DRUG DELIVERY – Recent Developments in Microneedle Technology for Transdermal Drug Delivery & Vaccination


INTRODUCTION

The skin has long been recognized as a potential target for local and systemic drug delivery as well as vaccination. The practical value of transdermal route of drug administration is, however, limited by substantial barrier properties of the skin. Physiologically beneficial protection that the skin provides against xenobiotic permeation is a drawback from the perspective of percutaneous transport of therapeutic agents. Thus, a delivery system that temporarily and reversibly permeabilizes the skin can enable delivery of a wide spectrum of molecules across the skin. One such technology is microneedles, which are micron-scale needles that can create microscopic pores in the stratum corneum and upper layers of the epidermis, thereby enhancing skin permeation up to several orders of magnitude.1 Four designs of microneedles have been developed: dissolving microneedles made of biodegradable polymers that encapsulate drugs or vaccines, solid coated microneedles in which the drugs or vaccines are coated on the microneedle surface, hollow microneedles for injection, and solid microneedles to pierce the skin followed by application of a drug patch (poke and patch approach) (Figure 1).2 The purpose of this article is to highlight some of the recent advances in the field of microneedle-mediated transdermal drug delivery and vaccination, including summaries of preclinical pharmacokinetic data and a brief overview of clinical studies. This review encompasses the time period from 2010 to present day. For more background on microneedles, interested readers should refer to the following references.3,4

MICRONEEDLE-ASSISTED TRANSDERMAL DRUG DELIVERY

Dissolving Microneedles

These types of microneedles are fabricated using biodegradable polymers in which drugs or vaccines are encapsulated in the microneedles.5 Once in the skin, the microneedles dissolve, thus releasing the drug. The enhancement of flux afforded by microneedles has generated significant interest in this transdermal delivery technology for delivery of peptides and proteins. Fukushima et al reported using two-layered dissolving microneedles for transdermal delivery of human growth hormone (rhGH) and desmopressin (DDAVP) in rats.6 rhGH (22 kDa) was formulated at 2-microgram doses in the dissolving microneedles composed of sodium chondroitin sulfate and dextran. The application of this microneedle formulation to the rat abdomen produced a PK profile characterized by fast attainment of peak concentration (tmax = 15 mins) and gradual decrease in the plasma rhGH level with terminal half-life approximating 25 mins. Chondroitin-based and dextranbased microneedles performed similarly. Importantly, the authors observed doseproportional increase in Cmax, and the area under concentration-time curve (AUC) as a function of dose. Also, the bioavailability was very high and amounted to approximately 95% in chondroinin microneedles and 73% in dextran microneedles. Interestingly, the IV bolus injection of rhGH revealed much shorter elimination half-life (4 mins) implying flip-flop kinetics for microneedle-mediated delivery. Hence, the terminal half-life of 25 mins following microneedle application was attributed to the absorption rather than elimination phase. On the other hand, DDAVP (1.07 kDa) chondroitin microneedles showed no flip-flop kinetics and an absorption phase half-life of 14 mins. PK profiles were characterized by tmax of 30 mins and terminal half-life of approximately 2 hrs. Approximately 0.1 mg rhGH could be formulated into a patch of 100 microneedles. The microneedle formulations were stable for at least 1 month under refrigeration or freezing conditions.

An independent, but related, study of hGH in rats was carried out by Lee et al.7 Carboxymethylcellulose-(CMC) and CMCtrehalose- based dissolving microneedles were administered to hairless rats reaching Cmax after approximately 30 mins and subsequently, plasma concentration decreased gradually with a half-life of 1.1 hrs. The authors subsequently compared microneedle-mediated hGH delivery with subcutaneous (SC) injections demonstrating markedly similar PK profiles.

Another study by Yukako et al reported low bioavailability of peptide leuprolide acetate (LA, 1.2 kDa) following administration to rats by dissolving microneedles and subcutaneous injection.8 In accordance with in vitro release data, in vivo PK profiles demonstrated short tmax (15 mins) for microneedles and 20 mins for SC injection. However, low bioavailabilities (32%) were observed due to the metabolic instability of LA in skin. This study highlights the potential limitations of microneedlemediated drug delivery related to the physiology of the skin rather than the microneedle technology itself.

Coated Microneedles

Daddona et al studied the pharmacokinetics and pharmacodynamics of parathyroid hormone (PTH, 4.1 kDa) microneedle-mediated delivery in humans.9 The microneedle arrays consisted of titanium microneedles coated with PTH (20 to 40 micrograms) attached to an adhesive patch. The patch is applied with a hand-held and reusable applicator. A once-daily subcutaneous injection of FORTEO®, which is used as a therapy for advanced osteoporosis in men and postmenopausal women, served as a reference for the evaluation of the coated microneedle system performance with a wear time of 30 mins. Clinical studies in post-menopausal women demonstrated that the microneedle system achieved shorter tmax of approximately 8 mins compared to 24 mins for SC injection. The terminal half-life after SC injection was longer compared to microneedle delivery, and implied flip-flop kinetics. The relative bioavailability of the PTH microneedle patch ranged from 40% to 80%. Interestingly, these differences were believed to be dictated by application to different anatomical body sites and not by the patch performance inconsistency, with the highest relative exposure that was obtained from the abdomen, followed by upper arm, and the lowest from the thigh. In each case, the residual PTH found on the microneedle array after application was < 20%. Dose-proportional increase in the AUC was observed in the clinic. The inter-subject and between-occasion intra-subject variability seen in the PTH-patch and FORTEO were comparable. Pharmacodynamically, the PTHcoated microneedle produced dose-proportional increase in the bone mineral density. The magnitude of this effect was higher compared to FORTEO and might be related to different PK achieved with the application of PTH microneedle. Moisture and oxygen-devoid packaged PTH microneedle patches were found to be stable in room temperature for 2 years, which is a significant advantage over FORTEO that needs to be stored at 5°C to 8°C.

Another study involving the Zosano microneedle patch system was carried out by Peters et al.10 In this study, the stability and preclinical performance of erythropoietincoated microneedles (EPO, approx. 34 kDa) was evaluated in rats. Pharmacokinetic profiles obtained following SC and microneedleassisted administration of EPO were alike and resulted in tmax of 6 to 12 hrs and a terminal elimination half-life of 9 to 12 hrs. A linear AUC dose-response curve was achieved within the 7- to 200-microgram dose range tested. Moreover, the relative bioavailability of EPO after microneedle administration was comparable to that obtained following SC injection.

Zhang et al investigated the potential of lidocaine-coated microneedles for local analgesic action in domestic swine.11 This study was unique because here, an attempt was made to use the microneedles to enhance local (dermal) delivery of a therapeutic agent. The authors used 3M’s 500 micrometer-long solid microneedles, termed sMTS, dip-coated with aqueous lidocaine (234 Da) solution. The local lidocaine concentration, obtained at the treatment site immediately following microneedle application was higher than the estimated level needed for analgesia and was maintained for an hour when co-administered with vasoconstrictive lidocaine.

Hollow Microneedles

Harvey et al investigated microneedlebased intradermal and subcutaneous delivery of protein drugs in Yucatan minipig.12 The authors employed a single microneedle device for injection of insulin and somatropin and a threemicroneedle device to inject etanercept in the dermal space. In the context of this study, microneedle refers to a manually assembled complex of 1-mm-long 34-gauge steel needle, flexible tubing, and an analytical microsyringe drug reservoir. Formulation volumes injected intradermally varied in the range of 50 to 250 microliters. Etanercept (132 kDa) injections demonstrated markedly shortened tmax (5 hrs) of microneedle-mediated delivery as compared to 18 hrs for subcutaneous injection. The absolute bioavailability was 75% for microneedles and 50% for SC administration. Similarly, somatropin (22 kDa) injections showed faster absorption kinetics following intradermal microneedle-mediated injections (tmax = 30 mins) as compared to subcutaneous administration (tmax = 2.75 hrs). The absolute bioavailability was found to be 100% for both routes. Moreover, insulin lispro (5.8 kDa) demonstrated rapid uptake to systemic circulation when administered intradermally (tmax = 22 mins) and slower uptake following SC injection (tmax = 61 mins). Interestingly, the absorption rate of regular and fast-acting insulin after microneedle-mediated intradermal injection was comparable. In all of the aforementioned studies, shorter tmax was accompanied by higher Cmax. The authors performed additional imaging studies that led to the hypothesis that rapid uptake seen for microneedle-mediated intradermal protein delivery is aided by fast lymphatic uptake in the dermis.

Pettis et al studied intradermal microneedle delivery of insulin lispro (5.8 kDa) versus its SC delivery in healthy human volunteers.13 Application of insulin intradermally through a single stainless steel hollow microneedle and an SC injection resulted in rapid systemic absorption and bioavailability of microneedle-mediated delivery comparable to SC injection. Tmax values increased (36 mins, 40 mins, and 46 mins) with increasing microneedle length (1.25 to 1.5 to 1.75 mm) and proved to be the highest for SC injection (64 mins).

Subcutaneous and intradermal delivery of liquid formulations through multiple hollow microneedles (hMTS, 3M Drug Delivery Systems) in domestic swine was studied by Burton et al.14 Pharmacokinetic studies compared microneedle-mediated and SC administration of three model compounds: naloxone (322 Da) hydrochloride, hGH (22kDa), and equine tetanus anti-toxin (ETAT, approx. 150 kDa). Interestingly, naloxone hydrochloride showed faster absorption following SC injection compared to microneedle administration. In contrast, hGH showed faster absorption following microneedle administration. The antibody (ETAT) showed similar PK profiles with both SC and microneedle administration.

Poke & Patch

This delivery approach involves piercing the upper layers of the skin with solid microneedles followed by application of a drug formulation (eg, patch, gel) at that site.2 The pretreatment creates microscopic pores in the skin, thereby enhancing flux of the molecules. Zhang et al reported increased bioavailability of L-carnitine (LC) in rats with the poke and patch method compared to its oral administration.15 Although LC is a small molecular weight compound (161 Da), it does not permeate at high rates through intestinal epithelium or skin due to its ionized and hydrophilic nature. Authors investigated in vitro permeation of LC from solution across untreated and microneedle pretreated skin and demonstrated a 17-fold increase in flux across skin pretreated with microneedles. Subsequently, several carbomer hydrogels were evaluated as formulation options. The CP980 gel showed an LC transport rate of 72% of that of the aqueous solution formulation across microneedle pretreated skin. Finally, a rat PK study comparing IV, oral, and poke and patch delivery methods was conducted. Oral delivery of LC resulted in only 8% bioavailability and tmax of 2 hrs. Following a 6-hr microneedleenhanced transdermal LC delivery, 22% bioavailability was achieved with tmax at 4 hrs. The bioavailability was calculated on the basis of the total dose applied in the patch. An alternative calculation based on the fraction of the total dose that was actually delivered into skin yields a bioavailability of 81%. Interestingly, the poke and patch PK profile resembled a traditional non-microneedle percutaneous profile in the sense that relatively steady plasma levels were obtained in the 2- to 6-hr time window. However, the likely reason for lack of more pronounced Cmax is that the timeline of the experiment was relatively short, and the microchannels did not close enough to significantly limit in vivo flux through the microchannels.

MICRONEEDLE-ASSISTED VACCINATION

The use of the skin as a target site for vaccination has been largely limited due to the difficulty in reliably performing intradermal injections. With the advent of microneedles, a minimally invasive and precise intradermal placement of vaccine has become possible. The skin is known to be a highly immunogenic tissue containing a wealth of antigen-presenting cells, including epidermal Langerhans’ cells and dermal dendritic cells.16 As such, it is a good vaccination target with additional dosesparing potential compared to lessimmunogenic muscle tissue.

A recent study by Weldon et al examined an influenza vaccine-coated microneedle in mice.17 The authors chose to utilize trehalosestabilized, solubilized viral protein antigens rather than more widely employed inactivated influenza virus and virus-like particles. Microneedles coated with stabilized recombinant trimeric soluble hemagglutinin (sHA) provided superior protection against influenza virus compared to the unmodified sHA. Moreover, post-challenge lung titers demonstrated that intradermal vaccination resulted in greater clearance of replicating virus compared to the SC vaccination.

In another study, Kommareddy et al investigated the use of dissolvable microneedle patches delivering influenza vaccine antigens in mice.18 The authors employed TheraJectTM VaxMatTM microneedle technology to incorporate microgram quantities of vaccine in a microneedle patch. A set of in vitro and in vivo studies confirmed the integrity and immunogenicity of the antigens incorporated into a dissolving microneedle patch.

However, perhaps most interestingly, 2011 proved a ground-breaking year for the commercialization of microneedle drug delivery technologies in the US. On May 9, 2011, the FDA approved a first, and so far the only, microneedle-based product: Fluzone Intradermal® influenza virus vaccine (Sanofi Pasteur).19 This constituted a major milestone in the transition of microneedle systems from the developmental stage to the market place. The vaccine is administered from a prefilled microinjection system consisting of a custom syringe with a 30-gauge, 1.5-mm-long, hollow single microneedle developed by Beckton Dickinson (Figure 2).20 The microneedle is about 10 times shorter than a traditional needle used to perform intramuscular injections of Fluzone. Moreover, Phase III clinical data confirmed that intradermal delivery of vaccine allows for dosesparing. A 9-microgram dose of vaccine delivered intradermally proved to be immunogenetically comparable to a 15- microgram dose delivered intramuscularly.21 Hence, a 40% reduction in the dose was achieved. In 2009, Intanza®, an influenza vaccine based on the same microneedle technology, was approved by the European Medicines Agency (EMA).22 These recent developments and the appearance of first products on the market have encouraged academic institutions and pharmaceutical industries to pursue the microneedle-based strategy for drug delivery and vaccination purposes as reflected by several ongoing clinical trials.

CLINICAL TRIALS INVOLVING MICRONEEDLES

A number of ongoing trails with microneedle-based delivery systems illustrate the significant interest in this new transdermal delivery technology. Several clinical studies are listed in ClinicalTrials.gov.23 A few of the notable studies are discussed here. Sanofi Pasteur has completed a Phase II trial that assessed non-inferiority of fractional doses of IMOVAX® Polio administered intradermally versus full doses of IMOVAX Polio administered intramuscularly (study results pending). Phase II and III studies sponsored by Emory University currently investigate the difference in glycemic control between the SC insulin catheter and microneedle for bolus delivery of insulin. NanoPass technologies’ ongoing studies evaluate the safety and efficacy of a MicronJet microneedle device as well as safety, pharmacokinetics, and pharmacodynamics of insulin injected via MicronJet. A Phase II study on pharmacokinetics and pharmacodynamics of basal insulin infusion administered either intradermally or subcutaneously has been completed by Becton Dickinson (study results pending). Furthermore, a Phase I clinical study assessing tolerability of the application of a 3M microstructure transdermal system has been completed (study results pending). Finally, Zosano Pharma microneedle-based PTH patch had completed Phase II clinical trials, and the results were published and summarized in the aforementioned Coated Microneedles section.

SUMMARY

Early research in the area of microneedleassisted transdermal drug delivery provided ample proof of its potential in enhancing and enabling transport of pharmaceuticals across thr skin. It also demonstrated its limitations related to the relatively small drug doses that can be used in conjunction with the microneedle technique and the microneedle fabrication complexity, including related challenges in therapeutic agents’ stability. Following extensive investigation of microneedle fabrication methods, later studies focused toward improving the performance of existing microneedle systems. Multiple preclinical and clinical studies demonstrated their successful application in vivo. The viability of the microchannels was assessed in vivo and its implications for the applicability of the poke and patch method were recognized. Clinical successes have resulted in significant interest from both academia and the pharmaceutical industry in this delivery area. A selective review of the recent advancements in this field reported herein highlights growing sophistication of this technology in its ability to deliver a wide variety of molecules and vaccines with precision. Importantly, at a time when biopharmaceuticals (biologics and vaccines) are becoming an integral part of pharmaceutical pipelines, microneedle-based devices offer a promising alternative to traditional SC and intramuscular injections.u

REFERENCES

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Dr. Mikolaj Milewski is a Senior Research Pharmacist at Merck. As a member of the Biopharmaceutics group, he assesses bioperformance risk of toxicology and clinical formulations. He also evaluates feasibility of alternative drug delivery routes for new chemical entities and existing products. He has authored research and review articles in peer-reviewed journals and is a member of AAPS and ACS. His areas of interest focus on oral and transdermal formulations, pharmacokinetics, and development of microneedle-based drug delivery systems. Prior to joining Merck, Dr. Milewski earned his PhD in Pharmaceutical Sciences from the University of Kentucky in Lexington.

Dr. Amitava Mitra earned his BS in Pharmacy from Birla Institute of Technology, India, his PhD in Pharmaceutical Sciences from the University of Maryland, and completed his post-doctoral fellowship from Fox Chase Cancer Center. Dr. Mitra has published research articles in peer-reviewed journals and has authored review articles and book chapters. He has numerous podium and poster presentations in national and international conferences. He is a member of the AAPS, CRS, and the Rho Chi Pharmacy Honor Society. He is a recipient of the Controlled Release Society-3M Drug Delivery Systems Graduate Student Outstanding Research Award in 2005 and American Association of Pharmaceutical Scientists- National Biotechnology Conference Graduate Student Award in 2006.