Volume 6, Issue 3 - Special issue: Nanomedicine at Biological Barriers / Guest editors: Kenneth Dawson and Louise Rocks

Access of therapeutic agents to the central nervous system is generally restricted by an intricate barrier system, which protects the brain and maintains homeostasis. Endogenous nutrients, however, engage with the available molecular systems and successfully cross the blood-brain barrier (BBB). One of the systems, receptor-mediated transcytosis (RMT), involves non-stereoselective vesicular trafficking and has been extensively investigated as suitable for transportation of large molecular cargos. This review focuses on recent developments in elucidating the molecular pathways that regulate RMT and the fashion in which they impact the design of therapeutic nanosystems for BBB transcytosis. The challenges of brain targeting, however, are not restricted to successful engagement with BBB receptors, also including species-specificity, selectivity and maintaining targeting capability in real biological media.

At epithelial barriers molecular pattern recognition mechanisms act as minesweepers against harmful environmental factors and thereby play a crucial role in the defense against invading bacterial and viral pathogens. However, it became evident that some of the proteins participating in these host defense processes may simultaneously function as regulators of tissue regeneration when in the extracellular matrix, thus coupling defense functions with regulation of stem cells. Although molecular pattern recognition has complex physiological roles and we just begin to understand its various functions, the simplicity of the underlying principles for recognition of specific classes of molecules may generate novel starting points for nanomedical approaches in drug delivery across epithelial barriers. The present article aims to provide an introduction into the biological context, processes, proteins, and general mechanisms of molecular pattern recognition in humans and, by using selected examples, to identify potential areas in nanomedicine for the exploitation of these mechanisms.

The pulmonary route offers an exceptional, non-invasive administration site for drug delivery. The principal characteristics that make the lungs an appealing route for drug administration include a large surface for drug dispersion (approximately 100 m 2 ), a low content of drug-metabolizing enzymes, and a high vascularization for systemic drug delivery. Recent advances in this field such as the development of modern inhalation devices, novel inhalation-adapted formulations, and innovative drug carriers have contributed to a significant improvement in the low level of lung aerosol deposition achieved in the past, and have allowed for an enhancement in aerosol penetration into the lungs. Less focus however has been placed on the fate of inhaled particles after they deposit onto lung surfaces. After first contact with a pulmonary surface therapeutic particles are exposed to complex microenvironments and biological barriers (both cellular and non-cellular) that may vary widely in composition depending on the region of the lung in which the particles deposit. Most of the current inhaled therapies aim to achieve deep lung deposition at the alveolar air-blood barrier. In this particular region, the epithelium is coated with the pulmonary surfactant, a thin liquid layer composed of lipids and proteins that reduces surface tension in the alveoli, but which also interacts with and may influence the fate of inhaled therapeutics within the alveolar region. In addition, alveolar macrophages efficiently engulf inhaled particulates in the 1–5 μm size range; these therefore also pose a significant barrier to the effective delivery of therapeutic micro- and nanoparticles (NPs). Furthermore, the tightly-joined epithelium of the airways is coated with a dynamic viscous mucus layer which forms the mucociliary escalator, an efficiently coordinated piece of machinery that entraps inhaled particulates including pollutants, pathogens and, eventually, therapeutic NPs, and removes them from the lungs. A better understanding of the complex processes to which inhaled particles are subjected within distinct regions of the lungs may allow for the design of innovative therapeutics, including biocompatible polymeric NPs, aimed to efficiently overcome the complex pulmonary barriers and thus enhance the therapeutic efficiency of NP-associated actives. The current review therefore discusses the structure of the pulmonary barriers, as well as some of most innovative strategies to overcome them in order to facilitate an enhanced delivery of inhaled therapeutics.

The use of nanoparticles (NPs) in nanomedicine is becoming an established therapeutic strategy for transport and delivery of bioactive molecules across biological barriers to elicit a specific cytological response in a target cell population. The utility of NP-derivatised nanomedicines and their potential therapeutic benefits, together with their biosafety, are being explored in cell models that, in order to accurately predict clinical potential, must reflect the architecture and function of the tissues from which the cells originate. These cell-based models, and their application in preclinical nanomedicine, are the subject of this review. Models are considered expressly from a commercial perspective, for their use as screening tools to identify and optimise therapeutic constructs, rather than as research tools requiring minimal throughput. Commercial utility is discussed in terms of the sourcing and assembly of cell-based model components, likelihood of data outputs highly predictive of clinical performance, and the compromise between need for advanced 3D culture architectures and the incremental difficulty in assembling those structures cost-effectively for medium-throughput analysis. The relative merits of cell lines and primary human cells are discussed, the latter with respect to their potential physiological relevance when cultured under permissive conditions conferred by a biologically-rich micro-environment. Tools for assembly of such microenvironments are reviewed, and the practitioner’s choices of commercial products enabling 3D cell culture are also critically evaluated, from the merits of cell aggregates and micro-tissues to the architectural attractions of recreating epithelial monolayers and multi-cell type reconstituted tissues. Additionally, pitfalls and practical challenges in co-assembly of cells and scaffolds/matrices are considered, with the intention of signposting routes to consistent, scalable production of 3D cell cultures capable of screening the nanomedical potential of NP-conjugated therapeutics.

In the past decade, the importance of quality and reproducibility within research has been re-emphasized, consequently becoming a crucial part of scientific experiments. Their implementation into in vitro and in vivo biological experiments is challenging due to various parameters that can influence the final scientific outcome. In parallel to these activities, there is a huge scientific effort to improve today’s medicines to make them safer and more efficient, and to cure untreatable diseases, such as many neurodegenerative diseases. Nanosized materials have been recognized as potential drug delivery systems in this arena due to their small size and surface properties, which enable the design and synthesis of safe and efficient delivery vehicles that might be able to cross the blood-brain barrier. However, the fundamental understanding behind their uptake mechanism and intracellular trafficking remains unknown. Simple and cost-effective in vitro blood-brain barrier models are widely used to address these questions. This paper aims to critically evaluate the current in vitro models using Transwell ™ systems and to discuss alternative approaches towards more reproducible in vivo features.