Difficulties in dopamine transporter radioligand PET analysis: the example of LBT-999 using [18F] and [11C] labelling Part I: PET studies.
BACKGROUND
LBT-999 (E)-N-(4-fluorobut-2-enyl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane is a dopamine transporter (DAT) ligand. [(18)F]LBT-999 was first labelled with carbon-11; we will now describe its in vivo behaviour in comparison to that of [(11)C]LBT-999.


RESULTS
Positron emission tomography (PET) experiments (baboons) confirmed the high affinity/specificity of [(18)F]LBT-999 for DAT. The brain regional distribution was in accordance with that of DAT. Pre-treatment with LBT-999 (1 mg/kg iv), but not with desipramine, a norepinephrine (NET) antagonist, reduced the striatum-to-cerebellum ratio by 96%, confirming the specificity for DAT vs. NET. The parent compound decreased rapidly and represented 24.3 ± 5.0% of plasma radioactivity at 30 min pi. Whole-body scans showed an important bone uptake of free fluorine following metabolism of [(18)F]LBT-999. In the cerebellum and striatum, distribution volumes increased by 30-40% between 80 and 230 min, suggesting the polluting role of a radiometabolite(s). [(11)C]LBT-999 exhibited a 40% higher standardized uptake value in the striata. This difference is likely due to N-dealkylation followed by [(18)F]fluoride release. 2β-Carbomethoxy-3β-(4'-tolyl) nortropane is then formed, while [(11)C]2β-carbomethoxy-3β-(4'-tolyl) nortropane is formed following injection of [(11)C]LBT-999. This metabolite has high affinity for the DAT. In one specific PET experiment, intravenous injection of this metabolite induced a strong displacement of [(18)F]LBT-999 in the striata, confirming that this metabolite readily crosses the blood-brain barrier (BBB) and binds to DAT.


CONCLUSIONS
[(18)F]LBT-999 is N-dealkylated in vivo to yield (1) a nonradioactive metabolite that crosses the BBB and has a high affinity for the DAT and (2) a [(18)F]fluoro-alkyl chain which is further defluorinated. The temporal changes in distribution volumes are consistent with the accumulation of a radiometabolite(s) in the brain. Therefore, the quantification of DAT density with [(18)F]LBT-999 is rather difficult.
CEA, I2BM, Service Hospitalier Frédéric Joliot, 4 place du Général Leclerc, F-91401 Orsay, France. wadad.saba@cea.fr
Wadad
W
Saba
Marie-Anne
MA
Peyronneau
Frédéric
F
Dollé
Sébastien
S
Goutal
Michel
M
Bottlaender
Héric
H
Valette
D000818:Animals; D001812:Blood-Brain Barrier; D002250:Carbon Radioisotopes / Q000493:pharmacokinetics*; D002531:Cerebellum / Q000000981:diagnostic imaging / Q000378:metabolism*; D003042:Cocaine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D003342:Corpus Striatum / Q000000981:diagnostic imaging / Q000378:metabolism*; D003891:Desipramine / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism; D005462:Fluorine Radioisotopes / Q000493:pharmacokinetics*; D008024:Ligands; D008297:Male; D010215:Papio; D049268:Positron-Emission Tomography / Q000379:methods*; D014018:Tissue Distribution
C508666:8-((E)-4-fluoro-but-2-enyl)-3beta-p-tolyl-8-aza-bicyclo(3.2.1)octane-2beta-carboxylic acid methyl ester; D002250:Carbon Radioisotopes; D050483:Dopamine Plasma Membrane Transport Proteins; D004791:Enzyme Inhibitors; D005462:Fluorine Radioisotopes; D008024:Ligands; D003042:Cocaine; D003891:Desipramine