Behavioral and biochemical investigations of bupropion metabolites

https://doi.org/10.1016/S0014-2999(03)02010-7Get rights and content

Abstract

The stimulus effects of bupropion metabolites were examined in a drug discrimination procedure using (−)nicotine- and (+)amphetamine-trained rats. (+)- and (−)threohydrobupropion partially substituted in each group. R,R-hydroxybupropion produced vehicle-appropriate responding in (−)nicotine animals but, when given in combination with the training dose of (−)nicotine, resulted in an attenuated effect. S,S-Hydroxybupropion partially (66%) substituted for (−)nicotine. In (+)amphetamine-trained animals, S,S-hydroxybupropion (ED50=4.4 mg/kg) generalized completely and was similar in potency to bupropion (ED50=5.4 mg/kg). Bupropion and its metabolites lacked affinity for nicotinic acetylcholinergic receptors, but all antagonized (−)nicotine-induced 86Rb+ efflux in cells expressing α3β4 nicotinic cholinergic receptors. S,S-Hydroxybupropion possessed affinity at the dopamine transporter comparable to bupropion, and was also found to bind at the norepinephrine transporter. Although it is unlikely that any metabolite isomer is chiefly responsible for the stimulus actions of bupropion, some probably play a role in the complex actions of this agent.

Introduction

Bupropion is an antidepressant medication and an agent used in the treatment of nicotine dependence (i.e., as an adjunct in smoking cessation therapy). Its mechanism of action, in both instances, is unknown. Current thinking is that bupropion might produce some of its effects via inhibition of biogenic amine reuptake; however, it is not particularly potent in this regard Ascher et al., 1995, Horst and Preskorn, 1998, Sanchez and Hyttel, 1999. Another more recently proposed theory is that the mechanism of action of bupropion, both as an antidepressant and in the treatment of nicotine dependence, might involve nicotinic acetylcholine (nACh or nicotinic) receptors that are linked to dopamine and norepinephrine release Fryer and Lukas, 1999, Miller et al., 2002, Slemmer et al., 2000. For example, bupropion inhibits nicotine-evoked [3H]dopamine and [3H]norepinephrine overflow from superfused striatal and hippocampal slices, respectively (Miller et al., 2002). In mice, bupropion antagonizes the antinociceptive, motor, hypothermic, and convulsive effects of nicotine (Slemmer et al., 2000). On the other hand, Young and Glennon (2002) and Wiley et al. (2002) have shown that stimulus substitution occurs in dose-dependent fashion when bupropion is administered to rats trained to distinguish (−)nicotine from saline vehicle in a drug discrimination task. Bupropion, then, might owe its therapeutic effectiveness to its ability to antagonize certain actions of nicotine while, at the same time, being able to mimic others. Bupropion itself, however, does not bind at α4β2 nicotinic receptors—the major population of nicotinic acetylcholinergic receptors in the brain—and its stimulus effects, unlike those of nicotine, are not attenuated by the noncompetitive nicotinic cholinergic antagonist mecamylamine Wiley et al., 2002, Young and Glennon, 2002.

A potentially important question that needs to be addressed is whether any of the above actions are due to specific optical isomers of bupropion or, perhaps, to a bupropion metabolite. Bupropion is a chiral substance, existing both as (+)- and (−)enantiomers; the racemic mixture is employed clinically. Although bupropion has been resolved, the optical isomers rapidly racemize even under neutral conditions Musso et al., 1993, Fang et al., 2000 suggesting that the observed effects of bupropion in vivo are probably not related to one of its individual enantiomers. Bupropion is rapidly and completely absorbed, and is extensively metabolized in vivo (Rotzinger et al., 1999); less than 10% of a bupropion dose is excreted unchanged (Lai and Schroeder, 1983). Pathways of bupropion metabolism involve hydroxylation of the tertiary-butyl group (with or without subsequent cyclization) and/or reduction of the carbonyl group to an alcohol (Schroeder, 1983). There are significant species differences in the metabolism of bupropion (Horst and Preskorn, 1998), but in humans, the two major metabolites are a phenylmorpholinol, hydroxybupropion (BW 306U), and an aminoalcohol, threohydrobupropion (sometimes referred to as threodihydrobupropion, and also known as R,R-2-(tert-butylamino)-1-(3-chlorophenyl)propanol and BW A494U) Rotzinger et al., 1999, Schroeder, 1983, Suckow et al., 1986, Welch et al., 1987. Another human metabolite, although formed in lesser amounts than the others, is erythrohydrobupropion. The same metabolites have been identified in special patient populations such as smokers (Hsyu et al., 1997), alcoholics (DeVane et al., 1990), and the elderly (Sweet et al., 1995). Bupropion metabolites can accumulate in plasma and achieve levels from 10 to 100 times greater than that of the administered agent Cooper et al., 1984, Suckow et al., 1986. These metabolites would seem to be prime candidates for investigation.

Metabolites of bupropion are not without pharmacologic action. In fact, the pharmacology of certain metabolites has seen some investigation and it has been suggested that the antidepressant actions of bupropion might be due to, or receive contribution(s) from, one or more bupropion metabolites Martin et al., 1990, Rotzinger et al., 1999, Young, 1991. Evidence suggests that certain bupropion metabolites might contribute to the antidepressant actions of bupropion Rotzinger et al., 1999, Young, 1991. For example, bupropion is active in several assays indicative of antidepressant action and hydroxybupropion is apparently more “antidepressant” than bupropion (Martin et al., 1990). In locomotor activity tests, bupropion produced dose-related increases in motor behavior in mice but hydroxybupropion and threohydrobupropion, tested at the same doses as bupropion, produced biphasic effects: the lowest doses produced increases in motor counts and the highest dose produced a decrease in activity (Martin et al., 1990). In biochemical assays hydroxybupropion, like bupropion, is a weak inhibitor of monoamine reuptake (Sanchez and Hyttel, 1999), but a fluoro derivative of a hydroxybupropion-related morpholinol is >30 times more potent than bupropion as an inhibitor of norepinephrine reuptake (Kelley et al., 1996).

The purpose of the present investigation was primarily two-fold. First, because bupropion substitutes for nicotine in rats trained to discriminate (−)nicotine from vehicle Wiley et al., 2002, Young and Glennon, 2002, we synthesized the individual optical isomers of bupropion metabolites (i.e. hydroxybupropion and threohydrobupropion; see Fig. 1 for chemical structures) and examined them in tests of stimulus generalization to determine if they might produce (or be responsible for) the nicotine-like stimulus effects of bupropion. Second, because bupropion possesses some stimulant character, bupropion and isomers of several of its metabolites were also examined in rats trained to discriminate (+)amphetamine from vehicle. Because bupropion is thought to produce some of its effects via nicotinic cholinergic receptors or via transporters, we took advantage of the availability of the metabolite isomers by submitting them to the NIMH Psychoactive Drug Screening Program for examination at several populations of nicotinic receptors and at the dopamine, norepinephrine, and dopamine transporters. Erythrohydrobupropion, however, was not examined because it lacks of significant bupropion-like actions (e.g. Martin et al., 1990) and because it is formed to a lesser extent than threohydrobupropion Posner et al., 1985, Rotzinger et al., 1999, Suckow et al., 1986.

Section snippets

Animals

The subjects were 16 male Sprague–Dawley rats (Charles River Laboratories) weighing 250–300 g at the beginning of the study. Nine animals were trained to discriminate 0.6 mg/kg (as the free base) of (−)nicotine, and seven animals were trained to discriminate 1.0 mg/kg of (+)amphetamine sulfate as previously described Glennon et al., 1995, Young and Glennon, 2002. In brief, the animals were housed individually and, prior to the start of the study, their body weights were reduced to approximately

Nicotine-trained animals

A group of nine animals was trained to discriminate 0.6 mg/kg of (−)nicotine from saline vehicle (ED50=0.11 mg/kg; 95% CL=0.06–0.20 mg/kg) (Fig. 2). The animals' response rates following administration of saline or the training dose of (−)nicotine were similar (i.e. approximately 20 resp/min). Seven doses of (−)threohydrobupropion (3–21 mg/kg) were examined in the (−)nicotine-trained rats; a dose of 16 mg/kg produced 43% (−)nicotine-appropriate responding (Fig. 2), and administration of higher

Discussion

The phenylmorpholinol hydroxybupropion possesses two chiral centers and can exist as two pairs of enantiomers. In theory, all four isomers are possible, but it is believed that the actual metabolite is a mixture of the S,S- and R,R-isomers (Fang et al., 2001) (see Fig. 1). Most pharmacological studies have simply identified or utilized hydroxybupropion without mention of its stereochemistry. In 1997 Suckow et al. (1997) reported that the actual human metabolite is predominantly the R,R isomer.

Acknowledgements

This work was supported in part by DA 01642 and DA 05274. We also wish to acknowledge the NIMH Psychoactive Drug Screening Program for providing the biochemical results on the bupropion metabolite isomers.

References (34)

  • D. Finney

    Probit Analysis

    (1952)
  • J.D. Fryer et al.

    Noncompetitive functional inhibition at diverse, human nicotinic acetylcholine receptor subtypes by bupropion, phencyclidine, and ibogaine

    J. Pharmacol. Exp. Ther.

    (1999)
  • P.H. Hsyu et al.

    Pharmacokinetics of bupropion and its metabolites in cigarette smokers versus nonsmokers

    J. Clin. Pharmacol.

    (1997)
  • J.L. Kelley et al.

    (2S,3S,5R)-2-(3,5-Difluorophenyl)-3,5-dimethyl-2-morpholinol: a novel antidepressant agent and selective inhibitor of norepinephrine uptake

    J. Med. Chem.

    (1996)
  • A.A. Lai et al.

    Clinical pharmacokinetics of bupropion: a review

    J. Clin. Psychiatry

    (1983)
  • P. Martin et al.

    Antidepressant profile of bupropion and three metabolites in mice

    Pharmacopsychiatry

    (1990)
  • D.K. Miller et al.

    Bupropion inhibits nicotine-evoked [3H]overflow from rat striatal slices preloaded with [3H]dopamine and from rat hippocampal slices preloaded with [3H]norepinephrine

    J. Pharmacol. Exp. Ther.

    (2002)
  • Cited by (0)

    View full text