Evaluation of chemotherapy response in VX2 rabbit lung cancer with ¹⁸F-Labeled C2A domain of synaptotagmin I

The C2A domain of synaptotagmin I can target apoptotic cells by binding to exposed anionic phospholipids. The goal of this study was to synthesize and develop ¹⁸F-labeled C2A-glutathione- S-transferase (GST) as a molecular imaging probe for the detection of apoptosis and to assess the response of paclitaxel chemotherapy in VX2 rabbit lung cancer. Methods: ¹⁸F-C2A-GST was prepared by labeling C2A-GST with N-succinimidyl 4-¹⁸F-fluorobenzoate (¹⁸F-SFB). ¹⁸F-C2A-GST was confirmed by high-performance liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. The binding of ¹⁸F-C2A-GST toward apoptosis was validated in vitro using camptothecin-induced Jurkat cells. Biodistribution of ¹⁸F-C2A-GST was determined in mice by a dissection method and small-animal PET. Single-dose paclitaxel was used to induce apoptosis in rabbits bearing VX2 tumors (n = 6), and 2 VX2 rabbits without treatment served as control. ¹⁸FC2A- GST PET was performed before and at 72 h after therapy, and ¹⁸F-FDG PET/CT was also performed before treatment. To confirm the presence of apoptosis, tumor tissue was analyzed and activated caspase-3 was measured. Results: ¹⁸FC2A- GST was obtained with more than 95% radiochemical purity and was stable for 4 h after formulation. ¹⁸F-C2A-GST bound apoptotic cells specifically. Biodistribution in mice showed that ¹⁸F-C2A-GST mainly excreted from the kidneys and rapidly cleared from blood and nonspecific organs. High focal uptake of ¹⁸F-C2A-GST in the tumor area was determined after therapy, whereas no significant uptake before therapy was found in the tumor with ¹⁸F-FDG-avid foci. The maximum standardized uptake value after therapy was 0.47±0.28, significantly higher than that in the control (0.009±0.001; P < 0.001). The apoptotic index was 79.81% ± 8.73% in the therapy group, significantly higher than that in the control (5.03%±0.81%; P < 0.001). Activated caspase-3 after paclitaxel treatment increased to 69.55%±16.27% and was significantly higher than that in the control (12.26%±5.39%; P < 0.001). Conclusion: ¹⁸F-C2A-GST was easily synthesized by conjugation with ¹⁸F-SFB and manifested a favorable biodistribution. Our results demonstrated the feasibility of ¹⁸FC2A- GST for the early detection of apoptosis after chemotherapy in a VX2 lung cancer model that could imitate.