Metabolism of mephentermine in male guinea pigs and male mice

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.

Fluoxetine increases the anorectic and long-term dopamine-depleting effects of phentermine

The anorectic drug phentermine produces dose-related toxic effects on brain dopamine (DA) neurons in animals. Until recently, phentermine was widely used in combination with fenfluramine for purposes of appetite suppression and weight loss. With the recent withdrawal of fenfluramine from the market, many people have begun combining phentermine with fluoxetine, a serotonin reuptake inhibitor which also produces mild anorectic effects. Fluoxetine, in addition to inhibiting serotonin reuptake, inhibits hepatic mixed function oxidase, which plays an important role in the metabolic degradation of amphetamines. The purpose of the present study was to assess the effects of fluoxetine on the anorectic and DA neurotoxic effects of phentermine in mice. Phentermine, in combination with fluoxetine, produced greater reductions in food intake and body weight than phentermine alone. The phentermine/fluoxetine combination also produced greater long-term reductions in brain DA levels than phentermine alone, likely reflecting greater DA neurotoxicity of the drug combination. Brain concentrations of phentermine were also found to be higher in animals pretreated with fluoxetine. These findings indicate that fluoxetine potentiates both the anorectic and DA neurotoxic effects of phentermine, probably by increasing phentermine brain levels. The clinical significance of these findings remains to be ascertained.

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.

Intestinal metabolism of mephentermine and its biliary metabolites in male Wistar rats

1. Intestinal metabolites produced in the incubation (0-24 h) of mephentermine (MP), phentermine (Ph), N-hydroxymephentermine (N-hydroxy-MP), N-hydroxyphentermine (N-hydroxy-Ph), p-hydroxymephentermine (p-hydroxy-MP) and p-hydroxyphentermine (p-hydroxy-Ph) with male Wistar rat intestinal contents under N2 were examined by g.l.c. and g.l.c.-electron impact (EI) mass spectrometry. Metabolites produced in the anaerobic incubation of bile from rats given MP, with the intestinal contents were also examined. In addition, urinary and biliary metabolites of p-hydroxy-MP and p-hydroxy-Ph dosed orally to rat were examined. 2. Metabolites in the anaerobic incubation of N-hydroxy-MP and N-hydroxy-Ph were MP and Ph, and Ph, respectively. No metabolites were detected in the incubation of MP, Ph, p-hydroxy-MP and p-hydroxy-Ph. 3. p-Hydroxy-MP and p-hydroxy-Ph (major), and MP and Ph (minor) were detected when bile from rats given MP was incubated with intestinal contents. 4. Unchanged p-hydroxy-MP, and conjugates of p-hydroxy-MP and p-hydroxy-Ph, were detected in the 24-h urine of rats dosed with p-hydroxy-MP, which accounted for about 3, 72 and 1% dose, respectively. Unchanged p-hydroxy-Ph and conjugated p-hydroxy-Ph were detected in the 24-h urine of rats dosed with p-hydroxy-Ph, which accounted for about 4 and 68% dose, respectively. 5. Conjugated p-hydroxy-MP and conjugated p-hydroxy-Ph, which accounted for about 3% doses, were detected in the 24-h bile of rats dosed with p-hydroxy-MP and p-hydroxy-Ph.(ABSTRACT TRUNCATED AT 250 WORDS)

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.