Synthesis And Preliminary Application Evaluation Of Tumor PET Molecular Probe Based On Ornithine Metabolism

Objective:The rapid proliferation of tumor cells depends on the high levels of polyamines in the cells,targeting interference with polyamine metabolic pathways has become an important direction of anti-tumor therapy.Over-expression of oncogenes leads to the up-regulation of the activity of the first-level rate-limiting enzyme,ornithine decarboxylase（ODC）,which regulated the intracellular polyamine biosynthesis and considered to be a bio-marker and important target of malignant tumors,a number of drugs targeting ODC are entering clinical research.In this study,we designed and synthesized a new type of ¹⁸F-labeled ornithine derivative,N⁵-(2-[¹⁸F]fluoropropionyl)ornithine(N⁵-[¹⁸F]FPO),which can bind with ODC and its potential utility for tumor PET imaging was evaluated preliminarily.Methods:The molecular probes N⁵-[¹⁸F]FPO was prepared by introducing small molecular groups containing radionuclides into the amino group at N5 position with ornithine,the main raw material of polyamine synthesis,as the lead compound.The in vitro and in vivo stability of N⁵-[¹⁸F]FPO were determined by HPLC,and the cellular uptake studies and preliminary study of uptake mechanisms were conducted in human hepatocarcinoma cells Hep G2.Also,the bio-distribution in vivo and small animal micro-PET/CT imaging were performed in normal ICR mice and three tumor-bearing mice models,respectively.Results:1.The total synthesis time of N⁵-[¹⁸F]FPO was about 80±5 min with a radiochemical yield of 15%±6%(uncorrected,based on ¹⁸F^–,n=8).The molar activity of N⁵-[¹⁸F]FPO was about 120-150 GBq/μmol and the radiochemical purity>98%.Also,a high stability can be seen both in vitro and vivo.2.N⁵-[¹⁸F]FPO exhibited a fast uptake in Hep G2 cells,reaching a peak value（20.3%±1.0%）at 100 min.And the cellular uptake ability of N⁵-[¹⁸F]FPO can be inhibited by L-ornithine and DFMO,and the inhibitory rates are 14.9%and 67.45%,respectively.The inhibition rate reached 75.8%when treated cells with ornithine and DFMO at the same time,which indicated that the transport pathway of N⁵-[¹⁸F]FPO is similar to that of L-ornithine,and interacted with ODC after being transported into the cell to a certain extent.3.The results of bio-distribution experiments showed that radioactivity is distributed throughout the body quickly after injection of N⁵-[¹⁸F]FPO.The radioactivity mainly accumulated in the kidney,followed by the gastrointestinal tissues.The intake of other tissues and organs is low,especially the brain and liver.The blood clearance rate from 2min to 60 min after injection was as high as 86%,which provides a low background for intravital imaging.The radioactive uptake of bone tissue did not increase significantly over time,indicating that it did not defluorinate in the body.4.The micro-PET/CT images of the DU-145 nude mice showed that the radioactive uptake value（SUVmean=2.5461±0.0312）at 60 min after the injection of N⁵-[¹⁸F]FPO was higher than 30 min（SUVmean=2.0336±0.0272）,90 min（SUVmean=1.9937±0.0012）and 120 min（SUVmean=1.1908±0.0250）.The tumor-to-muscle ratio（Tu/Mu）of H-22mice and DU-145 nude mice 60 min after injection were 7.79±0.48 and 7.98±0.55,respectively.5.The radioactive uptake value of the tumor in C6 glioma rats after injectived with N⁵-[¹⁸F]FPO（SUVmean=0.8962±0.0200）was lower than that of[¹⁸F]FDG（SUVmean=1.8538±0.0219）,but the ratio of the tumor to the normal brain tissue on the opposite side is about 2.54±0.19.After the injection of[¹⁸F]FDG,the uptake ratio between the tumor and the contralateral normal brain tissue was 1.72±0.99.Conclusion:In this study,the molecular probes N⁵-[¹⁸F]FPO was successfully prepared,the preliminary biological studies showed that N⁵-[¹⁸F]FPO had the potential to be a new tracer for ornithine metabolism imaging of ODC expression imaging of solid tumors.