Synthetic Cross-linking of Peptides: Molecular Linchpins for Peptide Cyclization.

Peptide-derived drugs constitute a significant fraction of therapeutic agents. In 2013, The global market of peptide therapeutics was ca. $19 billion; this value does not include revenue from insulin derivatives of $28 million. The combined sales of insulin and non-insulin peptide drugs is estimated to exceed $70 billion by 2019. A significant fraction of peptide-derived drugs is composed of an amino acid sequence and additional chemical functionalities that improve biological and pharmacological properties of the drug. In this review, we focus on synthetic cross-linkers that we refer to as "linchpins", which are commonly used to constrain the secondary structure of peptides and equip them with added benefits such as resistance to proteolytic degradation and conformational stability. The latter property leads to an increase in binding potency and increased bioavailability due to increased permeation through biological membranes. Some linchpins can even introduce properties not found in natural peptides such as light-responsiveness. Peptides cyclized by linchpins can be viewed as a sub-class of a larger family of peptide-derived drugs with desired pharmacological performance in vivo. To understand how chemical modifications by linchpins improve drug discovery, this review also briefly summarizes canonical examples of chemical modification used in modern peptide therapeutics.