Abstract
Purpose
To quantify the synaptic vesicle glycoprotein 2A (SV2A) changes in the whole central nervous system (CNS) under pathophysiological conditions, a high affinity SV2A PET radiotracer with improved in vivo stability is desirable to minimize the potential confounding effect of radiometabolites. The aim of this study was to develop such a PET tracer based on the molecular scaffold of UCB-A, and evaluate its pharmacokinetics, in vivo stability, specific binding, and nonspecific binding signals in nonhuman primate brains, in comparison with [11C]UCB-A, [11C]UCB-J, and [18F]SynVesT-1.
Methods
The racemic SDM-16 (4-(3,5-difluorophenyl)-1-((2-methyl-1H-imidazol-1-yl)methyl)pyrrolidin-2-one) and its two enantiomers were synthesized and assayed for in vitro binding affinities to human SV2A. We synthesized the enantiopure [18F]SDM-16 using the corresponding enantiopure arylstannane precursor. Nonhuman primate brain PET scans were performed on FOCUS 220 scanners. Arterial blood was drawn for the measurement of plasma free fraction (fP), radiometabolite analysis, and construction of the plasma input function. Regional time-activity curves (TACs) were fitted with the one-tissue compartment (1TC) model to obtain the volume of distribution (VT). Nondisplaceable binding potential (BPND) was calculated using either the nondisplaceable volume of distribution (VND) or the centrum semiovale (CS) as the reference region.
Results
SDM-16 was synthesized in 3 steps with 44% overall yield and has the highest affinity (Ki = 0.9 nM) to human SV2A among all reported SV2A ligands. [18F]SDM-16 was prepared in about 20% decay-corrected radiochemical yield within 90 min, with greater than 99% radiochemical and enantiomeric purity. This radiotracer displayed high specific binding in monkey brains and was metabolically more stable than the other SV2A PET tracers. The fP of [18F]SDM-16 was 69%, which was higher than those of [11C]UCB-J (46%), [18F]SynVesT-1 (43%), [18F]SynVesT-2 (41%), and [18F]UCB-H (43%). The TACs were well described with the 1TC. The averaged test–retest variability (TRV) was 7 ± 3%, and averaged absolute TRV (aTRV) was 14 ± 7% for the analyzed brain regions.
Conclusion
We have successfully synthesized a novel SV2A PET tracer [18F]SDM-16, which has the highest SV2A binding affinity and metabolical stability among published SV2A PET tracers. The [18F]SDM-16 brain PET images showed superb contrast between gray matter and white matter. Moreover, [18F]SDM-16 showed high specific and reversible binding in the NHP brains, allowing for the reliable and sensitive quantification of SV2A, and has potential applications in the visualization and quantification of SV2A beyond the brain.
摘要
目的
为了在病理生理条件下量化整个中枢神经系统(CNS)中突触囊泡糖蛋白2A(SV2A)的变化,需要一种具有改进的体内稳定性的高亲和力SV2A PET放射性示踪剂,以减少可能的辐射代谢物的混淆效应。本研究旨在基于UCB-A的分子支架开发这样一种PET示踪剂,并评估其在非人灵长类动物大脑中的药代动力学、体内稳定性、特异性结合以及非特异性结合信号,与[11C]UCB-A、[11C]UCB-J和[18F]SynVesT-1进行比较。
方法
合成了SDM-16(4-(3,5-二氟苯基)-1-((2-甲基-1H-咪唑-1-基)甲基)吡咯烷-2-酮)的外消旋体SDM-16及其两个对映体,并对其在体外与人类SV2A的结合亲和力进行测定。使用相应的外消旋芳基锡前体合成了对映纯[18F]SDM-16。在FOCUS 220扫描仪上进行了非人灵长类动物大脑PET扫描。抽取动脉血以测量血浆游离分数(fP)、放射代谢物分析和构建血浆输入功能。将区域时间活性曲线(TACs)与一组织室(1TC)模型拟合,以获得分布容积(VT)。使用非可替代结合势(BPND)计算非可替代分布容积(VND)或半卵圆中心(CS)作为参考区域。
结果
SDM-16经过3步合成,总产率为44%,在所有报道的SV2A配体中对人类SV2A的亲和力最高(Ki=0.9 nM)。[18F]SDM-16在90分钟内以大约20%的衰变校正放射化学产率制备,放射化学纯度和对映纯度均超过99%。该放射性示踪剂在猴脑中显示出高特异性结合,并且在代谢上比其他SV2A PET示踪剂更稳定。[18F]SDM-16的fP为69%,高于[11C]UCB-J(46%)、[18F]SynVesT-1(43%)、[18F]SynVesT-2(41%)和[18F]UCB-H(43%)。TACs很好地描述了1TC。分析的大脑区域的平均测试-重测变异性(TRV)为7±3%,平均绝对TRV(aTRV)为14±7%。
结论
我们成功合成了一种新型SV2A PET示踪剂[18F]SDM-16,其在已发表的SV2A PET示踪剂中具有最高的SV2A结合亲和力和代谢稳定性。[18F]SDM-16大脑PET图像显示出灰质和白质之间的出色对比。此外,[18F]SDM-16在NHP大脑中显示出高特异性和可逆的结合,可靠且敏感地定量SV2A,并具有在大脑之外可视化和定量SV2A的潜在应用。