Effect of formulation pH on transport of naltrexone species and pore closure in microneedle-enhanced transdermal drug delivery

Advancements in microneedle technology: current status and next-generation innovations

Microneedle technology is a pivotal component of third-generation transdermal drug delivery systems featuring tiny needles that create temporary microscopic channels in the stratum corneum which facilitate drug penetration in the dermis. This review offers a detailed examination of the current types of microneedles, including solid, coated, dissolving, hollow, and swelling microneedles, along with their preparation techniques as well as their benefits and challenges. Use of 3D printing technology is especially gaining significant attention due to its ability to achieve the high dimensional accuracy required for precise fabrication. Additionally, its customisability presents significant potential for exploring new designs and creating personalised microneedles products. Furthermore, this review explores next generation microneedles, especially stimuli-responsive microneedle, bioinspired microneedle and microneedles combined with other transdermal technology like sonophoresis, electroporation and iontophoresis. Regulatory aspects, characterisation techniques, safety considerations, and cost factors have also been addressed which are crucial for translation from lab to the market.

Breaking barriers: Innovative approaches for skin delivery of RNA therapeutics

RNA therapeutics represent a rapidly expanding platform with game-changing prospects in personalized medicine. The disruptive potential of this technology will overhaul the standard of care with reference to both primary and specialty care. To date, RNA therapeutics have mostly been delivered parenterally via injection, but topical administration followed by intradermal or transdermal delivery represents an attractive method that is convenient to patients and minimally invasive. The skin barrier, particularly the lipid-rich stratum corneum, presents a significant hurdle to the uptake of large, charged oligonucleotide drugs. Therapeutic oligonucleotides need to be engineered for stability and specificity and formulated with state-of-the-art delivery strategies for efficient uptake. This review will cover various passive and active strategies deployed to enhance permeation through the stratum corneum and achieve effective delivery of RNA therapeutics to treat both local skin disorders and systemic diseases. Some strategies to achieve selectivity between local and systemic administration will also be discussed.

Effect of Receptor Solution in Studies of In Vitro Permeation Test (IVPT)

In Vitro Permeation Test (IVPT) is commonly used to evaluate skin penetration of chemicals and performance of dermatological products. For a permeant with low aqueous solubility, an additive that is expected not to alter the skin barrier can be used in the receptor solution to improve permeant solubility. The objective of this study was to (a) evaluate the effects of these additives in IVPT receptor solution on skin permeability of model permeants and skin electrical resistance and (b) determine the solubility of the permeants in these receptor solutions. Bovine serum albumin (BSA), 2-hydroxypropyl-beta-cyclodextrin (HPCD), ethanol, nonionic surfactant Brij-98, and propylene glycol were the additives, and phosphate buffered saline (PBS) was the control. Steady-state skin permeability coefficients and resistances were determined. The receptor solutions examined in this study did not cause a significant increase in skin permeability or decrease in resistance (less than 40 % changes) except 25 % ethanol. The receptor solution containing 25 % ethanol induced an approximately twofold average increase in skin permeability and reduced skin electrical resistance by approximately threefold. The receptor solution of 2.5 % HPCD provided the highest levels of solubility for the model lipophilic permeants, while 0.2 % Brij-98 and 5 % ethanol showed the lowest solubility enhancement from those in PBS.

Thermosensitive biomaterial gels with chemical permeation enhancers for enhanced microneedle delivery of naltrexone for managing opioid and alcohol dependency

Naltrexone (NTX) can be transdermally delivered using microneedles (MN) to treat opioid and alcohol misuse disorders, but delivery is blunted by rapid in vivo micropore closure. Poloxamer (P407), a thermosensitive biocompatible hydrogel, sustains NTX delivery through MN-treated skin by generating a drug depot within the micropores. Optimizing P407 formulations could maintain sustained delivery after micropore closure while reducing required patch sizes, which would be more discreet and preferred by most patients. Here we developed NTX-loaded P407 gels with chemical permeation enhancers (CPEs) and used these novel formulations alongside MN treatment to enhance NTX permeation, utilizing parallel micropore and intact skin transport pathways. We analyzed physicochemical and rheological properties of CPE-loaded P407 formulations and selected formulations with DMSO and benzyl alcohol for further study. In vitro permeation tests demonstrated more consistent and sustained NTX delivery through MN-treated porcine skin from 16% P407 formulations vs. aqueous solutions. P407 with 1% benzyl alcohol and 10% DMSO significantly, P < 0.05, increased flux through MN-treated skin vs. formulations with benzyl alcohol alone. This formulation would require a smaller size patch than previously used to deliver NTX in humans, with half the NTX concentration. This is the first time poloxamer biomaterials have been used in combination with CPEs to improve MN-assisted transdermal delivery of an opioid antagonist. Here we have demonstrated that P407 in combination with CPEs effectively sustains NTX delivery in MN-treated skin while requiring less NTX than previously needed to meet clinical goals.

A Comprehensive Review of Microneedles: Types, Materials, Processes, Characterizations and Applications

Drug delivery through the skin offers many advantages such as avoidance of hepatic first-pass metabolism, maintenance of steady plasma concentration, safety, and compliance over oral or parenteral pathways. However, the biggest challenge for transdermal delivery is that only a limited number of potent drugs with ideal physicochemical properties can passively diffuse and intercellularly permeate through skin barriers and achieve therapeutic concentration by this route. Significant efforts have been made toward the development of approaches to enhance transdermal permeation of the drugs. Among them, microneedles represent one of the microscale physical enhancement methods that greatly expand the spectrum of drugs for transdermal and intradermal delivery. Microneedles typically measure 0.1-1 mm in length. In this review, microneedle materials, fabrication routes, characterization techniques, and applications for transdermal delivery are discussed. A variety of materials such as silicon, stainless steel, and polymers have been used to fabricate solid, coated, hollow, or dissolvable microneedles. Their implications for transdermal drug delivery have been discussed extensively. However, there remain challenges with sustained delivery, efficacy, cost-effective fabrication, and large-scale manufacturing. This review discusses different modes of characterization and the gaps in manufacturing technologies associated with microneedles. This review also discusses their potential impact on drug delivery, vaccine delivery, disease diagnostic, and cosmetics applications.

Innovative Systems to Deliver Allergen Powder for Epicutaneous Immunotherapy

Allergy is a disorder owing to hyperimmune responses to a particular kind of substance like food and the disease remains a serious healthcare burden worldwide. This unpleasant and sometimes fatal allergic disease has been tackled vigorously by allergen-specific immunotherapy over a century, but the progress made so far is far from satisfactory for some allergies. Herein, we introduce innovative, allergen powder-based epicutaneous immunotherapies (EPIT), which could potentially serve to generate a new stream of technological possibilities that embrace the features of super safety and efficacious immunotherapy by manipulating the plasticity of the skin immune system via sufficient delivery of not only allergens but also tolerogenic adjuvants. We attempt to lay a framework to help understand immune physiology of the skin, epicutaneous delivery of powdered allergy, and potentials for tolerogenic adjuvants. Preclinical and clinical data are reviewed showing that deposition of allergen powder into an array of micropores in the epidermis can confer significant advantages over intradermal or subcutaneous injection of aqueous allergens or other epicutaneous delivery systems to induce immunological responses toward tolerance at little risk of anaphylaxis. Finally, the safety, cost-effectiveness, and acceptability of these novel EPITs are discussed, which offers the perspective of future immunotherapies with all desirable features.

Characterization of microneedles and microchannels for enhanced transdermal drug delivery

Microneedle (MN)-based technologies are currently one of the most innovative approaches that are being extensively investigated for transdermal delivery of low molecular weight drugs, biotherapeutic agents and vaccines. Extensive research reports, describing the fabrication and applications of different types of MNs, can be readily found in the literature. Effective characterization tools to evaluate the quality and performance of the MNs as well as for determination of the dimensional and kinetic properties of the microchannels created in the skin, are an essential and critical part of MN-based research. This review paper provides a comprehensive account of all such tools and techniques.

Development of Hydrogels for Microneedle-Assisted Transdermal Delivery of Naloxone for Opioid-Induced Pruritus

Transdermal naloxone delivery could be a potential option for treating opioid-induced pruritus, but naloxone does not permeate skin well because of its hydrophilic nature. Microneedles (MNs) could overcome the skin barrier by painlessly creating microchannels in the skin to permit naloxone absorption to therapeutic levels. This study investigated how ionization correlates with naloxone permeation across MN-treated skin. Hydrogels containing 0.2, 0.5, or 1% naloxone were formulated with 1% cross-linked polyacrylic acid (polymer) and adjusted to pH 5, 6.5, or 7.4. Porcine skin was treated with MNs and naloxone gel, and in vitro permeation studies were performed using an in-line diffusion setup. Gel structural properties were evaluated using rheology. All gels had viscoelastic properties and good spreadability. Naloxone permeation through intact skin was highest from pH 7.4 gels when naloxone is unionized, in contrast with undetectable concentrations permeated from pH 5 gels with 100% ionization. Combining MN treatment with pH 5 gels significantly enhanced permeation and resulted in steady-state flux that would achieve therapeutic delivery. Absorption lag time was affected by MN length and naloxone gel concentration. Polymer concentration did not influence drug permeability. This study demonstrates that transdermal naloxone delivery with MNs is a viable treatment option for opioid-induced pruritus.

Space- and time-resolved investigation on diffusion kinetics of human skin following macromolecule delivery by microneedle arrays

Microscale medical devices are being developed for targeted skin delivery of vaccines and the extraction of biomarkers, with the potential to revolutionise healthcare in both developing and developed countries. The effective clinical development of these devices is dependent on understanding the macro-molecular diffusion properties of skin. We hypothesised that diffusion varied according to specific skin layers. Using three different molecular weights of rhodamine dextran (RD) (MW of 70, 500 and 2000 kDa) relevant to the vaccine and therapeutic scales, we deposited molecules to a range of depths (0-300 µm) in ex vivo human skin using the Nanopatch device. We observed significant dissipation of RD as diffusion with 70 and 500 kDa within the 30 min timeframe, which varied with MW and skin layer. Using multiphoton microscopy, image analysis and a Fick's law analysis with 2D cartesian and axisymmetric cylindrical coordinates, we reported experimental trends of epidermal and dermal diffusivity values ranging from 1-8 µm2 s-1 to 1-20 µm2 s-1 respectively, with a significant decrease in the dermal-epidermal junction of 0.7-3 µm2 s-1. In breaching the stratum corneum (SC) and dermal-epidermal junction barriers, we have demonstrated practical application, delivery and targeting of macromolecules to both epidermal and dermal antigen presenting cells, providing a sound knowledge base for future development of skin-targeting clinical technologies in humans.

Implantable polymeric microneedles with phototriggerable properties as a patient-controlled transdermal analgesia system

Adequate pain control can be achieved using a patient-controlled drug delivery system that can provide analgesia to patients as needed. To achieve this objective, we developed a phototriggered microneedle (MN) system that enables the on-demand delivery of pain medications to the skin under external near-infrared (NIR) light stimulation. In this system, polymeric MNs, containing NIR absorbers and analgesics, are combined with a poly(l-lactide-co-d,l-lactide) supporting array. A "removable design" of the supporting array enables the quick implantation of the MNs into the skin to act as a drug depot, thus shortening the patch application time. Upon irradiation with NIR light, the NIR absorbers in the implanted MNs can absorb light energy and induce a phase transition in the MNs to activate drug release. We demonstrated that lidocaine release can be modulated or repeatedly triggered by varying the duration of irradiation and controlling the on and off status of the laser. Lidocaine delivered by the implanted MNs can be rapidly absorbed into the blood circulation within 10 min and has a bioavailability of at least 95% relative to the subcutaneous injection, showing that the proposed system has the potential to provide a rapid onset of pain relief. Such an implantable device may allow pain sufferers receiving the painkiller without the need for multiple needle injections, and may enable controlling pain more conveniently and comfortably.

Near-Infrared Light-Activatable Microneedle System for Treating Superficial Tumors by Combination of Chemotherapy and Photothermal Therapy

Because of the aggressive and recurrent nature of cancers, repeated and multimodal treatments are often necessary. Traditional cancer therapies have a risk of serious toxicity and side effects. Hence, it is crucial to develop an alternative treatment modality that is minimally invasive, effectively treats cancers with low toxicity, and can be repeated as required. We developed a light-activatable microneedle (MN) system that can repeatedly and simultaneously provide photothermal therapy and chemotherapy to superficial tumors and exert synergistic anticancer effects. This system consists of embeddable polycaprolactone MNs containing a photosensitive nanomaterial (lanthanum hexaboride) and an anticancer drug (doxorubicin; DOX), and a dissolvable poly(vinyl alcohol)/polyvinylpyrrolidone supporting array patch. Because of this supporting array, the MNs can be completely inserted into the skin and embedded within the target tissue for locoregional cancer treatment. When exposed to near-infrared light, the embedded MN array uniformly heats the target tissue to induce a large thermal ablation area and then melts at 50 °C to release DOX in a broad area, thus destroying tumors. This light-activated heating and releasing behavior can be precisely controlled and switched on and off on demand for several cycles. We demonstrated that the MN-mediated synergistic therapy completely eradicated 4T1 tumors within 1 week after a single application of the MN and three cycles of laser treatment. No tumor recurrence and no significant body weight loss of mice were observed. Thus, the developed light-activatable MN with a unique embeddable feature offers an effective, user-friendly, and low-toxicity option for patients requiring long-term and multiple cancer treatments.

High aspect ratio elongated microparticles for enhanced topical drug delivery in human volunteers

Delivery of therapeutics into skin is hindered by the epidermal barriers. To overcome these barriers for the treatment of skin diseases, a cutaneous delivery method capable of field treatment using silica-elongated microparticles is developed. The microparticles are massaged into the skin using a 3D-printed microtextured applicator resulting in significant field-directed drug delivery enhancement.

Hydrogel-forming microneedles increase in volume during swelling in skin, but skin barrier function recovery is unaffected

We describe, for the first time, quantification of in-skin swelling and fluid uptake by hydrogel-forming microneedle (MN) arrays and skin barrier recovery in human volunteers. Such MN arrays, prepared from aqueous blends of hydrolyzed poly(methylvinylether/maleic anhydride) (15%, w/w) and the cross-linker poly(ethyleneglycol) 10,000 Da (7.5%, w/w), were inserted into the skin of human volunteers (n = 15) to depths of approximately 300 μm by gentle hand pressure. The MN arrays swelled in skin, taking up skin interstitial fluid, such that their mass had increased by approximately 30% after 6 h in skin. Importantly, however, skin barrier function recovered within 24 h after MN removal, regardless of how long the MN had been in skin or how much their volume had increased with swelling. Further research on closure of MN-induced micropores is required because transepidermal water loss measurements suggested micropore closure, whereas optical coherence tomography indicated that MN-induced micropores had not closed over, even 24 h after MN had been removed. There were no complaints of skin reactions, adverse events, or strong views against MN use by any of the volunteers. Only some minor erythema was noted after patch removal, although this always resolved within 48 h, and no adverse events were present on follow-up.

Microneedle integrated transdermal patch for fast onset and sustained delivery of lidocaine

Lidocaine as an analgesic is of particular interest in both acute and chronic pain conditions and is used via injections or transdermal patches. While injections are associated with problems such as patient incompliance, topical administration of lidocaine using patches is less efficient due to variability of drug absorption among individuals, slower drug permeation through the skin, and hence a resultant undesirable delay in analgesic effects. To address this clinical problem, we developed a microneedle integrated transdermal patch (MITP), using a photolithography based process, in which microneedles create micrometer-sized channels in the skin to deliver lidocaine rapidly, while the reservoir patch holding the bulk of the drug enables higher drug loading and carries on to release the drug for prolonged periods. We demonstrated a new approach of drug delivery using microneedles, where drugs diffuse out of microneedles through the porous channels left by dissolving drug particles. MITP was shown to be able to encapsulate up to 70 mg of lidocaine. In vitro permeation through rat skin demonstrated that MITP delivered a significantly higher amount of lidocaine than a commercial patch and with a faster onset of drug permeation.