Dual PET imaging of microtubules and synaptic density in a mouse model of Alzheimer’s Disease

Abstract

AbstractBackgroundWhile microtubule (MT)‐destabilization and synaptic loss hallmark AD pathological events, no study has systematically quantified their in vivo levels in the same brain. Any therapeutic intervention relying on MT destabilization and synaptic vesicle protein (SV2A) levels will benefit from a dual PET imaging strategy, providing target engagement, dosing, and occupancy studies for new drugs. Here, we report MT destabilization and synaptic density imaging results using [11C]MPC‐6827 and [18F]UCB‐H respectively in the same transgenic (TG) and wild‐type (WT) APP/PS1 mice.MethodMale APP/PS1 TG and WT mice (12‐14 mo, 20± 2g, n = 8/group) underwent 30‐min dynamic brain microPET/CT imaging with [11C]MPC‐6827 (0.2±0.02 mCi, iv tail). Four hours later, the same animals underwent another 30‐min brain PET/CT with [18F]UCB‐H (0.2±0.01 mCi, iv tail). A sub‐set of TG and WT mice (n = 4/group) were used to measure ex vivo destabilized MTs and SV2A levels using western blot to confirm MT destabilization and synaptic loss. Whole‐brain standard uptake values (SUVs) were calculated from co‐registered PET‐CT using PMOD. Ex vivo post‐PET biodistribution studies were performed, with percent injected dose‐pe‐ gram (%ID/g) calculated. 48h post‐biodistribution, in vitro autoradiography studies were performed on frozen brain samples from the same animals with [11C]MPC‐6827 and [18F]UCBH; regional uptake was quantified using phosphor‐stimulated luminescence.Result[11C]MPC‐6827 and [18F]UCBH were produced in high (>97±2%) radiochemical purities (n = 20 productions) and specific activities (∼3980±100 mCi/µmol), decay‐corrected to end‐of‐synthesis. Western blot analysis confirmed high destabilized MTs, and low SV2A levels in TG vs. WT. SUV analysis (Fig. 1) showed 56% higher [11C]MPC‐6827 and 66% lower [18F]UCB‐H in vivo brain uptake in TG compared to WT (**p = 0.0032). %ID/g analysis showed 58% higher [11C]MPC‐6827 and 61% lower [18F]UCB‐H ex vivo brain radioactive uptake in TG vs WT (*p = 0.04). Importantly, autoradiography demonstrated 44% higher [11C]MPC‐6827 and 34% lower [18F]UCB‐H in vitro uptake in cortical and hippocampal regions of TG vs. WT (*p = 0.05).ConclusionTG APP/PS1 mice demonstrated higher [11C]MPC‐6827 and lower [18F]UCB‐H brain uptake compared to WT littermates. In vivo PET, ex vivo biodistribution, and in vitro autoradiography data corroborated each other. Preliminary [11C]MPC‐6827 and [18F]UCB‐H PET evaluations showed negative correlations (*r = ‐0.778), suggesting MT destabilization and synaptic loss happen concurrently.