In vivo assessment of aminopeptidase N (APN/CD13) specificity of different 68Ga-labelled NGR derivatives using PET/MRI imaging

Aminopeptidase N (APN/CD13) plays an important role in neoangiogenic process in malignancies. Our previous studies have already shown that Ga-68-labelled NOTA conjugated asparagine-glycine-arginine peptide (c [KNGRE]-NH2) specifically bind to APN/CD13 expressing tumors. The aim of this study was to evaluate and compare the APN/CD13 specificity of newly synthesized Ga-68-labelled NGR derivatives in vivo by PET/MRI imaging using hepatocellular carcinoma (He/De) and mesoblastic nephroma (Ne/De) tumor models. PET/MRI and ex vivo biodistribution studies were performed 11 +/- 1 days after subcutaneous injection of tumor cells and 90 min after intravenous injection of Ga-68-NOTA-c(NGR), Ga-68-NODAGA-c(NGR), Ga-68-NODAGA-c(NGR) (MG1) or Ga-68-NODAGA-c(NGR) (MG2). The APN/CD13 selectivity was confirmed by blocking experiments and the APN/CD13 expression was verified by immunohistochemistry. Ga-68-labelled c(NGR) derivatives were produced with high specific activity and radiochemical purity. In control animals, low radiotracer accumulation was found in abdominal and thoracic organs. Using tumor-bearing animals we found that the Ga-68-NOTA-c(NGR), Ga-68-NODAGA-c(NGR), and Ga-68-NODAGA-c(NGR) (MG1) derivatives showed higher uptake in He/De and Ne/De tumors, than that of the accumulation of Ga-68-NODAGA-c(NGR) (MG2). APN/CD13 is a very promising target in PET imaging, however, the selection of the appropriate Ga-68-labelled NGR-based radiopharmaceutical is critical for the precise detection of tumor neo-angiogenesis and for monitoring the efficacy of anticancer therapy.