Cyclic Peptide

A cyclic peptide is a peptide whose main chain or side chains form a closed ring, either through an additional covalent bond or through closure between the N-terminus and C-terminus. This structural constraint profoundly modifies the physicochemical and biological properties of the peptide relative to its linear counterpart.

Main cyclization modes include: head-to-tail (N-terminus to C-terminus, amide bond), head-to-side-chain (N-terminus to a side chain), tail-to-side-chain (C-terminus to a side chain), side-chain-to-side-chain (between two side chains), and disulfide bridge (between two cysteines). More sophisticated cyclizations use thioether bridges (lanthipeptides), hydrocarbon staples (ring-closing metathesis on non-natural olefins), or Huisgen azides (click chemistry).

Pharmacokinetic advantages are well documented: better resistance to exopeptidases (no accessible free termini), prolonged plasma half-life, sometimes increased membrane permeability for small apolar rings, improved selectivity through conformational rigidification that favors a unique receptor binding mode and penalizes interactions with secondary targets. Cyclic peptides "beyond Lipinski's Rule of 5" (bRo5) are particularly studied for intracellular targets difficult to reach with small molecules.

Nature provides many examples: cyclosporin A (immunosuppressant, 11 cyclic residues), gramicidin S, bacitracin, vancomycin, daptomycin, oxytocin and desmopressin (disulfide bridge), native somatostatin (1-14 with disulfide bridge). In research, cyclic peptide libraries are explored through modified phage display (SICLOPPS, mRNA display RaPID), affinity screening, and cryo-EM-assisted drug design.

Notable research analogs include stabilized somatostatin lactams (octreotide, lanreotide), cyclic CXCR4 mimetics (T140 peptide), and various bicyclic scaffolds screened for inhibition of protein-protein interactions, a class of targets historically refractory to small molecules.

Cyclic peptides form a major pharmacological class with many approved drugs: ciclosporin A (immunosuppressant, 11-aa N-methylated cyclic), insulin (2 A/B chains linked by 2 disulfide bridges + 1 intrachain), octreotide (cyclic somatostatin analogue 8 aa), linaclotide (14 aa with 3 disulfide bridges). Cyclisation modes are multiple: head-to-tail (C-terminal linked to N-terminal forming an amide cycle), disulfide bridges between Cys, side-chain cycles (e.g., Lys-Asp lactam, Ser-Cys ether), hydrocarbon staple (stapled peptide synthesis with all-hydrocarbon Grubbs double bond).

Pharmacological advantages of cyclisation include: (1) conformational rigidification increasing target receptor affinity (10-1000x factor vs linear); (2) protease resistance exo- and endo-peptidic by blocking ends or flexibility; (3) improved membrane permeability for some cyclopeptides (ciclosporin, MK-677); (4) possible oral bioavailability unlike linear peptides. In NMR, cyclopeptides show characteristic NOE coalescence coefficients revealing conformational rigidity. AlphaFold 3 (2024) and RFdiffusion (2023) now allow de novo design of high-affinity cyclopeptide binders for a given target receptor.