Caspase-3 Substrates for Noninvasive Pharmacodynamic Imaging of Apoptosis by PET/CT

Abstract

Quantitative imaging of apoptosis in vivo could enable real-time monitoring of acute cell death pathologies such as traumatic brain injury, as well as the efficacy and safety of cancer therapy. Here, we describe the development and validation of F-18-labeled caspase-3 substrates for PET/CT imaging of apoptosis. Preliminary studies identified the O-benzylthreonine-containing substrate 2MP-TbD-AFC as a highly caspase 3-selective and cell-permeable fluorescent reporter. This lead compound was converted into the radiotracer [F]-TBD, which was obtained at 10% decay-corrected yields with molar activities up to 149 GBq/μmol on an automated radiosynthesis platform. [F]-TBD accumulated in ovarian cancer cells in a caspase- and cisplatin-dependent fashion. PET imaging of a Jo2-induced hepatotoxicity model showed a significant increase in [F]-TBD signal in the livers of Jo2-treated mice compared to controls, driven through a reduction in hepatobiliary clearance. A chemical control tracer that could not be cleaved by caspase 3 showed no change in liver accumulation after induction of hepatocyte apoptosis. Our data demonstrate that [F]-TBD provides an immediate pharmacodynamic readout of liver apoptosis in mice by dynamic PET/CT and suggest that [F]-TBD could be used to interrogate apoptosis in other disease states.

Molecular Imaging of Apoptosis: The Case of Caspase-3 Radiotracers

“…[ 18 F]TBD is a caspase-3 selective substrate obtained by CuAAC between [ 18 F]FEA and an alkyne precursor, and purified by solid-phase extraction, enabling a fast purification method [ 57 ]. The probe was based on the pan-caspase covalent inhibitor M808, containing two amino acids, aspartic acid and alanine, and the warhead FMK, which was optimized by combinatorial studies [ 58 ].…”

Overexpressed DEPDC1B contributes to the progression of hepatocellular carcinoma by CDK1

“…If CDK1 was knockdown, the pathway of DEPDC1B would be inhibited or blocked, which was consistent the results in other cancers. Caspase3 is a member of the caspase family, which was thought essential regulators in programmed cell apoptosis [ 32 , 33 ]. The cells in hepatoma and various phosphorylated forms can secret IGFBP-6.…”

Development of [18F]ICMT-11 for Imaging Caspase-3/7 Activity during Therapy-Induced Apoptosis

“…Otherwise, radiolabelled caspase substrates potentially have less susceptibility to saturation and a single enzyme can interact with more than one substrate molecule within the time frame of a PET study, which may result in the amplification of the radioactive signal produced by the active caspase. Various potential substrate candidates have been investigated in preclinical models and intracellular retention of the cleaved radiotracers has been achieved through different strategies, such as intracellular aggregation or the inclusion of cell-penetrating peptides in the molecule [ 49 , 50 , 51 , 52 , 53 ]. Preclinical results have suggested that it is possible to image apoptosis and therapy-induced increases of caspase-3/7 with substrate radiotracers.…”