Within the family of neuropeptide Y (NPY) receptors, the Y4 receptor (Y4R) is unique as it prefers pancreatic polypeptide over NPY and peptide YY. Today, low-molecular-weight Y4R ligands are lacking, in particular antagonists. We synthesized a series of peptidic NPY Y4R ligands, derived from the hexapeptide acetyl-Arg-Tyr-Arg-Leu-Arg-Tyr-NH2 (1), reported to be a Y4R partial agonist with high affinity (pK i Y4R: 8.43). Peptide 1 was N-terminally extended as well as truncated and subjected to a d-amino acid scan, and Leu was replaced by different amino acids. Compounds were characterized by radioligand competition binding and functional studies (Cai 2+ mobilization and β-arrestin 1/2 recruitment). N-terminal truncation of 1 resulted in a tetrapeptide (Arg-Leu-Arg-Tyr-NH2), being a Y4R partial agonist with unchanged Y4R affinity (pK i: 8.47). Remarkably, replacement of Leu in 1 and in derivatives of 1 by Trp turned Y4R agonism to antagonism, giving Y4R antagonists with pK i values ≤7.57.

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Our data indicate that early sunitinib-induced hypertension is associated with modest alterations in renal vascular function, but markedly increased renal sodium reabsorption, probably due to direct actions of the VEGF antagonist on the collecting duct, suggesting that VEGF receptors regulate renal Na+ absorption.

Over the past few years, an approach emerged that combines different receptor-specific peptide radioligands able to bind different target structures on tumor cells concomitantly or separately. The reason for the growing interest in this special field of radiopharmaceutical development is rooted in the fact that bispecific peptide heterodimers can exhibit a strongly increased target cell avidity and specificity compared to their corresponding monospecific counterparts by being able to bind to two different target structures that are overexpressed on the cell surface of several malignancies. This increase of avidity is most pronounced in the case of concomitant binding of both peptides to their respective targets but is also observed in cases of heterogeneously expressed receptors within a tumor entity. Furthermore, the application of a radiolabeled heterobivalent agent can solve the ubiquitous problem of limited tumor visualization sensitivity caused by differential receptor expression on different tumor lesions. In this article, the concept of heterobivalent targeting and the general advantages of using radiolabeled bispecific peptidic ligands for tumor imaging or therapy as well as the influence of molecular design and the receptors on the tumor cell surface are explained, and an overview is given of the radiolabeled heterobivalent peptides described thus far.