The Development of Peptide-Based Tools for the Analysis of Angiogenesis

Blocking vascular endothelial growth factor (VEGF) signaling has been validated as an effective approach for cancer therapy. VEGF overexpression in tumors is typically associated with increased angiogenesis and a poorer patient prognosis [1]. Recently, monitoring VEGF expression in human ovarian tumor xenografts has been achieved by utilizing the ⁸⁹Zr-labelled anti-VEGF mAb bevacizumab as a PET probe [2]. Unfortunately, the use of ⁸⁹Zr-mAbs may not be amenable for routine imaging of VEGF levels due to their long plasma half-life and the length of time required to achieve maximum contrast. Additionally, since the VEGF levels may be rapidly changing, a quick readout is often advantageous for diagnostic purposes. PET probes based on ¹⁸F-labeled peptides are more facile imaging agents because their faster plasma clearance allows optimal contrast within a few hours, facilitating repeated daily imaging. Target specific ¹⁸F-peptides have been obtained by combinatorial drug discovery methods combined with high throughput PET [3].

In this study we have utilized phage display technology to identify peptides that selectively recognize VEGF with nanomolar affinity. Two types of peptide-phage libraries were designed: one was based on three-helix Z-domain of staphylococcal protein A; the second library was based on the reengineered two-helix 39 residue “mini-Z” [4]. We report on the selection process, describe structural and biochemical characterization of the peptides selected, compare and contrast the image quality obtained from different peptides modified as PET probes and finally utilize a representative probe to investigate VEGF levels in a SKOV-3 ovarian cancer xenograph model. The PET images obtained with the peptide probe are comparable to those obtained using ⁸⁹Zr-B20 (anti-VEGF mAb).

[1] Volm M et al., International Jnl Cancer 1997 74, 64

[2] Nagengast WB et al., J. Nucl. Med. 2007, 48, 1313

[3] Gagnon MKJ et al., PNAS 2009, 106, 17904

[4] Braisted AC and Wells JA PNAS, 93, 5658