Cyclic peptides (or cyclic proteins) are polypeptide chains whose amino and carboxyl termini are themselves linked together with a peptide bond, forming a circular chain. A number of cyclic peptides have been discovered in nature and they can range anywhere from just a few amino acids in length to hundreds. The processes by which cyclic peptides are formed in cells are not yet fully understood. One interesting property of cyclic peptides is that they tend to be extremely resistant to digestion, allowing them to survive intact in the human digestive tract. This trait makes cyclic peptides attractive to protein based drug designers for use as scaffolds which, in theory, could be engineered to incorporate any arbitrary protein domain of medicinal value, in order to to allow those components to be delivered orally.

Examples include:

• amanitins
• bacitracin
• colistin
• cyclosporine
• dactinomycin
• daptomycin
• nisin
• polymyxin b
• pristinamycin
• octreotide
• valinomycin