Viable dopamine neurons in
Parkinson's disease express the
dopamine transporter (DAT) and release
dopamine (DA). We postulated that potent DAT inhibitors, with low affinity for the
serotonin transporter (SERT), may elevate endogenously released extracellular
dopamine levels to provide therapeutic benefit. The therapeutic potential of eight DAT inhibitors was investigated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated cynomolgus monkeys (Macaca fascicularis), with efficacy correlated with DAT occupancy as determined by positron emission tomography imaging in striatum. Four potent DAT inhibitors, with relatively high
norepinephrine transporter, but low SERT affinities, that occupied the DAT improved activity in parkinsonian monkeys, whereas three high-affinity DAT inhibitors with low DAT occupancy did not. 2beta-Carbomethoxy-3alpha-(3,4-dichlorophenyl)-7beta-hydroxy-8-methyl-8-azabicyclo[3.2.1.]
octane (O-1163) occupied the DAT but had short-lived pharmacological effects. The
benztropine analog
difluoropine increased general activity, improved posture, reduced body freeze, and produced sleep disturbances at high doses. (1R)-2beta-(1-Propanoyl)-3alpha-(4-fluorophenyl)tropane (O-1369) alleviated parkinsonian signs in advanced parkinsonian monkeys, by increasing general activity, improving posture, reducing body freeze, and sedation, but not significantly reducing
bradykinesia or increasing locomotor activity. In comparison with the D(2)-D(3) DA receptor agonist
quinelorane, O-1369 elicited
oral/facial dyskinesias, whereas
quinelorane did not improve posture or reduce balance and promoted stereotypy. In conclusion, DAT inhibitors with therapeutic potential combine high DAT affinity in vitro and high DAT occupancy of brain striatum in vivo with enduring day-time effects that do not extend into the nighttime. Advanced parkinsonian monkeys (80% DAT loss) respond more effectively to DAT inhibitors than mild parkinsonian monkeys (46% DAT loss). The therapeutic potential of
dopamine transport inhibitors for
Parkinson's disease warrants preclinical investigation.