Cathepsin S (Cat S) is a cysteine protease broadly expressed in the brain and a key mediator of neuroinflammatory processes, making it an attractive target for molecular imaging agents and drug development. Here, we report the development and evaluation of 18FLY3000328, a positron emission tomography (PET) radiotracer that selectively targets Cat S and is derived directly from the clinical Cat S inhibitor LY3000328. 18FLY3000328 was synthesized via a ruthenium-assisted late-stage 18F-deoxyfluorination of the corresponding phenol precursor. In vitro autoradiography, the marked reduction in tracer uptake under blocking conditions suggests specific binding of 18FLY3000328 to Cat S, and the significantly higher uptake in transgenic 5xFAD mouse brain sections indicates upregulation of Cat S in Alzheimer's disease (AD), consistent with previously reported findings. However, in vivo PET imaging in wild-type mice revealed low blood-brain barrier (BBB) penetration and no significant reduction under blocking conditions, suggesting that limited BBB permeability may constrain its efficiency as a brain PET tracer. Despite its limited brain penetration, these results suggest that 18FLY3000328 may serve as a useful starting scaffold for further optimization of physicochemical and pharmacokinetic properties toward Cat S-targeted PET imaging of neuroinflammation in AD and potentially other neurodegenerative disorders.
Yang et al. (Fri,) studied this question.