Synthesis and Evaluation of F-Enzalutamide, a New Radioligand for PET Imaging of Androgen Receptors: A Comparison with 16β-F-Fluoro-5α-Dihydrotestosterone.

16β-F-fluoro-5α-dihydrotestosterone (F-FDHT) is a radiopharmaceutical that has been investigated as a diagnostic agent for the assessment of androgen receptor (AR) density in prostate cancer using PET. However, F-FDHT is rapidly metabolized in humans and excreted via the kidneys into the urine, potentially compromising the detection of tumor lesions close to the prostate. Enzalutamide is an AR signaling inhibitor currently used in different stages of prostate cancer. Enzalutamide and its primary metabolite -desmethylenzalutamide have an AR affinity comparable to that of FDHT but are excreted mainly via the hepatic route. Radiolabeled enzalutamide could thus be a suitable candidate PET tracer for AR imaging. Here, we describe the radiolabeling of enzalutamide with F. Moreover, the in vitro and in vivo behavior of F-enzalutamide was evaluated and compared with the current standard, F-FDHT. F-enzalutamide was obtained by fluorination of the nitro precursor. In vitro cellular uptake studies with F-enzalutamide and F-FDHT were performed in LNCaP (AR-positive) and HEK293 (AR-negative) cells. Competition assays with both tracers were conducted on the LNCaP (AR-positive) cell line. In vivo PET imaging, ex vivo biodistribution, and metabolite studies with F-enzalutamide and F-FDHT were conducted on athymic nude male mice bearing an LNCaP xenograft in the shoulder. F-enzalutamide was obtained in 1.4% ± 0.9% radiochemical yield with an apparent molar activity of 6.2 ± 10.3 GBq/µmol. F-FDHT was obtained in 1.5% ± 0.8% yield with a molar activity of more than 25 GBq/µmol. Coincubation with an excess of 5α-dihydrotestosterone or enzalutamide significantly reduced the cellular uptake of F-enzalutamide and F-FDHT to about 50% in AR-positive LNCaP cells but not in AR-negative HEK293 cells. PET and biodistribution studies on male mice bearing a LnCaP xenograft showed about 3 times higher tumor uptake for F-enzalutamide than for F-FDHT. Sixty minutes after tracer injection, 93% of F-enzalutamide in plasma was still intact, compared with only 3% of F-FDHT. Despite its lower apparent molar activity, F-enzalutamide shows higher tumor uptake and better metabolic stability than F-FDHT and thus seems to have more favorable properties for imaging of AR with PET. However, further evaluation in other oncologic animal models and patients is warranted to confirm these results.