Assessment of the novel estrogen receptor PET tracer 4-fluoro-11β-methoxy-16α-[(18)F]fluoroestradiol (4FMFES) by PET imaging in a breast cancer murine model.

Purpose: The aim of this study was to compare the in vivo stability, uptake, and positron emission tomography (PET) imaging performance of a novel estrogen receptor PET tracer, 4-fluoro-11β-methoxy-16α-[(18)F]fluoroestradiol (4FMFES), with 16α-[(18)F]fluoroestradiol (FES).
Procedures: MC7-L1 and MC4-L2 (ER+) cell lines and their ERα-knockdown variants (ERαKD) were implanted subcutaneously in Balb/c mice. After 21 days, mice were imaged using either FES or 4FMFES. One hour post-injection, static images were acquired for 30 min and the tumor %ID/g uptake values were derived. Biodistribution data were also obtained 1 h following the injection of either FES or 4FMFES. Blood samples were taken at different times and analyzed on thin-layer chromatography to quantify the presence of radiometabolites for each radiotracer. To assess specific targeting to the estrogen receptors, mice bearing only ER+ tumors were treated with the competitive ER inhibitor fulvestrant 48 h prior to imaging with 4FMFES.
Results: Metabolic stability was found to be similar for both tracers in mice. Both FES and 4FMFES differentiated ER+ tumors from ERαKD tumors in biodistribution and PET imaging studies. 4FMFES achieved a significantly higher %ID/g uptake in ER+ tumors and MC4-L2 ERαKD tumors than FES in the PET imaging studies. Also, tumor-to-background ratio was higher in ER+ tumors using 4FMFES compared to FES. Dissection data showed a significantly higher %ID/g in all tested cell lines and ER-rich tissues using 4FMFES versus FES. Fulvestrant-treated mice had either low or undetectable tumor uptake.
Conclusion: In a tumor-bearing mouse model, 4FMFES achieves better specific tumor uptake and better contrast than FES, making it a promising candidate for ER imaging.