Evaluation of apoptosis imaging biomarkers in a genetic model of cell death.

Purpose: We have previously developed the caspase-based radiotracer, F-ICMT-11, for PET imaging to monitor treatment response. We further validated F-ICMT-11 specificity in a murine melanoma death-switch tumour model with conditional activation of caspase-3 induced by doxycycline.

Methods: Caspase-3/7 activity and cellular uptake of F-ICMT-11, F-ML-10 and F-FDG were assessed in B16ova and B16ovaRevC3 cells after death-switch induction. Death-switch induction was confirmed in vivo in xenograft tumours, and F-ICMT-11 and F-ML-10 biodistribution was assessed by ex vivo gamma counting of select tissues. PET imaging was performed with F-ICMT-11, F-ML-10 and F-FDG. Caspase-3 activation was confirmed by immunohistochemistry.

Results: Significantly increased caspase-3/7 activity was observed only in B16ovaRevC3 cells after death-switch induction, accompanied by significantly increased F-ICMT-11 (p < 0.001) and F-ML-10 (p < 0.05) and decreased F-FDG (p < 0.001) uptake compared with controls. B16ova and B16ovaRevC3 tumours had similar growth in vivo; however, B16ovaRevC3 growth was significantly reduced with death-switch induction (p < 0.01). Biodistribution studies showed significantly increased F-ICMT-11 tumour uptake following death-switch induction (p < 0.01), but not for F-ML-10. Tumour uptake of F-ICMT-11 was higher than that of F-ML-10 after death-switch induction. PET imaging studies showed that F-ICMT-11 can be used to detect apoptosis after death-switch induction, which was accompanied by significantly increased expression of cleaved caspase-3. F-FDG signal decreased in tumours after death-switch induction.

Conclusions: We demonstrate that F-ICMT-11 can be used to detect caspase-3 activation in a death-switch tumour model, independent of the confounding effects of cancer therapeutics, thus confirming its specificity and supporting the development of this radiotracer for clinical use to monitor tumour apoptosis and therapy response.