In Vivo Quantification of Cerebral Translocator Protein Binding in Humans Using 6-Chloro-2-(4′-123I-Iodophenyl)-3-(N,N-Diethyl)-Imidazo[1,2-a]Pyridine-3-Acetamide SPECT

This study provides the first comprehensive quantification of translocator protein (TSPO) binding using SPECT and 6-chloro-2-(4'-I-123-iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a] pyridine-3-acetamide (I-123-CLINDE) in neurologic patients. I-123-CLINDE is structurally related to well-known PET ligands such as F-18-PBR111 and F-18-DPA-714. Methods: Six patients with cerebral stroke and 4 patients with glioblastoma multi-forme (GBM) underwent 150-min dynamic SPECT scans with arterial blood sampling. Four of the patients were rescanned. All patients were genotyped for the rs6971 polymorphism. Volumes of interest were delineated on the individual SPECT scans and the coregistered MR images. Compartmental and graphical models using arterial input or the cerebellum as a reference region were used to quantify I-123-CLINDE binding. Results: Among the 6 models investigated, tlhe 2-tissue-compartment model with arterial input described the time-activity data best. Time-stability analyses suggested that acquisition time should be at least 90 min. Intersubject variation in the cerebellar distribution volume (V-T) was clearly related to the TSPO genotype. In the stroke patients the V-T in the periinfarction zone, compared with V-T in the ipsilateral cerebellum, ranged from 1.4 to 3.4, and in the GBM patients the V-T in the tumor, compared with the V-T in the cerebellum, ranged from 1.8 to 3.4. In areas of gadolinium extravasation, I-123-CLINDE binding parameters were not significantly changed. Thus, I-123-CLINDE binding does not appear to be importantly affected by blood-brain barrier disruption. Conclusion: As demonstrated within a group of stroke and GBM patients, I-123-CLINDE SPECT can be used for quantitative assessment of TSPO expression in vivo. Because of the absence of a region devoid of TSPO, reference tissue models should be used with caution. The 2-tissue-compartment kinetic analysis of a 90-min dynamic scan with arterial blood sampling is recommended for the quantification of I-123-CLINDE binding with SPECT.