Temporal and spatial evolution of therapy-induced tumor apoptosis detected by caspase-3-selective molecular imaging

Purpose: Induction of Apoptosis in tumors is considered a desired goal of Anticancer therapy. We investigated whether the dynamic temporal and spatial evolution of Apoptosis in response to cytotoxic and mechanism-based therapeutics could be detected noninvasively by the Caspase-3 radiotracer [(18)F]ICMT-11 and positron emission tomography (PET).

Experimental design: The effects of a single dose of the alkylating agent cyclophosphamide (CPA or 4-hydroperoxycyclophosphamide), or the mechanism-based small molecule SMAC mimetic birinapant on Caspase-3 activation was assessed in vitro and by [(18)F]ICMT-11-PET in mice bearing 38C13 B-cell lymphoma, HCT116 colon carcinoma, or MDA-MB-231 breast adenocarcinoma tumors. Ex vivo analysis of Caspase-3 was compared to the in vivo PET imaging data.

Results: Drug treatment increased the mean [(18)F]ICMT-11 tumor uptake with a peak at 24 hours for CPA (40 mg/kg; AUC40-60: 8.04 ± 1.33 and 16.05 ± 3.35 %ID/mL × min at baseline and 24 hours, respectively) and 6 hours for birinapant (15 mg/kg; AUC40-60: 20.29 ± 0.82 and 31.07 ± 5.66 %ID/mL × min, at baseline and 6 hours, respectively). Voxel-based spatiotemporal analysis of tumor-intrinsic heterogeneity suggested that discrete pockets of Caspase-3 activation could be detected by [(18)F]ICMT-11. Increased tumor [(18)F]ICMT-11 uptake was associated with Caspase-3 activation measured ex vivo, and early radiotracer uptake predicted Apoptosis, distinct from the glucose metabolism with [(18)F]fluorodeoxyglucose-PET, which depicted continuous loss of cell viability.

Conclusion: The proapoptotic effects of CPA and birinapant resulted in a time-dependent increase in [(18)F]ICMT-11 uptake detected by PET. [(18)F]ICMT-11-PET holds promise as a noninvasive pharmacodynamic biomarker of caspase-3-associated Apoptosis in tumors.