In Vivo Synaptic Density Imaging with C-UCB-J Detects Treatment Effects of Saracatinib in a Mouse Model of Alzheimer Disease.

C-UCB-J is a new PET tracer for synaptic density imaging. Recently, we conducted C-UCB-J PET on patients with mild cognitive impairment or early Alzheimer disease (AD) and found a 41% decrease in specific binding in the hippocampus compared with healthy subjects. We hypothesized that C-UCB-J may have potential to be a general biomarker for evaluating AD treatment effects via monitoring of synaptic density changes. In this study, we performed longitudinal C-UCB-J PET on AD mice to measure the treatment effects of saracatinib, which previously demonstrated synaptic changes with postmortem methods. Nine wild-type (WT) mice and 9 amyloid precursor protein and presenilin 1 double-transgenic (APPswe/PS1ΔE9 [APP/PS1]) mice underwent 3 C-UCB-J PET measurements: at baseline, after treatment, and during drug washout. After baseline measurements, saracatinib, a Fyn kinase inhibitor currently in clinical development for AD treatment, was administered by oral gavage for 41 ± 11 d. Treatment-phase measurements were performed on the last day of treatment, and washout-phase measurements occurred more than 27 d after the end of treatment. SUVs from 30 to 60 min after injection of C-UCB-J were calculated and normalized by the whole-brain (WB) or brain stem (BS) average values as SUV ratio (SUVR or SUVR-1). Hippocampal SUVR at baseline was significantly lower in APP/PS1 than WT mice (APP/PS1: 1.11 ± 0.04, WT: 1.15 ± 0.02, = 0.033, unpaired test). Using SUVR-1 in the hippocampus, there was also a significant difference at baseline (APP/PS1: 0.48 ± 0.13, WT: 0.65 ± 0.10, = 0.017, unpaired test). After treatment with saracatinib, hippocampal SUVR in APP/PS1 mice was significantly increased ( = 0.037, paired test). A trend-level treatment effect was seen with hippocampal SUVR-1 Saracatinib treatment effects may persist, as there were no significant differences between WT and APP/PS1 mice after drug washout. On the basis of the C-UCB-J PET results, hippocampal synaptic density was lower in APP/PS1 mice than in WT mice at baseline, and this deficit was normalized by treatment with saracatinib. These results support the use of C-UCB-J PET to identify disease-specific synaptic deficits and to monitor treatment effects in AD.