The ageing population, the high expectations for better quality of life and the changing lifestyle of European society call for improved, more efficient and affordable health care.
Nanotechnology can offer impressive resolutions, when applied to medical challenges like cancer, diabetes, Parkinson's or Alzheimer's disease, cardiovascular problems, inflammatory or infectious diseases.
Experts of the highest level from industry, research centers and academia convened to prepare the present vision regarding future research priorities in NanoMedicine. A key conclusion was the recommendation to set up a European Technology Platform on NanoMedicine designed to strengthen Europe's competitive position and improve the quality of life and health care of its citizens. This article has been extracted from the vision paper “European Technology Platform on NanoMedicine - Nanotechnology for Health” produced by the European Commission.
Targeted Drug Delivery and Release
The very slow progress in the treatment of severe diseases has led to the adoption of a multidisciplinary approach to the targeted delivery and release of drugs, underpinned by nanoscience and nanotechnology.
New drug delivery systems (DDS) combine polymer science, pharmaceutics, bioconjugate chemistry and molecular biology. The aim is to better control drug pharmacokinetics, pharmacodynamics, non-specific toxicity, immunogenicity and biorecognition of systems in the quest for improved efficacy.
Drug delivery and targeting systems under development aim to minimize drug degradation and loss, prevent harmful side effects and increase the availability of the drug at the disease site. Drug carriers include micro and nanoparticles, micro and nanocapsules, lipoproteins, liposomes, and micelles, which can be engineered to slowly degrade, react to stimuli and be site-specific. Targeting mechanisms can also be either passive or active. An example of passive targeting is the preferential accumulation of chemotherapeutic agents in solid tumors as a result of the differences in the vascularization of the tumor tissue compared with healthy tissue. Active targeting involves the chemical ‘decorating’ of the surface of drug carriers with molecules enabling them to be selectively attached to diseased cells.
The controlled release of drugs is also important for therapeutic success. Controlled release can be sustained or pulsatile. Sustained (or continuous) release of a drug involves polymers that release the drug at a controlled rate, by diffusion out of the polymer or by degradation of the polymer over time. Pulsatile release is often preferred, as it closely mimics the way by which the body naturally produces hormones such as insulin. It is achieved by using drug-carrying polymers that respond to specific stimuli (e.g. exposure to light, changes in pH or temperature).
Other nano-based approaches to drug delivery are focused on crossing a particular physical barrier, such as the blood-brain barrier; or on finding alternative and acceptable routes for the delivery of a new generation of protein-based drugs other than via the gastro-intestinal tract, where degradation can occur. Nanoscience and nanotechnology are thus the basis of innovative delivery techniques that offer great potential benefits to patients and new markets to pharmaceutical and drug delivery companies.
For over 20 years, researchers in Europe have used nanoscale technology as the basis of vast improvements in drug delivery and targeting and Europe is now well placed to build on this body of knowledge.
Drug Delivery Systems
A successful drug carrier system needs to demonstrate optimal drug loading and release properties, long shelf-life and low toxicity. Colloidal systems, such as micellar solutions, vesicle and liquid crystal dispersions, as well as nanoparticle dispersions consisting of small particles of 10 - 400 nm diameter show great promise as carriers in drug delivery systems.
Drugs can be trapped in the core of a micelle and transported at concentrations even greater than their intrinsic water solubility. A hydrophilic shell can form around the micelle, effectively protecting the contents. In addition, the outer chemistry of the shell may prevent recognition by the reticuloendothelial system, and therefore early elimination from the bloodstream. A further feature that makes micelles attractive is that their size and shape can be changed. Chemical techniques using crosslinking molecules can improve the stability of the micelles and their temporal control. Micelles may also be chemically altered to selectively target a broad range of disease sites.
Liposomes are vesicles that consist of one to several, chemically-active lipid bilayers. Drug molecules can be encapsulated and solubilised within the bilayers. Certain (channel) proteins can be incorporated in the membrane of the liposome, which act as size-selective filters only allowing the diffusion of small solutes such as ions, nutrients and antibiotics. Thus, drugs encapsulated within a liposome ‘nanocage’ that has been functionalized with channel proteins, are effectively protected from premature degradation. The drug molecule, however, is able to diffuse through the channel, driven by the concentration difference between the interior and the exterior of the ‘nanocage’.
Dendrimers are nanometre-sized, polymer macromolecules. They consist of a central core, branching units and terminal functional groups. The core chemistry determines the solubilizing properties of the cavity within the core, whereas external chemical groups determine the solubility and chemical behavior of the dendrimer itself. Targeting is achieved by attaching specific linkers to the external surface of the dendrimer which enable it to bind to a disease site, while its stability and protection from phagocytes is achieved by ‘decorating’ the dendrimers with polyethylene glycol chains.
Liquid Crystals combine the properties of both liquid and solid states. Liquid crystals can be made to form different geometries, with alternate polar and non-polar layers (i.e., lamellar phases), within which aqueous drug solutions can be incorporated.
Nanoparticles, including nanospheres and nanocapsules, can be amorphous or crystalline. They are able to adsorb and/or encapsulate a drug, thus protecting it against chemical and enzymatic degradation. In nanocapsules, the drug is confined to a cavity surrounded by a polymer membrane, while nanospheres are matrix systems within which the drug is physically and uniformly dispersed. In recent years, biodegradable polymeric nanoparticles have attracted considerable attention in the controlled release of drugs in targeting particular organs/tissues, as carriers of DNA in gene therapy and in their ability to deliver proteins, peptides and genes by the oral route.
Hydrogels are three-dimensional polymer networks that swell but do not dissolve in aqueous media. They are used to regulate drug release in reservoir-based systems or as carriers in swelling-controlled release devices. On the forefront of controlled drug delivery, hydrogels, as enviro-intelligent and stimuli-sensitive gel systems, can modulate drug release in response to pH, temperature, ionic strength, electric field, or specific analyte concentration differences. Release can be designed to occur within specific areas of the body. Hydrogels as drug delivery systems are very promising if combined with the technique of molecular imprinting.
Molecularly Imprinted Polymers
Molecularly imprinted polymers have an enormous potential for drug delivery systems. Examples include: rate-programmed drug delivery, where drug diffusion from the system has to follow a specific rate profile; activation-modulated drug delivery, where the release is activated by some physical, chemical or biochemical processes; and feedback-regulated drug delivery, where the rate of drug release is regulated by the concentration of a triggering agent, which is activated by the drug concentration in the body.
Despite already-developed applications, the incorporation of the molecular imprinting approach for the development of drug delivery systems is at an early stage. It can be expected that in the next few years significant progress will occur, taking advantage of the improvements in this technology in other areas.
Conjugation of Biological Molecules and Synthetic Polymers
The conjugation of biological molecules (peptides/proteins) and synthetic polymers is an efficient means of improving control over the nanoscale structure formation of synthetic polymers that can be used as drug delivery systems. The conjugation of suitable synthetic polymers to peptides or proteins can reduce toxicity, prevent immunogenic or antigenic side reactions, enhance blood circulation times and improve drug solubility. Modification of synthetic polymers with peptide sequences, which can act as antibodies to specific epitopes, can also prevent random distribution of drugs throughout a patient’s body by active targeting. The functionalisation of synthetic polymers with peptide sequences derived from extracellular matrix proteins is an efficient way to mediate cell adhesion, for example. In addition the ability of cationic peptide sequences to complex DNA and oligonucleotides offers prospects for the development of non-viral vectors for gene delivery, based on synthetic polymeric hybrid materials.
In-Situ Forming Implants
The field of in-situ forming implants has grown exponentially in recent years. Liquid formulations generating a (semi-) solid depot after subcutaneous injection are attractive delivery systems for parenteral (non-oral) application because they are less invasive and painful compared to implants. They enable drugs to be delivered locally or systemically over prolonged periods of time, typically up to several months. These depot systems could minimize side effects by achieving constant, ‘infusion-like’ drug profiles, especially important for delivering proteins with narrow therapeutic indices. They also offer the advantage of being relatively simple and cost effective to manufacture.
Microelectromechanical Systems (MEMS)
The ultimate goal in controlled release is the development of a microfabricated device with the ability to store and release multiple chemical substances on demand. Recent advancement in microelectromechanical systems (MEMS) have enabled the fabrication of controlled-release microchips, which have the following advantages:
• Multiple chemicals in any form (e.g. solid, liquid or gel) can be stored and released
• Chemical release is initiated by the disintegration of the barrier membrane by applying an electric potential
• A variety of highly potent drugs can potentially be delivered accurately and safely
• Complex release patterns (e.g. simultaneous constant and pulsatile release) can be achieved
• Local delivery is possible, achieving high concentrations of drug where needed, while keeping the systemic concentration of the drug at a low level
• Water penetration into the reservoirs is avoided by a barrier membrane and thus the stability of protein based drugs with limited shelf-life is enhanced.
The choice of drug is often influenced by the way it is administered, as this can make the difference between a drug’s success and failure. So the choice of a delivery route can be driven by patient acceptability, important properties of the drug (e.g. solubility), the ability to target the disease location, or effectiveness in dealing with the specific disease.
The most important drug delivery route is the peroral route. An increasing number of drugs are protein and peptide-based. They offer the greatest potential for more effective therapeutics, but they do not easily cross mucosal surfaces and biological membranes, they are easily denatured or degraded, they are prone to rapid clearance in the liver and other body tissues and they require precise dosing. At present, protein drugs are usually administered by injection, but this route is less accepted by patients and also poses problems of oscillating blood drug concentrations. So, despite the barriers to successful drug delivery that exist in the gastrointestinal tract (e.g. acid-induced hydrolysis in the stomach, enzymatic degradation throughout the gastrointestinal tract, bacterial fermentation in the colon), the peroral route is still the most intensively investigated as it offers advantages of convenience, cheapness of administration and manufacturing cost savings.
Parenteral routes (e.g. intravenous, intramuscular or subcutaneous) are very important. The only nanosystems presently on the market, liposomes, are administered intravenously. Nanoscale drug carriers have a great potential for improving the delivery of drugs through nasal and sublingual routes, both of which avoid first-pass metabolism; and for difficult access ocular, brain and intra-articular cavities.
It has been possible to deliver peptides and vaccines systemically using the nasal route through the association of active drug macromolecules with nanoparticles. In addition, there is the possibility of improving the ocular bioavailability of drugs if administered in a colloidal drug carrier.
Pulmonary delivery is also important and is effected in a variety of ways - via aerosols, metered dose inhaler systems, powders (dry powder inhalers) and solutions (nebulizers), which may contain nanostructures such as liposomes, micelles, nanoparticles and dendrimers. Aerosol products for pulmonary delivery comprise more than 30% of the global drug delivery market. Research into lung delivery is driven by the potential for successful protein and peptide drug delivery by this route and by the promise of an effective delivery mechanism for gene therapy (e.g. in the treatment of cystic fibrosis), as well as the need to replace chlorofluorocarbon propellants in metered dose inhaler systems. Pulmonary drug delivery offers local targeting for the treatment of respiratory diseases and increasingly appears to be a viable option for the delivery of drugs systemically. However, the success of pulmonary delivery of protein drugs is diminished by proteases in the lung, which reduce their overall bioavailability, and by the barrier between capillary blood and alveolar air (the air-blood barrier).
Transdermal Drug Delivery
Transdermal drug delivery avoids problems such as gastrointestinal irritation, metabolism, variations in delivery rates and interference due to the presence of food. It is also suitable for unconscious patients. The technique is generally non-invasive, well accepted by patients and can be used to provide local delivery over several days. Limitations include slow penetration rates, lack of dosage flexibility and/or precision, and a restriction to relatively low dosage drugs.
Trans-Tissue and Local Delivery Systems
Trans-tissue and local delivery systems are systems that require to be tightly fixed to resected tissue during surgery. The aim is to produce an elevated pharmacological effect, while minimizing systemic, administration-associated toxicity. Trans-tissue systems include: drug-loaded gelatinous gels, which are formed in-situ and adhere to resected tissues releasing drugs, proteins or gene-encoding adenoviruses; antibody-fixed gelatinous gels (cytokine barrier) that form a barrier that on a target tissue could prevent the permeation of cytokines into that tissue; cell-based delivery, which involves a gene-transduced oral mucosal epithelial cell-implanted sheet; device directed delivery - a rechargeable drug infusion device that can be attached to the resected site.
Gene delivery is a challenging task in the treatment of genetic disorders. Plasmid DNA has to be introduced into the target cells. It then needs to be transcribed, and the genetic information ultimately translated into the corresponding protein. To achieve this, a number of hurdles have to be overcome. The gene delivery system has to be targeted to the target cell, transported through the cell membrane, taken up and degraded in the endolysosomes, and the plasmid DNA trafficked intracellularly to the nucleus.
Basis For a Strategic Research Agenda
For all delivery routes formulation is essential and presents new challenges. Research into novel ways to introduce nanomedicines into the body is as important as the drug itself. Formulation research adds value in a competitive marketplace where change is now rapid. Present paradigms may not hold; an example is the increasing market share now occupied by needleless injections.
Nanoparticles and nanoformulations have already been used as drug delivery systems with great success and nanoparticulate drug delivery systems have still greater potential for many applications, including anti-tumour therapy, gene therapy, AIDS therapy, radiotherapy, in the delivery of proteins, antibiotics, virostatics, vaccines, and as vesicles to pass the blood-brain barrier.
Nanoparticles provide massive advantages regarding drug targeting, delivery and release, and with their additional potential to combine diagnosis and therapy, will emerge as one of the major tools in NanoMedicine. The main goals are to improve their stability in the biological environment, to mediate the bio-distribution of active compounds, to improve drug loading, targeting, transport, release and interaction with biological barriers. The cytotoxicity of nanoparticles or their degradation products remains a major problem and improvements in biocompatibility obviously are a main concern of future research.
The European Technology Platform on NanoMedicine needs to reflect on European strengths in this area in order to be competitive. In drug delivery, these are in polymer therapeutics, non-viral gene delivery, biological models for cells and tissues for in-vitro testing and in cancer targeting and therapy.
These form the basis for further developments, such as:
• Nano-drug delivery systems that deliver large but highly localized quantities of drugs to specific areas, to be released in controlled ways
• Controllable release profiles, especially for sensitive drugs
• Materials for nanoparticles that are biocompatible, biodegradable and non-toxic
• Architectures/structures, such as biomimetic polymers and nanotubes
• Technologies for self-assembly
• New functions (active drug targeting, on-command delivery, intelligent drug release devices/bioresponsive triggered systems, self-regulated delivery systems, smart delivery)
• Virus-like systems for intracellular delivery
• Nanoparticles to improve implantable devices
• MEMS (improved by nanotechnology) for nanoparticle release and multi-reservoir drug delivery systems
• Nanoparticles for tissue engineering, e.g. for the delivery of cytokines to control cellular growth and differentiation and to stimulate regeneration
• Biodegradable layered coatings on implants for sustained release of active molecules
• Advanced polymeric carriers for the delivery of therapeutic peptide/proteins (biopharmaceutics).
And also in the development of:
• Combined therapy and medical imaging, for example, nanoparticles for diagnosis and manipulation during surgery (e.g. thermotherapy with magnetic particles)
• Universal formulation schemes that can be used as intravenous, intramuscular or peroral drugs
• Cell and gene targeting systems
• Devices for detecting changes in magnetic or physical properties after specific binding of ligands on paramagnetic nanoparticles that can correlate with the amount of ligands
• Better disease markers in terms of sensitivity and specificity nanoanalytical instrumentation for improved understanding, engineering, and control of drug delivery systems
• High throughput in-vitro and in-vivo test systems for immunological and toxicological screening of nanoparticles and nanoformulations.