Comparison of radiolabeled isatin analogs for imaging caspase-3 activation with positron emission tomography

Objectives Isatin compounds bind with high affinity to caspase-3, an effector enzyme activated during apoptosis. We compared the in vivo behavior of 3 radiolabeled isatin compounds in a murine model of liver apoptosis.

Methods Three isatin compounds, [¹⁸F]WC-II-89, [¹⁸F]WC-IV-3, and [¹¹C]WC-98, were compared in a mouse model of massive liver apoptosis induced by injection of 10 ug i.v. of Jo2 anti-Fas antibody into female BALB/c mice (~18-20 g) 60-90 min prior to tracer injection for biodistribution studies. A blocking study with the pan-caspase inhibitor Q-VD-OPH and a separate dual tracer study with [¹⁸F]WC-II-89 and [^99mTc]mebrofenin were also performed. Caspase-3 activity levels were determined with a fluorometric enzyme assay.

Results There was increased uptake in all organs counted compared to controls, most prominent in the liver, with blood activity levels being relatively higher than in other organs (Figure 1). Fluorometric analysis revealed caspase-3 activity levels that varied between organs in a similar pattern to the varying levels of isatin uptake. Q-VD-OPH decreased liver uptake by 30% and blood activity by 60%. The levels of [^99mTc]mebrofenin uptake in non-liver organs were not as dramatically increased as that seen with [¹⁸F]WC-II-89.

Conclusions The radiolabeled isatin analogs appear to bind specifically to caspase-3 activity in vivo. However, the high levels of activity in the blood limit the ability to assess organ-specific caspase-3 activation with these tracers in this liver apoptosis model.

Research Support NIH K08 EB006702, P01 HL13851