Improving the Cell Permeability of Polar Cyclic Peptides by Replacing Residues with Alkylated Amino Acids, Asparagines, and d-Amino Acids

Gaussian accelerated molecular dynamics simulations facilitate prediction of the permeability of cyclic peptides

Despite their widespread use as therapeutics, clinical development of small molecule drugs remains challenging. Among the many parameters that undergo optimization during the drug development process, increasing passive cell permeability (i.e., log(P)) can have some of the largest impact on potency. Cyclic peptides (CPs) have emerged as a viable alternative to small molecules, as they retain many of the advantages of small molecules (oral availability, target specificity) while being highly effective at traversing the plasma membrane. However, the relationship between the dominant conformations that typify CPs in an aqueous versus a membrane environment and cell permeability remain poorly characterized. In this study, we have used Gaussian accelerated molecular dynamics (GaMD) simulations to characterize the effect of solvent on the free energy landscape of lariat peptides, a subset of CPs that have recently shown potential for drug development (Kelly et al., JACS 2021). Differences in the free energy of lariat peptides as a function of solvent can be used to predict permeability of these molecules, and our results show that permeability is most greatly influenced by N-methylation and exposure to solvent. Our approach lays the groundwork for using GaMD as a way to virtually screen large libraries of CPs and drive forward development of CP-based therapeutics.

Cell penetration of oxadiazole-containing macrocycles

Passive membrane permeability is an important property in drug discovery and biological probe design. To elucidate the cell-penetrating ability of oxadiazole-containing (Odz) peptides, we employed the Chloroalkane Penetration Assay. The present study demonstrates that Odz cyclic peptides can be highly cell-penetrant depending on the position of specific side chains and the chloroalkane tag. Solution NMR shows that Odz cyclic peptides adopt a β-turn conformation. However, despite observing high cell penetration, we observed low passive permeability in experiments with artificial membranes. These findings highlight the complexity of controlling cell penetration for conformationally sensitive macrocycles and suggest that Odz cyclic peptides may provide a framework for designing cell-penetrant cyclic peptides.

Macrocycles and macrocyclization in anticancer drug discovery: Important pieces of the puzzle

Increasing disease-related proteins have been identified as novel therapeutic targets. Macrocycles are emerging as potential solutions, bridging the gap between conventional small molecules and biomacromolecules in drug discovery. Inspired by successful macrocyclic drugs of natural origins, macrocycles are attracting more attention for enhanced binding affinity and target selectivity. Due to the conformation constraint and structure preorganization, macrocycles can reach bioactive conformations more easily than parent acyclic compounds. Also, rational macrocyclization combined with sequent structural modification will help improve oral bioavailability and combat drug resistance. This review introduces various strategies to enhance membrane permeability in macrocyclization and subsequent modification, such as N-methylation, intramolecular hydrogen bonding modulation, isomerization, and reversible bicyclization. Several case studies highlight macrocyclic inhibitors targeting kinases, HDAC, and protein-protein interactions. Finally, some macrocyclic agents targeting tumor microenvironments are illustrated.

Beyond Rule-of-five: Permeability Assessment of Semipeptidic Macrocycles

Compounds beyond the rule-of-five are generating interest as they expand the molecular toolbox for modulating targets previously considered "undruggable". Macrocyclic peptides are an efficient class of molecules for modulating protein-protein interactions. However, predicting their permeability is difficult as they differ from small molecules. Although constrained by macrocyclization, they generally retain some conformational flexibility associated with an enhanced ability to cross biological membranes. In this study, we investigated the relationship between the structure of semi-peptidic macrocycles and their membrane permeability through structural modifications. Based on a scaffold of four amino acids and a linker, we synthesized 56 macrocycles incorporating modifications in either stereochemistry, N-methylation, or lipophilicity and assessed their passive permeability using the parallel artificial membrane permeability assay (PAMPA). Our results show that some semi-peptidic macrocycles have adequate passive permeability even with properties outside the Lipinski rule of five. We found that N-methylation in position 2 and the addition of lipophilic groups to the side chain of tyrosine led to an improvement in permeability with a decrease in tPSA and 3D-PSA. This enhancement could be attributed to the shielding effect of the lipophilic group on some regions of the macrocycle, which in turn, facilitates a favorable macrocycle conformation for permeability, suggesting some degree of chameleonic behavior.

Controlling the Chameleonic Behavior and Membrane Permeability of Cyclosporine Derivatives via Backbone and Side Chain Modifications

Some macrocycles exhibit enhanced membrane permeability through conformational switching in different environmental polarities, a trait known as chameleonic behavior. In this study, we demonstrate specific backbone and side chain modifications that can control chameleonic behavior and passive membrane permeability using a cyclosporin O (CsO) scaffold. To quantify chameleonic behavior, we used a ratio of the population of the closed conformation obtained in polar solvent and nonpolar solvent for each CsO derivative. We found that β-hydroxylation at position 1 (1 and 3) can encode chameleonicity and improve permeability. However, the conformational stabilization induced by adding an additional transannular H-bond (2 and 5) leads to a much slower rate of membrane permeation. Our CsO scaffold provides a platform for the systematic study of the relationship among conformation, membrane permeability, solubility, and protein binding. This knowledge contributes to the discovery of potent beyond the rule of five (bRo5) macrocycles capable of targeting undruggable targets.

Amide-to-ester substitution as a stable alternative to N-methylation for increasing membrane permeability in cyclic peptides

Naturally occurring peptides with high membrane permeability often have ester bonds on their backbones. However, the impact of amide-to-ester substitutions on the membrane permeability of peptides has not been directly evaluated. Here we report the effect of amide-to-ester substitutions on the membrane permeability and conformational ensemble of cyclic peptides related to membrane permeation. Amide-to-ester substitutions are shown to improve the membrane permeability of dipeptides and a model cyclic hexapeptide. NMR-based conformational analysis and enhanced sampling molecular dynamics simulations suggest that the conformational transition of the cyclic hexapeptide upon membrane permeation is differently influenced by an amide-to-ester substitution and an amide N-methylation. The effect of amide-to-ester substitution on membrane permeability of other cyclic hexapeptides, cyclic octapeptides, and a cyclic nonapeptide is also investigated to examine the scope of the substitution. Appropriate utilization of amide-to-ester substitution based on our results will facilitate the development of membrane-permeable peptides.

A Model for the Rapid Assessment of Solution Structures for 24-Atom Macrocycles: The Impact of β-Branched Amino Acids on Conformation

Experiment and computation are used to develop a model to rapidly predict solution structures of macrocycles sharing the same Murcko framework. These 24-atom triazine macrocycles result from the quantitative dimerization of identical monomers presenting a hydrazine group and an acetal tethered to an amino acid linker. Monomers comprising glycine and the β-branched amino acids threonine, valine, and isoleucine yield macrocycles G-G, T-T, V-V, and I-I, respectively. Elements common to all members of the framework include the efficiency of macrocyclization (quantitative), the solution- and solid-state structures (folded), the site of protonation (opposite the auxiliary dimethylamine group), the geometry of the hydrazone (E), the C2 symmetry of the subunits (conserved), and the rotamer state adopted. In aggregate, the data reveal metrics predictive of the three-dimensional solution structure that derive from the fingerprint region of the 1D 1H spectrum and a network of rOes from a single resonance. The metrics also afford delineation of more nuanced structural features that allow subpopulations to be identified among the members of the framework. Well-tempered metadynamics provides free energy surfaces and population distributions of these macrocycles. The areas of the free energy surface decrease with increasing steric bulk (G-G > V-V ∼ T-T > I-I). In addition, the surfaces are increasingly isoenergetic with decreasing steric bulk (G-G > V-V ∼ T-T > I-I).

Oxineur, a novel peptide from Caspian cobra Naja naja oxiana against HT-29 colon cancer

Colon cancer ranks fourth in mortality. This cancer is still an important clinical challenge worldwide due to its high prevalence and poor prognosis. Proteomic studies revealed that snake venom is a diverse and variable mixture of enzymatic and non-enzymatic proteins and peptides. Despite the toxic effects of these molecules, several proteins and peptides have been isolated that have practical applications and appear to induce apoptosis and prevent cell metastasis. In this study, we worked on cytotoxic effects and anticancer activity of Naja naja oxiana (Iranian Caspian cobra) snake venom components on HT-29 cell line colon cancer. Separated Fraction-5 by FPLC indicated the high cytotoxicity on HT-29 cell line colon cancer by MTT test. Further isolation of F5 by HPLC showed that the purified peak 2, nominated as Oxineur that contains a cytotoxic effect on HT-29 cells and reduces cell viability at 8 μg/ml to 4% in 24 h. Oxineur has the least cytotoxic effect on HEK-293 normal cells. Further studies on Oxineur peptide confirmed the apoptotic effects with high expression of CASP9 gene and DNA fragmentation in cancerous cells. The partial sequence of Oxineur revealed 71% homology with the neurotoxin II from Naja naja oxiana. Since our target molecule is a peptide in the molecular weight range of 7 kDa, it has potentially a therapeutic value.

Anticancer peptides mechanisms, simple and complex

Peptide therapy has started since 1920s with the advent of insulin application, and now it has emerged as a new approach in treatment of diseases including cancer. Using anti-cancer peptides (ACPs) is a promising way of cancer therapy as ACPs are continuing to be approved and arrived at major pharmaceutical markets. Traditional cancer treatments face different problems like intensive adverse effects to patient's body, cell resistance to conventional chemical drugs and in some worse cases the occurrence of cell multidrug resistance (MDR) of cancerous tissues against chemotherapy. On the other hand, there are some benefits conceived for peptides usage in treatment of diseases specifically cancer, as these compounds present favorable characteristics such as smaller size, high activity, low immunogenicity, good biocompatibility in vivo, convenient and rapid way of synthesis, amenable to sequence modification and revision and there is no limitation for the type of cargo they carry. It is possible to achieve an optimum molecular and functional structure of peptides based on previous experience and bank of peptide motif data which may result in novel peptide design. Bioactive peptides are able to form pores in cell membrane and induce necrosis or apoptosis of abnormal cells. Moreover, recent researches have focused on the tumor recognizing peptide motifs with the ability to permeate to cancerous cells with the aim of cancer treatment at earlier stages. In this strategy the most important factors for addressing cancer are choosing peptides with easy accessibility to tumor cell without cytotoxicity effect towards normal cells. The peptides must also meet acceptable pharmacokinetic requirements. In this review, the characteristics of peptides and cancer cells are discussed. The various mechanisms of peptides' action proposed against cancer cells make the next part of discussion. It will be followed by giving information on peptides application, various methods of peptide designing along with introducing various databases. Future aspects of peptides for employing in area of cancer treatment come as conclusion at the end.

Impact of Cyclization and Methylation on Peptide Penetration through Droplet Interface Bilayers

Cell-penetrating peptides enter cells via diverse mechanisms, such as endocytosis, active transport, or direct translocation. For the design of orally delivered cell-penetrating peptides, it is crucial to know the contribution of these different mechanisms. In particular, the ability of a peptide to translocate through a lipid bilayer remains a key parameter for the delivery of cargos. However, existing approaches used to assess translocation often provide discrepant results probably because they have different sensitivities to the distinct translocation mechanisms. Here, we focus on the passive permeation of a range of hydrophobic cyclic peptides inspired by somatostatin, a somatotropin release-inhibiting factor. Using droplet interface bilayers (DIB), we assess the passive membrane permeability of these peptides and study the impact of the peptide cyclization and backbone methylation on translocation rates. Cyclization systematically improved the permeability of the tested peptides while methylation did not. By studying the interaction of the peptides with the DIB interfaces, we found membrane insertion and peptide intrinsic diffusion to be two independent factors of permeability. Compared to the industrial gold standard Caco-2 and parallel artificial membrane permeability assay (PAMPA) models, DIBs provide intermediate membrane permeability values, closer to Caco-2. Even for conditions where Caco-2 and PAMPA are discrepant, the DIB approach also gives results closer to Caco-2. Thereupon, DIBs represent a robust alternative to the PAMPA approach for predicting the permeability of peptides, even if the latter present extremely small structural differences.

Large-Scale Membrane Permeability Prediction of Cyclic Peptides Crossing a Lipid Bilayer Based on Enhanced Sampling Molecular Dynamics Simulations

Membrane permeability is a significant obstacle facing the development of cyclic peptide drugs. However, membrane permeation mechanisms are poorly understood. To investigate common features of permeable (and nonpermeable) designs, it is necessary to reproduce the membrane permeation process of cyclic peptides through the lipid bilayer. We simulated the membrane permeation process of 100 six-residue cyclic peptides across the lipid bilayer based on steered molecular dynamics (MD) and replica-exchange umbrella sampling simulations and predicted membrane permeability using the inhomogeneous solubility-diffusion model and a modified version of it. Furthermore, we confirmed the effectiveness of this protocol by predicting the membrane permeability of 56 eight-residue cyclic peptides with diverse chemical structures, including some confidential designs from a pharmaceutical company. As a result, a reasonable correlation between experimentally assessed and calculated membrane permeability of cyclic peptides was observed for the peptide libraries, except for strongly hydrophobic peptides. Our analysis of the MD trajectory demonstrated that most peptides were stabilized in the boundary region between bulk water and membrane and that for most peptides, the process of crossing the center of the membrane is the main obstacle to membrane permeation. The height of this barrier is well correlated with the electrostatic interaction between the peptide and the surrounding media. The structural and energetic features of the representative peptide at each vertical position within the membrane were also analyzed, revealing that peptides permeate the membrane by changing their orientation and conformation according to the surrounding environment.

Applications of amphipathic and cationic cyclic cell-penetrating peptides: Significant therapeutic delivery tool

A new class of peptides, cyclic cell-penetrating peptides (CPPs), has great potential for delivering a vast variety of therapeutics intracellularly for treating diverse ailments. CPPs have been used previously; however, their further use is limited due to instability, toxicity, endosomal degradation, and insufficient cellular penetration. Cyclic CPPs are being investigated in delivering therapeutics to treat various ailments, including multi-drug resistant microbial infections, HIV, and cancer. They can act as a carrier for a variety of cargos and target intracellularly. Approximately 40 cyclic peptides-based therapeutics are available in the market, and annually one cyclic peptide-based drug enters the market. Numerous research and review articles have been published in the last decade about linear and cyclic peptides separately. This review is the first to provide a comprehensive deliberation about cationic and amphipathic cyclic CPPs. Herein, we highlights their structures, significant advantages, translocation mechanisms, and delivery application in the area of biomedical sciences.

Interplay among Conformation, Intramolecular Hydrogen Bonds, and Chameleonicity in the Membrane Permeability and Cyclophilin A Binding of Macrocyclic Peptide Cyclosporin O Derivatives

A macrocyclic peptide scaffold with well-established structure-property relationship is desirable for tackling undruggable targets. Here, we adopted a natural macrocycle, cyclosporin O (CsO) and its derivatives (CP1-3), and evaluated the impact of conformation on membrane permeability, cyclophilin A (CypA) binding, and the pharmacokinetic (PK) profile. In nonpolar media, CsO showed a similar conformation to cyclosporin A (CsA), a well-known chameleonic macrocycle, but less chameleonic behavior in a polar environment. The weak chameleonicity of CsO resulted in decreased membrane permeability; however, the more rigid conformation of CsO was not detrimental to its PK profile. CsO exhibited a higher plasma concentration than CsA, which resulted from minimal CypA binding and lower accumulation in red blood cells and moderate oral bioavailability (F = 12%). Our study aids understanding of CsO, a macrocyclic peptide that is less explored than CsA but with greater potential for diversity generation and rational design.

Membrane Permeability in Cyclic Peptides is Modulated by Core Conformations

Cyclic peptides have the potential to bind to challenging targets, which are undruggable with small molecules, but their application is limited by low membrane permeability. Here, using a series of cyclic pentapeptides, we showed that established physicochemical criteria of permeable peptides are heavily violated. We revealed that a dominant core conformation, stabilized by amides' shielding pattern, could guide the design of novel compounds. As a result, counter-intuitive strategies, such as incorporation of polar residues, can be beneficial for permeability. We further find that core globularity is a promising descriptor, which can extend the capability of standard predictive models.

Cyclic Peptides as Drugs for Intracellular Targets: The Next Frontier in Peptide Therapeutic Development

Developing macrocyclic peptides that can reach intracellular targets is a significant challenge. This review discusses the most recent strategies used to develop cell permeable cyclic peptides that maintain binding to their biological target inside the cell. Macrocyclic peptides are unique from small molecules because traditional calculated physical properties are unsuccessful for predicting cell membrane permeability. Peptide synthesis and experimental membrane permeability is the only strategy that effectively differentiates between cell permeable and cell impermeable molecules. Discussed are chemical strategies, including backbone N-methylation and stereochemical changes, which have produced molecular scaffolds with improved cell permeability. However, these improvements often come at the expense of biological activity as chemical modifications alter the peptide conformation, frequently impacting the compound's ability to bind to the target. Highlighted is the most promising approach, which involves side-chain alterations that improve cell permeability without impact binding events.

Anticancer peptide: Physicochemical property, functional aspect and trend in clinical application (Review)

Cancer is currently ineffectively treated using therapeutic drugs, and is also able to resist drug action, resulting in increased side effects following drug treatment. A novel therapeutic strategy against cancer cells is the use of anticancer peptides (ACPs). The physicochemical properties, amino acid composition and the addition of chemical groups on the ACP sequence influences their conformation, net charge and orientation of the secondary structure, leading to an effect on targeting specificity and ACP-cell interaction, as well as peptide penetrating capability, stability and efficacy. ACPs have been developed from both naturally occurring and modified peptides by substituting neutral or anionic amino acid residues with cationic amino acid residues, or by adding a chemical group. The modified peptides lead to an increase in the effectiveness of cancer therapy. Due to this effectiveness, ACPs have recently been improved to form drugs and vaccines, which have sequentially been evaluated in various phases of clinical trials. The development of the ACPs remains focused on generating newly modified ACPs for clinical application in order to decrease the incidence of new cancer cases and decrease the mortality rate. The present review could further facilitate the design of ACPs and increase efficacious ACP therapy in the near future.

Real time monitoring of peptide delivery in vitro using high payload pH responsive nanogels

A pH responsive pMAA nanogel that demonstrates high loading capacity and rapid intracellular delivery of hydrophilic peptides.

Precision medicine by designer interference peptides: applications in oncology and molecular therapeutics

In molecular cancer therapeutics only 10% of known cancer gene products are targetable with current pharmacological agents. Major oncogenic drivers, such as MYC and KRAS proteins are frequently highly overexpressed or mutated in multiple human malignancies. However, despite their key role in oncogenesis, these proteins are hard to target with traditional small molecule drugs due to their large, featureless protein interfaces and lack of deep pockets. In addition, they are inaccessible to large biologicals, which are unable to cross cell membranes. Designer interference peptides (iPeps) represent emerging pharmacological agents created to block selective interactions between protein partners that are difficult to target with conventional small molecule chemicals or with large biologicals. iPeps have demonstrated successful inhibition of multiple oncogenic drivers with some now entering clinical settings. However, the clinical translation of iPeps has been hampered by certain intrinsic limitations including intracellular localization, targeting tissue specificity and pharmacological potency. Herein, we outline recent advances for the selective inhibition of major cancer oncoproteins via iPep approaches and discuss the development of multimodal peptides to overcome limitations of the first generations of iPeps. Since many protein–protein interfaces are cell-type specific, this approach opens the door to novel programmable, precision medicine tools in cancer research and treatment for selective manipulation and reprogramming of the cancer cell oncoproteome.

Cyclic Cell-Penetrating Peptides as Efficient Intracellular Drug Delivery Tools

Cyclic cell-penetrating peptides are relatively a newer class of peptides that have a huge potential for the intracellular delivery of therapeutic agents aimed at treating challenging ailments like multidrug-resistant bacterial diseases, cancer, and HIV infection. Cell-penetrating peptides (CPPs) have been extensively explored as intracellular delivery vehicles; however, they have some inherent limitations like poor stability, endosomal entrapment, toxicity, and suboptimal cell penetration. Owing to their favorable properties that avoid these limitations, cyclic CPPs can provide a good alternative to linear CPPs. Several Reviews have been published in the past decade that cover CPPs and cyclic peptides independently. To the best of our knowledge, this is one of the first Reviews that covers cyclic CPPs comprehensively in the light of studies published so far. In this Review, we have detailed examples of cyclic CPPs, their structures, and cyclization strategies followed by a detailed account of their advantages over their linear counterparts. A hot area in cyclic CPPs is the exploration of cell-penetration mechanisms; this Review highlights this topic in detail. Finally, we will review the applications of cyclic CPPs, followed by conclusions and future prospects.

Delivering bioactive cyclic peptides that target Hsp90 as prodrugs

The most challenging issue facing peptide drug development is producing a molecule with optimal physical properties while maintaining target binding affinity. Masking peptides with protecting groups that can be removed inside the cell, produces a cell-permeable peptide, which theoretically can maintain its biological activity. Described are series of prodrugs masked using: (a) O-alkyl, (b) N-alkyl, and (c) acetyl groups, and their binding affinity for Hsp90. Alkyl moieties increased compound permeability, P_app, from 3.3 to 5.6, however alkyls could not be removed by liver microsomes or in-vivo and their presence decreased target binding affinity (IC₅₀ of ≥10 µM). Thus, unlike small molecules, peptide masking groups cannot be predictably removed; their removal is related to the 3-D conformation. O-acetyl groups were cleaved but are labile, increasing challenges during synthesis. Utilising acetyl groups coupled with mono-methylated amines may decrease the polarity of a peptide, while maintaining binding affinity.

Polymer mediated transport of the Hsp90 inhibitor LB76, a polar cyclic peptide, produces an Hsp90 cellular phenotype

LB76 is a cyclic peptide that shows great promise as a selective heat shock protein 90 (Hsp90) inhibitor. However despite strong binding to and inhibition of Hsp90 in cell lysate its polar structure prevents it from crossing the cell membrane. We have developed a pH responsive polymer nanoparticle that effectively encapsulates LB76 from solution without need for purification. The nanoparticle releases the molecule upon crossing the cell membrane. Treatment of human colon cancer HCT116 cells with nanoparticles laden with LB76 produces the typical phenotype associated with Hsp90 inhibition, providing evidence of a therapeutically active payload.

Design Principles for Intestinal Permeability of Cyclic Peptides

One of the most exciting facets of cyclic peptides is that they have the potential to be orally bioavailable, despite having physical properties well beyond the traditional "Rule-of-5" chemistry space (Lipinski et al., Adv Drug Deliv Rev. 23(1): 3-25, 1997). An important component of meeting this challenge is to design cyclic peptides with good intestinal permeability. Here we discuss the design principles for intestinal permeability that have been developed in recent year. These principles can be subdivided into three regimes: physical property guidelines, design strategies for the macrocyclic ring, and design strategies for side chains. The most important overall aims are to minimize solvent-exposed polarity while keeping size, flexibility, and lipophilicity within favorable ranges, thereby allowing peptide chemists to achieve intestinal permeability in addition to other important properties for their compounds, such as solubility and binding affinity. Here we describe a variety of design strategies that have been developed to help peptide chemists in this endeavor.

RNA Display Methods for the Discovery of Bioactive Macrocycles

The past two decades have witnessed the emergence of macrocycles, including macrocyclic peptides, as a promising yet underexploited class of de novo drug candidates. Both rational/computational design and in vitro display systems have contributed tremendously to the development of cyclic peptide binders of either traditional targets such as cell-surface receptors and enzymes or challenging targets such as protein-protein interaction surfaces. mRNA display, a key platform technology for the discovery of cyclic peptide ligands, has become one of the leading strategies that can generate natural-product-like macrocyclic peptide binders with antibody-like affinities. On the basis of the original cell-free transcription/translation system, mRNA display is highly evolvable to realize its full potential by applying genetic reprogramming and chemical/enzymatic modifications. In addition, mRNA display also allows the follow-up hit-to-lead development using high-throughput focused affinity maturation. Finally, mRNA-displayed peptides can be readily engineered to create chemical conjugates based on known small molecules or biologics. This review covers the birth and growth of mRNA display and discusses the above features of mRNA display with success stories and future perspectives and is up to date as of August 2018.

Emerging Methods and Design Principles for Cell-Penetrant Peptides

Biomolecules such as antibodies, proteins, and peptides are important tools for chemical biology and leads for drug development. They have been used to inhibit a variety of extracellular proteins, but accessing intracellular proteins has been much more challenging. In this review, we discuss diverse chemical approaches that have yielded cell-penetrant peptides and identify three distinct strategies: masking backbone amides, guanidinium group patterning, and amphipathic patterning. We summarize a growing number of large data sets, which are starting to reveal more specific design guidelines for each strategy. We also discuss advantages and disadvantages of current methods for quantifying cell penetration. Finally, we provide an overview of best-odds approaches for applying these new methods and design principles to optimize cytosolic penetration for a given bioactive peptide.

Rapid Analysis of Cyclic Peptide Cyclosporine A by HPLC Using a Column Packed with Nonporous Particles

We developed a rapid and efficient analytical technique for cyclosporine A using HPLC on a column packed with 2-µm nonporous octadecylsilyl silica particles. Under optimized conditions, cyclosporine A was separated with high resolution from other cyclic peptides within 3 min, because the mass transfer resistance in the stationary phase was reduced by the use of the small, nonporous particles. Although the plate number increased greatly with the increase in the column temperature, the retention times were not affected. This behavior is different from other cyclic peptides or linear peptides. Based on its physicochemical characteristics, cyclosporine A is a poor hydrogen bond donor, and has a small topological polar surface area, low rotatable bond count, and high log P value. These results show that cyclosporine A is structurally rigid and undergoes poor water solvation even at high temperature. In the context of the rapid development of cyclic peptides with similar physicochemical characteristics to cyclosporine A, our developed method is useful for the development of cyclic peptide therapeutics.