Blanckaert 2008

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In vivo evaluation in rodents of [123

I]-3-I-CO as a potentialSPECT tracer for the serotonin 5-HT2A receptor

Peter B.M. Blanckaert, Ingrid Burvenich, Leonie Wyffels,Sylvie De Bruyne, Lieselotte Moerman, Filip De Vos

Laboratory for Radiopharmacy, Ghent University, B-9000 Ghent, Belgium

Received 22 May 2008; received in revised form 5 September 2008; accepted 8 September 2008

Abstract

Introduction: [123I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]methanone ([123I]-3-I-CO) is a potential single photon emission

computed tomography tracer with high affinity for the serotonin 5-HT2A receptor (Ki=0.51 nM) and good selectivity over other receptor (sub)

types. To determine the potential of the radioligand as a 5-HT2A tracer, regional brain biodistribution and displacement studies will be

performed. The influence of P-glycoprotein blocking on the brain uptake of the radioligand will also be investigated.

Methods: A regional brain biodistribution study and a displacement study with ketanserin were performed with [123I]-3-I-CO. Also, the

influence of cyclosporin A (50 mg/kg) on the brain distribution of the radioligand was investigated. For the displacement study, ketanserin

(1 mg/kg) was administered 30 min after injection of [123I]-3-I-CO.

Results: The initial brain uptake of [123I]-3-I-CO was quite high, but a rapid wash-out of radioactivity was observed. Cortex-to-cerebellum

binding index ratios were low (1.1 1.7), indicating considerable aspecific binding and a low specific signal of the radioligand. Tracer

uptake was reduced to the levels in cerebellum (a 60% reduction) after ketanserin displacement. Administration of cyclosporin A resulted in a

doubling of the brain radioactivity concentration.

Conclusions: Although [123I]-3-I-CO showed adequate brain uptake and could be displaced by ketanserin, high aspecific binding to brain

tissue was responsible for very low cortex-to-cerebellum binding index ratios, possibly limiting the potential of the radioligand as a serotonin

5-HT2A receptor tracer. We also demonstrated that [123I]-3-I-CO is probably a weak substrate for the P-glycoprotein efflux transporter.

2008 Elsevier Inc. All rights reserved.

Keywords: Serotonin; P-glycoprotein; 5-HT2A receptor; Brain tracer

1. Introduction

Serotonin is a well-known neurotransmitter, regulating

important functions such as sleep, mood and appetite [1].

It was demonstrated that serotonergic neurotransmission

malfunctioning is responsible for several psychiatric

conditions, for example depression. A subtype of the

serotonin receptor family, the serotonin 5-HT2A receptor,has been implicated in pathologies such as schizophrenia,

depression, anorexia and anxiety, both in humans [26]

and in animals [79]. Imaging of the 5-HT2A receptor with

SPECT is a valuable tool to aid psychiatrists in the

diagnosis of these pathologies. Several radioligands have

already been used to study the 5-HT2A receptor in the

brain, mostly PET tracers. [11C]-MDL100907 is the most

frequently used tracer for imaging of the 5-HT2A receptor

with PET [10]. [18F]-Altanserin [11] and [18F]-setoperone

[12] have also been used. Currently, only one SPECT

tracer ([123I]-R91150), has been used clinically for imaging

of the 5-HT2A receptor [13,14]. Since this radioligand

shows high aspecific binding in vitro, we decided todevelop a new potential SPECT tracer for imaging of the

5-HT2A receptor.

[123I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-

yl]methanone ([123I]-3-I-CO) [15] was synthesized from the

corresponding tributylstannylprecursor (1) using chlora-

mine-T in acetic acid (Fig. 1).

The radioligand is an antagonist with high affinity for the

5-HT2A receptor (Ki=0.51 nM) and selectivity of at least a

factor 20 over other receptor (sub)types, including the

Available online at www.sciencedirect.com

Nuclear Medicine and Biology 35 (2008) 861867www.elsevier.com/locate/nucmedbio

Corresponding author. Tel.: +32 9 264 8065; fax: +32 9 264 8071.

E-mail addresses: [email protected] ,

[email protected] (P.B.M. Blanckaert).

0969-8051/$ see front matter 2008 Elsevier Inc. All rights reserved.

doi:10.1016/j.nucmedbio.2008.09.004
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5-HT2C receptor[15]. The potential of the ligand as a single

photon emission computed tomography (SPECT) tracer for

the serotonin 5-HT2A receptor tracer was determined by

performing a regional brain biodistribution study with [123I]-

3-I-CO in SpragueDawley rats. To demonstrate specific

binding of the radioligand to the 5-HT2A receptor, a

displacement study using ketanserin as the displacing agent

was performed. Also, a metabolite analysis was executed to

exclude the presence of radiolabelled metabolites in brain.

Precursor synthesis, radiosynthesis and biodistribution

studies in mice have already been published elsewhere [16].

P-glycoprotein is an adenosine triphosphate-driven efflux

protein located amongst others in the blood-brain barrier. It is

also over expressed in different tumour types [17]. It was

demonstrated recently that blocking of P-glycoprotein

function with cyclosporin A had a profound effect on the

brain uptake of several radioligands [1820], resulting in a

strong increase in brain radioactivity concentration.

P-glycoprotein modulation could prove useful for

increasing drug concentrations (a.o. chemotherapeutics)

and in oncology where P-glycoprotein (Pgp) plays a role in

multidrug-resistance [21]. Also, several central nervoussystem-active drugs such as phenytoin (antiepileptic),

clomipramine (antidepressant) and chlorpromazine (neuro-

leptic) are substrates for Pgp. Possibly, their brain concen-

tration (and, hence, their therapeutic effect) can be altered by

modulation of Pgp [17]. Radioligands with demonstrated

Pgp affinity could be used for monitoring Pgp activity in the

blood-brain barrier. Also, Pgp efflux can reduce brain uptake

of radiotracers and thus hamper successful imaging [22]. For

these reasons, the influence of Pgp blocking with cyclos-

porin A on the regional brain biodistribution of [123I]-3-I-CO

in rodents was also investigated.

2. Materials and methods

2.1. Chemicals and radiochemicals

All chemicals and reagents were purchased from Acros

Organics (Beerse, Belgium) and were used without further

purification unless described otherwise. Solvents used were

of high-performance liquid chromatography (HPLC) quality

and were purchased from Chemlab (Belgium). No carried

added [123I]-sodium iodide (in 0.05M NaOH) was purchased

from GE Healthcare. Ketanserin was used as the tartrate salt

and was obtained from Tocris Cookson (Bristol, UK). It was

dissolved in 0.9% NaCl solution containing 10% ethanol

(v/v). Cyclosporin A was obtained from SigmaAldrich and

was dissolved in a mixture of ethanol and polyethoxylated

castor oil and diluted with 0.9% NaCl before injection. A

concentration of 50 mg/kg was used.

2.2. Chemistry

Synthesis and radiosynthesis of [123I]-3-I-CO were

performed as described previously [16]. Briefly, [

123

I]-3-I-CO was synthesized starting from the corresponding

tributylstannylprecursor. The precursor was iodinated

using chloramine-T in the presence of glacial acetic acid,

nca [123I]-NaI and ethanol as solvent. Radiosynthesis was

terminated by the addition of sodiummetabisulphite solu-

tion. The radioligand was purified on HPLC, using a

reversed-phase C18 column (Alltech Apollo C18 7250 mm,

5 m) and a 50/50 mixture of acetonitrile and phosphate

buffer (0.02 M, pH 7) as the eluent. Solvent was removed

with a C18 Sep-PAK cartridge, and the radioligand was

formulated for injection in 0.9% NaCl containing 10%

ethanol after sterile filtration. A radiochemical yield of

755% (n=3) was obtained. Radiochemical purity was

always higher then 95%.

2.3. Regional brain biodistribution study in rodents

All animal experiments were conducted according to the

regulations of the Belgian law and the Ghent University local

ethical committee (ECP 05/14). [123I]-3-I-CO (35 MBq)

was injected through the penile vein in male Sprague

Dawley rats (n=3 animals per time point, 200250 g

bodyweight). At predefined time points after injection of

the radioligand (20 min, 40 min, 1 h, 2 h and 4 h), the

animals were sacrificed by decapitation under isoflurane

anaesthesia. Blood was collected from the heart and the brainwas rapidly removed and dissected into different regions: the

cortex (which was further dissected into the frontal cortex,

parietal cortex, occipital cortex and temporal cortex),

cerebellum and a subcortical region. The different brain

and blood samples were weighed and counted for radio-

activity with an automated gamma counter [Cobra, Packard

Canberra, equipped with five 11 NaI(Tl) crystals].

Aliquots of the injected tracer solution (n=3) were weighed

and counted for radioactivity to determine the injected

radioactivity dose received by the animals. Results were

corrected for decay and tissue radioactivity concentrations

Fig. 1. Radiosynthesis of [123I]-3-I-CO.

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were expressed as a percentage of the injected dose per gram

of tissue (% ID/g tissue, meanS.D. values, n=3 per

time point).

2.4. Ketanserin displacement study

Ketanserin was used as the tartrate salt and was dissolved

in 0.9 % NaCl solution containing 10% ethanol at aconcentration of 2 mg/ml. A dosage of 1 mg/kg was used

for displacement experiments. [123I]-3-I-CO (35 MBq)

was injected through the penile vein in male Sprague

Dawley rats (n=3 animals per group). Thirty minutes after

injection of the radioligand, ketanserin tartrate was injected

through the penile vein. One hour after injection of the

radioligand, the animals were sacrificed by decapitation

under isoflurane anaesthesia.

Blood was collected and the brain was rapidly dissected.

All tissues were weighed and counted for radioactivity with

an automated gamma counter. Results were corrected for

decay and are expressed as % ID/g tissue (meanS.D.).

2.5. Influence of Pgp modulation

Cyclosporin A (50 mg) was dissolved in ethanol (100 l).

Polyethoxylated castor oil (900 l, Cremophore EL, Bayer,

Germany) was added and the mixture was shaken for 2 min

on a vortex mixer. The mixture was transferred to a sterile 10

ml injection vial, and sterile saline solution (9 ml) was added

to obtain an injectable solution containing 5 mg/ml

cyclosporin A. This solution could be stored in a refrigerator

for a maximum period of about two months. Male Sprague

Dawley rats (n=3 per group) were anesthetized with

isoflurane and injected in the penile vein with cyclosporin

A (50 mg/kg body weight). Thirty minutes after cyclosporinA administration, the rats were anesthetized with isoflurane

and injected in the penile vein with [123I]-3-I-CO (35

MBq). The animals were sacrificed by decapitation under

isoflurane anaesthesia at selected time points after injection

of the radioligand (20 min, 40 min, 60 min, 2 h and 4 h).

Blood was collected and the brain was rapidly removed and

dissected into different regions. Brain and blood samples

were weighed and counted for radioactivity with an

automated gamma counter. Results were corrected for

decay and expressed as % ID/g tissue (meanS.D., n=3).

The results were compared with the normal brain biodis-

tribution results.

2.6. Metabolite analysis in rodents

Male SpragueDawley rats (weight 200250 g, n=3)

were injected in the penile vein with [123I]-3-I-CO (1020

MBq). At 30 min post injection, the animals were sacrificed

by decapitation under isoflurane anaesthesia. Blood was

collected and the brain was rapidly dissected. Blood samples

were centrifuged for 3 min at 3000g, and the pellet was

discarded. Plasma (200 l) was mixed with acetonitrile (800

l), and the mixture was vortexed for 30 s, followed by

centrifugation for 3 min at 3000g. The supernatant (500 l)

was analysed on HPLC. The brain was transferred to a tube,

and acetonitrile (2 ml) was added. The mixture was cooled

on ice, homogenized for 30 s using a mixer (Ultra Turrax

T18 basic, IKA Works, Setting 5) and finally centrifuged at

3000gfor 3 min. The supernatant (500 l) was analysed on

HPLC. An Alltech Econosil C18 column (25010 mm; 10

m particle size) was used. The mobile phase was a 60/40

mixture of ethanol and buffer (0.02 M phosphate buffer pH

7.4) at a flow rate of 5 ml/min. A fraction collector was

placed at the end of the HPLC column, and the eluate was

collected in fractions of 5 ml (1 min). The fractions were then

counted for radioactivity with an automated gamma counter.

Extraction efficiency was always N90%.

3. Results and discussion

3.1. Regional brain biodistribution study in rodents

The results of the regional brain biodistribution study

with [123

I]-3-I-CO in Sprague

Dawley rats are shown inFig. 2. Brain radioactivity concentration values (% ID/g

tissue) for all brain regions are indicated in Table 1 as

meanS.D. values (n=3 animals per time point).

These results demonstrate that [123I]-3-I-CO readily

enters the brain. Brain radioactivity concentration was

about 0.25 % ID/g tissue for the whole brain at 1 hour post

injection. A maximum uptake in the target region (frontal

cortex) of 0.6740.074% ID/g tissue at 20 min post injection

was obtained. Uptake in the reference region (cerebellum)

was considerably lower (a maximum value of 0.380.072 %

ID/g tissue was obtained at 20 min post injection).

Initial brain uptake of [123I]-3-I-CO was highest in the

occipital cortex (0.9420.034% ID/g tissue at 20 min post

injection) and frontal cortex (0.6740.074% ID/g tissue at

20 min post injection). Uptake of [123I]-3-I-CO activity in

the blood was consistently low (maximum value was

0.0620.014% ID/g tissue at 20 min post injection). At 1 h

after tracer injection, brain uptake of [123I]-3-I-CO was the

Fig. 2. Regional brain biodistribution study with [123I]-3-I-CO in Sprague

Dawley rats. Results are expressed as % ID/g tissue (meanS.D., n=3 per

time point).

863P.B.M. Blanckaert et al. / Nuclear Medicine and Biology 35 (2008) 861867


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highest in the frontal cortex (0.3490.131 % ID/g tissue at

1 h post injection).

Fig. 2 demonstrates that radioactivity concentration results

for the frontal cortex were well above the radioactivity

concentrations in cerebellum up to 2 h after injection.

Compared to other brain tracers, [123I]-3-I-CO showed a quite

rapid washout out of the brain; the other brain regions showedroughly the same radioactivity clearance pattern (Table 1).

The highest cortex-to-cerebellum ratio was achieved in

the occipital cortex: a maximum ratio of 2.48 was obtained at

20 min post injection. Starting at 1 h after radioligand

injection, the ratio occipital cortex-to-cerebellum stabilised,

varying between 1.3 and 1.2. A maximal frontal cortex-to-

cerebellum ratio of 1.77 was reached at 20 min post injection,

decreasing to 1.66 at 1 h after radioligand injection. The ratio

stabilised around 1.1 at the later time points.

The data obtained in the [123I]-3-I-CO rat biodistribution

study were compared with the biodistribution data of a

clinically used 5-HT2A tracer, [123I]-R91150. Although brain

uptake of [123I]-3-I-CO in frontal cortex was high compared

to [123I]-R91150 (0.3490.131% ID/g and 0.240.01% ID/g,

respectively, at 1 h after injection), [123I]-3-I-CO also

showed considerably more aspecific binding to cerebellum

(0.2110.086% ID/g versus 0.040.002% ID/g for [123I]-

R91150 at 1 h after injection). Moreover, a maximal frontal

cortex-to-cerebellum ratio of only 1.7 was obtained through-

out the study, whereas the biodistribution study with [123I]-

R91150 revealed ratios varying between 7 and 18.

From these data, it is clear that [123I]-3-I-CO showed

considerable aspecific binding to brain tissue, possibly

limiting its potential as a 5-HT2A brain tracer. Also, the level

of specific binding of the radioligand was limited comparedto the aspecific binding of [123I]-3-I-CO in cerebellum.

3.2. Ketanserin displacement study

The influence of displacement with ketanserin on the

regional brain biodistribution of [123I]-3-I-CO in Sprague

Dawley rats can be seen in Fig. 3. [123I]-3-I-CO radioactivity

was displaced by ketanserin in the cortical areas of the brain,

decreasing the radioactivity concentration to the levels

observed in cerebellum. In frontal cortex, radioactivity

concentration decreased from 0.3490.131 % ID/g tissue at

1 h post injection to 0.170.036% ID/g tissue after

displacement with ketanserin. [123I]-3-I-CO activity also

decreased in other parts of the cortex (temporal, occipital and

parietal cortex), although the relative decrease in radio-

activity concentration was the highest in frontal cortex

(50% decrease after displacement with ketanserin).

No significant difference in radioactivity concentration

was found after ketanserin displacement in blood andcerebellum (cerebellum: 0.2110.086% ID/g tissue at 1 h

post injection for the normal group and 0.1840.02% ID/g

tissue after ketanserin displacement; blood: 0.0450.018%

ID/g tissue for the normal and 0.0420.008% ID/g tissue for

the ketanserin-treated group). This is in accordance with

using the cerebellum as a reference region when performing

PET or SPECT brain scans with 5-HT2A tracers. The results

of the displacement study of [123I]-3-I-CO with ketanserin

tartrate indicate that the radioligand binds specifically to the

central 5-HT2A receptor in vivo in SpragueDawley rats.

3.3. Influence of Pgp modulation

The influence of cyclosporin A pretreatment (50 mg/kg

intravenously, administered 30 min before injection of [123I]-

3-I-CO) on the regional brain biodistribution results with

[123I]-3-I-CO in SpragueDawley rats was investigated and

Table 1

Regional brain biodistribution with [123I]-3-I-CO in Sprague-Dawley rats

Time (min)

20 40 60 120 240

Blood 0.0620.01 0.0590.001 0.0450.02 0.0430.001 0.0260.001

Subcortical area 0.430.07 0.3430.08 0.280.13 0.0760.02 0.0420.002

Temporal cortex 0.500.04 0.370.07 0.300.1 0.0780.03 0.0460.002

Frontal cortex 0.670.07 0.430.08 0.350.1 0.0850.04 0.0520.004

Occipital cortex 0.940.03 0.660.2 0.280.09 0.0980.08 0.0740.009

Parietal cortex 0.380.03 0.310.02 0.230.01 0.0510.007 0.0270.001

Cerebellum 0.380.07 0.300.06 0.210.09 0.0810.02 0.0460.002

Results are expressed as % ID/g tissue (meanS.D., n=3 per time point).

Fig. 3. Ketanserin displacement study with [123I]-3-I-CO in Sprague

Dawley rats. Ketanserin was administered 30 min after radioligand injection.

Results are expressed as % ID/g tissue (meanS.D. values).



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can be seen in Fig. 4 (for reasons of clarity, only frontalcortex and cerebellum are shown). Brain radioactivity

concentration values are shown in Table 2.

On average, radioligand uptake doubled in all brain areas

after treatment of the animals with cyclosporin A. A

maximum radioactivity uptake of 0.9050.114% ID/g tissue

was obtained in frontal cortex at 40 min post injection. At

1 h after injection, radioactivity concentration in frontal

cortex was 0.5820.084% ID/g tissue. The radioactivity

concentration in cerebellum was 0.3550.094% ID/g tissue

at 1 h after injection of [123I]-3-I-CO. Radioactivity

concentration in blood remained low and was fairly

constant over time (0.0370.010% ID/g tissue at 2 h afterinjection, 0.0340.003% ID/g tissue at 4 h after injection).

It is clear from Fig. 4 that both frontal cortex and

cerebellum demonstrated an increased uptake of [123I]-3-I-

CO radioactivity after pre-treatment of the animals with

cyclosporin A.

For example, at 1 h after injection of [123I]-3-I-CO,

radioactivity concentration in frontal cortex increased

from 0.3490.131% ID/g tissue for the normal group

to 0.5820.084% ID/g tissue after treatment with

cyclosporin A.

A similar pattern was observed in the cerebellum:

radioactivity concentration increased from 0.2110.086%

ID/g tissue at 1 h after injection for the normal group to

0.3550.094% ID/g tissue at 1 h after injection for the

cyclosporin A group. The relative increase in [123I]-3-I-CO

radioactivity concentration was the same for the cerebellum

and the frontal cortex (a 67% increase). Other brain regions

followed more or less the same pattern. No significant effect

of cyclosporin A administration on the cortex-to-cerebellum

binding index ratios could be established.

Also, no significant difference in radioactivity concentra-

tion in blood could be found between the two treatment

groups. This could indicate that the increase in brain [123I]-3-

I-CO radioactivity concentration after treatment with

cyclosporin A is not the result of an increased influx of

tracer to the brain but, rather, the consequence of a decreased

tracer efflux out of the brain as a result of the Pgp blocking

effect of cyclosporin A.

However, studies with Pgp modulation and other brain

tracers revealed a much larger increase in brain radioactivityconcentration after cyclosporin A treatment (e.g., [123I]-

R91150 radioactivity concentration throughout the brain

increased five- to sixfold after treatment of the animals with

cyclosporin A). This is in contrast with the [123I]-3-I-CO

data, where only a twofold increase was observed.

From the above results, it can be concluded that [123I]-

3-I-CO is a substrate for efflux by Pgp but it is definitely a

much weaker substrate than for example [123I]-R91150.

3.4. Metabolite analysis in rodents

The metabolism pattern of [123I]-3-I-CO in Sprague

Dawley rats was roughly the same as the metabolism

pattern obtained in NMRI mice [16]. In blood, only two

radiolabelled components were present: one component

(90%) had the same HPLC retention time as authentic

[123I]-3-I-CO, and the other component (510%) was

identified as free radioiodide (data not shown). The same

pattern was observed in the brain, only with lower

concentrations of free radioiodide (35%). No lipophilic

radiolabelled metabolites were found that could possibly

interfere with later [123I]-3-I-CO imaging studies. The small

Fig. 4. Regional brain biodistribution study with [123I]-3-I-CO (with and

without cyclosporin A pretreatment). MeanS.D. values are shown (n=3 per

time point).

Table 2

Regional brain biodistribution with [123I]-3-I-CO in SpragueDawley rats after treatment with cyclosporin A

Time (min)

20 40 60 120 240

Blood 0.070.01 0.0470.01 0.0560.01 0.040.01 0.0340.003

Subcortical area 0.650.1 0.670.06 0.350.08 0.180.1 0.0750.01

Temporal cortex 0.560.26 0.810.23 0.320.06 0.140.09 0.0630.003

Frontal cortex 0.610.17 0.910.1 0.580.08 0.220.09 0.070.005

Parietal cortex 0.910.26 0.850.18 0.580.08 0.180.07 0.0710.01

Occipital cortex 0.610.16 0.720.12 0.360.06 0.170.07 0.0550.009

Cerebellum 0.630.11 0.650.09 0.350.09 0.160.06 0.0720.008

The dosage of cyclosporin A was 50 mg/kg. Results are expressed as % ID/g tissue (meanS.D., n=3 per time point).



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amount of free radioiodide found in blood should have no

negative effect.

4. Conclusion

The regional brain biodistribution study in Sprague

Dawley rats with [123I]-3-I-CO demonstrated that the

radioligand readily crossed the bloodbrain barrier, resulting

in concentration of the radioligand in areas of the brain

containing high concentrations of 5-HT2A receptors. Highest

brain radioactivity concentrations were found in the 5-HT2A-

rich areas of the brain (cortex, mostly frontal cortex and

occipital cortex). A maximum radioactivity concentration of

0.6740.074% ID/g tissue was found in frontal cortex at 20

min post injection. Lowest radioactivity concentrations were

found in areas of the brain containing low concentrations of

5-HT2A receptors (cerebellum). Uptake of [123I]-3-I-CO

radioactivity in the blood was consistently low (maximum

value was 0.0620.014% ID/g tissue at 20 min postinjection). A maximal frontal cortex-to-cerebellum binding

index ratio of 1.77 was obtained at 20 min post injection.

Starting 1 h after injection, the cortex-to-cerebellum binding

index ratio stabilised around 1.2. This value is very low

compared to other serotonin 5-HT2A brain SPECT tracers (in

studies with [123I]-R91150, the binding index ratio varied

between 7 and 18).

This low binding index ratio was caused by the

considerable amount of aspecific binding of [123I]-3-I-CO

to brain tissues, limiting the specific signal of the

radioligand. No radiolabelled metabolites were detected in

blood or brain.

Specific binding of the radioligand to the 5-HT2A receptor

was demonstrated in the displacement study with ketanserin.

[123I]-3-I-CO radioactivity was displaced in the 5-HT2A rich

areas in the brain (cortical areas), whereas no displacement

was observed in the brain areas lacking 5-HT2A receptors

(cerebellum). No significant difference was observed in

blood radioactivity concentrations after ketanserin treatment.

The biodistribution study with [123I]-3-I-CO after cyclos-

porin A treatment of SpragueDawley rats demonstrated that

the radioligand is a substrate for Pgp in vivo in rodents.

Blocking of Pgp function with cyclosporin A doubled the

[123I]-3-I-CO radioactivity concentration in brain. However,

other authors reported a five- to sixfold increase in brainradioactivity concentration after cyclosporin A treatment

[18,23]. Contrary to these results, the increase in brain [123I]-

3-I-CO uptake after cyclosporin A treatment was rather

limited, thus leading us to conclude that [123I]-3-I-CO is only

a weak substrate for Pgp efflux in vivo.

The potential of [123I]-3-I-CO as a possible radiotracer for

the serotonin 5-HT2A receptor is probably limited by a fairly

rapid washout of radioactivity out of the brain. The

radioligand demonstrated considerable aspecific binding to

brain tissues, resulting in cortex-to-cerebellum binding index

ratios that are probably too low for successful in vivo

imaging. We can also conclude that [123I]-3-I-CO is most

probably a weak substrate for the Pgp efflux transporter.

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