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.
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 MRI.
[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 3D-ordered 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 V
2 × (1 + k
4)) 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 endpoint.
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 .