Beyond Prevention: The Expanding Role of Vaccine Drug Delivery in Modern Therapeutics

Edward Jenner, one   of   the   pioneers   in   this   subject, coined   the   word   "vaccination" [1]. A vaccine is a substance that, while not always an illness, strengthens the body's defenses against disease. Typically, vaccines are made of DNA encoding pathogen antigenic proteins, killed or attenuated organisms, or subunits of organisms[2]. Subunit vaccines are quite selective and specific when it comes to responding with antibodies, but they frequently don't exhibit these reactions in situations like changes in the antibody's epitopic identification center and are not very immunogenic [3]. However, by providing the immune system with protein and carbohydrate subunits of the causative organism in a way that induces a specific and powerful immune response, the selectivity and specificity of these entities can be used to produce powerful and sustained immune responses [4]. Additionally, these epitopes may enable the development of vaccinations that protect against chronic illnesses like cancer or hepatitis C in addition to infectious diseases [5].  One of the difficulties in creating novel vaccines is determining the targets, which calls for knowledge of immunological principles and enables the logical creation of antigens, adjuvants, and their combinations [6]. Growing awareness of the dangers of pandemics and their catastrophic socioeconomic effects on both individual nations and the global community highlights how important it is to create safe and efficient methods for treating and preventing viral diseases [7]. The international scientific community has started to develop vaccines more quickly that can activate humoral and cellular immunity and create long-term immunological memory [8].  Inactivated or dead pathogens or live attenuated pathogens are used in traditional vaccines. Since they include many bacterial or viral components in addition to antigens, which efficiently activate multiple innate immune system components simultaneously, they do not require adjuvants [9].  Throughout the vaccine's development, the targeted product profile is crucial. The choice of vaccination dose types is influenced by various factors, including vaccine antigen classes, including live attenuated, inactivated, subunit, and most recently, mRNA-based. Its efficacy is ultimately impacted by the formulation development that results [10]. The formulation's intended route of administration must be taken into account when creating it. Choosing a vaccine delivery method has always involved taking into account the crucial balance between mucosal and systemic immune responses [11]. Various techniques are used to produce vaccines, ranging from conventional approaches that use live attenuated and inactivated viruses to next-generation approaches that include DNA, virus-like particles (VLP), adenoviral vectors, mRNA, and all or part of the spike protein. Infectious illness prevention was the original purpose of vaccines, but in more recent times, the idea of vaccines has expanded greatly [12]. Particularly in the area of cancer immunotherapy, vaccinations for prevention and treatment have been developed to treat various tumors with encouraging outcomes [13].

Classification of Vaccine Delivery Systems: Underlying Mechanisms

There are numerous ways to create vaccines that fight microorganisms. Usually, the foundational knowledge about the microbe such as how it infects host cells and how the immune system reacts to it is the basis for these decisions [14]. Also reviewed are other pragmatic factors, such as regions of the world where the vaccine should be administered. DNA vaccines and recombinant vector vaccines are two more vaccine types that are presently under development [15].

During the vaccine development process, a researcher may choose to pursue the following options:

Figure 1. Classification of Vaccines

i)  Live, attenuated vaccines:

Mechanism: The pathogenic microbe that is present in live, attenuated vaccines has been weakened or attenuated by being grown in a lab setting until it no longer possesses a significant level of pathogenicity. This is done by repeatedly introducing a pathogenic microbe into an unnatural host, such as tissue culture, embryonated eggs, or live animals for several generations [16,17].

• Advantages:

It is comparatively simple to develop live, attenuated vaccines [17].

The attenuated vaccines frequently provide lifetime immunity with just one or two doses and produce potent immune-protective cellular and antibody responses [18].

• Disadvantages:

Secondary mutations may result in disease and a reversion of virulence [17].

Live vaccinations are not recommended for individuals whose immune systems are compromised, damaged, or weakened as a result of chemotherapy, HIV infection, or pregnancy [18].

ii) Inactivated vaccines:

Mechanism: Antigens are usually rendered inactive by heat, chemicals like formaldehyde or β -propiolactone, or radiation. The chemical process eliminates the pathogen's capacity to multiply while maintaining the immunogenic structure in its native state. Maintaining the structural integrity of surface antigens' antigenic epitopes is vital. Therefore, by directly producing humoral and cell mediated immune responses against the natural pathogen, inactivated whole pathogen vaccines offer protection [18,19].

• Advantages:

Because inactivated microbes cannot revert to their pathogenic state, they are safer and   

more stable than live vaccines. [18] They can be readily stored and transported in a freeze-dried state and don't need to be   

refrigerated, which makes them more affordable and easily accessible for people in developing nations [19].

• Disadvantages:

Compared to live vaccines, the majority of inactivated vaccines elicit a weakened immune        response. Because there are many unrelated structural antigens of microorganisms present,   there is a higher chance of allergic reactions [20]. 

iii) Subunit Vaccines:

Mechanism: Vaccines are made from subunits of antigens that have the highest potential to    activate the immune system. Variable antigens in subunit vaccines can range from one to twenty. Because this type of vaccine only uses the specific antigenic determinants are very specific parts of the antigen that antibodies or T cells recognize and bind to the risk of side effects is greatly reduced, and the possibility of virulence reversal is totally eliminated [20,21].

• Advantages:

Very stable and safe because they don't have any live parts. Minimal chance of an adverse reaction [21].

• Disadvantages:

  Typically requires adjuvants [21].

iv) Toxoid Vaccines:

Mechanism: Toxoids, which are typically exotoxins produced by bacteria, are released by pathogens and cause a variety of disease symptoms following infection. Toxoid vaccines, such as those for tetanus and diphtheria, are made by purifying bacterial exotoxin. After that, the immunogenicity of the purified exotoxins is preserved while their toxicity is reduced or rendered inactive by formaldehyde or heat [22]. 

• Advantages:

Efficient in avoiding illnesses brought on by bacterial toxins [22].

• Disadvantages:

 Needs a booster shot to keep their immunity high.

v) Conjugate Vaccines:

Mechanism: In order to create conjugate vaccines, the polysaccharide was chemically conjugated to a more potent T-cell-stimulating antigen, such as tetanus or diphtheria toxoids. The linked protein and linked polysaccharide provide defenses against disease- causing bacteria by stimulating the immature immune system [23].

Uptake Of Antigen

Antigens: Molecules or components of pathogens (such as virus or bacteria) that the immune system recognizes as foreign. Any molecule with the potential to provoke an adaptive immune response, including proteins, polysaccharides, or other substances. Vaccines contain these antigens to help stimulate your immune system to recognize and fight the actual pathogen should you encounter it in future [24,25].

Antibodies: These are proteins made by B lymphocytes in response to antigens. They provide the immune response with specificity and are capable of binding to antigens, neutralizing the pathogen, and tagging it for destruction by other immune cells[26]. Vaccination induces the production of antibodies against the particular antigens in vaccine that circulate and protect from infection [25].

Uptake of Antigen: Uptake of the antigen is one of the primary events through which an adaptive immune response can be initiated. This process is, in large part, carried out by specialized immune cells called antigen-presenting cells (APCs). The arguably most critical task to perform in order to activate these T-cells is to take up and process antigen effectively [27].

 Mechanism of Antigen Uptake:

 i) Phagocytosis: Cells that engulf large particles, including pathogens [28].

Key Players: Macrophages and dendritic cells.

Process: Recognition of the presence of pathogens by receptors, such as toll-like receptors, leads     

to the engulfing of the invading agent in a phagosome, which then fuses with lysosomes for degradation [29].

 ii) Endocytosis: A means through which cells ingest extracellular material [28].

Key Players: B cells, dendritic cells.

Process: Antigen ligation of specific receptors, such as the B cell receptor, leads to the vesicular uptake of antigens into the interior of the cell, where they are processed and presented via MHC   class II [30].

iii) Receptor-Mediated Endocytosis: It occurs due to receptor binding of specific receptors on the membrane of the immune cells with antigens. This binding event instigates the ingestion of the complex; in this case, the receptor-antigen [31].

iv) Antigen Presentation: These antigens, when present inside the endocytic vesicle, will be further processed and brought to the surface of MHC molecules. MHC class II presents the exogenous antigens to CD4+ T cell and MHC class I displays the endogenous antigens to CD8+ T cell [32].

Figure 2. Mechanism of immune response after vaccination

 Drug Delivery Systems for Vaccines

Although the number of vaccination doses required is decreased when antigens are delivered using based on oil adjuvants like Freund's adjuvant, these adjuvants are not frequently utilized because of toxic issues such the formation of granulomas at the location of injection [33]. The FDA has authorized aluminum hydroxide and aluminum phosphate in the type of alum as adjuvants for consumption by human. As an outcome, antigen was developed into methods of delivery that distribute antigen in particle form instead of solution state in an effort to find healthier and more efficient adjuvants [34]. The following are other factors influencing the creation of vaccinations as regulated drug delivery systems such as:

• Vaccination failing with traditional vaccination regimens that include booster shots and prime shots because people forget to take the latter [35].
• Conversely, vaccine delivery techniques enable the assimilation of antigen doses, eliminating the need for booster shots because antigens are supplied gradually and under supervision [36].
• Manage the antigens' temporal and spatial exposure to the immune system in order to encourage direct targeting of the immune cells [37].

i)  Liposomal Delivery System: The phospholipid bilayers that make up liposomes and their derivatives, known as "lipoplexes" or liposome/DNA complexes, are hollow, spherical structures that can trap hydrophilic and hydrophobic molecules in the aqueous compartment and lipid bilayers, respectively, with cholesterol giving the bilayer its rigidity. The negative charge on DNA neutralizes the positive charge on liposomes, which causes lipoplexes to aggregate during storage. The development of liposomes/protamine/DNA (LPD) addresses this limitation. Protamine is a peptide that is rich in arginine. It gives stability to the preparation by condensing with DNA before it can complex with positive lipids [38].

• Utilizing the rodent malaria approach, P. yoelii 17X, liposomes are used as an adjuvant delivery method for a whole-parasite blood-stage vaccine. The clinically demonstrated cationic liposomal adjuvant composition, dimethyldioctadecylammonium (DDA)/trehalose 6,6′-dibehenate (TDB), is either set up in-house as DDA/TDB or purchased via Staten Serum Institute [SSI] as CAF01. CAF01 is made up of the synthesized mycobacterial glycolipid TDB stabilizing the cationic surfactants DDA. Significant cellular and antibody reactions have been demonstrated to be induced by it [39].
•  ii) Virosomes: These tiny, spherical, unilamellar lipid membrane vesicles (150 nm) are called virosomes. They contain viral membrane proteins like the influenza virus's hemagglutinin and neuraminidase, but they lack nucleocapsid, which contains the genetic material of the original virus [40]. These proteins allow the membranes of virosomes to fuse with immune system cells, delivering their contents—the particular antigens straight to their target cells and triggering a particular immune response even in the case of weak immunogenic antigens [40].
• Virosomes' exceptional qualities as an adjuvant and carrier system have been confirmed by approved vaccines against influenza (Inflexal® V) and hepatitis A (Epaxal®) [41]. Over 10 million people have received these two vaccines together, which are authorized in more than 45 countries.

iii) Polymeric nanoparticles:

Due to their size, polymeric nanoparticles are preferentially absorbed by the lymphoid tissue associated with the mucosa [42]. Since, synthetic polymer-based NPs and MPs are easy to prepare, biocompatible, biodegradable, stable in biological systems, and have low cytotoxic, protective, controlled, and sustained-release of enclosed drugs, they have drawn more interest in the past few years for their roles in vaccine delivery and adjuvanticity [43]. PLGA is among the most widely utilized synthesized biodegradable polymer-based NP for adjuvanticity and vaccine administration. Poly-lactic acid (PLA) and polyglycolic acid copolymerize to form PLGA, which is very suitable [44]. The American Food and Drug Administration (FDA) has licensed PLGA NPs for drug administration in humans and animals and they are easily packed with an extensive variety of compounds [45]. A significant advancement in vaccine development has been made recently with the creation of SARS-CoV-2 spike mRNA vaccines to contain the deadly pandemic. Lipid nanoparticles (LNPs), which are made up of various lipids in particular proportions, are typically used to construct mRNA vaccines; yet they typically lack specific delivery [46].  We described here the production of polymeric nanoparticles (PNPs) made of a guanidine copolymer with zwitterionic groups as well as an aryl-trimannoside ligand that targets dendritic cells (DC) for the encapsulation and selective delivery of an mRNA to dendritic cells in order to develop a selective delivery method for mRNA vaccine formulations [47].

iv) Emulsion Delivery System: Emulsions are heterogeneous liquid systems that can be water in-oil, oil-in-water, or more complex systems like microemulsions, nano emulsions, or water in-oil-in-water multiple emulsions. Antigens are emulsified in oil when a suitable emulsifier is present after being dissolved in a water phase. A number of variables, including the oil-to water phase ratio, emulsion droplet size, and oil phase viscosity, affect an emulsions controlled release properties [48].

Ovalbumin was used as the model antigen in the development of a novel emulsion-type vaccine delivery system for the amphiphilic bioresorbable polymers poly(ethylene glycol)-block-poly(lactide-co-epsilon-caprolactone) (PEGb-PLACL PEG-b-PLACL-emulsified formulation are made up of uniform fine particles and are stable and repeatable, which makes them superior to vaccines made with traditional adjuvants, according to the findings of physicochemical characterisation and in vitro release experiments [38].

v) Micellar System: Micelles as possible carriers of antigens have been thoroughly studied. Micelles are groups of amphiphilic surfactant molecules that self-aggregate. Above the critical micellar concentration, surfactants move into micellar structures to prevent contact with an incompatible external phase. They can also enclose hydrophilic or lipophilic cavities (reverse micelle), which helps to trap antigens and deliver them into the body [49].  In order to create a safe and efficient smallpox vaccination for people, Moyer's innovation outlines procedures and systems that use a genetically modified variant of the vaccinia virus to produce immunization after oral administration of the vaccine [50]. The vaccine can be administered as viral antigens or as live viruses that may produce viral proteins however cannot replicate entirely as a lytic virus. According to the stated procedures, the vaccine is prepared for use using micelles, microstarch elements, omega-3 fatty acids, and other nanoparticle and immuno-potentiators [38].

vi) Dendrimers Based Delivery System: Synthetic, branched polymers with layered structures are called dendrimers. The carrier system can be used to deliver a drug to a human subject by mixing a biologically active material with the multifunctional polymeric material in an aqueous loading environment [51].  Wright's innovation uses an ingredient called dendrimer as an additive in an influenza vaccination. An influenza antigen and a dendrimer in a biologically suitable carriers are both included in the vaccine. Because even a tiny quantity of the dendrimer functions as an efficient adjuvant, it is possible to adjuvant influenza without creating a hazardous compound [38].

Recent Advancement in Vaccine Delivery System

i) Advances in intranasal vaccine delivery: The effectiveness of the intranasal (IN) vaccination has been demonstrated by numerous investigations, some of which have also included clinical trials [52]. It is possible to use these vaccines for both respiratory and systemic infections. Commonly referred to as nasal-associated lymphoid tissue (NALT), the nasal cavity is abundant in lymphatic tissue that combines humoral and cellular immune responses to induce both mucosal and systemic immunity [53].

ii) Emerging Trends in Lipid-Based Vaccine Delivery: The absence of pathogen-associated molecular patterns (PAMPs) in subunit vaccines, however, results in low immunogenicity. Additionally, the peptides' tendency to degrade quickly in vivo tends to lessen the effectiveness of the dose that is administered [54]. Therefore, the development of delivery carriers based on nanotechnology is necessary to get around the aforementioned restrictions. Because of their capacity to release the biologically active agent both spatially and temporally, liposomes are among the frequently studied nanocarriers [55].

iii) Lipid- and Polymer-Based Nanocarrier Platforms for Cancer Vaccine Delivery: In recent years, cancer immunotherapy has grown in popularity as a means of treating a variety of cancers, especially those that don't respond to traditional chemotherapy and radiation therapy. By strengthening the patient's immune system to identify and eliminate cancer cells, cancer vaccines work to break the cycle between cancer and immunity. This makes them effective preventative and curative treatments [56].

iv) Edible Plant-Derived Extracellular Vesicles for Oral mRNA Vaccine Delivery: A humoral and cell- mediated immune response was induced in a number of investigations employing bacterial or human EVs expressing mRNA or recombinant SARS-CoV-2 proteins [57]. Additionally, vaccines based on EVs that display the natural arrangement of viral antigens have proven to be more effective than synthetic nanoparticles at providing long-lasting immunity and lowering toxicity. When it comes to vaccine delivery, especially when administered orally, edible plant-derived EVs have proven to be a viable substitute for human EVs [58].

v) Non-viral COVID-19 vaccine delivery systems: SARS-CoV-2 vaccines, from traditional viral and protein-based vaccines to more innovative ones like DNA- and mRNA-based vaccines, have been developed in a race by businesses and research institutes. Depending on the antigen design, adjuvant molecules, vaccine delivery systems, and immunization technique, each vaccine has a unique potency and duration of effectiveness [59].

vi) Nanocarriers based on bacterial membrane materials for cancer vaccine delivery: Over the past ten years, patients with glioblastoma, melanoma, and other cancers have shown strong tumor-specific immunogenicity and antitumor activity in therapeutic cancer vaccines, an emerging class of immune-oncology therapy [60].

The newer cancer vaccines elicit particular immune responses against neoantigens that are produced by genetic mutations in tumor cells, potentially preventing off-target effects, in contrast to the earlier cancer vaccines that targeted tumor-associated antigens that over express in tumor cells [61].

Table 1. Recent developments of delivery system in vaccines