Nanocarriers for Skin Applications: Where Do We Stand?

Skin penetration of active molecules for treatment of diverse diseases is a major field of research owing to the advantages associated with the skin like easy accessibility, reduced systemic‐derived side effects, and increased therapeutic efficacy. Despite these advantages, dermal drug delivery is generally challenging due to the low skin permeability of therapeutics. Although various methods have been developed to improve skin penetration and permeation of therapeutics, they are usually aggressive and could lead to irreversible damage to the stratum corneum. Nanosized carrier systems represent an alternative approach for current technologies, with minimal damage to the natural barrier function of skin. In this Review, the use of nanoparticles to deliver drug molecules, genetic material, and vaccines into the skin is discussed. In addition, nanotoxicology studies and the recent clinical development of nanoparticles are highlighted to shed light on their potential to undergo market translation.

Polymeric micelle mediated follicular delivery of spironolactone: Targeting the mineralocorticoid receptor to prevent glucocorticoid-induced activation and delayed cutaneous wound healing

Impaired wound healing in patients receiving glucocorticoid therapy is a serious clinical concern: mineralocorticoid receptor (MR) antagonists can counter glucocorticoid-induced off-target activation of MR receptors. The aim of this study was to investigate the cutaneous delivery of the potent MR antagonist, spironolactone (SPL), from polymeric micelle nanocarriers, prepared using a biodegradable copolymer, methoxy-poly(ethylene glycol)-di-hexyl-substituted-poly(lactic acid). Immunofluorescent labelling of the MR showed that it was principally located in the pilosebaceous unit (PSU), justifying the study rationale since polymeric micelles accumulate preferentially in appendageal structures. Cutaneous biodistribution studies under infinite and finite dose conditions, demonstrated delivery of pharmacologically relevant amounts of SPL to the epidermis and upper dermis. Preferential PSU targeting was confirmed by comparing amounts of SPL in PSU-containing and PSU-free skin biopsies: SPL nanomicelles showed 5-fold higher delivery of SPL in the PSU-containing biopsies, 0.54 ± 0.18 ng/mm²vs. 0.10 ± 0.03 ng/mm², after application of a hydrogel in finite conditions. Canrenone, an active metabolite of SPL, was also quantified in skin samples. In addition to being used for the treatment of delayed cutaneous wound healing by site-specific antagonism of the MR, the formulation might also be used to treat pilosebaceous androgen-related skin diseases, e.g. acne vulgaris, since SPL is a potent androgen receptor antagonist.

Technological Approaches for Improving Vaccination Compliance and Coverage

Vaccination has been well recognised as a critically important tool in preventing infectious disease, yet incomplete immunisation coverage remains a major obstacle to achieving disease control and eradication. As medical products for global access, vaccines need to be safe, effective and inexpensive. In line with these goals, continuous improvements of vaccine delivery strategies are necessary to achieve the full potential of immunisation. Novel technologies related to vaccine delivery and route of administration, use of advanced adjuvants and controlled antigen release (single-dose immunisation) approaches are expected to contribute to improved coverage and patient compliance. This review discusses the application of micro- and nano-technologies in the alternative routes of vaccine administration (mucosal and cutaneous vaccination), oral vaccine delivery as well as vaccine encapsulation with the aim of controlled antigen release for single-dose vaccination.

Current Progress in Particle-Based Systems for Transdermal Vaccine Delivery

Transcutaneous immunization (TCI) via needle-free and non-invasive drug delivery systems is a promising approach for overcoming the current limitations of conventional parenteral vaccination methods. The targeted access to professional antigen-presenting cell (APC) populations within the skin, such as Langerhans cells (LCs), various dermal dendritic cells (dDCs), macrophages, and others makes the skin an ideal vaccination site to specifically shape immune responses as required. The stratum corneum (SC) of the skin is the main penetration barrier that needs to be overcome by the vaccine components in a coordinated way to achieve optimal access to dermal APC populations that induce priming of T-cell or B-cell responses for protective immunity. While there are numerous approaches to penetrating the SC, such as electroporation, sono- or iontophoresis, barrier and ablative methods, jet and powder injectors, and microneedle-mediated transport, we will focus this review on the recent progress made in particle-based systems for TCI. This particular approach delivers vaccine antigens together with adjuvants to perifollicular APCs by diffusion and deposition in hair follicles. Different delivery systems including nanoparticles and lipid-based systems, for example, solid nano-emulsions, and their impact on immune cells and generation of a memory effect are discussed. Moreover, challenges for TCI are addressed, including timely and targeted delivery of antigens and adjuvants to APCs within the skin as well as a deeper understanding of the ill-defined mechanisms leading to the induction of effective memory responses.

Recent advances in follicular drug delivery of nanoparticles

Introduction: The improvement of percutaneous absorption represents a clear dermatopharmaceutical aim. Recently, the hair follicle was recognized to be an important penetration pathway. Especially nanoparticles show an enhanced intrafollicular penetration and can be utilized to target specific cell populations within the hair follicle.Areas covered: The present review briefly summarizes the recent advances in follicular drug delivery of nanoparticles. Moreover, the particularities of the hair follicle as a penetration pathway are summarized which include its structure and specific barrier properties. Recently, the mechanism of the follicular penetration process has been clarified.In the meantime, different strategies have been developed to successfully improve follicular drug delivery of nanoparticles. One approach is to equip the nanocarriers with a triggered release system enabling them to release their drug load at the right time and place.Expert opinion: Follicular drug delivery with smart nanocarrier-based drug delivery systems represents a promising approach to increase the percutaneous absorption of topically applied substances. Although technical achievements and efficacy proofs concerning an increased penetration of substances are already available, the practical implementation into clinical application still represents an additional challenge and should be in the focus of interest in future research.

Combined skin and muscle vaccination differentially impact the quality of effector T cell functions: the CUTHIVAC-001 randomized trial

Targeting of different tissues via transcutaneous (TC), intradermal (ID) and intramuscular (IM) injection has the potential to tailor the immune response to DNA vaccination. In this Phase I randomised controlled clinical trial in HIV-1 negative volunteers we investigate whether the site and mode of DNA vaccination influences the quality of the cellular immune responses. We adopted a strategy of concurrent immunization combining IM injection with either ID or TC administration. As a third arm we assessed the response to IM injection administered with electroporation (EP). The DNA plasmid encoded a MultiHIV B clade fusion protein designed to induce cellular immunity. The vaccine and regimens were well tolerated. We observed differential shaping of vaccine induced virus-specific CD4 + and CD8 + cell-mediated immune responses. DNA given by IM + EP promoted strong IFN-γ responses and potent viral inhibition. ID + IM without EP resulted in a similar pattern of response but of lower magnitude. By contrast TC + IM (without EP) shifted responses towards a more Th-17 dominated phenotype, associated with mucosal and epidermal protection. Whilst preliminary, these results offer new perspectives for differential shaping of desired cellular immunity required to fight the wide range of complex and diverse infectious diseases and cancers.

Commensal Microbes and Hair Follicle Morphogenesis Coordinately Drive Treg Migration into Neonatal Skin

Regulatory T cells (Tregs) are required to establish immune tolerance to commensal microbes. Tregs accumulate abruptly in the skin during a defined window of postnatal tissue development. However, the mechanisms mediating Treg migration to neonatal skin are unknown. Here we show that hair follicle (HF) development facilitates the accumulation of Tregs in neonatal skin and that upon skin entry these cells localize to HFs, a primary reservoir for skin commensals. Further, germ-free neonates had reduced skin Tregs indicating that commensal microbes augment Treg accumulation. We identified Ccl20 as a HF-derived, microbiota-dependent chemokine and found its receptor, Ccr6, to be preferentially expressed by Tregs in neonatal skin. The Ccl20-Ccr6 pathway mediated Treg migration in vitro and in vivo. Thus, HF morphogenesis, commensal microbe colonization, and local chemokine production work in concert to recruit Tregs into neonatal skin, thereby establishing this tissue Treg niche early in life.

Topical and cutaneous delivery using nanosystems

The goal of topical and cutaneous delivery is to deliver therapeutic and other substances to a desired target site in the skin at appropriate doses to achieve a safe and efficacious outcome. Normally, however, when the stratum corneum is intact and the skin barrier is uncompromised, this is limited to molecules that are relatively lipophilic, small and uncharged, thereby excluding many potentially useful therapeutic peptides, proteins, vaccines, gene fragments or drug-carrying particles. In this review we will describe how nanosystems are being increasingly exploited for topical and cutaneous delivery, particularly for these previously difficult substances. This is also being driven by the development of novel technologies, which include minimally invasive delivery systems and more precise fabrication techniques. While there is a vast array of nanosystems under development and many undergoing advanced clinical trials, relatively few have achieved full translation to clinical practice. This slow uptake may be due, in part, to the need for a rigorous demonstration of safety in these new nanotechnologies. Some of the safety aspects associated with nanosystems will be considered in this review.

Three dimensional distribution of Propionibacterium acnes biofilms in human skin

Propionibacterium acnes is regarded as a common member of the human skin microbiota, often occurring in biofilms. Little is known about the size of bacterial biofilms in hair follicles as a few sections of biopsy tissue are routinely evaluated. Transversal sectioning provides a better opportunity for histological analyses of hair follicles which can be followed through the different morphological levels. Direct visualization of P. acnes biofilms in hundreds of consecutive sections allowed insight into the 3D distribution in human hair follicles as well as investigating the depth of biofilm distribution within hair follicles. Four distinct colonization patterns of P. acnes biofilms were revealed. Results have shown that an individual P. acnes biofilm can spread for 1900 μm in a terminal hair follicle. This information can be of help while designing potential antibiofilm treatment.

Recent advances in topical delivery of proteins and peptides mediated by soft matter nanocarriers

Proteins and peptides are increasingly important therapeutics for the treatment of severe and complex diseases like cancer or autoimmune diseases due to their high specificity and potency. Their unique structure and labile physicochemical properties, however, require special attention in the production and formulation process as well as during administration. Aside from conventional systemic injections, the topical application of proteins and peptides is an appealing alternative due to its non-invasive nature and thus high acceptance by patients. For this approach, soft matter nanocarriers are interesting delivery systems which offer beneficial properties such as high biocompatibility, easiness of modifications, as well as targeted drug delivery and release. This review aims to highlight and discuss technological developments in the field of soft matter nanocarriers for the delivery of proteins and peptides via the skin, the eye, the nose, and the lung, and to provide insights in advantages, limitations, and practicability of recent advances.

Delivery and targeting of nanoparticles into hair follicles

It has been demonstrated that nanoparticles used for follicular delivery provide some advantages over conventional pathways, including improved skin bioavailability, enhanced penetration depth, prolonged residence duration, fast transport into the skin and tissue targeting. This review describes recent developments using nanotechnology approaches for drug delivery into the follicles. Different types of nanosystems may be employed for management of follicular permeation, such as polymeric nanoparticles, metallic nanocrystals, liposomes, and lipid nanoparticles. This review systematically introduces the mechanisms of follicles for nanoparticulate penetration, highlighting the therapeutic potential of drug-loaded nanoparticles for treating skin diseases. Special attention is paid to the use of nanoparticles in treating appendage-related disorders, in particular, nanomedical strategies for treating alopecia, acne, and transcutaneous immunization.