Synthesis and labeling of isoflavone phytoestrogens, including daidzein and genistein

H/D Exchange Processes in Flavonoids: Kinetics and Mechanistic Investigations

Several classes of flavonoids, such as anthocyanins, flavonols, flavanols, and flavones, undergo a slow H/D exchange on aromatic ring A, leading to full deuteration at positions C(6) and C(8). Within the flavanol class, H-C(6) and H-C(8) of catechin and epicatechin are slowly exchanged in D₂O to the corresponding deuterated analogues. Even quercetin, a relevant flavonol representative, shows the same behaviour in a D₂O/DMSOd₆ 1:1 solution. Detailed kinetic measurements of these H/D exchange processes are here reported by exploiting the time-dependent changes of their peak areas in the ¹H-NMR spectra taken at different temperatures. A unifying reaction mechanism is also proposed based on our detailed kinetic observations, even taking into account pH and solvent effects. Molecular modelling and QM calculations were also carried out to shed more light on several molecular details of the proposed mechanism.

Potential risk of isoflavones: toxicological study of daidzein supplementation in piglets

As a main component of soy isoflavones, daidzein is rich in soy-derived products, which are widely used as feed ingredients in farm animals. However, little research has been conducted on the side effects of dietary daidzein, especially in young animals. In this study, the safety of daidzein was evaluated. Results show that ingesting 400 mg/kg of dietary daidzein for 70 days is associated with a lower average daily weight gain (kilogram) (0.47 ± 0.03 vs 0.54 ± 0.04, P < 0.05) and a higher splenic damage index (1.00 ± 1.10 vs 0.00 ± 0.00, P < 0.05) in young pigs compared with control. Female pigs receiving 200 and 400 mg/kg daidzein showed reduced serum testosterone levels (ng/L) on days 35 and 70 compared with the control group (day 35, 246 ± 74 and 224 ± 20 vs 362 ± 48, P < 0.05; day 70, 252 ± 38 and 219 ± 77 vs 374 ± 38, P < 0.05). Daidzein residue (μg/kg) in pig livers increased (243 ± 80 vs 142 ± 47, P < 0.05, day 70). These results suggest that dietary supplements of 400 mg/kg of daidzein negatively affect the weight gain and splenic morphology of pigs.

The metabolism and analysis of isoflavones and other dietary polyphenols in foods and biological systems

Polyphenols in dietary and botanical matrices are usually present as simple and complex O-glycosides. In fermented dietary materials, the glycosidic moiety is removed and accompanied in some cases by more complex changes to the polyphenol. As for most xenobiotics, polyphenols undergo phase II conjugation in the intestinal wall during their absorption from the gut. In contrast, a few polyphenols, such as puerarin in the kudzu vine, are C-glycosides and are stable in the gut and during absorption, distribution and excretion. Large bowel bacteria reduce polyphenol aglycones, causing opening of the heterocyclic B-ring and ring cleavage. The products are mostly absorbed and enter the bloodstream. Phase I and II metabolism events occur in the intestine and the liver - most polyphenols predominantly circulate as β-glucuronides and sulfate esters with very little as the aglycones, the presumed active forms. In addition, metabolism can occur in non-hepatic tissues and cells including breast tumor cells that have variable amounts of cytochrome P450s, sulfatase and sulfotransferase activities. Inflammatory cells produce chemical oxidants (HOCl, HOBr, ONO(2)(-)) that will react with polyphenols. The isoflavones daidzein and genistein and the flavonol quercetin form mono- and dichlorinated products in reaction with HOCl. Genistein is converted to 3'-nitrogenistein in the lung tissue of lipopolysaccharide-treated rats. Whereas polyphenols that can be converted to quinones or epoxides react with glutathione (GSH) to form adducts, chlorinated isoflavones do not react with GSH; instead, they are converted to β-glucuronides and are excreted in bile. Analysis of polyphenols and their metabolites is routinely carried out with great sensitivity, specificity and quantification by LC-tandem mass spectrometry. Critical questions about the absorption and tissue uptake of complex polyphenols such as the proanthocyanins can be answered by labeling these polyphenols with (14)C-sucrose in plant cell culture and then purifying them for use in animal experiments. The (14)C signature is quantified using accelerator mass spectrometry, a technique capable of detecting one (14)C atom in 10(15) carbon atoms. This permits the study of the penetration of the polyphenols into the interstitial fluid, the fluid that is actually in contact with non-vascular cells.

Neuroprotective effects of puerarin against beta-amyloid-induced neurotoxicity in PC12 cells via a PI3K-dependent signaling pathway

Epidemiological data have indicated that estrogen replacement therapy (ERT) can decrease the risk of developing Alzheimer's disease (AD). Phytoestrogens have been proposed as potential alternatives to ERT. The aim of the present study was to assess the neuroprotective effects of puerarin, a phytoestrogen isolated from Pueraria lobata, against the toxicity of beta-amyloid (Aβ) in relation to the mitochondria-mediated cell death process, and to elucidate the role the activation of Akt and modulation of the pro- and antiapoptotic proteins in puerarin-induced neuroprotection. The present study shows that puerarin afforded protection against Aβ-induced toxicity through inhibiting apoptosis in PC12 cells. This result was also confirmed by the activated caspase-3 assay. P-Akt, Bcl-2 and p-Bad expression increased after pretreatment with puerarin in PC12 cells exposed to Aβ(25-35), whereas Bax expression and cytochrome c release decreased. Interestingly, these effects of puerarin against Aβ(25-35) insult were abolished by wortmannin, an inhibitor of PI3K phosphorylation. These findings suggest that puerarin prevent Aβ-induced neurotoxicity through inhibiting neuronal apoptosis, and might be a potential preventive or therapeutic agent for AD.

Equol: history, chemistry, and formation

Equol, first isolated from equine urine in 1932 and identified 50 years later in human urine as a metabolite of the soy isoflavones, daidzin and daidzein, is produced by intestinal bacteria in some, but not all, adults. This observation led to the term equol-producers to define those adults that could make equol in response to consuming soy isoflavones and the hypothesis that the health benefits of soy-based diets may be greater in equol-producers than in equol nonproducers. By virtue of a chiral center, equol occurs as a diastereoisomer and intestinal bacteria are enantiospecific in synthesizing exclusively the S-(-)equol enantiomer, an enantiomer that has selective affinity for the estrogen receptor-beta. Both enantiomers are of interest from a clinical and pharmacological perspective and are currently being developed as nutraceutical and pharmacological agents. The wide range of biological activities these enantiomers possess warrants their investigation for the treatment of a number of hormone-related conditions involving estrogen-dependent and androgen-related conditions. The following review describes the history, chemistry, and factors governing the intestinal bacterial formation of equol.

Breast, colorectal, and prostate cancer risk in the European Prospective Investigation into Cancer and Nutrition-Norfolk in relation to phytoestrogen intake derived from an improved database

BACKGROUND

The characterization of phytoestrogen intake and cancer risk has been hindered by the absence of accurate dietary phytoestrogen values.

OBJECTIVE

We examined the risk of breast, colorectal, and prostate cancers relative to phytoestrogen intake on the basis of a comprehensive database.

DESIGN

Demographic and anthropometric characteristics, a medical history, and 7-d records of diet were collected prospectively from participants (aged 40-79 y) in the European Prospective Investigation into Cancer and Nutrition-Norfolk (EPIC-Norfolk). Five hundred nine food items were analyzed by liquid chromatography-mass spectrometry/mass spectrometry, and (13)C(3)-labeled internal standards were analyzed for isoflavones (genistein, daidzein, glycitein, biochanin A, and formononetin), lignans (secoisolariciresinol and matairesinol), and enterolignans from gut microbial metabolism in animal food sources (equol and enterolactone). From the direct analysis, values for 10,708 foods were calculated. Odds ratios (ORs) for breast (244 cases, 941 controls), colorectal (221 cases, 886 controls), and prostate (204 cases, 812 controls) cancers were calculated relative to phytoestrogen intake.

RESULTS

Phytoestrogen intake was not associated with breast cancer among women or colorectal cancer among men. Among women, colorectal cancer risk was inversely associated with enterolactone (OR: 0.33; 95% CI: 0.14, 0.74) and total enterolignans (OR: 0.32; 95% CI: 0.13, 0.79), with a positive trend detected for secoisolariciresinol (OR: 1.60; 95% CI: 0.96, 2.69). A positive trend between enterolignan intake and prostate cancer risk (OR: 1.27; 95% CI: 0.97, 1.66) was attenuated after adjustment for dairy intake (OR: 1.19; 95% CI: 0.77, 1.82).

CONCLUSION

Dietary phytoestrogens may contribute to the risk of colorectal cancer among women and prostate cancer among men.

Pharmacokinetics of the soybean isoflavone daidzein in its aglycone and glucoside form: a randomized, double-blind, crossover study

BACKGROUND

There are conflicting results in the literature on the bioavailability of isoflavones in the aglycone and the glucoside forms.

OBJECTIVE

The objective was to investigate the pharmacokinetics of the soy isoflavone daidzein (DAI) on oral administration of both the aglycone and glucoside form in a human intervention study. In addition, the pharmacokinetics of the bacterial and oxidative metabolites of DAI was assessed.

DESIGN

Seven German men aged 22-30 y participated in a randomized, double-blind study in a crossover design. After ingestion of pure DAI or pure daidzein-7-O-beta-d-glucoside (DG) (1 mg DAI aglycone equivalent/kg body weight), blood samples were drawn before isoflavone administration and 1, 2, 3, 4.5, 6, 8, 10, 12, 24, and 48 h after the dose. Urine was collected before and 0-6, 6-12, and 12-24 h after the intake of the isoflavones. The concentrations of DAI and its major bacterial and oxidative metabolites in plasma and urine were measured with isotope dilution capillary gas chromatography-mass spectrometry.

RESULTS

The systemic bioavailability (area under the curve; AUC(inf)), the maximal plasma concentration (C(max)), and the cumulative recovery of DAI in urine after administration of DG were 3-6 times greater than after the ingestion of DAI. Except for equol, which was formed by only one volunteer, all other quantified metabolites exhibited 2-12 times greater AUC(inf), C(max), and urinary recoveries after consumption of DG.

CONCLUSION

Our results show that DG exhibits a greater bioavailability than its aglycone when ingested in an isolated form.

Extraction and quantification of phytoestrogens in foods using automated solid-phase extraction and LC/MS/MS

Phytoestrogens are a group of polyphenolic plant metabolites that can induce biological responses. Their bioactivity is based on their similarity to 17beta-estradiol and their ability to bind to the beta-estrogen receptor. Although epidemiological data are inconclusive, phytoestrogens are considered to be beneficial for a variety of conditions, for example, hormone-related cancers like breast and prostate cancer. To investigate the biological effects of these compounds and to assess the exposure of larger cohorts or the general public, reliable data on the phytoestrogen content of food is necessary. Previously, food analysis for phytoestrogens was performed using either HPLC-UV or GC/MS. Here, we describe the development of the first generic method for the analysis of phytoestrogens in food, using automated solid-phase extraction and liquid chromatography-tandem mass spectrometry. The presented method shows a good reproducibility and can be easily adapted to other phytoestrogens if required.

Reactivity of anthocyanins and pyranoanthocyanins. Studies on aromatic hydrogen-deuterium exchange reactions in methanol

Reactivity studies involving anthocyanin structures and their equilibrium forms will lead to better understanding of the properties of these antioxidants. Hydrogen-deuterium (H --> D) exchange reactions at various sites of the 3-glucosides of delphinidin (1), petunidin (2), malvidin (3), and the corresponding 3-glucosides of carboxypyranodelphinidin (4), carboxypyranopetunidin (5), carboxypyranomalvidin (6), and the flavonol quercetin 3-O-(6-alpha-rhamnopyranosyl-beta-glucopyranoside)(7) have been examined at room temperature in pure CD 3OD and in CD 3OD acidified with CF 3CO 2D. The H --> D exchange rate constants of H-6 and H-8 of 2 determined from (1)H NMR integration data were found to be independent upon pigment concentration (up to 4 x 10 (-2) M) and trifluoroactic acid concentration (0-15%, v/v), respectively. This suggest that these reactions follow first-order kinetics and unexpectedly to be independent of the acid concentration. H-6 and H-8 of the flavylium cation A-rings of 1- 3, and in the corresponding hydrogens of the hemiketal forms, exchanged with half-lives of approximately 100 h ( 1) and approximately 50 h ( 2 and 3), respectively. The pyranoanthocyanins (4-6) experienced no H --> D exchange for the analogous hydrogens, but H --> D exchange of H-beta (H-4)(t 1/2 approximately 25 h) for these compounds was observed. Only H-8 underwent significant H --> D exchange in 7. It is concluded that a stabilization of the sigma-complexes, assumed to be the intermediates in the reactions, takes place for the common anthocyanins (1-3) contrary to the pyranoanthocyanins (4-6).

Studies of the in vitro intestinal metabolism of isoflavones aid in the identification of their urinary metabolites

Soy isoflavones have recently gained considerable interest due to their possible health benefits. However, detailed studies on the metabolism of isoflavones are lacking. The aims of the investigation presented here were (1) to study the in vitro intestinal metabolism of isoflavones and their hydroxylated analogues 3'-OH-daidzein, 6-OH-daidzein, 8-OH-daidzein, and 3'-OH-genistein and (2) to characterize the structures of some earlier identified urinary metabolites of soy isoflavones, for which no authentic reference compounds have been available. Isoflavone standards (1-2 mg) were fermented with human fecal flora (16.7%) for 24 h. Metabolites formed during the fermentation were tentatively identified by interpretation of the mass spectra of trimethylsilylated compounds obtained by GC-MS. Compounds having hydroxyl groups at 5-position (i.e., genistein and 3'-OH-genistein) were completely converted to metabolites that could not be detected by the methods used in this study. The metabolism of daidzein and its hydroxylated analogues, 3'-OH-daidzein, 6-OH-daidzein, and 8-OH-daidzein, occurred to a much lesser extent. Minor amounts of reduced metabolites (i.e., isoflavanones and alpha-methyldeoxybenzoins) of these compounds were tentatively identified in fermentation extracts. The retention times and the mass spectra of reduced isoflavone metabolites, obtained from in vitro fermentations of pure compounds, were utilized to identify unknown urinary metabolites of soy isoflavones. Four novel isoflavone metabolites were identified in human urine collected after soy supplementation: 3' '-OH-O-desmethylangolensin, 3',4',7-trihydroxyisoflavanone, 4',7,8-trihydroxyisoflavanone, and 4',6,7-trihydroxyisoflavanone.

Experimental and DFT 1H NMR study of conformational equilibria in trans-4',7-dihydroxyisoflavan-4-ol and trans-isoflavan-4-ol

The solution-state conformational equilibria of trans-4',7-dihydroxyisoflavan-4-ol (1) and trans-isoflavan-4-ol (2) were assessed based on the temperature dependence of their vicinal coupling constants J(H)(-)(2)(alpha)(,H)(-)(3) and J(H-3,H)(-)(4) in comparison to values calculated with density functional theory (DFT) methods at the B3LYP/cc-pVTZ//B3LYP/6-31G(d,p) level of theory. For each half-chair conformer, several rotamers with respect to the C-4 hydroxyl and C-3 phenyl were calculated and the overall diequatorial-to-diaxial ratio at 298 K was assessed as 66:34 for 1 and 73:27 for 2. The syntheses of 1 and 2 are described.

Time-resolved fluoroimmunoassay for equol in plasma and urine

We present a method for the determination of the isoflavan equol in plasma and urine. This estrogenic isoflavan, which is formed by the action of the intestinal microflora, may have higher biological activity than its precursor daidzein. High urinary excretion of equol has been suggested to be associated with a reduction in breast cancer risk. The method is based on time-resolved fluoroimmunoassay, using a europium chelate as a label. After synthesis of 4'-O-carboxymethylequol the compound is coupled to bovine serum albumin (BSA), then used as antigen to immunize rabbits. The tracer with the europium chelate is synthesized using the same 4'-O-derivative of equol. After enzymatic hydrolysis (urine) or enzymatic hydrolysis and ether extraction (plasma) the immunoassay is carried out. The antiserum cross-reacted to variable extent with some isoflavonoids. For the plasma method the cross-reactivity does not seem to influence the results, which were highly specific. The overestimation of the values using the urine method (164%) compared to the results obtained by a gas chromatography-mass spectrometry (GC-MS) method is probably due to some influence of the matrix on the signal, and interference of structurally related compounds. It is suggested that plasma assays are used but if urine samples are measured a formula has to be used to correct the values making them comparable to the GC-MS results. The correlation coefficients between the time-resolved fluoroimmunoassay (TR-FIA) methods and GC-MS methods were high; r-values for the plasma and urine method, were 0.98 and 0.91, respectively. The intra-assay coefficient of variation (CV%) for the TR-FIA plasma and urine results at three different concentrations vary between 5.5-6.5 and 3.4-6.9, respectively. The inter-assay CV% varies between 5.4-9.7 and 7.4-7.7, respectively. The working ranges of the plasma and urine assay are 1.27-512 and 1.9-512nmol/l, respectively.

Review of the methods used in the determination of phytoestrogens

Interest in analytical methods for plant estrogens (phytoestrogens) has risen sharply in the past 10 years. In this review, we examine the existing analytical methods based on separations by gas-liquid chromatography, high-performance liquid chromatography and capillary electrophoresis in addition to methods of detection by ultraviolet absorption, fluorescence, electrochemical oxidation/reduction and mass spectrometry. These methods are compared with other methods of phytoestrogen analysis utilizing immunoassay approaches. The advantages and disadvantages of each of these methods are highlighted and potential areas for further development identified.

Soy isoflavonoid aglycons genistein and daidzein Do not increase the cytochrome P-450 content of the liver microsomes of mice

Mice (4-week-old, male ddy) were fed four isonitrogenic diets for 21 days: purified diet (C diet); fermented soybean (400 mg of soy isoflavonoids/kg; FSB); fermented soybean extract (400 mg of soy isoflavonoid aglycones/kg; FSBE); C with indole-3 carbinol (I3C) (2500 mg of I3C/kg; I3C). The I3C and FSB diets significantly increased the cytochrome P-450 content of hepatic microsomes in comparison with the C diet, while the FSBE diet did not. Other mice were fed seven diets for 21 days: C; C with 100 mg or 200 mg of genistein, 100 mg or 200 mg of daidzein, or 100 mg of genistein + 100 mg of daidzein/kg; I3C diet. Genistein and daidzein did not change the liver cytochrome P-450 content. There was no synergistic effect of the combined feeding of genistein and daidzein. The increase in the cytochrome P-450 content with the FSB diet depends on chemicals other than genistein and daidzein. Genistein and daidzein do not induce cytochrome P-450.

Time-resolved fluoroimmunoassay of plasma daidzein and genistein

We present a method for the determination of the phytoestrogens daidzein and genistein in plasma (serum). These weakly estrogenic isoflavones occur in soybeans and in smaller amounts in some other beans and plants. It has been suggested that they may afford protection against prostate and breast cancer. The method is based on time-resolved fluoroimmunoassay (TR-FIA) using a europium chelate as a label. After synthesis of 4'-O-carboxymethyl-daidzein and 4'-O-carboxymethyl-genistein the compounds are coupled to bovine serum albumin (BSA), then used as antigens to immunize rabbits. The tracers with the europium chelate are synthesized using the same 4'-O-derivative of the isoflavones. After enzymatic hydrolysis and ether extraction the immunoassay is carried out using the VICTOR 1420 multilabel counter (Wallac Oy, Turku, Finland). The antisera cross-reacted to some extent with some isoflavonoids but not with flavonoids. The cross-reactivity seems not to influence the results, which were highly specific for both compounds. The correlation coefficients between the TR-FIA methods and the reference method based on isotope dilution gas chromatography-mass spectrometry were high; r-values were about 0.95-0.99 depending on concentration. The intra-assay coefficients of variation (CV%) for daidzein and genistein at three different concentrations vary 3.2-4.5 and 3.2-4.1, respectively. The inter-assay CVs vary 5.0-6.3 and 4.5-5.3, respectively. The working ranges of the daidzein and genistein assays are 1.0-216 and 1.7-370 nmol/l, respectively. The plasma values (n = 80) of daidzein and genistein are very low in Finnish subjects (mean for daidzein, 3.8+/-6.8 and for genistein, 3.2+/-7.6 nmol/l; median value for daidzein 1.5 and for genistein 1.4 nmol/l).

Synthesis of haptens and conjugates for ELISAs of phytoestrogens. Development of the immunological tests

Seven carboxylic acid haptens of isoflavonoids were synthesized, with the spacer arm on the oxygen atom at the C7 position for one series, with formononetin, daidzein, equol, biochanin A, and genistein, and at the C8 position for a second series, with only formononetin and daidzein. The different haptens were coupled to bovine serum albumin (BSA) and to swine thyroglobulin (Thyr). Polyclonal antibodies were generated against the BSA conjugates. Enzyme-linked immunosorbent assays (ELISAs) were developed based on competition between free phytoestrogens and the Thyr-hapten conjugates for specific antibodies. IC(50) values of the standard curves ranged between 0.8 and 20 ng/mL that is, 0.3 and 9.2 pmol/well. The antibodies obtained should be useful for assays in vegetable matter as well as in biological fluids after a separation step. These ELISAs should be valuable also in the food industry to control phytoestrogen concentrations prior to and after processing.

Effect of dietary soy isoflavone aglycones on the urinary 16alpha-to-2-hydroxyestrone ratio in C3H/HeJ mice

Estradiol is metabolized through two mutually exclusive pathways. 2-Hydroxyestrone (2-OHE,) is antiestrogenic, while 16alpha-hydroxyestrone (16alpha-OHE1) is a potent estrogen. It is suggested that a high urinary 16alpha-OHE1-to-2-OHE1 rato is a biomarker of increased mammary tumor risk. Mice were fed one of the test diets for 21 days. Indole-3-carbinol (2,500 mg/kg diet) increased the cytochrome P-450 content of hepatic microsomes and liver weight and reduced the urinary 16alpha-OHE1-to-2-OHE1 ratio in comparison with the respective value in the control mice. Fermented soy extract (100, 200, or 400 mg isoflavonoid/kg diet), genistein (200 mg/kg diet), and daidzein (200 mg/kg diet) each reduced the urinary 16alpha-OHE1-to-2-OHE1 ratio without increasing the cytochrome P-450 content of hepatic microsomes or liver weight. The combination of genistein and daidzein (100 mg and 100 mg/kg diet) did not have a synergistic effect on the reduction in urinary 16alpha-OHE1-to-2-OHE1 ratio. These data suggest that the soy isoflavonoid aglycones genistein and daidzein and indole-3-carbinol each exert a cancer-preventive effect by shifting metabolism away from the production of genotoxic metabolites toward the production of inactive metabolites.

Radioimmunoassay of free genistein in human serum

Two radioimmunoassay (RIA) systems for genistein have been established, based on polyclonal antibodies against genistein-4'-O-(carboxymethyl)ether-bovine serum albumin and genistein-7-O-(carboxymethyl)ether-bovine serum albumin conjugates. The sensitivities of assays were 4.44 and 10.4 fmol (1.2 and 2.8 pg)/tube, respectively, the intraassay coefficients of variation ranged from 3.54 to 9.30%, the interassay C.V. varied from 6.72 to 19.7%, depending on the type of method and on genistein concentration. The cross-reactivities with other chemically related compounds (with exception of genistein derivatives at the position used for construction of the immunogen) were 5.5 and 6.1% for daidzein and 3.9 and 0.04% for formononetin in RIAs using reagents prepared through positions 4'- and 7- of genistein, respectively. The method was used for measurement of genistein levels in 26 omnivore subjects and in three volunteers after consumption of a meal prepared from 125 g of cooked whole soybeans. The values obtained in ether extracts from human sera were almost identical for both RIA systems, indicating that both RIAs measure the same entity.

The chemopreventive properties of soy isoflavonoids in animal models of breast cancer

Genistein (5,7,4'-trihydroxyisoflavone), one of two major isoflavonoids in soy, has anti-proliferative effects on mitogen-stimulated cell growth of human breast cancer cells in culture and is a candidate for use in the prevention of breast cancer. Soy protein preparations containing isoflavonoid conjugates have chemopreventive activity in carcinogen-induced rat models of breast cancer. Recent experiments in these models with purified genistein have revealed that the timing of the exposure of rats to this isoflavonoid is critical. Rats treated neonatally or prepuberally with genistein have a longer latency before the appearance of 7,12-dimethylbenz[a]anthracene-induced mammary tumors and a marked reduction in tumor number. The mechanism of genistein's preventive action is in part dependent on its estrogenic activity, which causes a more rapid differentiation of the cells of the mammary gland, and analogous to the effects of an early pregnancy. Rats administered genistein after 35 days of age have smaller alterations in breast cancer risk, with a maximum reduction in mammary tumor number of 27%. In ovariectomized nude mice, dietary genistein increases cell proliferation of human breast cancer MCF-7 cell xenografts compared with a control diet. This estrogen-like effect of genistein is not observed in non-ovariectomized rats. Future studies on the anticancer potential of soy isoflavonoids should examine their interaction with other phytochemical components of soybeans and exploit newly developed animal models of breast cancer in which specific genes have been activated or inactivated.