68Ga-labeled cyclic RGD dimers with Gly3 and PEG4 linkers: promising agents for tumor integrin αvβ3 PET imaging

Radiolabeled cyclic RGD (Arg-Gly-Asp) peptides have great potential for the early tumor detection and noninvasive monitoring of tumor metastasis and therapeutic response. F-18-labeled RGD analogs ([F-18]-AH111585 and [F-18]Galacto-RGD) have been investigated in clinical trials for positron emission tomography (PET) imaging of integrin expression in cancer patients. To develop new RGD radiotracers with higher tumor accumulation, improved in vivo kinetics, easy availability and low cost, we developed two new RGD peptides and labeled them with generator-eluted Ga-68 (t(1/2) = 68 min) for PET imaging of integrin alpha(v)beta(3) expression in tumor xenograft models. The two new cyclic RGD dimers, E[PEG(4)-c(RGDfK)](2) (P-4-RGD2, PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) and E[Gly(3)-c(RGDfK)](2) (G(3)-RGD2, G(3) = Gly-Gly-Gly) were designed, synthesized and conjugated with 1,4,7-triazacyclononanetriacetic acid (NOTA) for Ga-68 labeling. The microPET imaging and biodistribution of the Ga-68 labeled RGD tracers were investigated in integrin alpha(v)beta(3)-positive tumor xenografts. The new RGD dimers with the Gly(3) and PEG(4) linkers showed higher integrin alpha(v)beta(3) binding affinity than no-linker RGD dimer (RGD2). NOTA-G(3)-RGD2 and NOTA-P-4-RGD2 could be labeled with Ga-68 within 30 min with higher purity (> 98%) and specific activity (8.88-11.84 MBq/nmol). Both Ga-68-NOTA-P-4-RGD2 and Ga-68-NOTA-G(3)-RGD2 exhibited significantly higher tumor uptake and tumor-to-normal tissue ratios than Ga-68-NOTA-RGD2. Because of their high affinity, high specificity and excellent pharmacokinetic properties, further investigation of the two novel RGD dimers for clinical PET imaging of integrin alpha(v)beta(3) expression in cancer patients is warranted.