Development of agonist PET tracers for quantifying 5HT2AR

描述（由申请人提供）：5 -羟色胺2a受体（5HT2AR）在精神分裂症、抑郁症和神经退行性疾病的病理生理中具有重要作用，也是非典型抗精神病药物和抗抑郁药物的靶点。激动剂示踪剂可以测量与g蛋白结合的受体的高亲和力构象，并且可能对突触内内源性神经递质水平敏感。在我们的实验室使用激动剂放射性示踪剂[125I]LSD进行的体外放射自显影研究表明，与匹配的对照组相比，自杀受害者的5HT2R结合显著增加，然而，其他人和我们使用拮抗剂放射性配体进行的研究无法得出受体结合改变的确切性质。因此，开发一种激动剂放射性示踪剂可能为研究5HT2AR系统在神经精神疾病中的作用提供新的更敏感和准确的工具，并开发创新的治疗方法。拮抗剂和激动剂PET数据的组合将允许在高和低亲和构象状态中单独量化。我们使用拮抗剂放射性配体[11C] m100,907在狒狒和人体内进行了5HT2AR结合的PET定量。然而，由于亚型选择性差和非特异性结合，目前开发的激动剂放射性配体用于研究5HT2AR的潜力有限。因此，我们的目标是开发一种特定的激动剂放射性示踪剂，作为PET成像5HT2AR功能活性状态的探针。[3H]INBMeO是一种强效激动剂5HT2AR配体（1,5ht2ar Ki = 0.044 nM, EC50 = 0.44 nM, Emax = 81%），在体外结合研究中显示出在死后人脑中的特异性结合。INBMeO对许多受体具有优越的选择性，特别是对其他5HT2亚型。我们以高产率和比活性合成了[11C]INBMeO，我们在狒狒体内的PET初步研究表明，该示踪剂可穿透血脑屏障，并优先分布于富含5HT2AR的脑区。我们的目标是通过已知5HT2AR激动剂和拮抗剂的预处理研究，测试复测实验和示踪剂动力学建模来建立特异性，以了解[11C]1对高亲和位点5HT2AR成像的潜力。平行合成一组新的类似物2、3和4，筛选大鼠的5HT2AR亲和力、功能活性、血脑屏障渗透和分布，如果[11C]1被证明是次优结合，则表明最佳结合的配体将作为备份。我们还提议比较研究最成功的激动剂配体与拮抗剂[11C]M100907在狒狒脑中的结合，以评估5HT2AR的总受体浓度和HA构象浓度。在拟议的研究结束时，我们预计将有一种激动剂PET示踪剂作为5HT2AR功能性高亲和力状态的体内人体研究的潜在工具，并证明体内靶点参与治疗靶点的临床前验证。

DESCRIPTION (provided by applicant): Serotonin2A receptors (5HT2AR) have significant role in the pathophysiology of schizophrenia, depression and neurodegenerative diseases and are also targets for atypical antipsychotics and antidepressants. Agonist tracers allow measurement of the high affinity conformation of receptors that can be bound to G-proteins and are potentially sensitive to intra-synaptic levels of endogenous neurotransmitters. In vitro autoradiography studies performed in our laboratories using the agonist radiotracer [125I]LSD indicated a significant increase in 5HT2R binding in suicide victims compared to matched controls, whereas, studies by others and us using antagonist radioligands were unable to derive the precise nature of alterations of the receptor binding. Therefore, developing an agonist radiotracer may offer new more sensitive and accurate tool in investigating the role of 5HT2AR systems in neuropsychiatric disorders, and to develop innovative therapeutics. A combination of the antagonist and agonist PET data would allow separate quantification in the high and low affinity conformational states. We performed in vivo quantification of 5HT2AR binding by PET using the antagonist radioligand [11C] M100, 907 in baboons and human. However, the potential of agonist radioligands developed so far for studying 5HT2AR is limited due to poor subtype selectivity and nonspecific binding. Our objective is therefore, to develop a specific agonist radiotracer as a probe for imaging 5HT2AR in the functionally active state using PET. In vitro binding studies of [3H]INBMeO a potent agonist 5HT2AR ligand (1, 5HT2AR Ki = 0.044 nM, EC50 = 0.44 nM, Emax = 81%) demonstrated specific binding in postmortem human brain. INBMeO has superior selectivity over a number of receptors, particularly to the other 5HT2 subtypes. We synthesized [11C]INBMeO in high yield and specific activity and our preliminary investigation in baboon using PET indicated the tracer penetrates the BBB and distributes preferentially to 5HT2AR enriched brain regions. Our goal is to establish specificity by pretreatment studies with a known 5HT2AR agonist and antagonist, test retest experiments and tracer kinetic modeling to understand the potential of [11C]1 for imaging high affinity site 5HT2AR. A new set of analogues 2, 3 & 4 will be synthesized in parallel, screened for 5HT2AR affinity, functional activity, BBB penetration and distribution in rats and the ligand that indicate the best binding will serve as back up if [11C]1 is proven suboptimal. We are also proposing a comparative study of the binding of the most successful agonist ligand and the antagonist [11C]M100907 binding in baboon brain to evaluate the total receptor concentration and the concentration of HA conformation of 5HT2AR. At the end of the proposed studies we anticipate to have an agonist PET tracer as potential tool for in vivo human studies of the functional high affinity state of 5HT2AR and to demonstrate in vivo target engagement for preclinical validation of therapeutic targets.