Nanomedicine of the Future: transdermal delivery using nanoparticles
Anton Polyansky, "Biomolecule"
It is worth mentioning right away that in this article we will not talk about some miraculous remedies that are actively offered from the shelves of stores or on various dubious websites (often the word "nano" and "stem cells" are used in this case in the same context - apparently, to be sure). Here we will briefly discuss the existing and most promising variants of molecular structures (or further – nanoparticles) that are already being used or will be actively used in the future for the direct delivery of biologically active molecules through the skin.
IntroductionIn order to overcome the stratum corneum of the skin (Latin – stratum corneum), which is the basis of the barrier function of the body coverings, methods of physical and chemical exposure are used in practice.
At the same time, science does not stand still, and in laboratories around the world, scientists are actively developing new and highly effective approaches in transdermal delivery, which are so encouraging that it seems that in the near future almost any potentially active compound is hydrophilic or hydrophobic, low molecular weight or polymer (including proteins and nucleic acid molecules acids), – it will not be difficult to deliver exactly to the address. It is these achievements that I would like to take out of the laboratory lobby for everyone to see. And so, we will talk about nanotechnology and their application in medicine (nanomedicine). In Russia, this word, in the light of recent state initiatives, is probably familiar even to a schoolboy, and has practically become a household name. Therefore, it seems to me that readers will be interested in getting to know this area better in the context already mentioned.
Barrier properties of the stratum corneumThe skin is a natural barrier to foreign molecules and particles seeking to enter the body.
Its very structure prevents the penetration of large hydrophilic molecules, as well as water, preventing dehydration of the body, but allowing the skin to "breathe".
The main "line of defense" is the uppermost and thinnest layer of the skin – the stratum corneum.
The stratum corneum consists of corneocytes – dead cells filled with fibrillar protein keratin and forming horny "scales" 0.2–0.4 µm thick and ≈40 µm in diameter. Corneocytes are interconnected by corneodesmosomes that bind cells into a strong network. The stratum corneum is made impenetrable by the lipid matrix in which the corneocytes are "immersed"; it consists mainly of ceramides, cholesterol and fatty acids forming a system of multilamellar bilayers. Below is a layer of living epidermal cells and a layer of dermis permeated with capillaries capable of "spreading" the substance that has penetrated through the barrier throughout the body.
The arrows in the figure show the possible ways of penetration of substances through the skin – both through passive transfer and under the influence of physical and chemical factors.
A. The "tortuous" path of transdermal diffusion can be facilitated with the help of chemical enhancers – substances that relatively easily overcome the lipid barrier and "entrap" the molecules of the drug being delivered.
B. Low-voltage iontophoresis facilitates the penetration of substances by the transfollicular route – through hair follicles and ducts of sweat glands.
B. High-voltage electroporation temporarily destabilizes lipid bilayers, "opening" the door to the delivered substance. Sonophoresis (ultrasound) can additionally increase the efficiency of the A and B transport pathways.
G. Microneedles and thermal perforation create micron-sized holes in the skin through which transport can be carried out. Due to the smallness of the holes, these procedures are painless, and the holes themselves are very quickly tightened. – A. Ch. [20].
Nanoparticles as they areNanoparticles used in the delivery of therapeutic molecules.
1 – liposome (bottom) and adenovirus (top);
2 – polymer nanostructure;
3 – dendrimer;
4 – carbon nanotube.
"Nano" (Greek. – a billionth part) when applied to the described objects implies that their sizes are in the range of 1÷1000 nm (10-9 m), which corresponds to the levels of biological organization from atomic to subcellular. Thus, practically any supramolecular complexes fall under the definition of "nanoparticles". However, according to the already established tradition in the biological and medical literature, nanoparticles usually mean quite specific (and, above all, artificially created) molecular structures.
They can be divided into several classes.
Biological and biogenic nanoparticles. The biological world is literally filled with nanoparticles – these are enzymes (proteins with catalytic activity), DNA and RNA molecules, ribosomes, cellular vesicles, viruses, etc. A distinctive feature of such objects is their ability to aggregate and self-organize. This property is actively used when creating artificial structures that mimic real biological structures. A striking example is represented by various single-component and multicomponent liposomes, which are capable of forming under certain conditions from a solution of a mixture of lipids. Biological nanoparticles that already exist in nature are often used in practice. For example, various viruses are actively used for gene modification (transfection) of cells. It has been shown that adenoviruses with a suppressed replication system can also be effectively used for local noninvasive vaccination through the skin (delivery of antigens to Langerhans cells present in the skin) [1]. In addition to liposomes, artificial biogenic nanoparticles intended for targeted delivery usually also include lipid nanotubes [2], lipid nanoparticles and nanoemulsions, cyclic peptides [3], chitosans, nanoparticles based on nucleic acids [4].
Polymer nanoparticles. Polymer materials have a number of advantages that determine the effectiveness of their use in delivery technologies, such as biocompatibility, biodegradation ability, and functional compatibility. Typical compounds that form the basis for the creation of polymer nanoparticles are polylactic and polyglycolic acids, polyethylene glycol (PEG), polycapralactone, etc., as well as their various copolymers. PEG is often used to increase the stability of various molecular carriers. For example, liposomes coated with PEG ("stealth liposomes") are less susceptible to biodegradation compared to conventional ones, as a result of which they have a noticeable prolonged effect [5].
Dendrimers. Dendrimers are a unique class of polymers with a highly branched structure. At the same time, their size and shape can be very precisely specified during chemical synthesis [6]. Dendrimers are obtained from monomers by conducting successive convergent and divergent polymerizations (including using methods of peptide synthesis). Typical "monomers" used in the synthesis of dendrimers are polyamidoamine (PAMAM) and the amino acid lysine. The "target" molecules bind to dendrimers either by forming complexes with their surface, or by embedding deeply between their individual chains. The controlled dimensions and properties of the surface, as well as the stability of dendrimers, make them very promising for use as carriers. The effectiveness of their use for transdermal delivery of a number of drugs has been shown in animal models [7].
Carbon nanoparticles. Nanotubes and fullerenes are among the most "recognizable" nanostructures – almost no popular text about nanotechnology is complete without their images. For the discovery of this new form of carbon existence, R. Curl, R. Smalley and G. Kroto were awarded the Nobel Prize in Chemistry in 1996. These structures, formed only by carbon atoms, can be obtained using a voltaic arc, laser ablation (burning), chemical deposition from the gas phase, as well as in the gorenje process. Today, fullerenes are produced on an industrial scale by thermal spraying of carbon-containing soot in an inert gas atmosphere at reduced pressure in the presence of a catalyst. Nanotubes have an increased affinity for lipid structures; at the same time, they are able to form stable complexes with peptides and DNA oligonucleotides [8, 9], and even encapsulate these molecules [10, 11]. This determines their application in the field of creating effective delivery systems for vaccines and genetic material [12].
Inorganic nanoparticles. This class usually includes nanostructures obtained on the basis of silicon oxide, as well as various metals (gold, silver, platinum). At the same time, such a nanoparticle often has a silicon core and an outer shell formed by metal atoms. The use of metals makes it possible to create carriers with a number of unique properties. Thus, their activity (and in particular, the release of a therapeutic agent) can be modulated by thermal exposure (infrared radiation), as well as by a change in the magnetic field [12]. It has been shown that metal nanoparticles can effectively penetrate deep into the epidermis [13].
Not only deliveryThe use of the nanoparticles described above in medicine will not only effectively deliver biologically active molecules through various barriers of the body that they are not able to overcome independently (skin, blood-brain), but also significantly change the nature of the drug's action.
For example, transdermal delivery, compared with delivery through the bloodstream, avoids undesirable side effects, reduces the effective dose of the drug due to a significant increase in its local concentration. In addition, it has been shown that the pharmacokinetics of therapeutic molecules delivered to the body using nanoparticles changes. If for drugs entering the body orally or as a result of injection, an increase in concentration over time is described by a characteristic kinetic curve of the first order (the concentration increases exponentially over time), then in the case of using nanoparticles, an ideal time dependence of zero order is observed (a uniform increase in the concentration of the drug over time) [12]. This allows you to more accurately plan the dosage of the drug and prolong its effect.
Nanomedicine or nanocosmetics?The mentioned nanoparticle-based delivery methods, as well as the general level of development of modern molecular biology, biotechnology and pharmacology, significantly modify the ideas about the possibilities of skin therapy.
On the one hand, this ensures noticeable progress in the field of medicine (in particular, dermatology), on the other hand, it allows cosmetic preparations to reach a qualitatively new level. Indeed, we should expect from the nanocosmetics of the future that its action will be based not on masking undesirable effects, as it often happens today, but on eliminating their biological cause. But how, in this case, to distinguish the spheres of cosmetics and medicine? It is possible that such borders will disappear altogether in the future, but for now we will note the possible points of their contact.
Penetration of nanoparticles (40 nm) into the hair follicle.
The figure shows images obtained using fluorescent (black-and-white) and laser scanning (black-and-green) microscopy, as well as schematically shows the hair follicle. The red color corresponds to a fluorescent signal detected from nanoparticles. The work uses human skin preparations obtained from patients from the Department of plastic surgery. The drawing is adapted from [16].
We'll do without syringesNumerous vaccinations against all kinds of diseases have become familiar to modern man.
However, the methodology itself has not changed much over the last century. However, soon patients will be able to rightfully quote the famous poem by S. V. Mikhalkov: "I'm not afraid of injections." In the near future, syringes with a solution of antigens will be replaced by nanotransferers (sizes up to 500 nm) capable of delivering antigens through the stratum corneum to Langerhans cells. The effectiveness of such structures has been shown in laboratory studies, but the detailed mechanisms are still unknown. Nevertheless, experimentally established limitations in the size of effective carriers suggest that penetration into the inner layers of the skin is carried out through lipid channels between corneocytes [15]. It has also been shown that the use of small nanoparticles (only 40 nm) allows antigens to be delivered directly through hair follicles [16]. The use of such a delivery route is extremely promising, since not only a cluster of dendritic cells is located in the follicle area, but stem cells have also been found. This enables not only skin immunization, but also targeted dermatotherapy, including stimulation of cell proliferation [17].
Is "DNA cosmetics" real?The ability to influence the gene expression of skin cells, as well as deliver various "useful" genes, is a very tempting idea, so much so that modern cosmetic manufacturers are often engaged in its implementation...
however, so far only in words. Nevertheless, there are real developments in this area. Although the question "what to deliver" remains open, and it will take a considerable time to solve it, the question "how to deliver" already has concrete answers. For example, the combination of physical (radiofrequency) exposure approaches and the use of nanoparticles makes it possible to carry out effective epidermal delivery of DNA plasmids (ring DNA molecules capable of causing the expression of genes contained in them in target cells) [18]. At the same time, the researchers managed not only to deliver DNA molecules, but also to observe their expression in skin cells.
Delivery of genetic material (DNA plasmids) to skin cells.
DNA plasmids contain the β-galactosidase gene (a reporter enzyme whose expression can be detected by specific substrate staining). The blue color corresponds to the region in which the expression of the target plasmid is observed. Microscopic images of the cut (left) and the skin surface (right) are shown. Preparations of human skin cultured ex vivo were used in the work.
100 nm particles containing DNA plasmids were used as a delivery system. Their penetration into the skin is potentiated by radio frequency exposure using the ViaDerm™ device. The drawing is adapted from [18].
ConclusionDespite the promising results of the studies mentioned in this brief review, it should be noted that most of them are devoted only to experiments on laboratory animals or even model systems.
Nevertheless, given the increased interest in the described technologies on the part of pharmaceuticals and cosmetology, it will soon become quite possible to talk about nanocosmetics and skin nanomedicine not in the long term, but seriously.
The article was originally published in the journal "Cosmetics and Medicine" No. 2 for 2008 [19].
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