In vivo biokinetic and metabolic characterization of the ⁶⁸Ga-labelled α5β1-selective peptidomimetic FR366.
OBJECTIVE
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 αvβ3. However, targeting of other subtypes such as the integrin α5β1 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 α5β1 with high selectivity versus αvβ3, 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 α5β1 ligand in PET imaging.


METHODS
The free α5β1 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 α5β1, female BALB/c nude mice xenografted with α5β1-expressing RKO cells in the right shoulder and α5β1/αvβ3-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.


RESULTS
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 α5β1-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.


CONCLUSIONS
Our results show that PET imaging of α5β1 expression with the (68)Ga-labelled α5β1-specific ligand is feasible with good image quality. Thus, FR366 is a promising new tool for investigating the role of α5β1 in angiogenesis and the influence of this integrin subtype on cancer aggressiveness and metastatic potential.
Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar, Technische Universität München, Ismaninger Str. 22, 81675, München, Germany. calogero.dalessandria@tum.de.
Calogero
C
D'Alessandria
Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar, Technische Universität München, Ismaninger Str. 22, 81675, München, Germany.
Karolin
K
Pohle
Institute for Advanced Study (IAS) and Center of Integrated Protein Science (CIPSM), Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747, Garching, Germany.
Florian
F
Rechenmacher
Institute for Advanced Study (IAS) and Center of Integrated Protein Science (CIPSM), Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747, Garching, Germany.
Stefanie
S
Neubauer
Lehrstuhl für Pharmazeutische Radiochemie, Technische Universität München, Walther-Meißner-Str. 3, 85748, Garching, Germany.
Johannes
J
Notni
Lehrstuhl für Pharmazeutische Radiochemie, Technische Universität München, Walther-Meißner-Str. 3, 85748, Garching, Germany.
Hans-Jürgen
HJ
Wester
Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar, Technische Universität München, Ismaninger Str. 22, 81675, München, Germany.
Markus
M
Schwaiger
Institute for Advanced Study (IAS) and Center of Integrated Protein Science (CIPSM), Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747, Garching, Germany.
Horst
H
Kessler
Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar, Technische Universität München, Ismaninger Str. 22, 81675, München, Germany.|Department of Nuclear Medicine, Ulm University, Albert-Einstein-Allee 23, 89081, Ulm, Germany.
Ambros J
AJ
Beer
D000818:Animals; D045744:Cell Line, Tumor; D005260:Female; D005710:Gallium Radioisotopes / Q000493:pharmacokinetics*; D006146:Guanidines / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology; D006801:Humans; D039081:Integrin alpha5beta1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008024:Ligands; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D057786:Peptidomimetics / Q000493:pharmacokinetics*; D049268:Positron-Emission Tomography; D011485:Protein Binding; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D014018:Tissue Distribution; D014227:Triazines / Q000737:chemistry / Q000493:pharmacokinetics* / Q000494:pharmacology
C000612947:FR366 compound; D005710:Gallium Radioisotopes; D006146:Guanidines; D039081:Integrin alpha5beta1; D008024:Ligands; D057786:Peptidomimetics; D019275:Radiopharmaceuticals; D014227:Triazines
Integrin antagonist; PET imaging; Peptidomimetics; α5β1