Cancer, which is still a complicated puzzle for researchers, affects millions of individuals despite several breakthroughs in therapy. Even after many years of screening, it continues to be the one of the major causes of fatality [1]. According to Global Cancer Observatory (GLOBOCAN) 2022 data, nearly 20.0 million new cases and approximately 9.7 million cancer deaths were reported globally [2]. The World Health Organization (WHO) reported 2.48 million lung cancer cases and 2.30 million breast cancer cases approximately in 2022. Further, colorectal cancer, prostate cancer, stomach cancer, liver cancer, cervix cancer, bladder cancer, non-Hodgkin lymphoma (NHL), and esophageal cancers come under the top ten list. Not only incidence wise but in terms of mortality caused lung cancer is the cancer with largest deaths followed by colorectal cancer. Breast cancer, stomach and liver cancer are also under the category with high mortality rate (if analyzed for both sex), however, the breast cancer is reported to be associated with highest incidence rate and mortality in females. Global estimates for new cancer cases are projected to reach approximately 35 million by 2050 [3].

Disease burden estimates are projected after consideration of several risk factors involving changes in population growth and aging. Socioeconomic development risk factors, including poor nutrition, a sedentary lifestyle, and polluted air, also contribute to the risk of cancer [4–6]. A thorough awareness of this illness is vital for developing cutting-edge treatment and prevention techniques to lower cancer incidence globally. These techniques not only significantly lower disease rates but also help to reduce healthcare and economic burdens [7–9].

The standard or conventional methods for treating cancer include multiple therapies, such as chemotherapy, radiotherapy, surgery, and hormone therapy. However, the effectiveness of such mainstream treatments is associated with various side effects that vary with the type of cancer, cancer stage, general state of the patient, and specific therapy protocol [10–12]. Usually, surgery is associated with pain, fatigue, bleeding, nausea, and a high chance of injury to adjacent organs/tissues [13]. Some side effects may occur immediately after chemotherapy, with nausea, vomiting, and anorexia being the most common [14]. Chemotherapy is also characterized by other side effects, such as fatigue, stomatitis, alopecia, hair loss, diarrhea, and peripheral neuropathies [15, 16]. Radiotherapy is another conventional therapy with side effects such as fatigue, hair loss (in the treated area), skin changes (redness, itching), and organ damage due to the use of high-energy radiation [17, 18].

Furthermore, side effects are even associated with recent treatments, including immunotherapy, hormone therapy, and transplants such as rashes, fever, diarrhea, and fatigue [19–21]. These may also cause inflammation of organs, resulting in unfavorable and fatal results [22]. Immune checkpoint blockade therapy has exhibited success in clinical trials, however, this therapy has been dominated by antibodies and associated with limitations like adverse immune reactions and limited responsiveness [23]. Bone marrow transplant complications can lead to infections, hemorrhage, and organ damage [24, 25]. Graft-versus-host disease is also likely to occur after allogeneic transplantation [26]. Another alternative for cancer treatment could be angiogenesis inhibitors, however, they are also associated with side effects such as proteinuria (excess protein in the urine), high blood pressure, bleeding and altered wound healing, thus greatly impact the quality of life of patients [27–29].

Another recent breakthrough in cancer treatment is chimeric antigen receptor (CAR) T-cell therapy. This involves the isolation of T cells from patients, which are further engineered to possess artificial receptors for specific tumor antigens [30, 31]. This seems to be a better treatment than other treatments, but it also has limitations. Some of these include critical toxicity, only fair antitumor activity, restricted trafficking, and limited tumor penetration [32]. CAR-T-cell therapy also requires a significant workforce and time to prepare engineered T cells. These limitations make this therapy slightly more complicated and questionable [33]. These shortcomings and adverse effects associated with already available therapies are also responsible for unaddressed gap between cancer diagnosis and treatment, hence warrants the development of new approaches to overcome these challenges.

Anticancer peptides (ACPs), also regarded as immunomodulatory agents, are assisting in the development of new means to overcome the constraints of conventional therapy. These potential treatment candidates are suggested to have more specific actions and are being targeted to develop new anticancer treatment measures with fewer side effects [34]. ACPs can surpass the body’s natural defenses and reach tumors effectively. These peptides can be used individually or in combination with other therapies like immunotherapy (immune checkpoint inhibitors) and chemotherapy [23, 35]. ACPs can also be designed as self-assembling peptides or in conjugation with nanocarriers which can further improve their delivery and reduce their breakdown by the enzymes of the body [36]. This review will discuss the immunological constraints in cancer treatments, evidence of different peptides exhibiting anticancer potential and their mechanism of action in detail. Further this article also discusses about the ACPs in clinical trials and the challenges associated with ACPs for enhanced translation.

A thorough literature search was carried out for the review paper using a number of internet sources. Initially, Google Scholar was used to obtain overall information about the current literature on ACPs, followed by a literature search using different databases, including PubMed, Google Scholar, and Scopus. The keywords used for the literature search were “anticancer peptides”, “immunomodulatory warriors”, “azurin and anticancer”, “cathelicidin and anticancer”, “magainin and anticancer”, etc.

The search was restricted to English-language papers that were published mainly in recent years (2000–2024). Based on their titles, abstracts, and complete texts, the retrieved articles were scrutinized for relevance. An extensive summary of the present state of research on peptides with anticancer activity and immunomodulatory features was then provided using the information retrieved from the chosen papers. Papers that were not related to anticancer or immunomodulatory activities of microbial peptides were excluded.

Cancer is typically characterized by immunological challenges that hinder the complete elimination of tumors from the body. Some of the main immunological issues are involved in the development of metastasis, relapse, heterogeneity, resistance to conventional drugs, and limited response rates to other therapies.

Metastasis or the dissemination of cancer cells from the primary tumor to distant organs is a major obstacle to successful treatment. A recent study published by Skomedal and his colleagues [37] mentioned that metalloproteinases play a significant role in the metastasis of cancer. The marine microalga P. lutheri has been shown to inhibit metalloproteinase-9 in HT1080 fibrosarcoma cells [37]. Metastasis leads to a maximum number of deaths related to cancer. Cancer metastasis involves the separation of cancer cells from the main tumor present at a specific site, invasion of the extracellular matrix and basement membrane, and eventual dissemination to other organs, usually through capillary blood [38]. This can make treatment difficult since newly metastasized cells may contain receptors different from those of the primary tumor. Receptor-targeted treatment can kill only primary tumor cells and not metastasized cells. Metastatic cancer is a major cause of death in cancer patients with a very less survival rate. A recent study referring SEER database reported 1,030,937 patients were diagnosed to have metastatic cancer in USA (1992–2019) out of which 81% (837,811) were found dead in follow ups. Among the died patients further the majority of patients i.e., 82.7% (688,529) died due to diagnosed metastatic cancer suggesting the implication of metastasis Figure 1 [39].

Another crucial immunological problem pertaining to cancer is heterogeneity, which is prevalent even within the tumor and its own metastasized or circulating form [40]. Breast cancer is one of the leading causes of death due to its heterogeneity [41]. For example, human epidermal growth factor receptor 2 (HER2) expression shows heterogeneity between primary tumors as well as the cells that have metastasized from them [42].

The relapse of cancer after treatment is another major risk factor linked to cancer [43]. A study by Kriegmair et al. [44] (2018) showed that relapse commonly occurs within 5 years of primary treatment. Some cancer cells, called cancer stem cells or stemness-high cancer cells, possess greater tumorigenic and metastatic activity than normal cancer cells. These cells are more likely to relapse within a short period of time [45]. The mainstream methods of cancer therapy have failed to address this issue, except for gene therapy, which has been shown to have some positive effects on cancer relapse [46].

Both inflammatory components and continuous exposure to antigens also pose serious issues leading to the depletion of T cells [47]. Cancer cells that have evaded immune surveillance can be eliminated by immunological warriors such as cytotoxic T cells and natural killer (NK) cells, which can recognize and kill cancer cells even if they have downregulated the expression of tumor-associated antigens or recruited immunosuppressive cells to the tumor microenvironment (TME) [15, 48, 49]. Patients can be treated with interleukin 2 (IL-2) and IL-15 to induce the production of NK cells, but this can lead to life-threatening toxicity and capillary leak syndrome [50]. Additionally, treatment with IL-2 can sometimes decrease the transport of NK cells to tumor-containing regions of the body [51].

Two major strategies used by cancer cells to escape immune system regulation include evasion from immune identification and creation of an immunosuppressive TME. In the first approach, cancer cells may undergo a reduction or loss of surface tumor antigenic expression, which protects these tumor cells from being recognized and targeted by cytotoxic T lymphocytes. For example, loss of human leukocyte antigens (HLAs) in non-small cell lung cancers leads to immune escape through the presentation of fewer antigens [52]. Additionally, mutations and deletions may result in downregulation of the antigen-presenting machinery, conferring resistance to T-cell effector genes. As genomic tumor heterogeneity increases, the probability of sub clonal generations escaping immune attack also increases. Metastasis progression and therapeutic resistance usually occur due to the presence of rare clones in primary tumors. Furthermore, cancer cells evolve mechanisms that mimic peripheral tolerance, preventing the local cytotoxic response of effector T cells, as well as that of other cells, such as tumor-associated macrophages (TAMs), NK cells, and tumor-associated neutrophils (TANs) [52].

In the second strategy, cancer cell-derived factors instigate an immune-tolerant TME by secreting suppressive molecules, expressing inhibitory checkpoint molecules, and inducing the recruitment of immunosuppressive cells such as TAMs, myeloid-derived suppressor cells (MDSCs), and regulatory T cells (Tregs). These strategies result in a complex and efficient system for immune evasion. Therefore, cancer cells avoid destruction by immune attack through a combination of strategies that prevent immune recognition and create an immunosuppressive microenvironment as shown in Figure 2 [52].

A limited response rate is another major issue that is an obstacle during the treatment of cancer. Tumors have revolutionized current cancer treatments, leading to low response rates [53]. Due to primary and secondary resistance, single agent immunotherapy frequently fails, and only a small percentage of patients have lasting advantages promoting the use of combination therapies. With time, even combination therapy is losing its effect due to tumor mutations, leading to the demand for newer therapies for cancer treatment [54]. The various immunological challenges have been summarized in the Figure 3.

ACPs are short peptides that can specifically bind to tumor-associated cells (TAAs) and activate cytotoxic T cells and NK cells, leading to the killing of cancer cells, even those that have evaded the immune system. Cancer cell resistance to various existing therapies also demands the formulation of new ways to combat cancer [55]. Some malignancies become resistant to inhibitors and other immunotherapies. Immunotherapies can be used in combination with immunological warriors. This improves the effectiveness of therapy and aids in overcoming resistance [56].

Tumors require blood vessels to grow, which is another issue related to cancer. There are certain tumors that control angiogenesis to survive [57]. ACPs may target the vessels that lead to tumors to reduce the nutrient supply and access growth factors. This ultimately results in the fostering of anticancer immunity [58]. Persistent inflammation in the TME is another factor that promotes tumor growth. The activation of ACPs and immune cells limits the inflammatory response to control cancer growth by managing inflammation [59, 60]. Immunological stressors can establish immune memory. They can function by identifying and killing remaining cancer cells. This leads to long-term protection against tumor recurrence [61].

ACPs are commonly known as immunomodulatory agents due to their ability to generate a strong immune response and help the immune system fight tumor cells. These peptides are usually between 10 and 60 amino acids in length [62].

There are different ACPs that can be classified according to various parameters. Some of these classifications are according to origin, structure, mechanism of action, and target specificity. In terms of origin, these peptides can be obtained from plants, insects, microbes and mammals, where they serve as a part of the pathogen response system [63]. For example, Alfer is an anticancer microbial peptide obtained from insects that has been demonstrated to be effective against human papillomavirus [64]. Thiocoraline is a novel peptide of microbial origin (obtained from the actinomycete Micromonospora marina) with anticancer activity [65]. Bacteria also produce peptide bacteriocins that have been shown to have anticancer effects [66]. The cathelicidin family include peptides of mammalian origin. Additionally, synthetically designed peptides such as acyl-lysine oligomers are also reported to possess inhibiting properties [67].

Based on their structure, anticancer microbial peptides can be classified as either α-helices, β-sheets, random coils or cyclic peptides [64, 68]. ACPs can also be classified on the basis of other characteristics like mechanism of action, target specificity and origin. On the basis of their mechanisms of action ACPs can be classified as membrane disrupters, immune regulators, angiogenesis inhibitors, and apoptosis inducers [62, 69]. The different ACP classifications are summarized in Table 1.

They are also subdivided into two classes according to target specificity. The first category includes ACPs directed toward specific target cells without affecting healthy cells. A common example of such a type includes cecropins, whereas the second category comprises broad spectrum ACPs that act on various tumor types as well as normal cells. HNP-1 to 3 (human neutrophil defensins 1-3) are examples of broad-spectrum ACPs [68]. The knowledge of their classification is helpful in choosing various types of ACPs depending upon the cancer type, which leads to higher efficacy and greater success during clinical trials.

These peptides are being researched as potential treatments for different types of cancer because their precise tumor-targeting properties have been demonstrated in numerous trials. They also constitute an effective substitute for routine chemotherapy [80]. Some antimicrobial peptides with anticancer properties include magainin, nisin, and cecropins. Researchers have expressed interest in using these peptides for cancer treatment for many years. However, the efficacy and safety of these peptides have still not been demonstrated in clinical settings.

ACP-based therapy is gaining popularity in field of cancer treatment and research and usually involves targeting tumor cells at three different stages: initiation, growth and development. These peptides have great potential compared to other anticancer treatments due to their specific targeting properties and fewer side effects. Usually, tumors create a suppressive environment that hinders immune response and ACPs can reprogram the TME. M2-like TAMs promote tumor growth and these peptides inhibit their recruitment proving itself as a promising therapeutic strategy [135]. There are several modes of action of these peptides that help to prevent tumor cell growth, proliferation and spread.

Initially, researchers discovered that antimicrobial peptides that act as ACPs have the potential to inhibit bacteria, fungi, and viruses [143]. These agents have the ability to interrupt microbial membranes and hence can be candidates for treating various infectious diseases, including infections resistant to antibiotics [144].

Numerous studies have examined the ability of numerous ACPs, such as defensins, to inhibit viral entry, replication, and assembly. These are antiviral medicines that can be used for treating many viral diseases. Some major drugs include remdesivir and chloroquine. Both of these factors inhibit the replication of viruses [145]. Perishable goods can be made available for longer periods by employing ACPs to minimize the growth of deteriorating microorganisms such as bacteria and other pathogens. Many ACPs also have immunomodulatory effects because of increased cytokine production and increased immune cell activity [146, 147].

The coupling of ACPs to nanoparticles helps improve drug delivery to specific target sites, such as cancerous tissues [148]. This methodology increases the uptake of the drug and its therapeutic efficacy and stability. Several ACPs contribute to tissue repair and wound healing through accelerated regeneration of tissues, control of inflammation, and promotion of angiogenesis [149, 150]. With these qualities, they can be employed in making topical formulations and wound dressings for the management of chronic wounds.

Biofilms are a mass of microbes that are resistant to antibiotics. Since these peptides can penetrate and degrade biofilms, ACPs are suitable for the prevention and treatment of biofilm-related diseases [151]. Additionally, some ACPs have adjuvant properties that enhance the immune response to vaccines. Vaccines can be made more effective for immunization against cancer and infectious diseases [152]. Hence, not only anticancer agents but also antimicrobial peptides play significant roles in other areas.

Though, ACPs offer several advantages over other therapies, such as affordability, high effectiveness, lower toxicity, high tolerance, their development is a major challenge owing to its time-consuming designing, production, and higher cost. Another major challenge is their short half-life and proteolytic cleavage resulting in quick elimination from the body requiring frequent doses [153].

These limitations and challenges warrant more mechanistic studies to be conducted to bring these peptides into translational products for cancer treatment. Literature suggests mixing of these microbial peptides with other therapies, such as chemotherapy or immunotherapy to overcome these limitations, which can augment their anticancer effects and increase patient survival rates [154]. Understanding how the cells function under the influence of TME can be quite helpful to develop better combination therapies [155].

Further, to overcome the challenges of designing these peptides at a faster pace, different AI approaches such as deep learning (DL), machine leaning (ML) as well as hybrid learning (a combination of both ML and DL) can be used. It offers a more economical way to identify and separate ACPs from non-ACPs and then categorizing them based on their effectiveness [156]. To improve the half-life of such peptides, they can be conjugated with transport proteins such as transthyretin exerting protection from degradation. Apart from transthyretin, PEG or XTEN can also be used to improve half-life of ACPs. To overcome the issues of cell permeability, researchers attach CPPs (cellular delivery tags) to these peptides that help them enter cancer cell membrane easily [157]. immune checkpoint inhibitors (ICIs), that target PD-1 and CTLA-4, have recently shown progress in clinical trials. This gave rise to a new class of anti-cancer medications and in the field of immuno-oncology, however, these inhibitors are also associated with side effects like toxicity, need for intravenous administration and limited efficacy. The combination therapy of ICIs with ACPs is being explored to obtain better outcomes [23]. ACPs blocking immune checkpoints via surface interaction with checkpoint proteins, also referred as peptide inhibitors are suggested to activate T cell against tumor cells after releasing immune breaks [158]. Such inhibitor peptides, destabilizing the protein-protein interactions at immune checkpoints are designed using rational designing from known structures, empirical designing from sequence data and computational technology followed by phage display [23]. A recent study reported the designing and development of a helix-loop-helix peptide ERY 2-4 exhibiting the selective inhibition of immune checkpoint CTLA4-B7 and activating T cells [159].

Thus, the different combination therapies, carrier mediated delivery, rational designing of the peptides, combination of ACP and ICI approach as one are some of the approaches to deal with the challenges associated with ACP mediated cancer therapy.

Several anti-cancer peptides are currently under clinical trials or in the pre-clinical stage. Over the last decade, an increasing number of anti-cancer medications have been brought to the market for clinical studies. A gonadotropin-producing hormone analog, Lupron, had worldwide sales of over two hundred million dollars in the year 2011. There are 29 peptide-based drugs as in drug bank database out of which 5 have been approved and 11 are still under trials (Table 2). The approved peptides on tumour cells inhibit cancer growth by targeting anti-cancer immune response, hormone signalling, cancer cell death, and cellular transcription [140].

The first FDA-approved anti-cancer peptide, Bortezomib, demonstrates the potential this technique holds for cancer treatment. The knowledge about their mode of action and the clinical applications of anticancer microbial peptides is constantly expanding. These peptides could be used as effective weapons for combating cancer due to the increasing need for new cancer therapeutics. By using various modified amino-acids as well as other conjugation techniques, their stability can be enhanced leading to better pharmacokinetics. Scientists are also working towards personalized ACP therapies based on the characteristics of different cancer patients [153].

ACPs have shown revolutionary results as major therapeutics against different cancer types and with advances in computational methods and proper understanding of their structures and activities can lead to greater theranostic success [161].

The convergence of innate immunity and oncology points to a positive direction within which ACPs show great potential as immune modulators in the fight against cancer. ACPs act as several immunological modulators by modulating T-cell activation, inducing apoptosis, helping in membrane lysis, etc. These functions suggest the exploration of ACPs during the rising demand for novel cancer therapeutics. Further demonstrated success when used in combination with established treatments and immunotherapies also suggest the need to explore ACP based treatment approaches in detail. In combination with currently available treatments, the immunomodulatory activity of ACPs can reactivate dampened immune responses against tumors. This leads to an increase in immunity, which further helps in fighting cancer.

During the transition from preclinical studies to clinical studies of such combinations, various issues need to be thoughtfully considered. Some of the major issues include dose, formulation of proper combinations, and patient selection criteria. Similarly, investigations of any possible side effects and long-term consequences of ACP therapy have become necessary. One promising line of research relates to personalized treatment by means of ACPs that may be altered according to a specific patient’s immunogenomic profile.

This review emphasizes the evolutionary potential of ACPs as immunomodulatory drugs. In the ever-changing arena of anticancer therapies, these ACPs are on the verge of creating new grounds for a large discovery.