Analogs of benztropine (BZT) bind with high affinity to the dopamine transporter (DAT) but are distinct from cocaine in their behavioral activity. Because of these differences, and their potential as medical treatments for cocaine abuse, we have studied these compounds. Previously studied analogs of BZT have slow onsets of action that may contribute to their atypical effects. In one study BZTs with a relatively fast onset of effects were studied. Only one of the compounds increased locomotor activity, and the increases were modest compared to those of 10-20 mg/kg cocaine. In rats trained to discriminate 10 mg/kg cocaine from saline none of the compounds produced greater than 40% cocaine-like responding up to two hours after injection. None of the compounds produced place-conditioning when examined up to 90 min after injection, indicating minimal abuse liability. The compounds had high affinities at the DAT, with uniformly lower affinities at norepinephrine and serotonin transporters, suggesting minimal contribution of those sites to the behavioral effects of the compounds. The compounds also had affinity for sigma receptors that may have contributed to their behavioral effects. Together the results indicate that a slow onset of action is not a necessary condition for reduced cocaine-like effects of atypical DAT ligands. It is possible that the selective decreases in cocaine self-administration produced by BZT analogs are due to competition from excess behavioral activity (hyperlocomotion or stereotypy) induced by the atypical DA uptake inhibitors (BZT analogs) alone or with cocaine. Pretreatments with the typical dopamine uptake inhibitor methylphenidate dose-dependently shifted the cocaine self-administration dose-effect curve leftward. The shift in the dose-effect curve was obtained at doses of methylphenidate that, when administered alone, also decreased food-maintained behavior and increased locomotor activity and stereotypy. In contrast, the N-substituted BZT analogs decreased the maximum for the cocaine self-administration dose-effect curve, and did so at doses that were virtually without effects on food-maintained behavior. Further, the BZT analogs alone had minimal effects on locomotor activity and stereotypies and did not appreciably change the effects of cocaine on these measures. The present results suggest that the decrease in cocaine self-administration produced by the BZT analogs is due to an antagonism of the reinforcing effects of cocaine rather than due to interference from competing behavioral overstimulation. Modafinil is a DAT inhibitor that cannot be classified as atypical by the standards of the BZT analogs, but has some effects that are unusual for DAT inhibitors. A series of modafinil analogs were synthesized to investigate the effects of chemical modifications on DAT, serotonin transporter, and norepinephrine transporter binding. In addition, the locomotor-stimulant effects in mice of ()-modafinil, its R- and S-enantiomers, and its para-chloro sulfinylacetamide analog were compared to those of cocaine. The structure-activity relationships among these drugs suggest that the binding at the DAT is different from that for the atypical BZT DAT inhibitors. Studies of the behavioral effects suggest that the stimulant effects, although less than that of cocaine, were greater than that of many BZT analogs, consistent with the conclusion that modafinil analogs do not interact with the DAT in a manner similar to that of the BZT analogs. The unusual nature of the effects of modafinil may be due to actions at other targets, or it may be due to its manner of binding the DAT. Modafinil has also received some attention as having potential clinical efficacy in the treatment of ADHD and stimulant dependence. Both R- and S-enantiomers of modafinil were investigated with regard to its inhibition of DA uptake and its binding to the DAT. Both wild-type and mutant DATs were investigated. The Y335A mutant has the tyrosine at position 335 point mutated to alanine. Tyr-335 is a controlling access point critical for regulating conformational isomerization between inward and outward facing conforms. In the mutant the equilibrium is shifted to inward conformations. The Y156Fmutant has the tyrosine at position 156 point mutated to phenylalanine. The mutation disrupts the H bond between Tyr156 and Asp79. The bond allows a closed substrate binding pocket and thus functions as a gate for the transport pathway. Thus the mutant functions as an outward facing conformation. Data were compared to cocaine and the atypical dopamine uptake inhibitor, JHW 007. R- and S-modafinil were also evaluated using in vivo microdialysis to assess levels of dopamine in the mouse NAc shell and their cocaine-like behavioral effects were evaluated in mice trained to discriminate cocaine from saline injections. The racemic compound and the enantiomers bound to the DAT and inhibited dopamine uptake though less potently than cocaine. R-modafinil had approximately 3-fold higher affinity than its S-enantiomer. Molecular docking studies revealed subtle differences in binding modes for the enantiomers. R-modafinil was significantly less potent in the DAT Y156F mutant compared to wild-type DAT, whereas S-modafinil was affected less. Studies with the Y335A DAT mutant showed that the R- and S-enantiomers tolerated the inward facing conformation better than cocaine. Microdialysis studies demonstrated that both R- and S-modafinil produced increases in extracellular DA concentrations in the NAc shell less efficaciously than cocaine, and with a longer duration of action. Both enantiomers fully substituted in mice trained to discriminate cocaine from saline. R-modafinil displays an in vitro profile different from cocaine, however its behavioral effects currently appear to be similar to those of cocaine. Future studies of these compounds will be conducted to assess whether the binding differences between modafinil enantiomers have a more substantial behavioral consequence and whether preclinical evidence supports moving forward with these compounds or variants based on their chemical structure as potential treatments for behavioral disorders. Studies reported during the past year suggest that differences in pharmacology of BZT analogs and standard DAT inhibitors likely stem from fundamental differences between the manner in which they interact with the DAT. While modafinil is in several respects different from cocaine it is not similar to the BZT analogs. The possibility that dual actions of BZT analogs at σRs contribute to their atypical pharmacological effects is being further investigated. Regardless of mechanism, the results of studies with BZT analogs suggest the possibility for their development as medical treatments for cocaine abuse and the likelihood that the compounds themselves will have low liability for abuse. More generally, the present results confirm that the DAT can serve as an important target for the discovery of compounds with potential for development as cocaine abuse medical treatments.
Showing the most recent 10 out of 38 publications
