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.
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.