RT Journal Article SR Electronic T1 Biodistribution of 18F-FES in Patients with Metastatic ER+ Breast Cancer Undergoing Treatment with Rintodestrant (G1T48), a Novel Selective ER Degrader JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 694 OP 699 DO 10.2967/jnumed.121.262500 VO 63 IS 5 A1 Iqbal, Ramsha A1 Yaqub, Maqsood A1 Oprea-Lager, Daniela E. A1 Liu, Yeukman A1 Luik, Anne Marije A1 Beelen, Andy P. A1 Schuit, Robert C. A1 Windhorst, Albert D. A1 Boellaard, Ronald A1 Menke-van der Houven van Oordt, Catharina W. YR 2022 UL http://jnm.snmjournals.org/content/63/5/694.abstract AB 16α-18F-fluoro-17β-estradiol (18F-FES) is a PET tracer characterizing the expression of the estrogen receptor (ER). Because therapy can interfere with the kinetics and biodistribution of 18F-FES, the aim of this study was to describe the biodistribution of 18F-FES in patients with metastatic ER-positive (ER+) breast cancer undergoing treatment with rintodestrant (G1T48), a novel selective ER degrader. Methods: Eight patients underwent 18F-FES PET/CT imaging at baseline, 4–6 wk during treatment with rintodestrant (interim), and after treatment. After intravenous administration of 200 MBq (±10%) of 18F-FES, a 50-min dynamic PET/CT scan of the thorax was obtained, followed by a whole-body PET/CT scan 60 min after injection. Blood samples were drawn for measuring whole blood and plasma activity concentration and the parent fraction of 18F-FES. Volumes of interest were placed in the aorta ascendens and in healthy tissues on both dynamic and whole-body PET scans. SUVs and target-to-blood ratios (TBRs) were calculated. Areas under the curve (AUCs) of input functions and time–activity curves were calculated as a measure of uptake in different regions. Results: 18F-FES concentration in whole blood (and plasma) significantly (P < 0.05) increased at interim with median AUCs of 96.6, 116.6, and 110.3 at baseline, interim, and after treatment, respectively. In ER-expressing tissues, that is, the uterus and the pituitary gland, both SUV and TBR showed high 18F-FES uptake at baseline, followed by a decrease in uptake at interim (uterus: SUV −50.6% and TBR −58.5%; pituitary gland: SUV −39.0% and TBR −48.3%), which tended to return to baseline values after treatment (uterus: SUV −21.5% and TBR −37.9%; pituitary gland: SUV −14.2% and TBR −26.0%, compared with baseline). In other healthy tissues, tracer uptake remained stable over the 3 time points. Conclusion: The biodistribution of 18F-FES is altered in blood and in ER-expressing healthy tissues during therapy with rintodestrant. This indicates that rintodestrant alters the kinetics of the tracer, possibly affecting interpretation and quantification of 18F-FES uptake. Of note, 6 d or more after treatment with rintodestrant ended, the biodistribution returned to baseline values, consistent with recovery of ER availability after washout of the drug.