHEMISTRY https://doi.org/10.14233/ajchem.2018.21053 INTRODUCTION Propolis or “bee glue” is a resinous material collected by honeybees from tree buds, sap flows or other botanical exudates and is used for sealing and coating bee hives. For many centuries, aside from its use as food, beverage and food supplement, it has long been used extensively in traditional medicine because of its wide range of therapeutic values for the treatment of common cold, flu, upper respiratory tract infections, as dermatological preparations for wounds, burns, acne, herpes simplex, genitalis and neurodermatitis, as mouthwash and toothpaste to prevent caries and gingivitis and for cosmetic applications [1,2]. Propolis was shown to exhibit a broad spectrum of biological activities which include antimicrobial [3,4], antifungal [5,6], free radical scavenging [7,8], anti-HIV [9,10], anti-inflammatory [11,12] and anti-herpes [13,14]. A recent study reported that propolis, at a concentration of 0.01 % (w/v), exhibited antiproliferative and cytotoxic actions against human fibroblasts proliferation in cell culture, followed by mild cell necrosis [15]. High concentrations of ethanolic extracts of propolis were found to have an in vitro cytotoxic and geno- Cytotoxic Prenylflavanones from Philippine Stingless Bee (Tetragonula biroi Friese) Nests MARIQUIT M. DE LOS REYES 1,2,* , GLENN G. OYONG 1,3 , VIRGILIO D. EBAJO JR. 4 , CHIEN-CHANG SHEN 5 and CONSOLACION Y. RAGASA 4,6 1 Department of Biology, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines 2 Department of Biology, De La Salle University Manila - Laguna Campus, Biñan City, Laguna 4024, Philippines 3 Center for Natural Science and Environmental Research, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines 4 Department of Chemistry, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines 5 National Research Institute of Chinese Medicine, Ministry of Health and Welfare, 155-1, Li-Nong St., Sec. 2, Taipei 112, Taiwan 6 Department of Chemistry, De La Salle University Manila - Laguna Campus, Biñan City, Laguna 4024, Philippines *Corresponding author: Tel/Fax: +63 49 554 8900; E-mail: mariquit.delosreyes@dlsu.edu.ph Received: 19 September 2017; Accepted: 19 October 2017; Published online: 31 January 2018; AJC-18754 The dichloromethane crude extract and compounds, glyasperin A (1), propolin E (2) and propolin A (3), obtained from Philippine stingless bee (Tetragonula biroi Friese) nests, were evaluated for their cytotoxic potentials. The anti-proliferative activities of crude extract and compounds 1-3 against human cancer cell lines, breast (MCF-7) and colon (HT-29 and HCT-116), and a normal cell line, human dermal fibroblast neonatal (HDFn), were evaluated using the PrestoBlue ® cell viability assay. The crude extract was most anti-proliferative against HCT-116 cells (IC50 = 1.410 μg/mL). Glyasperin A (1) exhibited the strongest effect on MCF-7 cells (2.378 μg/mL). Propolin E (2) was most cytotoxic against HCT-116 cells (2.279 μg/mL), while propolin A (3) was most inhibitory against MCF-7 cells (2.815 μg/mL). Comparing the colorectal cancer cell lines, HCT-116 was generally more susceptible under crude extract, 2, and 3 (IC50 = 1.410, 2.279, 3.013 μg/mL, respectively) than HT-29 (5.620, 5.320, 5.359). Comparing the activities of propolin E and propolin A against HCT-116 cells, 2 (IC50 = 2.279 μg/mL) was more cytotoxic than 3 (3.013). The crude extract was more cytotoxic than Zeocin against HCT-116 cells. To our best of knowledge, this is the first report on the anti-proliferative properties of crude extract and compounds from T. biroi. Keywords: Cytotoxicity, Glyasperin A, Philippine stingless bee, Propolin A, Propolin E, Tetragonula biroi Friese. Asian Journal of Chemistry; Vol. 30, No. 3 (2018), 613-619 toxic effect on human peripheral lymphocytes [16]. Recently, focus has been given to the understanding of actual chemical constituents of propolis and their corresponding biological activities. The chemical composition of propolis is highly variable depending on the geographical area, prevailing environmental conditions and the plant species abundant around the bee hives [1,17-20]. The ethanolic extracts of propolis from different regions in Java were tested for cytotoxicity against tumor cell lines (T47D, MCF-7, Hela, Myeloma and Vero) using the MTT assay [21]. The propolis extract from Batang, Central Java showed the most potent activity against T47D and MCF-7 with IC50 values of 34.67 and 37.8 μg/mL, respectively, while the propolis extract from Sukabumi, West Java showed the most potent activity against Hela cell with an IC50 of 147.34 μg/mL. However, the propolis extracts were inactive against myeloma and Vero cells. Different types of propolis from Europe, New Zealand, Brazil and China were found to contain secondary metabolites such as flavonoids, phenolic acids, terpenoids, steroids and amino acids [3]. Propolis has been reported to contain β-amylase [22], polyphenolic compounds, flavones,