Integrins are transmembrane receptors responsible for cell-cell adhesion and cell-extracellular matrix binding and play an important role in angiogenesis and tumour metastasis. For this reason, integrins are increasingly used as targets for molecular imaging. Up to now interest has mostly been focused on the integrin subtype ?v?3. However, targeting of other subtypes such as the integrin ?5?1 is also of high interest due to its central role in colonization of metastatic cells, resistance of tumour cells to chemotherapy and ionizing radiation, and tumour aggressiveness. Recently, a highly active antagonist ligand (2,2'-(7-(1-carboxy-4-((6-((3-(4-(((S)-1-carboxy-2-(2-(3-guanidinobenzamido)acetamido)ethyl)carbamoyl)-3,5-dimethylphenoxy)propyl)amino)-6-oxohexyl)amino)-4-oxobutyl)-1,4,7-triazonane-1,4-diyl)diacetic acid, FR366) for the integrin subtype ?5?1 with high selectivity versus ?v?3, has been developed and tested successfully in preliminary in vitro and in vivo experiments. Here, we present our results of an investigation of the use of (68)Ga-labelled ?5?1 ligand in PET imaging.The free ?5?1 peptidomimetic ligand was functionalized with a spacer (6-aminohexanoic acid) and the bifunctional chelator 1-((1,3-dicarboxy)propyl)-4,7-(carboxymethyl)-1,4,7-triazacyclononane (NODAGA) to yield FR366 and labelled with (68)Ga. To confirm selective in vivo targeting of ?5?1, female BALB/c nude mice xenografted with ?5?1-expressing RKO cells in the right shoulder and ?5?1/?v?3-expressing M21 cells in the left shoulder were subjected to PET/CT scans and biodistribution experiments. Specificity of tracer uptake was proven by blocking studies. Metabolic stability of the injected tracer was measured in urine and in plasma.MicroPET/CT scans with radiolabelled FR366 showed a good tumour-to-normal tissue ratio with low uptake in the liver (0.32 ± 0.14 %ID/g) and good retention of (68)Ga-NODAGA-FR366 in the tumour (0.71 ± 0.20 %ID/g and 0.40 ± 0.12 %ID/g for RKO and M21 tumours, respectively, at 90 min after injection). Biodistribution experiments showed uptake in the ?5?1-expressing RKO tumour of 1.05 ± 0.23 %ID/g at 90 min after injection. Specificity of tracer uptake was demonstrated by injection of 5 mg/kg unlabelled ligand 10 min prior to tracer injection, resulting in a 67 % reduction in uptake in the RKO tumour. The tracer was found to be metabolically stable in urine and plasma 30 min after injection.Our results show that PET imaging of ?5?1 expression with the (68)Ga-labelled ?5?1-specific ligand is feasible with good image quality. Thus, FR366 is a promising new tool for investigating the role of ?5?1 in angiogenesis and the influence of this integrin subtype on cancer aggressiveness and metastatic potential.     «

Integrins are transmembrane receptors responsible for cell-cell adhesion and cell-extracellular matrix binding and play an important role in angiogenesis and tumour metastasis. For this reason, integrins are increasingly used as targets for molecular imaging. Up to now interest has mostly been focused on the integrin subtype ?v?3. However, targeting of other subtypes such as the integrin ?5?1 is also of high interest due to its central role in colonization of metastatic cells, resistance of tumo...     »