Hydroxylation of amphetamine to parahydroxyamphetamine by rat liver microsomes

Investigation of stereoselective metabolism of amphetamine in rat liver microsomes by microdialysis and liquid chromatography with precolumn chiral derivatization

The utility of microdialysis as a quantitative sampling technique for in vitro drug metabolism studies was demonstrated by investigating the stereoselective metabolism of D-, L- and DL-amphetamine by the cytochrome P-450 enzymes. Microdialysates containing the isomers of amphetamine and its metabolite were derivatized with the fluorescent chiral derivatizing agent, (-)-fluorenylethyl chloroformate. The diastereoisomers were isocratically separated by liquid chromatography (LC) on a reversed-phase C18, 3-micron (100 x 3.2 mm) column. The intra- and inter-assay relative standard deviation (R.S.D.) was below 10%. Michaelis-Menten parameters, K(m) and Vmax were obtained for the formation of both D- and L-hydroxyamphetamine from D-, L- and DL-amphetamine in the concentration range of 10-350 microM.

Enantiospecific drug analysis via the ortho-phthalaldehyde/homochiral thiol derivatization method

Pre-column derivatization with o-phthaldialdehyde and an optically active thiol has hitherto been used mainly for liquid-chromatographic chiral separation of amino acids. Chiral separation of non-amino-acid primary amines, especially of pharmaceuticals, via this approach has been largely ignored. We have therefore examined the applicability of the method to the chiral resolution of several pharmaceutical amines. o-Phthaldialdehyde and four commercially available homochiral thiols were used to study the separation of the enantiomers of amphetamine, p-hydroxyamphetamine, p-chloroamphetamine, 3-amino- 1-phenylbutane, 3-amino-1-(4-hydroxyphenyl)-butane, mexiletine, tocainide, tranylcypromine and rimantadine. The resulting highly fluorescent isoindole derivatives were resolved on a Waters Nova-Pak C18 column using mobile phases consisting of mixtures of methanol, a sodium acetate buffer and acetonitrile, and the column effluent was monitored using fluorescence or UV detection. In some cases the fluorescence and/or the UV absorbance of the two diastereomers were unequal. It was found that the resolution of most of the amines could be optimized by varying the homochiral thiol in the derivatization step. This method of chiral separation may have wide applicability in enantiospecific drug analysis of non-amino-acid primary amines due to its simplicity and the high sensitivity it provides.

Hydroxylation of phenylethylamine by rat liver preparations. Inhibition studies

1. Rat liver 100,000 g pellet microsomal fraction p-hydroxylate phenylethylamine to tyramine in a relatively slow proceeding, NADPH-requiring reaction; Km 2.1 x 10(-5) M and Vmax 0.32 nmol/mg protein/20 min. 2. This reaction is inhibited, either competitively, noncompetitively or uncompetitively by a number of behaviorally active monomethylated and monohalogenated derivatives of phenylethylamine. 3. Whereas formation of tyramine was not significantly affected by L-phenylalanine or its p-chloro derivative, it was competitively inhibited by imipramine, iprindole and alprazolam. 4. It is suggested that at least some of the effects of these drugs may result from their ability to interfere with phenylethylamine metabolism.

Participation of cytochrome P-450 isozymes in N-demethylation, N-hydroxylation and aromatic hydroxylation of methamphetamine

1. Five isozymes of cytochrome P-450 were purified from liver microsomes of phenobarbital-pretreated (P-450-SD-I and -II), 3-methylcholanthrene-pretreated (P-450-SD-III) and untreated rats (P-450-SD-IV and -V) to determine their catalytic activities in metabolic reactions of methamphetamine. 2. All the isozymes except P-450-SD-III showed considerably high N-hydroxylating activity of methamphetamine. The cytochromes P-450 initiate N-demethylation of this drug by two metabolic pathways, C-hydroxylation and N-hydroxylation. 3. Both N-demethylation and N-hydroxylation of methamphetamine were efficiently catalysed by the phenobarbital-inducible forms P-450-SD-I and -II and constitutive forms P-450-SD-IV and -V. 4. The constitutive forms P-450-SD-IV and -V revealed high catalytic activities of p-hydroxylation of methamphetamine, but phenobarbital- and 3-methylcholanthrene-inducible isozymes showed only low activities. 5. The present results indicate that the different extents of the metabolic intermediate complex formation with cytochrome P-450 (455 nm complex) in the microsomes from phenobarbital-, 3-methylcholanthrene-pretreated, and untreated rats is not attributable to the activities of the respective isozymes of cytochrome P-450 to form the precursor of the complex, N-hydroxymethamphetamine.

Reversible inhibition of aromatic hydroxylation of methamphetamine in rat liver microsomal preparations pretreated with methamphetamine

The effects of a single and repeated administration of methamphetamine (MP) in vivo in rats on its own metabolism in vitro were investigated. In both cases, the p-hydroxylation of MP to p-hydroxymethamphetamine by a microsomal fraction from rat liver was inhibited for a period of 16 hr after the last injection of MP. This inhibition was diminished by dialysis of the microsomal preparations. In contrast, the reduced level of cytochrome P-450 in hepatic microsomes from rats pretreated with the SKF 525-A did not revert to the control value after dialysis. When microsomes were preincubated with N-hydroxymethamphetamine, which is the metabolite of MP and a potent substrate for the formation of a metabolic intermediate (MI) complex with cytochrome P-450, the content of the MI was increased and the MP-hydroxylation activity decreased in direct proportion to the length of the preincubation. These results suggest that the inhibition of MP-hydroxylation may be due to reduction of the level of cytochrome P-450 that accompanies the formation of the MI complex. Furthermore, it appears that the complex can be dissociated by dialysis.

The in-vitro metabolism of [14C]pentobarbitone and [14C]phenobarbitone by hamster liver microsomes

The metabolism of [14C]pentobarbitone and [14C]phenobarbitone has been reinvestigated using an in-vitro hepatic microsomal system (Syrian hamsters, Aroclor 1254 induction). The incubation system was routinely supplemented with EDTA (1 mM) and a substrate concentration study revealed the metabolism of [14C]pentobarbitone to be concentration-dependent, with the greatest overall metabolism (greater than 50%) occurring at 0.054 mumol per 3.5 mL. With [14C]phenobarbitone as substrate, overall metabolism was extremely low (3%) and independent of substrate concentration. Addition of further cofactors to the incubation mixture at 20 min intervals over an extended period resulted in almost complete metabolism of [14C]pentobarbitone (100 min), 3'-hydroxypentobarbitone and 3'-oxopentobarbitone being identified as metabolites together with many minor, unidentified products. With [14C]phenobarbitone as the substrate, cofactor addition up to 120 min resulted in 8% overall metabolism; p-hydroxyphenobarbitone was identified as a product of metabolism; other minor products were unidentified. The metabolism studies failed to produce a metabolite having the properties of the N-hydroxylated product of either [14C]pentobarbitone or [14C]phenobarbitone within the detection limits available (0.02% of 0.5 mumol per incubate).

The metabolism of 1-phenyl-2-(N-methyl-N-furfurylamino)propane (furfenorex) in the rat in vivo and in vitro

The metabolism of 1-phenyl-2-(N-methyl-N-furfurylamino)propane (furfenorex) was studied in the rat in vivo and in vitro. Nine metabolites with only traces of the unchanged drug were obtained from urine after oral administration of furfenorex to rats. The major metabolite was an acidic compound, isolated and identified as 1-phenyl-2-(N-methyl-N-gamma-valerolactonylamino)propane. Amphetamine, methamphetamine and their hydroxylated metabolites were excreted as minor metabolites. Metabolites excreted in two days after administration of the drug amounted to about 20% of dose. The acidic metabolite, a major metabolite in vivo, was not detected after incubation of furfenorex with rat-liver microsomes. The major metabolic routes of furfenorex in vitro were N-demethylation and N-defurfurylation which produced 1-phenyl-2-(N-furfurylamino)propane (furfurylamphetamine) and methamphetamine, respectively. The formation of furfurylamphetamine and methamphetamine were catalysed by rat-liver microsomes supplemented with NADPH and O2, and were inhibited by either SKF 525-A or CO. The formation of both metabolites were inhibited by 2-methyl-1,2-bis-(3-pyridyl)-1-propanone (metyrapone), but not by 7,8-benzoflavone. They were enhanced by pretreatment of rats with phenobarbitone, but not with 3-methylcholanthrene. These data suggested that N-demethylation and N-defurfurylation of furfenorex were mainly mediated by cytochrome P-450 but not cytochrome P-448.

In-vitro and in-vivo metabolism of the presynaptic dopamine agonist 3-PPP to a catecholic analogue in rats

The dopamine agonist 3-PPP and its enantiomers are hydroxylated in-vitro by rat liver microsomes to the catecholamine 3-(3,4-dihydroxyphenyl)-N-n-propylpiperidine (4-OH-3-PPP) with Km and Vmax values of about 1 microM and 2 nmol (mg protein)-1 min-1 respectively. As the catecholamine formed appears to be a good substrate for catechol-O-methyltransferase, in-vivo catecholamine formation in rats from 3-PPP was only detectable after inhibition of COMT by tropolone. The resulting brain levels of 4-OH-3-PPP, as measured by HPLC with electrochemical detection 45 min after administration, were about 350 pmol g-1 after i.p., and about 100 pmol g-1 after s.c. injection of 45 mumol kg-1 3-PPP, with no significant difference between racemic, ( + ) or (-) 3-PPP. It was estimated that these catecholamine levels represent about 1-5% of the 3-PPP levels after i.p., and about 0.2-0.5% after s.c. administration of 3-PPP. The relevance of this metabolic conversion of 3-PPP for its pharmacological profile is discussed.

Metabolism of methamphetamine, amphetamine and p-hydroxymethamphetamine by rat-liver microsomal preparations in vitro

Methamphetamine N-demethylation and p-hydroxylation activities of rat liver were located mainly in the microsomal fraction. The Km values for methamphetamine and p-hydroxymethamphetamine demethylations were 1.0 and 1.6 mM, respectively. The Km value for amphetamine p-hydroxylation was 10.2 microM; substrate inhibition occurred at high substrate concn. Two Km values were obtained for the aromatic hydroxylation of methamphetamine (10.6 microM and 2.2 mM). N-Demethylation of methamphetamine and p-hydroxymethamphetamine were depressed in rats pretreated with 3-methylcholanthrene, CoCl2 or SKF 525-A. In rats pretreated with phenobarbital, methamphetamine demethylase was induced and p-hydroxymethamphetamine demethylase was depressed. The p-hydroxylation of methamphetamine and amphetamine in rats pretreated with phenobarbital, CoCl2, SKF 525-A or iprindole were depressed.

The metabolism of 1-phenyl-2-(N-methyl-N-benzylamino)propane (benzphetamine) in vitro in rat

The metabolism of 1-phenyl-2-(N-benzylamino)propane (benzphetamine) in vitro was studied using rat-liver microsomes. Five metabolites were isolated from the incubation mixture and identified as 1-phenyl-2-(N-benzylamino)propane (benzylamphetamine), (1-(p-hydroxyphenyl)-2-(N-methyl-N-benzylamino)propane, 1-(p-hydroxyphenyl)-2-(N-benzylamino)propane, methamphetamine and amphetamine. This metabolism in vitro was compared with that in vivo which was reported previously. The formation of all five metabolites were catalysed by liver microsomes supplemented with NADPH and O2, and inhibited by either SKF 525-A or CO. N-Demethylation was inhibited by either 2-methyl-1,2-bis-(3-pyridyl)-1-propanone (metyrapone) or n-octylamine, while aromatic hydroxylation was inhibited by 7,8-benzoflavone and N-debenzylation was depressed by all these inhibitors. N-Demethylation was enhanced by pretreatment of rats with phenobarbitone, while aromatic hydroxylation was induced by pretreatment with 3-methylcholanthrene, and N-debenzylation was Induced by pretreatment with either phenobarbitone or 3-methylcholanthrene. These data suggested that the metabolism of benzphetamine was mediated by three slightly different enzyme systems.

Conversion of N-hydroxyamphetamine to phenylacetone oxime by rat liver microsomes

These in vitro studies indicate that N-oxidation of N-hydroxyamphetamine (NOHA) by rat liver homogenates yields phenylacetone oxime (PAOx) as the major metabolite. This oxidation was NADPH and oxygen dependent but was not appreciably increased in microsomes from phenobarbital-pretreated animals. The addition to microsomal incubations of superoxide dismutase (SOD), catalase (CAT), azide or mannitol did not alter the rate of oxidation, suggesting that O2-, H2O2, or OH' are not involved in this process. The reaction was minimally inhibited by a 2:1 ratio of CO/O2, and there was no significant reduction in the formation of product by the presence of diethylaminoethyl diphenylvalerate (SKF-525A) or 2,4-dichloro-6-phenylphenoxyethylamine (DPEA) in micromolar concentrations. Thus, although this NADPH-dependent N-oxidation pathway was catalyzed by rat hepatic microsomes, the data suggest that is was not a cytochrome P-450 mediated monooxygenase reaction.

Differences in the availability of d- and l-enantiomers after administration of racemic amphetamine to rats

1. Rats were treated with racemic amphetamine or separately with the single enantiomers. The two optical isomers were determined in several brain areas, in plasma and urine. 2. The concentration of d-enantiomer significantly exceeds that of the l-enantiomer in brain and plasma but not in urine, following administration of racemic amphetamine. In contrast, when the two isomers are given separately, their brain concentrations are similar. 3. Such a difference does not appear in the brain of mice treated with racemic amphetamine or in the brain of rats pre-treated with SKF 525-A, an inhibitor of amphetamine hydroxylation. 4. The possibility that the l-isomer can interfere with hydroxylation of d-amphetamine is discussed.

Stereotyped behavior and hyperthermia in dogs: correlation with the levels of amphetamine and p-hydroxyamphetamine in plasma and CSF

A gas-chromatographic method for simultaneously measuring p-hydroxyamphetamine (pOA) against amphetamine (A) in plasma and CSF is presented. The time course of body temperature (Tb), stereotyped behavior (St), and A and pOA levels in plasma and CSF were studied after administration of 0.6 and 1.5 mg/kg p.o. of A to dogs. Stereotyped behavior reached maximal value 2.5 h after A, as did levels of A in CSF. The A levels in CSF decreased steadily in the following hours and simultaneously with the levels of A in plasma. St remained elevated and began to decrease after 6.5 h. The relationship between St and amounts of A was not linear but exponential. This suggest that both A and its metabolite contributed to this effect. In fact, a linear relationship was found between St and the amounts of pOA in CSF. Body temperature had a time course similar to A plasma levels, reaching peak value after 1.5 h and declining thereafter simultaneously with A. A linear relationship was found between Tb and the amounts of A in plasma. Thus Tb seems to be a peripheral A effect related to the presence of the drug in plasma.

The stereoselective metabolism of ethylamphetamine with fortified rabbit liver homogenates

1. The liver enzyme systems of rabbits involved in the oxidative metabolism of ethylamphetamine were shown to be localized in the microsomal fraction. 2. The effect of substrate concentration, incubation period and storage of the microsomal preparations at 4 degrees was investigated. 3. The results indicate the involvement of at least two stereoselective enzyme systems or different conformations of binding of different substrates to the same enzyme system. R-(-)-Ethylamphetamine is the preferred substrate for N-oxidation and dealkylation, whereas the S-(+)-isomer is preferred for deamination. When racemic ethyl amphetamine is metabolized, the enantiomers act as independent compounds and compete for the enzymes. 4. Neither alpha-C- nor N-oxidation is induced by pre-treatment with phenobarbitone or 3-methylcholanthrene.

The metabolism of amphetamine in vitro by rabbit liver preparations: a comparison of R(-) and S(+) enantiomers

1. Incubation of R(-), S(+) and RS(+/-) amphetamines with rabbit liver 9000 g supernatant indicated that R(-) was metabolized at a faster rate than S(+), but that racemic amphetamine was metabolized at the same rate as S(+) during one hour incubations. 2. N-Hydroxyamphetamine and 1-phenyl-2-propranol were the major compounds detected in both R(-) and S(+) amphetamine incubations. 3. Phenylacetone oxime was detected in significant quantities after 3 h incubations of R(-) amphetamine, but only in minor quantities from S(+). 4. A fall in the amount of N-hydroxyamphetamine present in R(-) amphetamine incubations after a 3 h period as compared to a 1 h incubation, paralleled by a rise in the amount of phenylacetone oxime during 3 h suggested that the oxime was derived as a secondary metabolics from N-hydroxyamphetamine. 5. R(-) and S(+) N-hydroxyamphetamines were both metabolized to phenylacetone oxime by rabbit liver 9000 g supernatant, but the R(-) enantiomer was converted at a faster rate than S(+).

Metabolic and experimental factors in the behavioral response to repeated amphetamine

Previous studies have shown that repeated administration of d-amphetamine results in a progressive augmentation of locomotor activity and stereotypy. The present studies demonstrate that rats also exhibit an enhanced behavioral response following multiple daily injections of l-amphetamine and methylphenidate. Furthermore, behavioral augmentation is shown to persist for at least six days after a single injection of d-amphetamine. These results demonstrate the generality of the reverse tolerance phenomenon and indicate that metabolic factors, such as the formation of false neurotransmitters, do not account for the enhanced behavioral responsiveness observed with multiple injections of these drugs. The role of experiential factors in the behavioral augmentation was studied by (1) varying the amount of continuous exposure to the experimental environment prior to d-amphetamine administration, and (2) examining the effects of repeated injections of saline or d-amphetamine in different environments prior to testing in the experimental chambers. The results, which revealed a behavioral augmentation independent of pretreatment condition, indicate that neither acclimation to the test chamber nor state-dependent conditioning to external stimuli accounts for the enhanced locomotor activity and stereotypy observed with repeated administration of psychomotor stimulants.

Distribution and occurrence of amphetamine and p-hydroxyamphetamine in tissues of the rat after injection of d-amphetamine sulfate

The distribution of amphetamine and the formation and distribution of its principal metabolite p-hydroxyamphetamine after the injection of relatively small amounts of d-amphetamine was examined in the rat. Amphetamine entered all organs readily and was rapidly eliminated. p-Hydroxyamphetamine was rapidly synthesised and it is suggested that substrate inhibition of hepatic hydroxylating enzymes occurs. In the brain, amphetamine was homogeneously distributed between the seven brain regions examined whereas p-hydroxyamphetamine was predominantly concentrated in the striatum.

The p-hydroxylation of amphetamine and phentermine by rat liver microsomes

1. The products of p-hydroxylation of amphetamine and phentermine by two different preparations of rat liver microsomes were identified and quantitatively determined. At low concentrations (muM) significant proportions of the substrates were metabolized to the p-hydroxy derivatives by an NADPH-dependent system. The enzyme system was inhibited by higher substrate concentrations (mM) and was not induced by either phenobarbital or 3-methylcholanthrene. 2. The properties of this in vitro system are consistent with reports on in vivo studies of this reaction.