Effective specific activities determined by scintillation proximity counting for production runs of [18F]FES and 4F-M[18F]FES

Introduction16α-[18F]Fluoro-17β-estradiol ([18F]FES) and various derivatives can be used to image noninvasively the expression of estrogen receptors in breast cancer. A high specific activity is required for successful visualization of ER expression in vivo, particularly for small animal imaging. We describe a simple method for effective specific activity (ESA) measurements of ER-binding ligands.MethodsScintillator-coated polystyrene microplates (FlashPlate) were coated with purified ER of the alpha subtype. [18F]FES and 4-fluoro-11β-methoxy-16α-[18F]fluoroestradiol (4F-M[18F]FES) were prepared by stereoselective opening of their respective cyclic sulfate precursors. After decay of the radioactivity, samples at various dilutions were put in the wells of the FlashPlate along with buffer and [3H]estradiol. On the same FlashPlate, nonradioactive estradiol was placed at concentrations ranging from 10−11 to 10−6 M to provide a standard competition curve.ResultsThe average effective specific activities of different batches of [18F]FES and 4F-M[18F]FES were 1169 (range, 49–6251) and 4695 (range, 413–15,261) Ci/mmol, respectively.ConclusionScintillation proximity technology allows for simple and reproducible measurements of the ESA of receptor-binding radiopharmaceutical for which purified receptors are available.