Exploring Novel Anticancer Approaches: Design, Synthesis and Biological Evaluation of Small Molecules and NGR Tumor-Homing Peptides

In anticancer drug discovery DNA secondary structures, called G-quadruplexes (G4s), emerged as targets of interest, owing to their localization in crucial positions of the genome, such as in telomers and oncogene promoter regions. It has been widely accepted that G4 stabilization by small molecules at these levels, may represent a realistic anticancer strategy. Since a diimidazo[1,2-a:1,2-c]pyrimidine bis-guanylhydrazone derivative was found to be a potent stabiliser of several quadruplex DNAs over duplex DNA, a library of analogues was designed and synthesized to shed light on the structural requirements for G4 binding. The newly synthesized hydrazones were evaluated for their ability to stabilize several G4 with different topology and also for their antiproliferative activity. In particular the most promising compound of the series showed the capability to selectively stabilize a specific G4 structure and to inhibit cancer cell growth in the nanomolar range. Tumor-homing peptides have been considered promising tools for imaging and therapy of tumors over-expressing their target ligand. In this context, the cyclic CNGRC peptide has been extensively used as tumor-homing carrier for its ability to specifically recognise aminopeptidase N (APN/CD13), a zinc-dependent ectoenzyme, upregulated in tumor blood vessels undergoing angiogenesis and in the majority of cancer cells. Considering the potential of APN-directed tumour homing strategies and taking into account the recently reported crystal structure of porcine APN complexed with cCNGRCG, a library of C-terminus and N-terminus functionalized cCNGRC peptides was designed and synthesized with the aim to confirm the importance of N-terminal amine group of the cCNGRC in the APN binding. The synthesized peptides were assayed for their ability to inhibit APN in vitro. The C-terminus was identified as the right conjugation site of the cCNGRC peptide, which was consequently conjugated with a fluorescent probe for APN imaging studies in cancer cells.