Effect of substituents on microsomal reduction of benzo(c)fluorene N-oxides

The potential benzo(c)fluorene antineoplastic agent benfluron (B) displays high activity against a broad spectrum of experimental tumours in vitro and in vivo. In order to suppress some of its undesirable properties, its stru cture has been modified. Benfluron N-oxide (B N-oxide) is one of benfluron derivatives tested. The main metabolic pathway of B N-oxide is its reductio n to tertiary amine B. A key role of cytochrome P4502B and P4502E1 in B N-o xide reduction has been proposed in the rat. Surprisingly, B N-oxide is red uced also in the presence of oxygen although all other N-oxides undergo red uction only under anaerobic conditions, With the aim to determine the influ ence of the N-oxide chemical structure and its redox potential on reductase affinity, activity and oxygen sensitivity five relative benzo(c)fluorene N -oxides were prepared. A correlation between the redox potential measured a nd the non-enzymatic reduction ability of the substrate was found, hut no e ffect of the redox potential on reductase activity was observed. Microsomal reductases display a high affinity to B N-oxide (apparent K-m congruent to 0.2 mM). A modification of the side-chain or nitrogen substituents has led to only a little change in apparent K-m values, but a methoxy group substi tution on the benzo(c)fluorene moiety induced a significant K-m increase (t en-fold). Based on kinetic study results, the scheme of mechanism of cytoch rome P350 mediated benzo(c)fluorene N-oxides reduction have been proposed. All benzo(cl)fluorene N-oxides under study were able to be reduced in the p resence of oxygen. Changes in the B N-oxide structure caused an extent of a naerobic conditions preference. The relationship between the benzo(c)fluore ne N-oxide structure and the profile of metabolites in microsomal incubatio n was studied and important differences in the formation of individual N-ox ide metabolites were found. (C) 2000 Elsevier Science Ireland Ltd. All righ ts reserved.