Application of quantitative GC-mass spectrometry to study of pharmacokinetics of amphetamine and phentermine

A Chemical Perspective of Pharmacology and Toxicology

My chemical training provided a somewhat different perspective of biolo-gical problems, in the problem itself and approaches to its solution. I was fortunate to have in my laboratory postdocs and students who shared this perspective and used appropriate tools to address problems in amphetamine pharmacology and air pollution toxicology. These apparently disparate areas of research shared two chemical reactions: prooxidant-based generation of reactive oxygen and formation of covalent bonds between electrophiles and biological nucleophiles. This article is an attempt to summarize that research and to identify those individuals who made the contributions.

The neurotoxic mechanisms of amphetamine: Step by step for striatal dopamine depletion

Amphetamine (AMPH) is a commonly abused psychostimulant that induces neuronal cell death/degeneration in humans and experimental animals. Although multiple neurotoxic mechanisms of AMPH have been intensively investigated, the interplay between these mechanisms has remained elusive. In this study, we used a rat model of AMPH-induced long-lasting striatal dopamine (DA) depletion and identified mechanisms of neurotoxicity, energy failure, excitotoxicity, and oxidative stress. Pretreatment with nicotinamide (NAM, a co-factor for the electron transport chain) blocked AMPH-induced free radical formation, energy failure, and striatal DA decrease. Also, MK-801 (a NMDA receptor antagonist) blocked AMPH-induced free radical formation and striatal DA but not energy failure decrease, indicating excitotoxicity may occur before free radical formation and after energy failure. Thus, these results show that during AMPH intoxication, energy failure, excitotoxicity, and free radical formation are orchestrated consecutively to mediate the depletion of striatal DA.

Amphetamine treatment during early postnatal development transiently restricts somatic growth

AIMS

Restricted somatic growth during fetal or early postnatal periods has been suggested to serve as a predictive indicator for neuroanatomical changes and behavioral impairments during adulthood. Here, the effects of d-amphetamine sulfate (AMPH) exposure during the brain growth spurt period on this potential indicator were evaluated.

MAIN METHODS

Rats received 0, 5, 15 or 25mg/kg/day of AMPH via two daily intragastric intubations from PD4-9. Body weight data were collected every other day from PD1 to 21, and then weekly until PD59. On PD9, a subset of animals was terminated 90min after the last amphetamine treatment and the weights of the cortex, cerebellum, and brainstem were collected. Weights of these brain regions from young adult rats were also assessed on PD68.

KEY FINDINGS

AMPH exposure during early postnatal development limited somatic growth in a dose-related manner, with significantly lower body weights in animals assigned to the AMPH 25 and AMPH 15 groups. However, this was transient in nature, with no significant difference in weight observed after pups were weaned on PD21. Further, no differences in brain weight were observed at either age as a result of AMPH exposure.

SIGNIFICANCE

These findings support the idea that developmental AMPH exposure transiently restricts somatic growth. Moreover, the lack of effect on brain weight shows that AMPH differentially affects somatic and brain growth. The current findings suggest that in addition to the immediate effects on body weight, amphetamine may alter the rate of growth, and increase the risk for weight-related adult diseases.

GBR 12909 administration as a mouse model of bipolar disorder mania: mimicking quantitative assessment of manic behavior

RATIONALE

Mania is a core feature of bipolar disorder (BD) that traditionally is assessed using rating scales. Studies using a new human behavioral pattern monitor (BPM) recently demonstrated that manic BD patients exhibit a specific profile of behavior that differs from schizophrenia and is characterized by increased motor activity, increased specific exploration, and perseverative locomotor patterns as assessed by spatial d.

OBJECTIVES

It was hypothesized that disrupting dopaminergic homeostasis by inhibiting dopamine transporter (DAT) function would produce a BD mania-like phenotype in mice as assessed by the mouse BPM.

METHODS

We compared the spontaneous locomotor and exploratory behavior of C57BL/6J mice treated with the catecholamine transporter inhibitor amphetamine or the selective DAT inhibitor GBR 12909 in the mouse BPM. We also assessed the duration of the effect of GBR 12909 by testing mice in the BPM for 3 h and its potential strain dependency by testing 129/SvJ mice.

RESULTS

Amphetamine produced hyperactivity and increased perseverative patterns of locomotion as reflected in reduced spatial d values but reduced exploratory activity in contrast to the increased exploration observed in BD patients. GBR 12909 increased activity and reduced spatial d in combination with increased exploratory behavior, irrespective of inbred strain. These effects persisted for at least 3 h.

CONCLUSIONS

Thus, selectively inhibiting the DAT produced a long-lasting cross-strain behavioral profile in mice that was consistent with that observed in manic BD patients. These findings support the use of selective DAT inhibition in animal models of the impaired dopaminergic homeostasis putatively involved in the pathophysiology of BD mania.

Activations of c-fos/c-jun signaling are involved in the modulation of hypothalamic superoxide dismutase (SOD) and neuropeptide Y (NPY) gene expression in amphetamine-mediated appetite suppression

Amphetamine (AMPH) is known as an anorectic agent. The mechanism underlying the anorectic action of AMPH has been attributed to its inhibitory action on hypothalamic neuropeptide Y (NPY), an appetite stimulant in the brain. This study was aimed to examine the molecular mechanisms behind the anorectic effect of AMPH. Results showed that AMPH treatment decreased food intake, which was correlated with changes of NPY mRNA level, but increased c-fos, c-jun and superoxide dismutase (SOD) mRNA levels in hypothalamus. To determine if c-fos or c-jun was involved in the anorectic response of AMPH, infusions of antisense oligonucleotide into the brain were performed at 1 h before daily AMPH treatment in freely moving rats, and the results showed that c-fos or c-jun knockdown could block this anorectic response and restore NPY mRNA level. Moreover, c-fos or c-jun knockdown could partially block SOD mRNA level that might involve in the modulation of NPY gene expression. It was suggested that c-fos/c-jun signaling might involve in the central regulation of AMPH-mediated feeding suppression via the modulation of NPY gene expression.

Effects of anorectic drugs on food intake under progressive-ratio and free-access conditions in rats

The effects of two anorectic drugs, dexfenfluramine and phentermine, on food intake under different food-access conditions were examined. Experiment 1 compared the effects of these drugs on food intake under a progressive-ratio (PR) schedule and free-access conditions. Dexfenfluramine decreased food intake under both conditions, but the doses required to decrease intake under free-access conditions were higher than those required to reduce intake under the PR condition. Intermediate doses of phentermine sometimes increased breaking points, and higher doses decreased them. Phentermine decreased food intake at the same doses under both access conditions. Thus the potency of dexfenfluramine, but not phentermine, to decrease food-maintained behavior depended upon the food-access condition. Experiment 2 used a novel mixed progressive-ratio schedule of food delivery to study the duration of drug effects. Sessions consisted of five components separated by 3-hr timeouts. The ratio requirement reset at the beginning of each component and a new breaking point was obtained. Both dexfenfluramine and phentermine dose-dependently decreased breaking points early in the session. In some rats, compensatory increases in breaking point were observed. That is, breaking points later in the session increased over control levels, resulting in no change in the total number of food pellets earned for the session compared to control. The present findings suggest that the effects of some anorectic drugs depend upon the access conditions for food; increasing the effort to obtain food may enhance their ability to decrease food-maintained behavior.

Fluorometric determination of DL-fenfluramine, DL-norfenfluramine and phentermine in plasma by achiral and chiral high-performance liquid chromatography

High-performance liquid chromatography (HPLC) with fluorescence detection has been developed for the simultaneous determination of sympathomimetic amines including ephedrine, norephedrine, 2-phenylethylamine, 4-bromo-2,5-dimethoxyphenylethylamine, phentermine (Phen) and DL-fenfluramine (Fen) in spiked human plasma. Furthermore, an enantioselective HPLC method for the separation of D-Fen (dexfenfluramine) and L-Fen (levofenfluramine) in addition to their active metabolites D- and L-norfenfluramine (Norf) is described. The detection was achieved at emission wavelength of 430 nm with excitation wavelength of 325 nm for both methods. The analytes were extracted from plasma (100 microl) at pH 10.6 with ethyl acetate using fluoxetine as the internal standard. The extracts were evaporated and derivatized with the fluorescence reagent 4-(4,5-diphenyl-1H-imidazole-2-yl)benzoyl chloride in the presence of carbonate buffer (pH 9.0). A gradient separation was achieved on a C18 column for the achiral separation or on a Chiralcel OD-R column for the chiral separation. The methods were fully validated, and shown to have excellent linearity, sensitivity and precision. The chiral method has been applied for the determination of D- and L-enantiomers of Fen and Norf, in addition to Phen in rat plasma after an intraperitoneal administration of DL-Fen and Phen, simultaneously.

Simultaneous determination of fenfluramine and phentermine in urine using gas chromatography mass spectrometry with pentafluoropropionic anhydride derivatization

Fenfluramine and phentermine ('fen-phen') are stimulants used primarily for weight loss that have a causative association with serious health problems. Though voluntarily removed from the market by their manufacturers and the FDA in September of 1997, both drugs occasionally reappear in the clinical and forensic setting from individuals who retained old prescriptions, transported the drugs from foreign countries, or 'stockpiled' the medications immediately before their removal from the market. The authors describe an analytical method for simultaneous detection of fenfluramine, phentermine, and the internal standard, N-propylamphetamine, in urine using pentafluoropropionic anhydride derivatization. Detection using and mass spectrometry is described. Baseline resolution of the analytes was achieved in the presence of four other common sympathomimetic amines. The increased molecular weights, better peak profiles, and characteristic fragments containing portions of both derivatizing agent and parent compound aid in the unambiguous identification of these analytes.

Caudate-putamen dopamine and stereotypy response profiles after intravenous and subcutaneous amphetamine

We compared the behavioral and caudate-putamen extracellular dopamine responses following intravenous (3.6 mg/kg) and subcutaneous (8 mg/kg) amphetamine administration using 2-min microdialysate sampling intervals, and doses of the drug selected to achieve comparable maximal brain concentrations. Following intravenous amphetamine, dopamine peaked within the first 2 min, then declined with a first-order decay rate of 0.018+/-0.007 min(-1). Following subcutaneous amphetamine, dopamine achieved maximum concentrations at 9 min and remained near peak levels for about 30 min before declining with a first-order decay rate of 0.019+/-0.008 min(-1). Maximal brain amphetamine levels and peak dopamine concentrations were equivalent following either route of drug administration. In contrast to the short latency to maximal extracellular dopamine, the onset of oral stereotypies was delayed until about 30 min following both routes of drug administration. Furthermore, in contrast to the behavioral response to amphetamine, apomorphine administration resulted in the rapid appearance of oral stereotypies within 5-10 min after drug administration. These results suggest that although caudate-putamen dopamine receptor activation may be a critical factor in the expression of focused oral stereotypies, other effects of amphetamine may interfere with the ability of animals to exhibit these behaviors.

Amphetamine induces hydroxyl radical formation in the striatum of rats

Amphetamine-induced hydroxyl radical formation in the striatum of rats was investigated in this study. With the utilization of the microdialysis and HPLC-ECD, the striatal dopamine (DA) release and the formation of 2,3-dihydroxybenzoic acid (2,3-DHBA), derived from the reaction of hydroxyl radicals (.OH) and salicylate in perfusion, were monitored and detected during desipramine and/or amphetamine (AMPH) administration. Our data revealed that after desipramine treatment AMPH injections not only amplified striatal DA release and 2,3-DHBA formation, but also intensified the stereotyped behaviors induced by AMPH. Furthermore, we discovered that alpha-methyl-para-tyrosine (alpha-MT) pretreatment prevented the onset of the above responses. In desipramine-treated rats, the tissue homogenization study demonstrated that a single dose of AMPH produced long-term depletion of striatal DA; this was not seen in saline-treated rats. Moreover, striatal DA depletion could be lessened by pretreatment with mannitol, a .OH scavenger. These results indicate that AMPH-induced striatal .OH formation might be DA-related in desipramine-treated rats, and suggest that .OH formation might be correlated with AMPH-induced neurodegeneration.

Computer analyzed EEG in amphetamine-responsive hyperactive children

Electroencephalographic (EEG) frequency analysis and evoked potentials (EPs) of 27 hyperactive children were examined to determine which, if any, of these electrophysiological parameters might be useful for the selection of those children likely to respond to stimulant medication. The children were treated with placebo, d-amphetamine, and l-amphetamine in double-blind fashion and in random order. EEGs for frequency analysis, visual EPs, and auditory EPs were obtained before and during treatment. Off medication, hyperactive responders to amphetamine had higher predominant beta frequency and shorter latencies of some EP waves than did nonresponders. Although the findings reported here would not select all responders, they would allow the exclusion of most nonresponders.

Kinetics of distribution of amphetamine in cats

The distribution and metabolic fate of amphetamine were studied in cats. In the brain, high levels of drug were detected in the grey matter structures at short intervals after administration, while at longer intervals distribution between white and grey matter areas was more uniform. In peripheral tissues the greatest concentration of the drug was seen in the highly vascularized organs. Para-hydroxy-amphetamine was found in minimal amounts in the liver and kidneys and only at trace quantities in the brain.

Pharmacokinetics of methylphenidate in the rat using single-ion monitoring GLC-mass spectrometry

A GLC-mass spectrometric assay for methylphenidate in biological fluids was developed using the ethyl ester homolog of the drug as the internal standard. The procedure has a lower level of sensitivity of 1.2 ng/ml and is based on GLC-mass spectrometic monitoring of the m/e 180 ion common to the mass spectra of the N-trifluoroacetyl derivatives of the drug and internal standard. The brain and plasma levels of methylphenidate in rats were determined after intravenous administration of 0.5 mg/kg of the drug. The two-compartment open pharmacokinetic model fit the data.

Specific deuteromethylation by the Eschweiler-Clarke reaction. Synthesis of differently labelled variants of trimethylamine and their use for the preparation of labelled choline and acetylcholine

Reductive methylation (the Eschweiler-Clarke reaction) was used as a route for the synthesis of differently deuterium labelled variants of trimethylamine with the ultimate aim of preparing labelled variants of choline and acetylcholine. Combinations of unlabelled and labelled formaldehyde and formic acid yielded symmetrically labelled trimethylamines of high isotopic purity. The labelled congeners to acetylcholine and choline that were prepared subsequently provide suitable internal standards and tracers to be used in mass spectral analysis and in the evaluation of the pharmacokinetics of the parent compounds.

The identification and analysis of the metabolic products of mephentermine

Phentermine (Ib), N-hydroxymephentermine (Ic) and N-hydroxyphentermine (Id) were identified as metabolic products after in vitro incubation of mephentermine (Ia) with rabbit liver microsomal fractions. Compounds Ia, Ib and Ic were also identified as excretion products in the urine of a human subject given a single dose of mephentermine (Ia) sulphate. Derivatization with acetic anhydride, trifluoroacetic anhydride and the trimethylsilyl donor reagent N,O-bis-(trimethylsilyl)-trifluoroacetamide (BSTFA) or hexamethyldisilazane (HMDS) were used for qualitative identification of the metabolic products Ib-Id by g.l.c.-mass spectrometry and for quantitative determination of Ia-Id after extraction from rabbit hepatic homogenates. The synthesis of N-hydroxymephentermine (Ic) and the properties of the metabolic products are reported.

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.

Submicrogram assay for scopolamine in plasma and urine

A GLC-mass spectrometric method for scopolamine, sensitive to 50 pg/ml for a 4-ml plasma or urine sample, was developed. The method used a deuterated internal standard to minimize variability in absolute recovery in the extraction procedure. Scopoline and deuterated scopoline were formed from the base-catalyzed hydrolysis of scopolamine and the internal standard and were analyzed as the heptafluorobutyrates, using a GLC-mass spectrometric system by monitoring the m/e 138 and 141 fragments, respectively.