Pharmacological characterisation of (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl)nortropane (PE2I) binding to the rat neuronal dopamine transporter expressed in COS cells

SPECT molecular imaging in Parkinson's disease

Parkinson's disease (PD) is a common disorder, and the diagnosis of Parkinson's disease is clinical and relies on the presence of characteristic motor symptoms. The accuracy of the clinical diagnosis of PD is still limited. Functional neuroimaging using SPECT technique is helpful in patients with first signs of parkinsonism. The changes detected may reflect the disease process itself and/or compensatory responses to the disease, or they may arise in association with disease- and/or treatment-related complications. This paper addresses the value of SPECT in early differential diagnosis of PD and its potential as a sensitive tool to assess the pathophysiology and progression, as well as the therapeutic efficacy of PD.

PE2I: a radiopharmaceutical for in vivo exploration of the dopamine transporter

The membrane dopamine transporter (DAT) has a pivotal role in the regulation of dopamine (DA) neurotransmission involved in a number of physiological functions and brain disorders. Molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT), are relevant tools to explore the DAT, and we developed the cocaine derivative N-(3-iodopro-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) that has proved to be a very potent radiopharmaceutical to image the DAT by these techniques. Several methods are available to obtain PE2I labeled with iodine-123 or -125, carbon-11 and tritium. The pharmacological properties of PE2I have demonstrated that it has good affinity for the DAT (4 nM) and is one of the most selective DAT ligands. [(125)I]PE2I characterized postmortem in human brains has revealed very intense and selective binding in the basal ganglia. Ex vivo autoradiography in rats has shown that high level of [(125)I]PE2I accumulates in the striatum and also in the substantia nigra and ventral tegmental area. [(125)I]PE2I accumulation in the rat striatum is rapid, high, and selective, providing a maximum striatum/cerebellum ratio of 10 during the first 30 min post injection. Using SPECT or PET, rapid, high, and selective accumulation of PE2I was found in the caudate nucleus and putamen in monkeys, whereas rapid wash out from the cerebellum was observed. In vivo investigations in healthy humans have demonstrated that PE2I has high striatal uptake, low nonspecific binding, low radiation exposure, and a fairly short scanning time. A number of findings in various animal models of Parkinson's disease in rats and monkeys have demonstrated the high efficacy of PE2I for detection of reduction in the density of DAT, thus showing the potential value of PE2I for early diagnosis and evaluation of treatment of this disease. The excellent properties of PE2I are basis for the development of new DAT tracers for use in future PET explorations using fluor-18.

Measurement of striatal and extrastriatal dopamine transporter binding with high-resolution PET and [11C]PE2I: quantitative modeling and test-retest reproducibility

[(11)C]PE2I is a novel positron emission tomography (PET) radiotracer for the dopamine transporter (DAT). The reproducibility and reliability of [(11)C]PE2I measurements, especially in the small DAT-rich brain regions, is unknown and of critical importance to the interpretation of the data. Five healthy volunteers were scanned twice during the same day using [(11)C]PE2I and the HRRT PET scanner. Methods based on metabolite-corrected arterial plasma curve and reference region were used to estimate distribution volumes (V(T)) and binding potential (BP). Within-subject and between-subject variabilities were compared. [(11)C]PE2I accumulated in the DAT-rich striatum and the midbrain. Equilibrium of specific binding appeared late in the striatum, whereas it was reached earlier in the midbrain. Plasma metabolite analysis showed that the potentially brain-penetrant 4-hydroxymethyl metabolite represented 15% to 20% of total plasma radioactivity. V(T) and BP measurements were associated with low within-subject variability. Measurement of DAT binding in small brain regions, including the substantia nigra, is reproducible and reliable using [(11)C]PE2I and high-resolution research tomograph. A scanning time of more than 70 mins is required for the striatum, while less is sufficient for DAT quantification in the midbrain. The previously suggested involvement of the potentially brain-penetrant radioactive metabolite in the quantification should be further studied.

Slow isomerization step in the interaction between mouse dopamine transporter and dopamine re-uptake inhibitor N-(3-iodoprop-2E-enyl)-2beta-carbo-[3H]methoxy-3beta-(4'-methylphenyl)nortropane

The kinetics of the association and dissociation of the tritium-labeled selective and potent dopamine transporter inhibitor N-(3-iodoprop-2E-enyl)-2beta-carbo-[3H]methoxy-3beta-(4'-methylphenyl)nortropane ([3H]PE2I) with the transporter of mouse striatal membranes was studied. The analysis revealed that the specific binding of [3H]PE2I occurs within a homogeneous population of binding sites in these membranes. The relatively slow binding process was characterized by the pseudo-first-order rate constant kobs. The plot of these rate constants versus free radioligand concentration was hyperbolic, demonstrating that at least two kinetically distinguishable steps can be identified in the interaction of dopamine transporter with this inhibitor. The fast and reversible binding step, characterized by dissociation constant KA = 51 +/- 23 nM, is followed by a slow but also reversible isomerization step of the complex, characterized by the isomerization rate constant ki = (7 +/- 2)10(-2) s(-1) and by the rate constant k(-i) = (3.9 +/- 0.5)10(-3) s(-1) for the reverse process. This isomerization step increases the apparent affinity of the ligand and probably consists of a conformational transition of the transporter protein, induced by the inhibitor molecule.

Post-mortem human brain autoradiography of the norepinephrine transporter using (S,S)-[18F]FMeNER-D2

The binding of the norepinephrine transporter radioligand, (S,S)-[18F]FMeNER-D2, to human brain post-mortem was examined in vitro by whole hemisphere autoradiography. The rank order for the density of labelling was: locus coeruleus>>cortex approximately cerebellum approximately thalamus>caudate approximately putamen. The NET-selectivity of binding was confirmed by co-incubation with desipramine. The dual NET/SERT inhibitor duloxetine also inhibited specific binding, whereas PE2I or citalopram had no evident effect.