Automated radiosynthesis and biodistribution of [¹⁸F]fluoroflutamide

The androgen receptor (AR) is upregulated in multiple cancers, such as salivary gland cancer, glioblastoma, triple-negative breast cancer, and, most commonly, prostate cancer. Imaging expression of the AR using a radiolabeled AR antagonist would enable non-invasive clinical staging and assessment of tumor heterogeneity via positron emission tomography (PET), complementing the findings of a [⁶⁸Ga]PSMA-11 scan. Previous efforts described the radiolabeling of enzalutamide with fluorine-18 but were limited by low molar activity. In this study, we showcase an efficient, reproducible automated synthesis of [¹⁸F]fluoroflutamide via zinc-mediated radiofluorination and assess its biodistribution in preclinical models of prostate cancer.

From 1.87 Ci (69.19 GBq) of [¹⁸F]fluoride, [¹⁸F]fluoroflutamide was isolated with a non-decay-corrected radiochemical yield of 3.46 ± 1.30% (n = 4) and a radiochemical purity of > 99% in an average 79 min from end-of-beam using a bromoamide precursor. The average A_m was 1.57 Ci/µmol (or 58.23 GBq/µmol) at the end of synthesis, and the radiochemical purity after 4 h was > 97%, indicating negligible radiolysis. A biodistribution and imaging study was performed using nude athymic mice with 22Rv1 human prostate cancer xenografts. From the results of this study, we observed a 'mass effect' in which carrier-added mice displayed an increased uptake of the tracer across most organs.

Herein, we report the optimized automated radiosynthesis of [¹⁸F]fluoroflutamide with high purity and sufficient yield for preclinical evaluation of AR-targeted imaging in mouse models of prostate cancer. The lack of uptake in androgen receptor-rich tissues and xenografts suggests impaired in vivo target engagement. While α-tertiary fluorination may attenuate CYP1A2-mediated oxidation to the higher-affinity 2-hydroxyflutamide metabolite, the observed signal is also consistent with reduced intrinsic AR affinity and/or altered pharmacokinetics of the fluorinated analog.