Chiral resolution of serial potent and selective σ₁ligands and biological evaluation of (−)-[¹⁸F]TZ3108 in rodent and the nonhuman primate brain

Twelve optically pure enantiomers were obtained using either crystallization or chiral high performance liquid chromatography (HPLC) separation methodologies to resolve six racemic sigma-1 (σ₁) receptor ligands. The in vitro binding affinities of each enantiomer for σ₁, σ₂receptors and vesicular acetylcholine transporter (VAChT) were determined. Out of the 12 optically pure enantiomers, five displayed very high affinities for σ₁(K_i < 2 nM) and high selectivity for σ₁versus σ₂and VAChT (>100-fold). The minus enantiomer, (−)-14a ((−)-TZ3108) (K_{i- σ 1} = 1.8 ± 0.4 nM, K_{i- σ 2} = 6960 ± 810 nM, K_i-VAChT = 980 ± 87 nM), was chosen for radiolabeling and further in vivo evaluation in rodents and nonhuman primates (NHPs). A biodistribution study in Sprague Dawley rats showed brain uptake (%ID/gram) of (−)-[¹⁸F]TZ3108 reached 1.285 ± 0.062 at 5 min and 0.802 ± 0.129 at 120 min. NHP microPET imaging studies revealed higher brain uptake of (−)-[¹⁸F]TZ3108 and more favorable pharmacokinetics compared to its racemic counterpart. Pretreatment of the animal using two structurally different σ₁ligands significantly decreased accumulation of (−)-[¹⁸F]TZ3108 in the brain. Together, our in vivo evaluation results suggest that (−)-[¹⁸F]TZ3108 is a promising positron emission tomography (PET) tracer for quantifying σ₁receptor in the brain.