Preparation of [64Cu]CuCl2
High specific activity 64Cu in the chemical form of copper dichloride was produced at the Unidad Radiofarmacia-Ciclotrón, Facultad de Medicina, UNAM, via the 64Ni(p,n)64Cu reaction with 11-MeV protons, by methods previously reported [9, 10]. After radiochemical purification, the copper fraction was evaporated to dryness and recovered with 5 ml of physiological saline solution and sterilized by passing it through a 0.22-μm syringe filter (Millex-GV). The pH of the reconstituted solution of [64Cu]CuCl2 in physiological saline was around 5.5, with a typical apparent molar activity in the range of 150–700 GBq/μmol as determined by titration of [64Cu]CuCl2 with the chelator 2-S-(4-Aminobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic-acid (p-NH2-Bn-NOTA). Radionuclidic purity at the time of injection was > 99% as determined by gamma spectroscopy in a HPGe detector.
Six healthy volunteers were included (3 women and 3 men; mean age ± SD, 54.3 ± 8.6 years; age range, 47–70 years; mean weight ± SD, 77.2 ± 12.4 kg; weight range, 60–97 kg). Volunteers were recruited at the Instituto Nacional de Neurología y Neurocirugía with the approval of the Bioethical Committee. Written informed consent was obtained from each subject. Prescreening consisted of a detailed review of medical history and a physical examination. Subjects with evidence of clinical disease or a history of organ removal surgery were excluded.
Imaging was performed with a Biograph mCT 20 PET/CT scanner (Siemens Medical Solutions, USA) at the Instituto Nacional de Cancerología. Before administration of the tracer a low-dose CT scan without contrast was acquired for anatomical localization and attenuation correction. After intravenous injection of [64Cu]CuCl2 (4.0 MBq/kg), three consecutive WB emission scans were acquired at 5, 30, and 60 min post-injection (p.i.). In order to keep the X-ray dose as low as possible, subjects remained motionless on the bed of the scanner until the first series of emission scans was completed, so that the same CT scan could be used for the 3 emission scans. Additional PET/CT scans were acquired at 5, 9, and 24 h p.i. The WB PET scans were acquired from the vertex to mid thighs, with 2–3 min per bed position. PET images were reconstructed using a 2D ordered subset expectation maximization (OSEM2D) algorithm, and were corrected for scatter and random events, dead time, and decay. Resulting voxels were stored in units of Bq/cm3. The verification of the cross-calibration between the PET scanner and the dose calibrator was performed by a uniform phantom filled with a 18F solution.
All PET/CT images were archived in Digital Imaging and Communications in Medicine (DICOM) format and were analyzed using OsiriX MD Imaging Software (Pixmeo SARL, Bernex, Switzerland). Only organs with a percentage uptake greater than their mass percentage of total body weight [(organ weight/total body weight)×100] were considered as source organs. Liver, pancreas, kidneys, and bowels met this criterion. The rest of the injected activity was assumed to be homogeneously distributed over the rest of the organs and tissues and was accounted as remainder of the body. Volumes of interest (VOIs) were manually drawn around the organs (slice by slice on the axial plane of CT images) onto each frame of the six acquired scans. Total activity in each organ was determined by multiplying the mean activity concentration (Bq/cm3) by the volume of interest. Tissue distribution expressed as percentage-injected dose per organ (%ID/organ) was plotted against time to obtain the time-activity curves (TACs) of measured organs, including the brain and red bone marrow as organs of interest. As for the bone marrow, the estimation of the amount of activity within this tissue was based on the evaluation of the lumbar vertebrae (L1-L5), as described by McParland .
Radiation dosimetry estimates
In this study, radiation absorbed doses and effective doses were calculated based on the RADAR method  by entering the time-integrated activity coefficient (formerly known as residence time) of each source organ into OLINDA/EXM 1.1 software (Organ Level Internal Dose Assessment Code, Vanderbilt University, Nashville, USA), using the reference adult male and female models . The OLINDA/EXM kinetic analysis module was used to calculate the time-integrated activity coefficient by applying a three-exponential fit to the six data points, switching ON the option for decay corrected data. Organ volumes derived from CT were converted to mass using density values from the International Commission on Radiological Protection Publication 89 (ICRP-89). Finally, the time-integrated activity coefficients for the gastrointestinal tract were estimated using the ICRP 30 gastrointestinal (GI) tract model included in the OLINDA/EXM code assuming an activity fraction of 0.095 (men) and 0.096 (women) enters the small intestine, as determined from the PET images at 24 h p.i. This model assumes that a fraction of injected activity enters the small intestine with no reabsorption.
Data are presented as means ± standard deviation (SD) unless otherwise stated.