In vitro and in vivo evaluation of the caspase-3 substrate-based radiotracer [¹⁸F]-CP18 for PET imaging of apoptosis in tumors

Purpose: A novel caspase-3 substrate-based probe [¹⁸F]-CP18 was evaluated as an in vivo positron emission tomography (PET) imaging agent for monitoring apoptosis in tumors. Methods: Uptake of [¹⁸F]-CP18 in cell assays and tumors was measured. Caspase-3/7 activities in cell lysates and tumor homogenates were determined. Autoradiography,Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and cleaved caspase-3 immunostaining were performed on adjacent tumor sections to identify areas of apoptosis. Results: The in vitro cell assays showed caspase-3-dependent uptake of [ ¹⁸F]-CP18 in tumor cells when treated with an apoptosis inducer. The in vivo microPET imaging signal of [¹⁸F]-CP18 in xenograft tumors correlated with the ex vivo caspase-3/7 activities in these tumors. Furthermore, tumor autoradiographies of [¹⁸F]-CP18 in tumor sections matched adjacent sections stained by TUNEL and caspase-3 immunohistochemistry (IHC). Conclusions: [¹⁸F]-CP18 demonstrated high affinity and selectivity for activated caspase-3 both in vitro and in vivo, and the results support [¹⁸F]-CP18 as a promising new PET imaging agent for apoptosis.