RESEARCH HIGHLIGHT

Enabling skin vaccination using new delivery technologies

Yeu-Chun Kim & Mark R. Prausnitz

Published online: 14 December 2010# Controlled Release Society 2010

Abstract The skin is known to be a highly immunogenicsite for vaccination, but few vaccines in clinical use targetskin largely because conventional intradermal injection isdifficult and unreliable to perform. Now, a number of newor newly adapted delivery technologies have been shownto administer vaccine to the skin either by non-invasive orminimally invasive methods. Non-invasive methods in-clude high-velocity powder and liquid jet injection, aswell as diffusion-based patches in combination with skinabrasion, thermal ablation, ultrasound, electroporation, andchemical enhancers. Minimally invasive methods aregenerally based on small needles, including solid micro-needle patches, hollow microneedle injections, and tattooguns. The introduction of these advanced delivery tech-nologies can make the skin a site for simple, reliablevaccination that increases vaccine immunogenicity andoffers logistical advantages to improve the speed andcoverage of vaccination.

Keywords Vaccine . Skin vaccination . Intradermalvaccination .Microneedle . Review

Introduction

Morbidity and mortality due to infectious diseases havebeen dramatically reduced by improved vaccination, whichis the most cost-effective public health measure to preventspread of disease (Lambert et al. 2005; Levine and Sztein2004). Most vaccines are given by intramuscular injectioneven though the muscle is not a highly immunogenic organ(Hutin et al. 2003; Hohlfeld and Engel 1994). The skin, incontrast, is a much more attractive site for vaccination froman immunologic perspective because of its many residentdendritic cells and efficient drainage to lymph nodes(Debenedictis et al. 2001; Kupper and Fuhlbrigge 2004).However, skin vaccination has made relatively little impacton medical practice because intradermal injection requiresspecialized training and, even with training, does notreliably target the skin (Flynn et al. 1994; Mitragotri 2005).

Skin vaccination was used heavily during the smallpoxeradication campaign by employing the bifurcated needlewith two sharp vaccine-holding prongs that are repeatedlyinserted into the skin to deposit a dose of live vaccine in theskin (Baxby 2002). Although the bifurcated needle is easyto administer, the small and variable dose it delivers limitsits continued use for vaccination. Bacillus Calmette-Guérinvaccine against tuberculosis is currently administeredintradermally using the Mantoux method, in which aconventional hypodermic needle is inserted at a shallowangle into the skin (Andersen and Doherty 2005; Flynn etal. 1994). This method requires specially trained personneland typically achieves inconsistent delivery, which hasmotivated some to recommend abandoning intradermalinjection in favor of a simple-to-use percutaneous puncturedevice (Hawkridge et al. 2008). Since 1991, the WorldHealth Organization has recommended intradermal injec-tion using the Mantoux method as a cost-saving measure in

Y.-C. Kim :M. R. Prausnitz (*)School of Chemical and Biomolecular Engineering,Georgia Institute of Technology,Atlanta, GA 30332, USAe-mail: prausnitz@gatech.edu

Y.-C. KimDepartment of Chemical and Biomolecular Engineering,Korea Advanced Institute of Science and Technology (KAIST),Daejeon, Republic of Korea

Drug Deliv. and Transl. Res. (2011) 1:7–12DOI 10.1007/s13346-010-0005-z

developing countries for vaccination against rabies becausefractional doses of vaccine are effective when injected inthe skin (WHO 2010).

Many more vaccines would be candidates for vaccina-tion via the skin if simple, reliable methods of intradermaldelivery were available. Because skin is often the firstorgan of the body to face microbial or viral invasion, skinprotects the body from infection using not only its physicalbarrier of the stratum corneum layer but also its strongimmunological function enabled by resident antigen-presenting cells (Kupper and Fuhlbrigge 2004). Langerhanscells in epidermis and dermal dendritic cells in the dermisare the main immunological skin cells with the essentialrole to capture foreign antigens and present them indraining lymph nodes. Additionally, skin keratinocytesand other cells in epidermis and dermis produce cytokinesand chemokines which can stimulate and control immuneresponses. Antigen trafficking studies have shown thatvaccination through the skin leads to more efficient antigenmigration into lymph nodes than conventional intramuscu-lar delivery (Steinman and Banchereau 2007; Valladeau andSaeland 2005; Sugita et al. 2007).

Vaccine dose sparing by delivery via the skin is wellestablished in clinical practice for rabies vaccine and hasbeen demonstrated in many clinical trials for a number ofother vaccines as well, which is a practical outcome ofskin’s enhanced immunogenicity (PATH 2009). Morerecently, intradermal influenza vaccine has been introducedin Europe and other parts of the world due to its improvedprotective immunity seen in the elderly compared toconventional intramuscular vaccination (Arnou et al.2009). In this case, intradermal injection of a reformulatedvaccine is achieved by a novel microneedle syringe thatenables medical personnel to reliably inject into the skinwith minimal additional training (Laurent et al. 2007). Asdiscussed below, a number of novel non-invasive andminimally invasive technologies have been developed forskin vaccination. These technologies are poised to nowmake the skin a viable route for vaccination.

Non-invasive skin vaccination

Hypodermic needles are not only difficult to use forintradermal injection but also their intentional re-use andunintentional needle sticks cause more than one half milliondeaths annually due to transmission of HIV and hepatitis Band C from dirty needles (WHO 2004). Thus, an ideal skinvaccination method would not only be reliable but wouldalso eliminate the dangers and pain associated withhypodermic needles. Non-invasive skin vaccination meth-ods seek to achieve this ideal by eliminating the needle andreplacing it with methods to increase skin permeability that

do not involve the generation of sharps waste (Mitragotri2005).

Liquid jet injection is the best known needle-freevaccination method, which involves directing a pressurizedliquid to make a pathway into the skin and thereby depositvaccine intradermally. This method was in widespread usein the mid-twentieth century for intramuscular and subcu-taneous vaccinations and has been adapted for intradermalinjections too. Intradermal jet injection is gaining renewedinterest and is the subject of clinical trials for inactivatedpoliovirus vaccination (Mohammed et al. 2010).

Epidermal powder immunization (EPI) is conceptuallysimilar to liquid jet injection using high pressure flow, butaccelerates dried-powder particles of vaccine, rather thanliquid, into the skin at supersonic speed. Immunization viathis route has been shown to derive similar immuneresponses compared to intramuscular immunization inhuman study (Dean and Chen 2004). As another variationon this theme, particle-mediated epidermal delivery(PMED) administers DNA vaccine coated on gold micro-particles that are shot into the skin. Clinical studies ofPMED immunization showed promising results but lessimmunogenic responses compared to standard vaccinedelivery methods (Jones et al. 2009).

These first methods described above actively depositvaccine into the skin. Other approaches seek to remove theskin’s main barrier, stratum corneum, as a first step, and thenallow vaccine entry into the skin by subsequent diffusionfrom a patch or other topical formulation. The stratumcorneum can be removed by abrasive methods, such astape-stripping or sanding with emery paper. Transcutaneousimmunization in this way induced robust humoral andmucosal immune response and migration of activatedantigen-presenting cell from skin to draining lymph nodes(Guebre-Xabier et al. 2003). Clinical trials using thisapproach seek to develop a new vaccine against traveler’sdiarrhea and influenza (Frech et al. 2005; Frerichs et al.2008). Skin abrasion using a razor and a toothbrush alsoshowed promising human clinical results (Van Kampen etal. 2005). Microdermabrasion is a cosmetic approach toremove stratum corneum; a recent study demonstratedvaccination using this approach in animals (Gill et al. 2009).

Other approaches developed for transdermal delivery ofdrugs have also been adapted for skin vaccination. Forexample, thermal ablation generates microscopic holes instratum corneum by vaporizing it at high temperatureproduced by highly focused thermal energy (Park et al.2008). Resistive heating from electrical energy or radio-frequency has been developed and shown to enableprotective immune responses to vaccine antigens in animalstudies (Bramson et al. 2003; Fagnoni et al. 2008).

Ultrasound has also been shown to increase skin perme-ability and thereby used for vaccine delivery into the skin. The

8 Drug Deliv. and Transl. Res. (2011) 1:7–12

effects of ultrasound not only enhanced the vaccine deliverybut also stimulated activation of antigen-presenting cells in theepidermis in animals (Dahlan et al. 2009).

Electroporation has been used for transdermal drugdelivery by increasing skin permeability and one studydelivered peptide-based vaccine using an electroporation asa permeability-enhancing tool (Zhao et al. 2006), butmostly, electroporation has been used primarily to increasepermeability of skin cells to enhance intracellular deliveryof DNA vaccine for increased cell transfection for effectiveantigen protein production (Drabick et al. 2001). Electro-poration has been used to derive effective immuneresponses in DNA vaccine studies in animal and morerecently in human clinical trials for DNA vaccinationagainst prostate cancer (http://clinicaltrials.gov/ct2/show/NCT00859729).

The use of chemical enhancers is the best known methodto disrupt stratum corneum lipid structure and therebyincreases skin permeability, mostly only for small mole-cules. However, a recent study has shown the potential useof mixtures of chemical enhancers as a vaccination toolthat, when properly optimized through high-throughputscreening, can not only increase skin permeability to thevaccine antigen but also play a novel role as an adjuvant(Karande et al. 2009).

Overall, non-invasive vaccination methods mostly fallinto two categories. The high-velocity, needle-free injectionsystems are generally able to quickly and efficiently drivevaccines into the skin while avoiding the dangers ofhypodermic needles. These methods do, however, oftenrequire bulky devices, can cause pain similar to hypodermicneedles, and sometimes do not achieve reliable injections(Baxter and Mitragotri 2006, Hogan et al. 2010). The othernon-invasive methods increase skin permeability to varyingextents and then require a slow and often inefficient processof vaccine antigen diffusion into the skin that typicallyleaves a large fraction of the vaccine behind on the skinsurface (Prausnitz and Langer 2008).

Minimally invasive skin vaccination

To overcome the limitations of non-invasive skin vaccina-tion methods while still avoiding the dangers and unpleas-antness of hypodermic needles, minimally invasivemethods to administer vaccine to the skin have beendeveloped, primarily using very small hollow and solidneedles. Because this approach directly and activelydeposits vaccine in the skin, it can deliver vaccine dosesfaster and more reliably than non-invasive vaccinations.

Most work on minimally invasive skin vaccination hasinvolved the use of microneedles, which are long enough tocross the stratum corneum barrier but short enough to avoid

pain and to reliably remain within the skin for targeteddelivery (Gill et al. 2008). There are four different types ofmicroneedles that have been studied for vaccine delivery:hollow, solid, coated, and dissolving microneedles (Prausnitzet al. 2009).

Hollow microneedles look like miniature hypodermicneedles that can be inserted at an angle perpendicular to theskin, which permits healthcare professionals to giveintradermal injections without special training. Hollowmicroneedles have been used as single needles or asmulti-needle arrays for vaccination against influenza,anthrax, and other diseases in animals and human subjects(Arnou et al. 2009; Van Damme et al. 2009; Mikszta et al.2005; Dean et al. 2005). A single hollow microneedledevice has recently been introduced into clinical practicefor intradermal influenza vaccination because skin vaccina-tion was shown to provide superior immunogenicity in theelderly, who have the highest rates of morbidity andmortality from seasonal influenza (Holland et al. 2008).Additional human trials with hollow microneedles alsoshowed significant dose sparing compared to intramuscularimmunization (Leroux-Roels et al. 2008; Van Damme et al.2009).

Solid microneedles have been used to pierce holes in theskin for subsequent delivery of vaccine from a topicalformulation. This approach is similar to many of the non-invasive methods that rely on slow and typically inefficientdelivery by diffusion into the skin. Microneedles have beenused in this way for transcutaneous vaccination usingdiphtheria toxoid, but did not generate strong immuneresponses for influenza vaccine (Ding et al. 2009). Scrapingthe skin with solid microneedles by a method similar to thenon-invasive abrasive techniques has also been used forDNA vaccine delivery, which derived better humoral andcellular immune responses against hepatitis B than intra-muscular or intradermal vaccination by injection (Miksztaet al. 2002).

Recently, microneedles have been coated with vaccinefor rapid dissolution in the skin within minutes. Metalmicroneedles have been coated with a number of differenttypes of influenza vaccines, including inactivated virus(Kim et al. 2010a; Fernando et al. 2010) and virus-likeparticle (Quan et al. 2010b) as well as other antigens, suchas hepatitis C (Gill et al. 2010) and ovalbumin (Matriano etal. 2002). Vaccination in mice using inactivated influenzavirus vaccine induced similar virus-specific IgG antibodyresponse, hemagglutination inhibition titer, and neutralizingactivity as a primary response to vaccination and generatedrobust protective immunity to influenza after challengecompared to conventional intramuscular immunization(Kim et al. 2009; Kim et al. 2010c). Notably, influenzavirus was cleared from the lungs of microneedle-immunized mice after challenge much more efficiently than

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intramuscularly immunized mice. This was explained byenhanced humoral and cellular recall immune responsesamong microneedle-immunized mice. Microneedle vacci-nation also induced significantly higher levels of antibodiesand MHC II-associated CD4+ T helper cells post-challenge(Kim et al. 2009; Kim et al. 2010c).

Mice vaccinated by coated microneedles stored at roomtemperature for 1 month generated similar antibodyresponses to those of mice vaccinated by freshly coatedmicroneedles, which suggests the possibility of a thermo-stable vaccine that does not require refrigeration (Kim et al.2010b). In addition, microneedle vaccination generateddose-sparing effects using an influenza virus-like particlevaccine and a model ovalbumin vaccine (Matriano et al.2002; Quan et al. 2010a)

Dissolving microneedles have been developed to offerthe simplicity and effectiveness of coated metal micro-needles, but eliminate the generation of sharp microneedlewaste because dissolving microneedles made of water-soluble polymers and sugars completely dissolve away inthe skin. In a recent vaccination study, dissolving-microneedle immunization induced similar humoral andcellular immune responses compared to intramuscularimmunization and showed better lung virus clearance andenhanced cellular recall responses after challenge, which issimilar to the results seen with coated metal microneedleimmunization (Sullivan et al. 2010).

Finally, tattooing is a well-established method ofdepositing materials in the skin for cosmetic purposes,which has now been adapted for DNA vaccination in theskin. High-frequency oscillating tattoo needles can piercethe skin to deliver vaccine in the dermis. DNA tattooinginduced better humoral and cellular immune responses thanintramuscular immunization in animal studies (Bins et al.2005).

Minimally invasive methods of skin vaccination, notablythrough the use of various microneedle designs, offeradvantages over non-invasive approaches by activelydelivering vaccines into the skin in a rapid, reliable, andefficient manner. They also offer advantages over hypoder-mic needles by reducing or eliminating pain, biohazardoussharps waste, and the need for specially trained medicalpersonnel. Minimally invasive methods are, nonetheless,invasive and therefore have greater safety concerns andsterility requirements than non-invasive methods do.

Logistical advantages of skin vaccination methods

In addition to the immunological advantages of vaccina-tion in the skin, skin vaccination can also offer importantlogistical advantages compared to conventional hypoder-mic injection. Most of the skin vaccination methods

described in the preceding sections avoid the pain andapprehension felt by patients when receiving hypodermicinjections, eliminate or reduce the risk of needlestickinjury and re-use of needle and/or syringe, and may beadministered by minimally trained personnel or possiblyby patients themselves, thereby enabling increased vacci-nation coverage. This becomes especially importantduring rapid mass vaccination against a possible pandemicor during immunization campaigns because hypodermicinjection is relatively slow and assembly of populations atcentralized sites for vaccination risks cross-contaminationand spread of pathogens.

Many of the skin vaccination methods, including EPI,many transdermal patch designs, and solid microneedles,use vaccine in a dry state. This enables vaccine stabilizationin the dry state, possibly without the need for refrigeration,and avoids the need for vaccine reconstitution by medicalpersonnel, which is time-consuming, can lead to vaccinewastage, and is a source of medical errors. Reformulationof the vaccine for dry-state presentation, however, willincur additional costs and technical challenges (Hickling etal. 2010).

Skin vaccination methods also often reduce the packagesize of the dosage form compared to a hypodermic needle,syringe, and vaccine vial, which facilitates storage, trans-portation, and disposal. This reduces cost and infrastructureneeds, and, for vaccines requiring refrigeration, this reducesthe space required in the cold chain.

Finally, skin vaccination has the potential to reduce thecost of vaccination due to these logistical advantages, aswell as possible dose sparing and increased vaccineimmunogenicity that reduces overall healthcare costs bybetter preventing disease. These savings need to bebalanced against the possibly increased cost of thevaccination method. In most cases, there will be initialcosts to introduce the new technology, but at steady statemass production, many of the skin vaccination methodsneed not cost more than manufacturing of conventionalinjectable vaccines. This is because the dominant cost isoften the cost of the antigen and its sterile manufacturingwhereas the novel dosage form costs no more than a needle,syringe, and vial. However, in other cases, especially whena reusable device is involved, there may be added costs.

Conclusion

Overall, skin vaccination has the potential to improveboth the immunology and logistics of vaccination.Although skin-targeted immunization has been used fora long time, its application beyond a few vaccines hasbeen hindered due to the lack of simple and reliable skinvaccination technology. In recent years, a number of

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technologies developed largely for drug delivery havebeen applied to skin vaccination and shown to administervaccines easily and reliably into the skin. Non-invasivetechnologies offer the safety of a needle-free method ineither a rapid, high-velocity injection format or a slow,diffusion-based patch. Minimally invasive methods, most-ly in the form of various microneedle designs, offersimple, rapid, and efficient vaccination that have advan-tages over non-invasive methods, but with the trade-off ofintroducing safety concerns associated with their invasivenature. With these vaccine delivery tools, we can nowadminister vaccines to the skin for possible application inthe clinic, carry out studies to better understand skinimmunology, and design skin-based vaccines that harnessand optimize skin immunology for improved immuniza-tion. Skin vaccination can now transition from a topic ofimmunological interest with limited clinical utility to aviable method of vaccination to increase vaccine immu-nogenicity and broaden vaccination coverage.

Acknowledgements We thank our colleagues at the Georgia TechCenter for Drug Design, Development and Delivery, the EmoryUniversity Vaccine Center, and PATH for helpful discussions. Thiswork was supported in part by the National Institutes of Health.

Conflict of interest M.R.P. serves as a consultant and is an inventoron patents licensed to companies developing microneedle-basedproducts. This possible conflict of interest has been disclosed and isbeing managed by Georgia Tech and Emory University.

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