NGR Tumor-Homing Peptides: Structural Requirements for Effective APN (CD13) Targeting

Cyclic CNGRC (cCNGRC) peptides are very important targeting ligands for Aminopeptidase N (APN or CD13), which is overexpressed on the surface of many cancer cells. In this work we have (1) developed an efficient solid phase synthesis and (2) tested on purified porcine APN and APN-expressing human cells two different classes of cCNGRC peptides: the first carrying a biotin affinity tag or a fluorescent tag attached to the carboxyl Arg-Cys-COOH terminus and the second with the tags attached to the amino H2N-Cys-Asn terminus. Carboxyl-terminus functionalized cCNGRC peptides 3, 6, and 8 showed good affinity for porcine APN and very good capacity to target and be internalized into APN-expressing cells. In contrast, amino-terminus functionalized cCNGRC peptides 4, 5, and 7 displayed significantly decreased affinity and targeting capacity. These results, which are in agreement with the recently reported X-ray structure of a cCNGRC peptide bound to APN showing important stabilizing interactions between the unprotected cCNGRC amino terminus and the APN active site, indicate that the carboxyl and not the amino-terminus of cCNGRC peptides should be used as a "handle" for the attachment of toxic payloads for therapy or isotopically labeled functions for imaging and nuclear medicine.