The fundamental understanding of tumor immunology and its plausible mechanism of action has opened the route to employing the body’s immunity against cancer 
. Immunotherapy in the form of a vaccine has great potential for breast cancer treatment over chemotherapy and endocrine therapies due to several issues, including relapse and drug resistance. Recent reports demonstrate that about 80% of treatment failures are due to metastases and drug resistance from several mechanisms of action, such as genetic mutation 
. The management of advanced malignant breast cancer, with median overall survival ranging from 4 to 5 years for luminal-like tumors to 1 year for triple-negative disease, remains minimal and is considered short 
. Until now, many scientists have tried to discover how to overcome therapeutic resistance because more than one mechanism may be responsible for oncogenesis. Even though there are several immunotherapy forms, including adoptive cell transfer, checkpoint blockage, and antibody-based drugs, vaccines are seemingly more tempting due to their wide safety profile and lifelong protection 
. However, until now, no breast cancer vaccine has been authorized by the U.S. Food and Drug Administration (FDA) for either therapeutic or prophylactic purposes. The immune system in humans is incredibly complex. Even though, until now, no breast cancer vaccine has been authorized, many are still in clinical trials. It is only a matter of time. The tumor microenvironment (TME) in tumors remains one of the major hurdles in developing a therapy against breast cancer. Breast cancer is generally infiltrated by immune cells triggered by the cancer cells, which can create an immunosuppressive microenvironment that encourages tumor growth by inhibiting immune cells 
. Besides TME, the following factors are thought to be responsible for the challenges of breast cancer vaccine development: (i) the stage of breast cancer; (ii) the choice of TAAs to target; and (iii) the vaccine’s low immunogenicity as a result of the antigen selected or as a result of the vaccine delivery platform used 
There is, however, increasing attention in clinical research that evaluates vaccines derived from the peptide. The rationale for this interest is based on the aberrant expression of proteins or antigens by breast cancer. With the discovery of breast cancer antigens, the peptide-based vaccine is becoming a potential alternative to conventional therapies, which are known to have serious drawbacks. The vaccines would modulate the immune system of the body to specifically attack cancer cells based on the recognition of tumor associate antigens (TAAs) or tumor-specific antigens (TSAs) on the surface of cancer. Interestingly, recognition of these antigens eventually allows the immune system to recognize these antigens, to have long-lasting immunity, and to solve the relapse issue after completion of the treatment. A robust, fundamental, and precise comprehension mechanism for the action of peptide vaccines is required to establish potent and effective cancer vaccines.
A peptide-based cancer vaccine is a short chain of amino acids that contain epitopes that are reactive to T cells. The major objective of peptide-based cancer vaccines is to induce the necessary host immune response to recognize and eliminate targeted cancer cells based on a defined set of TAAs and TSAs. The peptide-based vaccine follows the principle of immunotherapy, which modulates the immune system of the body to specifically attack cancer cells based on the recognition of aberrant expression of tumor antigens or proteins in the cancer cell. Interestingly, recognition of these antigens eventually allows the immune system to recognize these antigens, to have long-lasting immunity, and to solve the relapse issue after completion of the treatment.
Administration of a peptide vaccine functionalized CD8+ cytotoxic T lymphocytes (CTL) cells to attack tumor cells through the release of granozymes, granulysin, perforin, and Fas ligand (FasL) through Fas death receptor binding to cancer cells for apoptosis to occur (Figure 2
). Cytokine release helps with lymphocyte migration, B cell development, T cell activation, and expansion. After activation, CD4+ T cells further differentiate to develop dominant anticancer pathways and responses. In order to modulate tumor-specific immune responses and inhibit proliferation in the body, significant interventions have been rendered to identify tumor-expressed antigen cells or those recognized as tumor-associated antigens (TAAs) utilizing T cells 
. Identified as an example of HER2 antigen in breast cancer and transformed into a vaccine component capable of triggering a specific and systemic immune response that may contribute to the suppression, removal, and destruction of cancer in body tissues 
. When TAAs are found in the body from cancer cells, specific fragments of cancer proteins are expressed on the cell surface and then attached to the MHC 1 complex 
. It would ultimately be recognized by NK cells and CD8+ cells. The dissimilarity between endogenous and exogenous peptides is crucial for a functional immune response.
Peptide-based vaccines offer several advantages over other types of cancer vaccines. Peptide-based vaccines can be easily customized with minimal epitopes while still being able to induce desired immune responses safely. Peptides are manufactured almost entirely using synthetic chemical approaches. Therefore, peptide antigens can be completely and specifically identified as chemical entities. Hence, all issues associated with the biological contamination of antigens are effectively eliminated.
4. Identified Tumor-Associated Antigens in Peptides Vaccine Development for Breast Cancer
HER2, also known as ERBB2, NEU, and CD34, is a human epidermal growth factor receptor 2 and a component of transmembrane glycoprotein that is overexpressed in approximately 20–30% of primary breast carcinomas 
for tyrosine kinase activity. The HER2/neu cell surface receptor is the most frequently targeted TAA; thus, the HER2-derived peptide vaccine has shown excellent potential in developing breast cancer vaccines. Upon dimerization of the antigen receptor, the numerous intracellular signaling pathways are activated by transphosphorylation, which mediates cell proliferation and differentiation. However, when inappropriate activation happens, it contributes to the production of many malignancies 
. Slamon et al. 
first discovered the function of HER2 as a marker with a prognosis value for treating breast cancer in 1987. It has been confirmed and proved by several scholars 
. Studies conducted by Clynes and colleagues reported monoclonal antibodies targeting HER2 to provide clinical benefits against HER2 overexpressing breast carcinomas 
. To date, the application of HER2 as a therapeutic marker and predictor in invasive breast carcinomas has been commonly utilized and continues to develop.
To sum up, HER2/neu is a well-known therapeutic target that is a hallmark of HER2-positive breast cancer. With HER2-targeted vaccinations, targeting HER2 appears to be a reasonable strategy for the dysregulation of numerous signaling cascades that promote oncogenesis. It has been extensively used with a GM-CSF adjuvant. They offer little or no risk and the chance of producing a memory antibody against the same disease. Ex vivo expansion of cellular immunity, including activation of CD8+ CTL against breast carcinoma, will be enabled by the production of anti-HER2 immunization. Vaccinated patients showed high levels of CD8+ and mediated delayed-type hypersensitivity reactions 
. Established by Mansourian et al. 
, p5 peptide encapsulated with liposomes co-administrated with CpG-ODN has been shown to decrease tumor size and, at the same time, improve animal survival period in mice of the breast cancer model. This was confirmed by Farzad et al.’s study 
, which displayed another peptide, the P435 HER2-derived peptide, conjugated to liposomes capable of inducing CTL responses, therefore improving prognosis in the TUBO murine breast cancer model. Another study revealed that Nelipepimut-S (E75) was a nine-amino acid peptide extracted from the HER2 protein capable of increasing the patient’s survival rate. Research from recent clinical trials has demonstrated positive effects of HER2-specific vaccinations that, when paired with chemo-drugs, could synergistically inhibit the recurrence of breast cancer, creating robust immunity and sustaining elevated CTL rates. One of the hurdles to the HER2-based vaccine is against TNBC subtypes due to the lack of HER-2 (ERBB2), progesterone, and estrogen receptors. Costa and colleagues suggested that combinations of HER2-based vaccines with pembrolizumab or nivolumab (immune checkpoint inhibitor antibodies) merit promising outcomes 
. Combinations with other therapies might produce synergistic effects and resensitize other cancer cell death programs.
Transmembrane mucin-like glycoprotein Mucin 1 (MUC-1) is a type of glycoprotein comprising a single polypeptide chain with multiple oligosaccharide side chains with oxygen linkages to serine, proline, and threonine residues 
frequently overexpressed in glandular and epithelial mammary, lung, and colon cancers 
. MUC-1 overexpression can be used as a marker for cancer that suggests that the cancer is progressing 
Several studies have demonstrated that targeting MUC-1 was a successful option for the cancer vaccine because it is broadly dispersed in all tumors and cancers, including stem cell cancer 
. Covalently bound to the TLR agonist, the completely synthetic glycosylated MUC-1 peptide vaccine exhibited good humoral and cellular immune responses 
. The pioneered MUC-1 peptide vaccine study was performed in 1995 against patients with breast carcinoma. The majority of patients reacted to the medication, and no toxicity was found 
. The extension of the research was carried out up until the Phase III clinical trial. A pilot Phase III analysis of 31 early Stage II breast cancer patients utilizing oxidized mannan-MUC-1 immunotherapy found that MUC1 immunotherapy is effective 
. The MUC-1 peptide vaccine candidates have demonstrated an improved survival rate. MUC-1 is proposed as a potential biomarker to be targeted in breast cancer therapy because patients would typically overexpress the MUC-1 biomarker (approximately 90%) for immune system detection. Antibodies against MUC-1 can efficiently cause CTL and TLR. Hence, reasonable disease regulation is accomplished as patients produce strong antibody titers of MUC1 IgG. In clinical studies on women with Stage I and Stage II breast cancer, MUC-1 IgG and IgM antibodies were tested and assessed for their association with disease-specific survival 
. MUC1 is a possible antigen to be utilized as a site-specific target for the deployment of therapeutic agents as a vaccine against MUC1 for breast carcinoma. Recent studies have shown that MUC1 can induce antigen-specific cellular and humoral responses not only to trigger MUC1-specific CD4+ and CD8+ T-cells but also to generate antibodies 
EphA is a type of transmembrane glycoprotein with tyrosine kinase (RTK) receptors on the surface that play a significant role as tumor-specific cell-surface receptors for drug-targeting sites. It is the largest group among tyrosine kinase receptor families, and among them, EphA2 is commonly overexpressed in breast cancer. The activation and overexpression of EphA2 frequently lead to its ligand-independent oncogenic and angiogenesis activation, which are triggered by dwindled contact with the ligand, ephrin-A (EphA2). Loss of the ligated EphA2 receptor decreases the intrinsic tumor-suppressive signaling pathways, accompanied by downregulation of the PI3K/Akt and the ERK pathways, thus decreasing the tumor volume and size.
As a therapeutic target, EphA2 receptors remain an essential marker. Overexpression of EphA2 receptors has been correlated with low survival in all patients with breast cancer subtypes due to the EphA2 activity that enhances tumorigenesis and the progression of metastases 
. A monoclonal antibody (mAb EA5) has been studied to suppress EphA2 receptors in ER-positive breast cancer and to minimize cancer invasiveness 
. The outcome was promising, and the study proceeded in the presence of tamoxifen. Furthermore, the monoclonal antibody EPhA2 can specifically target antigens and suppress the development of breast cancer cells and tumorigenesis.
YSA and SWL are peptide-based EpHAs that target EpHA2 receptors on the surface of tumor cells. Scarberry et al. 
reported the success of using a magnetic CoFe2
nanoparticle-YSA peptide conjugate to extract ovarian cells from blood and fluid in mice. Even though the EpHA antigens are overexpressed in blood cancer and tumor cells, further exploration with regard to EphA as a peptide-based vaccine is very limited. Perhaps an EpHA-based vaccine does not elicit a potent immune response to eliminate various classifications of breast cancer. This may be triggered by cross-reactions between drugs and other proteins or by incomplete subcellular internalization of antibody-drug conjugates (ADCs). Thus, in order to address this issue, Salem et al. 
proposed peptide-based targeting drugs that were less harmful but efficient and inexpensive. The aim of breast cancer therapy may be to merge EphA2 expression with carcinogenesis. Strategies focused on EphA2 targeting have been groundbreaking developments in therapeutic discovery. The targeted drug, for example, trastuzumab, is yet to be used; the concern of cardiotoxicity persists. Immunotherapy, similar to a cancer vaccination, tends to be an effective solution to treating metastatic breast carcinoma.
Survivin, a 16.5 kDa intracellular acidic protein of 142 amino acids encoded by the BIRC5 gene, is a multifunctional protein that belongs to the smallest member of the inhibitory apoptosis protein family (IAPs). It regulates cell cycle progression through inhibition of the apoptosis pathway 
. A high level of survivin expression is significantly associated with breast, urothelial, and colorectal cancer invasiveness and its low prognosis 
. Survivin is undetectable in healthy tissue, indicating that it is exclusively presented as a biomarker when there is tumor transformation and acts as a transcriptome that is expressed in breast cancer. It seems to play a role in the antiapoptotic function of a protein, preventing the cell program from happening. A study by Ryan et al. 
showed that a high level of survivin expression patterns is often associated with HER2/neu positive breast cancer and correlates with the prognosis.
This was confirmed by Lyu et al. 
, who reported that dysregulation of survivin was found in HER2/neu breast cancer, and survivin was identified as a desirable therapeutic target for blocking its IAP functions. A gene and immunotherapy named sepantronium bromide (YM155) have been developed to block survival. They provided a positive outcome for in vivo research by lowering the expression of survivin, raising the regression of the tumor, and prolonging the life of the mouse. However, YM155 failed to demonstrate an improvement in treatment response in metastatic non-small-cell lung cancer patients (NSCLC). This failure is probably due to the presence of multiple pathways linking survivin with other regulated proteins, making it more complicated 
Another research performed by Tanaka et al. 
found that cytoplasm-responsive nanocarriers conjugated with a functional cell-penetrating peptide could facilitate the delivery of anti-vascular endothelial growth factor siRNA (siVEGF) complexes to tumor tissues after systemic injection and could elicit a potent anti-tumor effect. In a study from Rodel et al. 
, survivin as an antigen vaccine conferred peptide-specific CTL induction of urothelial cancers in patients without significant adverse reactions. On the other hand, the latest research indicated that the presence of the survivin antigen in breast carcinoma revealed a connection between expression and therapeutic outcomes 
SART3 is a tumor rejection antigen consisting of 3806 bp of nucleotides encoded by a 140-kilodalton (kDa) protein expressed in the cytosol of most of the cell proliferation and has been shown during gene transcription and mRNA synthesis of cancer cells. Similar to survivin, the SART3 oncogene is absent in normal tissues except for the fetal liver and testicles 
. This antigen exhibits strong binding with HLA-A24-restricted CTL epitopes and may be useful for specific immunotherapy.
A Phase 1 clinical trial was recorded by Miyagi et al. 
utilizing the SART3 peptide vaccine in colorectal cancer patients. The findings revealed a significant induction of cellular immune responses in 7 out of 11 patients. However, no explicit activation of the humoral immune response (IgG or IgE) has been recorded for peptides. SART3 led to the regulation of pro-inflammatory cytokine expression and the association of the degree of expression with malignancies and the prognosis for patients with breast cancer 
. Given its positive outcome against SART antigen-expressed cancer, no clinical trials of SART-associated breast cancer vaccine goals have been reported.
CEA is a 180-kDa glycoprotein widely recognized as an oncofetal antigen that is found in numerous cancers, including colorectal, breast, gastric, pancreatic, and non-small cell lung cancers. CEA is one of the earliest tumor markers used to identify and anticipate the recurrence of tumors following surgical resection 
. Its overexpression leads to the progression of the tumor. High secretions of CEA from cancer cells in the blood serum and over-expressed CEA on the surface of tumor cells make it accessible for use as a selective marker for cancer immunotherapy. CEA has been used as the foundation for numerous cancer vaccines, including DNA-based vaccines, dendritic cell-based vaccines, recombinant vector-based vaccines, protein-based vaccines, and anti-idiotype antibody vaccines, with the potential to induce both humoral and cell-mediated immunity that contributes to the killing of cancer cells 
. Ojima et al. 
demonstrated that genetically modified dendritic cells that express CEA administered simultaneously with interleukin 12 (IL-12), GM-CSF enhanced the therapeutic effects in CEA transgenic mice through the improvement of CEA-specific T-cell responses. Interestingly, the vaccination therapy eliminated colon cancer up to 2 × 103
sizes. Furthermore, no detrimental results were found after the experiment. The research performed by Gulley et al. 
found that 9 out of 16 patients diagnosed with recombinant CEA-MUC-1-TRICOM poxviral-based robust tumor vaccines had an increase in both CD8+ and CD4+ immune responses. To sum up, targeting CEA could be a successful vaccination technique for the clinical application of peptide vaccines to achieve a positive antitumor response.
p53 is a tumor suppressor protein that plays a vital role in regulating genomic stability by controlling the cell cycle and inducing apoptosis when cell damage is beyond repair. p53 mutations occur in about 18–25% of primary breast cancer, rendering them potential biomarkers for cancer immunotherapy. Missense mutations within the p53 gene could potentially cause the accumulation of mutant proteins within the cell nucleus through posttranscriptional modification. The prognosis value of the patient appeared to be associated with the p53 level. Approximately 80% of TNBC patients have been identified with high p53 gene levels, and so far, no immunotherapeutic medication scientifically used for TNBC has been proven to develop a peptide-derived vaccine 
. Many 2-sulfonyl pyrimidine compounds, such as PK11007 and PK11000, are successful in killing cancer cells by explicitly attacking mutant P53 thiol groups, thereby reducing oxidative stress levels (e.g., ROS) and eventually retaining a redox state 
PRIMA-1MET (APR-246) has been clinically studied in a Phase I clinical trial and is currently undergoing more clinical review. It inhibits cancer cell growth by targeting mutant p53 and inactivating it in triple-negative breast cancer (TNBC) 
. In the Phase I/II clinical trial, ten colorectal cancer patients were vaccinated with p53-derived synthetic long peptides (SLPs). Rapid p53-specific T-cell responses were observed in blood samples obtained six months after the last vaccine 
. Although the theory suggests that SLP would activate a high level of T cells in vaccinated patients, the clinical findings have unfortunately not been compatible. Perhaps targeting p53 alone is not enough to eliminate breast cancer cells. Thus, multiple peptides that target multiple antigens while stimulating multi-antigenic immune responses tend to be the right approach to improving the immunogenicity and clinical efficacy of p53-directed immunotherapies