Synthesis and Evaluation of 99m Tc-Labelled 2-Nitroimidazole Derivatives with Different Linkers for Tumour Hypoxia Imaging.

When developing novel radiopharmaceuticals, a linker moiety between the chelator and targeting vector can have a crucial influence on adjusting the affinity of the tracer and its biodistribution in organisms. To develop novel 99m Tc-labelled hypoxia imaging radiotracers, in this study, five isocyanide-containing 2-nitroimidazole derivatives with different linkers (L1, L2, L3, L4 and L5) were synthesised and radiolabelled with technetium-99m to obtain five stable 99m Tc-complexes ([ 99m Tc]Tc-L1, [ 99m Tc]Tc-L2, [ 99m Tc]Tc-L3, [ 99m Tc]Tc-L4 and [ 99m Tc]Tc-L5). Corresponding rhenium analogues of [ 99m Tc]Tc-L1 were synthesised and suggested the structures of these 99m Tc-complexes would be a monovalent cation with a technetium (I) core surrounded by six ligands. [ 99m Tc]Tc-L1 is hydrophilic, while the lipophilicities of [ 99m Tc]Tc-L2, [ 99m Tc]Tc-L3, [ 99m Tc]Tc-L4 and [ 99m Tc]Tc-L5 are close. In vitro cell experiments showed that all five novel 99m Tc-complexes had higher uptake in hypoxic cells compared with aerobic cells, which indicates the complexes have good hypoxia selectivity. The biodistribution of the five 99m Tc-complexes in S180 tumour-bearing mice showed that they all had certain uptake in the tumours. Among them, [ 99m Tc]Tc-L1 had the highest tumour-to-muscle (4.68 ± 0.44) and tumour-to-blood (3.81 ± 0.46) ratios. The introduction of polyethylene glycol (PEG) chains effectively reduced the lipophilicity and decreased uptake by the liver, intestine and blood but also increased clearance from the tumours. In vivo metabolic studies showed [ 99m Tc]Tc-L1 kept intact and remained stable in tumour, blood and urine at 2 h post-injection. The results of SPECT imaging showed that [ 99m Tc]Tc-L1 had significant tumour uptake at 2 h post-injection, but there was still high uptake in abdominal organs such as the liver and kidney, suggesting that this complex needs to be further optimised before being used for tumour hypoxia imaging.