Metabolism of the isoflavones genistein and biochanin A in human breast cancer cell lines

There is substantial variation in the growth inhibition of different human breast cancer cell lines by the isoflavones genistein and biochanin A. ZR-7 5-1 and BT-20 cells are greater than or equal to 2- to 4-fold less sensitiv e to these isoflavones than are MCF-7 cells, whereas T47D cells have a sens itivity similar to that of MCF-7 cells. To determine whether these differen ces are related to isoflavone metabolism by these cancer cells, each of the cell lines was incubated with [4-C-14]genistein and [4-C-14]biochanin A. M etabolites in the cell culture media were identified by radio-HPLC electros pray ionization mass spectrometry. One metabolite of genistein (genistein 7 -sulfate) and 2 metabolites of biochanin A (genistein and genistein 7-sulfa te) were detected by radio-HPLC. Further analysis by mass spectrometry iden tified 3 other metabolites, a hydroxylated methylated form of each isoflavo ne and a biochanin A sulfate. IC50 (the concentration at which the growth r ate was halved) values of the breast cancer cell lines did not correlate we ll with production of genistein 7-sulfate from genistein or with biochanin A sulfate, genistein 7-sulfate, or genistein from biochanin A. However, IC5 0 values correlated with the production of the hydroxylated and methylated forms of the isoflavones. Only T47D cells produced these metabolites in thi s study, and only T47D cells had IC50 values similar to those of MCF-7 cell s, which also produced the hydroxylated and methylated metabolites. These d ata suggest that the hydroxylated and methylated metabolites may be the act ive forms of genistein in human breast cancer cells and emphasize the impor tance of isoflavone metabolism in the mechanism of action of isoflavones.