S. De Bruyne et al.
authenticity of the tracer was determined by co-elution with
[
123I]-7
Biodistribution studies in mice
the nonradioactive reference compound. In vitro,
remained stable for at least 24 h.
Male NMRI mice of 4–6 weeks old, weighing 22ꢀ30 g, were
injected in the tail vein with 200 mL of 8:92 (v:v) – ethanol:saline
solution containing approximately 185 kBq of [123I]-7. At various
time points p.i., mice (three animals per time point) were
sacrificed under isoflurane anesthesia and dissected. Blood,
urine and organs were removed, weighed and counted for
radioactivity in the gamma counter. All organs were rinsed with
water prior to weighing and counting. For calculation of the
injected dose, five aliquots of the injection solution were
weighed and counted for activity. Results are decay corrected
and expressed as a percentage of the injected dose per gram of
tissue (%ID/g) with standard deviation (SD).
Upon intravenous administration, low levels of activity were
observed in mouse brain indicating low blood–brain penetra-
tion of [123I]-7. At none of the selected time points, radioactivity
concentration of brain exceeded blood activity. A possible
reason for the low brain uptake of [123I]-7 is the contribution of
P-gp pumps or other multidrug resistance protein transporters
in the brain distribution of [123I]-7. We examined the effect of
pretreatment with Cs A, a modulator of the P-gp pumps, on the
brain uptake of [123I]-7. A significant increase in the brain uptake
(4.1–5.4-fold) of [123I]-7 was observed compared with the control
group indicating that [123I]-7 is transported by the P-gp pumps
out of the brain. These findings might be a possible explanation
for the low brain accumulation of [123I]-7. In rat brain, the
regional distribution of DAT was not reflected in the radio-
activity distribution. Although 7 is selective for DAT in vitro, it
did not display the expected selective distribution in vivo in rat
brain. Based on these results, one can conclude that [123I]-7 is
not suitable as a radioligand for in vivo SPECT visualization of
DAT but could be a useful tracer for the P-gp transporter.
Further research to validate the usefulness of [123I]-7 as a SPECT
tracer for P-gp pumps in vivo is ongoing.
Blood–brain barrier transporter inhibition study
To investigate whether the low brain uptake of [123I]-7 is due to
P-gp interference, a biodistribution study with pretreatment of
the mice with Cs A was performed. A 50 mg/kg dose and a
30 min time period between Cs A and tracer injection have been
reported to inhibit the efflux action of P-gp pumps in rodent
brain.31 Male NMRI mice with a body mass of 20–25 g (n = 3)
were intravenously injected with either 50 mg/kg Cs A (test
group) or the same volume of physiological saline (control
group). After 30 min, [123I]-7 (185 kBq, 150 mL) was injected in the
tail vein. The mice were sacrificed and dissected at 2 min and 3 h
p.i. of [123I]-7. Blood and organs were weighed and counted for
radioactivity. Results are expressed as %ID/g7SD. Statistical
analyses were performed using the one-side unpaired Student’s
t-test. A p value o0.05 was interpreted as statistically significant.
Acknowledgement
The authors thank the PhD student Liesbet Vervoort (Laboratory
for Radiopharmacy, Ghent University, Belgium) for her assistance
in the mice studies. This work was supported in part by the
Intramural Research Programs of the National Institute on Drug
Abuse, the National Institute on Alcohol Abuse and Alcoholism
and the National Institute on Diabetes, Digestive and Kidney
Diseases of the National Institutes of Health.
Cerebral biodistribution studies in rats
Male rats (250–300 g, Sprague–Dawley) were injected in the tail
vein with 7.4 MBq of [123I]-7 dissolved in 300 mL 8:92 (v:v) –
ethanol:water. At 10, 30 min, 1, 3 and 6 h p.i., animals (n = 3)
were sacrificed by intravenous injection of T61. Blood was taken
and the brain was removed and dissected. Blood samples and
the different brain parts were weighed and counted for
radioactivity with the gamma counter. Radioactivity is decay
corrected and expressed as %ID/g7SD.
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Conclusion
It has been demonstrated that DAT neuroimaging is useful in PD
diagnosis, providing information on the integrity of the
dopaminergic neurotransmitter system in vivo.18–20 Because of
the crucial role of DAT in the etiology of PD as well as other
neurological disorders and addiction, several DAT ligands have
been developed for in vivo SPECT imaging.21,22 These ligands,
for example, [123I]-FP-CIT, are not selective toward SERT.23–25 The
investigated SPECT tracer, [123I]-4-(2-(bis(4-fluorophenyl)methoxy)-
ethyl)-1-(4-iodobenzyl)piperidine, has good in vitro selectivity over
the other monoamine transporters (108 for SERT/DAT and 2163
for NET/DAT).28 We therefore evaluated the potential of [123I]-7 as
a suitable tracer for DAT in vivo.
The preparation and purification of the precursor molecule, as
well as the reference compound, were accomplished without
any significant problems. The radiosynthesis of [123I]-7 was
achieved by a one-pot synthetic procedure in a 40710% yield
and with good specific activity. After purification, the radio-
chemical purity appeared to be higher than 98%. The
Copyright r 2009 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2009, 52 304–311