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

Abstract

Purpose

We have previously developed the caspase-based radiotracer, ¹⁸F-ICMT-11, for PET imaging to monitor treatment response. We further validated ¹⁸F-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 ¹⁸F-ICMT-11, ¹⁸F-ML-10 and ¹⁸F-FDG were assessed in B16ova and B16ovaRevC3 cells after death-switch induction.

Death-switch induction was confirmed in vivo in xenograft tumours, and ¹⁸F-ICMT-11 and ¹⁸F-ML-10 biodistribution was assessed by ex vivo gamma counting of select tissues. PET imaging was performed with ¹⁸F-ICMT-11, ¹⁸F-ML-10 and ¹⁸F-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 ¹⁸F-ICMT-11 (p < 0.001) and ¹⁸F-ML-10 (p < 0.05) and decreased ¹⁸F-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 ¹⁸F-ICMT-11 tumour uptake following death-switch induction (p < 0.01), but not for ¹⁸F-ML-10. Tumour uptake of ¹⁸F-ICMT-11 was higher than that of ¹⁸F-ML-10 after death-switch induction. PET imaging studies showed that ¹⁸F-ICMT-11 can be used to detect apoptosis after death-switch induction, which was accompanied by significantly increased expression of cleaved caspase-3. ¹⁸F-FDG signal decreased in tumours after death-switch induction.

Conclusions

We demonstrate that ¹⁸F-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.

http://bit.ly/2GxnTwE