The microsomal N-oxidation of phentermine.

The microsomal N-oxidation of phentermine (Ia) to N-hydroxyphentermine (Ib) and to alpha,alpha-dimethyl-alpha-nitroso-beta-phenylethane (Ic was investigated. Maximum activities were obtained with microsomal (9000 g supernatant and microsomes) fractions of rabbit liver in the presence of an NADPH generating system. Incubation of Ia with hepatic washed microsomes from a phenobarbitone pretreated rabbit increased the formation of Ib and decreased that of Ic but the total amount of N-oxidized metabolites (i.e. Ib + Ic) was not affected. The ratio of the metabolically produced Ic to Ib but not the total amount of N-oxygenated metabolites varied greatly depending of the liver microsomal fractions used in the incubation mixtures of Ia; more Ib was produced from Ia using 9000 g supernatant and conversely, more Ic was formed using the washed microsomes of the same liver. The nitroso compound (Ic) was metabolically reduced to Ib and Ib to Ia by the hepatic 9000 g supernatant and soluble fraction; under the same conditions, washed microsomes had only limited reductive properties towards Ic and Ib. N-Hydroxyphentermine (Ib) was not metabolically oxidized to Ic when incubated with washed microsomes from rabbit liver. The use of known carbon-oxidation inhibitors showed that cytochrome P-450 is not involved in the incorporation of oxygen at the nitrogen centre of Ia. The metabolic characteristics and kinetic behavior of the microsomal N-oxidation of Ia supported a recently proposed mechanism explaining the independent formation of Ib and Ic from a common precursor resulting from metabolic N-oxidation of Ia.