Screening technologies for cyclic peptide libraries based on gene encoding have significantly advanced, offering robust platforms for the discovery of bioactive peptides with potential therapeutic applications. Two prominent methodologies in this domain are Split-Intein Circular Ligation of Peptides and Proteins (SICLOPPS) and phage display.
Split-Intein Circular Ligation of Peptides and Proteins (SICLOPPS)
SICLOPPS is a biotechnology technique that facilitates the intracellular production of cyclic peptides through a ribosomal synthesis mechanism followed by an intein-mediated splicing event. In this approach, a gene encoding the desired peptide sequence is inserted into a vector that expresses a split intein system. Upon expression in a host organism, the intein fragments facilitate the excision and ligation of the peptide, resulting in a cyclic structure. This method allows for the creation of vast libraries of cyclic peptides, each encoded by a unique DNA sequence, enabling the direct correlation between genotype and phenotype. The cyclic nature of these peptides often enhances their stability and resistance to proteolytic degradation, making them attractive candidates for drug development.
Phage Display
Phage display is a technique that presents peptides or proteins on the surface of bacteriophages, linking the displayed peptide to its encoding DNA within the phage particle. By inserting random or specific peptide-encoding sequences into the phage genome, researchers can construct extensive libraries of peptides displayed on the phage surface. These libraries are subjected to biopanning—a process where phages are exposed to target molecules, and those with high-affinity interactions are isolated and amplified. Phage display has been instrumental in identifying peptides that bind to specific proteins, aiding in drug discovery and the development of diagnostic tools.
Bacterial Display
Bacterial display systems involve expressing peptides on the surface of bacterial cells, providing an alternative platform for screening peptide libraries. This method is particularly useful for affinity-based screening, antibody epitope mapping, and the identification of cell-binding peptides. By displaying cyclic peptides on the bacterial surface, researchers can utilize fluorescence-activated cell sorting (FACS) to isolate cells presenting peptides with desired binding characteristics. Bacterial display offers advantages such as the ability to perform high-throughput screening and the potential for vaccine development by presenting antigens on the bacterial surface.
Applications and Implications
The development of gene-encoded cyclic peptide libraries and their corresponding screening technologies have broad implications in biomedical research:
Drug Discovery: These technologies enable the identification of peptides that can modulate protein-protein interactions, serving as leads for therapeutic development.
Vaccine Development: By displaying antigenic peptides on bacterial surfaces, novel vaccine candidates can be rapidly identified and tested.
Protein-Protein Interaction Studies: Cyclic peptides can serve as probes to dissect complex protein interaction networks within cells.
In summary, gene-encoded cyclic peptide library screening technologies like SICLOPPS, phage display, and bacterial display have revolutionized the identification and development of bioactive peptides, offering versatile tools for advancing biomedical science.
Summary and Analysis of Marketed Cyclic Peptide Drugs
Current Status of Cyclic Peptide Drug Development
Despite over 100 cyclic peptides progressing through clinical trials, only 18 have been approved in the past two decades. Most approved cyclic peptide drugs originate from natural sources, requiring extensive optimization to enhance their therapeutic potential. However, this process is labor-intensive and inefficient, limiting drug development speed.
Advancements in Screening Technologies
The emergence of high-throughput screening methods, such as phage display, yeast two-hybrid, and mRNA display, has revolutionized peptide drug discovery. These technologies allow rapid identification of target peptides, reducing production costs and accelerating development timelines.
FDA-Approved Cyclic Peptide Drugs
Most approved cyclic peptide drugs are derived from natural compounds, such as:
Zikonotide: A calcium channel blocker from Conus magus venom for chronic pain.
Daptomycin: A cyclic peptide antibiotic against Gram-positive bacteria.
Vasopressin: A natural hormone used to treat hypotension.
High-Throughput Screening in Drug Development
Few cyclic peptides from high-throughput screening have reached the market, but notable examples include:
Romiplostim (phage display technology): Treats idiopathic thrombocytopenic purpura by stimulating platelet production.
Zilucoplan (mRNA display technology): A complement C5 inhibitor for myasthenia gravis.
Future Prospects
Several promising cyclic peptides are in late-stage clinical trials, such as:
Pol6326 (CXCR4 inhibitor, Phase 3)
BT1718 (bicyclic peptide targeting MMP-dm1)
PTG-300 (treatment for β-thalassemia and anemia)
Although cyclic peptide drug development remains slow, high-throughput screening technologies are expected to improve efficiency and expand the range of approved drugs in the future.