In vivo imaging of brain androgen receptors in rats: a [(18)F]FDHT PET study.
BACKGROUND
Steroid hormones like androgens play an important role in the development and maintenance of several brain functions. Androgens can act through androgen receptors (AR) in the brain. This study aims to demonstrate the feasibility of positron emission tomography (PET) with 16β-[(18)F]fluoro-5α-dihydrotestosterone ([(18)F]FDHT) to image AR expression in the brain.


METHODS
Male Wistar rats were either orchiectomized to inhibit endogenous androgen production or underwent sham-surgery. Fifteen days after surgery, rats were subjected to a 90-min dynamic [(18)F]FDHT PET scan with arterial blood sampling. In a subset of orchiectomized rats, 1mg/kg dihydrotestosterone was co-injected with the tracer in order to saturate the AR. Plasma samples were analyzed for the presence of radioactive metabolites by radio-TLC. Pharmacokinetic modeling was performed to quantify brain kinetics of the tracer. After the PET scan, the animals were terminated for ex-vivo biodistribution.


RESULTS
PET imaging and ex vivo biodistribution studies showed low [(18)F]FDHT uptake in all brain regions, except pituitary. [(18)F]FDHT uptake in the surrounding cranial bones was high and increased over time. [(18)F]FDHT was rapidly metabolized in rats. Metabolism was significantly faster in orchiectomized rats than in sham-orchiectomized rats. Quantitative analysis of PET data indicated substantial spill-over of activity from cranial bones into peripheral brain regions, which prevented further analysis of peripheral brain regions. Logan graphical analysis and kinetic modeling using 1- and 2-tissue compartment models showed reversible and homogenously distributed tracer uptake in central brain regions. [(18)F]FDHT uptake in the brain could not be blocked by endogenous androgens or administration of dihydrotestosterone.


CONCLUSIONS
The results of this study indicate that imaging of AR availability in rat brain with [(18)F]FDHT PET is not feasible. The low AR expression in the brain, the rapid metabolism of [(18)F]FDHT in rats and the poor brain penetration of the tracer likely contributed to the poor performance of [(18)F]FDHT PET in this study.
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
M A
MA
Khayum
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
J
J
Doorduin
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
I F
IF
Antunes
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
C
C
Kwizera
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
R
R
Zijlma
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands; PRA-Health Sciences, PO Box 200, 9470 AE Zuidlaren, The Netherlands.
J A
JA
den Boer
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands.
R A J O
RA
Dierckx
Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen, PO BOX 3100, 9700RB, The Netherlands. Electronic address: e.f.j.de.vries@umcg.nl.
E F J
EF
de Vries
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging* / Q000378:metabolism*; D013196:Dihydrotestosterone / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D005462:Fluorine Radioisotopes / Q000493:pharmacokinetics*; D007091:Image Processing, Computer-Assisted; D008297:Male; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D051381:Rats; D017208:Rats, Wistar; D011944:Receptors, Androgen / Q000378:metabolism*; D014018:Tissue Distribution
D005462:Fluorine Radioisotopes; D019275:Radiopharmaceuticals; D011944:Receptors, Androgen; D013196:Dihydrotestosterone
16β-[(18)F]fluoro-5α-dihydrotestosterone ([(18)F]FDHT); Androgen receptor; Neuroimaging; Positron emission tomography; Small animal imaging