Cancer Chemoprevention & Herbal Antioxidants 87 compound curcumin, to inhibit carcinogen induced mutation in the Ames assay and tumorigenesis in several experimental systems suggesting anti-initiating and/or anti-promoting activity against several chemical carcinogens [13–15]. Both in vitro and in vivo studies have shown the efficacy of turmeric/ curcumin in inhibiting the carcinogen induced activity of various CYP450s isozymes [16, 17] and enhancement of phase II enzymes [18] resulting in decreased levels of carcinogen derived DNA adducts [17, 19]. However, the molecular basis of observed antiinitiation mechanism needs to be evaluated further. Chemoprevention and Black Tea Tea is fast emerging as a potential chemopreventive beverage and green tea is now a well established chemo- preventive beverage and its biological activities are attributed to certain active flavanols like epigallocatechin gallate (EGCG) etc. Similar chemopreventive efficacy needs to be investigated for black tea (and its polyphenols), which is a widely consumed beverage. Since polymeric black tea polyphenols (PBPs) or thearubigins (TRs) comprise almost 54% of the total polyphenols in black tea, studies on chemo- prevention in our lab are focused on these fractions. PBPs were isolated from black tea brew as five different fractions (PBP1 to PBP5) by liquid-liquid and solid-liquid extraction by exploiting their property of selective solubility in different solvents and were partially characterized by NMR and FTIR analysis [20]. In vivo studies indicated that, like green tea polyphenols, PBPs can inhibit carcinogen-induced up regulation of phase I enzymes like CYP450s and induce phase II detoxifying enzymes thus decreasing the load of DNA adduct formation [21–24]. Furthermore, PBPs have also shown anti-promoting properties in experimental skin carcinogenesis [unpublished observations]. Screening for Chemopreventive Efficacy Putative chemopreventive agents are subjected to rigorous in vitro and in vivo screening assays to determine their efficacies against different stages of carcinogenesis, in defined model systems and investigate the mechanism(s) of chemomodulation [25]. In vitro assays are employed as rapid screens for determining the chemopreventive efficacy based on modulation of different events presumed to be mechanistically linked to carcinogenesis. In these evaluations, biochemical assays, bacterial or mammalian cell systems, cell lines, cell free extracts etc. are employed. Chemo- preventive agents are screened for their abilities to inhibit carcinogen/mutagen-induced effects (mutagenic/clastogenic effects, adduct and free radical formation, effect on metabolic and repair enzymes etc.) or to inhibit cell proliferation or to enhance cell differentiation and apoptosis [26, 27]. Vol. 40, No. 2, 2007 These tests have generated voluminous and useful infor- mation about the chemopreventive properties and mechanism of action of large number of environmental agents (under defined conditions). However, these properties are not always reproducible in in vivo systems. Probable reasons for these differences are: (a) doses of chemopreventive agent(s) employed in in vitro studies are not achievable in vivo, (b) metabolic incompetance and lower inducibility of metabolic enzymes in cell lines as compared to tissues, (c) pharma- cokinetics in the two system(s) and (d) lack of proper controls (e.g. normal cell counterpart for studies on cancer cell lines). Therefore, in vivo assay systems are essential for evaluation of chemopreventive potential. Animal models are central to the theme of disease prevention as the data of short-term or long-term exposure of chemo- preventive agents can be accrued by exploiting the short lifespan of animal models. In vivo studies allow us to take a closer look at signaling mechanisms and drug metabolism, forming an integral part of preclinical studies. Many animal models are available for testing the efficacy of chemo- preventive agents. Selection of model system is generally governed by their rapidity, expression of multi stage carcinogenesis, organ/ tissue specificity, hormone responsiveness, invasiveness, slow/fast growth of tumor, histological types and particular relevance to most common human cancers. In these assays, chemopreventive activity of an agent is generally investigated against carcinogen(s)-induced tumors or appropriate biomarkers in experimental animals. In experimental animals chemopreventive activity is judged either by increase in latency period or decrease in incidence and multiplicity of carcinogen or spontaneously induced tumors or by inhibition of carcinogen-induced alterations or development of pre-malignant lesions [25]. In some instances regression of tumor or attenuation of xenograft growth and metastasis is also investigated [28, 29]. Genetically engineered mouse models have also been employed to develop specific types of cancer or to sub- stantially mimic human cancers e.g. APCmin, carries a germline truncation of one APC allele and develop multiple intestinal adenomas [30]. This approach is very helpful in demonstrating the role of defined molecular target or pathway in carcinogenesis and chemoprevention. Studies employing animal models have contributed signif- icantly in identification of number of chemopreventive agents and also helped in understanding the complexity of gene- environment interactions. However, a major drawback of assays employing animal models to study chemopreventive efficacies is shorter experimental duration than the inherent lifespan of animals. Due to this, adverse effects (if any) of chemopreventive agents exhibited at later time point may be missed. Several experimental studies have shown that some of the chemopreventive agent(s) have been observed to be