Radio-labeled Isoquinoline Derivatives As Specific Caspase 3 Inhibitors For Apoptosis Imaging

Objective: The study designed, radio-labeled, and biological evaluated of 11C-HIQPM, an Isoquinoline-1,3,4-trione derivative as potent Caspase 3 inhibitor, as a novel PET tracer for imaging apoptosis.Methods: The effect of alkali and reaction temperature to the stability of precursor and standard compounds were investigated; 18F labeling was tried to perform by PET-MF-IT-1 Multi-Tracer Production Platform. 11C-HIQPM was obtained by N-methylation of HIQPH at home-made synthesis module and was identified by analytic HPLC. In vitro, binding assay of 11C-HIQPM with Cisplatin-induced apoptosis of Human Lung Adenocarcinoma Cell Line A549 was performed at different times.Correlation between cells uptake of radiotracer and apoptotic rate of A549 cells was analyzed. The specificity of 11C-HIQPM binding to Caspase 3 was studied by blocking experiment, the potency of inhibition of Caspase 3 was also detected with 12C-HIQPM.Ex vivo bio-distribution of 11C-HIQPM were study in both normal SD rats and D-GalN/LPS-induced fulminant hepatic apoptosis model of SD rats at 1, 15, 30, 45 and 60 min after injection, respectively. 11C-HIQPM PET/CT images were acquired of A549 bearing tumor mice before and 24 h after chemotherapy with cisplatin. The changes of the uptakes of tumor and muscle were measured and expressed in %ID/g.Results: The precursor and the standards are not stable in high temperature and strong alkali conditions. Consequently, 11F-HIQPF failed to be obtained by 18F labeling under different reaction conditions. 11C-HIQPM was synthesized by N-methylation at nitrogen position of HIQPH with 11C-methyl triflate. The radiochemical purity exceeded 98 % and the specific activity was 41.81±9.89 GBq/?mol (n = 5). After 20 min incubation, apoptotic A549 cells showed the maximum amount of 11C-HIQPM up-taking, then the radiotracer was partly excreted. Cellular uptakes of 11C-HIQPM were correlated with the apoptosis rate of A549 cells (y = 0.0192 x + 0.1310, R2 =0.9257). 11C-HIQPM binding with A549 cells was inhibited by Ac-DEVD-CHO with IC50 of 113.4 nM, which indicate that could 11C-HIQPM specifically bind caspase 3,meanwhile, 12C-HIQPM showed excellent inhibition of caspase 3 (IC50 = 36 nM).From the bio-distribution experiments, the radioactivity was pro-dominantly metabolized through the kidneys and nonspecific uptake of the radio-tracer was observed in most of the other organs of SD rats. The uptake of the liver of model group was 1.7 fold compared with control group. PET imaging results showed that the uptake ratio of tumor to muscle (T/M) was 1.9 of A549 tumor bearing mice before chemotherapy, and climbed to 5.8 after cisplatin treated. After chemotherapy, the uptake of apoptotic tumor was 3.6 times increased, which showed "highlight" region in the PET image with high-contrast.Conclusions: 11C-HIQPM, a 1,3,4-trione-isoquinoline derivative as novel PET tracer for imaging caspase-3 activation in apoptosis, demonstrated high affinity and selectivity of caspase 3 of apoptotic cells in vitro and vivo by various biological experiment, which preliminarily indicate that radio-labeled 1,3,4-trione-isoquinoline derivatives may suitable for apoptosis imaging.