Development of a novel self-dissolving microneedle array of alendronate, a nitrogen-containing bisphosphonate: evaluation of transdermal absorption, safety, and pharmacological effects after application in rats

Drug Delivery and Therapy Strategies for Osteoporosis Intervention

With the advent of the aging society, osteoporosis (OP) risk increases yearly. Currently, the clinical usage of anti-OP drugs is challenged by recurrent side effects and poor patient compliance, regardless of oral, intravenous, or subcutaneous administration. Properly using a drug delivery system or formulation strategy can achieve targeted drug delivery to the bone, diminish side effects, improve bioavailability, and prolong the in vivo residence time, thus effectively curing osteoporosis. This review expounds on the pathogenesis of OP and the clinical medicaments used for OP intervention, proposes the design approach for anti-OP drug delivery, emphatically discusses emerging novel anti-OP drug delivery systems, and enumerates anti-OP preparations under clinical investigation. Our findings may contribute to engineering anti-OP drug delivery and OP-targeting therapy.

Transdermal drug delivery via microneedles for musculoskeletal systems

As the population is ageing and lifestyle is changing, the prevalence of musculoskeletal (MSK) disorders is gradually increasing with each passing year, posing a serious threat to the health and quality of the public, especially the elderly. However, currently prevalent treatments for MSK disorders, mainly administered orally and by injection, are not targeted to the specific lesion, resulting in low efficacy along with a series of local and systemic adverse effects. Microneedle (MN) patches loaded with micron-sized needle array, combining the advantages of oral administration and local injection, have become a potentially novel strategy for the administration and treatment of MSK diseases. In this review, we briefly introduce the basics of MNs and focus on the main characteristics of the MSK systems and various types of MN-based transdermal drug delivery (TDD) systems. We emphasize the progress and broad applications of MN-based transdermal drug delivery (TDD) for MSK systems, including osteoporosis, nutritional rickets and some other typical types of arthritis and muscular damage, and in closing summarize the future prospects and challenges of MNs application.

Advances in microneedle-based therapy for bone disorders

Bone-related disorders treatment is a serious public health concern, imposing a significant social and economic burden on patients and healthcare systems. Although conventional drug delivery systems have made advances in bone diseases prevention and management, the limited delivery efficiency and convoluted focal environment lead to inadequate drug absorption and lack of specificity to achieve the intended therapeutic impact. Microneedle-based therapy represents an extraordinarily safe and well-tolerable therapeutic approach for treating bone disorders, providing improved efficacy by breaking down the barriers and delivery of therapeutic components to the target sites with programable release profiles in a less invasive manner. Over the past decades, numerous significant achievements in the development of various types of drug-carried microneedles have been made to address the obstacles encountered in the bone-treating procedure, enabling the microneedle-based therapy to take an important step in practical applications. In this light, this review summarizes these remarkable researches in terms of microneedles types and drug delivery strategies, with the goal of demonstrating the benefits of exploiting microneedle-based therapy as a novel strategy for treating bone-related disorders. The remaining challenges and future perspectives are also discussed in the hope of inspiring more efficient and intelligent bone treatment strategies.

Polysaccharide-Based Coatings as Drug Delivery Systems

Therapeutic polysaccharide-based coatings have recently emerged as versatile strategies to transform a conventional medical implant into a drug delivery system. However, the translation of these polysaccharide-based coatings into the clinic as drug delivery systems still requires a deeper understanding of their drug degradation/release profiles. This claim is supported by little or no data. In this review paper, a comprehensive description of the benefits and challenges generated by the polysaccharide-based coatings is provided. Moreover, the latest advances made towards the application of the most important representative coatings based on polysaccharide types for drug delivery are debated. Furthermore, suggestions/recommendations for future research to speed up the transition of polysaccharide-based drug delivery systems from the laboratory testing to clinical applications are given.

Recent advancements in the expedition of microneedles: from lab worktops to diagnostic care centers

Microneedle (MN) technology plays a significant role in bioengineering as it allows for minimally invasive exposure to the skin via the non-invasive procedure, increased drug permeability, and improved biological molecule detectability in the epidermal layers, all while improving therapeutic safety and effectiveness. However, MNs have several significant drawbacks, including difficulty scaling up, variability in drug delivery pattern regarding the skin's external environment, blockage of dermal tissues, induction of inflammatory response at the administration site, and limitation of dosing based on the molecular weight of drug and size. Despite these drawbacks, MNs have emerged as a special transdermal theranostics instrument in clinical research to assess physiological parameters. Bioimaging technology relies on microneedles that can measure particular analytes in the extracellular fluid effectively by crossing the stratum corneum, making them "a unique tool in diagnostics detection and therapeutic application inside the body." This review article discusses the recent advances in the applications especially related to the diagnostics and toxicity challenges of microneedles. In addition, this review article discusses the clinical state and commercial accessibility of microneedle technology-based devices in order to provide new information to scientists and researchers.

Hyaluronic Acid: A Review of the Drug Delivery Capabilities of This Naturally Occurring Polysaccharide

The inclusion of physiologically active molecules into a naturally occurring polymer matrix can improve the degradation, absorption, and release profile of the drug, thus boosting the therapeutic impact and potentially even reducing the frequency of administration. The human body produces significant amounts of polysaccharide hyaluronic acid, which boasts exceptional biocompatibility, biodegradability, and one-of-a-kind physicochemical features. In this review, we will examine the clinical trials currently utilizing hyaluronic acid and address the bright future of this versatile polymer, as well as summarize the numerous applications of hyaluronic acid in drug delivery and immunomodulation.

Bone Tissue Engineering in the Treatment of Bone Defects

Bones play an important role in maintaining exercise and protecting organs. Bone defect, as a common orthopedic disease in clinics, can cause tremendous damage with long treatment cycles. Therefore, the treatment of bone defect remains as one of the main challenges in clinical practice. Today, with increased incidence of bone disease in the aging population, demand for bone repair material is high. At present, the method of clinical treatment for bone defects including non-invasive therapy and invasive therapy. Surgical treatment is the most effective way to treat bone defects, such as using bone grafts, Masquelet technique, Ilizarov technique etc. In recent years, the rapid development of tissue engineering technology provides a new treatment strategy for bone repair. This review paper introduces the current situation and challenges of clinical treatment of bone defect repair in detail. The advantages and disadvantages of bone tissue engineering scaffolds are comprehensively discussed from the aspect of material, preparation technology, and function of bone tissue engineering scaffolds. This paper also summarizes the 3D printing technology based on computer technology, aiming at designing personalized artificial scaffolds that can accurately fit bone defects.

Zoledronic Acid Deteriorates Soft and Hard Tissue Healing of Murine Tooth Extraction Sockets in a Dose-Dependent Manner

The pathophysiology, histopathology, and immunopathology of bisphosphonate-related osteonecrosis of the jaw (BRONJ) Stage 0 remain unclear. The aim of this study was to investigate the effects of high-dose bisphosphonates on tooth extraction socket healing by creating a murine model of BRONJ Stage 0-like lesions using 8-week-old female C57BL/6J mice. Zoledronic acid (Zol) was administered subcutaneously twice a week for 7 weeks at doses of 0.1 mg/kg/week (moderate dose; Zol-M), 0.5 mg/kg/week (high dose; Zol-H1), and 1.0 mg/kg/week (higher dose; Zol-H2). Saline was used as a control (VC). Both maxillary first molars were extracted 3 weeks after drug treatment. Maxillae, long bones, and sera were collected 4 weeks post-extraction (n = 7 mice/group). Microcomputed tomography, histological, immunohistochemical, and ELISA analyses were performed. A ceiling effect for Zol was noted at the Zol-H1 dose. Osseous healing of extraction sites was significantly impaired with increased necrotic bone and the number of empty lacunae in a Zol dose-dependent manner. Zol significantly decreased epithelial thickness, due to a decrease in thickness of the stratum spinosum, in both Zol-H1 and Zol-H2. Both Zol-H1 and Zol-H2 significantly suppressed the distribution of F4/80⁺ macrophages in the connective tissue of tooth extraction sockets, although gross healing appeared to be normal. Intriguingly, both Zol-H1 and Zol-H2 significantly increased the numbers of TRAP⁺ mononuclear cells and detached osteoclasts in the connective tissue and bone marrow of extraction sites compared to VC and Zol-M, correlated with serum TRAcP5b levels. The created murine model of BRONJ Stage 0-like lesions becoming more severe in a dose-dependent manner may help to understand the pathophysiology and histopathology of BRONJ Stage 0 in humans.

Hyaluronic acid based microneedle array: Recent applications in drug delivery and cosmetology

Microneedles are micron-sized arrays of needles that facilitate drug delivery for local and systemic effects. Hyaluronic acid (HA) is a glycosaminoglycan and is an indigenous component of the connective tissues and dermis. Owing to its versatility and biocompatibility, it has widely been used against various bone, eye, and skin disorders. Therefore, fabricating HA-microneedles is fetching massive global attention. HA based dissolvable microneedles have been immensely explored due to their biodegradable nature. Its degradation residues are very safe. Several attempts have been made to deliver vitamins, proteins, DNAs, and biological macromolecules by HA-microneedles. Here we present the recent advancements in HA-microneedles based application on drug delivery and cosmetology. Its bio-degradation pathways, the receptors on which HA and its derivatives interact, the biological half-lives, and their importance as useful materials for various applications are highlighted. The literature reports identify HA-microneedle as an useful carrier for the delivery of pharmaceuticals.

Strategies for transdermal drug delivery against bone disorders: A preclinical and clinical update

The transdermal drug delivery system is an exceptionally safe and well-tolerable therapeutic approach that has immense potential for delivering active components against bone-related pathologies. However, its use is limited in the current clinical practices due to the low skin permeability of most active drugs in the formulation. Thus, innovations in the methodologies of skin permeation enhancement techniques are suggested to overcome this limitation. Although various transdermal drug delivery systems are studied to date, there are insufficient studies comparing the therapeutic efficacy of transdermal delivery systems to oral delivery systems. Thus, creating a decision-making dilemma between oral or transdermal therapies. Therefore, a timely review is inevitable to develop a platform for future researchers to develop next-generation transdermal drug delivery strategies against skeletal diseases that must be convenient and cost-effective for the patients with improved therapeutic efficacy. Here, we will outline the most recent strategies that can overcome the choice limitation of the drug and enhance the transdermal adsorption of various types of drugs to treat bone disorders. For the first time, in this review paper, we will highlight the preclinical and clinical studies on the different transdermal delivery methods. Thus, providing insight into the current therapeutic approaches and suggesting new directions for the advancements in transdermal drug delivery systems against bone disorders.

Emerging era of microneedle array for pharmaceutical and biomedical applications: recent advances and toxicological perspectives

Background

Microneedles (MNs) are the utmost unique, efficient, and minimally invasive inventions in the pharmaceutical field. Over the past decades, many scientists around the globe have reported MNs cautious because of their superb future in distinct areas. Concerning the wise use of MNs herein, we deal in depth with the present applications of MNs in drug delivery.

Main text

The present review comprises various fabrication materials and methods used for MN synthesis. The article also noted the distinctive advantages of these MNs, which holds huge potential for pharmaceutical and biomedical applications. The role of MNs in serving as a platform to treat various ailments has been explained accompanied by unusual approaches. The review also inculcates the pharmacokinetics of MNs, which includes permeation, absorption, and bioavailability enhancement. Besides this, the in vitro/in vivo toxicity, biosafety, and marketed product of MNs have been reviewed. We have also discussed the clinical trials and patents on the pharmaceutical applications of MNs in brief.

Conclusion

To sum up, this article gives insight into the MNs and provides a recent advancement in MNs, which pave the pathway for future pharmaceutical and biomedical applications.

Graphical abstract

Pharmaceutical and biomedical applications of MNs

Alendronate Sodium Intercalation in Layered Double Hydroxide/Poly (ε-caprolactone): Application in Osteoporosis Treatment

Background:

Osteoporosis is a bone disease alters the amount and variety of proteins in bone tissue and increases the potential of bone fracture. Antiresorptive therapy is one of the most popular treatment methods for osteoporosis. To reduce side effects and enhance the bioavailability of drug agents, the controlled delivery of drug is commonly utilized.

Objectives:

We investigated the controlled release of Alendronate in different composites of layered double hydroxide (LDH) using poly (ε-caprolactone) (PCL) as a matrix.

Materials and Methods:

We prepared different microsphere composites of ALD intercalated in various amounts of LDH, using PCL as a matrix. The controlled release of ALD from these composites is subsequently investigated. Samples are characterized and in vitro cell cytotoxicity, attachment, osteogenic activity including alkaline phosphatase activity and mineralization are examined using MG-63 human osteosarcoma cells.

Results:

The results showed that the release of ALD is more desirable and controlled in the samples having a higher amount of LDH incorporated into the PCL matrix. MG63 cells show a significant increase in viability, attachment, and mineralization while alkaline phosphatase activity remains almost at a constant level after 3 weeks.

Conclusions:

Overall, the findings showed that by incorporation of 15 wt% of LDH, the composite microsphere is capable of holding the antiresorptive drug longer and release it in a more controlled manner. This is an advantageous and promising characteristic for a carrier that could be used as a potential candidate for osteoporosis treatment.

Non-invasive drug delivery technology: development and current status of transdermal drug delivery devices, techniques and biomedical applications

Pay-load deliveries across the skin barrier to the systemic circulation have been one of the most challenging delivery options. Necessitated requirements of the skin and facilitated skin layer cross-over delivery attempts have resulted in development of different non-invasive, non-oral methods, devices and systems which have been standardized, concurrently used and are in continuous upgrade and improvements. Iontophoresis, electroporation, sonophoresis, magnetophoresis, dermal patches, nanocarriers, needled and needle-less shots, and injectors are among some of the methods of transdermal delivery. The current review covers the current state of the art, merits and shortcomings of the systems, devices and transdermal delivery patches, including drugs' and other payloads' passage facilitation techniques, permeation and absorption feasibility studies, as well as physicochemical properties affecting the delivery through different transdermal modes along with examples of drugs, vaccines, genes and other payloads.

Development of a Phosphoric Acid-Mediated Hyaluronic Acid Gel Sheet for Efficient Transdermal Delivery of Alendronate for Anti-Osteoporotic Therapy

For efficient transdermal delivery of alendronate (ALN) for anti-osteoporotic therapy, we developed a hyaluronic acid (HA) gel sheet that was prepared simply by enhancing HA noncovalent interactions using phosphoric acid and polyhydric alcohol (propanediol and glycerin). HA solution viscosity increased after addition of phosphoric acid, and the HA gel sheet formed after heated drying. The HA gel sheet could be converted to high viscosity state by addition of water. These results indicate that phosphoric acid enhances the noncovalent interactions of HA molecules. The HA gel sheet elicited no skin irritation over 7 days after a 24-h application. The permeation of ALN across rat and human skin was 109 and 7.17 µg/cm², respectively, up to 24 h after application of the ALN-loaded HA gel sheet, which is sufficient for clinical treatment of osteoporosis. The bioavailability of ALN in rats was ~20% after application of the ALN-loaded HA gel sheet, and plasma calcium levels were effectively reduced 3 days after sheet application. Furthermore, in a rat osteoporosis model, the reduction in tibial bone density was suppressed by treatment with the ALN-loaded HA gel sheet. These results indicate that our phosphoric acid-mediated HA gel sheet is a promising transdermal formulation for efficient ALN delivery.

Self-Dissolving Microneedle Arrays for Transdermal Absorption Enhancement of Human Parathyroid Hormone (1-34)

Human parathyroid hormone (1-34) (PTH) has been widely used as the subcutaneous injection formulation for the treatment of osteoporosis. In the present study, we developed an efficient transdermal delivery system of PTH by using dissolving microneedle arrays (MNs) composed of hyaluronic acid (HA) for the treatment of osteoporosis. PTH-loaded MNs, with needle length 800 µm, were fabricated via a micro-molding method. The stability of PTH in MNs was found to be 6-fold higher than that of PTH solution when stored at room temperature (15–20 °C) for one month. Micron-scale pores were clearly visible in rat skin following application of PTH-loaded MNs. PTH-loaded MNs were completely dissolved by 60 min following application to rat skin. The bioavailability (BA) of PTH relative to subcutaneous injection was 100 ± 4% following application of PTH-loaded MNs in rats. In addition, PTH-loaded MNs were found to effectively suppress decreases in bone density in a rat model of osteoporosis. Furthermore, no skin irritation was observed at the site of application in rats. These findings indicate that our dissolving MNs have a potential use in formulations for the transdermal delivery of PTH and for the treatment of osteoporosis.

Microneedles containing cross-linked hyaluronic acid particulates for control of degradation and swelling behaviour after administration into skin

Microneedles (MN) containing cross-linked hyaluronic acid (X-linked HA) particulates were prepared to control the degradation and swelling behaviour after transdermal drug delivery. The X-linked HA particulates were prepared by cross-linking HA chains and then passing the particulates through a sieve. Then, microneedles were prepared by micromolding method. The rheological properties of X-linked HA were studied. The penetration success rate, mechanical failure and dissolution rate of microneedles containing only hyaluronic acid (HA MN) and microneedles containing X-linked HA were compared. The delivery of fluorescein into the skin with X-linked HA MN was also observed using a confocal microscope. The size of the pulverised particulates in water ranged between 29 and 82 μm in diameter. The HA MN and X-linked HA MN were 270 μm in length. X-linked HA MN with fluorescein was inserted to a depth of 90% of the microneedle length successfully. There was no decrease in the penetration success rate for MN with up to 20% content of X-linked HA particulates. X-linked HA MN with up to 20% of particulate content did not change the dissolution time. Delay in degradation of HA, sustained drug release, and swelling behaviour of the skin layer can be obtained by X-linked HA MN.

Functionalised nanostructures for transdermal delivery of drug cargos

Nanotechnology has burgeoned over last decade exploring varieties of novel applications in all areas of science and technology. Utilisation of bio-friendly polymers for engineering nanostructures (NS) improves safety and efficacy in drug delivery. Biopolymers not merely employed for fabricating drug carriers but also leveraged for surface functionalisation of other NS to impart bio-mimicking properties. Biopolymer functionalised NS garnered researcher's attention because of their potential to enhance skin permeability of drug cargo. Biopolymers, i.e. cell-penetrating peptides (CPP), chitosan and hyaluronic acid not only enhance skin permeability but also add multiple functions due to their intrinsic biomimetic properties. This multifunctional drug delivery system is a promising tool to achieve skin delivery of large number of therapeutic agents. In this review, functionalisation of NS with biopolymers particularly polysaccharides and polypeptides is discussed in detail. In particular, applications of these functionalised NS for TDDS is elaborated. Moreover, this review provides framework for elaborating importance of functionalisation of NS to enhance skin permeability and depicts advantages of biopolymers to construct more biocompatible carriers for drug cargos.

Efficient Transdermal Delivery of Alendronate, a Nitrogen-Containing Bisphosphonate, Using Tip-Loaded Self-Dissolving Microneedle Arrays for the Treatment of Osteoporosis

To improve the transdermal bioavailability and safety of alendronate (ALN), a nitrogen-containing bisphosphonate, we developed self-dissolving microneedle arrays (MNs), in which ALN is loaded only at the tip portion of micron-scale needles by a dip-coating method (ALN(TIP)–MN). We observed micron-scale pores in rat skin just after application of ALN(TIP)–MN, indicating that transdermal pathways for ALN were created by MN. ALN was rapidly released from the tip of MNs as observed in an in vitro release study. The tip portions of MNs completely dissolved in the rat skin within 5 min after application in vivo. After application of ALN(TIP)–MN in mice, the plasma concentration of ALN rapidly increased, and the bioavailability of ALN was approximately 96%. In addition, the decrease in growth plate was effectively suppressed by this efficient delivery of ALN in a rat model of osteoporosis. Furthermore, no skin irritation was observed after application of ALN(TIP)–MN and subcutaneous injection of ALN, while mild skin irritation was induced by whole-ALN-loaded MN (ALN–MN)—in which ALN is contained in the whole of the micron-scale needles fabricated from hyaluronic acid—and intradermal injection of ALN. These findings indicate that ALN(TIP)–MN is a promising transdermal formulation for the treatment of osteoporosis without skin irritation.

Repeat application of microneedles does not alter skin appearance or barrier function and causes no measurable disturbance of serum biomarkers of infection, inflammation or immunity in mice in vivo

We address, for the first time, the impact of skin insertion on multiple occasions of polymeric microneedle arrays in an animal model in vivo. Dissolving microneedle arrays prepared from aqueous blends of 20% w/w Gantrez® S-97 BF and 40% w/w poly(vinyl pyrrolidone) 58kDa and hydrogel-forming microneedle arrays prepared from aqueous blends of and poly(ethyleneglycol) 10kDa were repeatedly applied to the skin of hairless mice in vivo. Skin appearance and skin barrier function, as illustrated by measurement of transepidermal water loss, were not measurably altered during the entire study period. Biomarkers of infection, immunity and inflammation/irritation were also statistically unchanged, regardless of the microneedle formulation, needle density or number of applications. Mice remained healthy throughout and continued to gain weight during the study. For example, transepidermal water loss values were typically in the range 10-15gm-2h-1 immediately prior to microneedle insertion and 15-25gm-2h-1 immediately following microneedle removal, regardless of when they were measured during the study periods. Serum biomarker levels, measured immediately post-mortem were always in the range 10-20µgml-1 for C-reactive protein, 0.5-1.5mgml-1 for Immunoglobulin G and 1000-2500pgml-1 for interleukin 1-β and were never statistically different from untreated controls. No measurable levels of tumour necrosis factor-α were found in any animals. These findings are encouraging for the formulations investigated, suggesting that their repeated use by patients will not cause undesirable side-effects. By beginning to address potential regulatory questions at an early stage, the microneedles field will be ideally-placed to take advantage of the potential market. This work illustrates a potential pre-clinical strategy for development of regulatory dossiers on microneedle technologies.

Nanocapsules engineered from polyhedral ZIF-8 templates for bone-targeted hydrophobic drug delivery

Biodegradable nanocapsules for bone-targeted simvastatin delivery were developed by using catechol modified gelatin as wall material and ZIF-8 as templates.

Development of a drug-coated microneedle array and its application for transdermal delivery of interferon alpha

Interferon alpha (IFNα) is one of the most famous drugs for the treatment of chronic hepatitis C and various types of human malignancy. Protein drugs, including IFNα, are generally administered by subcutaneous or intramuscular injection due to their poor permeability and low stability in the bloodstream or gastrointestinal tract. Therefore, in the present study, novel IFNα-coated polyvinyl alcohol-based microneedle arrays (IFNα-MNs) were fabricated for the transdermal delivery of IFNα without the painful injection. IFNα was rapidly released from MNs in phosphate buffered solution and these MNs presented piercing ability in the rat skin. Slight erythema and irritation were observed when MNs were applied to the rat skin, but these skin damages completely disappeared within 24 h after removing the IFNα-MNs. Furthermore, the pharmacokinetic parameters of IFNα-MNs were similar to those of IFNα subcutaneous administration. Finally, IFNα-MNs showed a significant antitumor effect in tumor bearing mice similar to that of IFNα subcutaneous administration. These results indicate that IFNα-MNs are a useful biomaterial tool for protein drug therapy and can improve the quality of life in patients by avoidance of painful injections.

Improvement of transdermal delivery of sumatriptan succinate using a novel self-dissolving microneedle array fabricated from sodium hyaluronate in rats

The purpose of the present study was to develop an alternative transdermal formulation containing sumatriptan succinate (SS) for the treatment of migraine. Novel self-dissolving SS-loaded microneedle arrays (MNs) were fabricated from sodium hyaluronate and their efficacy for transdermal delivery of SS was characterized. The resulting MNs maintained their skin piercing abilities for at least 30 min after being placed at a high relative humidity of 75%. Rapid release of SS from the MNs was also observed in vitro. Optical coherence tomography images demonstrated that MNs were able to successfully pierce into rat skin without any bending or cracking, and needles were completely dissolved within 1 h. MNs significantly increased transepidermal water loss; however, skin barrier function gradually recovered to control levels within 24 h, in contrast to the skin damage observed after tape stripping treatment. These findings indicated that the micropores created by MNs quickly resealed, and that the skin damage was reversible. Furthermore, a dose-dependent plasma concentration of SS was obtained after transdermal delivery using SS-loaded MNs in rats. Absorption of SS delivered by MNs was similar to that observed after subcutaneous injection and was associated with high bioavailability (ca. 90%), which was much higher than that produced by oral administration. These findings suggested that application of SS-loaded MNs to the skin provided an effective alternative approach to enhance the transdermal delivery of SS without serious skin damage, and would be likely to improve patient compliance.

Transdermal Drug Delivery: Innovative Pharmaceutical Developments Based on Disruption of the Barrier Properties of the stratum corneum

The skin offers an accessible and convenient site for the administration of medications. To this end, the field of transdermal drug delivery, aimed at developing safe and efficacious means of delivering medications across the skin, has in the past and continues to garner much time and investment with the continuous advancement of new and innovative approaches. This review details the progress and current status of the transdermal drug delivery field and describes numerous pharmaceutical developments which have been employed to overcome limitations associated with skin delivery systems. Advantages and disadvantages of the various approaches are detailed, commercially marketed products are highlighted and particular attention is paid to the emerging field of microneedle technologies.

[Development of a novel transdermal delivery system of peptide and protein drugs using microneedle arrays]

Transdermal delivery of peptide and protein drugs may be limited by the stratum corneum, which is a protective barrier against the entry of microorganisms and water. Many approaches have been utilized to promote peptide and protein drugs delivery across the stratum corneum, including chemical enhancer modification and physical disruption of barrier function. However, it has been difficult to achieve therapeutic levels of peptide and protein drugs via this route without any skin irritation. Recently, attention has been paid to the possibility of using microneedle arrays in delivering peptide and protein drugs into the skin. As a novel and minimally invasive approach, microneedle arrays are capable of creating superficial pathways across the skin for peptide and protein drugs to achieve enhanced transdermal drug delivery. This method combines the efficacy of conventional injection needles with the convenience of transdermal patches, while minimizing the disadvantages of these administration methods. Therefore, microneedle arrays are a very useful alternative method for delivering peptide and protein drugs from the skin into the systemic circulation without any serious damage to skin. In this review, recent challenges in the developments of microneedle arrays for the delivery of peptide and protein drugs are summarized. Then, future developments of microneedle arrays for the delivery of peptide and protein drugs are also discussed in order to improve their therapeutic efficacy and safety.

Microneedle/nanoencapsulation-mediated transdermal delivery: Mechanistic insights

A systematic study was undertaken to gain more insight into the mechanism of transdermal delivery of nanoencapsulated model dyes across microneedle (MN)-treated skin, a complex process not yet explored. Rhodamine B (Rh B) and fluorescein isothiocyanate (FITC) as model hydrophilic and hydrophobic small/medium-size molecules, respectively, were encapsulated in poly lactic-co-glycolic acid (PLGA) nanoparticles (NPs) and delivered through full thickness porcine skin pretreated with MN array. Permeation through MN-treated skin was affected by physicochemical characteristics of NPs and the encapsulated dyes. Dye flux was enhanced by smaller particle size, hydrophilicity, and negative zeta potential of NPs. Regarding encapsulated dyes, solubility at physiological pH and potential interaction with skin proteins proved to outweigh molecular weight as determinants of skin permeation. Data were verified using confocal laser scanning microscopy imaging. Findings coupled with the literature data are supportive of a mechanism involving influx of NPs, particularly of smaller size, deep into MN-created channels, generating depot dye-rich reservoirs. Molecular diffusion of the released dye across viable skin layers proceeds at a rate determined by its molecular characteristics. Data obtained provide mechanistic information of importance to the development of formulation strategies for more effective intradermal and transdermal MN-mediated delivery of nanoencapsulated therapeutic agents.

Nanostructured lipid carrier-loaded hyaluronic acid microneedles for controlled dermal delivery of a lipophilic molecule

Nanostructured lipid carriers (NLCs) were employed to formulate a lipophilic drug into hydrophilic polymeric microneedles (MNs). Hyaluronic acid (HA) was selected as a hydrophilic and bioerodible polymer to fabricate MNs, and nile red (NR) was used as a model lipophilic molecule. NR-loaded NLCs were consolidated into the HA-based MNs to prepare NLC-loaded MNs (NLC-MNs). A dispersion of NLCs was prepared by high-pressure homogenization after dissolving NR in Labrafil and mixing with melted Compritol, resulting in 268 nm NLCs with a polydispersity index of 0.273. The NLC dispersion showed a controlled release of NR over 24 hours, following Hixson-Crowell's cube root law. After mixing the NLC dispersion with the HA solution, the drawing lithography method was used to fabricate NLC-MNs. The length, base diameter, and tip diameter of the NLC-MNs were approximately 350, 380, and 30 μm, respectively. Fluorescence microscopic imaging of the NLC-MNs helped confirm that the NR-loaded NLCs were distributed evenly throughout the MNs. In a skin permeation study performed using a Franz diffusion cell with minipig dorsal skin, approximately 70% of NR was localized in the skin after 24-hour application of NLC-MNs. Confocal laser scanning microscopy (z-series) of the skin at different depths showed strong fluorescence intensity in the epidermal layer, which appeared to spread out radially with the passage of time. This study indicated that incorporation of drug-loaded NLCs into MNs could represent a promising strategy for controlled dermal delivery of lipophilic drugs.

Biomedical applications of bisphosphonates

Since their discovery over 100 years ago, bisphosphonates have been used industrially as corrosion inhibitors and complexing agents. With the discovery of their pharmacological activity in the late 1960s, implicating their high affinity for hydroxyapatite, bisphosphonates have been employed in the treatment of bone diseases and as targeting agents for colloids and drugs. They have notably been investigated for the treatment of Paget's disease, osteoporosis, bone metastases, malignancy-associated hypercalcemia, and pediatric bone diseases. Currently, they are first-line medications for several of these diseases and are taken by millions of patients worldwide, mostly postmenopausal women. A major problem associated with their use is their low oral bioavailability. Several delivery systems have been proposed to improve their absorption and to direct them to sites other than bone tissues. Beyond their important pharmacological role, the medical applications of bisphosphonates are numerous. In addition, their metal-chelating properties have been exploited to coat and stabilize implants, nanoparticulates, and contrast agents. In this contribution, we review the pharmacological and clinical uses of bisphosphonates and highlight their novel applications in the pharmaceutical and biomedical fields.

Polymer microneedles for transdermal drug delivery

A microneedle system has been developed to deliver chemical and biological agents through the stratum corneum, which is the main barrier to drug delivery. Recently, microneedles have been fabricated from various kinds of polymers, including biocompatible polymer, biodegradable polymer, and water-soluble polymer. Polymer microneedles offer the benefits of ease of fabrication, cost-effectiveness, and mass production, as well as controlled drug release using the water solubility and degradation properties of polymer. In this review, the key features of polymer microneedles are discussed, including fabrication, materials, mechanical properties, drug delivery properties, and applications. Polymer microneedles provide a promising method for transdermal drug delivery by utilizing various physical and chemical properties of polymer.