[F-18]- and [C-11]-Labeled N-benzyl-isatin sulfonamide analogues as PET tracers for Apoptosis: synthesis, radiolabeling mechanism, and in vivo imaging study of apoptosis in Fas-treated mice using [C-11]WC-98

The radiolabeled isatin sulfonamide caspase-3 inhibitor, [F-18]2 (WC-II-89), is a potential PET radiotracer for noninvasive imaging of apoptosis. The radiolabeling mechanism was studied by C-13 NMR, ESI/MS, and computational calculations. It was found that the high electrophilicity of the C3 carbonyl group in the isatin ring, which served as a trap for [F-18]fluoride, was responsible for the failure of the radiolabeling via nucleophilic substitution of the mesylate group in 7a by [F-18]fluoride. Once treated with a strong base, 7a opened the isatin ring completely to form an isatinate intermediate 16, which lost the ability to trap [F-18]fluoride, thereby allowing the displacement of the mesylate group to afford the F-18-labeled isatinate 17. [F-18]17 can be converted to isatin [F-18]2 efficiently under acidic conditions. The ring-opening and re-closure of the isatin ring under basic and acidic conditions were confirmed by reversed phase HPLC analysis, ESI/MS and C-13 NMR studies. Computational studies of model compounds also support the above proposed mechanism. Similarly, the ring-opening and re-closure method was used successfully in the synthesis of the C-11 labeled isatin sulfonamide analogue [C-11]4 (WC-98). A microPET imaging study using [C-11]4 in the Fas liver apoptosis model demonstrated retained activity in the target organ (liver) of the treated mice. Increased caspase-3 activation in the liver was verified by the fluorometric caspase-3 enzyme assay. Therefore, this study provides a useful method for radio-synthesis of isatin derivative radiotracers for PET and SPECT studies, and [C-11]4 is a potential PET radiotracer for noninvasive imaging of apoptosis.