NP5526_proof ■ 24 June 2014 ■ 1/22 Neuropharmacology xxx (2014) 1e22 55 Contents lists available at ScienceDirect 56 57 Neuropharmacology 58 59 60 journal homepage: www.elsevier.com/locate/neuropharm 61 62 63 64 65 1 Dopamine reuptake transporter (DAT) “inverse agonism” e Anovel 66 2 67 3 hypothesis to explain the enigmatic pharmacology of cocaine 68 4 * 69 5 Q5 David J. Heal , Jane Gosden, Sharon L. Smith 70 6 71 RenaSci Limited, BioCity, Pennyfoot Street, Nottingham NG1 1GF, UK 7 72 8 73 9 article info abstract 74 10 75 11 76 Article history: The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. 12 Available online xxx However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, 77 13 which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable 78 14 Keywords: subjective effects such as drug “highs” or euphoria in drug-experienced human volunteers. Moreover 79 15 Cocaine they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate 80 16 Dopamine reuptake inhibitor that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed 81 Dopamine releasing agent 17 monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to 82 Dopamine transporter fi 18 Inverse agonist conclude that the highly unusual, stimulant pro le of cocaine and related compounds, eg methylphe- 83 19 DAT inverse agonist nidate, is not mediated by monoamine reuptake inhibition alone. 84 We describe the experimental findings which suggest cocaine serves as a negative allosteric 20 Novel mechanism 85 21 modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of fi 86 22 transport. This results in a ring-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric 87 23 modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of 88 24 inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the 89 25 action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an in- 90 26 verse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a 91 27 new class of monoaminergic drugs; DAT “inverse agonists”. 92 28 This article is part of a Special Issue entitled ‘CNS Stimulants’. 93 © 29 2014 Elsevier Ltd. All rights reserved. 94 30 95 31 96 32 97 33 98 1. Introduction 10-fold more potent as a dopamine reuptake inhibitor (Cheetham 34 99 et al., 1993, 1996; Heal et al., 1998b) than cocaine (Kula and 35 100 It was our experience in developing the centrally-acting, anti- Baldessarini, 1991; Hyttel, 1982; Richelson and Pfenning, 1984; 36 ® ® 101 obesity drug, sibutramine (Meridia , Reductil ), which raised Rothman et al., 2001) and very rapidly penetrated the brain (Heal 37 102 serious doubts about the hypothesis that cocaine's pharmacological et al., 1992), fail to mimic the psychostimulant actions of cocaine? 38 103 effects were solely mediated by monoamine reuptake inhibition. As a consequence, an enormous amount of preclinical and clinical 39 104 Sibutramine is an interesting molecule because it contains the b- testing was performed to attempt to demonstrate that sibutramine 40 105 phenylethylamine substructure that is present in many monoamine was pharmacologically different from both D-amphetamine and 41 106 releasing agents, eg D-amphetamine, methamphetamine and cocaine. The most compelling evidence came from studies in drug- 42 107 MDMA. In addition, sibutramine's active metabolites inhibited the experienced human volunteers where sibutramine did not produce 43 108 reuptake of noradrenaline (norepinephrine), 5-hydroxytryptamine positive effects, eg “Drug liking”, “High”, “Euphoria” or “Want to 44 109 (5-HT, serotonin) and dopamine (Cheetham et al., 1993, 1996; Heal take drug again” (Cole et al., 1998; Schuh et al., 2000). On the 45 110 et al., 1998b), which raised the question of its pharmacological contrary, sibutramine produced no discernable subjective effects at 46 111 similarity to cocaine. How could sibutramine, which was in effect low dose and was dysphoric and aversive at high dose (Cole et al., 47 112 1998; Schuh et al., 2000). 48 113 Subsequently, it has emerged that the clinical findings with 49 114 sibutramine were not an anomaly, and have now been replicated 50 * Corresponding author. Tel.: þ44 (0) 115 9124260; fax: þ44 (0) 115 9124263. 115 many times with other monoamine reuptake inhibitors, eg bupro- 51 E-mail addresses: [email protected] (D.J. Heal), [email protected] 116 (S.L. Smith). pion, mazindol and tesofensine. Furthermore, in vivo experiments in 52 117 53 http://dx.doi.org/10.1016/j.neuropharm.2014.06.012 118 54 0028-3908/© 2014 Elsevier Ltd. All rights reserved. 119 Please cite this article in press as: Heal, D.J., et al., Dopamine reuptake transporter (DAT) “inverse agonism” e A novel hypothesis to explain the enigmatic pharmacology of cocaine, Neuropharmacology (2014), http://dx.doi.org/10.1016/j.neuropharm.2014.06.012 NP5526_proof ■ 24 June 2014 ■ 2/22 2 D.J. Heal et al. / Neuropharmacology xxx (2014) 1e22 1 animals have revealed that cocaine's monoaminergic pharmacology following sections, we will critically assess validity of the evidence 66 2 is profoundly different from that of clinically used monoamine re- in support of the DAT inhibitor hypothesis for cocaine. 67 3 uptake inhibitor drugs with the exception of methylphenidate. 68 4 Together, these observations have led us to the conclusion that the 2.1. All dopamine reuptake inhibitors are stimulant euphoriants 69 5 highly unusual, stimulant profile of cocaine and related compounds 70 6 is not mediated by reuptake inhibition alone. In this review, we put Although observations made using animal models have excel- 71 7 forward the hypothesis that cocaine allosterically modulates the lent predictive validity with regard to the recreational abuse lia- 72 8 function of the dopamine reuptake transporter (DAT) to reverse its bility of compounds from many pharmacological classes, the 73 9 direction of transport, resulting in a firing-dependent retrotransport monoamine reuptake inhibitors are problematic in this respect (see 74 10 of dopamine into the synaptic cleft. The proposed action of cocaine review by Freeman et al., this issue). In recent years, there has been 75 11 is, therefore totally different from that of related, small molecule, increasing pressure from regulators to explore recreational abuse 76 12 negative allostereric modulators of monoamine reuptake trans- liability of novel CNS-active drug-candidates in drug-experienced 77 13 porters, eg SoRI-6238, which only reduce the rate of inward trans- human volunteers. From these observations, there is a growing 78 14 port (Nandi et al., 2004). Since the physiological role of DAT is to body of definitive information from reuptake inhibitors with a widely 79 15 remove dopamine from the synapse, we have postulated that co- differing balance of reuptake inhibitory actions across the spectrum 80 16 caine's effect is analogous to that of an inverse agonist. If this hy- of monoamines that sheds light on the question of whether all DAT 81 17 pothesis proves to be valid then cocaine is the prototypical inhibitors have the capacity to be stimulant euphoriants. 82 18 compound that exemplifies a new class of monoaminergic drugs; As shown in Table 1, cocaine, which is a non-selective mono- 83 19 the DAT “inverse agonists”. amine reuptake inhibitor (Hyttel, 1982; Richelson and Pfenning, 84 20 1984; Andersen, 1989), unequivocally and reproducibly produces 85 21 2. Exploring the myths pleasurable energisation (stimulation), and feelings of wellbeing 86 22 and elation (euphoria) that are reflected in subjective reports of 87 23 When a drug has been the subject of scientific research for de- “drug liking”. 88 24 cades, there is often a prevailing view that its mechanism of action Methylphenidate, which is a catecholamine reuptake inhibitor 89 25 has been fully elucidated and there is nothing new to be learnt. In (Hyttel, 1982; Richelson and Pfenning, 1984; Andersen, 1989), also 90 26 this situation there is a tendency for a hypothesis that is based on evokes subjective effects in drug-experienced human volunteers 91 27 sound scientific observations to become dogma. Dogma which is and normal healthy subjects that are similar to those produced by 92 28 often accompanied by an unwillingness to concede that when the the highly abused psychostimulants, i.e. cocaine or amphetamine 93 29 facts no longer fit the hypothesis, it is the hypothesis and not the (Martin et al., 1971; Smith and Davis, 1977; Heishman and 94 30 facts that needs to be re-evaluated. Cocaine may turn out to be Henningfield, 1991; Rush et al., 2001; Rush and Baker, 2001; see 95 31 another classic example. also reviews by Kollins et al., 2001; Huss and Lehmkuhl, 2002; Klein 96 32 Although it was already well established that cocaine potenti- Schwartz, 2002; Kollins, 2003, Table 1). Specifically, under “blin- 97 33 ated the effects of noradrenaline in peripheral tissues and this ef- ded” experimental conditions, methylphenidate has been reported 98 34 fect was pronounced in areas where there was dense sympathetic to generalise to the cocaine cue in normal human volunteers and 99 35 innervation (Trendelenburg, 1965, 1966, 1969), it was the seminal stimulant abusers who have been trained to discriminate between 100 36 work of Iversen and colleagues that led to the discovery that cocaine and placebo (Rush and Baker, 2001; Rush et al., 2002b). 101 37 cocaine was a competitive inhibitor of high-affinity, noradrenaline These subjects also reported levels of “drug liking” and “stimula- 102 38 uptake transport (Uptake-1) in the heart.