Advances in mRNA Vaccines for Breast Cancer

Current Status of mRNA Vaccines for Breast Cancer

Breast cancer ranks as the most prevalent malignant tumor globally. Standard treatment modalities encompass surgery, radiotherapy, chemotherapy, hormone therapy, and targeted therapy. Despite the burgeoning popularity of immunotherapy, hailed as the forefront in anti-tumor treatments, its efficacy in breast cancer remains suboptimal. This inadequacy primarily stems from breast cancer's characteristic low mutation load, rendering it an immunologically cold tumor. However, advancements in therapeutic messenger ribonucleic acid (mRNA) tumor vaccines offer promise in transitioning breast cancer from a cold to a hot tumor, thereby enhancing its responsiveness to immunotherapy. Unlike protein or cell vaccines, therapeutic mRNA tumor vaccines boast simplified production and design processes. Leveraging deoxyribonucleic acid (DNA) templates, they can be cost-effectively mass-produced via in vitro transcription using RNA polymerase. Nonetheless, certain structures within mRNA and double-stranded RNA impurities are prone to activating the innate immune system and are susceptible to degradation by ubiquitous RNA enzymes.

A diagram of the mRNA vaccine.

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Important Breakthrough in mRNA Vaccines

There have been several major breakthroughs in recent years in mRNA vaccine production processes as well as delivery systems.

• The emergence of HPLC purification technology and nucleotide modification techniques has greatly reduced the activation of nonspecific immune responses.
• Traditional mRNA vaccines reference the mature mRNA structure of eukaryotic organisms, encoding the target protein in the open reading frame. In contrast, the novel mRNA vaccines mimic the structure of single-stranded RNA virus genomes, allowing for self-replication and the generation of large quantities of virus particles upon entry into host cells. Therefore, these novel mRNA vaccines retain their self-replication machinery, replacing the sequence of viral structural proteins with that of the vaccine target protein, resulting in antigen expression levels ten to a hundred times higher than traditional vaccines, and are deemed extremely safe, referred to as self-replicating mRNA vaccines.
• The lipid nanoparticle delivery system further enhances the competitiveness of mRNA vaccines. On one hand, its high loading efficiency and large surface area, combined with good biocompatibility, facilitate cellular uptake of mRNA and endosomal escape. On the other hand, lipid nanoparticle carriers protect the stability of mRNA during delivery in vivo, preventing enzymatic degradation, prolonging drug circulation time, and increasing delivery efficiency.

With continuous technological advancements, mRNA vaccines' ability to activate the body's immune response has been significantly enhanced and widely applied, especially after their successful application in combating the novel coronavirus, garnering tremendous attention.

Design of mRNA Vaccines for Breast Cancer

Target Selection

Target selection is crucial for designing breast cancer vaccines, with tumor antigens primarily including tumor-associated antigens (TAAs), tumor-specific antigens (TSAs), and tumor microenvironment (TME) antigens. TAAs, mainly expressed in tumor cells and to some extent in normal cells, are commonly targeted for protein vaccines against breast cancer. HER2 is the preferred antigen choice for HER2-positive breast cancer. In triple-negative breast cancer, with the highest tumor mutation burden and poorest prognosis, tumor immunotherapy focuses on this subtype. Tumor neoantigens, specific to tumors, result from somatic non-synonymous or frameshift mutations, some of which can activate the host's immune system. The high payload of mRNA vaccines allows simultaneous expression of multiple tumor neoantigens, enhancing efficacy in activating anti-tumor immune responses for precision individualized breast cancer treatment. Compared to peptide vaccines, mRNA vaccines simplify the process of introducing TAAs (and TSAs) into the body by providing more antigen information in a single immunization without the need for selecting HLA-restricted epitopes.

Selection of mRNA Vaccine Structures

According to existing research, there are two main structures of breast cancer vaccines: traditional non-replicating mRNA vaccines and virus-derived self-amplifying mRNA vaccines.

• Self-amplifying mRNA (SAM) or replicon vaccines based on viral structural features are a popular type of vaccine. However, they are developed through genetic recombination technology, replacing the gene sequence encoding viral structural proteins with the target gene sequence, and do not contain viral particles.
• Virus-derived self-replicating mRNA vaccines are gaining more attention. The most commonly used viral structure is alphavirus self-replicating structure with other options including flavivirus and measles virus. Studies in breast cancer have found that these vaccines not only exhibit significant anti-tumor effects at extremely low doses but also target breast cancer cells and act as oncolytic viruses.

Selection of Vaccine Carriers

Currently, research on vaccine carriers primarily includes autologous dendritic cells as carriers and lipid nanoparticles (LNPs).

• Dendritic cells derived from monocytes can be obtained from the peripheral blood cells of patients. Immature dendritic cells are isolated and cultured in vitro with maturation factors. Then, they are loaded with mRNA encoding antigens via electroporation and subsequently injected intravenously into the patient's body.
• LNPs, on the other hand, are formed by mixing cationic or ionizable lipids, cholesterol, and phospholipids complexes with mRNA encoding antigens to form spherical nanoparticles. The surface lipids are then modified with polyethylene glycol to create a colloidal barrier with protective functions.

The former, as a form of cellular therapy, introduces mRNA into mature dendritic cells to achieve optimal T-cell stimulation activity. While effective, this approach is expensive. In contrast, LNPs have low production costs and high efficiency in carrying mRNA, making them a more favored choice among researchers. LNPs have demonstrated good immunogenicity, stability, targeting ability, and therapeutic effects in preclinical stages.

Selection of Injection Method

Since breast cancer is a relatively superficial solid tumor, intratumoral injection is highly feasible. This approach not only broadens the selection of antigen targets but also enables the direct expression of specific immune-active mediators within the tumor microenvironment.

Prospects of Breast Cancer Vaccines Development

The combination of mRNA vaccines with other immunotherapy approaches is highly necessary in breast cancer treatment. The immune ecology of the tumor microenvironment in breast cancer is closely linked to the efficacy of immunotherapy, with tumor heterogeneity and strong immune suppression being the primary reasons for the suboptimal response to immunotherapy. mRNA vaccines can carry multiple tumor-associated or tumor-specific antigens simultaneously to stimulate immune responses, effectively overcoming the issue of tumor heterogeneity. Additionally, combining immunotherapy with checkpoint inhibitors and other treatments can better alter the overall immune-suppressive microenvironment of breast cancer, significantly enhancing the efficacy of mRNA tumor vaccines. Although breast cancer is not the cancer type that benefits the most from immunotherapy primarily based on checkpoint inhibitors, with the development of mRNA vaccine technology, breast cancer is expected to transition from an "immune cold tumor" to a "hot tumor," thereby greatly benefiting from immunotherapy to a greater extent.