Design, Synthesis and Evaluation of the First 2-Alkynyl(aza)indole <sup>18</sup>F Probe Targeting α-Synuclein Aggregates.

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Background/Objectives: The role of α-synuclein (α-syn) in the pathogenesis of Parkinson's disease (PD) or neurodegenerative diseases such as Lewy body dementia (LBD) and multiple system atrophy (MSA) is commonly accepted. Through different physiological dysfunctions, abnormal forms of α-syn are generated. These abnormal aggregates accumulate and alter pre- and postsynaptic transmission, in particular that of dopamine. Thus, the development of a diagnostic biomarker of synucleinopathies remains crucial and challenging. The development of an α-syn positron emission tomography (PET) radiopharmaceutical may be suitable to early diagnose and stratify patients, follow up disease progression, and evaluate future therapies. Methods: To develop a selective α-syn PET tracer, we synthesized an original series based on alkynyl(aza)indoles. Fifteen final ligands were synthesized bearing indoles or azaindoles from one side of the alkyne and a substituted phenyl ring for the opposite side of the alkyne. The final ligands were tested to determine Ki and/or Kd toward α-syn, tau, and Aβ. Results: The SAR showed that the indole series exhibited moderate to low affinity for α-syn and, moreover, lower Ki toward Aβ and tau (i.e., compound 39, Ki(αsyn) 21.7 nM, Ki(Aβ) 64.4 nM, Ki(Tau) 27.6 nM), highlighting the low potency of these series to afford an α-syn tracer. The introduction of a nitrogen on the different positions of the phenyl to obtain the corresponding azaindoles resulted for most of the compounds in better affinity for α-syn and selectivity towards Aβ compared to the indole analogs (i.e., compound 43, Ki(αsyn) 4.7 nM, Ki(Aβ) 24.4 nM, and Ki(Tau) 4.61 nM). A fluorinated azaindole derivative was prepared with a view to obtaining a 18F tracer and exhibited the highest affinity for α-syn but without selectivity against tau and Aβ. The radiosynthesis of [18F]45 was performed in a two-step procedure starting from the tosylated and protected precursor. [18F]45 was obtained in 85 ± 5 min with a radiochemical yield of 32 ± 3%. Molar activity, determined from a calibration with stable 45, was around 130 GBq/µmole. The dynamic PET imaging showed that [18F]45 was able to cross the blood-brain barrier, but non-specific uptake was observed, confirming the in vitro results. Conclusions: Although promising nanomolar affinity for the target, the new tracer showed mainly non-specific in vivo uptake in the rat brain, indicating that further pharmacomodulations on the azaindole series are required.

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