Wounds can generally be grouped into several categories including amputation, avulsion, crush injury, puncture, abrasion, and lacerations. Each wound type can affect multiple tissues—a laceration or puncture may include skin and muscle damage—and could also become chronic wounds. In the United States, chronic skin wounds affect approximately 6.5 million patients, requiring around $25 billion annually on treatment (Sen et al., 2009). Such treatment currently relies on optimizing controllable healing factors rather than administering effective targeted therapies (Liu et al., 2022), (Ezhilarasu et al., 2020). Several approaches for medicated as well as non-medicated dressings have been proposed. Such innovative dressings include hydrogel and alginate materials, electrospun nanofibers, reactive oxygen species scavengers, and microelectronic sensors in smart dressings that aim to facilitate wound healing and provide wound care management (Rezvani Ghomi et al., 2019), (Liu et al., 2021), (Zhao et al., 2020), (Farahani and Shafiee, 2021). Targeted therapies for wound healing that are specific to the extent of damage and tissues affected could add to the current treatment protocols with the potential of restoring both structure and function to patients as well as improving quality of life. Effective therapies may even help to decrease the amount of time patients spend in a healthcare setting, reducing costs for both patients and providers. Innovative wound healing therapeutic and regenerative interventions are much needed, and peptides represent a potential source of lead compound discovery and subsequent development. Owing to their enormous diversity in structure and function, peptides are among the most prevalent and biologically important class of compounds in biology, nature, and modern medicine. Cyclic peptides are polypeptide chains formed by amide bonds in a circular sequence between proteinogenic or non-proteinogenic amino acids but can also occur with disulfide bonds that bring distal residues in a peptide sequence together. One of the most commonly used cyclic peptides in medicine is insulin which has been used clinically for more than 100 years (Bliss, 1993). Currently, cyclic peptides comprise more than two-thirds of the FDA and EMA-approved peptides used in the pharmaceutical industry and approved therapeutic cyclic peptides are derived from natural sources (Zorzi et al., 2017). Cyclization confers many benefits such as promoting peptide stability which improves target protein binding affinity and specificity (Sohrabi et al., 2020); reducing conformational flexibility (Craik, 2006); and increasing peptide chain efficacy by increasing the interacting surface area for protein-protein interactions (Angelini et al., 2012). In addition to improving structural properties, cyclization also improves peptide pharmacokinetics (Bockus et al., 2013). The constrained structure reduces the energy barrier required to bind to transport proteins which increases passive diffusion and active transport (Dougherty et al., 2019). Thus, cyclic peptides are an attractive platform from which wound healing therapeutics could be based. In this review we provide an overview of cyclic peptides that have been shown to promote wound healing as well as recovery and tissue injury prevention. We also discuss the current state of the field clinically, describe applications, and provide an assessment including both advantages and challenges in incorporating cyclic peptides into clinical care. To the best of our knowledge, this is the first review that attempts to collect cyclic peptides from the literature that address wound healing.

Injury causing tissue damage can occur throughout the body, and depending on the severity, can involve connective, epithelial, muscle, and nervous tissue. Here, we organize cyclic wound healing peptides into several categories—general wounds of the skin and blood vessels, bone, nervous tissue, and ischemic reperfusion injuries that include the liver and kidneys (Figure 1; Table 1).

A variety of peptides have been characterized that possess both antimicrobial and wound healing properties. In particular, multiple antimicrobial peptides (AMPs) implicated in wound healing have been identified within the epidermis of the skin, which is likely due to its critical function as part of the innate immune system. Thus, as a major interface between the inside and outside world, the epidermis, and, namely, the keratinocytes within the epidermis, play an important role in protecting against infection and facilitating injury remediation. Here, we provide a brief overview of AMPs with dual function as wound healing peptides. While many of these peptides are not cyclic, there are ample opportunities to cyclize and could be subject to peptide engineering efforts.

The linear peptide LL-37, also known as cathelicidin, is one such wound healing AMP. In vitro, cathelicidin has been shown to promote the proliferation of multiple cell types including HaCaT cells, a cell line used to model epidermal homeostasis, while also augmenting macrophage recruitment—an important feature of the wound healing process (Wang et al., 2022). Moreover, cathelicidin was shown to boost activity of the wound healing MAPK signaling pathway in vitro, and topical cathelicidin was able to accelerate wound healing in in vivo, whole skin rodent models of both S. aureus-infected and non-infected wounds. Interestingly, CD4-PP—a cyclic derivative of cathelicidin created via its dimerization and backbone cyclization—has been developed and shown to also possess immunomodulatory and antimicrobial activity (White et al., 2022). In particular, CD4-PP has exhibited activity against known pathogens including E. coli, K. pneumoniae, and P. aeruginosa. Furthermore, unlike its linear counterpart, CD4-PP has enhanced resistance to proteolytic degradation which increases its utility in a clinical setting.

Human β defensins are another class of AMP that are associated with wound healing activity. Specifically, human β defensin-3 (hBD-3)—a skin derived AMP—has been shown to boost the migration and proliferation of keratinocytes thereby initiating wound healing (Takahashi et al., 2021). These activities occur via the phosphorylation of the proteins EGFR and STAT, which triggers cascades that stimulate cell migration and proliferation, respectively. Importantly, these effects aren’t limited to the skin alone, with evidence pointing to the efficacy of hBDs in accelerating wound healing in the gut (Otte et al., 2008). In addition to the activities of hBD-3 in keratinocytes, hBD-3 also exerts prominent effects on human fibroblasts as well (Takahashi et al., 2021). Indeed, hBD-3 stimulates the production of pro-angiogenic growth factors including VEGF and FGF, increasing fibroblast proliferation and migration and hastening wound healing in an in vitro model. Moreover, in vivo models have also demonstrated that hBD-3 enhances immune cell and fibroblast recruitment to skin wound sites as well as angiogenesis. Although very promising therapeutically, there are some barriers that have made human defensins poor therapeutic candidates. For example, their antimicrobial activity is reduced dramatically by ionic strength (Hoover et al., 2003). However, the development of cyclic analogs of the defensins have thus far yielded encouraging results, with some cyclic variants of α defensins exhibiting salt insensitivity (Yu et al., 2000). Additional work in this space is required to understand whether a similar approach may be feasible with β defensins like hBD-3 and if wound healing activity persists.

Catestatin is a neuroendocrine AMP secreted by the chromaffin cells of the adrenal glands. It was first characterized as an inhibitor of catecholamine release and is derived from the proteolytic cleavage of the protein chromogranin A (CgA), which serves as the precursor protein for multiple AMPs. More recent research has revealed pleitropic effects of catestatin, identifying its roles in cardiovascular, metabolic, and immune regulation (Zalewska et al., 2022). In the context of wound healing, it has been shown to stimulate the proliferation and migration of keratinocytes and while also boosting Ca²⁺ levels in these cells (Hoq et al., 2011). These activities were shown to involve multiple signaling pathways including EGFR, Akt, MAPK, and PLC. The active core of catestatin, known as cateslytin, has been shown to be resistant to degradation by S. aureus-secreted enzymes and, thus, may have particular relevance in the context of the remediation of infection-prone wounds and growth inhibition of antibiotic-resistant bacteria (Scavello et al., 2021). Additionally, research also suggests that catestatin may also have relevance in the context of inflammatory conditions like ulcerative colitis where it appears to regulate the dynamics of intestinal mucus, reduce severity in an experimental model of colitis, and promote intestinal cell proliferation and migration to facilitate wound healing in the intestinal environment (Eissa et al., 2018). Engineering efforts have been made to cyclize catestatin, however the cyclic derivatives synthesized lost their biological activity, likely due to deviations in the structure of the amino terminal domain (Preece et al., 2004). Therefore, additional efforts are warranted to engineer cyclized forms of catestatin that retain their therapeutic potential.

A number of cyclic peptides have been approved and used in the clinic as antimicrobial, anticancer, and immunosuppressive agents and have a broad range of cellular targets (Choi and Joo, 2020; Zhang and Chen, 2022). The cyclic structure of these agents confers multiple advantages that include an increase in stability and resistance to degradation, specificity, and movement across membranes. Cyclic peptides can also be used in tissue healing applications. The natural process of wound healing can be accelerated with various wound healing agents, and cyclic peptides represent a unique source of potential wound healing therapeutics. In this review, cyclic peptide compounds are presented in the context of not only classic wound healing but also towards preventing reperfusion injuries, anabolism, and regeneration in the context of various tissue types. While typical wound healing can readily be applied to skin traumas such as burns and other open wounds, by broadening our scope we also show that applications to bone and nervous tissue can be accounted for in the context of complex medical traumas. In the context of patient care, a multi-pronged approach to wound healing can be envisioned where cyclic peptides are administered through hydrogels, intravenously, or orally. Multiple cyclic peptides that promote growth of skin, blood vessel, bone, muscular, and nervous tissue could be delivered simultaneously depending on the type and extent of tissue damage in the patient.

Delivery of the cyclic wound healing peptides can be facilitated by nanoparticles and lipid-based systems such as micelles. Peptides used for internal injuries would need to undergo rigorous pharmacokinetic/pharmacodynamic studies to ensure that a therapeutic dose is able to reach the target organ and provide its intended effect without eliciting adverse reactions or clearing from the organ too rapidly. A number of peptides in this review have been for the nervous system and associated support cells, and while drug delivery to the central nervous system is challenging, translocation of compounds across the blood-brain barrier (BBB) can be accomplished using BBB-permeabilizing short peptide tags that could act as delivery vehicles for therapeutic cargo. In addition, cyclic wound healing peptides could be designed and screened to allow for both repair and BBB permeability without the need for adding an additional tag.

Wound healing cyclic peptides and approaches can also be used to accelerate growth of specific tissues in an ex vivo setting for therapeutic purposes. Patients who have extensive wounds such as skin trauma may need to receive tissue grafts. Viable patient tissue could be harvested and grown in the laboratory in cell culture supplemented with growth promoting cyclic peptides before being grafted or transplanted back into the patient with the goal of restoring pre-injury functionality and barrier protection. The use of these peptides would help patients recover faster and perhaps even decrease the risk of infection from an open wound.

While cyclic peptides are a promising therapeutic intervention for wound healing, there are several limitations preventing their immediate use in clinical settings. There is currently a dearth of randomized control trials evaluating the vast majority of proposed cyclic peptide therapeutics which makes it difficult to determine if they will provide a clinically significant means to improve metrics such as patient mortality or rate of adverse events. As with any therapy, there will be risk of side effects such as allergic reactions to application of these foreign proteins. While immunogenicity is a much greater concern in protein-based therapeutics, that risk is still present for peptides. These challenges are compounded by the increasing use of non-pharmacological interventions in the wound healing space, which may not pose the same degree of risk and are an attractive alternative (Ongarora, 2022). An example of such a technology is Negative Pressure Wound Therapy, which has been shown to improve wound healing times in surgical and traumatic injuries (De Pellegrin et al., 2023; Liebman et al., 2023). Additionally, although studies claim that the cost of these wound healing peptides would be low, currently approved peptide drugs such as man-made insulin often come with significant price tags that are a cause of noncompliance amongst patients (Chiang et al., 2023; Li et al., 2023). Another example is the drug Plecanatide, which was recently approved for treating chronic constipation, and can cost over $500 for a pack (Zhang and Chen, 2022; Plecanatide Prices, 2023). These costs must be considered, especially in the context of the actual benefit achieved in clinical outcomes and not just from laboratory assays. Thus, peptide therapeutics could be limited in use if cost barriers are not properly addressed. It is also important to note that while many of the peptides in this review are categorized as wound healing or cytoprotective for specific tissues, these peptides could also have similar activities in other parts of the body. Such peptides could even be detrimental to the healing process through inducing dysregulation within cells, inflammation, and associated immune responses. More research in relevant models and carefully designed clinical trials are needed to fully characterize bioactivity, safety, and efficacy of these compounds.

Despite these challenges, cyclic peptides are amenable to computational design tools as well as high-throughput functional screening approaches that contribute to lead compound identification and optimization. Computational methods have greatly enhanced our ability to modify existing proteins and design de novo proteins to achieve a precise molecular goal. Software tools such as ProtCAD allow for the intelligent design of de novo proteins with optimized predicted binding energies and stabilities (Summa, 2002; Pike and Nanda, 2015). Various research groups have been able to leverage existing technologies to create cell-permeable cyclic peptides, as well as develop cyclic peptides to specifically target enzyme active sites (Hewitt et al., 2015; Hosseinzadeh et al., 2021). Many of the screening methodologies used for protein identification provide great advantages to therapeutic cyclic peptide discovery programs. One of these advantages is the ability to use phage display to enable rapid, high-throughput screening of many different cyclic peptides at one time. An example of such a platform is the RaPID platform, which can enable the display of even quite complex cyclic peptides with post-translational modifications (Goto and Suga, 2021). RaPID, and other similar peptide display technologies will synergize with traditional peptide therapeutic engineering methods. These optimization methods can help increase oral bioavailability, provide effective cyclization strategies for efficient manufacturing of cyclic peptides, and improve the therapeutic properties of promising peptide lead compounds (Räder et al., 2018; Bechtler and Lamers, 2021).

Cyclic peptide wound healing, cytoprotectants, and regenerative compounds may help complement current wound treatment approaches and increase the rate of recovery after injury. Here, we show that cyclic peptides can promote healing in various tissue types and even prevent ischemic reperfusion injury. Often, these peptides are derived from host-intrinsic growth factors and target associated receptors. However, developments in computational methods have allowed for de novo protein design and extensive screening approaches could all aid in the design and testing of cyclic peptide wound healing agents. While there are challenges in clinical implementation, cyclic peptides have several native advantages that make it an attractive category of potential wound healing therapeutics.