Preclinical PET Imaging of Tumor Cell Death following Therapy Using Gallium-68-Labeled C2Am

Simple Summary There is an unmet clinical need for imaging agents capable of detecting the presence, extent, and distribution of tumor cell death following treatment. We describe here a gallium-68-labeled derivative of the C2A domain of Synaptotagmin-I (Ga-68-C2Am), which binds to the phosphatidylserine exposed by dying cells, for imaging tumor cell death in vivo using positron emission tomography (PET). Since PET is a highly sensitive tomographic imaging technique that is widely used in clinical oncology and gallium-68 has emerged as a cost-effective radiotracer that can be eluted on site from a benchtop generator, Ga-68-C2Am could find clinical application for the rapid assessment of tumor responses to treatment. There is an unmet clinical need for imaging agents capable of detecting early evidence of tumor cell death, since the timing, extent, and distribution of cell death in tumors following treatment can give an indication of treatment outcome. We describe here Ga-68-labeled C2Am, which is a phosphatidylserine-binding protein, for imaging tumor cell death in vivo using positron emission tomography (PET). A one-pot synthesis of Ga-68-C2Am (20 min, 25 degrees C, >95% radiochemical purity) has been developed, using a NODAGA-maleimide chelator. The binding of Ga-68-C2Am to apoptotic and necrotic tumor cells was assessed in vitro using human breast and colorectal cancer cell lines, and in vivo, using dynamic PET measurements in mice implanted subcutaneously with the colorectal tumor cells and treated with a TRAIL-R2 agonist. Ga-68-C2Am showed predominantly renal clearance and low retention in the liver, spleen, small intestine, and bone and generated a tumor-to-muscle (T/m) ratio of 2.3 +/- 0.4, at 2 h post probe administration and at 24 h following treatment. Ga-68-C2Am has the potential to be used in the clinic as a PET tracer for assessing early treatment response in tumors.