Radiosynthesis and ex vivo evaluation of (R)-(−)-2-chloro-N-[1-¹¹C-propyl]n-propylnorapomorphine☆

Abstract 

Introduction

Several dopamine D₂ agonist radioligands have been used with positron emission tomography (PET), including [¹¹C-]-(−)-MNPA, [¹¹C-]-(−)-NPA and [¹¹C]-(+)-PHNO. These radioligands are considered particularly powerful for detection of endogenous dopamine release, but they either provide PET brain images with limited contrast or have affinity for both D₂ and D₃ receptors. We here present the carbon-11 radiolabeling and ex vivo evaluation of 2-Cl-(−)-NPA, a novel PET-tracer candidate with high in vitro D₂/D₃ selectivity.

Methods

2-Cl-[¹¹C]-(−)-NPA and [¹¹C]-(−)-NPA were synthesized by a two step N-acylation-reduction process using [¹¹C]-propionyl chloride. Awake rats were injected with either tracer, via the tail vein. The rats were decapitated at various times, the brains were removed and quickly dissected, and plasma metabolites were measured. Radioligand specificity, and P-glycoprotein involvement in brain uptake, was also assessed.

Results

2-Cl-[¹¹C]-(−)-NPA and [¹¹C]-(−)-NPA were produced in high specific activity and purity. 2-Cl-[¹¹C]-(−)-NPA accumulated slower in the striatum than [¹¹C]-(−)-NPA, reaching maximum concentrations after 30 min. The maximal striatal uptake of 2-Cl-[¹¹C]-(−)-NPA (standard uptake value 0.72±0.24) was approximately half that of [¹¹C]-(−)-NPA (standard uptake value 1.37±0.18). Nonspecific uptake was similar for the two compounds. 2-Cl-[¹¹C]-(−)-NPA was metabolized quickly, leaving only 17% of the parent compound in the plasma after 30 min. The specific binding of 2-Cl-[¹¹C]-(−)-NPA was completely blocked and inhibition of P-glycoprotein did not alter the brain uptake.

Conclusion

Ex vivo experiments showed, despite a favorable D₂/D₃ selectivity, that 2-Cl-[¹¹C]-(−)-NPA is inferior to [¹¹C]-(−)-NPA as a PET tracer in rat, because of slower brain uptake and lower specific to nonspecific binding ratio.

Keywords: PET, Agonist, D₂, D₃, Dopamine, Apomorphine, 2-Cl-[¹¹C]-(−)-NPA, [¹¹C]-(−)-NPA, Radioligand, P-glycoprotein, Amphetamine