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

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, 3 beta-(4-methylphenyl)-2 beta-[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 (hCB(1)) receptor. 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 hCB(1) receptor. Selected compounds were also characterized in a similar assay for human mu opioid receptor activation to assess the specificity of their effects. RTI-371 and several other DAT-selective inhibitors with atypical actions on locomotor behaviour increased the efficacy of CP55940 in a concentration-dependent manner. 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 hCB(1) receptor.