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 structure 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 reduction to tertiary amine B. A key role of cytochrome P4502B and P4502E1 in B N-oxide reduction has been proposed in the rat. Surprisingly, B N-oxide is reduced also in the presence of oxygen although all other N-oxides undergo reduction only under anaerobic conditions. With the aim to determine the influence 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 and the non-enzymatic reduction ability of the substrate was found, but no effect of the redox potential on reductase activity was observed. Microsomal reductases display a high affinity to B N-oxide (apparent K(m) congruent with0. 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 substitution on the benzo(c)fluorene moiety induced a significant K(m) increase (ten-fold). Based on kinetic study results, the scheme of mechanism of cytochrome P450 mediated benzo(c)fluorene N-oxides reduction have been proposed. All benzo(c)fluorene N-oxides under study were able to be reduced in the presence of oxygen. Changes in the B N-oxide structure caused an extent of anaerobic conditions preference. The relationship between the benzo(c)fluorene N-oxide structure and the profile of metabolites in microsomal incubation was studied and important differences in the formation of individual N-oxide metabolites were found.
Skálová
L
L
Faculty of Pharmacy, Charles University, Heyrovského 1203, CZ-500 05, Hradec Králové, Czech Republic.
Nobilis
M
M
Szotáková
B
B
Wsól
V
V
Kubícek
V
V
Baliharová
V
V
Kvasnicková
E
E
D000332:Aerobiosis; D000693:Anaerobiosis; D000818:Animals; D000970:Antineoplastic Agents / Q000737:chemistry / Q000378:metabolism*; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005449:Fluorenes / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D051381:Rats; D017208:Rats, Wistar
D000970:Antineoplastic Agents; D005449:Fluorenes; D003577:Cytochrome P-450 Enzyme System; C059230:benfluron N-oxide