Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, PR China; Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104, USA.
Nucl Med Biol. 2019 Apr;71:54-64. doi: 10.1016/j.nucmedbio.2019.05.002. Epub 2019 May 16.
Alzheimer's disease is a common neurodegenerative disease that is characterized by the presence of Aβ plaques in the brain. The FDA has approved the use of Amyvid (florbetapir f18, AV-45) as a PET imaging agent for detecting Aβ plaques in the living human brain. In an attempt to reduce N-demethylation in vivo by taking advantage of more stable C-D bonds, an analog of AV-45, [F]D3FSP ([F]7), was synthesized to improve image contrast for detecting and monitoring the Aβ plaques. A convenient and improved preparation of [F]D3FSP ([F]7) is needed for widespread clinical application. We report herein the optimization of the radiosynthesis and solid-phase extraction (SPE) procedure.
Radiosyntheses of [F]D3FSP ([F]7) under different fluorination conditions were evaluated, and the intermediate, containing an N-Boc protecting group, was deprotected using different acids. One of the major objectives was to simplify the final purification step via SPE to avoid the commonly employed HPLC purification and maximize the radiochemical yields of [F]D3FSP ([F]7) while simultaneously removing several chemical impurities (pseudocarriers). Washing various solid-phase cartridges with different combinations of ethanol/water and acetonitrile/water was explored to optimize the purification step. To evaluate the potential interference in Aβ plaques imaging from the presence of pseudocarriers, each chemical was identified and quantified by LC/MS and HPLC. An in vitro binding assay was employed to evaluate the binding affinities of [F]D3FSP ([F]7) and the pseudocarriers to Aβ plaques using postmortem AD brain tissue.
Using the optimized radiosynthesis method and SPE purification, the final dose of [F]D3FSP ([F]7) was obtained in 50 min with a very low content of pseudocarriers (21.7 ± 5.5 μg). The radiochemical yield was 44.4 ± 5.7% (decay corrected), and the radiochemical purity was >95%. SPE-purified doses of [F]D3FSP ([F]7) displayed excellent binding affinity and specificity for Aβ plaques as measured in an in vitro binding assay using AD brain homogenates, and the OH-pseudocarrier, 8 (K = 19.5 ± 0.5 nM), and the Cl-pseudocarrier, 10 (K = 18.6 ± 3.9 nM), showed lower binding affinities for Aβ plaques than those of AV-45 (K = 8.6 ± 0.5 nM) and D3FSP, 7 (K = 9.8 ± 0.5 nM).
An optimized radiosynthesis and fast SPE purification method suitable for the preparation of clinical doses of [F]D3FSP ([F]7) was accomplished. The results of quality control tests and binding studies suggested that the SPE-purified doses of [F]D3FSP ([F]7) are appropriate for imaging Aβ plaques in the human brain.
阿尔茨海默病是一种常见的神经退行性疾病，其特征是大脑中存在 Aβ 斑块。美国食品和药物管理局已批准使用 Amyvid（氟比他滨 F18，AV-45）作为正电子发射断层扫描（PET）成像剂，用于检测活体人脑中的 Aβ 斑块。为了通过利用更稳定的 C-D 键来减少体内的 N-去甲基化，合成了 AV-45 的类似物[F]D3FSP([F]7)，以提高检测和监测 Aβ 斑块的图像对比度。需要一种方便且改进的[F]D3FSP([F]7)的放射合成方法用于广泛的临床应用。在此，我们报告了放射合成和固相萃取（SPE）程序的优化。
评估了[F]D3FSP([F]7)在不同氟化条件下的放射合成，并使用不同的酸对含有 N-Boc 保护基团的中间体进行脱保护。主要目标之一是通过 SPE 简化最终的纯化步骤，避免常用的 HPLC 纯化，并在同时去除几种化学杂质（伪载体）的情况下，最大限度地提高[F]D3FSP([F]7)的放射化学产率。研究了用不同的乙醇/水和乙腈/水组合洗涤各种固相萃取柱，以优化纯化步骤。为了评估伪载体存在对 Aβ 斑块成像的潜在干扰，通过 LC/MS 和 HPLC 对每种化学物质进行了鉴定和定量。使用 AD 脑匀浆进行体外结合测定，评估[F]D3FSP([F]7)和伪载体与 Aβ 斑块的结合亲和力。
使用优化的放射合成方法和 SPE 纯化，最终剂量的[F]D3FSP([F]7)在 50 分钟内获得，伪载体含量非常低（21.7±5.5μg）。放射化学产率为 44.4±5.7%（经衰变校正），放射化学纯度>95%。SPE 纯化的[F]D3FSP([F]7)剂量在使用 AD 脑匀浆进行的体外结合测定中显示出对 Aβ 斑块的优异结合亲和力和特异性，而 OH-伪载体 8（K=19.5±0.5nM）和 Cl-伪载体 10（K=18.6±3.9nM）对 Aβ 斑块的结合亲和力低于 AV-45（K=8.6±0.5nM）和 D3FSP（K=9.8±0.5nM）。
完成了适合制备[F]D3FSP([F]7)临床剂量的优化放射合成和快速 SPE 纯化方法。质量控制测试和结合研究的结果表明，SPE 纯化的[F]D3FSP([F]7)剂量适合在人体大脑中成像 Aβ 斑块。
