[¹⁸F]- and [¹¹C]-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 [ ¹¹C]WC-98

The radiolabeled isatin sulfonamide caspase-3 inhibitor, [¹⁸F]2 (WC-II-89), is a potential PET radiotracer for noninvasive imaging of apoptosis. The radiolabeling mechanism was studied by ¹³C 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 [ ¹⁸F]fluoride, was responsible for the failure of the radiolabeling via nucleophilic substitution of the mesylate group in 7a by [ ¹⁸F]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 [¹⁸F]fluoride, thereby allowing the displacement of the mesylate group to afford the ¹⁸F-labeled isatinate 17. [¹⁸F]17 can be converted to isatin [¹⁸F]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 ¹³C 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 ¹¹C labeled isatin sulfonamide analogue [¹¹C]4 (WC-98). A microPET imaging study using [¹¹C]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 [¹¹C]4 is a potential PET radiotracer for noninvasive imaging of apoptosis.