Syntheses and radiosyntheses of two carbon-11 labeled potent and selective radioligands for imaging vesicular acetylcholine transporter

Abstract

Purpose: The vesicular acetylcholine transporter (VAChT) is a specific biomarker for imaging presynaptic cholinergic neurons. The syntheses and C-11 labeling of two potent enantiopure VAChT inhibitors are reported here.

Procedures: Two VAChT inhibitors, (±)-2 and (±)-6, were successfully synthesized. A chiral HPLC column was used to resolve the enantiomers from each corresponding racemic mixture for in vitro characterization. The radiosyntheses of (-)-[(11)C]2 and (-)-[(11)C]6 from the corresponding desmethyl phenol precursor was accomplished using [(11)C]methyl iodide or [(11)C]methyl triflate, respectively.

Results: The synthesis of (-)-[(11)C]2 was accomplished with 40-50 % radiochemical yield (decay-corrected), SA > 480 GBq/μmol (EOB), and radiochemical purity >99 %. Synthesis of (-)-[(11)C]6 was accomplished with 5-10 % yield, SA > 140 GBq/μmol (EOB), and radiochemical purity >97 %. The radiosynthesis and dose formulation of each tracer was completed in 55-60 min.

Conclusions: Two potent enantiopure VAChT ligands were synthesized and (11)C-labeled with good radiochemical yield and specific activity.

Scheme 1

Syntheses of standard compounds (−)-2 and (+)-2. Reagents and conditions: a) 1a,2,7,7a-tetrahydronaphtho[2,3-b]oxirene, K₂CO₃, EtOH reflux, b) HPLC separation of enantiomers: Chiralcel OD column, mobile phase: 1 % hexane/20 % IPA70% MeOH; flow rate: 4.0 ml/min (+)-2 at 14 min and (−)-2 at 30 min.

Scheme 2

Syntheses of enantiomers of (+)-6 and (−)-6, and the desmethyl counterpart (−)-7. Reagents and conditions: a) 1 eq. (BOC)₂O, K₂CO₃, THF/H₂O, 0 °C to rt, 66 %; b) N,O-dimethylhydroxylamine hydrochloride, NEt₃, BOPCl, DMF, 0 °C to rt, overnight, 54 %; c) 5-bromo-2-methoxypyridine, BuLi, THF, −78 °C, 75 %; d) TFA, CH₂Cl₂, 98 %; e) 1a,2,7,7a-tetrahydronaphtho[2,3-b]oxirene, NEt₃, MeOH, 24 %; f) Separation of enantiomers on chiral HPLC (Chiralcel OD 250×10 mm, mobile phase: 100 % acetonitrile, 4 ml/min), R_t (+)-enantiomer at 12 min and (−)-enantiomer at 18 min; g) TMSI, CHCl₃, 55 °C, 99 %.

Scheme 3

Synthesis of precursor (−)-9. Reagents and conditions: a) Conc. HCl, 100 °C, 8 days, 96 %; b) 1a,2,7,7a-tetrahydronaphtho[2,3-b]oxirene, K₂CO₃, EtOH, reflux, 4 days, 62 %; c) BnBr, K₂CO₃, acetone, 45 °C, overnight, 62 %; d) Separation of enantiomers of 10 on HPLC: Chiralcel OD column 250×10 mm; mobile phase: 60 % isopropanol in hexanes; flow rate: 4.0 ml/min; (+)-enantiomer at 22 min and (−)-enantiomer at 49 min; e) (−)-10, 10 % Pd/C, MeOH, 70 psi, 21 h, 90 %.

Scheme 4

Radiosyntheses of (−)-[¹¹C]2 and (−)-[¹¹C]6.