Identification of new flavone-8-acetic acid metabolites using mouse microsomes and comparison with human microsomes.

Flavone-8-acetic acid (FAA) is a potent anticancer agent in mouse but has not shown activity in humans. Because FAA metabolism could play a role in this interspecies difference, our aim was to identify the metabolites formed in vitro using mouse microsomes compared with those in human microsomes. Mouse microsomes produced six metabolites as detected by reversed-phase high-performance liquid chromatography-mass spectrometry (MS). Three metabolites were identified as the 3'-, 4'-, or 6-hydroxy-FAA, by comparison with retention times and UV and MS spectra of standards. Two metabolites presented a molecular weight of 296 (FAA = 280) indicating the presence of one oxygen but did not correspond to any monohydroxylated FAA derivative. These two metabolites were identified as epoxides because they were sensitive to epoxide hydrolase. The position of the oxygen was determined by the formation of the corresponding phenols under soft acidic conditions: one epoxide yielded the 3'- and 4'-hydroxy-FAA, thus corresponding to the 3',4'-epoxy-FAA, whereas the other epoxide yielded 5- and 6-hydroxy-FAA, thus identifying the 5,6-epoxy-FAA. The last metabolite was assigned to the 3',4'-dihydrodiol-FAA because of its molecular weight (314) and sulfuric acid dehydration that indicated that the 3'- and 4'-positions were involved. Compared with mouse microsomes, human microsomes (2 pools and 15 individual microsomes) were unable to metabolize FAA to a significant extent. In conclusion, we have identified six new FAA metabolites formed by mouse microsomes, whereas human microsomes could not metabolize this flavonoid to a significant extent. The biological importance of the new metabolites identified herein remains to be evaluated.