Optimized Quantification of Translocator Protein Radioligand 18F-DPA-714 Uptake in the Brain of Genotyped Healthy Volunteers

International audience; Translocator protein (TSPO) is expressed at a low level in healthy brain and is upregulated during inflammatory processes that may occur in neurodegenerative diseases. Thus, TSPO may be a suitable in vivo indicator of neurodegeneration. Here, we quantified the $^{18}$F-DPA-714 radioligand in healthy TSPO-genotyped volunteers and developed a method to eliminate the need for invasive arterial blood sampling. Methods: Ten controls (7 high-affinity binders [HABs] and 3 mixed-affinity binders [MABs]) underwent $^{18}$F-DPA-714 PET with arterial and venous sampling. $^{18}$F-DPA-714 binding was quantified with a metabolite-corrected arterial plasma input function, using the 1-and 2-tissue-compartment models (TCMs) as well as the Logan analysis to estimate total volume distribution (V$_T$) in the regions of interest. Alternative quantification methods were tested, including tissue-to-plasma ratio or population-based input function approaches normalized by late time points of arterial or venous samples. Results: The distribution pattern of $^{18}$F-DPA-714 was consistent with the known distribution of TSPO in humans, with the thalamus displaying the highest binding and the cerebellum the lowest. The 2-TCM best described the regional kinetics of 18 F-DPA-714 in the brain, with good identifiability (percentage coefficient of variation , 5%). For each region of interest, V$_T$ was 47.6% ± 6.3% higher in HABs than in MABs, and estimates from the 2-TCM and the Logan analyses were highly correlated. Equilibrium was reached at 60 min after injection. V$_T$ calculated with alternative methods using arterial samples was strongly and significantly correlated with that calculated by the 2-TCM. Replacement of arterial with venous sampling in these methods led to a significant but lower correlation. Conclusion: Gen-otyping of subjects is a prerequisite for a reliable quantification of $^{18}$F-DPA-714 PET images. The 2-TCM and the Logan analyses are accurate methods to estimate $^{18}$F-DPA-714 V T in the human brain of both HAB and MAB individuals. Population-based input function and tissue-to-plasma ratio with a single arterial sample are promising alternatives to classic arterial plasma input function. Substitution with venous samples is promising but still requires methodologic improvements.