Cyclic Peptides: Promising Scaffolds for Biopharmaceuticals

Abstract

To date, small molecules and macromolecules, including antibodies, have been the most pursued substances in drug screening and development efforts. Despite numerous favorable features as a drug, these molecules still have limitations and are not complementary in many regards. Recently, peptide-based chemical structures that lie between these two categories in terms of both structural and functional properties have gained increasing attention as potential alternatives. In particular, peptides in a circular form provide a promising scaffold for the development of a novel drug class owing to their adjustable and expandable ability to bind a wide range of target molecules. In this review, we discuss recent progress in methodologies for peptide cyclization and screening and use of bioactive cyclic peptides in various applications.

Keywords: biopharmaceuticals; cyclic peptides; mRNA display; yeast two hybrid.

Figure 1

Natural cyclic peptides. (A) Cyclosporine A. (B) Romidepsin. (C) Largazole. (D) Murepavadin.

Figure 2

Scaffold-based cyclic peptide synthesis. (A) Organobromides selectively react with sulfhydryl group of cysteine. (B) N-hydroxysuccimide group reacts with the primary amine in p peptide and forms a stable amide bond. (C) Click reaction between azide and alkyne groups can be used for cyclization of chemically synthesized peptide.

Figure 3

Enzyme-based cyclic peptide synthesis. (A) Cyclization of synthetic peptide using thioetsterase domain of nonribosomal peptide synthetase (NRPS). (B) Butelase 1-mediated cyclization requires C-terminal Asn-His-Val (NHV) tripeptide. After cyclization, only asparagine residue remains. (C) Split intein-mediated circular ligation for the synthesis of cyclic peptide. (D) Semi-synthetic method for peptide cyclization via oxime ligation using chemically synthesized N-intein and biologically prepared C-intein carrying the unnatural amino acid p-acetophenylalanine.

Figure 4

Screening of biologically active cyclic peptides. (A) Messenger RNA (mRNA) display-based cyclic peptide library screening. (B) Bacterial reverse two-hybrid system used for screening of active cyclic peptides. RNAP: RNA polymerase.

Figure 5

Optimization of active cyclic peptide. Ala-scanning of cyclic peptide 1a identified two essential amino acids, arginine and tyrosine, for target binding. Subsequent optimization led to a more potent dipeptide, Cpd14 (compound 14).