**5.4 Peptides**

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers, has been provided (Ferro-Flores et al., 2010). Peptides used for tumor targeting show some advantages over antibodies. Peptides are small and show rapid diffusion into the target tissues resulting in rapid pharmacokinetics (Ferro-Flores et al., 2010). Their fast blood clearance can lead to high tumor to background ratio shortly after administration of the radiopeptide. In addition, they can tolerate harsh chemical conditions and are easy to be purified and modified. The common peptides that are used in molecular imaging and therapy of breast cancer are somatostatin, bombesin, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) (Ferro-Flores et al., 2010). The important disadvantage of peptides is rapid degrading from blood by endogenous peptidases and proteases (Ferro-Flores et al., 2010).
