Development of Radiolabeled Ligands Targeting the Glutamate Binding Site of the N‑Methyl‑d‑aspartate Receptor as Potential Imaging Agents for Brain

Abnormal activity of various N-methyl-d-aspartate receptor (NMDAR) subtypes has been implicated in a wide variety of neurological disorders such as Alzheimer’s disease, schizophrenia, and epilepsy. Imaging agents for PET and SPECT that target NMDARs in a subtype-selective fashion may enable better characterization of those disorders and enhance drug development. On the basis of a pyrazoline derivative that demonstrated neuroprotective effects in vivo, we synthesized a series of para-substituted analogues and measured their affinities to various NMDAR subtypes. Compounds 4a–c and 4e showed greater, nanomolar affinity for the GluN1/2A subtype versus GluN1/2B. Dicarbomethoxy (pro-drug) analogues of [124/125I]4d and [11C]4e (i.e., [124/125I]11d and [11C]11e) were generated and tested for NMDAR binding specificity in ex vivo autoradiography and brain biodistribution studies. Although NMDAR-specific binding could be demonstrated for [125I]11d and [11C]11e through autoradiography and biodistribution studies, imaging of neither [124I]11d nor [11C]11e could demonstrate brain penetration sufficient for detection by PET.多种N-甲基-D-天冬氨酸受体（N-methyl-d-aspartate receptor, NMDAR）亚型的异常活化与阿尔茨海默病、精神分裂症、癫痫等多种神经系统疾病密切相关。以亚型选择性方式靶向NMDAR的正电子发射断层显像（PET）与单光子发射计算机断层显像（SPECT）显像剂，可实现对这类疾病的更精准表征，并助力药物研发进程。本研究基于一种在体内展现出神经保护作用的吡唑啉衍生物，合成了一系列对位取代类似物，并测定了它们对不同NMDAR亚型的结合亲和力。实验结果显示，化合物4a–c与4e对GluN1/2A亚型展现出较GluN1/2B亚型更强的纳摩尔级结合亲和力。我们进一步制备了[124/125I]4d与[11C]4e的二甲氧羰基前体药物（prodrug）类似物（即[124/125I]11d与[11C]11e），并通过体外放射自显影术与脑部生物分布实验，测试了其NMDAR结合特异性。尽管通过放射自显影与生物分布实验可证实[125I]11d与[11C]11e具备NMDAR特异性结合能力，但[124I]11d与[11C]11e均无法达到足以被PET检测的脑部摄取量。