Immune modulation by adjuvants combined with diphtheria toxoid administered topically in BALB/c mice after microneedle array pretreatment

Pharmaceutical and Immunological Evaluation of Cholera Toxin A1 Subunit as an Adjuvant of Hepatitis B Vaccine Microneedles

PURPOSE

For successful delivery of a solid vaccine formulation into the skin using microneedles, the solubility of an adjuvant should be considered because the decrease in the dissolution rate by the addition of adjuvant decreases the delivery efficiency of the vaccine.

METHODS

In this study, cholera toxin A subunit 1 (CTA1) was examined as an adjuvant to Hepatitis B vaccine (HBV) microneedles because of its good water solubility, improved safety, and positive effect as shown in intramuscular administration of a liquid vaccine.

RESULTS

All solid formulations with CTA 1 dissolved in in vivo mouse skin within 30 min, and they were successfully delivered into the skin. In experiments with mice, the addition of CTA1 led to improved IgG immune response compared to the use of an aluminum hydroxide-based formulation and intramuscular administration of HBV. In addition, CTA1 induced CD8 + T cell response as much as in which the aluminum hydroxide-based formulation induced.

CONCLUSIONS

CTA1 is an adjuvant that satisfies both the delivery efficiency and the immunological characteristics required for vaccine microneedles. CTA1 will be used as a potential adjuvant through vaccine microneedles.

Non-Invasive Vaccines: Challenges in Formulation and Vaccine Adjuvants

Given the limitations of conventional invasive vaccines, such as the requirement for a cold chain system and trained personnel, needle-based injuries, and limited immunogenicity, non-invasive vaccines have gained significant attention. Although numerous approaches for formulating and administrating non-invasive vaccines have emerged, each of them faces its own challenges associated with vaccine bioavailability, toxicity, and other issues. To overcome such limitations, researchers have created novel supplementary materials and delivery systems. The goal of this review article is to provide vaccine formulation researchers with the most up-to-date information on vaccine formulation and the immunological mechanisms available, to identify the technical challenges associated with the commercialization of non-invasive vaccines, and to guide future research and development efforts.

A Single-Administration Microneedle Skin Patch for Multi-Burst Release of Vaccine against SARS-CoV-2

The necessity for multiple injections and cold-chain storage has contributed to suboptimal vaccine utilization, especially in pandemic situations. Thermally-stable and single-administration vaccines hold a great potential to revolutionize the global immunization process. Here, a new approach to thermally stabilize protein-based antigens is presented and a new high-throughput antigen-loading process is devised to create a single-administration, pulsatile-release microneedle (MN) patch which can deliver a recombinant SARS-CoV-2 S1-RBD protein-a model for the COVID-19 vaccine. Nearly 100% of the protein antigen could be stabilized at temperatures up to 100 °C for at least 1 h and at an average human body temperature (37 °C) for up to 4 months. Arrays of the stabilized S1-RBD formulations can be loaded into the MN shells via a single-alignment assembly step. The fabricated MNs are administered at a single time into the skin of rats and induce antibody response which could neutralize authentic SARS-CoV-2 viruses, providing similar immunogenic effect to that induced by multiple bolus injections of the same antigen stored in conventional cold-chain conditions. The MN system presented herein could offer the key solution to global immunization campaigns by avoiding low patient compliance, the requirement for cold-chain storage, and the need for multiple booster injections.

Microneedle patch as a new platform to effectively deliver inactivated polio vaccine and inactivated rotavirus vaccine

We recently reported a lack of interference between inactivated rotavirus vaccine (IRV) and inactivated poliovirus vaccine (IPV) and their potential dose sparing when the two vaccines were administered intramuscularly either in combination or standalone in rats and guinea pigs. In the present study, we optimized the formulations of both vaccines and investigated the feasibility of manufacturing a combined IRV-IPV dissolving microneedle patch (dMNP), assessing its compatibility and immunogenicity in rats. Our results showed that IRV delivered by dMNP alone or in combination with IPV induced similar levels of RV-specific IgG and neutralizing antibody. Likewise, IPV delivered by dMNP alone or in combination with IRV induced comparable levels of neutralizing antibody of poliovirus types 1, 2, and 3. We further demonstrated high stability of IRV-dMNP at 5, 25, and 40 °C and IPV-dMNP at 5 and 25 °C, and found that three doses of IRV or IPV when co-administered at a quarter dose was as potent as a full target dose in inducing neutralizing antibodies against corresponding rotavirus or poliovirus. We conclude that IRV-IPV dMNP did not interfere with each other in triggering an immunologic response and were highly immunogenic in rats. Our findings support the further development of this innovative approach to deliver a novel combination vaccine against rotavirus and poliovirus in children throughout the world.

Microneedle-Mediated Vaccination: Innovation and Translation

Vaccine administration by subcutaneous or intramuscular injection is the most commonly prescribed route for inoculation, however, it is often associated with some deficiencies such as low compliance, high professionalism, and risk of infection. Therefore, the application of microneedles for vaccine delivery has gained widespread interests in the past few years due to its high compliance, minimal invasiveness, and convenience. This review focuses on recent advances in the development and application of microneedles for vaccination based on different delivery strategies, and introduces the current status of microneedle-mediated vaccination in clinical translation. The prospects for its application including opportunities and challenges are further discussed.

Microneedle Systems for Vaccine Delivery: the story so far

INTRODUCTION

Vaccine delivery via a microneedle (MN) system has been identified as a potential alternative to conventional vaccine delivery. MN can be self-administered, is pain-free and is capable of producing superior immunogenicity. Over the last few decades, significant research has been carried out in this area, and this review aims to provide a comprehensive picture on the progress of this delivery platform.

AREAS COVERED

This review highlights the potential role of skin as a vaccine delivery route using a microneedle system, examines recent advancements in microneedle fabrication techniques, and provides an update on potential preclinical and clinical studies on vaccine delivery through microneedle systems against various infectious diseases. Articles for the review study were searched electronically in PubMed, Google, Google Scholar, and Science Direct using specific keywords to cover the scope of the article. The advanced search strategy was employed to identify the most relevant articles.

EXPERT OPINION

A significant number of MN mediated vaccine candidates have shown promising results in preclinical and clinical trials. The recent emergence of cleanroom free, 3D or additive manufacturing of MN systems and stability, together with the dose-sparing capacity of the Nanopatch® system, have made this platform, commercially, highly lucrative.

Progress in microneedle array patch (MAP) for vaccine delivery

A microneedle array patch (MAP) has been developed as a new delivery system for vaccines. Preclinical and clinical trials with a vaccine MAP showed improved stability, safety, and immunological efficacy compared to conventional vaccine administration. Various vaccines can be delivered with a MAP. Currently, microneedle manufacturers can mass-produce pharmaceutical MAP and cosmetic MAP and this mass-production system can be adapted to produce a vaccine MAP. Clinical trials with a vaccine MAP have shown comparable efficacy with conventional administration, and discussions about regulations for a vaccine MAP are underway. However, there are concerns of reasonable cost, mass production, efficacy, and safety standards that meet FDA approval, as well as the need for feedback regarding the best method of administration. Currently, microneedles have been studied for the delivery of many kinds of vaccines, and preclinical and clinical studies of vaccine microneedles are in progress. For the foreseeable future, some vaccines will continue to be administered with syringes and needles while the use of a vaccine MAP continues to be improved because of the advantages of less pain, self-administration, improved stability, convenience, and safety.

Materials for Immunotherapy

Breakthroughs in materials engineering have accelerated the progress of immunotherapy in preclinical studies. The interplay of chemistry and materials has resulted in improved loading, targeting, and release of immunomodulatory agents. An overview of the materials that are used to enable or improve the success of immunotherapies in preclinical studies is presented, from immunosuppressive to proinflammatory strategies, with particular emphasis on technologies poised for clinical translation. The materials are organized based on their characteristic length scale, whereby the enabling feature of each technology is organized by the structure of that material. For example, the mechanisms by which i) nanoscale materials can improve targeting and infiltration of immunomodulatory payloads into tissues and cells, ii) microscale materials can facilitate cell-mediated transport and serve as artificial antigen-presenting cells, and iii) macroscale materials can form the basis of artificial microenvironments to promote cell infiltration and reprogramming are discussed. As a step toward establishing a set of design rules for future immunotherapies, materials that intrinsically activate or suppress the immune system are reviewed. Finally, a brief outlook on the trajectory of these systems and how they may be improved to address unsolved challenges in cancer, infectious diseases, and autoimmunity is presented.

Immune response and reactogenicity of an unadjuvanted intradermally delivered human papillomavirus vaccine using a first generation Nanopatch™ in rhesus macaques: An exploratory, pre-clinical feasibility assessment

The human papillomavirus (HPV) 9-valent, recombinant vaccine (Gardasil™9) helps protect young adults (males and females) against anogenital cancers and genital warts caused by certain HPV genotypes (ref. Gardasil™9 insert). This vaccine is administered intramuscularly (IM). The aim of this study was to determine preclinically whether intradermal (ID) vaccination with an unadjuvanted 9-valent recombinant HPV vaccine using a first-generation ID delivery device, the Nanopatch™, could enhance vaccine immunogenicity compared with the traditional ID route (Mantoux technique). IM injection of HPV VLPs formulated with Merck & Co., Inc., Kenilworth, NJ, USA Alum Adjuvant (MAA) were included in the rhesus study for comparison. The Nanopatch™ prototype contains a high-density array comprised of 10,000 microprojections/cm², each 250 µm long. It was hypothesized the higher density array with shallower ID delivery may be superior to the Mantoux technique. To test this hypothesis, HPV VLPs without adjuvant were coated on the Nanopatch™, stability of the Nanopatch™ with unadjuvanted HPV VLPs were evaluated under accelerated conditions, skin delivery was verified using radiolabelled VLPs or FluoSpheres®, and the immune response and skin site reaction with the Nanopatch™ was evaluated in rhesus macaques. The immune response induced by Nanopatch™ administration, measured as HPV-specific binding antibodies, was similar to that induced using the Mantoux technique. It was also observed that a lower dose of unadjuvanted HPV VLPs delivered with the first-generation Nanopatch™ and applicator or Mantoux technique resulted in an immune response that was significantly lower compared to a higher-dose of alum adjuvanted HPV VLPs delivered IM in rhesus macaques. The study also indicated unadjuvanted HPV VLPs could be delivered with the first-generation Nanopatch™ and applicator to the skin in 15 s with a transfer efficiency of approximately 20%. This study is the first demonstration of patch administration in non-human primates with a vaccine composed of HPV VLPs.

Phosphorothioated antisense oligodeoxynucleotide suppressing interleukin-10 is a safe and potent vaccine adjuvant

While vaccination is highly effective for the prevention of many infectious diseases, the number of adjuvants licensed for human use is currently very limited. The aim of this study was to evaluate the safety, efficacy, and to clarify the mechanism of a phosphorothioated interleukin (IL)-10-targeted antisense oligonucleotide (ASO) as an immune adjuvant in intradermal vaccination. The cytotoxicity of IL-10 ASO and its ability to promote T cell proliferation were assessed by Cell Counting Kit-8 (CCK-8) assay. The contents of IL-6, IL-8, TNF-α, IL-1β, and IL-10 in inoculated local tissue and the antigen-specific antibody titers in mouse serum samples were determined by ELISA. The target cells of IL-10 ASO were observed using immunofluorescent staining. The results showed that the specific antibody titer of ovalbumin (OVA), a model antigen, was increased 100-fold upon addition of IL-10 ASO as an adjuvant compared to that of OVA alone. IL-10 ASO showed an immunopotentiation efficacy similar to that of Freund's incomplete adjuvant, with no detectable cell or tissue toxicity. In vitro and in vivo experiments confirmed that IL-10 ASO enhances immune responses by temporarily suppressing IL-10 expression from local dendritic cells and consequently promoting T cell proliferation. In conclusion, IL-10 ASO significantly enhances immune responses against co-delivered vaccine antigens with high efficacy and low toxicity. It has the potential to be developed into a safe and efficient immune adjuvant.

Coated and Hollow Microneedle-Mediated Intradermal Immunization in Mice with Diphtheria Toxoid Loaded Mesoporous Silica Nanoparticles

PURPOSE

To examine the immunogenicity of diphtheria toxoid (DT) loaded mesoporous silica nanoparticles (MSNs) after coated and hollow microneedle-mediated intradermal immunization in mice.

METHODS

DT was loaded into MSNs and the nanoparticle surface was coated with a lipid bilayer (LB-MSN-DT). To prepare coated microneedles, alternating layers of negatively charged LB-MSN-DT and positively charged N-trimethyl chitosan (TMC) were coated onto pH-sensitive microneedle arrays via a layer-by-layer approach. Microneedle arrays coated with 5 or 3 layers of LB-MSN-DT were used to immunize mice and the elicited antibody responses were compared with those induced by hollow microneedle-injected liquid formulation of LB-MSN-DT. Liquid DT formulation with and without TMC (DT/TMC) injected by a hollow microneedle were used as controls.

RESULTS

LB-MSN-DT had an average size of about 670 nm and a zeta potential of -35 mV. The encapsulation efficiency of DT in the nanoparticles was 77%. The amount of nano-encapsulated DT coated onto the microneedle array increased linearly with increasing number of the coating layers. Nano-encapsulated DT induced stronger immune responses than DT solution when delivered intradermally via hollow microneedles, but not when delivered via coated microneedles.

CONCLUSION

Both the nano-encapsulation of DT and the type of microneedles affect the immunogenicity of the antigen.

Microneedle arrays for vaccine delivery: the possibilities, challenges and use of nanoparticles as a combinatorial approach for enhanced vaccine immunogenicity

INTRODUCTION

Vaccination is one of the greatest breakthroughs of modern preventative medicine. Despite this, there remain problems surrounding delivery, efficacy and compliance. Thus, there is a pressing need to develop cost-effective vaccine delivery systems that could expand the use of vaccines, particularly within developing countries. Microneedle (MN) arrays, given their ease of use, painlessness and ability to target skin antigen presenting cells, provide an attractive platform for improved vaccine delivery and efficacy. Studies have demonstrated enhanced immunogenicity with the use of MN in comparison to conventional needle. More recently, dissolving MN have been used for efficient delivery of nanoparticles (NP), as a means to enhance antigen immunogenicity.

AREAS COVERED

This review introduces the fields of MN technology and nanotechnology, highlighting the recent advances which have been made with these two technologies combined for enhanced vaccine delivery and efficacy. Some key questions that remain to be addressed for adoption of MN in a clinical setting are also evaluated.

EXPERT OPINION

MN-mediated vaccine delivery holds potential for expanding access to vaccines, with individuals in developing countries likely to be the principal beneficiaries. The combinatorial approach of utilizing MN coupled with NP, provides opportunities to enhance the immunogenicity of vaccine antigens.

Immunogenicity of diphtheria toxoid and poly(I:C) loaded cationic liposomes after hollow microneedle-mediated intradermal injection in mice

In this study, we aimed to investigate the immunogenicity of cationic liposomes loaded with diphtheria toxoid (DT) and poly(I:C) after hollow microneedle-mediated intradermal vaccination in mice. The following liposomal formulations were studied: DT loaded liposomes, a mixture of free DT and poly(I:C)-loaded liposomes, a mixture of DT-loaded liposomes and free poly(I:C), and liposomal formulations with DT and poly(I:C) either individually or co-encapsulated in the liposomes. Reference groups were DT solution adjuvanted with or without poly(I:C) (DT/poly(I:C)). The liposomal formulations were characterized in terms of particle size, zeta potential, loading and release of DT and poly(I:C). After intradermal injection of BALB/c mice with the formulations through a hollow microneedle, the immunogenicity was assessed by DT-specific ELISAs. All formulations induced similar total IgG and IgG1 titers. However, all the liposomal groups containing both DT and poly(I:C) showed enhanced IgG2a titers compared to DT/poly(I:C) solution, indicating that the immune response was skewed towards a Th1 direction. This enhancement was similar for all liposomal groups that contain both DT and poly(I:C) in the formulations. Our results reveal that a mixture of DT encapsulated liposomes and poly(I:C) encapsulated liposomes have a similar effect on the antibody responses as DT and poly(I:C) co-encapsulated liposomes. These findings may have implications for future design of liposomal vaccine delivery systems.

Insulin delivery systems combined with microneedle technology

Diabetes, a metabolic disorder of glucose, is a serious chronic disease and an important public health problem. Insulin is one of the hormones for modulating blood glucose level and the products of which is indispensable for most diabetes patients. Introducing microneedles (MNs) to insulin delivery is promising to pave the way for modulating glucose level noninvasively of diabetes patients, as which born to be painless, easy to handle and no need of any power supply. In this work, we review the process of insulin delivery systems (IDSs) based on MN technology in terms of two categories: drug free MNs and drug loaded MNs. Drug free MNs include solid MNs ("poke and patch"), hollow MNs ("poke and flow") and reservoir-based swelling MNs ("poke and swell R-type"), and drug loaded MNs include coated MNs ("coat and poke"), dissolving MNs ("poke and release") and insulin incorporated swelling MNs ("poke and swell I-type"). Majority researches of MN-based IDSs have been conducted by using hollow MNs or dissolving MNs, and almost all clinical trials for MN-based IDSs have employed hollow MNs. "Poke and patch" approach dramatically increase skin permeability compared to traditional transdermal patch, but MNs fabricated from silicon or metal may leave sharp waste in the skin and cause a safety issue. "Poke and flow" approach, similar to transitional subcutaneous (SC) injection, is capable of producing faster insulin absorption and action than SC injection but may associate with blockage, leakage and low flow rate. Coated MNs are able of retaining the activity of drug, which loaded in a solid phase, for a long time, however have been relatively less studied for insulin application as the low drug dosing. "Poke and release" approach leaves no biohazardous sharp medical waste and is capable of rapid drug release. "Poke and swell R-type" can be seen as a combination of "poke and flow" and "poke and patch" approach, while "poke and swell I-type" is an approach between "coat and poke" and "poke and release" approach. Insulin MNs are promising for painless diabetes therapeutics, and additional efforts for addressing fundamental issues including the drug loading, the PK/PD profile, the storage and the safety of insulin MNs will accelerate the clinical transformation.

Novel strategy for immunomodulation: Dissolving microneedle array encapsulating thymopentin fabricated by modified two-step molding technology

Thymopentin (TP5) is commonly used in the treatment for autoimmune diseases, with a short plasma half-life (30s) and a long treatment period (7 days to 6 months). It is usually administrated by syringe injection, resulting in compromised patient compliance. Dissolving microneedle array (DMNA) offers a superior approach for transdermal delivery of biological macromolecules, as it allows painless penetration through the stratum corneum and generates minimal biohazardous waste after dissolving in the skin. Despite recent advances in DMNA as a novel approach for transdermal drug delivery, problem of insufficient mechanical strength remains to be solved. In this study, TP5-loaded DMNA (TP5-DMNA) was uniquely developed using a modified two-step molding technology. The higher mechanical strength was furnished by employing bovine serum albumin (BSA) as a co-material to fabricate the needles. The obtained TP5-DMNA containing BSA displayed better skin penetration and higher drug loading efficiency than that without BSA. The in vivo pharmacodynamics study demonstrated that TP5-DMNA had comparative effect on immunomodulation to intravenous injection of TP5, in terms of ameliorating the CD4+/CD8+ ratio, SOD activity and MDA value to the basal level. Only mild irritation was observed at the site of administration. These results suggest that the novel TP5-DMNA utilizing BSA provides an alternative approach for convenient and safe transdermal delivery of TP5, which is a promising administration strategy for future clinical application.

Microfabrication for Drug Delivery

This review is devoted to discussing the application of microfabrication technologies to target challenges encountered in life processes by the development of drug delivery systems. Recently, microfabrication has been largely applied to solve health and pharmaceutical science issues. In particular, fabrication methods along with compatible materials have been successfully designed to produce multifunctional, highly effective drug delivery systems. Microfabrication offers unique tools that can tackle problems in this field, such as ease of mass production with high quality control and low cost, complexity of architecture design and a broad range of materials. Presented is an overview of silicon- and polymer-based fabrication methods that are key in the production of microfabricated drug delivery systems. Moreover, the efforts focused on studying the biocompatibility of materials used in microfabrication are analyzed. Finally, this review discusses representative ways microfabrication has been employed to develop systems delivering drugs through the transdermal and oral route, and to improve drug eluting implants. Additionally, microfabricated vaccine delivery systems are presented due to the great impact they can have in obtaining a cold chain-free vaccine, with long-term stability. Microfabrication will continue to offer new, alternative solutions for the development of smart, advanced drug delivery systems.

The success of microneedle-mediated vaccine delivery into skin

Microneedles (MNs) are designed to specifically target the outermost, skin barrier layer, the stratum corneum, creating transient pathways for minimally invasive transcutaneous delivery. It is reported that MNs can facilitate delivery without stimulating the pain receptors or damaging blood vessels that lie beneath, thus being perceived as painless and associated with reduced bleeding. This immunocompetence of the skin, coupled with its ease of access, makes this organ an attractive vaccination site. The purpose of this review was to collate primary scientific literature pertaining to MN-mediated in vivo vaccination programmes. A total of 62 original research articles are presented, compiling vaccination strategies in 6 different models (mouse, rat, guinea pig, rabbit, pig, macaque and human). Vaccines tested span a wide range of viral, bacterial and protozoan pathogens and includes 7 of the 13 vaccine-preventable diseases, as defined by the WHO. This review highlights the paucity of available clinical trial data. MN-delivered vaccines have demonstrated safety and immunogenicity in pre-clinical models and boast desirable attributes such as painless administration, thermostability, dose-sparing capacity and the potential for self-administration. These advantages should contribute to enhanced global vaccine access.

Microneedle patches for vaccination in developing countries

Millions of people die of infectious diseases each year, mostly in developing countries, which could largely be prevented by the use of vaccines. While immunization rates have risen since the introduction of the Expanded Program on Immunization (EPI), there remain major challenges to more effective vaccination in developing countries. As a possible solution, microneedle patches containing an array of micron-sized needles on an adhesive backing have been developed to be used for vaccine delivery to the skin. These microneedle patches can be easily and painlessly applied by pressing against the skin and, in some designs, do not leave behind sharps waste. The patches are single-dose, do not require reconstitution, are easy to administer, have reduced size to simplify storage, transportation and waste disposal, and offer the possibility of improved vaccine immunogenicity, dose sparing and thermostability. This review summarizes vaccination challenges in developing countries and discusses advantages that microneedle patches offer for vaccination to address these challenges. We conclude that microneedle patches offer a powerful new technology that can enable more effective vaccination in developing countries.

Nucleic acid delivery into skin for the treatment of skin disease: Proofs-of-concept, potential impact, and remaining challenges

Nucleic acids (NAs) hold significant potential for the treatment of several diseases. Topical delivery of NAs for the treatment of skin diseases is especially advantageous since it bypasses the challenges associated with systemic administration which suffers from enzymatic degradation, systemic toxicity and lack of targeting to skin. However, the skin's protective barrier function limits the delivery of NAs into skin after topical application. Here, we highlight strategies for enhancing delivery of NAs into skin, and provide evidence that translation of topical NA therapies could have a transformative impact on the treatment of skin diseases.

Microneedles: bench to bedside

Microneedles are tiny micron-sized structures, made of a variety of materials, used to minimally disrupt the outermost layer of the skin for enhancing the delivery of therapeutic molecules across the skin. They are sufficiently long enough just to breach the stratum corneum barrier but too short to reach the nerve endings that perceive pain. Treating the skin using microneedles results in the creation of aqueous microchannels that promote delivery of molecules practically of any size. Small molecules, proteins, vaccines and diagnostic agents can be delivered using microneedles. This technology that has started with microstructures made of metal and silicon has now undergone significant advances in the last decade and currently there are microneedle products in the market.

Inactivated polio vaccination using a microneedle patch is immunogenic in the rhesus macaque

The phased replacement of oral polio vaccine (OPV) with inactivated polio vaccine (IPV) is expected to significantly complicate mass vaccination campaigns, which are an important component of the global polio eradication endgame strategy. To simplify mass vaccination with IPV, we developed microneedle patches that are easy to administer, have a small package size, generate no sharps waste and are inexpensive to manufacture. When administered to rhesus macaques, neutralizing antibody titers were equivalent among monkeys vaccinated using microneedle patches and conventional intramuscular injection for IPV types 1 and 2. Serologic response to IPV type 3 vaccination was weaker after microneedle patch vaccination compared to intramuscular injection; however, we suspect the administered type 3 dose was lower due to a flawed pre-production IPV type 3 analytical method. IPV vaccination using microneedle patches was well tolerated by the monkeys. We conclude that IPV vaccination using a microneedle patch is immunogenic in rhesus macaques and may offer a simpler method of IPV vaccination of people to facilitate polio eradication.

Animal models for cutaneous vaccine delivery

Main challenges in skin vaccination are overcoming the stratum corneum (SC) barrier and targeting the antigen presenting cells (APC) in the epidermis and the dermis. For this purpose many delivery techniques are being developed. In vivo immunogenicity and safety studies in animals are mandatory before moving to clinical trials. However, the results obtained in animals may or may not be predictive for humans. Knowledge about differences and similarities in skin architecture and immunology within a species and between species is crucial. In this review, we discuss variables, including skin morphology, skin barrier function, mechanical properties, site of application and immunology, which should be taken into account when designing animal studies for vaccination via the skin in order to support the translation to clinical trial outcomes.

Microneedle patches for vaccine delivery

In today's medical industry, the range of vaccines that exist for administration in humans represents an eclectic variety of forms and immunologic mechanisms. Namely, these are the live attenuated viruses, inactivated viruses, subunit proteins, and virus-like particles for treating virus-caused diseases, as well as the bacterial-based polysaccharide, protein, and conjugated vaccines. Currently, a new approach to vaccination is being investigated with the concept of DNA vaccines. As an alternative delivery route to enhance the vaccination efficacy, microneedles have been devised to target the rich network of immunologic antigen-presenting cells in the dermis and epidermis layers under the skin. Numerous studies have outlined the parameters of microneedle delivery of a wide range of vaccines, revealing comparable or higher immunogenicity to conventional intramuscular routes, overall level of stability, and dose-sparing advantages. Furthermore, recent mechanism studies have begun to successfully elucidate the biological mechanisms behind microneedle vaccination. This paper describes the current status of microneedle vaccine research.

Transcutaneous vaccines--current and emerging strategies

INTRODUCTION

Vaccination, which is the major fundamental prophylaxis against illness and death from infectious disease, has greatly contributed to the global improvement of human health. However, the disadvantages of conventional injection systems hamper the delivery of vaccination technologies to developing countries. The imminent practice of easy-to-use vaccination methods is expected to overcome certain issues associated with injectable vaccinations. One innovative method is the transcutaneous immunization (TCI) system.

AREAS COVERED

Two major strategies for TCI are discussed in this review. One is to promote antigen permeation of the skin barrier by patch systems or nanoparticles. The other is the delivery of antigens into the skin by electroporation and microneedles in order to physically overcome the skin barrier. Moreover, adjuvant development for TCI is discussed.

EXPERT OPINION

Many different approaches have been developed for TCI, which have the potential to be effective, easy-to-use and painless methods of vaccination. However, in practical terms, the guidelines concerning the manufacturing processes and clinical trial evaluation of the procedures have not kept pace with the development of these novel formulations. The accumulation of information regarding skin characteristics and the properties of TCI devices will help refine TCI system development guidelines and thus lead to the improvement of transcutaneous vaccination.

Microneedles for drug and vaccine delivery

Microneedles were first conceptualized for drug delivery many decades ago, but only became the subject of significant research starting in the mid-1990's when microfabrication technology enabled their manufacture as (i) solid microneedles for skin pretreatment to increase skin permeability, (ii) microneedles coated with drug that dissolves off in the skin, (iii) polymer microneedles that encapsulate drug and fully dissolve in the skin and (iv) hollow microneedles for drug infusion into the skin. As shown in more than 350 papers now published in the field, microneedles have been used to deliver a broad range of different low molecular weight drugs, biotherapeutics and vaccines, including published human studies with a number of small-molecule and protein drugs and vaccines. Influenza vaccination using a hollow microneedle is in widespread clinical use and a number of solid microneedle products are sold for cosmetic purposes. In addition to applications in the skin, microneedles have also been adapted for delivery of bioactives into the eye and into cells. Successful application of microneedles depends on device function that facilitates microneedle insertion and possible infusion into skin, skin recovery after microneedle removal, and drug stability during manufacturing, storage and delivery, and on patient outcomes, including lack of pain, skin irritation and skin infection, in addition to drug efficacy and safety. Building off a strong technology base and multiple demonstrations of successful drug delivery, microneedles are poised to advance further into clinical practice to enable better pharmaceutical therapies, vaccination and other applications.

Microneedle-mediated vaccine delivery: harnessing cutaneous immunobiology to improve efficacy

INTRODUCTION

Breaching the skin's stratum corneum barrier raises the possibility of the administration of vaccines, gene vectors, antibodies and even nanoparticles, all of which have at least their initial effect on populations of skin cells.

AREAS COVERED

Intradermal vaccine delivery holds enormous potential for improved therapeutic outcomes for patients, particularly those in the developing world. Various vaccine-delivery strategies have been employed, which are discussed in this review. The importance of cutaneous immunobiology on the effect produced by microneedle-mediated intradermal vaccination is also discussed.

EXPERT OPINION

Microneedle-mediated vaccines hold enormous potential for patient benefit. However, in order for microneedle vaccine strategies to fulfill their potential, the proportion of an immune response that is due to the local action of delivered vaccines on skin antigen-presenting cells, and what is due to a systemic effect from vaccines reaching the systemic circulation, must be determined. Moreover, industry will need to invest significantly in new equipment and instrumentation in order to mass-produce microneedle vaccines consistently. Finally, microneedles will need to demonstrate consistent dose delivery across patient groups and match this to reliable immune responses before they will replace tried-and-tested needle-and-syringe-based approaches.

An update on the application of physical technologies to enhance intradermal and transdermal drug delivery

A large number of biopharmaceuticals and other macromolecules are being developed for therapeutic applications. Conventional oral delivery is not always possible due to first-pass metabolism and degradation in the GI tract. Parenteral delivery is invasive and has poor patient compliance. Transdermal delivery provides one attractive route of administration. Transdermal administration can achieve the continuous and non-invasive delivery of drugs. However, passive transdermal delivery is restricted to small lipophilic molecules. Active physical-enhancement technologies are being investigated to increase the scope of transdermal delivery to hydrophilic molecules and macromolecules. Recent developments in transdermal technologies, such as microporation, iontophoresis and sonophoresis can enable therapeutic delivery of many drug molecules, biopharmaceuticals, cosmeceuticals and vaccines. This review provides an update of recent developments in transdermal delivery focusing on physical-enhancement technologies.

A combined approach of vesicle formulations and microneedle arrays for transcutaneous immunization against hepatitis B virus

In the search for an optimal approach for the transcutaneous immunization (TCI) of hepatitis B surface antigen (HBsAg), two vesicle formulations, L595 vesicles (composed of sucrose-laurate ester and octaoxyethylene-laurate ester) and sPC vesicles (composed of soybean-phosphatidylcholine and Span-80) were prepared and characterized in vitro and in vivo. HBsAg was associated to the vesicles, resulting in sPC-HBsAg vesicles (±170nm) with 79% HBsAg association and L595-HBsAg vesicles (±75nm) with only 29% HBsAg association. The vesicles induced in mice via TCI an antibody response only when the skin was pretreated with microneedles. This response was improved by the adjuvant cholera toxin. The sPC-HBsAg vesicle formulations showed to be the most immunogenic for TCI, which was related to the higher degree of HBsAg association.

Effect of vesicle size on tissue localization and immunogenicity of liposomal DNA vaccines

The formulation of plasmid DNA (pDNA) in cationic liposomes is a promising strategy to improve the potency of DNA vaccines. In this respect, physicochemical parameters such as liposome size may be important for their efficacy. The aim of the current study was to investigate the effect of vesicle size on the in vivo performance of liposomal pDNA vaccines after subcutaneous vaccination in mice. The tissue distribution of cationic liposomes of two sizes, 500 nm (PDI 0.6) and 140 nm (PDI 0.15), composed of egg PC, DOPE and DOTAP, with encapsulated OVA-encoding pDNA, was studied by using dual radiolabeled pDNA-liposomes. Their potency to elicit cellular and humoral immune responses was investigated upon application in a homologous and heterologous vaccination schedule with 3 week intervals. It was shown that encapsulation of pDNA into cationic lipsomes resulted in deposition at the site of injection, and strongest retention was observed at large vesicle size. The vaccination studies demonstrated a more robust induction of OVA-specific, functional CD8+ T-cells and higher antibody levels upon vaccination with small monodisperse pDNA-liposomes, as compared to large heterodisperse liposomes or naked pDNA. The introduction of a PEG-coating on the small cationic liposomes resulted in enhanced lymphatic drainage, but immune responses were not improved when compared to non-PEGylated liposomes. In conclusion, it was shown that the physicochemical properties of the liposomes are of crucial importance for their performance as pDNA vaccine carrier, and cationic charge and small size are favorable properties for subcutaneous DNA vaccination.

Delivery systems for intradermal vaccination

Intradermal (ID) vaccination can offer improved immunity and simpler logistics of delivery, but its use in medicine is limited by the need for simple, reliable methods of ID delivery. ID injection by the Mantoux technique requires special training and may not reliably target skin, but is nonetheless used currently for BCG and rabies vaccination. Scarification using a bifurcated needle was extensively used for smallpox eradication, but provides variable and inefficient delivery into the skin. Recently, ID vaccination has been simplified by introduction of a simple-to-use hollow microneedle that has been approved for ID injection of influenza vaccine in Europe. Various designs of hollow microneedles have been studied preclinically and in humans. Vaccines can also be injected into skin using needle-free devices, such as jet injection, which is receiving renewed clinical attention for ID vaccination. Projectile delivery using powder and gold particles (i.e., gene gun) have also been used clinically for ID vaccination. Building off the scarification approach, a number of preclinical studies have examined solid microneedle patches for use with vaccine coated onto metal microneedles, encapsulated within dissolving microneedles or added topically to skin after microneedle pretreatment, as well as adapting tattoo guns for ID vaccination. Finally, technologies designed to increase skin permeability in combination with a vaccine patch have been studied through the use of skin abrasion, ultrasound, electroporation, chemical enhancers, and thermal ablation. The prospects for bringing ID vaccination into more widespread clinical practice are encouraging, given the large number of technologies for ID delivery under development.

Pentapeptide commonality between Corynebacterium diphtheriae toxin and the Homo sapiens proteome

Cross-reactivity may affect diagnostic tests and cause harmful autoimmune reactions following immunotherapy. To predict potential cross-reactivity and search for safe immunotherapeutic approaches, we analyzed sequence identity between microbial antigens and the human proteome. Using diphtheria toxin (DT) as a model, we examined its patterns of identity with human proteins at the pentapeptide level. DT shares 503 pentapeptides with the human proteome, while only 31 pentapeptides are unique to the toxin. DT pentapeptide identity involves multiple/repeated matches in human proteins (a total of 4966 occurrences). Human proteins containing bacterial peptide matches include antigens linked to fundamental cellular functions, such as cell cycle control, proliferation, development and differentiation. The data presented in this article offer a rational basis for designing peptide-based vaccines that specifically target DT and thus eliminate the potential risk of cross-reactivity with human proteins. More generally, this study proposes a methodological approach for avoiding cross-reactivity in immune reactions.

Dose sparing enabled by skin immunization with influenza virus-like particle vaccine using microneedles

To address the limitations of conventional influenza vaccine manufacturing and delivery, this study investigated administration of virus-like particle (VLP) influenza vaccine using a microneedle patch. The goal was to determine if skin immunization with influenza VLP vaccine using microneedles enables dose sparing. We found that low-dose influenza (A/PR/8/34 H1N1) VLP vaccination using microneedles was more immunogenic than low-dose intramuscular (IM) vaccination and similarly immunogenic as high-dose IM vaccination in a mouse model. With a 1μg dose of vaccine, both routes showed similar immune responses and protective efficacy, with microneedle vaccination being more effective in inducing recall antibody responses in lungs and antibody secreting cells in bone marrow. With a low dose of vaccine (0.3μg), microneedle vaccination induced significantly superior protective immunity, which included binding and functional antibodies as well as complete protection against a high dose lethal infection with A/PR/8/34 virus, whereas IM immunization provided only partial (40%) protection. Therefore, this study demonstrates that microneedle vaccination in the skin confers more effective protective immunity at a lower dose, thus providing vaccine dose-sparing effects.

Influence of microneedle shape on the transport of a fluorescent dye into human skin in vivo

Microneedles can enhance the penetration of vaccines into the skin for transcutaneous vaccination. In this study for the first time the influence of microneedle geometry on the transport through the formed conduits was visualised in human volunteers by confocal laser scanning microscopy. Three differently shaped 300 μm long microneedle arrays were selected and fluorescein was applied either before or after piercing. Based on the intensity a distinction was made between regions with high and low intensity fluorescence (HIF and LIF). The areas of both intensities were quantified over time. In most cases HIF areas were only present in the stratum corneum, while LIF areas were also present in the viable epidermis. The areas were larger if fluorescein was applied after piercing compared to before piercing. After 15 min almost no HIF was present anymore at the skin surface. The microneedle geometry, but not the manner of application affected the shape and depth of the conduits. In conclusion we showed that the different microneedle arrays are able to form conduits in the skin, but the geometry of the microneedles influences the penetration of the fluorescent dye. This is the first step towards a more rational design of microneedle arrays for transcutaneous vaccination.