Monitoring of brown adipose tissue with β3-adrenergic receptor agonist using [18F]FDG PET/CT and [123I]mIBG SPECT/CT

Brown adipose tissue (BAT) is a thermogenic organ regulated by sympathetic nervous activation via β3-adrenergic receptors (β3-AR) and serves as a target for metabolic imaging. This study evaluated BAT activation induced by the selective β3-AR agonist CL316,243 using [ 18 F]FDG PET/CT and [ 123 I]mIBG SPECT/CT in male BALB/c mice. Following daily intraperitoneal treatment with CL316,243, uptake of two tracers in interscapular BAT was quantified using volume-of-interest analysis and ex vivo biodistribution. Uncoupling protein 1 (UCP1) expression in BAT was measured by Western blot. Both [ 18 F]FDG PET/CT and [ 123 I]mIBG SPECT/CT successfully monitored CL316,243 induced BAT activation. [ 18 F]FDG PET/CT demonstrated visually conspicuous and quantitatively higher CL316,243-induced uptake differences compared with [ 123 I]mIBG SPECT/CT. Conversely, ex vivo biodistribution revealed a higher relative fold increase in [ 123 I]mIBG uptake in interscapular BAT than in [ 18 F]FDG uptake following treatment. White adipose tissue also showed increased uptake of two tracers with CL316,243 treatment. Western blot confirmed elevated UCP1 expression in BAT after CL316,243 treatment. These findings indicate that under the imaging conditions used in this study, [ 18 F]FDG PET/CT showed greater visual and image-derived contrast than [ 123 I]mIBG SPECT/CT, demonstrating complementary roles for both tracers in evaluating BAT biology.