Isolation of urinary p-hydroxylated metabolites of mephentermine and phentermine in male Wistar rats

Curiosities in drug metabolism

1. It is inevitable that during some xenobiotic biotransformation studies, a certain metabolite or degradation product arises of which the identity is uncertain, the route of formation is ambiguous, or it is just a plain mystery. 2. The following communication draws attention to three drugs reported in the literature, chlorphentermine, phenothiazine and aminopyrine, where after many years of investigation there still exists uncertainty over some of their metabolites. Noticeably, these three examples probably involve (potential) interaction of a nitrogen centre within the drug molecule. 3. It is hoped that the resurrection and assemblage of these data will offer interesting reading and that these examples may prove sufficiently intriguing to motivate further exploration and some resolution of these lingering concerns.

Metabolism of phentermine and its derivatives in the male Wistar rat

1. Urinary metabolites of the male Wistar rat dosed i.p. and orally with phentermine (Ph), N-hydroxyphentermine (N-hydroxy-Ph) and p-hydroxyphentermine (p-hydroxy-Ph) were examined by g.l.c. and g.l.c.-mass spectroscopy. 2. N-hydroxy-Ph which accounted for about 3% dose was identified in the urine of rat dosed i.p. and orally with Ph. The major urinary metabolite of Ph dosed i.p. and orally was a p-hydroxy-Ph conjugate (51% dose). 3. The major urinary metabolite of N-hydroxy-Ph dosed i.p. and orally was a p-hydroxy-Ph conjugate (40% dose). A N-hydroxy-Ph conjugate (12% dose) was identified following i.p. administration of N-hydroxy-Ph, but was not detected following oral administration. Small amounts of Ph (< 10% dose) and p-hydroxy-Ph (3% dose) were also identified after i.p. and oral administration of N-hydroxy-Ph. 4. The only urinary metabolite of p-hydroxy-Ph after either i.p. or oral dosing was a p-hydroxy-Ph conjugate (65% dose). 5. These results indicate that N-hydroxy-Ph is a urinary metabolite of Ph in rat; p-hydroxy-Ph is produced by the hydroxylation of Ph itself and partly by the hydroxylation of Ph formed from N-hydroxy-Ph; the p-hydroxy-Ph conjugate is the major and final metabolite of Ph dosed i.p. and orally.

Metabolism of mephentermine and its derivatives by the microsomal fraction from male Wistar rat livers

1. The N-demethylation of mephentermine (MP), p-hydroxymephentermine (p-hydroxy-MP) and N-hydroxymephentermine (N-hydroxy-MP), to produce phentermine (Ph), p-hydroxyphentermine (p-hydroxy-Ph) and N-hydroxyphentermine (N-hydroxy-Ph), and the p-hydroxylation of MP and Ph, to produce p-hydroxy-MP and p-hydroxy-Ph, were examined using rat liver microsomal preparations containing NADPH. Microsomal reduction of N-hydroxy-Ph to Ph was also examined using various cofactors. In addition, enzymic system for the N-demethylation and p-hydroxylation were examined using various inhibitors. 2. N-Hydroxy-MP demethylation to N-hydroxy-Ph proceeded at a rate almost 10-fold faster than other reactions. MP demethylation to Ph, MP oxidation to P-hydroxy-MP, Ph oxidation to p-hydroxy-Ph proceeded at similar rates, whilst p-hydroxy-MP demethylation to p-hydroxy-Ph was catalysed at the slowest rate. Microsomal reduction of N-hydroxy-Ph to Ph required NADH, and the activity was similar to that of MP oxidation to p-hydroxy-MP. 3. N-Demethylation of MP, p-hydroxy-MP and N-hydroxy-MP were inhibited not only by inhibitors of cytochrome P450, but also by methimazole, an inhibitor of the FAD-monooxygenase system. p-Hydroxylations of MP and Ph were inhibited only by inhibitors of cytochrome P450.

The formation of metabolites of mephentermine by microsomal and cytosolic preparations of male Wistar rat livers

1. Metabolites of mephentermine (MP), phentermine (Ph), p-hydroxy-MP, p-hydroxy-Ph, N-hydroxy-MP and N-hydroxy-Ph on incubation with rat liver microsomal and cytosolic preparations were identified by g.l.c. and g.l.c.-mass spectrometry. 2. Identification of the metabolites indicated the following new metabolic routes of MP: NADPH-dependent microsomal formation of p-hydroxy-MP from MP, of p-hydroxy-Ph from p-hydroxy-MP, and the NADH-dependent microsomal formation of Ph from N-hydroxy-Ph.

Metabolism of mephentermine in male guinea pigs and male mice

1. Urinary metabolites of mephentermine (MP), after i.p. administration of MP to male Hartley guinea pigs and mice, were identified by g.l.c.-electron impact (EI) mass spectrometry. Excretion of urinary radioactivity, and metabolites of 3H-MP, after i.p. administration, were determined by preparative t.l.c.-liquid scintillation counting. 2. About 27% of the radioactivity administered was excreted in the 24 h urine of guinea pigs, and 36% dose was excreted in 5 days. In mice, about 47% of the radioactivity was excreted in the 24 h urine, and 52% in 5 days. 3. Excretion rates of metabolites detected in the 24 h urine of guinea pigs were phentermine (Ph, 7.8%), a conjugate of N-hydroxyphentermine (N-hydroxy-Ph, 3.6%), p-hydroxyphentermine (p-hydroxy-Ph, 1.0%) and its conjugate (2.9%), and other metabolites (conjugates of MP and Ph, N-hydroxymephentermine (N-hydroxy-MP) and its conjugate, p-hydroxymephentermine (p-hydroxy-MP) and its conjugate, and N-hydroxy-Ph; less than 1.0%). The rates of excretion for mice were Ph (11.7%), conjugates of p-hydroxy-MP (3.1%), Ph (2.7%) and p-hydroxy-Ph (1.6%), and N-hydroxy-Ph (1.2%) and other metabolites (conjugates of MP and N-hydroxy-Ph, N-hydroxy-MP and its conjugate, p-hydroxy-Ph, and p-hydroxy-MP; less than 1.0%). 4. These results indicate that MP administered to mice is metabolized mainly to Ph and p-hydroxy-MP by N-demethylation and p-hydroxylation of the parent compound, and in guinea pigs the primary metabolic reaction of MP is N-demethylation producing Ph.(ABSTRACT TRUNCATED AT 250 WORDS)