Radiolabeled NGR-Based Heterodimers for Angiogenesis Imaging: A Review of Preclinical Studies

Since angiogenesis/neoangiogenesis has a major role in tumor development, progression and metastatic spread, the establishment of angiogenesis-targeting imaging and therapeutic vectors is of utmost significance. Aminopeptidase N (APN/CD13) is a pivotal biomarker of angiogenic processes abundantly expressed on the cell surface of active vascular endothelial and various neoplastic cells, constituting a valuable target for cancer diagnostics and therapy. Since the asparagine-glycine-arginine (NGR) sequence has been shown to colocalize with APN/CD13, the research interest in NGR-peptide-mediated vascular targeting is steadily growing. Earlier preclinical experiments have already demonstrated the imaging and therapeutic feasibility of NGR-based probes labeled with different positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radionuclides, including Gallium-68 (68Ga), Copper-64 (64Cu), Technetium-99m (99mTc), Lutetium-177 (177Lu), Rhenium-188 (188Re) or Bismuth-213 (213Bi). To improve the tumor binding affinity and the retention time of single-receptor targeting peptides, NGR motifs containing heterodimers have been introduced to identify multi-receptor overexpressing malignancies. Preclinical studies with various tumor-bearing experimental animals provide useful tools for the investigation of the in vivo imaging behavior of NGR-based heterobivalent ligands. Herein, we review the reported preclinical achievements on NGR heterodimers that could be highly relevant for the development of further target-specific multivalent compounds in diagnostic and therapeutic settings.

Aminopeptidase N (CD13) targeted MR and NIRF dual-modal imaging of ovarian tumor xenograft

The development of tumor-specific imaging nanoprobes with the potential to improve the accuracy of cancer diagnosis has become an area of intense research. Aminopeptidase N (CD13) predominantly expresses on the surface of ovarian tumor cells and can be specifically recognized by Asn-Gly-Arg (NGR) peptide. The applicability of CD13 as a target for specific ovarian tumor imaging, however, remains unexploited so far. In this study, Cy5.5-labeled, NGR-conjugated iron oxide nanoparticles (Cy5.5-NGR-Fe₃O₄ NPs) were prepared as an ovarian tumor specific bimodal imaging nanoprobe. It is demonstrated that the conjugation of NGR targeting moiety leads to a higher cellular uptake toward ES-2 cells, the human ovarian carcinoma cells that highly express CD13. Moreover, magnetic resonance imaging of ovarian tumor xenograft reveals that the Fe₃O₄-Cy5.5-NGR NPs results in a significant T₂* signal reduction in the tumor. Meanwhile, near infrared fluorescence imaging indicates a higher accumulation of Fe₃O₄-Cy5.5-NGR NPs in the tumor xenograft. Therefore, CD13 could be applied as a novel and efficient target for constructing ovarian tumor specific nanoprobes with improved accuracy for ovarian tumor diagnosis.

13F-1, a novel 5-fluorouracil prodrug containing an Asn-Gly-Arg (NO2) COOCH3 tripeptide, inhibits human colonic carcinoma growth by targeting Aminopeptidase N (APN/CD13)

13F-1 is a 5-fluorouracil prodrug containing an Asn-Gly-Arg (NO2) COOCH3 tripeptide. 13F-1 might possess the activity against cancer growth by targeting Aminopeptidase N (APN/CD13). Our goal in this study was to evaluate the inhibitory effect of 13F-1 on the growth of human colonic carcinoma by both in vitro and in vivo studies. Experiments were performed in colonic carcinoma Colo205 cells, which highly express APN/CD13 on cell surface. The inhibition of 13F-1 on cancer cell growth was estimated by the colorimetric and clonogenic assays. The assays of Annexin V-FITC/PI and JC-1 fluorescence probe were employed to determine the apoptotic cells. Further experiment was performed in mice bearing Colo205 xenografts. 13F-1 was injected for three consecutive weeks. The specimens of Colo205 xenografts were removed for TUNEL staining and western blotting analysis. The expressions of APN/CD13 were analyzed by immunofluorescent flow cytometry and western blotting assays. 13F-1 significantly inhibited Colo205 cell proliferation. 13F-1 by injection delayed the expansion of Colo205 xenografts without significant toxicity to mice. The inhibitory effect of 13F-1 might arise from its role in apoptotic induction. Further analysis indicated that 13F-1 strongly inhibited APN/CD13 expression on cancer cell surface. In contrast, 5-FU did not affect APN/CD13 expression. These results indicated the mechanism of 13F-1 action that 13F-1׳s effect was associated with its role in suppression of APN/CD13 expression. Conclusion, 13F-1 could be developed as a promising agent for treatment of cancers with high expression of APN/CD13.