Macrocyclic peptides (MCPs)

Macrocyclic peptides (MCPs) are a class of compounds characterised by their cyclic backbone structures formed through covalent bonds between amino acid residues. Unlike linear peptides, MCPs contain a macrocyclic ring, typically comprising 12–40 atoms, which imparts enhanced stability, rigidity, and biological activity. These molecules have gained increasing attention in medicinal chemistry, structural biology, and pharmaceutical research due to their ability to target protein–protein interactions and their potential in drug discovery.

Structural Features

Macrocyclic peptides are distinguished by the presence of a closed ring structure within the peptide backbone. This structural constraint reduces conformational flexibility and stabilises specific three-dimensional shapes. Key structural characteristics include:

• Ring Size: The macrocyclic ring may vary in size depending on the number of amino acid residues involved.
• Linkage Types: The ring closure can occur via peptide bonds, disulphide bridges, lactam linkages, or chemical modifications such as stapling.
• Side-Chain Incorporation: Amino acid side chains can contribute to the cyclisation process, introducing additional stability or functional groups.
• Conformational Restriction: The cyclic nature limits rotations, leading to enhanced binding affinity and selectivity toward biological targets.

Biosynthesis and Chemical Synthesis

Macrocyclic peptides can be produced both naturally and synthetically.

• Natural Sources: Many MCPs are synthesised by microorganisms, particularly bacteria and fungi, as secondary metabolites. Examples include cyclosporine, a fungal cyclic peptide, and vancomycin, an antibiotic peptide with a complex cyclic framework.
• Ribosomal Synthesis: Certain MCPs are ribosomally synthesised and subsequently undergo post-translational modifications such as cyclisation or heteroatom incorporation.
• Non-Ribosomal Peptide Synthesis (NRPS): Many macrocyclic peptides are produced through NRPS, an enzymatic assembly line process independent of ribosomes, allowing incorporation of non-standard amino acids.
• Synthetic Methods: Chemical synthesis strategies involve solid-phase peptide synthesis (SPPS) followed by cyclisation steps. Techniques include head-to-tail cyclisation, side-chain-to-side-chain cyclisation, and chemical stapling. Advances in peptide ligation and click chemistry have improved synthetic accessibility.

Biological Activity and Mechanisms

Macrocyclic peptides exhibit a broad spectrum of biological activities due to their high binding specificity and stability. Their major mechanisms of action include:

• Enzyme Inhibition: MCPs can inhibit enzymes by binding to catalytic sites or allosteric regions.
• Protein–Protein Interaction Modulation: The structural rigidity enables MCPs to disrupt or stabilise protein–protein interactions, which are often difficult to target with small molecules.
• Antimicrobial Properties: Several MCPs act as antibiotics, antifungals, or antivirals due to their membrane-targeting or enzymatic inhibition activities.
• Immunosuppression: Certain MCPs such as cyclosporine A are widely used in immunosuppressive therapy for organ transplantation.

Applications in Drug Discovery

The drug development potential of macrocyclic peptides lies in their ability to occupy the space between small molecules and biologics.

• Therapeutic Targets: MCPs are effective against challenging drug targets including transcription factors, receptors, and signalling proteins.
• Oral Bioavailability: Their conformational rigidity enhances metabolic stability and sometimes improves oral absorption compared to linear peptides.
• Approved Drugs: Examples include cyclosporine A (immunosuppressant), vancomycin (antibiotic), and romidepsin (anticancer agent).
• Drug Delivery Systems: MCPs are being explored as carriers for targeted delivery of drugs due to their cell-penetrating ability.

Advantages and Limitations

Macrocyclic peptides offer a range of advantages over linear peptides and conventional small molecules:
Advantages:

• Enhanced stability against enzymatic degradation.
• High target specificity and strong binding affinity.
• Potential for oral availability and favourable pharmacokinetics.
• Ability to modulate protein–protein interactions.

Limitations:

• Synthesis can be technically demanding and costly.
• Structural complexity limits large-scale production.
• Oral bioavailability, though improved, remains a challenge for many MCPs.
• Potential immunogenicity in therapeutic use.

Current Research and Future Prospects

Ongoing research focuses on developing novel synthetic methodologies, expanding libraries of MCPs, and improving pharmacokinetic properties. Emerging strategies include:

• Combinatorial Chemistry: Generating large libraries of MCPs for screening against diverse biological targets.
• Phage Display and mRNA Display: Biotechnological approaches to evolve MCPs with high affinity to specific targets.
• Stapled Peptides: Introduction of non-natural amino acids or chemical linkers to enhance stability and cell penetration.
• Computational Modelling: Use of molecular dynamics and machine learning to design peptides with optimised binding characteristics.