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.