Introduction Translocator protein (TSPO) or a peripheral benzodiazepine receptor is established as a marker of microglial activation in the process of neuroinflammation in the brain. Activated microglia have been reported to increase in number with age in many studies with the most common TSPO PET tracer [11C]PK11195. This tracer, however, is reputed to suffer from high nonspecific binding. In contrast, a new TSPO PET tracer [11C]DPA713 has been recently reported to bind to TSPO more specifically than [11C]PK11195. To examine the power of [11C]DPA713 in their depiction, we estimated binding potentials (BPs) of [11C]DPA713 for the multiple brain regions in young and old healthy adults and compared them with [11C]PK11195 BPs.
Methods Thirteen healthy young (mean age 21.5±1.8) and 12 healthy elderly (71.6±2.6) adults underwent [11C]DPA713 PET and MRI measurements. BP was estimated by a simplified reference tissue model analysis in which a normalized normal input was used as a reference tissue time activity curve as reported previously. We manually set regions of interest (ROIs) bilaterally on multiple brain regions in individual MRI images and obtained the values of BP by transferring ROIs onto corresponding PET BP images. Then, we compared [11C]DPA713 BP levels between young and old subjects in all ROIs and also examined the differences in magnitude between currently estimated [11C]DPA713 BP and previously estimated [11C]PK11195 BP.
Results In all ROIs, the levels of [11C]DPA713 BP were significant higher in old subjects than in young subjects. Comparison between two tracers showed that levels of [11C]DPA713 BP in the old group were significantly higher than those of [11C]PK11195 BP in a different set of healthy old subjects.
Discussion A new [11C]DPA713 tracer can depict a greater activation of microglia in old subjects than young subjects and improves delineation of activated microglia occurring in elderly compared with a [11C]PK11195 tracer.