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Synthesis and in vivo evaluation of [123I]-3-I-CO: a potential SPECT tracer for the serotonin 5-HT2A receptor

PETER BLANCKAERT UGent (2008)
abstract
The aim of this doctoral dissertation was the precursor synthesis, radiosynthesis and in vivo evaluation of [123I]-3-I-CO as a possible new tracer for imaging of the central serotonin 5-HT2A receptor with SPECT. [123I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl] methanone ([123I]-3-I-CO) demonstrates good affinity for the 5-HT2A receptor (Ki = 0.51 nM) and good selectivity ratios over other receptor types and was therefore selected as the ligand. First, the in vivo behaviour of a currently used 5-HT2A SPECT tracer, [123I]-R91150, was evaluated (chapter 3), and brain uptake of the tracer was assessed in rodents, as a standard for comparison. The influence of P-glycoprotein blocking (with cyclosporin A) on the biodistribution and brain uptake of [123I]-R91150 was also evaluated in rodents, and these results were compared with the data obtained from the normal biodistribution studies with [123I]-R91150. Also, the influence of P-glycoprotein blocking on pinhole μSPECT imaging with [123I]-R91150 in rodents was investigated. In NMRI mice, a dose-dependent influence of cyclosporin A on the brain uptake of [123I]-R91150 was observed, indicating that the increased brain uptake is the result of a decreased efflux of tracer out of the brain after blocking of the P-glycoprotein efflux transporter with cyclosporin A. Pre-treatment of Sprague-Dawley rats with cyclosporin A resulted in a drastically increased brain uptake of [123I]-R91150 (brain uptake increased seven-fold after Pglycoprotein blocking) and a vastly improved pinhole μSPECT imaging quality. From these results it can be concluded that [123I]-R91150 is a substrate for P-glycoprotein efflux in vivo, and that it’s brain efflux can be blocked by administration of cyclosporin A. Organic synthesis of the tributylstannyl precursor for the radiosynthesis of [123I]-3-I-CO was performed in adequate yield. An average yield of about 85 % was obtained in the radiosynthesis reaction. The radioligand was purified with semi-preparative HPLC, and radiochemical purities of > 95 % were obtained consistently. The radioligand was stable at room temperature until 48 h after synthesis. A logP value of 3.10 ± 0.10 was obtained for [123I]-3-I-CO (chapter 4). In vivo evaluation of [123I]-3-I-CO in NMRI mice revealed high initial brain uptake (6.26 ± 1.36 % ID/g tissue at 10 min post injection), but radioactivity concentrations in brain decreased rapidly over time. No radiolabelled metabolites were observed in blood or brain of NMRI mice. Brain uptake of [123I]-3-I-CO was also investigated in Sprague-Dawley rats (chapter 5): highest brain radioactivity concentrations were obtained in the occipital (0.942 ± 0.034 % ID/g tissue at 20 min post injection) and frontal cortex (0.674 ± 0.074 % ID/g tissue at 20 min post injection). Blood radioactivity concentrations were consistently low (a maximum value of 0.062 ± 0.014 % ID/g tissue was obtained at 20 min post injection). An average frontal cortex-to-cerebellum ratio of about 1.7 was obtained. In the Sprague-Dawley rat biodistribution studies, a rapid washout of radioactivity from the brain was observed. [123I]-3-I-CO was displaced from the 5-HT2A receptor by ketanserin: radioactivity concentration in the 5-HT2A rich areas of the brain decreased by 50 % after ketanserin displacement. Nevertheless, the residual radioactivity levels in cerebellum after ketanserin displacement remained high, especially compared to the results obtained with [123I]-R91150, and are probably caused by aspecific binding of the radioligand to brain tissues. No radiolabelled metabolites could be detected in the blood or brain of Sprague-Dawley rats. The influence of P-glycoprotein modulation with cyclosporin A on the brain uptake of [123I]-3-I-CO was also investigated. On average, a 67 % increase in [123I]-3-I-CO radioactivity concentration was observed throughout the brain after treatment of the animals with cyclosporin A. We can conclude from these results that [123I]-3-I-CO is at least a partial substrate for P-glycoprotein efflux, but the increase in brain radioactivity concentration after cyclosporin A treatment was not as large as the increase observed with [123I]-R91150 (chapter 3). Although cortical tissues could be visualized using pinhole μSPECT imaging with [123I]-3-ICO, aspecific binding of the radioligand was observed in the cerebellum, probably limiting its application as a serotonin 5-HT2A receptor tracer in humans. μSPECT imaging quality also did not improve after cyclosporin A pre-treatment of the animals. Although the initial rodent studies demonstrated promising brain uptake of the radioligand, it can be concluded that [123I]-3-I-CO probably has very limited potential as a 5-HT2A tracer for SPECT, due to high aspecific binding and rapid washout of the radioligand out of the brain. Also, compared to other clinically used brain tracers (for example [123I]-R91150), the specific ‘signal’ of [123I]-3-I-CO in brain is too limited for application as a tracer in brain receptor imaging studies.
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author
promoter
UGent
organization
year
type
dissertation (composite)
subject
keyword
P-glycoprotein, R91150, 5-HT2A, radiosynthesis, serotonin, 3-I-CO
pages
VI, 213 pages
publisher
Ghent University. Faculty of Pharmaceutical Sciences
place of publication
Ghent, Belgium
defense location
Gent : Farmaceutisch Instituut (auditorium I)
defense date
2008-10-20 18:00
language
English
UGent publication?
yes
classification
D1
copyright statement
I have retained and own the full copyright for this publication
id
1008697
handle
http://hdl.handle.net/1854/LU-1008697
date created
2010-07-12 01:30:53
date last changed
2010-07-12 13:30:28
@phdthesis{1008697,
  abstract     = {The aim of this doctoral dissertation was the precursor synthesis, radiosynthesis and in vivo evaluation of [123I]-3-I-CO as a possible new tracer for imaging of the central serotonin 5-HT2A receptor with SPECT. [123I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]
methanone ([123I]-3-I-CO) demonstrates good affinity for the 5-HT2A receptor (Ki = 0.51 nM) and good selectivity ratios over other receptor types and was therefore selected as the ligand.
First, the in vivo behaviour of a currently used 5-HT2A SPECT tracer, [123I]-R91150, was evaluated (chapter 3), and brain uptake of the tracer was assessed in rodents, as a standard for comparison. The influence of P-glycoprotein blocking (with cyclosporin A) on the biodistribution and brain uptake of [123I]-R91150 was also evaluated in rodents, and these results were compared with the data obtained from the normal biodistribution studies with [123I]-R91150. Also, the influence of P-glycoprotein blocking on pinhole \ensuremath{\mu}SPECT imaging with [123I]-R91150 in rodents was investigated.
In NMRI mice, a dose-dependent influence of cyclosporin A on the brain uptake of [123I]-R91150 was observed, indicating that the increased brain uptake is the result of a decreased efflux of tracer out of the brain after blocking of the P-glycoprotein efflux transporter with cyclosporin A. Pre-treatment of Sprague-Dawley rats with cyclosporin A resulted in a drastically increased brain uptake of [123I]-R91150 (brain uptake increased seven-fold after Pglycoprotein blocking) and a vastly improved pinhole \ensuremath{\mu}SPECT imaging quality. From these results it can be concluded that [123I]-R91150 is a substrate for P-glycoprotein efflux in vivo, and that it{\textquoteright}s brain efflux can be blocked by administration of cyclosporin A.
Organic synthesis of the tributylstannyl precursor for the radiosynthesis of [123I]-3-I-CO was performed in adequate yield. An average yield of about 85 \% was obtained in the radiosynthesis reaction. The radioligand was purified with semi-preparative HPLC, and radiochemical purities of {\textrangle} 95 \% were obtained consistently. The radioligand was stable at room temperature until 48 h after synthesis. A logP value of 3.10 {\textpm} 0.10 was obtained for [123I]-3-I-CO (chapter 4).
In vivo evaluation of [123I]-3-I-CO in NMRI mice revealed high initial brain uptake (6.26 {\textpm} 1.36 \% ID/g tissue at 10 min post injection), but radioactivity concentrations in brain decreased rapidly over time. No radiolabelled metabolites were observed in blood or brain of NMRI mice. Brain uptake of [123I]-3-I-CO was also investigated in Sprague-Dawley rats (chapter 5): highest brain radioactivity concentrations were obtained in the occipital (0.942 {\textpm} 0.034 \% ID/g tissue at 20 min post injection) and frontal cortex (0.674 {\textpm} 0.074 \% ID/g tissue at 20 min post injection). Blood radioactivity concentrations were consistently low (a maximum value of 0.062 {\textpm} 0.014 \% ID/g tissue was obtained at 20 min post injection).
An average frontal cortex-to-cerebellum ratio of about 1.7 was obtained. In the Sprague-Dawley rat biodistribution studies, a rapid washout of radioactivity from the brain was observed. [123I]-3-I-CO was displaced from the 5-HT2A receptor by ketanserin: radioactivity concentration in the 5-HT2A rich areas of the brain decreased by 50 \% after ketanserin displacement. Nevertheless, the residual radioactivity levels in cerebellum after ketanserin displacement remained high, especially compared to the results obtained with [123I]-R91150, and are probably caused by aspecific binding of the radioligand to brain tissues. No radiolabelled metabolites could be detected in the blood or brain of Sprague-Dawley rats.
The influence of P-glycoprotein modulation with cyclosporin A on the brain uptake of [123I]-3-I-CO was also investigated. On average, a 67 \% increase in [123I]-3-I-CO radioactivity concentration was observed throughout the brain after treatment of the animals with cyclosporin A. We can conclude from these results that [123I]-3-I-CO is at least a partial substrate for P-glycoprotein efflux, but the increase in brain radioactivity concentration after cyclosporin A treatment was not as large as the increase observed with [123I]-R91150 (chapter 3).
Although cortical tissues could be visualized using pinhole \ensuremath{\mu}SPECT imaging with [123I]-3-ICO, aspecific binding of the radioligand was observed in the cerebellum, probably limiting its application as a serotonin 5-HT2A receptor tracer in humans. \ensuremath{\mu}SPECT imaging quality also did not improve after cyclosporin A pre-treatment of the animals.
Although the initial rodent studies demonstrated promising brain uptake of the radioligand, it can be concluded that [123I]-3-I-CO probably has very limited potential as a 5-HT2A tracer for SPECT, due to high aspecific binding and rapid washout of the radioligand out of the brain. Also, compared to other clinically used brain tracers (for example [123I]-R91150), the specific {\textquoteleft}signal{\textquoteright} of [123I]-3-I-CO in brain is too limited for application as a tracer in brain receptor imaging studies.},
  author       = {BLANCKAERT, PETER},
  keyword      = {P-glycoprotein,R91150,5-HT2A,radiosynthesis,serotonin,3-I-CO},
  language     = {eng},
  pages        = {VI, 213},
  publisher    = {Ghent University. Faculty of Pharmaceutical Sciences},
  school       = {Ghent University},
  title        = {Synthesis and in vivo evaluation of [123I]-3-I-CO: a potential SPECT tracer for the serotonin 5-HT2A receptor},
  year         = {2008},
}

Chicago
BLANCKAERT, PETER. 2008. “Synthesis and in Vivo Evaluation of [123I]-3-I-CO: a Potential SPECT Tracer for the Serotonin 5-HT2A Receptor”. Ghent, Belgium: Ghent University. Faculty of Pharmaceutical Sciences.
APA
BLANCKAERT, P. (2008). Synthesis and in vivo evaluation of [123I]-3-I-CO: a potential SPECT tracer for the serotonin 5-HT2A receptor. Ghent University. Faculty of Pharmaceutical Sciences, Ghent, Belgium.
Vancouver
1.
BLANCKAERT P. Synthesis and in vivo evaluation of [123I]-3-I-CO: a potential SPECT tracer for the serotonin 5-HT2A receptor. [Ghent, Belgium]: Ghent University. Faculty of Pharmaceutical Sciences; 2008.
MLA
BLANCKAERT, PETER. “Synthesis and in Vivo Evaluation of [123I]-3-I-CO: a Potential SPECT Tracer for the Serotonin 5-HT2A Receptor.” 2008 : n. pag. Print.