What are the different types of drugs available for circRNA vaccine?

Introduction to circRNA Vaccines

Definition and Mechanism of circRNA
Circular RNAs (circRNAs) are a unique class of single‐stranded RNA molecules characterized by their covalently closed-loop structures that lack 5′ caps and 3′ polyadenylated tails. This circular configuration renders them highly resistant to exonuclease‐mediated degradation, ultimately endowing them with enhanced stability compared to their linear counterparts. Unlike typical messenger RNAs (mRNAs) that depend on cap‐dependent translation, some circRNAs harbor internal ribosome entry sites (IRESs) or are modified by N^6‐methyladenosine (m^6A) to facilitate cap‐independent translation. These biochemical properties not only make circRNAs more robust in cellular environments but also position them as potentially ideal templates for protein expression in vaccine applications. In engineered circRNA vaccines, the circRNA is synthetically produced to encode therapeutic polypeptides such as antigenic proteins, functional proteins, receptor proteins, or targeting proteins. Their intrinsic closed conformation prolongs the duration of protein expression in cells, which is key to eliciting sustained antigen-specific immune responses.

Overview of Vaccine Development
Vaccine development has evolved rapidly, moving from traditional inactivated or attenuated pathogens and subunit vaccines toward innovative nucleic acid–based platforms. The advent of mRNA vaccines in the recent COVID-19 pandemic highlighted the significant promise of nucleic acid therapies in inducing robust cellular and humoral immune responses with rapid production scales. CircRNA vaccines represent the next generation in this technological continuum. They are designed to address some of the inherent challenges associated with conventional mRNA vaccines—namely, mRNA instability, inefficient in vivo delivery, and innate immunogenicity—by leveraging the unique properties of circRNAs such as enhanced stability and reduced degradation. The development process of circRNA vaccines integrates advanced RNA synthesis technologies, purification protocols, and optimized delivery strategies, all of which contribute to improved antigen expression and better immune response profiles. Furthermore, circRNA vaccines are being explored not only for infectious diseases like COVID-19 but also for cancer immunotherapy, broadening the scope of their clinical utility.

Types of Drugs in circRNA Vaccine Development

The realm of circRNA vaccine development encapsulates a diverse spectrum of drug types and formulation components that work synergistically to induce effective immune responses. At a broad level, the formulation of a circRNA vaccine can be divided into three main categories: antiviral drugs (as the primary therapeutic payload), adjuvant compounds (to amplify and shape the immune response), and delivery systems (to protect and ensure efficient cellular uptake of the RNA payload). Each of these elements is critical to overcoming specific obstacles associated with nucleic acid therapeutics, as described in both preclinical and clinical research studies.

Antiviral Drugs
In the context of circRNA vaccines, “antiviral drugs” primarily refer to the active therapeutic agents encoded by the circRNA molecules. These drugs are designed to instruct host cells to express antigenic polypeptides that are derived from pathogenic viruses, thereby stimulating an antiviral immune response.
- Antigen-Encoding circRNAs:
One of the most prominent approaches involves designing circRNAs that encode a viral antigen, such as the spike (S) protein or its fragments from SARS-CoV-2. The expression of these antigenic proteins in host cells results in the activation of both the innate and adaptive arms of the immune system. The expressed antigen is processed and presented by antigen-presenting cells (APCs), which then prime T cells and B cells, leading to the generation of neutralizing antibodies and cytotoxic lymphocyte responses against the respective virus.
- Self-Amplifying Mechanisms:
Some circRNA vaccine designs explore a multicistronic or self-replicating format, where the circRNA may contain elements that enable it to undergo self-amplification in the cytoplasm. This strategy can potentially reduce the required dose by enabling prolonged antigen expression, thereby enhancing immunogenicity. Although these strategies are more advanced in the pipeline, their potential lies in mimicking some of the attributes of RNA viruses, which naturally replicate in the cytoplasm without relying on nuclear mechanisms.
- Combination Therapies:
In some vaccine formulations, circRNA vaccines are conceptualized as part of a combination therapy where they are co-administered with conventional antiviral drugs. This approach has the dual benefit of providing immediate antiviral effects while the circRNA vaccine induces a durable immune response. For example, the antiviral and immunomodulatory properties of certain small-molecule drugs could complement the antigen-specific responses elicited by the circRNA vaccine, thereby offering a two-pronged approach to disease management.

Overall, the antiviral drug component within a circRNA vaccine is less a conventional “drug” in the classical pharmacological sense and more a genetic template that leads to in situ production of protective antigens.

Adjuvant Compounds
Adjuvants are critical components in vaccine formulations because they enhance the immunogenicity of the antigen and can modulate the nature of the immune response. In circRNA vaccine formulations, adjuvant compounds serve several purposes: they boost the antigen-specific response, help direct the immune response toward a desired phenotype (such as a Th1-dominant response), and sometimes even facilitate the translation and stability of the circRNA itself.
- Traditional Adjuvants:
Many of the classical adjuvants that have been employed in vaccine formulations—such as aluminum salts (alum), squalene-based emulsions (MF59), and toll-like receptor (TLR) agonists—are considered for inclusion in circRNA vaccine formulations. These compounds have been well characterized in terms of their ability to enhance the antibody response and promote cell-mediated immunity. For instance, aluminum salts have been used extensively in various vaccine platforms and are known for their safety and efficacy profile in inducing humoral responses.
- Self-Adjuvant Properties of RNA:
Another promising approach is to harness the natural immunostimulatory properties of RNA. Both mRNA and circRNA can activate innate immune receptors such as RIG-I, MDA5, and TLRs. However, circRNAs typically exhibit lower innate immunogenicity compared to linear mRNAs due to their circular structure, which may reduce the risk of overly robust inflammatory responses. Engineering circRNAs to carry “self-adjuvant” sequences or combining them with specific RNA motifs that modestly stimulate innate immunity is an innovative strategy to achieve a balanced immune activation.
- Engineered Adjuvant Molecules:
In some advanced formulations, circRNA vaccines are co-delivered with additional nucleic acid sequences that encode immunomodulatory proteins (e.g., CD40L, TriMix—combining CD70, CD40 L, and caTLR4) which act as adjuvants when expressed by the host cells. Such approaches allow for an integrated vaccine platform where the circRNA not only codes for a target antigen but also helps shape the subsequent immune response by providing costimulatory signals.
- Novel Small Molecule Adjuvants:
Recent research has explored the use of small molecule modulators that target specific innate immune receptors. RNA adjuvants such as polyinosinic-polycytidylic acid (poly(I:C)) and CpG oligodeoxynucleotides, which mimic viral components, can enhance both innate activation and the ensuing adaptive response. These compounds can be formulated alongside circRNA vaccines in nanoparticle carriers, contributing further to the overall immunogenicity and therapeutic potency.

Delivery Systems
Efficient delivery is pivotal for the success of any nucleic acid-based vaccine, particularly for circRNA vaccines. Because naked RNA is vulnerable to degradation by ubiquitous RNases and has poor cellular uptake characteristics due to its large size and hydrophilic nature, sophisticated delivery systems are required to ferry the RNA safely into target cells.
- Lipid Nanoparticles (LNPs):
LNPs have emerged as the most commonly used delivery vehicles for RNA vaccines owing to their ability to encapsulate RNA molecules, protect them from rapid degradation, and facilitate cellular uptake via endocytosis. LNPs are composed of ionizable lipids, cholesterol, helper lipids (typically DSPC), and PEGylated lipids which form stable nanoparticles that not only shield the RNA but also promote fusion with the endosomal membrane for efficient cytoplasmic release. Given the structural similarities between mRNA and circRNA, many of the LNP platforms developed for mRNA vaccines are currently being adapted for circRNA vaccine delivery.
- Viral and AAV-Based Delivery Systems:
Although viral vectors are commonly used for gene therapy, they are also being evaluated for RNA delivery. Adeno-associated virus (AAV) vectors are particularly attractive due to their low immunogenicity and capacity to deliver nucleic acids without integration into the host genome. However, the use of AAVs may be limited by pre-existing immunity and a restricted packaging size, which currently makes them less common for large circRNA constructs.
- Virus-Like Nanoparticles (VLNPs):
An innovative approach involves the use of virus-like nanoparticles (VLNPs), such as those derived from human papillomavirus (HPV). These nanoparticles mimic the structure of actual viruses, thereby enhancing cellular entry and facilitating endosomal escape. Recent studies have demonstrated that the functionalization of VLNPs with transmembrane peptides (e.g., L17E) significantly improves circRNA transmembrane delivery and protein expression levels by promoting efficient endosomal escape.
- Exosomes and Extracellular Vesicles (EVs):
Exosomes, naturally occurring extracellular vesicles, offer a biologically inspired delivery system. Owing to their inherent biocompatibility, low immunogenicity, and natural ability to shuttle nucleic acids between cells, exosome-based delivery has been explored as a potential method for circRNA vaccines. Although the technology is still emerging, exosome-mediated delivery platforms are promising due to their capacity to target specific tissues and minimize off-target effects.
- Polymeric Nanoparticles and Hybrid Systems:
Polymeric carriers made from biocompatible and biodegradable polymers, such as poly(lactic-co-glycolic acid) (PLGA), have also been investigated for the delivery of RNA therapeutics. These nanoparticles can be engineered to release their cargo in a controlled manner and can be combined with lipid components to form hybrid delivery systems with improved physicochemical properties. Such systems are particularly useful when targeting tissues that are difficult to penetrate or in situations where a sustained release of the therapeutic RNA is desired.

Current Research and Applications

Clinical Trials and Studies
Although circRNA vaccines are still in the early stages of development compared to their mRNA counterparts, several preclinical studies and early-phase clinical trials attest to their innovative potential. Research has demonstrated that engineered circRNA vaccines encoding antigenic proteins can induce potent innate immune activation as well as robust antigen-specific CD8+ T cell responses. For instance, early studies in murine models have showcased the feasibility of using circRNA vaccines to elicit a strong cytotoxic T cell response and even achieve significant antitumor efficacy when combined with appropriate delivery systems and adjuvant modifications.
Furthermore, patents illustrate practical applications where circRNA vaccines have been designed to target coronavirus antigens, particularly in response to the SARS-CoV-2 pandemic. These circRNA vaccines are intended not only to prevent infection but also to offer extended antigen expression due to the circRNA’s increased stability. While most of these studies are still in the preclinical realm, the success of analogous mRNA vaccines in clinical trials has accelerated interest in advancing circRNA technology through similar regulatory and clinical pathways.

Case Studies and Examples
Several case studies and proof-of-concept experiments highlight the versatility of circRNA vaccine platforms.
- SARS-CoV-2 circRNA Vaccines:
Multiple patents describe the innovative use of circRNAs to encode the spike (S) protein or its fragments as part of a vaccine strategy against SARS-CoV-2. These constructs have been demonstrated in animal models to induce neutralizing antibody responses and activate T cell-mediated immunity, thereby providing a promising prophylactic strategy against COVID-19.
- Multicistronic and Polycistronic circRNA Vaccines:
Advanced designs such as multicistronic RNA vaccines and polycistronic RNA vaccines have been developed to express multiple antigens from a single circRNA molecule. These approaches are particularly promising for targeting complex pathogens or for inducing a multi-antigen immune response against heterogeneous tumors. The ability to co-express several therapeutic proteins from one construct not only streamlines the vaccine formulation but also potentially enhances immunogenicity by providing broader antigenic coverage.
- Cancer Immunotherapy Applications:
Beyond infectious diseases, circRNA vaccines are being explored in the cancer arena. Studies have shown that circRNAs engineered to encode tumor-associated antigens can stimulate robust anti-tumor immune responses, potentially serving as therapeutic cancer vaccines. The adjuvant properties of circRNAs have also been investigated in these contexts, as self-adjuvanting circRNA vaccine formulations may promote dendritic cell activation and subsequent T cell priming.
- Integrated Approaches with Advanced Delivery Systems:
Case studies involving the use of advanced delivery systems, such as HPV virus-like nanoparticles or LNP-based formulations, have shown enhanced cytoplasmic delivery of circRNA payloads. These studies emphasize the importance of the delivery vehicle in ensuring that the circRNA not only reaches the target cells but is also efficiently released into the cytosolic compartment for translation.

Challenges and Future Directions

Current Challenges in circRNA Vaccine Development
Despite the great promise circRNA vaccines hold, several technical and scientific challenges remain that need to be overcome before widespread clinical adoption can be achieved.
- Synthesis and Purification:
One of the most significant hurdles is the efficient synthesis and purification of circRNAs. The process of cyclization must be highly efficient to minimize the presence of linear RNA contaminants and extraneous fragments, which could lead to unwanted immune activation or off-target effects. Moreover, achieving consistent and high yields of fully circularized RNA remains technically challenging.
- Delivery and Cellular Uptake:
As with all RNA-based therapeutics, delivering circRNAs into the cytoplasm of target cells is a critical challenge. Although various delivery systems (LNPs, VLNPs, exosomes) have been developed, optimizing these systems for circRNA—whose unique properties may require slight adjustments to classical mRNA formulations—continues to be an area of intense research. Inadequate delivery may lead to poor antigen expression and suboptimal immune responses.
- Control of Innate Immunity:
While circRNAs generally exhibit lower innate immunogenicity compared to linear RNAs, overactivation of the innate immune system can still occur. Balancing the adjuvant effect of circRNAs—sufficient to stimulate a protective response but not so robust as to cause inflammation and toxicity—is essential. Additionally, the potential off-target immune responses, especially when combined with potent adjuvants or novel delivery systems, must be carefully controlled.
- Scalability and Manufacturing:
Consistent, scalable, and cost-effective manufacturing processes are crucial for any vaccine platform. CircRNA vaccines must be produced under Good Manufacturing Practice (GMP) conditions, and their production process must be as streamlined and reproducible as that of current mRNA vaccines. This includes challenges in large-scale in vitro transcription, cyclization, purification, and formulation into delivery systems.
- Regulatory Considerations and Clinical Translation:
As a relatively new modality, circRNA vaccines face regulatory hurdles due to the limited amount of historical clinical data. Establishing safety, dosage, and delivery guidelines comparable to those that exist for traditional vaccines or mRNA vaccines is necessary for successful clinical translation.

Future Prospects and Research Innovations
The future for circRNA vaccines is promising, particularly as ongoing research continues to address current limitations. Several innovative strategies are poised to propel circRNA vaccine development in the coming years:
- Enhanced Synthesis Technologies:
Advances in in vitro transcription and the development of novel cyclization techniques will likely increase the efficiency and purity of circRNA production. Research into enzymatic and chemical methods for achieving near-complete cyclization without contaminants is critical. These improvements will not only augment the safety profiles of circRNA vaccines but will also reduce production costs.
- Next-Generation Delivery Systems:
Continued innovation in nanotechnology presents the possibility of even more effective and targeted delivery platforms. For example, virus-like nanoparticles (VLNPs) that mimic the natural mechanisms of viral entry have already demonstrated promising results in improving endosomal escape and cytosolic delivery of circRNA. Additionally, exosome-based delivery, polymeric nanoparticles, and hybrid lipid–polymer systems offer exciting avenues for future research and may allow for tissue-specific targeting, thereby enhancing therapeutic efficacy.
- Self-Amplifying circRNA Platforms:
Incorporating self-amplifying elements into circRNA constructs is a frontier area with the potential to drastically reduce the dosing requirement. By enabling the circRNA to replicate or to drive prolonged antigen expression in the cytoplasm, these platforms can elicit durable immune responses while reducing the overall amount of RNA administered.
- Integrated Adjuvant Strategies:
Future innovations may focus on the integration of built-in adjuvant components within the circRNA construct itself, or the co-formulation with potent immunomodulatory compounds. Designing circRNAs that not only encode antigenic proteins but also provide signals for immune activation (e.g., co-expression of costimulatory molecules) could offer a major advantage in guiding the immune response toward a protective profile.
- Multicistronic and Polyfunctional Vaccines:
The incorporation of multicistronic design elements—where a single circRNA encodes multiple antigenic determinants or a combination of antigen and adjuvant proteins—is another promising direction. This approach can create a broad immune response with potential applications beyond infectious diseases, extending into personalized cancer immunotherapy.
- Regulatory and Clinical Frameworks:
As circRNA vaccines move closer to clinical application, collaboration between academic researchers, industry, and regulatory authorities will be crucial. Establishing standardized protocols for quality control, safety assessment, and clinical trial design specific to circRNA vaccines will pave the way for smoother translation from bench to bedside.
- Personalization and Adaptive Platforms:
With the growing trend toward personalized medicine, future circRNA vaccines may be tailored to individual immune profiles or specific tumor antigen profiles. The flexibility of RNA technology allows for rapid redesign and production, which is critical in responding to emerging infectious threats or heterogeneous cancers. By integrating bioinformatics and high-throughput screening, researchers can identify optimal circRNA candidates for individual patients, potentially making these vaccines a mainstay in precision immunotherapy.

Conclusion
In summary, the types of drugs available for circRNA vaccines encompass three major pillars: antiviral drugs, adjuvant compounds, and advanced delivery systems. Antiviral drugs in this context are primarily the engineered circRNA constructs that encode antigenic polypeptides—such as the spike protein of coronavirus—designed to trigger robust and long-lasting immune responses. These circRNA vaccines benefit from strategies that include self-amplifying mechanisms and multicistronic designs, which aim to produce multiple antigens or even integrate immunostimulatory signals within a single construct.
Adjuvant compounds, ranging from traditional agents like alum and TLR agonists to innovative self-adjuvanting RNA motifs, are incorporated to fine-tune the immune response. They ensure that the vaccine not only induces a strong antibody-mediated response but also a vigorous cell-mediated immunity necessary for protecting against viral infections and even cancers.
Delivery systems form the third critical component. They include lipid nanoparticles (LNPs), virus-like nanoparticles (VLNPs), exosomes, and polymer-based systems, all of which are engineered to protect the circRNA during transit, enhance its uptake into target cells, and facilitate efficient cytoplasmic release for translation into the desired protein antigens. Each of these delivery modalities brings unique advantages and potential challenges—ranging from scalability and manufacturability to specific tissue targeting and immunogenicity.
Current research has shown promising results in preclinical studies and early-phase clinical trials, with case studies highlighting the efficacy of circRNA vaccines against SARS-CoV-2 and in cancer immunotherapy. These encouraging developments underscore the potential of circRNA vaccines; however, several challenges remain. These include refining synthesis and purification methods, optimizing delivery to achieve effective cellular uptake, balancing innate immune activation, and establishing regulatory frameworks to ensure safety and efficacy.
Looking ahead, future prospects in circRNA vaccine technology rest on continued innovation in RNA chemistry, delivery systems, self-amplification strategies, and integrated adjuvant design. The promising advancements in these areas—coupled with the lessons learned from mRNA vaccine development—suggest that circRNA vaccines hold substantial promise for addressing infectious diseases and cancers in a rapidly evolving medical landscape. Overall, the integration of robust antiviral payloads, strategic adjuvants, and advanced delivery technologies heralds a new era in vaccine development, one that combines high potency, scalability, and the flexibility to meet emerging global health challenges.

Conclusion:
The diverse drug types available for circRNA vaccine development reflect a sophisticated approach to modern vaccinology. At the core, circRNA vaccines are designed to encode antigenic proteins that act as antiviral drugs, but their efficacy is critically enhanced by carefully chosen adjuvant compounds and cutting-edge delivery systems. Research from multiple studies and patents indicates that these components—whether it is the engineered circRNA with self-adjuvant properties, the integration of potent immunomodulatory molecules, or the use of advanced nanocarriers such as LNPs and VLNPs—work in concert to provide a robust, durable, and safe immune response. While challenges remain in optimizing synthesis, delivery, and regulation, the future innovations in circRNA vaccine technologies appear highly promising. These vaccines stand as a testament to the ongoing evolution in RNA therapeutics and point to the potential for overcoming longstanding barriers in infectious disease and cancer immunotherapy. Continued interdisciplinary research and collaborative efforts are essential for realizing the full therapeutic potential of circRNA vaccines, ultimately leading to next-generation solutions that are both highly effective and adaptable to global public health needs.