Phage Selection of Mono- and Bicyclic Peptide Ligands
Cyclic peptides represent an attractive molecule format for the development of ligands with potential pharmaceutical application. With phage display technology, large combinatorial libraries of polycyclic peptides can be generated and screened. Monocyclic peptides, cyclized by a disulfide bridge of two cysteine residues flanking the peptide, have been isolated to numerous protein targets. A first such monocyclic peptide – a mimetic of erythropoietin – was recently approved for clinical use. Peptide ligands with two macrocyclic peptide rings – the bicyclic peptides – can also be developed by phage display. Bicyclic peptide phage libraries are generated by cyclizing linear peptides displayed on phage with a chemical reagent. This strategy has led to the isolation of tight bicyclic peptide ligands of a range of targets. Phage display technologies and combinatorial libraries applied to generated mono- and bicyclic peptide ligands are discussed. Four case studies illustrating the phage selection procedures and the properties of the isolated mono- and bicyclic peptide ligands are reviewed.