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Since reactive oxygen radicals play an important role in carcinogenesis and other human disease states, antioxidants present in consumable fruits, vegetables, and beverages have received considerable attention as cancer chemopreventive agents. Thus, in order to identify antioxidants in plant extracts, test materials were assessed for potential to scavenge stable 1,2-diphenyl-2-picrylhydrazyl (DPPH) free radicals, reduce TPA-induced free radical formation in cultured HL-60 human leukemia cells, and inhibit responses observed with a xanthine/xanthine oxidase assay system. Approximately 700 plant extracts were evaluated, and 28 were found to be active in the DPPH free radical scavenging assay. Based on secondary analyses performed to assess inhibition of 7,12-dimethylbenz(a)anthracene-induced preneoplastic lesion formation with a mouse mammary organ culture model, Chorizanthe diffusa Benth. (Polygonaceae), Mezoneuron cucullatum Roxb. (Leguminosae), Cerbera manghas L. (Apocynaceae) and Daphniphyllum calycinum Benth. (Daphniphyllaceae) were selected and subjected to bioassay-guided fractionation. 5,7,3',5'-Tetrahydroxy-8,4'-dimethoxyflavonol, 5,8,4'-trihydroxy-7,3'-dimethoxyflavonol, 5,3',4'-trihydroxy-7-methoxyflavonol, and 6,3',4'-trihydroxy-7-methoxyflavonol were identified as active principles from C. diffusa. Piceatannol, trans-resveratrol, apigenin and scirpusin A were found as the active principles of M. cucullatum, olivil, (-)-carinol, and (+)-cycloolivil were active principles from C. manghas, and 5,6,7,4'-tetrahydroxyflavone 3-O-rutinoside and kaempferol 3-O-neohesperidoside were active principles from D. calycinum. Of these substances, the hydroxystilbenes piceatannol and transresveratrol have thus far been shown to inhibit carcinogen-induced preneoplastic lesion formation in the mouse mammary gland organ culture model.

The mammalian lignans enterodiol (ED) and enterolactone (EL) produced from colonic bacterial action on dietary precursors have exhibited anticarcinogenic effects in vitro. The major lignan precursor in flaxseed (a rich source) has been identified as secoisolariciresinol diglycoside (SDG). The purpose of this study was to first isolate SDG and determine whether 1) SDG accounted for all the lignan production from flaxseed; 2) this production was dose-related; and 3) a relationship between in vitro production and in vivo urinary excretion existed. Extraction of flaxseed with dioxane:ethanol (1:1, v/v) followed by chromatographic separations yielded the purified SDG. Rats were fed a high fat diet without/with 2.5, 5 or 10 g/100g ground flaxseed or 1.1, 2.2 or 4.4 micromol SDG/d (equivalent to levels in the respective flaxseed diets) for 4 wk. In vitro lignan production was assessed by fermenting flaxseed or SDG for 24 h with human fecal inoculum. Urinary lignan excretion increased linearly with doses from 0-5% flaxseed and 0-2.2 micromol SDG/d followed by a plateau, indicating a threshold response. When all doses were considered, a curvilinear relationship was observed. A similar trend was seen in vitro for SDG, resulting in a high correlation between in vitro production and in vivo excretion of lignans (r = 0.990, P < 0.02). Thus in vivo response can be predicted with confidence based on in vitro results. Theoretical urinary ED + EL from the SDG present in flaxseed correlated with the actual excretion in flaxseed-fed animals (r = 0.655, P < 0.005). However, urinary ED + EL of SDG-fed rats was only 20% of levels of flaxseed-fed rats, indicating the presence of other precursors or incomplete conversion of SDG to ED and EL.

Terminal differentiation of human promyelocytic leukemia (HL-60) cells can be induced by a variety of chemical agents and this process can be monitored readily by the generation of morphologically, histochemically, and functionally mature granulocytes and monocytes/macrophages. The availability of this model has heightened interest in the possible therapeutic role of inducers of myeloid differentiation for the treatment of leukemia and other neoplasms. In addition, however, potent cancer chemopreventive agents induce HL-60 cell differentiation at very low dose levels. Thus, as part of our search for natural product chemopreventive agents, extracts derived from nearly 400 plants were tested for their potential to induce HL-60 cell differentiation. As a result, 17 plant extracts were judged to be active (ED50 values < or = 4 micrograms/ml). One of most potent leads was an extract derived from Dirca occidentalis Gray (Thymelaeaceae) (ED50, 0.14 micrograms/ml), and bioassay-guided fractionation led to the identification of genkwanin (I), (+/-)-lariciresinol (II) and sitoindoside II (IV) as active principles, with ED50 values of 18.3, 1.1 and 0.069 microM, respectively. Based on these data, we conclude that the HL-60 cell differentiation system is a valid and useful model for the discovery of natural product cancer chemopreventive or chemotherapeutic agents.

For the discovery of new cancer chemopreventive agents, we have studied the potential of plant extracts to inhibit phorbol ester-induced ornithine decarboxylase (ODC) activity in cell culture. Four active rotenoids were obtained from the African plant Mundulea sericea (Leguminosae). These isolates were highly potent when evaluated for inhibition of chemically induced preneoplastic lesions in mammary organ culture and inhibition of papillomas in the two-stage mouse skin model, and they appear to function by a unique mechanism at the level of ODC messenger RNA expression. Based on our findings, rotenoids can be regarded as promising new chemopreventive or anticancer agents.

From the defatted meal of flaxseed (Linum usitatissimum), a novel phenylpropanoid glucoside, linusitamarin [1], was isolated, along with a number of known compounds. The structure of 1 was determined by spectroscopic analysis.