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Posted: Aug 24, 2011
Nanoparticles can hinder intracellular transport
(Nanowerk News) New medicines containing nanoparticles are proven to have clear curative value, but complications can sometimes arise. Researchers at the Norwegian Radium Hospital in Oslo have shown how nanoparticles can interfere with the transport of vital substances in cells.
Intensive research is being dedicated to developing new medicines that seek out diseased cells and leave the healthy ones alone. Nanotechnology enables scientists to customise medicines and the vehicles that deliver them to where the body needs them.
Numerous animal trials have shown some promising results – but have also revealed the occurrence of undesired reactions that negatively affect cell function. The role of nanoparticles in such reactions is not yet well understood.
Tore-Geir Iversen is Senior Scientist at the Centre of Cancer Biomedicine at the Norwegian Radium Hospital, part of Oslo University Hospital. Basic research on cell cultures, such as that carried out at the Norwegian Radium Hospital, clearly demonstrates that nanoparticles affect the cells.
Dr Iversen and his colleagues began by posing some very fundamental questions: What happens once the nanoparticles enter the cells? Do they accumulate or degrade in the cell? Do they clear the cell to be recycled? Is their effect toxic to cells?
Tore-Geir IversenAfter four years of experimentation, the researchers are zeroing in on how nanoparticles behave in cells. Dr Iversen's group is the first to show that uptake and accumulation of nanoparticles in cells can disrupt important intracellular transport pathways.
The project has received funding under two of the Research Council of Norway's Large-scale Programmes: Functional Genomics in Norway (FUGE) and Nanotechnology and New Materials (NANOMAT). The findings were first published in the US journal Nano Letters.
Researchers working on the project have studied nanoparticles 30-100 nanometres in diameter, a typical size used for delivering medicines and DNA into cells.
The nanoparticles were dyed so as to fluoresce (light up) when irradiated by a laser. By dying various particles with different fluorescent substances and irradiating them with different laser wavelengths, the researchers were able to locate the various particles within cells using a microscope.
One much-used particle type is fluorescent quantum dots, which light up when irradiated by light of wavelengths approaching the ultraviolet range. Another type is iron oxide particles, which bind to fluorescent substances so that researchers can study their uptake and where they are transported within cells. Iron oxide particles have been used in magnetic resonance imaging (MRI) diagnostics for 20 years.
Trials have shown that a protein that transports iron into a cell is taken up in the usual way even when bound to a nanoparticle. However, while 99% of a protein not bound to a nanoparticle will make its way out of the cell and can be recycled, a nanoparticle-bound protein remains in the cell.
This photo shows uptake of fluorescent nanoparticles inside cells. The yellow and violet dots are particles that have been transported into the cell and have accumulated. (Photo: Radiumhospitalet)
There it accumulates in the endosomes, which have an important function in the cell's internal transport system. Endosomes are bubble-like compartments encased in a membraneThus the researchers discovered that the nanoparticles interrupt the transport of vital substances in and out of a cell, causing undesirable changes in the cell's physiology and disrupting normal cell functioning.
"The likely explanation," says Dr Iversen, "is that the protein has to enter through very thin tubes (called tubules) in the endosomes. Nanoparticles of the size we are researching either cannot enter the tubule or they lodge inside and plug it up."
This is critical knowledge when it comes to designing future particles.
Cautions against jumping to conclusions
The Norwegian research confirms that there are no shortcuts to developing a medicine that targets diseased tissue. Even when a protein itself is targeted and has a positive effect, that same protein bound to a nanoparticle may be less effective – or perhaps even harmful.
"We find it frustrating that a number of international scientific articles confidently conclude, on poorly verified grounds, that nanoparticles effectively transport medicines to the nucleus," laments Dr Iversen. He and his colleagues recently published a review article in the journals Nanomedicine and Nano Today that details their criticisms of such claims.
"Hopefully our reviews will lead to higher quality in future studies of uptake in the cells."
Tore-Geir Iversen is concerned that the pharmaceutical industry is rushing its product development.
If a nanomedicine is used to extend the life of a patient with terminal cancer, then the accumulation of nanoparticles might be insignificant, he reasons. But when a medicine is developed to treat a chronic disorder, so that patients will take that medicine over years, then pharmaceutical companies should have to demonstrate that their medicine is fully degraded and excreted from the body.
The challenge is that even clinical studies carried out on patients with chronic diseases will not provide the whole truth. The negative effects of a nanomedicine may not show up in a short-term study, but patients who use that medicine over many years to fight a chronic disorder may end up exhibiting an over-occurrence of certain cancer types due to the nanoparticles being incompletely excreted, disrupting transport in the body's cells.
"We shouldn't skip over understanding basic cell biology and go straight to clinical trials or animal trials," cautions Dr Iversen.
Next, the Norwegian Radium Hospital researchers want to find out whether nanoparticles smaller than 30 nanometres in diameter will navigate the transport system any better.
Furthermore, in close cooperation with materials researchers they aim at creating particles with sizes and surface compositions that allow them to stably circulate in the blood stream and interact specifically with their target cells yet ensures they can be broken down within the cells.
The cell biologists will also collaborate with immunologists before moving on to animal trials.
How nanoparticles are transported into cells
Nanoparticles, proteins and other macromolecules are taken up in the body's cells via various mechanisms. The nanoparticles are first encapsulated in fatty bubbles, called vesicles, which form on the cell surface.
These vesicles merge with larger vesicles known as sorting endosomes, which can then mature into multivesicular bodies (MVBs). These MVBs, in turn, merge with lysosomes, where proteins and other macromolecules are broken down by proteases and other enzymes. Nanoparticles can be transported out of the cell via either sorting endosomes or recycling endosomes.
Source: By Claude Olsen, The Research Council of Norway