Synthesis and biological evaluation of a fluorine-18-labeled nonsteroidal androgen receptor antagonist, N-(3-[¹⁸F]fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide

Introduction: Androgen receptor (AR), which is overexpressed in most prostate cancers, is the target of androgen ablation and antiandrogen therapies: it is also the target for the receptor-mediated imaging of AR-positive prostate cancer using radiolabeled ligands. Previous AR imaging agents were based on a steroidal core labeled with fluorine. To develop a novel class of nonsteroidal imaging agents, with binding and pharmacological characteristics that are more similar to those of clinically used AR antagonists, we synthesized N-(3-fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide (3-F-NNDI), an analog of recently reported AR antagonist ligands. Methods: 3-F-NNDI was synthesized in six steps starting with 1-nitronaphthalene, with fluorine incorporation as the final step. The labeling of 3-F-NNDI with fluorine-18 was achieved through a novel, extremely mild, S_NAr displacement reaction of an o-nitro-activated arene trimethylammonium salt, and 3-[¹⁸F]F-NNDI was prepared in high specific activity. Results and Discussion: 3-F-NNDI was found to have an AR-binding affinity similar to that of its parent compound. In vitro assays demonstrated high stability of the labeled compound under physiological conditions in buffer and in the blood. Androgen target tissue uptake in diethylstilbestrol-pretreated male rats, however, was minimal, probably because of extensive metabolic defluorination the radiolabeled ligand. Conclusions: This study is part of our first look at a novel class of nonsteroidal AR antagonists as positron emission tomography (PET) imaging agents that are alternatives to steroidal AR agonist-based imaging agents. Although 3-[¹⁸F]F-NNDI has significant affinity for AR, it showed limited promise as a PET imaging agent because of its poor target tissue distribution properties.