Radiosynthesis and evaluation of a fluorine-18 labeled radioligand targeting vesicular acetylcholine transporter

Vesicular acetylcholine transporter (VAChT) is a reliable biomarker for assessing the loss of cholinergic neurons in the brain that is associated with cognitive impairment of patients. 5-Hydrotetralin compound (±)-5-OH-VAT is potent (K_i = 4.64 ± 0.32 nM) and selective for VAChT (>1800-fold and 398-fold for σ₁ and σ₂ receptor, respectively) with favorable hydrophilicity (LogD = 1.78), while (−)-5-OH-VAT originally serves as the radiolabeling precursor of (−)-[¹⁸F]VAT, a promising VAChT radiotracer with a logD value of 2.56. To evaluate (−)-5-OH-[¹⁸F]VAT as a radiotracer for VAChT, we performed in vitro binding assay to determine the potency of the minus enantiomer (−)-5-OH-VAT and plus enantiomer (+)-5-OH-VAT, indicating that (−)-5-OH-VAT is a more potent VAChT enantiomer. Radiosynthesis of (−)-5-OH-[¹⁸F]VAT was explored using three strategies. (−)-5-OH-[¹⁸F]VAT was achieved with a good yield (24 ± 6%) and high molar activity (∼37 GBq/µmol, at the end of synthesis) using a microwave assisted two-step one-pot procedure that started with di-MOM protected nitro-containing precursor (−)-6. MicroPET studies in the brain of nonhuman primate (NHP) suggest that (−)-5-OH-[¹⁸F]VAT readily penetrated the blood brain barrier and specifically accumulated in the VAChT-enriched striatum with improved washout kinetics from striatum compared to [¹⁸F]VAT. Nevertheless, the lower target to non-target ratio may limit its use for in vivo measurement of the VAChT level in the brain.