Nanoemulsions as potential vehicles for transdermal and dermal delivery of hydrophobic compounds: an overview

INTRODUCTION: In recent years, nanoemulsions have been investigated as potential drug delivery vehicles for transdermal and dermal delivery of many compounds especially hydrophobic compounds in order to avoid clinical adverse effects associated with oral delivery of the same compounds. Droplet size and surface properties of nanoemulsions play an important role in the biological behavior of the formulation. AREAS COVERED: In this review, current literature of transdermal and dermal delivery of hydrophobic compounds both in vitro as well as in vivo has been summarized and analyzed. EXPERT OPINION: Nanoemulsions have been formulated using a variety of pharmaceutically acceptable excipients. In many cases of dermal and transdermal nanoemulsions, the skin irritation or skin toxicity issues on human beings have not been considered which needs to be evaluated properly. In the last decade, much attention has been made in exploring new types of nanoemulsion-based drug delivery system for dermal and transdermal delivery of many hydrophobic compounds. This area of research would be very advantageous for formulation scientists in order to develop some nanoemulsion-based formulations for their commercial exploitation and clinical applications.     

Low-frequency sonophoresis: application to the transdermal delivery of macromolecules and hydrophilic drugs

Importance of the field: Transdermal delivery of macromolecules provides an attractive alternative route of drug administration when compared to oral delivery and hypodermic injection because of its ability to bypass the harsh gastrointestinal tract and deliver therapeutics non-invasively. However, the barrier properties of the skin only allow small, hydrophobic permeants to traverse the skin passively, greatly limiting the number of molecules that can be delivered via this route. The use of low-frequency ultrasound for the transdermal delivery of drugs, referred to as low-frequency sonophoresis (LFS), has been shown to increase skin permeability to a wide range of therapeutic compounds, including both hydrophilic molecules and macromolecules. Recent research has demonstrated the feasibility of delivering proteins, hormones, vaccines, liposomes and other nanoparticles through LFS-treated skin. In vivo studies have also established that LFS can act as a physical immunization adjuvant. LFS technology is already clinically available for use with topical anesthetics, with other technologies currently under investigation.

Areas covered in this review: This review provides an overview of mechanisms associated with LFS-mediated transdermal delivery, followed by an in-depth discussion of the current applications of LFS technology for the delivery of hydrophilic drugs and macromolecules, including its use in clinical applications.

What the reader will gain: The reader will gain an insight into the field of LFS-mediated transdermal drug delivery, including how the use of this technology can improve on more traditional drug delivery methods.

Take home message: Ultrasound technology has the potential to impact many more transdermal delivery platforms in the future due to its unique ability to enhance skin permeability in a controlled manner.     

Biomedical applications of microneedles in therapeutics: recent advancements and implications in drug delivery

Introduction: The skin, as the largest organ, is a better option for drug delivery in many diseases. However, most transdermal delivery is difficult due to the low permeability of therapeutics across the various skin layers. There have been many innovations in transdermal drug delivery to enhance the therapeutic efficacy of the drugs administered. Microneedles (MN), micron sized needles, are of great interest to scientists as a new therapeutic vehicle through transdermal routes, especially for vaccines, drugs, small molecules, etc.

Areas covered: This review covers new insights into different types of MNs such as solid, hollow, coated and dissolving MNs (SMNs, HMNs, CMNs, and DMNs) for selected biomedical applications in detail. Specific focus has been given to CMNs and DMNs for vaccine and drug delivery applications with recent developments in new MNs covered.

Expert opinion: This review explores the feasibility of innovative MNs used as a drug delivery carrier. Because most of the SMNs and HMNs have many limitations, it is difficult to achieve therapeutic efficacy. Therefore, many scientists are investigating functional modifications of MNs through covalent and non-covalent methods, especially for CMNs and DMNs. The biomedical applications of MNs are growing and new exciting improvements could be achieved, thus resulting in better micro/nano technologies in the near future.     

Utilization of nanoemulsions to enhance bioactivity of pharmaceuticals,supplements, and nutraceuticals: Nanoemulsion delivery systems and nanoemulsion excipient systems

ABSTRACT

Introduction: The efficacy of many hydrophobic bioactives (pharmaceuticals, supplements, and nutraceuticals) is limited due to their relatively low or highly variable bioavailability. Nanoemulsions consisting of small lipid droplets (r < 100 nm) dispersed in water can be designed to improve bioavailability.

Areas covered: The major factors limiting the oral bioavailability of hydrophobic bioactive agents are highlighted: bioaccessibility, absorption and transformation. Two nanoemulsion-based approaches to control these processes and improve bioavailability are discussed: nanoemulsion delivery systems (NDS) and nanoemulsion excipient systems (NES). In NDS, hydrophobic bioactives are dissolved within the lipid phase of oil-in-water nanoemulsions. In NES, the bioactives are present within a conventional drug, supplement, or food, which is consumed with an oil-in-water nanoemulsion. Examples of NDS and NES utilization to improve bioactive bioavailability are given.

Expert opinion: Considerable progress has been made in nanoemulsion design, fabrication, and testing. This knowledge facilitates the design of new formulations to improve the bioavailability of pharmaceuticals, supplements, and nutraceuticals. NDS and NES must be carefully designed based on the major factors limiting the bioavailability of specific bioactives. Research is still required to ensure these systems are commercially viable, and to demonstrate their safety and efficacy using animal and human feeding studies.     

Spray-on transdermal drug delivery systems

Introduction: Transdermal drug delivery possesses superior advantages over other routes of administration, particularly minimizing first-pass metabolism. Transdermal drug delivery is challenged by the barrier nature of skin. Numerous technologies have been developed to overcome the relatively low skin permeability, including spray-on transdermal systems.

Areas covered: A transdermal spray-on system (TSS) usually consists of a solution containing the drug, a volatile solvent and in many cases a chemical penetration enhancer. TSS promotes drug delivery via the complex interplay between solvent evaporation and drug–solvent drag into skin. The volatile solvent carries the drug into the upper layers of the stratum corneum, and as the volatile solvent evaporates, an increase in the thermodynamic activity of the drug occurs resulting in an increased drug loading in skin.

Expert opinion: TSS is easily applied, delivering flexible drug dosage and associated with lower incidence of skin irritation. TSS provides a fast-drying product where the volatile solvent enables uniform drug distribution with minimal vehicle deposition on skin. TSS ensures precise dose administration that is aesthetically appealing and eliminates concerns of residual drug associated with transdermal patches. Furthermore, it provides a better alternative to traditional transdermal products due to ease of product development and manufacturing.     

Future of the transdermal drug delivery market – have we barely touched the surface?

ABSTRACT

Introduction: Transdermal drug delivery is the movement of drugs across the skin for absorption into the systemic circulation. Transfer of the drug can occur via passive or active means; passive transdermal products do not disrupt the stratum corneum to facilitate delivery whereas active technologies do. Due to the very specific physicochemical properties necessary for successful passive transdermal drug delivery, this sector of the pharmaceutical industry is relatively small. There are many well-documented benefits of this delivery route however, and as a result there is great interest in increasing the number of therapeutic substances that can be delivered transdermally.

Areas Covered: This review discusses the various transdermal products that are currently/have been marketed, and the paths that led to their success, or lack of. Both passive and active transdermal technologies are considered with the advantages and limitations of each highlighted. In addition to marketed products, technologies that are in the investigative stages by various pharmaceutical companies are reviewed.

Expert Opinion: Passive transdermal drug delivery has made limited progress in recent years, however with the ongoing intense research into active technologies, there is great potential for growth within the transdermal delivery market. A number of active technologies have already been translated into marketed products, with other platforms including microneedles, rapidly progressing towards commercialisation.     

Topical delivery of acyclovir and ketoconazole

Abstract

Context: Viral and fungal cutaneous manifestations are regularly encountered in immunocompromised human immunodeficiency virus/acquired immunodeficiency syndrome individuals and can be treated by drugs such as acyclovir and ketoconazole, respectively.

Objective: The aim of this study was to determine whether the novel Pheroid? delivery system improved the transdermal delivery and/or dermal delivery of acyclovir and ketoconazole when incorporated into semi-solid formulations.

Materials and methods: Semi-solid products (creams and emulgels) containing these drug compounds were formulated, either with or without (control) the Pheroid? delivery system. The stability of the formulated semi-solid products was examined over a period of six months and included the assay of the actives, pH, viscosity, mass loss and particle size observation. Vertical Franz cell diffusion studies and tape stripping methods were used to determine the in vitro, stratum corneum (SC)-epidermis and epidermis-dermis delivery of these formulations.

Results and discussion: Stability tests showed that none of the formulations were completely stable. Acyclovir showed a biphasic character during the in vitro skin diffusion studies for all the tested formulations. The Pheroid? cream enhanced the transdermal, SC-epidermis and epidermis–dermis delivery of acyclovir the most. The average amount of ketoconazole diffused over 12?h showed improved delivery of ketoconazole, with the Pheroid? emulgel exhibiting the best transdermal and epidermis–dermis delivery.

Conclusion: The Pheroid? formulae increased transdermal penetration as well as delivery to the dermal and epidermal skin layers. The Pheroid? emulgel and the Pheroid? cream increased the topical delivery of ketoconazole and acyclovir, respectively.     

The role of microneedles for drug and vaccine delivery

Introduction: Transdermal drug delivery offers a number of advantages for the patient, not only due to its non-invasive and convenient nature, but also due to factors such as avoidance of first-pass metabolism and prevention of gastrointestinal degradation. It has been demonstrated that microneedles (MNs) can increase the number of compounds amenable to transdermal delivery by penetrating the skin’s protective barrier, the stratum corneum, and creating a pathway for drug permeation to the dermal tissue below.

Areas covered: MNs have been extensively investigated for drug and vaccine delivery. The different types of MN arrays and their delivery capabilities are discussed in terms of drugs, including biopharmaceutics and vaccines. Patient usage and effects on the skin are also considered.

Expert opinion: MN research and development is now at the stage where commercialisation is a viable possibility. There are a number of long-term safety questions relating to patient usage which will need to be addressed moving forward. Regulatory guidance is awaited to direct the scale-up of the manufacturing process alongside provision of clearer patient instruction for safe and effective use of MN devices.     

Microstructured bicontinuous phase formulations: their characterization and application in dermal and transdermal drug delivery

Introduction: The development of approaches to increase drug solubility and partitioning into the skin is an active area of research in topical and transdermal delivery. In addition to forming spherical aggregates, e.g., conventional oil in water or water in oil microemulsions, the combination of an oil, surfactant and water can create bicontinuous structures where the self-assembly properties of surfactants mean that the boundaries between oil and water are no longer random. This leads to the formation of specific microstructures whose intrinsic properties and interactions with the drug will determine the ability to formulate a given drug, its stability once formulated and its subsequent delivery.

Areas covered: The review explores the relationship between the microstructure of biphasic formulations, present in microemulsions and liquid crystalline phases, and drug delivery into the skin. An overview of possible internal microstructures is followed by a summary of the methods used for structure characterization. The final section presents the work to-date and discusses the efficacy of such vehicles in enhancing dermal and transdermal delivery.

Expert opinion: The combination of water, surface agent and oil generates a broad range of three dimensional structures differing in both chemical and physical proprieties. Knowledge of the microstructure is important in understanding the behavior of a formulation and its effect on drug delivery into the skin. Microstructure complexity, interactions between the drug and the vehicle (i.e., location and mobility) and those between the vehicle and the skin are key determinants of drug delivery.     

Biomaterials as novel penetration enhancers for transdermal and dermal drug delivery systems

Abstract

The highly organized structure of the stratum corneum provides an effective barrier to the drug delivery into or across the skin. To overcome this barrier function, penetration enhancers are always used in the transdermal and dermal drug delivery systems. However, the conventional chemical enhancers are often limited by their inability to delivery large and hydrophilic molecules, and few to date have been routinely incorporated into the transdermal formulations due to their incompatibility and local irritation issues. Therefore, there has been a search for the compounds that exhibit broad enhancing activity for more drugs without producing much irritation. More recently, the use of biomaterials has emerged as a novel method to increase the skin permeability. In this paper, we present an overview of the investigations on the feasibility and application of biomaterials as penetration enhancers for transdermal or dermal drug delivery systems.     

Charged liposomes as carriers to enhance the permeation through the skin

Introduction: In recent years, there has been increased interest in developing charged liposomes as carriers for transdermal drug delivery. It is necessary to modify the basic composition of the liposomes in order to enhance the penetration properties of the vesicles through the skin. Charged liposomes offer several advantages compared with previous drug delivery systems.

Areas covered: This paper provides a brief overview of the different drug delivery systems that exist which aim to improve the permeation of drugs through the skin, focusing on the use of charged liposomes for transdermal delivery. We propose a classification of such liposomes based on the origin of the charge given to the vesicles.

Expert opinion: Despite the advances that are occurring in the design of charged liposomes for transdermal drug delivery, the long-term stability continues to be a drawback in such systems. The presence of charge on the surface of the vesicles favors the electrostatic repulsion among them, creating a ζ potential positive or negative that prevents their aggregation and flocculation. However, there is loss of the encapsulated drug, which limits the in vivo use of these systems. It should be emphasized that charged liposomes are indeed a promising candidate for use in gene therapy and vaccine targeting, in a great diversity of diseases, for which drugs are administered by the percutaneous route.     

Heat effects on drug delivery across human skin

ABSTRACT

Introduction: Exposure to heat can impact the clinical efficacy and/or safety of transdermal and topical drug products. Understanding these heat effects and designing meaningful in vitro and in vivo methods to study them are of significant value to the development and evaluation of drug products dosed to the skin.

Areas covered: This review provides an overview of the underlying mechanisms and the observed effects of heat on the skin and on transdermal/topical drug delivery, thermoregulation and heat tolerability. The designs of several in vitro and in vivo heat effect studies and their results are reviewed.

Expert opinion: There is substantial evidence that elevated temperature can increase transdermal/topical drug delivery. However, in vitro and in vivo methods reported in the literature to study heat effects of transdermal/topical drug products have utilized inconsistent study conditions, and in vitro models require better characterization. Appropriate study designs and controls remain to be identified, and further research is warranted to evaluate in vitro-in vivo correlations and the ability of in vitro models to predict in vivo effects. The physicochemical and pharmacological properties of the drug(s) and the drug product, as well as dermal clearance and heat gradients may require careful consideration.     

Current aspects of formulation efforts and pore lifetime related to microneedle treatment of skin

Importance of the field: The efficacy of microneedles in the area of transdermal drug delivery is well documented. Multiple studies have shown that enhancement of skin permeation by means of the creation of microscopic pores in the stratum corneum can greatly improve the delivery rates of drugs. However, skin pretreatment with microneedles is not the only factor affecting drug transport rates. Other factors, including drug formulation and rate of micropore closure, are also important for optimizing delivery by this route.

Areas covered in this review: This review aims to highlight work that has been done in these areas, with an emphasis on drug formulation parameters that affect transdermal flux.

What the reader will gain: This review creates an appreciation for the many factors affecting microneedle-enhanced delivery. Most results clearly indicate that microneedle skin pretreatment by itself may have different effects on drug transport depending on the formulation used, and formulation characteristics have different effects on the transport through untreated skin and microneedle-treated skin. Several formulation approaches are reported to optimize microneedle-enhanced drug delivery, including co-solvent use, vesicular, nanoparticulate and gel systems.

Take home message: In addition to well-established factors that affect microneedle-assisted delivery (geometry, type of microneedle, etc.), formulation and pore viability are also critical factors that must be considered.     

Permeation enhancer strategies in transdermal drug delivery

Abstract

Today, ～74% of drugs are taken orally and are not found to be as effective as desired. To improve such characteristics, transdermal drug delivery was brought to existence. This delivery system is capable of transporting the drug or macromolecules painlessly through skin into the blood circulation at fixed rate. Topical administration of therapeutic agents offers many advantages over conventional oral and invasive techniques of drug delivery. Several important advantages of transdermal drug delivery are prevention from hepatic first pass metabolism, enhancement of therapeutic efficiency and maintenance of steady plasma level of the drug. Human skin surface, as a site of drug application for both local and systemic effects, is the most eligible candidate available. New controlled transdermal drug delivery systems (TDDS) technologies (electrically-based, structure-based and velocity-based) have been developed and commercialized for the transdermal delivery of troublesome drugs. This review article covers most of the new active transport technologies involved in enhancing the transdermal permeation via effective drug delivery system.     

Topical delivery for the treatment of psoriasis

Importance of the field: Psoriasis is one of the most common human skin diseases. Topical therapy forms the cornerstone in the management of mild-to-moderate psoriasis. Topical therapies are also used as adjunctive to systemic therapy in moderate and severe forms of the disease.

Areas covered in this review: In this review, an overview of psoriasis pathogenesis, new topical medications for psoriasis, new targets and molecules, combination topical therapies and combination of topical and phototherapy is provided. Over the past decade several efficacious and acceptable treatment options have emerged from the age-old therapies. The development of sophisticated formulation options has led to an enhancement in the rate and extent of drug delivery across the skin, increasing therapeutic value and improving patient compliance.

What the reader will gain: Readers will learn about monotherapy and combination topical products as well as new topical drug delivery technology to achieve optimal clinical outcomes. This review will highlight the need to generate more dermal pharmacokinetic data for better understanding of the impact of formulation change on skin pharmacokinetics to help design improved topical drug delivery systems.

Take home message: New topical formulations have the potential to achieve better efficacy with improved safety profile.     

Transdermal delivery of calcium channel blockers for hypertension

Introduction: Calcium channel blockers are a very important class of antihypertensive drugs. Most calcium channel blockers (CCBs) exhibiting low oral bioavailability are required to be taken more than once a day due to their short half-lives which result in poor patient compliance. There is an ineluctable requirement for improved drug-delivery devices for CCBs because of the quantum of their utilization and shortcoming associated with their conventional dosage forms.

Areas covered: There have been worthwhile research endeavors worldwide to investigate the skin permeation and to develop transdermal formulations of various categories of CCBs. This review explores the investigations on the feasibility and applicability of systemic delivery of various CCBs via skin.

Expert opinion: Transdermal delivery of CCBs has been particularly acknowledged as a potential drug-delivery route in the therapy of hypertension. Several overtures have been made to enhance delivery of these drugs via skin barrier. There have been remarkable research endeavors worldwide to investigate the skin permeation and to develop transdermal systems of various CCBs. Persistent advancement in this area holds promise for the long-term success in technologically advanced transdermal dosage forms being commercialized sooner rather than later.     

Superiority of liquid crystalline cubic nanocarriers as hormonal transdermal vehicle: comparative human skin permeation-supported evidence

ABSTRACT

Objectives: The aim of this investigation was to explore the feasibility of various nanocarriers to enhance progesterone penetration via the human abdominal skin.

Methods: Four progesterone-loaded nanocarriers; cubosomes, nanoliposomes, nanoemulsions and nanomicelles were formulated and characterized regarding particle size, zeta potential, % drug encapsulation and in vitro release. Structural elucidation of each nanoplatform was performed using transmission electron microscopy. Ex vivo skin permeation, deposition ability and histopathological examination were evaluated using Franz diffusion cells.

Results: Each nanocarrier was fabricated with a negative surface, nanometric size (≤ 270 nm), narrow size distribution and reasonable encapsulation efficiency. In vitro progesterone release showed a sustained release pattern for 24 h following a non-Fickian transport diffusion mechanism. All nanocarriers exhibited higher transdermal flux relative to free progesterone. Cubosomes revealed a higher skin penetration with transdermal steady flux of 48.57.10^–2 ± 0.7 µg/cm² h. Nanoliposomes offered a higher percentage of skin progesterone deposition compared to other nanocarriers. Based on the histopathological examination, cubosomes and nanoliposomes were found to be biocompatible for transdermal application. Confocal laser scanning microscopy confirmed the ability of fluoro-labeled cubosomes to penetrate through the whole skin layers.

Conclusion: The elaborated cubosomes proved to be a promising non-invasive nanocarrier for transdermal hormonal delivery.     

Chemical penetration enhancers: a patent review

Background: Ever since transdermal drug delivery came into existence, it has offered great promises, although most of them are yet to be fulfilled owing to some intrinsic restrictions of the transdermal route. On the positive side, transdermal drug delivery systems present advantages including non-invasiveness, prolonged therapeutic effect, reduced side effects, improved bioavailability, better patient compliance and easy termination of drug therapy. The greatest hindrance in the percutaneous delivery is the obstruction property of the stratum corneum, the outermost layer of the skin, in addition to usual problems such as skin binding, skin metabolism, cutaneous toxicity and prolonged lag times. Objective: This paper reviews investigations on the feasibility and application of penetration enhancers as described in recent patents, which help in the selection of a suitable sorption promoter(s) for enhanced delivery of medicaments through the skin. Method: The patents granted under various categories of penetration enhancers have been discussed including fatty acids, terpenes, fatty alcohol, pyrrolidone, sulfoxides, laurocapram, surface active agents, amides, amines, lecithin, polyols, quaternary ammonium compounds, silicones, alkanoates and so on. Conclusion: Scores of promising chemicals have been harnessed for their skin permeation promoting capacity as mentioned earlier. In future, many more chemicals and putative enhancers are likely be documented and patented.     

脂质体经皮局部给药研究进展

目的：探讨脂质体在皮肤局部给药系统中的作用.方法：通过阐述脂质体在皮肤给药系统中透皮吸收的作用机制、影响因素以及在各领域的应用,了解脂质体在皮肤局部给药系统中的作用.结果：脂质体应用于皮肤局部给药系统具有许多优势.结论：脂质体在皮肤局部给药系统中有很大的发展潜力.     

Liposomes in cosmeceutics

Introduction: Cosmeceuticals are cosmetic products with biologically active ingredients purporting to have medical or drug-like benefits. Some cosmeceuticals can act effectively when reaching their target sites in the deeper layers of the skin. However, the barrier nature of skin causes significant difficulties for compounds to be delivered through. Therefore, scientists are investigating various strategies to overcome these barrier properties. Liposomes have been claimed to improve the topical delivery of compounds.

Areas covered: This paper offers a brief overview of current approaches in the research and development of liposomal formulations to improve the performance of cosmeceuticals, from recent literature. This review deals with the potential of liposomes as a skin delivery system for cosmeceuticals, with a focus on the clinical application of liposomes.

Expert opinion: Liposomes are well-known vesicular cosmetic delivery systems. The topical application of liposomes offers a wide range of advantages including increased moisturization, restoring action, biodegradability, biocompatibility and extended and slow dermal release. Their similar structure to biological membranes allows penetration into the epidermal barrier, compared with other delivery systems. The incorporation of cosmeceuticals using suitable delivery systems is important in the management of cosmetic disorders.