Cyclization strategies in peptide derived drug design

The choice of peptides as prototype modulators of biological function, is justified on the grounds that peptides are natural constituents of living systems. They exist as hormones, biochemical inhibitors, antigens, growth factors, transmembrane carriers and, indeed they are comprised of the building blocks of all proteins. As such, the natural and mutated analogs of these functional entities provide a rich variety of pharmacophore models for further development. Peptidomimetic modification of active peptides can provide biostable analogs. Moreover, cyclization of linear peptides is frequently used as an attractive venue to provide both conformationally more restricted as well as more biostable analogs. The objective of this review is to report an updated summary of the more recently developed methodologies for the design and synthesis of cyclized peptides, citing selected examples of the effect of cyclization on both proteolytic stability and biological activity.