N-Fluoroalkylated and N-Alkylated Analogues of the Dopaminergic D-2 Receptor Antagonist Raclopride

A series of raclopride [(S)-2-[(3,5-dichloro-6-methoxy-2-hydroxybenzamido)methyl]-l-ethylpyrrolidine] derivatives bearing pyrrolidino N-fluoroalkyl or -alkyl substituents were synthesized and evaluated as potential dopaminergic receptor-based positron tomography radiopharmaceuticals. Radiosynthetic procedures for producing the corresponding N-[¹⁸F]fluoroalkylated analogues of raclopride from ¹⁸F- (β+, t1/2 = 110 min) in high specific activity were also developed. In vitro binding assays using competitive displacement of [³H]spiperone from primate caudate tissue indicated that the N-alkylated analogues of raclopride had K_ivalues of 5–40 nM, whereas the corresponding values for analogous N-fluoroalkylated derivatives ranged from 90–160 nM. The relatively low D-2 binding affinity of these fluorinated salicylamides was corroborated by in vivo tissue biodistribution results in rodents. On the basis of structure-binding correlations, the impact of intramolecular hydrogen bonding, ligand basicity, and steric bulk on the affinity of the benzamides for D-2 receptor binding are discussed. Strategies are presented for the development of alternative fluorinated salicylamides that are both receptor active and metabolically stable.