Unlocking cancer vaccine potential: What are the key factors?

ABSTRACT

Cancer is a global health challenge, with changing demographics and lifestyle factors producing an increasing burden worldwide. Screening advancements are enabling earlier diagnoses, but current cancer immunotherapies only induce remission in a small proportion of patients and come at a high cost. Cancer vaccines may offer a solution to these challenges, but they have been mired by poor results in past decades. Greater understanding of tumor biology, coupled with the success of vaccine technologies during the COVID-19 pandemic, has reinvigorated cancer vaccine development. With the first signs of efficacy being reported, cancer vaccines may be beginning to fulfill their potential. Solid tumors, however, present different hurdles than infectious diseases. Combining insights from previous cancer vaccine clinical development and contemporary knowledge of tumor immunology, we ask: who are the ‘right’ patients, what are the ‘right’ targets, and which are the ‘right’ modalities to maximize the chances of cancer vaccine success?

KEYWORDS:

• Cancer vaccine
• solid tumors
• immunotherapy
• cancer antigen
• tumor biology
• clinical development
• oncology

Introduction

Cancer is a major contributor to human mortality and morbidity, responsible for in one in six deaths worldwide.^1,2 Its burden continues to increase due to shifts in demographic trends and the influence of globalization on several risk factors, such as aging populations, tobacco and alcohol use, physical activity, and environmental pollution.^1,3 Technological advances, such as circulating tumor DNA (ctDNA) assays and low-dose computed tomography (LDCT) scans, are enabling population-level screening programs to be established for progressively lower costs. With the growing impact and accessibility of screening programs globally, there is a general trend toward earlier stage diagnoses across a range of solid tumor types.^4–6 Concurrently, increasingly complex cancer therapies – including antibody-drug conjugates, bioengineered cell therapies, and immune checkpoint inhibitors (ICIs) – have improved patient outcomes and survival significantly, but only for small proportions of patients.⁷ Moreover, the high market price of these drugs has contributed to a snowballing in the cost of cancer care and, as a result, many are inaccessible to lower resourced settings.^8–10 The culmination of these trends in oncology – a growing global burden of cancer, a shift to earlier stage diagnosis, and the increasing cost of advanced therapeutics – highlights the need, specifically, for accessible treatments that are suitable for administration in the earlier stages of a cancer’s development.

Cancer vaccines offer a potential solution to the problems facing the field of oncology in the coming decade. These therapies benefit from economies of scale, simple administration, and may be more effective in earlier disease settings than later.^11,12 Traditional vaccines aim to prevent or ameliorate infectious diseases through priming immune responses to exogenous antigens associated with given pathogens before a natural infection occurs. The induced adaptive immune response consists of humoral immunity (mediated predominantly by antibodies and memory B lymphocytes), and cellular immunity (typically carried out by CD4⁺ and CD8⁺ T lymphocytes). Similarly, cancer vaccines seek to induce an anti-tumor immunity using an appropriate delivery vector to prime responses to cancer antigens presented on the surface of tumor cells.^13,14

Cancer antigens are most often variations of endogenous peptides (or their levels of expression) caused by alterations in the DNA of cancer cells during carcinogenesis. The immunogenic epitopes produced are broadly classified as Tumor-associated antigens (TAAs), Tumor-specific antigens (TSAs), and viral antigens.^15–17 TAAs are self-antigens that are encoded within an individual’s germline DNA, which have an abnormal level of expression on tumor cells relative to comparable normal cells. Examples of TAAs include the cancer testis antigens, prostate-specific antigen (PSA), and human epidermal growth factor receptor 2 (HER2).¹⁶ TSAs are, in contrast, highly tumor-specific and are typically more immunogenic due to their complete absence in normal cells. Neoantigens are the best characterized type of TSA, which result from mutations that cause a nonsynonymous change in translated peptides. However, TSAs can emerge through a number of other less understood routes, such as post-translational modifications (e.g. glycosylation or phosphorylation) and from the translation of non-coding genomic regions.^18,19 Viral antigens, unlike TAAs and TSAs, are completely foreign to the body and introduced to human cells via infection. These immunogenic targets can be clinically relevant cancer antigens in tumors resulting from oncogenic viruses – such as human papillomavirus (HPV).^20–22

A range of platforms have been explored over the decades of cancer vaccine development. Early attempts that directly administered naked, tumor-associated peptides to patients have given way to precision tumor antigen identification platforms, with the targets encoded in modalities such as bespoke mRNA and genetically modified viral constructs.^23,24 Yet, to date, only one interventional cancer vaccine has been licensed for use: Sipuleucel-T, a Dendritic Cell (DC) vaccine created from autologous, peripheral-blood mononuclear cells (PBMCs) pulsed ex vivo with a recombinant fusion protein comprising prostatic acid phosphatase (PAP) fused to granulocyte – macrophage colony-stimulating factor (GM-CSF). Despite its approval in 2010 for use in castration-resistant prostate cancer, Sipuleucel-T was quickly superseded in common practice by hormone blockade agents that benefited from greater overall survival (OS), and a less complex and costly manufacturing process.^25,26 However, the advances of vaccine technology made during the COVID-19 pandemic have since reinvigorated exploration of their use within oncology.²¹ As of December 2023, there were 258 active, interventional studies of therapeutic cancer vaccines in solid tumor indications registered on ClinicalTrials.gov. While peptide and cellular modalities account for the majority of active studies, there are an increasing number of studies using mRNA and viral vectors following their efficacy in the pandemic and rapid regulatory approvals.²⁷

The type of immune response required to have a clinically significant impact on solid tumors is different to that needed in acute infections. For certain viral infections, for example, strong humoral responses are sufficient to ameliorate clinical symptoms upon later infection, even in the absence of significant cellular immunity.²⁸ The importance of humoral immunity, particularly the induction of neutralizing antibodies, was evidenced during COVID-19 vaccination where neutralization levels were associated with protection from symptomatic infection.²⁹ However, tumor targeting is instead largely dependent on T cell responses, and for cancer vaccines to be effective, there is a need for a high magnitude expansion of specific, cytotoxic T cell populations that can infiltrate and destroy tumors. It is also important that these proliferating, effector T cells do not enter an exhausted state – either as a result of persistent antigen stimulation or the immunosuppressive effects of the tumor microenvironment (TME).^30,31 Ideally, this would be followed by a corresponding memory T cell kinetic, able to prevent clinical recurrence for several years.^32,33 In contrast to natural infection – which tends to be intrinsically pro-inflammatory (e.g. via display of pathogen-associated molecular patterns (PAMPs)) – cancer can acquire immune evasive attributes quickly, making a durable immune memory crucial in the longer term to ensure rapid expansion and cell killing to prevent recurrent disease becoming established.^34,35 While aspects of humoral responses have been associated with positive outcomes in the treatment of solid tumors, biomarkers of cellular immunity strongly correlate with positive prognosis.^36–38 Similarly, in observations of patients that have gone into long-term remission, significant proportions of tumor-reactive T cells may be found in the circulating volume many years after tumor regression.^39–42

Lack of understanding as to what constitutes the ‘right’ patients, targets, and modalities has hindered the progress of cancer vaccine development.^27,43,44 As discussed in detail elsewhere, improved knowledge of tumor biology and lessons learned from the development of other cancer immunotherapies have provided insights that can be applied to cancer vaccines.^12,19 While these are transferable to selecting the right patients and targets, uncertainties around optimal modality choice remain (Figure 1). We discuss all of these key considerations in detail below, with particular focus on their practicality, and ability to generate the required immune response profile to effectively target solid tumors.

Figure 1. Challenges facing cancer vaccine development.

Patient factors

While the general principle of a cancer ‘vaccine’ is seemingly well suited for their intended use, the vast majority of therapeutic cancer vaccine studies over the past decade have reported disappointing clinical outcomes. Early clinical studies of cancer vaccines tended to focus on patients with treatment-refractory disease. This has been standard practice for many first-in-human oncology studies but, as a consequence, these tend to recruit patients who are less physiologically fit with more advanced disease.^45,46 As tumor burden increases, cancer can induce systemic immunosuppressive effects (e.g. increased production of neutrophils, eosinophils, and monocytes with reductions in dendritic cell, B cell and T cell populations from hemopoietic cells) as well as nurturing a localized, immune-evasive TME.^47,48 The TME matures to a state of hypoxia, reduced pH, increased hydrostatic pressure, and erratic neo-angiogenic vasculature, which is also able to recruit immunosuppressive bystander cells.^49–51 These effects may reduce the ability of cancer vaccines to elicit a robust immune response.^20,47 However, in the last 12 months alone, we have begun to see some promising results for mRNA-based cancer vaccines in melanoma and pancreatic cancer.^11,52 Unlike many previous cancer vaccine studies that have focused on later disease settings, both of these studies have involved the adjuvant administration of treatment subsequent to curative-intent surgery. Further evidence to support the use of cancer vaccines in earlier disease settings includes HOOKIPA Pharma’s recent positive results using an arenavirus in combination with pembrolizumab in the first-line treatment of metastatic Head and Neck Squamous Cell Carcinoma (HNSCC).⁵³ These suggest that less established solid tumors may be more susceptible to the adaptive immune response generated by vaccines. Moreover, even with the ‘perfect’ cancer vaccine, primary T cell expansion in response to an antigen takes time (typically peaking around day 14) – not an insignificant length of time for patients with advanced, poor prognosis tumors.⁵⁴ This reframes the possible utility of cancer vaccines, and is supported by the growing understanding of immune-evasive mechanisms that solid tumors exploit as they become more advanced and able to cultivate an immune-evasive phenotype and TME.^11,52 Exploring the window of opportunity offered in the adjuvant setting, in particular, is a strategy being adopted by a range of key players in this space during 2023 – including BioNTech, Merck, Genentech, Gritstone Bio, and Evaxion.⁵⁵

Beyond treatment setting, consideration must also be given to anti-cancer therapies that patients have received previously, as well as those administered in conjunction with cancer vaccines. Previous exposure to immunotherapies may induce a selection pressure toward clones with less immunogenic antigens and/or reduced antigen presentation, while cytotoxic agents and radiotherapy may produce immunosuppression that could hamper vaccine efficacy.^45,56,57 The early signs of clinical efficacy seen in the aforementioned studies are in protocols that combined vaccines with ICIs over a long duration.⁵⁵ While there is logic in suppressing checkpoint activity whilst administering a treatment designed to expand specific anti-cancer populations of T cells, the addition of ICIs is costly and not without the risk of significant toxicity.⁵⁸ Studies exploring the synergistic role of these interventions have suggested that ICIs help overcome immunosuppressive elements of the TME, however the impact of the TME and cancer-related systemic immunosuppression may be lessened by administering vaccines at earlier disease stages in immunocompetent patients.⁵⁹ Similarly, discussion around ‘hot’ and ‘cold’ tumor types (i.e. tumors that do, or do not, show infiltration by lymphocytes) that have surrounded translational analysis of ICI studies may be less relevant to cancer vaccines dosed in their earlier setting. It has also been noted that in controlled studies, cancer vaccines have demonstrated the ability to increase CD8+ T cell levels within tumors – suggesting a possible utility in cold tumors, particularly.^60,61

Human leucocyte antigen (HLA) molecules are a crucial patient factor when considering antigen presentation, as core determinants of antigen sequences that can be recognized by an individual’s cellular immunity. The importance of these molecules in immunity has long been established in transplant medicine, but their implications on the immunogenicity of cancer antigens are yet to be fully understood.^13,17,18 Importantly, the genes encoding these molecules are diverse and vary greatly within populations. There is a historical bias in HLA research that has meant that HLA haplotypes that are more common in ethnic minorities are less understood.⁶² If cancer vaccines are to be successful – particularly on a global scale – this must be considered, and concerted efforts be made to better characterize the impact of diverse HLA genotypes on cancer antigen presentation.

Target selection

Cancer vaccines utilize either a ‘personalized’ or ‘off-the-shelf’ model when delivering target antigens. Personalized vaccines require a sample of a patient’s tumor to produce a vaccine that delivers TSAs and TAAs specific to an individual’s cancer, while off-the-shelf platforms target TSAs, TAAs, or viral antigens that are known to be associated with a given cancer type.^63,64 Personalized platforms typically use next-generation sequencing to compare an individual’s germline DNA to that of a tumor sample, in order to find specific mutations that could lead to TSAs/TAAs. All, or a selection of these, are then delivered to the patient in a bespoke vaccine. However, this process can be costly and require many months to manufacture.^65,66 Another technique is to personalize vaccines using ‘anonymous’ antigens (i.e. without determining what TSAs/TAAs are present), for example by producing tumor lysate ex vivo to create a peptide vaccine. However, these still require tumor samples and are difficult to measure the immunogenicity of as the antigen payload is unknown.⁶⁵ Off-the-shelf vaccines use a selection of TSAs/TAAs that are associated with a particular tumor type, rather than patient-specific mutations. A major benefit of this approach is the ability to manufacture the same vaccine in bulk for many patients. Off-the-shelf methodology enables patients who have yet to undergo resection, or cannot due to co-morbidity or anatomical inaccessibility, to be treated in a semi-tailored manner while avoiding treatment delays resulting from manufacture.¹³ This format would also allow for neoadjuvant treatment that would deliver antigens prior to surgical resection of tumors, avoiding postoperative immunosuppression – a period associated with cancer progression.⁶⁷ However, ‘off-the-shelf’ vaccines are reliant on a robust, preexisting understanding of cancer antigen expression and presentation within a particular cancer.⁶⁵ While this limitation may have held this methodology back previously, there are promising results emerging from off-the-shelf vaccine studies as understanding improves.^68,69

The choice of target antigens must give careful consideration to intrinsic patient factors (e.g. HLA genotype) and tumor biology (e.g. peptide processing and presentation).^70,71 While antigen discovery methods have been well reviewed elsewhere, it is important to understand the limitations of current techniques.⁷² Broadly, the technologies and approaches to identifying these potential immunotherapeutic targets – which span point mutations, novel gene isoforms, expression of non-coding genomic regions, fusion, and frameshift peptides – have rapidly evolved. As mentioned, a key approach in cancer antigen identification is the use of next-generation sequencing to identify DNA or RNA-encoded tumor mutations and aberrant expression. Sequencing approaches coupled with machine learning models – to estimate the likelihood of translation and presentation – have been used in the majority of personalized neoantigen vaccines to date.^21,55,73 However, these have been unable to reliably predict immunogenicity and presentation of delivered targets – limiting their suitability to a larger rollout in a diverse population.⁵² A fundamental approach gaining traction is immunopeptidomics, wherein mass spectrometry is employed to identify eluted peptides from tumor samples. Currently, immunopeptidomics is one of the only methods that can directly detect presented cancer antigens. Yet, it suffers from lower sensitivity and, alone, is unable to resolve tumor-specific peptides from those found in the normal human proteome.^18,74 A combination of these techniques in the future may be able to improve the accuracy of target selection for cancer vaccines – as well as other cancer immunotherapies.^12,14

Disagreement persists on the best approaches for discovering the ‘ideal’ cancer antigens, but there is consensus that these need to be robustly tumor-expressed and capable of eliciting cytotoxic T cell responses.^13,17,75 The gold standard approach is to perform ex vivo cultures of patient T cells with candidate targets (using either primed DCs or bead-based assays) with readouts via antigen-specific tetramer staining, detection of cytotoxic markers through ELISPOT, or intracellular cytokine staining. Increasingly sensitive assays for peptide-T cell recognition and stimulation have also been developed – these are of particular importance to personalized platforms wherein high throughput is needed to assess individuals’ unique sets of antigens.^76,77

Cancer antigen presentation is required to provide an opportunity for immune cells to recognize and kill malignant cells, through an adaptive immune response. All intracellular proteins eventually undergo degradation with a subset of the resulting peptides being loaded onto HLA molecules. The subsequent complex is then trafficked to the cell surface for extracellular display.⁷⁸ T cells – particularly cytotoxic CD8+ cells – with corresponding T cell receptors (TCRs) are able to recognize presented TSAs and TAAs, enabling an immune response against the cancer cells. This occurs when epitopes are presented by HLA class I on the surface of cancer cells directly, or indirectly by antigen-presenting cells (APCs) that engulf and present immunogenic epitopes on HLA class II molecules. The activation of these cells via the recognition of presented TSAs/TAAs leads to T cell mediated cancer cell death. This cell death, in turn, causes an agnostic release of other cancer antigens that may be present in the tumor that can prime further adaptive immune responses. This series of steps is known as the cancer immunity cycle. This concept highlights that anti-cancer immunity can be self-reinforcing – a property that cancer vaccines may be able to exploit by initiating this cycle, even in instances where only a subset of antigens are targeted by the vaccine^47,79 (Figure 2).

Figure 2. The updated cancer immunity cycle annotated with cancer vaccine technologies and how their mechanisms interact with this cycle [adapted from Mellman et al.].⁷⁹ APCs, antigen-presenting cells; CTLs, cytotoxic T lymphocytes; TAA, Tumor-Associated Antigen; TSA, Tumor-Specific Antigen.

Modality choice

The COVID-19 pandemic enabled relatively novel vaccine platforms to accelerate their clinical development to address the global threat that SARS-CoV-2 posed. This has led to billions of doses of vaccines derived from these technologies to be administered in less than 3 years, providing a robust understanding of their safety profile and enabling technical optimization of their manufacture.^13,63 Despite differing in mechanism and construction, the two most widely deployed vaccine technologies used in the COVID-19 pandemic – mRNA and viral vectors – have demonstrated the ability to rapidly scale up regulatory-compliant manufacturing capacity, establish viable international supply chains, and have begun to benefit from economies of scale.⁸⁰ Whilst there are lessons to be learned from the deployment of vaccine technologies during the pandemic, there are still a number of questions that need to be asked to best support the clinical development of these therapies as they are translated into solid tumors. How generalizable are the results of COVID-19 studies to malignancies? What are the optimal characteristics of a vector delivering cancer antigens? Are these quintessentially different to those needed in infectious disease prophylaxis?

Cancer vaccines, by various means, all seek to introduce TSAs and/or TAAs to the immune system. This can be achieved by direct presentation of cancer antigens or the induction of expression via normal cell machinery (Figure 2). A range of technologies have been harnessed to achieve the antigen delivery that is central to cancer vaccines; these include peptides, virus-like particles, cells (e.g. DCs), DNA, mRNA, and microbial vectors (e.g. viruses).¹⁹ Despite a common objective, these modalities have been shown to provoke differing immune responses. We have begun to understand the relative strengths of different vaccine platforms in inducing humoral and cellular immunity through COVID-19 vaccines. For example, the Pfizer BNT162b2 mRNA vaccine stimulated higher magnitude B cell responses compared to viral vectors, which were superior in producing T cell responses.^81,82 While variations in dose, and route and frequency of administration have also been shown to vary with the immune response seen, the intrinsic immunokinetics of each vaccine platform must be considered cognizant of the conditions required for an anti-tumor effect.^83,84

Peptide vaccines

Peptide vaccines consist of sequences of amino acids that are designed to directly mimic tumor antigens of interest. Therapeutic cancer vaccines using this modality have been the longest in development; initially, these vaccines consisted of recombinant proteins and epitope-length peptides, but novel approaches like synthetic long peptides (SLPs) have become more common to improve on the stability of this type of vaccine. The use of longer peptides reduces their susceptibility to enzymatic digestion and allows for multi-epitope payloads. Human studies of these vaccines have shown them to be tolerable, however they suffer from unfavorable pharmacodynamics due to their charge and solubility.¹³ Also, owing to a specific requirement for cross presentation of epitopes by cross-presenting dendritic cells to generate vaccine-specific CD8+ T cell responses, peptide vaccines typically induce higher proportions of CD4+ T cell vaccine-specific responses.^85,86 While these vaccines benefit from easy synthesis and low cost, no peptide-based cancer vaccines have made it beyond Phase II trials due to a lack of efficacy.^87,88 A recent study did, however, make note of the low-level persistence of some antigen-specific CD8+ clones in a small cohort (n = 5) of long-term survivors of hepatocellular carcinoma who had received a peptide cancer vaccine in a study (n = 60) almost a decade prior. Yet, the clinical significance is unclear due to the confounding effect of other treatments received by these patients.⁸⁹ Many ongoing peptide vaccine studies are now focused on exploring the use of adjuvants and immunotherapy-combinations in order to improve the poor efficacy elicited by this class of vaccine.⁹⁰

Virus-like particles

Virus-like particles (VLPs) are nanostructures that spontaneously assemble from viral structural proteins that lack the viral genetic material. They are hollow shells that mimic the overall structure of a virus but are incapable of causing a viral infection.^91,92 Several VLPs – designed to display cancer-specific proteins or epitopes on their surface – have been explored as cancer vaccines in preclinical studies. Like peptide vaccines, VLP-based vaccines also rely on complicated cross-presentation mechanisms to generate a CD8+ T cell response. Similarly, VLPs tend to skew toward CD4+ rather than CD8+ T cell responses that may impact their clinical efficacy in solid tumors.^91,93 There are currently a small number of active trials using VLPs as therapeutic cancer vaccines, but these have yet to read out any mature data.⁹³

Cellular vaccines

Cell-based vaccines use autologous immune cells from patients – typically derived from PBMCs – to deliver TSAs/TAAs. Most often these have utilized monocyte-derived DCs, taken from peripheral blood sampling or by leukapheresis. These are then exposed, ex vivo, to target antigens (e.g. via incubation with tumor lysate or by transfection) before being expanded and reinfused into a patient, wherein cells carry out APC functions to elicit an adaptive immune response. This class of vaccine has been studied for over three decades and is the technology behind Sipuleucel-T, discussed earlier. A major restriction of current cell-based vaccines is their autologous origin. Extracting and stimulating these cells is a laborious process and, for immunocompromised patients in particular, can result in low yields in the final product. Co-opting the induced APCs ability to sort through a multitude of antigens within, for example, whole cell lysate provides a means of targeting anonymous antigens while avoiding the need for costly genetic analysis.^26,77,94 However, as mentioned, this method limits the ability to understand which antigens are most immunogenic and, as a consequence, the generalizability of results.¹³

DNA vaccines

DNA-based vaccines work via ‘reverse vaccination’ strategies, wherein the vaccine does not contain the antigen(s) itself, rather the genetic blueprint with which to produce them. Once this is transfected into the nucleus of host cells, it can then exploit native machinery to produce the desired antigenic peptide. Typically, these use DNA plasmids that encode both the target antigen and stimulatory adjuvants (e.g. cytokines, colony stimulating factor). This is an established platform used experimentally in infectious disease and cancer settings and, as such, has a well characterized and tolerable safety profile.^95,96 These compounds enjoy high stability but, due to the intrinsic negative charge of DNA, they suffer from poor cell penetrance and immunogenicity – the latter addressed somewhat by the encoding co-stimulatory molecules within the DNA sequence, but still requiring efficient protein translation for this to take effect. This requires crossing both the cell membrane and the nuclear envelope to access the necessary cellular machinery, whereas mRNA vaccines need only cross the cell membrane. This hurdle can be improved by electroporation, however this is a time-consuming and clinically skilled task limited to accessible anatomical areas.¹⁹ Even so, the optimal way to deliver a DNA vaccine is still an open question and the need for specialized delivery devices adds to the overall cost of treatment. Clinical studies of a DNA vaccine encoding PAP – the same target as Sipuleucel-T – demonstrated an improved PSA doubling time in phase I. Yet, this did not translate to improved PFS in later phases.²⁵ Chimeric DNA vaccines – encoding antigens with xenogeneic domains – seek to improve immunogenicity using extra-species elements.⁹⁷ However, aside from the emergency approval of ZyCov-D – an DNA vaccine against SARS-Cov-2 in India – there have been no approvals of DNA vaccines in humans. Despite this, a DNA-based cancer vaccine for use in canine melanoma has been recently approved by the FDA.⁹⁸

mRNA vaccines

In contrast to DNA, mRNA is very unstable. It is subject to rapid degradation at room temperature and, in vivo, free mRNA is rapidly broken down by nucleases.^21,43 As a result, mRNA vaccines consist of mRNA molecules bound to a biochemical agent (typically lipidic) to improve stability. Both elements contribute to the overall immunogenicity of the vaccine. Native uracil nucleosides in mRNA sequences trigger pattern recognition receptors in APCs, inducing an innate immune response that results in reduced antigen presentation due to rapid clearance. Replacing these with nucleosides modified analogues to avoid triggering APCs has been demonstrated to increase antigen production.⁹⁹ Liposome delivery formulations – such as the lipid nanoparticles (LNPs) used in BioNTech’s COVID-19 vaccine – have been developed to reduce the intrinsic immunogenicity of free mRNA, and encourage preferential endocytosis by APCs. Lipid nanoparticles also help direct the immune response by stimulating the production of IL-6, thereby promoting T follicular helper cell differentiation, and enhancing B cell and antibody responses.¹⁰⁰ This proclivity for professional APCs, including DCs, gives mRNA vaccines access to the cytoplasmic machinery needed to be translated into peptides as well as the opportunity to be presented on APC-restricted HLA-I and II molecules. HLA-I and II can readily stimulate both CD4+ and CD8+ cells, which produces a more balanced immune response than DNA-based vaccines – albeit with a shorter duration of expression and still with a disproportionate CD4+ effect.⁵⁸ Solutions such as self-replicating mRNA vaccines may improve these outcomes by increasing the duration of protein expression, but further refinements are needed to bolster CD8+ activation for solid tumors.¹⁰¹ Another challenge with mRNA vaccines in oncology is the issue of rapid dispersion at the site of injection, which – depending on anatomical location – may limit their ability to reach lymphoid tissue and circulating APCs in order to maximize response. Intravenous administration allows for systemic circulation of vaccine and has been demonstrated to improve antigen production. However, this is associated with an increased risk of toxicity – although they are still well tolerated relative to other anti-cancer treatments.¹⁰² Finally, the durability of immune responses generated by mRNA vaccines remains unclear with many current approaches using high-frequency repeat dosing in order to generate continuous T cell stimulation, although aforementioned developments like amplifying and circular mRNA may reduce this requirement.¹⁰³

Prior to the pandemic, the initial mRNA vaccines platforms were, predominately, intended for the treatment of cancer. These included longer mRNA molecules encoding multiple TSAs/TAAs, yet many of these failed to demonstrate efficacy. This resulted in cancer vaccines losing momentum relative to other immunotherapy classes.^{19,43,104,105} Two mRNA studies this year have reignited the interest in cancer vaccines at large, namely, the academic-led, neoadjuvant pancreatic ductal adenocarcinoma trial at Memorial Slone Kettering (co-sponsored by BioNTech) and KEYNOTE-942 (NCT03897881), the cutaneous melanoma study from a Merck and Moderna collaboration. The former is a Phase I trial of a personalized mRNA vaccine (encoding a maximum of 20 neoantigens) that was administered to 16 patients in 8 weekly doses, starting nine-week post-surgical resection, prior to atezolizumab and mFOLFIRINOX adjuvant immunochemotherapy. While data is not yet mature, the eight patients who were considered ‘responders’ (based on evidence of T cell response on ELISpot) are recurrence-free at 18 months follow-up. Interestingly, only 11% of the predicted neoantigens produced an immunogenic response, and the resulting T cell responses were not significantly impaired by the subsequent cytotoxic chemotherapy.⁵² The Merck study combined another personalized mRNA vaccine (encoding a maximum of 34 neoantigens) with pembrolizumab in the adjuvant setting of recently resected melanoma. This study showed a 44% decreased risk of disease recurrence or death at 18 months. In February 2023, the FDA granted a breakthrough therapy designation to mRNA-4157 plus pembrolizumab as an adjuvant treatment for patients with high-risk melanoma following a complete resection.¹¹ These are significant developments, but both studies rely on combination therapy with long courses of ICIs – adding significantly to the costs of these regimens.¹⁰⁶

Viral vector vaccines

Viral vector vaccines work by reprogramming viruses – which have been attenuated or rendered replication-defective – to deliver TSAs and/or TAAs using recombinant DNA technology. Like DNA and RNA vaccines, these require in vivo protein synthesis to generate antigenic peptides to be subsequently presented. In contrast, they do not require artificial constructs (e.g. LNPs) or adjuvants to produce strong innate and adaptive immune responses. Co-evolution with viral species has resulted in the human immune system producing vigorous CD4+ and CD8+ T cell responses to viral PAMPs. Virally transduced host cells present both viral peptides and the vaccine’s encoded antigens, acting as a natural adjuvant to the payload of the vaccine.^19,95,107 Common species used in this application include Adenoviruses (Ad), Adeno-associated viruses (AAV), Arenaviruses, Chimpanzee adenovirus (ChAd), and modified Vaccinia Ankara (MVA).^108,109 The biology behind different viral vectors and their immune activity is nuanced and has been extensively reviewed elsewhere.¹¹⁰ However, viral biology affords the opportunity to engineer vectors that enjoy advantages over other vaccine types.¹⁰⁷ Compared to mRNA, for example, viral vaccines benefit from cost efficiency, scalability, and thermostability. These features were highlighted by the significant cost-effectiveness advantage that viral vector vaccines – such as ChAdOx1 nCov-19 and Jcovden – demonstrated during the COVID-19 pandemic.^111–113

The immune system has evolved to respond to viruses with strong T cell responses; natural viral infections can confer immunokinetics that can persist over many decades.¹¹⁴ The typical kinetics of a T cell response can be divided into three phases: rapid expansion of the primary effector response, contraction of the responding population upon resolution of infection, and establishment of a long‐lived memory population. Unlike the typical T cell response with distinct phases, there are observations in adenovirus vectors of “memory inflation.”¹¹⁵ This response – also described in other natural viral infections – is characterized by persistent, high-frequency, antigen-specific CD8+ T cells driven by the prolonged presentation of low-level antigen to the immune system.¹¹⁶ This attribute lends viral vectors a key advantage in producing the desired duration of CD8+ activation required for effective cancer vaccines.

Recombinant viral vectors have been used to prime immune responses to a range of pathogens for over 40 years; recent highlights include prophylactic vaccines against Malaria, Ebola, and COVID 19.¹⁰⁷ Their utility in cancer vaccines has also benefited from advances in cancer immunology and they, too, are beginning to show signs of efficacy.^44,49 The viral species that are chosen for use in viral vectors tend to, deliberately, be those that are not known to cause life-threatening complications. These may be commensal species or their zoonotic variants, the latter being more prevalent in non-human hosts that also reduces the risk of preexisting immunity.^107,117,118 The immune system may already possess a primed response against a viral vector or can quickly generate one – such as neutralizing antibodies (NAbs). The existing, or resultant, immune neutralization of the viral vector has previously caused concern regarding antigen presentation efficacy and subsequent immunogenicity. However, viral vaccines in those with preexisting immunity have been shown to be immunogenic in Ebola (Ad5-EBOV) and COVID-19 (CanSino’s Ad5-nCoV vaccine).^49,119,120 Viral vector vaccines can also be modified to circumvent anti-vector immunity, improve transgene expression in target cells, and deliver multiple antigens.^49,121 Another strategy to avoid neutralization by the immune system is heterologous dosing, a method that consists of using the same antigen payload in different vectors; subsequent ‘booster’ doses, if required, are delivered by an alternative vector avoiding the anti-vector immune response primed by the first dose. This technique consists of two or more different viral species, or multiple vaccine modalities. This strategy has been shown to precipitate superior immune responses relative to homologous regimens of both mRNA and viral vector platforms in a number of COVID-19 studies.^122–124

Cancer vaccines studies using viral vectors are beginning to see positive results, including in more advanced settings. As mentioned, HOOKIPA Pharma has demonstrated success in a phase I study of its two-vector, off-the-shelf, arenavirus vaccine targeting HPV viral antigens in HPV+, locally advanced or metastatic HNSCC. In patients with refractory disease, they demonstrated a disease control rate of 80% - exceeding the historical comparison of 35% with pembrolizumab monotherapy.^53,125 Similarly, Barinthus Biotherapeutics (previously Vaccitech) are advancing an off-the-shelf, prostate cancer vaccine into phase II. Targeting TAAs associated with prostate cancer, a prior study using an early version of their ChAd and MVA platform reported that 22% of patients with metastatic, castration-resistant prostate cancer experienced biochemical remission (i.e. decrease of PSA by >50%) following treatment.¹²⁶ A TSA targeting vaccine has also shown promise. An off-the-shelf Great Apes Adenovirus and MVA viral platform – ‘Nous-209’ produced by Nouscom – has been used to target 209 common frameshift antigens in colorectal and gastric cancer patients with high microsatellite-instability. In combination with pembrolizumab, this treatment has resulted in clinically significant increases in progression-free survival in patients with refractory, metastatic disease. However, this study has yet to publish mature data.¹²⁷ Viral vectors are also being utilized in personalized platforms. Gritstone Bio’s GRANITE is a heterologous vaccine approach using ChAd68 and a self-amplifying RNA, encoding up to 20 neoantigens. Phase I/II results in advanced metastatic solid tumors demonstrated consistent induction of CD8+ T cells against multiple neoantigens and IFNγ responses, detectable by ELISpot in 100% of the patients who received both prime and boost doses. While data is immature, there were some early signs of efficacy in patients with metastatic, microsatellite-stable colorectal cancer.^125,128

Discussion

After taking a backseat to the meteoric rise of ICIs, it is encouraging to see the first signs of efficacy within cancer vaccines. Cancer vaccines present selected antigens to the immune system outside of the immunosuppressive TME, in order to induce a specific, adaptive immune response. This offers the potential to overcome the limitations of currently licensed immunotherapies and, simultaneously, improve their safety profile.¹³ The lackluster results seen in early cancer vaccine studies, it seems, may be attributable to trial design and contemporary understanding of target selection and antigen presentation rather than fundamental issues with the modalities used. Despite their early failure in solid tumors, these studies provided useful insights when these platforms were repurposed during the pandemic.¹²⁹

There is a clear shift of focus toward the adjuvant setting for cancer vaccines. This seems like a logical choice given the discussed obstacles that come with more advanced disease settings. Earlier disease settings minimize the impact of the TME, treat patients when they are most immunocompetent, and remove the confounding factor of prior anti-cancer treatment.¹² This environment can be used to better study the adaptive immune responses generated in a disease-free setting. Focusing on this niche also addresses the growing need for cost-effective treatments for patients increasingly diagnosed in the early stages of cancer.^130,131 However, cancer vaccines may well be efficacious in more advanced settings, too, when combined with other treatments – as is already suggested by a number of viral vector vaccines beginning to report positive results in more advanced settings in combination with ICIs.^55,124

As target selection and the understanding of antigen presentation continue to advance, cancer vaccines will likely improve in efficacy and safety. While there is a current trend toward personalized vaccines, these require significant resources and time to manufacture. However, the detailed analysis required by these bespoke platforms will lead to a deeper understanding of TSA/TAA prevalence across tumor types and ethnic groups. This, in turn, may enable more effective off-the-shelf vaccines to be produced, which can benefit from economies of scale. Not dissimilar to the current direction of travel toward allogenic cellular therapies, it is pragmatic to develop platforms that can deliver effective cancer treatment in a semi-tailored manner rather than a fully bespoke one. This strategy may offer the benefits of personalized medicine without the cost and infrastructure required for fully personalized therapies.¹³² Additionally, effective off-the-shelf vaccines would enable neoadjuvant dosing to be explored; using the window of opportunity while patients are treatment-naïve and physiologically fitter before surgery and subsequent treatment (Figure 3). This is a paradigm shift that has recently been effectively exploited in ICI therapies.¹³³

Figure 3. Opportunities for ‘off the shelf’ and personalized cancer vaccines in the neoadjuvant and adjuvant settings.

Many technical questions remain in regard to the use of different cancer vaccine modalities. Seemingly simple clinical parameters in other drugs, like dose and frequency of administration, are made more difficult in this setting by the variable immunogenicity of antigens and unclear half-life of presentation, factors that may be further complicated by heterologous dosing regimens.^83,134 However, clinical optimization of these will come as they progress toward market authorization. In regard to the route of administration, at least, intravenous appears to be the most effective across modalities.¹²³ Considering the immunological characteristics of the modalities discussed, it seems the enthusiasm surrounding mRNA and viral vectors is not unfounded. It is impressive to see signs of efficacy in the adjuvant setting – especially in pancreatic cancer – and particularly remarkable to see similar suggestions in advanced, refractory cancers treated with off-the-shelf, viral vectors.^13,55 The latter potentially owing its success to the superior ability of viral vaccines to produce cytotoxic T cell responses.⁴⁹ However, these results are in the context of ICI combination therapy and whether cancer vaccines are able to elicit clinically significant responses as monotherapies is yet to be seen. Regardless, it is reassuring to see – even with long courses of ICIs – both modalities have demonstrated strong safety and tolerability profiles.^25,124,135

We are, seemingly, in the midst of a cancer vaccine renaissance. Given the limitations and cost of ICIs – as well as the stalled progress in other solid tumor immunotherapies – it is important to see signs of efficacy in cancer vaccines studies.^136–138 The silver lining of the global COVID-19 pandemic may well be the advancement of vaccine technology to the point where it now has the potential to address the growing burden of cancer. However, it is vital that the differences between acute viral infections and solid tumors are appreciated when interpreting these results; while vaccines producing high magnitude – but short-term – responses may be efficacious in the former, evidence suggests solid tumors require high-magnitude and long-lived, anti-tumor T cell responses. It is important, too, that we learn not only from the logistics of the platforms used in the COVID-19 vaccine roll-out but also the issues that arose around equitable access and cost-effectiveness. To make a meaningful impact on the increasing global incidence of cancer, we need to place a specific focus on developing treatments that are cost-effective and efficacious across diverse populations.^10,139

Author contributions statement

MG and LNL were involved in the conception and design, analysis, and interpretation of the data; the drafting of the paper, revising it critically for intellectual content; and the final approval of the version to be published.

SC, JK, OT, VPA, and CL were involved in the drafting of the paper, revising it critically for intellectual content; and the final approval of the version to be published.

NC, LT, and AS were involved in the drafting of the paper.

All authors agree to be accountable for all aspects of the work.

Acknowledgments

The authors would like to thank Professors Teresa Lambe and Paul Klenerman (University of Oxford) for their input and guidance on the content of this article.

Disclosure statement

MG is a shareholder in Achilles Therapeutics Ltd. and has previously received honoraria from Merck Sharp & Dohme.

LNL, SC, and JK are co-founders and shareholders of Infinitopes Ltd.

Additional information

Funding

The author(s) reported that there is no funding associated with the work featured in this article.