LAUSR

Fluorescently labelled ghrelin is an effective imaging probe for ex vivo biopsy analysis, in vivo distribution studies, and cell‐based analyses. The objective of our study was to improve the receptor affinity and stability of this ghrelin probe through cyclization, thereby providing a chemical probe with advantages in specificity and sensitivity as compared to immunohistochemical approaches. Truncation of ghrelin to its first 20 essential binding amino acids simplifies chemical synthesis, but reduces the α‐helical content of the peptide, which is important for receptor recognition. To overcome this limitation, we used a “staple scan” to synthesize stable α‐helical cyclic ghrelin(1‐20) analogues using a lactam bridge in either the i, i + 4 or i, i + 7 position. Stapling improved helicity in every case when compared to the linear sequence; however, the binding affinity to the receptor was dependent on the staple position. The peptide with the greatest improvement resulted in a [θ]222/[θ]208 ratio of 0.84, and an IC50 of 7.85 nM. The lead analogue was fluorescently labeled on the C‐terminal lysine of the peptide and microscopy experiments confirmed receptor binding in cells expressing GHS‐R1a. We postulate that the lead stapled peptide can be used as a cancer cell‐specific fluorescent stain with potential research and clinical applications.