Synthesis and evaluation of 7α-(3-[¹⁸F]fluoropropyl) estradiol

Introduction: Several lines of evidence suggest that C-7α-substituted estradiol derivatives are well tolerated by estrogen receptor (ER). In line with this hypothesis, we are interested in the design and synthesis of C-7α-substituted estrogens as molecular probes to visualize ER function. Methods: We have synthesized 7α-(3-[¹⁸F]fluoropropyl) estradiol (C3-7α-[¹⁸F]FES) as a potential radiopharmaceutical for ER imaging by positron emission tomography (PET). In vitro receptor binding and in vivo biodistribution and blocking studies in mature female mice, and in vivo metabolite analysis were carried out. Furthermore, in vivo ER-selective uptake was confirmed using ER-positive T-47D and ER-negative MDA-MB-231 tumor-bearing mice. We also compared the in vivo biodistribution of C3-7α-[¹⁸F]FES with 16α-[¹⁸F]FES. Results: C3-7α-[¹⁸F]FES was produced in moderate yields (30.7%±15.1%, decay corrected) with specific activity of 32.0±18.1GBq/μmol (EOS). The in vitro binding affinity of C3-7α-FES to the ERα isoform was sufficient and equivalent to that of estradiol. C3-7α-[¹⁸F]FES showed selective uptake in ER-rich tissues, such as the uterus (4.7%ID/g±1.2%ID/g at 15minutes) and ovary (4.0%ID/g±1.0%ID/g at 5minutes). The tissue time activity curves of these organs showed reversible kinetics, indicating suitability for quantitative analysis. The highest contrast was obtained at 120minutes after injection of C3-7α-[¹⁸F]FES in the uterus (uterus/blood=18, uterus/muscle=17.3) and ovary (ovary/blood=6.3, ovary/muscle=6.0). However, the level of selective uptake of C3-7α-[¹⁸F]FES was significantly lower than that of 16α-[¹⁸F]FES. Most radioactivity in the uterus was detected in unchanged form, although peripherally C3-7α-[¹⁸F]FES was rapidly degraded to hydrophilic metabolites. In accordance with this peripheral metabolism, gradual increases in bone radioactivity were observed, indicating defluorination. Coinjection with estradiol dose-dependently inhibited C3-7α-[¹⁸F]FES uptake in the uterus and ovary. The in vivo IC<inf>50</inf> values of estradiol in the uterus and ovary were 34.4 and 38.5nmol/kg, respectively. Furthermore, in vivo tumor uptake of C3-7α-[¹⁸F]FES was significantly higher (unpaired t test with Welch's correction; p=0.015) in ER-positive T-47D tumors (2.3%ID/g±0.4%ID/g) than ER-negative MDA-MB-231 tumors (0.9%ID/g±0.1%ID/g). Conclusions: Although extensive metabolism was observed in rodents, C3-7α-[¹⁸F]FES showed promising results for quantitative analysis of ER density in vivo. However, the selective uptake of C3-7α-[¹⁸F]FES was lower than that of 16α-[¹⁸F]FES. Further optimizations and structure-activity relationship studies of the C-7α-substituted estradiol are needed.