Positive allosteric modulation of the human cannabinoid (CB₁) receptor by RTI‐371, a selective inhibitor of the dopamine transporter

Abstract

Background and purpose: In our search for an indirect dopamine agonist as therapy for cocaine addiction, several selective inhibitors of the dopamine transporter (DAT), which are 3-phenyltropane analogues, were assayed for their effect on locomotor activity in mice. Interestingly, several of the compounds showed a poor correlation between stimulation of locomotion and DAT inhibition. One of the compounds, 3b-(4-methylphenyl)-2b-[3-(4-chlorophenyl)isoxazol-5-yl]tropane (RTI-371), was shown to cross the blood-brain barrier, by binding studies in vivo, and block cocaine-induced locomotor stimulation. As poor pharmacokinetics could not explain the behavioural effects of RTI-371, this compound was screened through our functional assays for activity at other CNS receptors. Initial screening identified RTI-371 as a positive allosteric modulator of the human CB1 (hCB1) receptor. Experimental approach: The effect of RTI-371 and other DAT-selective inhibitors on CP55940-stimulated calcium mobilization was characterized in a calcium mobilization-based functional assay for the hCB1 receptor. Selected compounds were also characterized in a similar assay for human m opioid receptor activation to assess the specificity of their effects. Key results: RTI-371 and several other DAT-selective inhibitors with atypical actions on locomotor behaviour increased the efficacy of CP55940 in a concentration-dependent manner. Conclusions and implications:These results suggest that the lack of correlation between the DAT-binding affinity and locomotor stimulation of RTI-371 could be due at least in part to its activity as a positive modulator of the hCB1 receptor. ( British Journal of Pharmacology

2-Isoxazol-3-Phenyltropane Derivatives of Cocaine: Molecular and Atypical System Effects at the Dopamine Transporter

“…Although minimal requirements of affinity for sR and the DAT for antagonism of cocaine self-administration are currently unclear, the ratios of affinity at sR for RTI-336 and RTI-371 are similar and are substantially lower than those for rimcazole and its analogs, casting doubt on a role for sR in the present effects. Navarro et al (2009) proposed that unique effects of RTI-371 and the BZT analog JHW007 were due at least in part to positive allosteric modulation of CB 1 R. If a positive allosteric modulation of CB 1 R indeed modified the otherwise stimulant effects of RTI-371, greater stimulant effects in CB 1 R KO mice compared with WT mice would be expected. Across the range of doses studied, the effects of RTI-371 did not differ in the mice with different genotypes, indicating that positive allosteric modulation of CB 1 R is unlikely as a factor in the unique effects of RTI-371.…”

“…Further, a preliminary report indicated that RTI-371 blocked cocaine-induced locomotor stimulation (Navarro et al, 2005). Finally, Navarro et al (2009) suggested that the antagonism of cocaine by RTI-371 was due at least partly to positive-allosteric modulation of the cannabinoid CB 1 receptor (CB 1 R), as both RTI-371 and the BZT analog JHW007 had this activity.…”

International Union of Basic and Clinical Pharmacology. LXXIX. Cannabinoid Receptors and Their Ligands: Beyond CB1and CB2

“…The ability of CB 1 and CB 2 receptors to signal through G i/o proteins and, further downstream, through adenylyl cyclase is frequently exploited in two widely used in vitro bioassays: the [ 35 S]GTP␥S binding assay and the cAMP assay Pertwee, 2005a). As well as orthosteric site(s), the CB 1 receptor possesses one or more allosteric sites that can be targeted by ligands in a manner that enhances or inhibits the activation of this receptor by direct agonists (Price et al, 2005a;Adam et al, 2007;Horswill et al, 2007;Navarro et al, 2009). …”

N-Substituted Benztropine Analogs: Selective Dopamine Transporter Ligands with a Fast Onset of Action and Minimal Cocaine-Like Behavioral Effects

“…For example, benztropine (BZT) analogs have been reported that did not stimulate locomotor activity as effectively as cocaine, did not fully substitute for cocaine in rats trained to discriminate cocaine from saline injections (Newman et al, 1995;Katz et al, 1999Katz et al, , 2004, were not self-administered to the same degree as cocaine or other DA uptake inhibitors (Woolverton et al, 2000;Ferragud et al, 2009;Hiranita et al, 2009b), did not produce place conditioning comparable with cocaine (Li et al, 2005;Velá zquez-Sá nchez et al, 2009), and were less effective than cocaine in stimulating nucleus accumbens shell DA levels (Tanda et al, 2005. Although BZT analogs may be the most thoroughly studied atypical DAT inhibitors, examples from other structural classes exist, including analogs of the receptor ligand, rimcazole (Katz et al, 2003), and some analogs of cocaine (Navarro et al, 2009).…”