A Review: The Antiviral Activity of Cyclic Peptides

The Cyclic Antimicrobial Peptide C-LR18 Has Enhanced Antibacterial Activity, Improved Stability, and a Longer Half-Life Compared to the Original Peptide

Background: LR18 is an α₋helical AMP with high antimicrobial activity, low hemolytic activity, and low cytotoxicity. However, the susceptibility to degradation of the peptidase enzyme and a short half-life hinder its application as a therapeutic agent. Improving the stability and prolonging the half-life of LR18 are crucial to accelerate its application in the treatment of infectious diseases. Methods: A new cyclic peptide, C-LR18, was designed and synthesized through end-to-end cyclization of LR18 via disulfide bonds. The biological activity, half-life, and therapeutic effect of C-LR18 on Escherichia coli₋infected mice were studied. Results: C-LR18 maintained the characteristics of low cytotoxicity and low hemolytic activity of the original LR18 peptide and had higher antibacterial activity and significantly improved stability. After treatment with 1 mg/mL of trypsin, carboxypeptidase, and papain for 1 h, the MIC of C-LR18 against E. coli ATCC25922 was 4 μM, while that of LR18 had increased to 128 μM. After exposure to 50% serum or artificial gut solution for 30 min, the MIC of C-LR18 against E. coli ATCC25922 increased 4-fold, while that of LR18 increased 16-fold. The half-life of C-LR18 in plasma and in rats was extended to 3.37-fold and 4.46-fold, respectively, that of LR18. The acute toxicity of C-LR18 in mice is lower than many AMPs reported so far (LD50 = 37.8 mg/kg). C-LR18 has a therapeutic effect on E.coli-infected mice. Conclusions: The cyclic peptide C-LR18 has higher antibacterial activity and stability and a longer half-life than LR18 in rats in vitro and in vivo. C-LR18 also has a therapeutic effect on KM mice infected with E. coli and is expected to become a therapeutic drug for bacterial diseases and applied to the treatment of human and veterinary diseases.

Harnessing marine antimicrobial peptides for novel therapeutics: A deep dive into ocean-derived bioactives

Marine antimicrobial peptides (AMPs) are potent bioactive compounds with broad-spectrum activity against bacteria, viruses, and fungi, offering a promising alternative to traditional antibiotics. These small, cationic, and amphiphilic peptides (3-50 amino acids) are key components of marine organisms' immune defenses, adapted to harsh oceanic environments. Discovered in the 1980s, marine AMPs have garnered interest for their unique structures and potential applications in human health. However, despite the ocean's vast biodiversity, they remain underexplored compared to land-based AMPs. This review emphasizes the therapeutic potential of marine AMPs, including their modes of action, structural variety, and applications in drug development, tissue regeneration, and cancer treatment. Moreover, it discusses their antibacterial, antiviral, antifungal, and antiparasitic properties. Additionally, the review addresses strategies to enhance the therapeutic potential of marine AMPs and the challenges associated with their development. By examining the promising future of marine AMPs, this review aims to pave the way for new approaches to combat antimicrobial resistance and develop innovative treatments for various infectious diseases. The potential of marine AMPs as the "medicine bank of the new millennium" remains vast, providing a valuable resource for future drug discovery and sustainable practices across industries.

Future applications of cyclic antimicrobial peptides in drug delivery

INTRODUCTION

Cyclic antimicrobial peptides (CAMPs) are gaining attention as promising candidates in advanced drug delivery systems due to their structural stability, resistance to proteolytic degradation, and versatile therapeutic potential. Their unique properties enable applications that extend beyond combating multidrug-resistant (MDR) pathogens. Their amphipathic and cell-penetrating properties allow them to efficiently transport drugs across cellular membranes.

AREAS COVERED

This review explores the structural advantages and mechanisms of action of CAMPs, emphasizing their role in drug delivery. The literature analysis (2010-2024) from PubMed, Scopus, and Web of Science highlights developments in CAMP-conjugated therapies, liposomal formulations, and encapsulation systems. The review also examines their antimicrobial potency, amphipathic and cell-penetrating properties, and integration into nanocarrier technologies to enhance drug stability, bioavailability, and precision targeting. Challenges such as toxicity, scalability, and cost are also discussed. CAMPs have the potential to revolutionize drug delivery through their robustness and multifunctionality, particularly in precision medicine.

EXPERT OPINION

Future advancements in peptide engineering, nanotechnology, and AI-driven design are expected to enhance CAMPs' therapeutic specificity, reduce toxicity, and broaden their applications, including oncology and gene therapy, paving the way for their integration into next-generation therapeutics.

Peptide-Bismuth Tricycles: Maximizing Stability by Constraint

Constrained peptides possess excellent properties for identifying lead compounds in drug discovery. While it has become increasingly straightforward to discover selective high-affinity peptide ligands, especially through genetically encoded libraries, their stability and bioavailability remain significant challenges. By integrating macrocyclization chemistry with bismuth binding, we generated series of linear, cyclic, bicyclic, and tricyclic peptides with identical sequences. Utilizing bismuth to rigidify the peptide structure allows for a better comparison of different constraint levels, reducing confounding effects of interactions often seen with hydrophobic stapling reagents. Our study facilitated the identification of a peptide-bismuth tricycle that fully withstands cellular levels of glutathione, acts as a nanomolar protease inhibitor without being proteolytically digested by its target, and is fully stable in human plasma. Importantly, this multicyclic peptide does not possess any non-canonical amino acid modifications. Using oxime ligation, we conjugated an analogue of this tricycle to the N-terminus of two nanobodies to demonstrate potential applications in targeted therapy.

Cyclic natural product oligomers: diversity and (bio)synthesis of macrocycles

Cyclic compounds are generally preferred over linear compounds for functional studies due to their enhanced bioavailability, stability towards metabolic degradation, and selective receptor binding. This has led to a need for effective cyclization strategies for compound synthesis and hence increased interest in macrocyclization mediated by thioesterase (TE) domains, which naturally boost the chemical diversity and bioactivities of cyclic natural products. Many non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) derived natural products are assembled to form cyclodimeric compounds, with these molecules possessing diverse structures and biological activities. There is significant interest in identifying the biosynthetic pathways that produce such molecules given the challenge that cyclodimerization represents from a biosynthetic perspective. In the last decade, many groups have pursued the characterization of TE domains and have provided new insights into this biocatalytic machinery: however, the enzymes involved in formation of cyclodimeric compounds have proven far more elusive. In this review we focus on natural products that involve macrocyclization in their biosynthesis and chemical synthesis, with an emphasis on the function and biosynthetic investigation on the special family of TE domains responsible for forming cyclodimeric natural products. We also introduce additional macrocyclization catalysts, including butelase and the CT-mediated cyclization of peptides, alongside the formation of cyclodipeptides mediated by cyclodipeptide synthases (CDPS) and single-module NRPSs. Due to the interdisciplinary nature of biosynthetic research, we anticipate that this review will prove valuable to synthetic chemists, drug discovery groups, enzymologists, and the biosynthetic community in general, and inspire further efforts to identify and exploit these biocatalysts for the formation of novel bioactive molecules.

Cyclic peptides: A powerful instrument for advancing biomedical nanotechnologies and drug development

Cyclic peptides have emerged as an essential tool in the advancement of biomedical nanotechnologies, offering unique structural and functional advantages over linear peptides. This review article aims to highlight the roles of cyclic peptides in the development of biomedical fields, with a particular focus on their application in drug discovery and delivery. Cyclic peptides exhibit exceptional stability, bioavailability, and binding specificity, making them ideal candidates for therapeutic and diagnostic applications. We explore the synthesis and design strategies that enable the precise control of cyclic peptide structures, leading to enhanced performance in targeting specific cellular pathways. The article also highlights recent breakthroughs in the use of cyclic peptides for creating innovative drug delivery systems, including nanoparticle conjugates and peptide-drug conjugates, which have shown promise in improving the efficacy and safety profiles of existing traditional treatments. The integration of cyclic peptides into nanotechnological frameworks holds significant promise for addressing unmet medical needs, providing a foundation for future advancements in personalized medicine and targeted drug delivery.

Identification and Characterization of CC-AMP1-like and CC-AMP2-like Peptides in Capsicum spp

Antimicrobial peptides (AMPs) are compounds with a variety of bioactive properties. Especially promising are their antibacterial activities, often toward drug-resistant pathogens. Across different AMP sources, AMPs expressed within plants are relatively underexplored with a limited number of plant AMP families identified. Recently, we identified the novel AMPs CC-AMP1 and CC-AMP2 in ghost pepper plants (Capsicum chinense x frutescens), exerting promising antibacterial activity and not classifying into any known plant AMP family. Herein, AMPs related to CC-AMP1 and CC-AMP2 were identified within both Capsicum annuum and Capsicum baccatum. In silico predictions throughout plants were utilized to illustrate that CC-AMP1-like and CC-AMP2-like peptides belong to two broader AMP families, with three-dimensional structural predictions indicating that CC-AMP1-like peptides comprise a novel subfamily of α-hairpinins. The antibacterial activities of several closely related CC-AMP1-like peptides were compared with a truncated version of CC-AMP1 possessing significantly more activity than the full peptide. This truncated peptide was further characterized to possess broad-spectrum antibacterial activity against clinically relevant ESKAPE pathogens. These findings illustrate the value in continued study of plant AMPs toward characterization of novel AMP families, with CC-AMP1-like peptides possessing promising bioactivity.

Influence of heterochirality on the structure, dynamics, biological properties of cyclic(PFPF) tetrapeptides obtained by solvent-free ball mill mechanosynthesis

Cyclic tetrapeptides c(Pro-Phe-Pro-Phe) obtained by the mechanosynthetic method using a ball mill were isolated in a pure stereochemical form as a homochiral system (all L-amino acids, sample A) and as a heterochiral system with D configuration at one of the stereogenic centers of Phe (sample B). The structure and stereochemistry of both samples were determined by X-ray diffraction studies of single crystals. In DMSO and acetonitrile, sample A exists as an equimolar mixture of two conformers, while only one is monitored for sample B. The conformational space and energetic preferences for possible conformers were calculated using DFT methods. The distinctly different conformational flexibility of the two samples was experimentally proven by Variable Temperature (VT) and 2D EXSY NMR measurements. Both samples were docked to histone deacetylase HDAC8. Cytotoxic studies proved that none of the tested cyclic peptide is toxic.

Development and Challenges of Cyclic Peptides for Immunomodulation

Cyclic peptides are polypeptide chains formed by cyclic sequences of amide bonds between protein-derived or non-protein-derived amino acids. Compared to linear peptides, cyclic peptides offer several unique advantages, such as increased stability, stronger affinity, improved selectivity, and reduced toxicity. Cyclic peptide has been proved to have a promising application prospect in the medical field. In addition, this paper mainly describes that cyclic peptides play an important role in anti-cancer, anti-inflammatory, anti-virus, treatment of multiple sclerosis and membranous nephropathy through immunomodulation. In order to know more useful information about cyclic peptides in clinical research and drug application, this paper also summarizes cyclic peptides currently in the clinical trial stage and cyclic peptide drugs approved for marketing in the recent five years. Cyclic peptides have many advantages and great potential in treating various diseases, but there are still many challenges to be solved in the development process of cyclic peptides.

Mixture-Based Screening of Focused Combinatorial Libraries by NMR: Application to the Antiapoptotic Protein hMcl-1

We report on an innovative ligand discovery strategy based on protein NMR-based screening of a combinatorial library of ∼125,000 compounds that was arranged in 96 distinct mixtures. Using sensitive solution protein NMR spectroscopy and chemical perturbation-based screening followed by an iterative synthesis, deconvolutions, and optimization strategy, we demonstrate that the approach could be useful in the identification of initial binding molecules for difficult drug targets, such as those involved in protein-protein interactions. As an application, we will report novel agents targeting the Bcl-2 family protein hMcl-1. The approach is of general applicability and could be deployed as an effective screening strategy for de novo identification of ligands, particularly when tackling targets involved in protein-protein interactions.

Native and Engineered Cyclic Disulfide-Rich Peptides as Drug Leads

Bioactive peptides are a highly abundant and diverse group of molecules that exhibit a wide range of structural and functional variation. Despite their immense therapeutic potential, bioactive peptides have been traditionally perceived as poor drug candidates, largely due to intrinsic shortcomings that reflect their endogenous heritage, i.e., short biological half-lives and poor cell permeability. In this review, we examine the utility of molecular engineering to insert bioactive sequences into constrained scaffolds with desired pharmaceutical properties. Applying lessons learnt from nature, we focus on molecular grafting of cyclic disulfide-rich scaffolds (naturally derived or engineered), shown to be intrinsically stable and amenable to sequence modifications, and their utility as privileged frameworks in drug design.

Short Antimicrobial Peptides: Therapeutic Potential and Recent Advancements

There has been a lot of interest in antimicrobial peptides (AMPs) as potential next-generation antibiotics. They are components of the innate immune system. AMPs have broad-spectrum action and are less prone to resistance development. They show potential applications in various fields, including medicine, agriculture, and the food industry. However, despite the good activity and safety profiles, AMPs have had difficulty finding success in the clinic due to their various limitations, such as production cost, proteolytic susceptibility, and oral bioavailability. To overcome these flaws, a number of solutions have been devised, one of which is developing short antimicrobial peptides. Short antimicrobial peptides do have an advantage over longer peptides as they are more stable and do not collapse during absorption. They have generated a lot of interest because of their evolutionary success and advantageous properties, such as low molecular weight, selective targets, cell or organelles with minimal toxicity, and enormous therapeutic potential. This article provides an overview of the development of short antimicrobial peptides with an emphasis on those with ≤ 30 amino acid residues as a potential therapeutic agent to fight drug-resistant microorganisms. It also emphasizes their applications in many fields and discusses their current state in clinical trials.