Medicosnext
Words by Dr Hitesh Bhattarai
During the COVID-19 pandemic, there was a race to create vaccine against SARS-CoV-2. The reins of the vaccine manufacture were taken by a number of big pharmaceutical companies worldwide. Pfizer/BioNTec and Moderna focused their attention on mRNA vaccine manufacture, while Johnson and Johnson and Astrazenca focused their attention on viral vector based vaccine manufacture. Sanofi synthesized vaccine based on recombinant protein and Chinese manufactures synthesized vaccine based on inactivated virus. It was a race against time and in a short duration of 1 year all these companies produced vaccines. The mRNA vaccines, which were synthesized for the first time against any pathogen, had a very good safety record and efficacy records in Phase III clinical trials. These mRNA vaccines were widely used in developed countries and started a new era of vaccine manufacture.
Evolution of Vaccine Development
The theory behind vaccine development has not changed much since Edward Jenner first used cowpox to prevent smallpox. When the patients were infected with the body developed immunity against a similar type of virus, smallpox. Instead of using live organism as vaccine, inactivated organisms, proteins of organisms, nucleic acids that encode parts of organisms can be used as vaccine. These vaccines stimulate the immune system of the patient. The patient produces T-cells and B-cells primed to attack the antigens described earlier that stimulate the immune system. When the real infection happens at a later date the immunity from earlier vaccination helps the body fight against pathogens. When mRNA is used as vaccine it produces antigen protein, which stimulates the immune system. Since the discovery of mRNA they have been attempted to use as vaccines. But this did not succeed for a long time because mRNA stimulates vigorous immune response against itself and cannot be converted to antigen protein by the translation machinery. Kariko and Weismann discovered that when modified bases found in natural systems were used in the mRNA, these mRNAs avoided violent immune response and could be robustly translated into proteins, which acted as the desired antigens. For their work, the duo was awarded the Nobel Prize in physiology in 2020.
Traditional Approaches vs. mRNA Vaccine
In recent years, it has been proven over and over again, that the immune system modulation is not only important in treating infectious diseases but also the emperor of all maladies, cancer. Cancer was known to inhibit immune cells so that they could not attack cancer. Antibodies against CTLA4 and PD1, which are the molecules that deactivate immune system, were successful in treating some cancers. While for other cancers, especially liquid cancers, body’s own T cells were taken out, activated by adding chimeric antigen receptor and put back in to attack cancer cells. This treatment known as CAR-T therapy was able to cure certain other cancers. Cancers are especially difficult to treat because of two reasons: the lack of difference between cancer cells and normal cells and the heterogeneity of cancer cells. Radiotherapy, surgery and chemotherapy that target cancers cells also affect the normal cells. It is very difficult to hit molecular targets of cancers only. Additionally, cancer cells can be very heterogenous once they metastasize. Some cancers express one antigen, while others express other antigen. Targeting one antigen or a few antigens cannot treat certain metastatic cancers. mRNA vaccine theoretically has the potential to solve both these issues of cancer. On one hand, T and B cells attack antigens very specifically and have the potential to kill the specific antigen expressing cancer cells. On the other hand mRNA vaccine can be tailor made to express multiple antigens of the cancer cells targeting heterogenous population even in late metastasis.
Unveiling the Potential of mRNA Vaccines
The MIT technology review of 2021 placed mRNA vaccines at the top of medical breakthroughs revolutionizing the medical field. The mRNA vaccine has the potential to produce proteins that can serve as antigens without integrating into the genome. Since mRNA is only transient, expressed repeated innoculations is also possible. Encoded units as in RNA vaccines are versatile and can be used to encode a large number of antigens. Other advantages of RNA vaccines are rapid development time and easy modification potential, high immunogenicity potential, cell independent production and intrinsic adjuvant effect. mRNA vaccines trigger strong humoral and cell mediated immune response. They can also lead to enduring immunological memory effective in preventing tumor recurrence. While mRNA might offer the most potential advantage over other platforms of vaccine development other, platforms are also in clinical stages of development. DNA vaccine technology has the benefit of low cost of production, cell independent production, long lasting immune response, the potential to target multiple antigens and easier storage conditions. DNA vaccine has the risk of integrating into the host genome, risk of autoimmune reaction and low transfection efficiency. Viral and bacterial vector based vaccines trigger strong immune response but have safety concerns associated with pre-existing immunity. Peptide based vaccines are easy to manufacture, but elicit weaker immune response, necessitating additional measures for efficacy. Another platform called cell based vaccines isolate dendritic cells that present antigens from the blood, treat these cells with antigens, and transfer them back to the body so that immune system is stimulated. While this type of vaccine leads to strong immune stimulation and multi form antigen loading, it has high cost, potential for immunogenicity of the cells and there is a need for patient specific customization.
A Dual Approach to Antigen Targeting
mRNA cancer vaccines generally target two types of cancer antigens: tumor associated antigens and tumor specific antigens. They can also encode for immunostimulatory factors. Tumor associated antigens or shared antigens are expressed at elevated levels by the cancer cells but are found at very low levels in other normal cells. Such factors were identified before tumor specific factors and have been targets of vaccines before. A dendritic cell vaccine, Provenge, was approved in 2010 by FDA against castration resistant prostrate cancer. It targeted prostrate acid phosphatase, a tumor associated antigen, expressed mostly in prostrate cells. Mutated KRAS antigen vaccine, MAGE-A3 antigen vaccine, HPV E6 and E7 vaccine all target tumor associated antigens. A number of Phase III clinical trials targeting tumor associated antigen failed in 2013 leading to a setback in the development of vaccines. However as of April 2023, 15 clinical trials are ongoing using mRNA vaccines against tumor associated antigens. For developing a tumor associated antigen mRNA cancer vaccine multiple antigens can be combined in 1 vaccine making it easier to synthesize and administer vaccine.
Tumor associated antigen, also called neoantigens, are only found in cancer cells and are absent in normal cells. They were first identified and their potential to elicit immune response was recorded after the advent of next generation sequencing technology in 2013. They are somatic cell variants and include single nucleotide variant, insertion or deletion mutation, gene fusion mutation, copy number variant, splice variant and microsatellite instability variants. They are predicted after tumor tissue RNA-seq, tumor tissue whole exome sequencing and peripheral blood mononuclear cell whole exome sequencing. Not all variants serve as antigens so predicting variants that serve as antigens is a computationally challenging task that requires the use of artificial intelligence and various softwares. Once antigens are identified they need to be checked for their capability of presentation and recognition by T cell receptor. Such antigens are tailor made for specific individual and the cost of vaccine production is high. Tumors with high mutational burden such as melanoma can have more neoantigens that can be used to produce mRNA vaccine. As of April 2023, 13 clinical trials were ongoing using mRNA vaccine against tumor specific antigen. To prepare vaccine for one patient over 20 antigens are often used.
The last class of mRNA vaccine encode for the immunostimulatory factors. As of April 2023, 7 clinical trials were ongoing using these factors in mRNA vaccine. mRNA vaccines encode for immunostimulants that modify the tumor microenvironment and enhance the efficacy of immune checkpoint inhibitors. OX40L, IL23, IL36g are different proinflammatory molecules that stimulated immune system and display some level of antitumor activity when used along with immune checkpoint inhibitors.
Among these three molecules encoded by mRNA vaccine, the use of personalized tumor associated antigen has shown promising result in two clinical trials and are now proceeding to the next round. On February 2023, FDA gave the personalized mRNA vaccine along with Pembrolizumab that targets PD-1 receptor a breakthrough designation against advanced stage, high-risk melanoma. In a Phase 2b trial mutant antigen treated arm had 44% higher chance of recurrence free survival than the placebo arm. The mRNA vaccine used 34 tumor specific, personalized antigens. Phase 3 trial will now be carried out.
Dr. Balachandran’s Vision for Personalized Cancer Vaccines （box）
Dr. Vinod Balachandran and his team at Memorial Sloan Kettering Cancer Center carried out a study against pancreatic ductal adenocarcinoma (PDAC), a highly lethal form of pancreatic cancer. Even with contemporary treatments, the survival rate for individuals diagnosed with this cancer after five years of treatment is only around 12%. Their groundbreaking approach involves a personalized autologous mRNA cancer-treatment vaccine tailored to engage the immune system against patient-specific neoantigens found in pancreatic cancer cells. In a small clinical trial published in Nature, tumor samples from 19 PDAC patients were sequenced by BioNTech to identify potential immune-triggering proteins. Subsequently, personalized vaccines were developed for 18 patients, targeting up to 20 neoantigens, in a process spanning about nine weeks from surgery to vaccine delivery. Administered alongside an immune checkpoint inhibitor drug called Atezolizumab, the vaccines were given in nine doses over several months. Astonishingly, half of the patients showcased robust immune responses, with their T cells recognizing and targeting the pancreatic cancer cells. The activated T cells, previously absent before vaccination, persisted and effectively suppressed cancer recurrence for over a year and a half post-treatment. Dr. Balachandran expresses optimism about this personalized vaccine’s potential to revolutionize pancreatic cancer treatment and potentially address other aggressive cancers, although further research is underway to comprehend why some individuals didn’t mount strong immune responses. Plans for an expanded clinical trial are in progress, marking a promising leap forward in the fight against this challenging cancer type.
Hurdles in mRNA Delivery
There are two main reasons why it is difficult to target naked mRNA to cells. First they get easily degraded by RNAase, widely found in biological systems. Second naked mRNA is negatively charged and the cell membrane is negatively charged as well making it difficult for the mRNA to cross the membrane barrier. The mRNA delivery often happens when it is packaged with positively charged polymer or protein. Recently lipid nanoparticles have been developed that have been successful at targeting mRNA to different parts of the body. Lipid nanoparticles generally contain four components: phospholipids, sterols, inonizable lipids and PEGylated lipids. Phospholipids and sterols are necessary to encase the naked RNA. Ionizable lipids form positive charge in low pH and are important for delivery of mRNA. PEGylated lipids prolong the half-life of nanolipids through the liver and other biological systems. Most of the nanolipids prepared so far use passive means of targeting. Designing actively targeting nanolipids is the holy grail of mRNA delivery.
Nanoparticles as Medical Game-Changers
The researchers at Johns Hopkins Medicine worked on improving the delivery of mRNA vaccines through the use of nanoparticles. They have made a breakthrough with the development of a nanoparticle designed to revolutionize the delivery of mRNA-based vaccines. These minuscule, biodegradable particles have shown immense potential in improving the administration of mRNA vaccines for various infectious diseases, including COVID-19, and have promising applications in treating non-infectious diseases like cancer. The study, reported in the Proceedings of the National Academy of Sciences, highlighted that these polymer-based nanoparticles, when injected into mice, efficiently traveled to the spleen, activating specific immune cells that combat cancer. In fact, mice with melanoma showed double the survival rate, and those with colorectal cancer had significantly prolonged survival following injections of these nanoparticles compared to control treatments. The nanoparticles aimed to precisely target dendritic cells, crucial for instructing the immune system to identify and eliminate cancer cells. This groundbreaking research holds immense potential for enhancing vaccine delivery and transforming cancer treatment paradigms, presenting an exciting avenue for medical advancements.
Navigating Delivery Routes
The mRNA vaccine can be delivered through various routes. Intradermal delivery method targets dermal and lymph node antigen presenting cells. It can lead to direct access of antigen presenting cells, but the volume that can be delivered in very low. In subcutaneous method more volume can be delivered but it leads to degradation of mRNA. Intramuscular route has less side effects and directly targets dense blood networks but the delivery volume is very little. Intranodal delivery has the most direct access to antigen presenting cells. But it is a complicated process requiring ultrasound to direct injection. Finally, intravenous method targets splenic and lymph node antigen presenting cells and can deliver the largest volume among different delivery method. However the mRNA has to pass thorough liver and can be degraded easily. Additionally there is risk of systemic side effects.
Monitoring Treatment Response
The introduction of mRNA cancer vaccines represents an innovative approach to cancer treatment, necessitating novel methods to evaluate their efficacy. Unlike conventional therapies targeting tumor cells directly, mRNA vaccines induce an immune response, challenging traditional radiographic imaging evaluations that focus on tumor shrinkage. This unique mechanism may trigger inflammatory reactions and tumor swelling, complicating efficacy assessments. Additionally, these vaccines might not exhibit conspicuous radiographic responses typically observed with cytotoxic therapy, especially in cases of low disease burden. Tailored to each patient’s unique condition and cancer type, personalized mRNA cancer vaccines demand diverse treatment plans and dosages, underscoring the need for distinct efficacy evaluation standards. Biomarkers capable of accurately monitoring treatment response become imperative to discern which vaccines should progress from early-phase to larger clinical trials. Various immune monitoring techniques, including flow cytometry analysis and assays measuring cytokine release and immune cell activation, are being employed to evaluate mRNA therapy effectiveness. Advancements in single-cell RNA sequencing (scRNA-seq) technology hold promise in evaluating immunotherapy responses. scRNA-seq enables untargeted quantification of transcripts in individual cells, offering insights into immune cell subsets and their gene expression patterns associated with treatment response.
Conclusion
The advent of therapeutic mRNA cancer vaccines has ushered in a new era in the realm of oncology, demonstrating unparalleled potential to revolutionize cancer treatment strategies. The dual targeting approach of mRNA cancer vaccines—addressing both tumor-associated antigens and tumor-specific antigens—showcases versatility in combating various cancers. The personalized nature of these vaccines, particularly those targeting neoantigens, represents a significant step forward in cancer immunotherapy. Despite great promise, serious challenges lie ahead. Some of the challenges are hurdles like tumor heterogeneity, an immunosuppressive microenvironment, and practical obstacles in administration and evaluation. Currently trials are targeting late stage metastatic cancers. It is quite possible that targeting early stage cancer using mRNA vaccine might be more optimal. In the late stage the immune system is weak and might not be able to surmount a strong response against cancer. The coming days will see more optimizations in antigen choice, delivery method, route of administration and evaluation in clinical outcomes.
