Gallium-68 Complex of a Macrobicyclic Cage Amine Chelator Tethered to Two Integrin-Targeting Peptides for Diagnostic Tumor Imaging

Tumor-targeting peptides radiolabeled with positron-emitting ⁶⁸Ga are promising candidates as new noninvasive diagnostic agents for positron emission tomography (PET). The targeting peptides are tethered to a chelator that forms a stable coordination complex with Ga³⁺ that is inert to dissociation of Ga³⁺in vivo. Metal complexes of macrobicyclic hexaamine “sarcophagine” (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) ligands exhibit remarkable stability as a result of the encapsulating nature of the cage amine ligand. A Ga³⁺ sarcophagine complex, [Ga-(1-NH₃-8-NH₂-sar)]⁴⁺, has been characterized using X-ray crystallography, demonstrating that Ga³⁺ is coordinated to six nitrogen atoms in a distorted octahedral complex. A bifunctional derivative of (NH₂)₂sar, possessing two aliphatic linkers with carboxylic acid functional groups has been attached to two cyclic-RGD peptides that target the α_vβ₃ integrin receptor that is overexpressed in some types of tumor tissue. This dimeric species can be radiolabeled with ⁶⁸Ga³⁺ in >98% radiochemical yield and ⁶⁸Ga³⁺ does not dissociate from the ligand in the presence of transferrin, an endogenous protein with high affinity for Ga³⁺. Biodistribution and micro-PET imaging studies in tumor-bearing mice indicate that the tracer accumulates specifically in tumors with high integrin expression. The high tumor uptake is coupled with low nonspecific uptake and clearance predominantly through the kidneys resulting in high-quality PET images in animal models.