Comparison of radiolabeled isatin analogs for imaging apoptosis with positron emission tomography

Introduction: Caspase-3 is one of the executioner caspases activated as a result of apoptosis. Radiolabeled isatins bind to caspase-3 with high affinity and are potential tracers for use with positron emission tomography to image apoptosis. We compared the ability of two novel radiolabeled isatins, [¹⁸F]WC-IV-3 and [¹¹C]WC-98, to detect caspase-3 activation in a rat model of cycloheximide-induced liver injury. Methods: Male Sprague-Dawley rats were treated with cycloheximide and then imaged with microPET 3 h later with [¹⁸F]WC-IV-3 and [¹¹C]WC-98. Biodistribution studies were also performed simultaneously, with caspase-3 activation verified by fluorometric enzyme assay and Western blots. Results: MicroPET imaging studies demonstrated similar behavior of both tracers but with a lower maximum peak with [¹¹C]WC-98 than with [¹⁸F]WC-IV-3. Biodistribution studies demonstrated increased uptake of both tracers in the liver and spleen, but this was statistically significant only in the liver with both compounds. The level of [¹⁸F]WC-IV-3 uptake appeared to correlate roughly with rates of caspase-3 activation by the enzyme assay, but the magnitude of difference between treated and control groups was lower than that observed in previously published data with [¹⁸F]WC-II-89, another radiolabeled isatin analog. Activation was also confirmed in the liver and spleen but not in fat by Western blot. Conclusion: [¹⁸F]WC-IV-3 uptake appears to correlate with increased caspase-3 enzyme activity, but the dynamic range of uptake of these two tracers appears to be less than that seen with [¹⁸F]WC-II-89. Studies are ongoing to verify these results in other animal models of apoptosis.