Flavonoids are polyphenolic compounds found in a wide variety of plants. In recent years, flavonoids have been found to be beneficial to human health. Our lab has investigated the ability of flavonoids to induce cytotoxicity in human breast cancer cell lines. We have discovered that some compounds can induce cytotoxicity very readily, whereas others cannot. We hypothesized that this difference is attributable to a compound\u27s ability to transverse the cell membrane. Therefore, we used LC-MS to measure the amount of each flavonoid that is able to enter into MDA-MB-231 cells, human breast cancer cells. We tested 14 flavonoids. MDA-MB-231 cells were plated and treated with 100 μM flavonoids for 24 and 48 hr. After treatment, cells were extracted using methanol. Flavonoids from extracts were characterized and quantitated by LC-MS. We found that intracellular flavonoid concentration increased over time for some of the flavonoids. We could not detect quercetin, myricetin, and catechin in cell extracts. However, quercetin was found to be a moderate inducer of cytotoxicity, whereas myricetin and catechin were not. We found that quercetin dimerized over time in the cell medium only in the presence of serum. The quercetin dimer could not be detected in cellular extracts, which suggested that it was unable to cross the membrane. Additionally, synthetic quercetin dimer did not induce cell death. Our results suggest that only fresh quercetin killed cells; once quercetin dimerized, it could not induce cell death
