From micromolar to nanomolar affinity: a systematic approach to identify the binding site of CGRP at the human calcitonin gene-related peptide 1 receptor.

CGRP Y0-28-37 is known as a selective CGRP1 receptor antagonist. In order to elucidate the essential requirements for its receptor interaction, we performed a variety of systematic approaches by modifying the C-terminal segments CGRP Y0-28-37 and CGRP 27-37. N-Terminal and C-terminal segments have been synthesized, as well as chimeras which combine segments of CGRP, adrenomedullin, and amylin. Furthermore, we carried out an Ala scan, a Phe scan, a D-amino acid scan and a Pro scan of CGRP 27-37. Additionally, single amino acids were replaced by those with similar biophysical properties. Receptor binding studies of all analogs were performed at human neuroblastoma cells SK-N-MC, which selectively express the hCGRP1 receptor. On the basis of the obtained results, we synthesized a series of ligands with multiple amino acid replacements in order to optimize the exchange at each position. This approach yielded to a series of high affinity ligands, including [D31,P34,F35] CGRP 27-37 which exhibits a 100-fold increased affinity compared to the unmodified segment. So far, this is the smallest CGRP analog that shows affinity in the nanomolar range.