Pharmacological Characterization of Low-to-Moderate Affinity Opioid Receptor Agonists and Brain Imaging with ¹⁸F‑Labeled Derivatives in Rats

The 3,4-dichloro-N-(1-(dimethylamino)­cyclohexyl)­methyl benzamide scaffold was studied as a template for 18F-positron emission tomography (18F-PET) radiotracer development emphasizing sensitivity to changes in opioid receptor (OR) occupancy over high affinity. Agonist potency, binding affinity, and relevant pharmacological parameters of 15 candidates were investigated. Two promising compounds 3b and 3e with μ-OR (MOR) selective agonist activity in the moderate range (EC50 = 1–100 nM) were subjected to 18F-fluorination, autoradiography, and small-animal PET imaging. Radioligands [18F]3b and [18F]3e were obtained in activity yields of 21 ± 5 and 23 ± 4% and molar activities of 25–40 and 200–300 GBq/μmol, respectively. Displaceable binding matching MOR distribution in the brain was confirmed by imaging. Radioligands showed a rapid pharmacokinetic profile; however, metabolite-corrected, blood-based modeling was required for data analysis. Observed BPND was low, although treatment with naloxone leads to a marked decrease in specific binding, confirming the discovery of a new template for 18F-labeled OR-agonist PET ligands.本研究以3,4-二氯-N-((1-(二甲氨基)环己基)甲基)苯甲酰胺（3,4-dichloro-N-(1-(dimethylamino)cyclohexyl)methyl benzamide）骨架为模板，开展18F-正电子发射断层显像（18F-positron emission tomography, 18F-PET）用放射性示踪剂的开发工作，研究重点聚焦于对阿片受体（opioid receptor, OR）占位变化的敏感性，而非高亲和力。对15个候选化合物的激动剂效价、结合亲和力及相关药理学参数进行了系统考察，最终筛选得到2个兼具中等活性μ型阿片受体（μ-opioid receptor, MOR）选择性激动活性的化合物3b与3e，其半最大效应浓度（EC50）为1~100 nM。随后对上述化合物开展18F氟化修饰、放射自显影及小动物PET成像实验。放射性配体[18F]3b与[18F]3e的放化产率分别为21±5%与23±4%，摩尔活度分别为25~40 GBq/μmol与200~300 GBq/μmol。成像实验证实，脑组织内的可置换结合模式与MOR的分布特征相符。该类放射性配体展现出快速的药代动力学特征，但数据分析需采用经代谢物校正的血液模型。虽观测到的非位移结合电位（BPND）较低，但经纳洛酮处理后特异性结合出现显著下降，证实本研究成功开发出一种可用于18F标记OR激动剂类PET配体的新型骨架。