Fluorine-18-FPH for PET imaging of nicotinic acetylcholine receptors.
Visualization of central nicotinic acetylcholine receptors (nAChRs) with modern PET or SPECT imaging techniques has been hampered by the lack of a radioligand with suitable in vivo binding characteristics (i.e., high target-to-nontarget ratios and kinetics appropriate for the half-life of the tracer and imaging modality used). This paper describes in vivo binding, kinetics and pharmacology of a highly potent 18F-labeled analog of epibatidine, (+/-)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([18F]FPH), in the mouse brain with the view towards application of this tracer for PET imaging of nAChR in human brain.


METHODS
Fluorine-18-FPH was administered intravenously to mice, and time-activity curves were determined for several regions in the brain and other organs. Saturation and pharmacology of [18F]FPH binding was demonstrated in vivo by preinjecting unlabeled FPH or other drugs with known pharmacological action before [18F]FPH was injected. The effect of the drugs on [18F]FPH accumulation was evaluated.


RESULTS
[18F]FPH was rapidly incorporated into the mouse brain; peak activity (2.4% of the injected dose) was measured at 5 min after intravenous administration, followed by washout to 1.1% injected dose (ID) at 60 min. Highest concentrations of 18F occurred at 15 min in areas known to contain high densities of nAChR ¿e.g., thalamus [9.7% of injected dose per gram tissue (ID/g¿] and superior colliculus (8.3% ID/g)]. Accumulation of the 18F tracer in hippocampus, striatum, hypothalamus and cortical areas was intermediate (5.0, 5.6, 4.2 and 5.6% ID/g, respectively) and low in the cerebellum (2.8% ID/g). The distribution of [18F]FPH in the mouse brain matched that of other in vivo nAChR probes such as 3H-labeled epibatidine or norchloroepibatidine, [3H](-)-nicotine and [3H]cytisine and that of nAChR densities determined in postmortem autoradiographic studies in rodents. Preinjection of blocking doses of unlabeled epibatidine, (-)-nicotine, lobeline and cytisine significantly inhibited [18F]FPH binding in thalamus and superior colliculus, but not in cerebellum, whereas drugs that interact with binding sites other than acetylcholine recognition sites of nAChR (e.g., mecamylamine, scopolamine, N-methylspiperone and ketanserin) had no effect on [18F]FPH accumulation in any of the brain regions examined.


CONCLUSIONS
Fluorine-18-FPH labels nAChR in vivo in the mouse brain. Because of its high uptake into the brain and high ratios of specific-to-nonspecific binding, this radioligand appears to be ideally suited for PET imaging of nAChR in the mammalian brain.
Horti
A
A
Intramural Research Program, National Institute on Drug Abuse, Baltimore, Maryland, USA.
Scheffel
U
U
Stathis
M
M
Finley
P
P
Ravert
H T
HT
London
E D
ED
Dannals
R F
RF
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000493:pharmacokinetics; D005462:Fluorine Radioisotopes; D008297:Male; D051379:Mice; D011725:Pyridines / Q000493:pharmacokinetics; D011978:Receptors, Nicotinic / Q000032:analysis*; D013997:Time Factors; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed
D019086:Bridged Bicyclo Compounds, Heterocyclic; D005462:Fluorine Radioisotopes; D011725:Pyridines; D011978:Receptors, Nicotinic; C107635:2-(2-fluoro-5-pyridyl)-7-azabicyclo(2.2.1)heptane