Multicomponent chemical enhancer formulations for transdermal drug delivery: More is not always better

Drug delivery to and through the skin

Drug delivery technology has advanced significantly over >50 years, and has produced remarkable innovation, countless publications and conferences, and generations of talented and creative scientists. However, a critical review of the current state-of-the-art reveals that the translation of clever and sophisticated drug delivery technologies into products, which satisfy important, unmet medical needs and have been approved by the regulatory agencies, has - given the investment made in terms of time and money - been relatively limited. Here, this point of view is illustrated using a case study of technology for drug delivery into and through the skin and aims:  to examine the historical development of this field and the current state-of-the-art;  to understand why the translation of drug delivery technologies into products that improve clinical outcomes has been quite slow and inefficient; and  to suggest how the impact of technology may be increased and the process of concept to approved product accelerated.

Glycols: The ubiquitous solvent for dermal formulations

Glycols stand out as one of the most commonly employed safe and effective excipients for pharmaceutical and cosmeceutical products. Their widespread adoption can be attributed to their exceptional solvency characteristics and their ability to interact effectively with skin lipids and keratin for permeation enhancement. Notably, propylene glycol enjoys significant popularity in this regard. Ongoing research endeavours have been dedicated to scrutinising the impact of glycols on dermal drug delivery and shedding light on the intricate mechanisms by which glycols enhance skin permeation. This review aims to mitigate the discordance within the existing literature, assemble a holistic understanding of the impact of glycols on the percutaneous absorption of active compounds and furnish the reader with a profound comprehension of the foundational facets pertaining to their skin permeation enhancement mechanisms, while simultaneously delving deeper into the intricacies of these processes.

Confocal Raman spectroscopy at different laser wavelengths in analyzing stratum corneum and skin penetration properties of mixed PEGylated emulsifier systems

Emulsifier mixtures are widely used in cosmetics and pharmaceutics and thus, brought extensive studies for their performances on skin applications. PEG-20cetyl ether (C20) is recently proposed to induce skin irritation and is of interest to study its skin interactions when mixed with other emulsifiers. PEG-2oleyl ether (O2) and PEG-20stearyl ether (S20) are selected and in specific, 50 mM of C20, O2, S20 as well as Mix1 (50 mM C20 mixed with 50 mM O2) and Mix2 (50 mM C20 mixed with 50 mM S20) solutions were applied on skin samples. Confocal Raman spectroscopy (CRS) analyses of stratum corneum (SC) thickness and SC lipid content were performed after 4 h skin treatments. In parallel, skin penetration properties were also evaluated via CRS by applying procaine solutions with/without emulsifiers on skin samples for 24 h. In terms of the CRS measurements, two excitation wavelengths of 532 nm and 785 nm are both utilized in this study and we secondly aimed to compare their results and suitability in SC and skin analyses. Based on the experimental observations, comparable results are obtained by using both excitation wavelengths of 532 nm and 785 nm demonstrating their suitability in analyzing SC and skin samples. Thereinto, 785 nm laser wavelength shows the advantage of deeper skin penetration and allows the measurements of fluorescent skin samples; 532 nm laser wavelength enables simple measurement performance without substrate and coverslip interference. With regards to the results of emulsifier mixtures, the addition of S20 and O2 reduced the skin interactions and penetration enhancing ability of C20, giving us the hint to build milder systems with emulsifier mixtures. Besides, the CRS results of stronger skin interruption were also correlated with the higher critical micelle concentration (CMC) values of emulsifiers and their mixtures, which may provide evidence in explaining the interactions between emulsifiers and skin.

Current Status of Amino Acid-Based Permeation Enhancers in Transdermal Drug Delivery

Transdermal drug delivery (TDD) presents many advantages compared to other conventional routes of drug administration, yet its full potential has not been achieved. The administration of drugs through the skin is hampered by the natural barrier properties of the skin, which results in poor permeation of most drugs. Several methods have been developed to overcome this limitation. One of the approaches to increase drug permeation and thus to enable TDD for a wider range of drugs consists in the use of chemical permeation enhancers (CPEs), compounds that interact with skin to ultimately increase drug flux. Amino acid derivatives show great potential as permeation enhancers, as they exhibit high biodegradability and low toxicity. Here we present an overview of amino acid derivatives investigated so far as CPEs for the delivery of hydrophilic and lipophilic drugs across the skin, focusing on the structural features which promote their enhancement capacity.

Emerging skin-targeted drug delivery strategies to engineer immunity: A focus on infectious diseases

INTRODUCTION

Infectious pathogens are global disrupters. Progress in biomedical science and technology has expanded the public health arsenal against infectious diseases. Specifically, vaccination has reduced the burden of infectious pathogens. Engineering systemic immunity by harnessing the cutaneous immune network has been particularly attractive since the skin is an easily accessible immune-responsive organ. Recent advances in skin-targeted drug delivery strategies have enabled safe, patient-friendly, and controlled deployment of vaccines to cutaneous microenvironments for inducing long-lived pathogen-specific immunity to mitigate infectious diseases, including COVID-19.

AREAS COVERED

This review briefly discusses the basics of cutaneous immunomodulation and provides a concise overview of emerging skin-targeted drug delivery systems that enable safe, minimally invasive, and effective intracutaneous administration of vaccines for engineering systemic immune responses to combat infectious diseases.

EXPERT OPINION

In-situ engineering of the cutaneous microenvironment using emerging skin-targeted vaccine delivery systems offers remarkable potential to develop diverse immunization strategies against pathogens. Mechanistic studies with standard correlates of vaccine efficacy will be important to compare innovative intracutaneous drug delivery strategies to each other and to existing clinical approaches. Cost-benefit analyses will be necessary for developing effective commercialization strategies. Significant involvement of industry and/or government will be imperative for successfully bringing novel skin-targeted vaccine delivery methods to market for their widespread use.

Transdermal delivery of tolterodine tartrate for overactive bladder treatment: In vitro and in vivo evaluation

The purpose of the study was to develop a transdermal tolterodine tartrate (TT) patch and to analyse its efficacy for overactive bladder (OAB) treatment. Patches were prepared using various polymers and plasticizers via the solvent casting method. The patches were characterized for tensile strength, thickness, moisture content, modulus of elasticity and water absorption capacity. Differential scanning calorimetry and Fourier transform infrared analyses were also performed. To determine patch effectiveness, in vitro release, permeation and animal studies were performed. The patches showed satisfactory percentage of release, up to 89.9 %, and their mechanical properties included thickness (0.10-0.15 mm), tensile strength (4.62-9.98 MPa) and modulus of elasticity (20-29 MPa). There were no significant interactions between TT and other excipients. Animal studies indicated that the TT patch reduced the incidence of side effects; however, studies of longer duration are required to determine the effectiveness in treating OAB.

Topical formulations of miltefosine for cutaneous leishmaniasis in a BALB/c mouse model

Cutaneous leishmaniasis (CL) is caused by several species of the protozoan parasite Leishmania and affects approximately 10 million people worldwide. Currently available drugs are not ideal due to high cost, toxicity, parenteral administration and suboptimal efficacy. Miltefosine is the only oral treatment (Impavido®) available to treat CL, given over a period of 28 days with common side effects such as vomiting and diarrhoea.

OBJECTIVE

To explore the local application of miltefosine as a topical formulation to enhance activity and reduce the drug's adverse effects.

METHODS

The antileishmanial activity of miltefosine was confirmed in vitro against several Leishmania species. The permeation of miltefosine, in different solvents and solvent combinations, through BALB/c mouse skin was evaluated in vitro using Franz diffusion cells. The topical formulations which enabled the highest drug permeation or skin disposition were tested in vivo in BALB/c mice infected with L. major.

KEY FINDINGS

The overall permeation of miltefosine through skin was low regardless of the solvents used. This was reflected in limited antileishmanial activity of the drug formulations when applied topically in vivo. All topical formulations caused skin irritation.

CONCLUSIONS

We conclude that miltefosine is not an appropriate candidate for the topical treatment of CL.

Niosomes containing hydroxyl additives as percutaneous penetration enhancers: effect on the transdermal delivery of sulfadiazine sodium salt

The aim of this study was to improve the transdermal permeation of sulfadiazine sodium, employing synergistic combination of surfactants (in the form of niosomes) and additives with different number of hydroxylic groups, (following referred to as "alcohol"), as component of the bilayer. In particular the effect of different concentration of each alcohol (ethanol, propylene glycol or glycerol, from 5%, to 40% v/v) on niosomes size and distribution, drug entrapment efficiencies and ex vivo drug percutaneous permeation were evaluated, identifying formulations giving the best performances. The findings revealed that the presence of alcohol critically affect the physico-chemical properties of niosomes, with regards to dimensions, drug encapsulation and permeation. Vesicular size increased with the amount of alcohol and at the same alcohol concentration, follow the sequence ethanol>propylene glycol>glycerol. Loaded niosomes were larger than empty ones. Low E% values were found for ethanol, even less in propylene glycol and glycerol based samples, confirming that the chemical structure of the alcohol and its physico-chemical properties, affected the sulfadiazine entrapment efficiency. The comparative evaluation of percutaneous permeation profiles showed that the cumulative amount of permeated drug increases with alcohol concentration up to 20% v/v. Higher concentration (40% v/v) resulted in a strong decrease of the potential skin permeation. Best performances were obtained with glycerol. In all cases ex vivo sulfadiazine percutaneous permeations are controlled and improved respect to the corresponding free drug solutions and traditional niosomes used as controls.

Single compartment drug delivery

Drug design is built on the concept that key molecular targets of disease are isolated in the diseased tissue. Systemic drug administration would be sufficient for targeting in such a case. It is, however, common for enzymes or receptors that are integral to disease to be structurally similar or identical to those that play important biological roles in normal tissues of the body. Additionally, systemic administration may not lead to local drug concentrations high enough to yield disease modification because of rapid systemic metabolism or lack of sufficient partitioning into the diseased tissue compartment. This review focuses on drug delivery methods that physically target drugs to individual compartments of the body. Compartments such as the bladder, peritoneum, brain, eye and skin are often sites of disease and can sometimes be viewed as "privileged," since they intrinsically hinder partitioning of systemically administered agents. These compartments have become the focus of a wide array of procedures and devices for direct administration of drugs. We discuss the rationale behind single compartment drug delivery for each of these compartments, and give an overview of examples at different development stages, from the lab bench to phase III clinical trials to clinical practice. We approach single compartment drug delivery from both a translational and a technological perspective.

Practical considerations for optimal transdermal drug delivery

PURPOSE

The properties of various transdermal drug delivery system (TDDS) products are reviewed, with safety recommendations and guidance on addressing questions frequently posed by patients and caregivers.

SUMMARY

Drug delivery via a TDDS can offer many advantages over other methods of administration, but those benefits can be compromised by improper use or alteration of medication patches or a lack of awareness of the properties of different patch types (reservoir, matrix, drug-in-adhesive). To assess current TDDS technologies and recommended practices for safe and effective use of medication patches, a literature search for articles on commonly used TDDS products available in the United States was conducted; supplemental information was obtained from package inserts and through direct communication with manufacturers. In addition to recommendations on the site and duration of TDDS application and proper patch disposal, clinicians must consider (1) potential problems with cutting patches as a method of dosage adjustment, (2) safety concerns related to the electric conductivity of metal-containing patches, (3) appropriate strategies for managing patch adhesion failures, and (4) the advisability of writing on patches for medication safety or compliance reasons. Clinicians should also be prepared to counsel patients about TDDS-specific recommendations on the avoidance of sunlight and other external heat sources during the use of a medication patch.

CONCLUSION

Practical considerations related to transdermal drug delivery include the appropriateness of cutting patches, the implications of their containing metallic components, and whether they may be covered with tape or written on. Manufacturers of patches provide some useful information on these topics.