[18F]FEDAA1106 was synthesized, as described previously
. The mean injected radioactivity was 153.4 ± 10.2 MBq. The mass injected was less than 2.7 μg.
The injection of the radioligand was performed as a bolus over 30 s via an inserted intravenous line, and the cannula was flushed with 10 mL of saline.
The study was conducted in line with the Helsinki Declaration and approved by the Independent Ethics Committee and the Radiation Safety Committee of the Karolinska University Hospital as well as the Swedish Medical Products Agency. The study was registered at www.ClinicalTrials.gov. Nine patients (seven females/two males; age 34.2 ± 9.1 years old) with relapsing-remitting MS in acute relapse were included in this study; six of them were drug-naive, and three patients were on beta-interferon treatment at the time of the PET measurements. All the patients fulfilled the following criteria: clinical signs of a relapse within a maximum of 21 days before screening, at least two T2 lesions at different locations and at least one Gd-enhanced lesion in the MRI screening and a score between 0 and 5 points on the Kurtzke expanded disability status scale. Five healthy volunteers (four females/one male, age 38.0 ± 9.7 years old) were included in this study. Subjects who had other chronic diseases or conditions or who used concomitant medication that could interfere with measurements, such as treatment with benzodiazepines, were excluded. Likewise, any unstable medical conditions as well as patients with either evidence or history of a severe psychiatric disorder were judged ineligible for this study. Healthy volunteers with abnormal MRI findings for their ages were also excluded. Informed written consent was obtained from all the subjects. The time span between the MRI screening and the PET scan was 3.6 ±1.4 days (1 to 6 days) for MS patients and 9.8 ± 3.9 days (5 to 15 days) for healthy volunteers.
MRI T1-weighted images were acquired using the 1.5 T GE Signa system (GE Medical Systems, Milwaukee, WI, USA). T1-weighted, T2-weighted, and fluid-attenuated inversion recovery (FLAIR) images were acquired for both MS patients and healthy controls. Gd-enhanced T1 images were also acquired for MS patients.
PET measurements with arterial blood sampling
PET measurements were performed on an ECAT Exact HR 47 PET system (CTI/Siemens, Knoxville, TN, USA) operated in 3D mode. The scanner’s three-ring detector block architecture provides a 15-cm-wide field of view. The transversal resolution in the reconstructed image is about 3.8 mm full width at half maximum (FWHM), and the axial resolution is 3.125 mm
. Attenuation correction was obtained with three rotating 68Ge line sources. Emission data of [18F]FEDAA1106 were acquired over 150 min with a 30-min break between 60 and 90 min after the radioligand injection. The frame times were 20 s × 6 frames, 1 min × 4 frames, 3 min × 6 frames, 6 min × 6 frames in the first part, and 6 min × 10 frames in the second part. Images were reconstructed using the standard filtered back projection with a 2-mm Hanning filter.
A catheter was inserted in the radial artery, and arterial blood was collected continuously for 5 min using an automated blood-sampling system at a speed of 5 mL/min (ABSS; Allog AB, Mariefred, Sweden). Blood samples (4 mL) were drawn at 2.5, 9, 20, 30, 40, 60, 90, 120, and 150 min for blood and plasma radioactivity and metabolite correction.
A reversed-phase high-performance liquid chromatography (HPLC) method was used to determine the percentages of radioactivity in plasma that corresponded to both unchanged [18F]FEDAA1106 and its radioactive metabolites during the course of the PET measurement. Plasma (0.5 mL), obtained after centrifugation of blood at 2,000×g for 2 min, was mixed with acetonitrile (0.7 mL). After additional centrifugation of the acetonitrile-plasma mixture (1.1 mL) at 2,000×g for 2 more min, the supernatant was measured in a NaI well counter and then analyzed by radio-HPLC.
The radio-HPLC system consisted of an interface module (D-7000; Hitachi, Chiyoda-ku, Japan), an L-7100 pump (Hitachi), an injector (Rheodyne model 7125 with a 1.0-mL loop; IDEX Corporation, Oak Harbor, WA, USA), and an absorbance detector (L-7400; 254 nm; Hitachi) in series with a radiation detector (Radiomatic 150TR; Packard, Meriden, CT, USA) equipped with a PET flow cell (600 μL cell). A μ-Bondapak-C18 column (300 mm × 7.8 mm, 10 μm; Waters, Milford, MA, USA) was used for metabolite analysis. The following gradient settings were used with a flow rate of 6.0 mL/min: solvents - acetonitrile (A) and phosphoric acid (10 mM) (B); time 0 min 25 (A) and 75 (B); time 4.5 min 80 (A) and 20 (B); time 8.0 min 30 (A) and 70 (B); and time 10.0 min 25 (A) and 75 (B).
Image data analysis
Data analyses were performed focusing on the following three main points: kinetic compartment model analysis for brain anatomical regions, feasibility of estimation of outcome measures for MS lesions, and visual inspection of PET images. Group differences between patients with MS and control subjects were intended to be investigated by the evaluation of brain anatomical regions. As Gd (+) and/or T2/FLAIR high-intensity lesions were considered to reflect on inflammatory demyelization, and the increase of TSPO binding was reported in such areas in previous PET studies
[2, 8], the feasibility of a quantitative evaluation of MRI-defined MS lesions was explored. In one of the previous PET studies
, high TSPO binding was reported in the areas beyond the MRI-defined MS lesions. Additionally, visual inspection of the PET images was performed in order to detect such changes.
Kinetic analysis for brain anatomical regions
PET images of the first and second parts were coregistered to the T1-weighted MRI using an SPM5 software (Wellcome Department of Imaging Neuroscience, London, UK). Regions of interest were manually delineated on the individual T1-weighted MRIs. The following regions were defined: frontal cortex, orbitofrontal cortex, lateral temporal cortex, insular cortex, parietal cortex, occipital cortex, anterior cingulate cortex, posterior cingulate cortex, caudate, putamen, thalamus, hippocampus, cerebellum, midbrain, and pons. A VOI for the whole grey matter was obtained from the segmented MRI. Time-activity curves for regions of interest (ROIs) were generated by applying the ROIs to the corresponding dynamic PET data.
In this study, binding potential (BPND) and total distribution volume (VT), as outcome measures for [18F]FEDAA1106 binding and brain distribution, were calculated using a two-tissue compartment (2TC) model
. Comparisons between patients with MS and control subjects were made for BPND and VT values, respectively. In addition, a medication-dependent stratification of the MS group was made.
Feasibility of estimation of BPND and VT for MS lesions
In the MRI brain images of MS patients, ROIs were delineated on MRI-defined MS lesions revealed by FLAIR and T2 and T1 Gd-enhanced scans. We focused on the MRI-defined MS lesions of the following two groups: Gd-enhanced MS lesions (total 49 lesions) and non-Gd-enhanced-but-T2/FLAIR-high-intensity MS lesions (total 159 lesions).
Time-activity curves for each MS lesion as well as for summed subgroups were generated by applying the ROIs to the dynamic PET data. BPND and VT were calculated by 2TC.
Visual inspection of PET images
For visual inspection of PET images, VT and standard uptake value (SUV) images were generated. VT parametric images were generated using the Logan plot (T* = 48 min)
. SUV images were generated from the 90-to-150-min PET images.
The parametric images were visually compared with MRI images, focusing on MRI-defined MS lesions.
Kinetic analysis and generation of parametric images were performed using the software package PMOD (PMOD 3.0, PMOD Group, Zurich, Switzerland).