Development of agonist PET tracers for quantifying 5HT2AR

描述（由申请人提供）：5-羟色胺2A受体（5-HT 2AR）在精神分裂症、抑郁症和神经退行性疾病的病理生理学中具有重要作用，也是非典型抗精神病药和抗抑郁药的靶点。激动剂示踪剂允许测量受体的高亲和力构象，所述受体可以结合至G蛋白并且对内源性神经递质的突触内水平潜在敏感。在我们的实验室使用激动剂放射性示踪剂[125 I]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5 HT 2 R结合显著增加，而其他人和我们使用拮抗剂放射性配体的研究无法得出受体结合改变的确切性质。因此，开发激动剂放射性示踪剂可能为研究5 HT 2AR系统在神经精神疾病中的作用提供新的更灵敏和更准确的工具，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亲和力和低亲和力构象状态下单独定量。我们使用拮抗剂放射性配体[11 C] M100，907在狒狒和人中通过PET进行了5 HT 2AR结合的体内定量。然而，由于亚型选择性差和非特异性结合，迄今为止开发的用于研究5 HT 2AR的激动剂放射性配体的潜力是有限的。因此，我们的目标是开发一种特异性激动剂放射性示踪剂作为使用PET在功能活性状态下成像5 HT 2AR的探针。[3 H]INBMeO是一种有效的激动剂5 HT 2AR配体（1，5 HT 2AR Ki = 0.044 nM，EC 50 = 0.44 nM，Emax = 81%）的体外结合研究表明，在死后人脑中具有特异性结合。INBMeO对许多受体具有上级选择性，特别是对其他5-HT 2亚型。我们以高产率和比活性合成了[11 C]INBMeO，并且我们使用PET在狒狒中的初步研究表明示踪剂穿透BBB并优先分布到5 HT 2AR富集的脑区域。我们的目标是建立特异性的预处理研究与已知的5 HT 2AR激动剂和拮抗剂，测试复检实验和示踪动力学建模，以了解潜在的[11 C]1成像高亲和力网站5 HT 2AR。将平行合成一组新的类似物2、3和4，筛选大鼠中的5 HT 2AR亲和力、功能活性、BBB渗透和分布，并且如果[11 C]1被证明是次优的，则指示最佳结合的配体将用作备份。我们还建议对狒狒脑中最成功的激动剂配体和拮抗剂[11 C]M100907的结合进行比较研究，以评估总受体浓度和5 HT 2AR的HA构象浓度。在拟议的研究结束时，我们预计将有一个激动剂PET示踪剂作为潜在的工具，在体内人体研究的功能性高亲和力状态的5 HT 2AR，并证明在体内的目标接合的临床前验证的治疗目标。 公共卫生相关性：5 HT 2AR与精神分裂症、抑郁症、自杀行为的病理生理学有关，并且也是抗精神病作用的推定靶标。目前，还没有成功的放射性示踪剂可用于定量的功能性高亲和力状态的5 HT 2AR，以了解疾病状态的变化的精确性质，因此，我们的目标是开发一种特定的PET示踪剂的5 HT 2AR的体内成像。

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. PUBLIC HEALTH RELEVANCE: 5HT2AR are implicated in the pathophysiology of schizophrenia, depression, suicidal behavior and is also a putative target for antipsychotic action. Currently there is no successful radiotracer available for the quantification of the functional high affinity state of 5HT2AR to understand the precise nature of alterations in diseased states and therefore our goal is to develop a specific PET tracer for the in vivo imaging of 5HT2AR.