Assessment of safety, efficacy, and dosimetry of a novel 18-kDa translocator protein ligand, [11C]CB184, in healthy human volunteers
Sakata et al. EJNMMI Research
Assessment of safety, efficacy, and dosimetry of a novel 18-kDa translocator 11 protein ligand, [ C]CB184, in healthy human volunteers
Muneyuki Sakata 0
Kenji Ishibashi 0
Masamichi Imai 2
Kei Wagatsuma 0
Kenji Ishii 0
Kentaro Hatano 1
Kiichi Ishiwata 0 3 4
Jun Toyohara 0
0 Research Team for Neuroimaging, Tokyo Metropolitan Institute of Gerontology , 35-2 Sakae-cho, Itabashi-ku, 173-0015 Tokyo , Japan
1 Faculty of Medicine, University of Tsukuba , Tsukuba , Japan
2 Department of Radiology, Toranomon Hospital , Tokyo , Japan
3 Department of Biofunctional Imaging, Fukushima Medical University , Fukushima , Japan
4 Institute of Cyclotron and Drug Discovery Research, Southern TOHOKU Research Institute for Neuroscience , Koriyama , Japan
Background: N,N-di-n-propyl-2-[2-(4-[11C]methoxyphenyl)-6,8-dichloroimidazol[1,2-a]pyridine-3-yl]acetamide ([11C]CB184) is a novel selective radioligand for the 18-kD translocator protein (TSPO), which is upregulated in activated microglia in the brain, and may be useful in positron emission tomography (PET). We examined the safety, radiation dosimetry, and initial brain imaging with [11C]CB184 in healthy human volunteers. Results: Dynamic [11C]CB184 PET scans (90 min) were performed in five healthy male subjects. During the scan, arterial blood was sampled at various time intervals, and the fraction of the parent compound in plasma was determined with high-performance liquid chromatography. No serious adverse events occurred in any of the subjects throughout the study period. [11C]CB184 was metabolized in the periphery: 36.7% ± 5.7% of the radioactivity in plasma was detected as the unchanged form after 60 min. The total distribution volume (VT) was estimated with a two-tissue compartment model. The VT of [11C]CB184 was highest in the thalamus (5.1 ± 0.4), followed by the cerebellar cortex (4.4 ± 0.2), and others. Although regional differences were small, the observed [11C]CB184 binding pattern was consistent with the TSPO distribution in the normal human brain. Radiation dosimetry was determined in three healthy male subjects using a serial whole-body PET scan acquired over 2 h after [11C]CB184 injection. [11C]CB184 PET demonstrated high uptake in the gallbladder at a later time (>60 min). In urine obtained approximately 100 min post-injection, 0.3% of the total injected radioactivity was recovered, indicating hepatobiliary excretion of radioactivity. The absorbed dose (μGy/MBq) was highest in the kidneys (21.0 ± 0.5) followed by the lungs (16.8 ± 2.7), spleen (16.6 ± 6.6), and pancreas (16.5 ± 2.2). The estimated effective dose for [11C]CB184 was 5.9 ± 0.6 μSv/MBq. Conclusions: This initial evaluation indicated that [11C]CB184 is feasible for imaging of TSPO in the brain.
Microglia are the resident macrophages in the central
nervous system (CNS) and are activated in response to
pathological events such as infectious disease,
inflammation, neuronal injury, ischemia, brain tumors, and
neurodegenerative and neuropsychiatric disorders [1–4]. Therefore,
activation of microglia in response to brain insults could
be used as a disease marker for multiple CNS disorders.
Microglia express the 18-kDa translocator protein
(TSPO), formerly called the peripheral benzodiazepine
receptor, in the outer mitochondrial membrane . In
the healthy brain, the expression level of TSPO in
microglia is low. When microglia are activated in response to
brain injury, TSPO expression is markedly upregulated
. Therefore, overexpression of TSPO is considered a
marker of activated microglia. Thus, radiolabeled TSPO
ligands have been developed as in vivo imaging probes for
detecting activated microglia with positron emission
tomography (PET) in lesioned areas of the brain. This strategy
may be useful for understanding the pathogenesis of
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various CNS disorders and assessing the efficacy of
treatment for neuroinflammation.
The prototype compound,
has been widely used as a PET tracer for imaging
TSPO expression in humans [1, 5, 7, 8]. However,
(R)[11C]PK11195 has several limitations, including its low
signal-to-noise ratio, highly variable kinetics, and
apparent lack of sensitivity for detecting low levels of
microglial activation. These drawbacks of
(R)[11C]PK11195 are mainly due to its low binding affinity
to TSPO and high lipophilicity, which result in high
levels of nonspecific binding and extensive binding to
plasma proteins . Thus, several chemically diverse
radioligands with high affinity for TSPO and lower
lipophilicity have been developed as alternatives to
(R)[11C]PK11195 [9, 10] and evaluated in humans [11–16].
These ligands include phenoxy arylamides (e.g.,
[11C]DAA1106 , [11C]PBR06 , [11C]PBR28 ,
and [18F]FEPPA ) and pyrazolopyrimidines (e.g.,
[11C]DPA-713  and [18F]DPA-714 ). However,
clinical trials with these new TSPO ligands showed
variable results in patients [17, 18], indicating the
importance of developing a variety of new radiotracers
with appropriate sensitivity and specificity. So another
candidate of new TSPO ligand with different structural
class should be considered, which may avoid individual
difference of TSPO binding.
Recently, Hatano et al. developed the
([11C]CB184), as a novel selective radioligand for TSPO
. The affinity of CB184 for TSPO is 7.9 times higher
than that of (R)-PK11195 (Ki = 0.54 and 4.27 nM,
respectively). The relative TSPO binding affinity of CB184
(7.9-fold) to (R)-PK11195 is higher than those of PBR28
(2–5-folds) , DPA-713 (2-fold) , and DPA-714
(1.3-fold) . In addition, CB184 has lower lipophilicity
than (R)-PK11195 (logP = 2.06 and 2.54, respectively).
Preclinical efficacy studies showed that the regional
uptake of [11C]CB184 into inflamed areas is comparable to
uptake of (R)-[11C]PK11195 in the
6-hydroxydopamineinjured striatum  but higher in the herpes
encephalitis rat model . Furthermore, the radiosynthesis of
[11C]CB184 was straightforward with high production
yield  that will meet the GMP standards for human
Very recently, Toyohara et al. conducted preclinical
safety, radiation dosimetry, and the first PET imaging
studies of [11C]CB184 in a normal volunteer . The
radiation-absorbed dose estimated from murine
distribution data is highest in the lung but similar in magnitude
to most other 11C-labeled PET tracers . The absence
of any abnormalities in rats in the acute toxicity test and
the absence of mutagenicity of CB184 together
demonstrated the clinical suitability of [11C]CB184 for use in
PET studies in humans. Furthermore, the first brain
imaging with PET following administration of [11C]CB184
was performed safely in a normal human volunteer.
These findings prompted us to further undertake initial
evaluation of [11C]CB184 in more human subjects in a
phase 1 study. Here, we report the safety, radiation
dosimetry, and initial brain imaging with [11C]CB184 in
healthy human subjects.
The mean ± SD of the administered mass of [11C]CB184
was 4.9 ± 2.1 μg (range, 2.7–8.1 μg). Administration of
[11C]CB184 was well tolerated by all subjects. No adverse
or clinically detectable pharmacologic effects were seen in
any of the eight subjects. No clinically important trends
indicative of a safety concern were noted for laboratory
parameters, vital signs, or electrocardiogram parameters.
Brain PET scanning
Figure 1 shows the representative static [11C]CB184
images (upper row) and magnetic resonance imaging (MRI)
Fig. 1 Representative magnetic resonance and static images of [11C]CB184 PET obtained from a 28-year-old male subject. (Upper) [11C]CB184 PET
images (SUV summed 40–60 min). (Lower) Magnetic resonance images. PET images were smoothed with a Gaussian filter of 4 mm in FWHM
(lower row) of the corresponding slices obtained from a
typical subject. The tracer was homogeneously distributed
in the brain gray matter regions.
Figure 2a shows the mean time activity curves (TACs)
in five brain regions of typical subjects (n = 4) after
intravenous injection of [11C]CB184. Radioactivity in all gray
matter regions peaked at about 5 min. In contrast, one
atypical subject showed faster brain kinetics, resulting in
lower brain uptake than the other four subjects (Fig. 2b).
The preliminary kinetic analysis of the comparison of
Akaike’s information criterion (AIC) (paired t test, P < 0.05)
in all regions investigated showed that the two-tissue
compartment model provided significantly better AIC scores
than the one-tissue compartment model. The rank order of
total distribution volume (VT) values (mL/cm ) of gray
matter regions from the two-tissue compartment model
(n = 4) was thalamus (5.1 ± 0.4) > cerebellum (4.4 ±
0.2) ≈ occipital cortex (4.3 ± 0.2) ≈ putamen (4.0 ± 0.2) ≈
frontal cortex (4.0 ± 0.2) ≈ temporal cortex (3.9 ± 0.4) ≈
parietal cortex (3.9 ± 0.3) > caudate (3.2 ± 0.1). One
atypical subject showed significantly lower VT than that
of the four typical subjects (Fig. 3). However, the
distribution pattern of radioactivity in the brain was similar
among all five subjects.
Plasma radioactivity rapidly decreased after a bolus
injection (Fig. 4a). The concentrations of radioactivity and
the overall shapes of the TACs in blood and plasma were
well matched (Fig. 4b). The results of high-performance
liquid chromatography (HPLC) analysis of plasma are
summarized in Table 1. We found no differences in the
metabolite profile or plasma kinetics between the one
atypical and four typical subjects.
The extraction ratio of plasma radioactivity into
acetonitrile was >93%. In HPLC analysis, the recovery in the
eluate was quantitative. [11C]CB184 was eluted at a
retention time of 8.3 min. Three hydrophilic metabolites
(HM1, 3.2 min; HM2, 4.6 min; and HM3, 6.4 min) and a
lipophilic metabolite (LM1, 9.7 min) were detected. At
60 min after injection, [11C]CB184 was still the main
compound detected (36.7% ± 5.7%, n = 5). The mean
radioactivity voided into urine at 111 ± 17 min (range,
97–132; n = 8) was 0.3% ± 0.0% of the injected activity
(range, 0.2–0.3, n = 8). In urine, broad hydrophilic
metabolites that eluted between the elution front (2.6 min)
and 6.4 min on the chromatogram were dominant
(99.4% ± 0.6%, n = 3). The parent radioligand was not
detected in voided urine.
The representative whole-body distribution of [11C]CB184
in one subject is shown in Fig. 5.
Figure 6 shows the decay-corrected TACs of source
organs for the same subject. The distribution of
[11C]CB184 was consistent with the expected known
distribution of TSPO in the body and was similar to that of
other radioligands for TSPO [23–25].
The lungs had the highest uptake (25% injected activity)
in the first frame. Uptake in the lung decreased thereafter
however still dominant (4.2% injected activity) until the
last frame (Fig. 6a), which reflects the high density of
TSPO. Other organs with high densities of TSPO
including the kidneys (Fig. 6a) and heart wall (Fig. 6b) were
visible and retained radioactivity. The brain, which had a
low TSPO density, was clearly visualized in the early
time course and indicates higher brain permeability of
[11C]CB184 than in other organs (Fig. 6b). The
radioactivity in liver peaked at 20 min and showed the highest
radioactivity of all organs thereafter (Fig. 6a). The
gallbladder was clearly visible, and the radioactivity gradually
increased (Fig. 6c), illustrating the hepatobiliary excretion of
radioactivity. The radioactivity in the urinary bladder was
very low (Fig. 6d) and not visible at any time. The mean ±
SD of radioactivity voided into urine at 132 ± 0 min (n = 3)
was only 0.3% ± 0% (n = 3) of the injected activity.
The normalized number of disintegrations is shown in
Additional file 1: Table S1, and the organ absorbed and
effective doses are shown in Table 2.
Fig. 2 Mean decay-corrected TACs of five brain regions after intravenous injection of [11C]CB184 into typical human subjects (a). Comparison of
TACs in the frontal cortex for typical subjects and an atypical subject (b). Data for typical subjects represent the mean ± SD of four subjects
Fig. 3 Comparison of the VT for typical subjects and an atypical
subject. Data for typical subjects represent the mean with the 95%
confidential interval for four subjects
The highest absorbed dose was observed in the kidneys,
followed by the lungs, spleen, and pancreas. The mean ±
SD estimated effective dose was 5.9 ± 0.6 μSv/MBq.
This is the first clinical study to assess the safety,
radiation dosimetry, and initial brain imaging of [11C]CB184
in a small number of healthy human subjects.
We found that [11C]CB184 was safe and well tolerated,
with no adverse effects in the eight subjects included in
this study. The radiation-absorbed doses were higher in
Fig. 4 Mean decay-corrected TACs of whole blood, plasma,
metabolitecorrected plasma, and fraction of intact [11C]CB184 after intravenous
injection of [11C]CB184 into human subjects (a). Values for 5 min
(b) were extracted from a. Data for the fraction of intact [11C]CB184
represent the mean ± SD of five subjects
Table 1 Percentages of radiolabeled metabolites in plasma after
intravenous injection of [11C]CB184
Data are the mean ± SD for healthy male subjects (n = 5)
HM hydrophilic metabolite, LM lipophilic metabolite
the kidneys, lungs, spleen, pancreas, heart wall, thyroid,
and liver than in the other organs studied but was
nonetheless sufficiently low for clinical use. The individual
organ and total-body doses associated to [11C]CB184
PET were comparable to other 11C-labeled TSPO
ligands [23, 25, 26].
[11C]CB184 was distributed in the gray matter regions.
The regional distribution of [11C]CB184 was consistent
with the TSPO density in the healthy human brain. The
localization of TSPO in the normal human brain has
been demonstrated by in vitro autoradiographic studies
with 3H-PK11195 . The highest signal level of TSPO
was observed in the thalamus, followed by the cerebellum
and other brain regions. Furthermore, the VT values in the
gray matter and the regional distribution patterns of
[11C]CB184 closely resembled those of recently developed
TSPO ligands such as [11C]PBR28  and [11C]DPA-713
 (Table 3). Importantly, inter-individual variations of
VT values were much smaller for [11C]CB184 than those
of [11C]PBR28 and [11C]DPA-713. This small
interindividual variation of VT might have benefits for clinical
studies such as in statistical parametric mapping.
Although the peripheral metabolism of [11C]CB184
was faster than that of (R)-[11C]PK11195 , VT values
of [11C]CB184 were 10 times higher than those of
(R)[11C]PK11195 . This higher VT of [11C]CB184 may
be due to the 7.9 times higher affinity and lower
lipophilicity of CB184 compared to (R)-PK11195 .
In this small number study, we observed unusually
lower binding of [11C]CB184 in the entire brain. We found
no differences in plasma input function between the
unusually lower binding and typical binding. This finding
may indicate mixed-binding affinity of [11C]CB184 to
TSPO in humans, due to the presence of an rs6971
polymorphism in the gene encoding TSPO . Although
imidazopyridineacetamides, like [11C]CB184, have different
structural skeletons from newly developed other TSPO
ligands, recently published data indicate that the
imidazopyridineacetamides, [18F]PBR111  and [123I]CLINDE
, show the influence of a genetic polymorphism on the
Fig. 5 Representative whole-body decay-corrected maximum-intensity-projection images of [11C]CB184. Images were obtained at 1–5, 38–45, and 95–
102 min after intravenous injection of [11C]CB184 into a healthy male subject
TSPO binding. Our human data suggest an approximately
1.4-fold difference between atypical binding and typical
binding. This small difference may indicate the difference
between high-affinity binding and mixed-affinity binding. If
so, the effect of a genetic polymorphism for [11C]CB184
binding may be slightly weaker than that of other
secondgeneration TSPO ligands . To clarify these points,
future studies should be performed to determine whether
significant differences are present in [11C]CB184 binding
among different types of rs6971 polymorphisms. For this
purpose, in vitro autoradiographic analyses in postmortem
human brain  will be effective to prove the influence of
rs6971 polymorphisms on the binding affinity of CB184.
We used a two-tissue compartment model and
calculated VT as the outcome measure related to the cerebral
TSPO density. To estimate the binding potential of
[11C]CB184, a reference tissue model may be useful for
quantification of TSPO. However, as shown in Fig. 3, the
VT in the entire brain region was decreased in an atypical
subject. This indicates that specific binding is present in
Fig. 6 Regional decay-corrected TACs of 16 source organs (a-d) after intravenous injection of [11C]CB184 into the same subject as shown in Fig. 5.
TACs were expressed as percent of injected dose (%ID). Each panels showed the TACs of source organs with high (a), moderate (b), low (c) and
very low (d) radioactivity. Activities in bone marrow were estimated from thoracic and lumbar vertebrae. The initial time point for the lungs
(25%ID at 3.3 min) was deleted because this high value unnecessarily extended the y-axis
Table 2 Organ absorbed doses
Lower large intestine wall
Upper large intestine wall
Data are the mean ± SD for healthy male subjects (n = 3)
the entire brain and avoids the assumption of reference
tissue modeling. Estimation of specific binding requires
estimation of the non-displaceable volume of distribution
(VND) . Estimation of VND requires a pharmacological
blocking study . If nonspecific binding (VND) is
homogeneous between subjects and within an individual
subject, one can estimate the VND values by applying the
polymorphism plot to the [11C]CB184 PET data across
the population of high-affinity and mixed-affinity binding
subjects . We preliminary performed the
polymorphism plot on the current data (Fig. 7). VND was estimated
as the x-intercept value of 1.647.
The limitations of this study are the small sample size
and lack of analysis of rs6971 polymorphisms. Further
clinical studies with a larger sample size and genetic
analysis are planned in our laboratory. The other limitation is
the low density of TSPO in the healthy normal brain,
which obscured confirmation of the sensitivity and
specificity of [11C]CB184. Therefore, an additional study
demonstrating strong TSPO expression in patients, such as
glioma patients , is needed. The most critical
comment is the short half-life of 11C, which considerably
complicates the widespread use of [11C]CB184. The benefit of
using 11C-labeled tracers is their lower radiation burden
when serial PET scans are performed in the same subject.
Very recently, 18F-fluoroethyl derivatives of CB184 were
synthesized and show promising properties for TSPO
imaging to detect neuroinflammation .
The initial findings of the present study in a small group
of subjects indicated that [11C]CB184 PET is feasible for
imaging TSPO expression in the brain with an acceptable
radiation dose and pharmacological safety at the dose
required for adequate PET imaging. The brain uptake of
[11C]CB184 can be calculated as VT, which is an index of
TSPO density. The VT values of [11C]CB184 corresponded
well to the estimated TSPO density in the healthy human
brain. In this small group analysis, we experienced
unusually lower uptake of [11C]CB184, which may suggest
the effect of a genetic polymorphism on the binding of
[11C]CB184 to TSPO.
All experiments were approved by the Tokyo
Metropolitan Institute of Gerontology institutional review board
(IRB) and were performed in accordance with the IRB
rules and policies. All subjects gave study-specific
informed consent to participate in the study, and all
experiments were carried out in accordance with the relevant
guidelines. The study was registered in UMIN-CTR
(UMIN000020139) on December 9, 2015.
Table 3 Comparison of regional gray matter VT of four TSPO ligands measured in human subjects
Data represent the mean ± SD for healthy typical male subjects (n = 4). Data for [11C]DPA-713, (R)-[11C]PK11195, and [11C]PBR28 were taken from previously
published reports [13, 15]
Fig. 7 Estimation of VND of [11C]CB184. Polymorphism plot with
x-intercept representing VND. In this plot, the x-axis is the mean VT
for typical subjects and the y-axis is the difference between VT for
typical and atypical subjects
Eight healthy male subjects, aged 22–34 years (mean
age ± SD, 26 ± 4 years), were enrolled in this study. The
subject inclusion criteria included age between 20 and
60 years old, male, the ability to provide informed
consent, and normal medical history, physical examination
and vital-sign findings. The subject exclusion criteria
included who has dysfunction in the liver and kidneys,
abnormal findings in the CNS, cardiac failure, history of
drug or food allergy, and judged by the clinical
investigator to be inappropriate as a participant in this study. Five
of the eight subjects were recruited into a dynamic brain
PET study. The subjects weighed 50.1–70.6 kg (mean
weight ± SD, 64.6 ± 8.8 kg). For anatomical co-registration,
a three-dimensional (3D) fast spoiled gradient-echo
(repetition time = 7.6 ms, echo time = 3.1 ms, inversion time =
400 ms, matrix = 256 × 256 × 196 voxels) T1-weighted
whole-brain image was acquired for each subject on a
GE Discovery MR750w 3.0T scanner (GE Healthcare,
Wauwatosa, WI). The other three subjects participated
in a whole-body distribution study. The subjects weighed
59.7–84.4 kg (mean weight ± SD, 69.2 ± 13.3 kg). All eight
subjects were free of somatic and neuropsychiatric
illnesses according to their medical history and findings of
physical examination and had no brain abnormalities on
[11C]CB184 was prepared by O-methylation of the
corresponding desmethyl precursor using [11C]methyl triflate
as described previously .
Safety data were collected after administration of
[11C]CB184 and throughout the follow-up period of
1 week in five subjects. Safety monitoring included the
recording of adverse events, changes in vital signs,
physical examination, electrocardiogram, and laboratory
parameters (serum biochemistry and hematology analysis).
The detailed protocol for investigating safety monitoring
was the same as that reported previously .
Brain PET scanning
PET scanning was performed using a Discovery PET/
computed tomography (CT) 710 scanner (GE Healthcare,
Milwaukee, WI) in 3D mode. This scanner has an axial
field of view of 15.7 cm, a spatial resolution of 4.5 mm full
width at half maximum (FWHM), and a Z-axis resolution
of 4.8 mm FWHM. We acquired 47 slices. After low-dose
computed tomography (LD-CT) scanning to correct for
attenuation, [11C]CB184 (609 ± 117 MBq/12.1 ± 6.1 nmol)
was injected into the antecubital vein of each subject as a
bolus for 1 min, and a 90-min dynamic scan (20 s × three
frames, 30 s × three frames, 60 s × five frames, 150 s × five
frames, and 300 s × 14 frames) was performed. Arterial
blood (0.5 mL each) was sampled at 10, 20, 30, 40, 50, 60,
70, 80, 90, 100, 110, 120, 135, 150, and 180 s, as well as at
5, 7, 10, 15, 20, 30, 40, 50, 60, 75, and 90 min. The whole
blood and separated plasma were weighed, and
radioactivity was measured with a NaI (Tl) well scintillation counter
(BeWell Model-QS03 F/B; Molecular Imaging Labo, Suita,
Japan). To analyze the labeled metabolites, 1.5 mL
additional blood was obtained at 3, 10, 20, 30, 40, and 60 min.
After the PET scan, urine was obtained from each subject,
and radioactivity was measured. Unaltered [11C]CB184 in
the plasma was analyzed with HPLC, and the
metabolite-corrected TAC of plasma was obtained as
described previously .
Tomographic images were reconstructed using a
3Dordered subset expectation maximization algorithm
(subset, 16; iteration, 4) with incorporated time-of-flight
information. The dynamic images were post-smoothed
with a 4-mm FWHM Gaussian filter. The data were
reconstructed in 128 × 128 × 47 voxels, and the voxel size
was 2 × 2 × 3.27 mm. Partially overlapping circular
regions of interests (ROIs) that were 10 mm in diameter
were placed on the frontal, temporal, parietal, occipital,
and cerebellar cortices, thalamus, putamen, and head of
the caudate nucleus with reference to the co-registered
MRI. TACs for these ROIs were calculated as becquerel
per milliliter or as standardized uptake value (SUV):
(activity/ml tissue)/(injected activity/g body weight).
Using the TACs of tissues and the metabolite-corrected
TAC of plasma, the VT (K1/k2 × (1 + k3/k4)) for [11C]CB184
was evaluated using the one- and two-tissue compartment
models. The goodness of fit by the two-model analysis was
evaluated using AIC.
The protocol for investigating radiation dosimetry in
human subjects using whole-body imaging was essentially
the same as that reported previously .
Whole-body PET/CT scans were obtained using a
Discovery 710 PET/CT scanner (GE Healthcare) in 3D
mode. LD-CT was used for attenuation correction of the
PET emission scan. The first PET acquisition was started
1 min after the intravenous bolus injection of 763 ±
40 MBq (9.9 ± 1.9 nmol) of [11C]CB184. Then, 128-min
scans (18 frames, 13 bed positions per frame, overlap of
23 of 47 slices per bed, 15 s/bed × four frames, 30 s/
bed × 12 frames, and 60 s/bed × two frames) from the
top of the head to mid-thigh were performed. Images
were reconstructed using a 3D-ordered subset
expectation maximization algorithm (subset, 24; iteration, 2)
with a 6.4-mm Gaussian filter. The recovery of
radioactivity in whole-body PET/CT scans (total activity in
the image/injected radioactivity) was quantitative at the
first frame (89% ± 8%, at 1–5 min after injection, n = 3)
and gradually only a little decreased to the last frame
(76% ± 5%, at 115–128 min after injection, n = 3).
ROIs were manually placed over 16 organs that could
be identified on PET or LD-CT: adrenals, brain,
gallbladder, small intestine, stomach, heart wall, kidneys, liver,
lungs, pancreas, bone marrow (thoracic and lumbar
vertebrae), spleen, testes, thymus, thyroid, and urinary
bladder. The decay-uncorrected and decay-corrected TACs
of organs were calculated as the percent injected dose
(%ID) per milliliter and the %ID per organ. The volume
of bone marrow, in which only part of the organ could
be measured, was substituted by the volume that was
calculated from the mass of red marrow in the adult
male phantom (1.12 kg for 73.7 kg of body weight)
adjusted by the subject’s body weight and 1.04 g/mL as the
specific gravity . The normalized number of
disintegrations (MBq-h/MBq administered) for each source
organ is equal to the area under the time course of
decay-uncorrected curve (%ID/mL) multiplied by the
volume of the organ ROI. The area under the time
course curve was calculated by summing the area from
time zero to the endpoint of the scan and the area from
the endpoint of the scan to infinity. The former area was
calculated by trapezoidal integration. The latter area was
calculated by integration of radioactive decay from the
The absorbed doses in 25 target organs of the adult
male phantom were estimated from the normalized
number of disintegrations of source organs by implementing
the Medical Internal Radiation Dose method using
OLINDA/EXM (Vanderbilt University) . The effective
dose was also calculated by OLINDA/EXM using the
methodology described in International Commission on
Radiological Protection Publication 60 .
Additional file 1: Table S1. Normalized number of disintegrations
calculated from whole-body [11C]CB184 PET in human subjects. (DOC 37 kb)
%ID: Percent injected dose; (R)-[11C]PK11195:
3D: Three-dimensional; AIC: Akaike’s information criteria; CNS: Central
nervous system; CT: Computed tomography; FWHM: Full width at half
maximum; HM: Hydrophilic metabolite; HPLC: High-performance liquid
chromatography; IRB: Institutional review board; LD-CT: Low-dose computed
tomography; LM: Lipophilic metabolite; MRI: Magnetic resonance imaging;
PET: Positron emission tomography; ROIs: Regions of interests; SD: Standard
deviation; SUV: Standardized uptake value; TACs: Time activity curves;
TSPO: 18-kD Translocator protein; VND: Non-displaceable volume of
distribution; VT: Total distribution volume
We thank Mr. Kunpei Hayashi and Mr. Masanari Sakai for the technical support
with the cyclotron operation and radiosynthesis, Mr. Shotaro Yamaguchi for the
assistance in the data acquisition, Ms. Kiyomi Miura for the care of the subjects
during the PET scanning, and Ms. Airin Onishi for the coordination of the
clinical study. This work was supported in part by a Grant-in-Aid for Scientific
Research (B) No. 25293271 from the Japan Society for the Promotion of Science
and a grant from Research Funding for Longevity Sciences from the National
Center for Geriatrics and Gerontology, Japan (21-5).
This work was supported in part by a Grant-in-Aid for Scientific Research (B)
No. 25293271 from the Japan Society for the Promotion of Science and a
grant from Research Funding for Longevity Sciences from the National
Center for Geriatrics and Gerontology, Japan (21-5).
JT, KI (Ishii), and KI (Ishiwata) conceived and designed the experiments. JT, KI
(Ishibashi), MI, KW, and KI (Ishiwata) performed the experiments. JT, MS, MI,
and KW analyzed the data. KH and KI (Ishii) contributed the reagents/
materials/analysis tools. JT and MS wrote the paper. All authors read and
approved the final manuscript.
Consent for publication
Ethics approval and consent to participate
All procedures performed in studies involving human participants were in
accordance with the ethical standards of the Tokyo Metropolitan Institute of
Gerontology institutional review board and/or national research committee
and with the 1964 Helsinki declaration and its later amendments or
comparable ethical standards. Informed consent was obtained from all
individual participants included in the study. The study was registered in
UMIN-CTR (UMIN000020139) on December 9, 2015.
1. Venneti S , Lopresti B , Wiley CA . The peripheral benzodiazepine receptor in microglia: from pathology to imaging . Prog Neurobiol . 2006 ; 80 : 308 - 22 .
2. Kreutzberg GW . Microglia: a sensor for pathological events in the CNS . Trends Neurosci . 1996 ; 19 : 312 - 8 .
3. Graeber MB , Li W , Rodriguez ML . Role of microglia in CNS inflammation . FEBS Lett . 2011 ; 585 : 3798 - 805 .
4. Perry VH , Nicoll JA , Holmes C. Microglia in neurodegenerative disease . Nat Rev Neurol . 2010 ; 6 : 193 - 201 .
5. Papadopoulos V , Baraldi M , Guilarte TR , Kundsen TB , Lacapère JJ , Lindemann P , et al. Translocator protein (18 kDa ): new nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function . Trends Pharmacol Sci . 2006 ; 27 : 402 - 9 .
6. Banati RB . Visualizing microglia activation in vivo . Glia . 2002 ; 40 : 206 - 17 .
7. Doorduin J , de Vries EFJ , Dierckx RA , Klein HC . PET imaging of the peripheral benzodiazepine receptor: monitoring disease progression and therapy response in neurodegenerative disorders . Curr Pharm Des . 2008 ; 14 : 3297 - 315 .
8. Schweitzer PJ , Fallon BA , Mann JJ , Kumar JSD . PET tracers for the peripheral benzodiazepine receptor and uses thereof . Drug Discov Today . 2010 ; 15 : 933 - 42 .
9. Trapani A , Palazzo C , de Candia M , Lasora FM , Trapani G. Targeting of the translocator protein 18 kDa (TSPO): a valuable approach for nuclear and optical imaging of activated microglia . Bioconjug Chem . 2013 ; 24 : 1415 - 28 .
10. Owen DRJ , Matthews PM . Imaging brain microglial activation using positron emission tomography and translocator protein-specific radioligands . Int Rev Neurobiol . 2011 ; 101 : 19 - 39 .
11. Ikoma Y , Yasuno F , Ito H , Suhara T , Ota M , Toyama H , et al. Quantitative analysis for estimating binding potential of the peripheral benzodiazepine receptor with [11C]DAA1106 . J Cereb Blood Flow Metab . 2007 ; 27 : 173 - 84 .
12. Fujimura Y , Zoghbi SS , Simeon FG , Taku A , Pike VW , Innis RB , et al. Quantification of translocator protein (18 kDa) in the human brain with PET and a novel radioligand, 18F-PBR06 . J Nucl Med . 2009 ; 50 : 1047 - 53 .
13. Fujita M , Imaizumi M , Zoghbi SS , Fujimura Y , Farris AG , Suhara T , et al. Kinetic analysis in healthy human of a novel positron emission tomography radioligand to image the peripheral benzodiazepine receptor, a potential biomarker for inflammation . Neuroimage . 2008 ; 40 : 43 - 52 .
14. Rusjan PM , Wilson AA , Bloomfield PM , Vitcu I , Meyer JH , Houle S , et al. Quantification of translocator protein binding in human brain with the novel radioligand [18F]-FEPPA and positron emission tomography . J Cereb Blood Flow Metab . 2011 ; 31 : 1807 - 16 .
15. Endres CJ , Pomper MG , James M , Uzuner O , Hammoud DA , Watkins CC , et al. Initial evaluation of 11C-DPA-713, a novel TSPO PET ligand, in humans . J Nucl Med . 2009 ; 50 : 1276 - 82 .
16. Arlicot N , Vercouillie J , Riberio MJ , Tauber C , Venel Y , Baulieu JL , et al. Initial evaluation in healthy humans of [18F]DPA-714, a potential PET biomarker for neuroinflammation . Nucl Med Biol . 2012 ; 39 : 570 - 8 .
17. Gulyás B , Vas A , Tóth M , Takano A , Varrone A , Cselényi Z , et al. Age and disease related changes in the translocator protein (TSPO) system in the human brain: positron emission tomography measurements with [11C]vinpocetine . Neuroimage . 2011 ; 56 : 1111 - 21 .
18. Varrone A , Mattsson P , Forsberg A , Takano A , Nag S , Gulyás B , et al. In vivo imaging of the 18-kDa translocator protein (TSPO) with [18F]FEDAA1106 and PET does not show increased binding in Alzheimer's disease patients . Eur J Nucl Med Mol Imaging . 2013 ; 40 : 921 - 31 .
19. Hatano K , Sekimata K , Yamada T , Abe J , Ito K , Ogawa M , et al. Radiosynthesis and in vivo evaluation of two imidazopyridineacetamides, [11C]CB184 and [11C]CB190, as a PET tracer for 18 kDa translocator protein: direct comparison with [11C](R)-PK11195 . Ann Nucl Med . 2015 ; 29 : 325 - 35 .
20. Vállez Garcia D , de Vries EFJ , Toyohara J , Ishiwata K , Hatano K , Dierckx RAJO , et al. Evaluation of [11C] CB184 for imaging and quantification of TSPO overexpression in a rat model of herpes encephalitis . Eur J Nucl Med Mol Imaging . 2015 ; 42 : 1106 - 18 .
21. Toyohara J , Sakata M , Hatano K , Yanai S , Endo S , Ishibashi K , et al. Preclinical and first-in-human studies of [11C]CB184 for imaging the 18-kDa translocator protein by positron emission tomography . Ann Nucl Med . 2016 ; 30 : 534 - 43 .
22. van der Aart J , Hallett WA , Rabiner EA , Passhier J , Comley RA . Radiation dose estimates for carbon-11-labeled PET tracers . Nucl Med Biol . 2012 ; 39 : 305 - 14 .
23. Brown AK , Fujita M , Fujimura Y , Liow JS , Stabin M , Ryu YH , et al. Radiation dosimetry and biodistribution in monkey and man of 11C-PBR28: a PET radioligand to image inflammation . J Nucl Med . 2007 ; 48 : 2072 - 9 .
24. Fujimura Y , Kimura Y , Siméon FG , Dickstein LP , Pike VW , Innis RB , et al. Biodistribution and radiation dosimetry in humans of a new PET ligand, 18F-PBR06, to image translocator protein (18 kDa) . J Nucl Med . 2010 ; 51 : 145 - 9 .
25. Hirvonen J , Roivainen A , Virta J , Helin S , Någren K , Rinne JO . Human biodistribution and radiation dosimetry of 11C-(R)-PK11195, the prototypic PET ligand to image inflammation . Eur J Nucl Med Mol Imaging . 2010 ; 37 : 606 - 12 .
26. Brody AL , Okita K , Shieh J , Liang L , Hubert R , Mamoun M , et al. Radiation dosimetry and biodistribution of the translocator protein radiotracer [11C]DAA1106 determined with PET/CT in healthy human volunteers . Nucl Med Biol . 2014 ; 41 : 871 - 5 .
27. Doble A , Malgorius C , Daniel M , Daniel N , Imbault F , Basbaum A , et al. Labelling of peripheral-type benzodiazepine binding sites in human brain with [3H]PK11195: anatomical and subcellular distribution . Brain Res Bull . 1987 ; 18 : 49 - 61 .
28. Owen DRJ , Gunn RN , Rabiner EA , Bennacef I , Fujita M , Kreisl WC , et al. Mixed-affinity binding in humans with 18-kDa translocator protein ligands . J Nucl Med . 2011 ; 52 : 24 - 32 .
29. Guo Q , Colasanti A , Owen DR , Onega M , Kamalakaran A , Bennacef I , et al. Quantification of the specific translocator protein signal of 18F-PBR111 in healthy humans: a genetic polymorphism effect on in vivo binding . J Nucl Med . 2013 ; 54 : 1915 - 23 .
30. Feng L , Svarer C , Thomsen G , de Nijs R , Larsen VA , Jensen P , et al. In vivo quantification of cerebral translocator protein binding in human using 2-chloro-2-(4′-123I-iodophenyl)-3-(N, N-diethyl)-imidazo[1,2-a]pyridine-3- acetamide SPECT . J Nucl Med . 2014 ; 55 : 1966 - 72 .
31. Tiwari AK , Ji B , Yui J , Fujinaga M , Yamasaki T , Xie L , et al. [18F] FEBMP: positron emission tomography imaging of TSPO in a model of neuroinflammation in rats, and in vitro autoradiograms of the human brain . Theranostics . 2015 ; 5 : 961 - 9 .
32. Innis RB , Cunningham VJ , Delforge J , Fujita M , Gjedde A , Gunn RN , et al. Consensus nomenclature for in vivo imaging of reversibly binding radioligands . J Cereb Blood Flow Metab . 2007 ; 27 : 1533 - 9 .
33. Owen DR , Guo Q , Kalk NJ , Colasanti A , Kalogiannopoulou D , Dimber R , et al. Determination of [11C] PBR28 binding potential in vivo: a first human TSPO blocking study . J Cereb Blood Flow Metab . 2014 ; 34 : 989 - 94 .
34. Su Z , Roncaroli F , Durrenberger PF , Coope DJ , Karabatsou K , Hinz R , et al. The 18-kDa mitochondrial translocator protein in human gliomas: an 11C-(R)PK11195 PET imaging and neuropathology study . J Nucl Med . 2015 ; 56 : 512 - 7 .
35. Perrone M , Moon BS , Park HS , Laquintana V , Jung JH , Cultrignelli A , et al. A novel PET imaging probe for the detection and monitoring of translocator protein 18 kDa expression in pathological disorders . Sci Rep . 2016 ; 6 : 20422 .
36. Toyohara J , Sakata M , Oda K , Ishii K , Ito K , Hiura M , et al. Initial human PET studies of metabotropic glutamate receptor type 1 ligand 11C-ITMM . J Nucl Med . 2013 ; 54 : 1302 - 7 .
37. Ito K , Sakata M , Oda K , Wagatsuma K , Toyohara J , Ishibashi K , et al. Comparison of dosimetry between PET/CT and PET alone using 11C-ITMM . Australas Phys Eng Sci Med . 2016 ; 30 : 89 - 96 .
38. Kirschner AS , Ice RD , Beierwalters WH . Radiation dosimetry of 131I-19- iodocholesterol: the pitfalls of using tissue concentration data-reply . J Nucl Med . 1975 ; 16 : 248 - 9 .
39. Stabin MG , Sparks RB , Crowe E. OLINDA/EXM: the second-generation personal computer software for internal dose assessment in nuclear medicine . J Nucl Med . 2005 ; 46 : 1023 - 7 .
40. ICRP, 1991 . 1990 Recommendations of the International Commission on Radiological Protection . ICRP publication 60. Ann ICRP . 1990 ; 21 ( 1-3 ): 4 - 11 .