Characterization of the adrenomedullin receptor acting as the target of a new radiopharmaceutical biomolecule for lung imaging

Abstract

Direct labeling of linear adrenomedullin (AM) with 99mTc ([ 99mTc]AM) displayed excellent selectivity for imaging the pulmonary circulation system in dogs. Hence, we investigated this particular selectivity and characterized the binding sites found in dog lungs. AM and other peptides belonging to the calcitonin peptide family, including calcitonin-gene related peptide (CGRP), adrenomedullin-2 (AM2), amylin and pro-adrenomedullin N-terminal peptide (PAMP), were prepared by solid-phase peptide synthesis. Receptor binding assays were performed by using [ 125I]AM as a radioligand on dog lung homogenates. It was found that AM bound with potent affinity, displaying in fact a high and a low affinity binding site. Moreover, competition binding assays using peptide ligands showed the following ranking for displacement: AM > AM(13–52) > CGRP ≈ AM2 ≥ AM(22–52) ≥ AM2(16–47) > CGRP(8–37) > amylin ≈ PAMP. Thus, these results strongly suggested that the AM binding site found in dog lungs and acting as a clearance receptor is mainly the adrenomedullin AM 1 receptor subtype. The pharmacophores underlying AM 1 binding affinity and specificity were studied by determining the key amino acids, the minimal peptide fragment, and some aspects of the secondary structures. So far, it appeared that the C-terminal segment of human AM is an essential feature for binding. Also, the α-helix secondary structure found in the AM molecule would facilitate the ligand recognition process with the AM receptor in dog lungs. Our results demonstrated that AM or some analogs or fragments could be suitable radiopharmaceutical agents for lung imaging.