Metabolism and autoradiographic evaluation of [(18)F]FE@CIT: a Comparison with [(123)I]beta-CIT and [(123)I]FP-CIT.
OBJECTIVE
Since the late 1980s, cocaine analogues based on the phenyltropane structure, such as [(11)C]CFT and [(123)I]beta-CIT have been used for the imaging of the dopamine transporter. FE@CIT (fluoropropyl ester) and FP-CIT (N-fluoropropyl derivative) are further analogues. The aim of this study was to (1) evaluate and compare the metabolic stability of beta-CIT, FP-CIT and FE@CIT against carboxyl esterases and (2) evaluate selectivity of [(18)F]FE@CIT compared to [(123)I]beta-CIT and [(123)I]FP-CIT using autoradiography.


METHODS
In vitro enzymatic hydrolysis assays were performed using different concentrations of beta-CIT, FE@CIT and FP-CIT with constant concentrations of carboxyl esterase. Autoradiography was performed on coronal 20-microm rat brain sections incubated with different radioactivity concentrations of [(123)I]beta-CIT, [(123)I]FP-CIT or [(18)F]FE@CIT and, additionally, with 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile [serotonin transporter (SERT)] and nisoxetine [norepinephrine transporter (NET)] for blocking experiments.


RESULTS
In vitro assays showed Michaelis-Menten constants of 175 micromol (beta-CIT), 183 micromol (FE@CIT) and 521 micromol (FP-CIT). Limiting velocities were 0.1005 micromol/min (beta-CIT), 0.1418 micromol/min (FE@CIT) and 0.1308 micromol/min (FP-CIT). This indicates a significantly increased stability of FP-CIT, whereas carboxyl esterase stability of beta-CIT and FE@CIT showed no significant difference. Autoradiographic analyses revealed a good correlation between dopamine transporter (DAT)-rich regions and the uptake pattern of FE@CIT. Blocking experiments showed a higher DAT selectivity for [(18)F]FE@CIT than for the other two tracers.


CONCLUSIONS
We found that (1) the metabolic stability of FE@CIT was comparable to that of beta-CIT, whereas FP-CIT showed higher resistance to enzymatic hydrolysis; and (2) the overall uptake pattern of [(18)F]FE@CIT on brain slices was comparable to that of [(123)I]beta-CIT and [(123)I]FPCIT. After blocking of NET and SERT binding, a significantly higher DAT selectivity was observed for [(18)F]FE@CIT. Hence, [(18)F]FE@CIT may be of interest for further clinical application.
Ettlinger
Dagmar E
DE
Department of Nuclear Medicine, Medical University of Vienna, A-1090 Vienna, Austria.
Häusler
Daniela
D
Wadsak
Wolfgang
W
Girschele
Friedrich
F
Sindelar
Karoline M
KM
Mien
Leonhard-Key
LK
Ungersböck
Johanna
J
Viernstein
Helmut
H
Kletter
Kurt
K
Dudczak
Robert
R
Mitterhauser
Markus
M
D000814:Aniline Compounds / Q000494:pharmacology; D000818:Animals; D001345:Autoradiography; D001667:Binding, Competitive; D043182:Carboxylesterase / Q000494:pharmacology; D003042:Cocaine / Q000031:analogs & derivatives* / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins / Q000187:drug effects / Q000378:metabolism; D004355:Drug Stability; D005473:Fluoxetine / Q000031:analogs & derivatives / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D050484:Norepinephrine Plasma Membrane Transport Proteins / Q000037:antagonists & inhibitors; D009662:Nortropanes / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology; D013440:Sulfides / Q000494:pharmacology; D014326:Tropanes / Q000378:metabolism*
D000814:Aniline Compounds; D043182:Carboxylesterase; D003042:Cocaine; D050483:Dopamine Plasma Membrane Transport Proteins; D005473:Fluoxetine; D050484:Norepinephrine Plasma Membrane Transport Proteins; D009662:Nortropanes; D017367:Selective Serotonin Reuptake Inhibitors; D013440:Sulfides; D014326:Tropanes; C412822:3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile; C097058:N-(2-fluoroethyl)-2-carbomethoxy-3-(4-iodophenyl)nortropane; C087552:2-carbomethoxy-8-(3-fluoropropyl)-3-(4-iodophenyl)tropane; C011386:nisoxetine; C069723:2beta-carbomethoxy-3beta-(4-iodophenyl)tropane