Quantification of TSPO overexpression in a rat model of local neuroinflammation induced by intracerebral injection of LPS by the use of [18F]DPA-714 PET

Purpose [F-18]DPA-714 is a radiotracer with high affinity for TSPO. We have characterized the kinetics of [F-18]DPA-714 in rat brain and evaluated its ability to quantify TSPO expression with PET using a neuroinflammation model induced by unilateral intracerebral injection of lipopolysaccharide (LPS). Methods Dynamic small-animal PET scans with [F-18]DPA-714 were performed in Wistar rats on a FOCUS-220 system for up to 3 h. Both plasma and perfused brain homogenates were analysed using HPLC to quantify radiometabolites. Full kinetic modelling of [F-18]DPA-714 brain uptake was performed using a metabolite-corrected arterial plasma input function. Binding potential (BPND) calculated as the distribution volume ratio minus one (DVR-1) between affected and healthy brain tissue was used as the outcome measure and evaluated against reference tissue models. Results The percentage of intact [F-18]DPA-714 in arterial plasma samples was 92 +/- 4 % at 10 min, 75 +/- 8 % at 40 min and 52 +/- 6 % at 180 min. The radiometabolite fraction in brain was negligible (< 3 % at 30 min). Among the models investigated, the reversible two-tissue (2T) compartment model best described [F-18]DPA-714 brain kinetics. BPND values obtained with a simplified and a multilinear reference tissue model (SRTM, MRTM) using the contralateral striatum as the reference region correlated well (Spearman's r=0.96, p <= 0.003) with 2T BPND values calculated as DVR-1, and showed comparable bias (bias range 17.94 %, 20.32 %). Analysis of stability over time suggested that the acquisition time should be at least 90 min for SRTM and MRTM. Conclusion Quantification of [F-18]DPA-714 binding to TSPO with full kinetic modelling is feasible using a 2T model. SRTM and MRTM can be suggested as reasonable substitutes with the contralateral striatum as the reference region and a scan duration of at least 90 min. However, selection of the reference region depends on the disease model used.