The nonpeptidyl caspase binding radioligand (S)-1-(4-(2-[~(18)F]Fluoroethoxy)-benzyl)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin ([~(18)F]CbR) as potential positron emission tomography-compatible apoptosis imaging agent

摘要

Aim. Radiolabeled Avnnexin V-derivatives are well characterized phosphatidylserine-targeting biomarkers and considered as state-of-the-art tracers for non-invasive molecular imaging of apoptosis. In contrast to Annexin V-derived imaging agents being surrogate markers of apoptosis, activated cysteinyl aspartate-specific proteases (caspases) represent the common final path of apoptosis being a suitable in vivo target for the exclusive imaging of apoptotic tissues in vivo. Methods. We suggest 5-pyrrolidinylsulfonyl isatins as a potential nonpeptidyl class of caspase inhibitors for the design of caspase binding radioligands (CbRs), that could be used for in vivo visualization of activated effector caspases. The caspase inhibitor (S)-(+)-5-[1-(2-Methoxy-methylpyrrolidmyl)sulfonyl]isatin 1 (K_(i, caspaSe-3) (1)-60 nM) was chosen as lead structure for the development of nonpeptidyl CbRs. Its structural expansion at the N-1-position the yields moderate lipophilic p-(2-fluo-roethoxy)benzyl variant 2 (log D=2.2), without loss of caspase binding potency (IC_(50, Caspase-3) (2)=36.4 nM). Results. Subsequent automated radiosynthesis of the corresponding ~(18)F-labeled target CbR [~(18)F]2 was performed by direct ~(18)F-labeling of tosylate precursor 4. Conclusion. As shown by biodistribution studies and small animal positron emission tomography a nonpeptidyl 5-pyrrolidinylsulfonyl isatin-type caspase inhibitor (S)-1-(4-(2-[~(18)F]Fluoroetho-xy)benzyl)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin [~(18)F]2 with rapid blood clearance characteristics could potentially detect apoptosis in vivo.