Shedding light on drug delivery: Detecting protein-coated polymer particles
- Published: Apr 17, 2017
- Author: Jon Evans
- Channels: Detectors
Nanoparticles potentially provide a highly effective way to transport drugs into the body, as they can protect drug molecules and stop them being broken down too quickly. Unfortunately, as foreign bodies, nanoparticles tend to be attacked by the immune system, rather hampering their delivery abilities. One solution is to hide the nanoparticles from the immune system by covering them in proteins, which can also have the added benefit of allowing the nanoparticles to be targeted at specific tissues, such as tumors.
The problem then becomes how to cover the nanoparticles in proteins, because nanoparticles are often hydrophobic but proteins are hydrophilic. This makes it difficult to find a solvent that both nanoparticles and proteins will disperse in, which is required for them to interact with each other. Now, however, a team of Chinese chemists have come up with a novel method for coating polymer nanoparticles with proteins, which they assessed by studying the resultant coated particles with several different analytical techniques.
Their novel method involves using polymer nanoparticles containing pyridyl disulphide groups and proteins containing thiol groups, as these groups naturally react with each other. The proteins are dissolved in water while the polymer is dissolved in an organic solvent that is miscible in water. When the water and organic solvent are mixed, the polymers naturally clump together to form particles that normally exclude any proteins. In this case, however, rapid reactions between the pyridyl disulphide groups and the thiol groups cause the proteins to bind to the surface of the polymer particles, forming a coating.
Electrophoresis and scattered light
As a first test of this method, the chemists, led by Hanying Zhao at Nankai University, tried it out with a polymer called poly(tert-butyl methacrylate-co-pyridyl disulfide methacrylamide) (PtBMA-co-PPDSMA) and the protein bovine serum albumin (BSA). Studies with a transmission electron microscope revealed that the gray PtBMA-co-PPDSMA particles did appear to become covered in a dark layer of protein.
To confirm the dark layer was indeed BSA, Zhao and his team analyzed the particles with gel electrophoresis. They found that the particles were too massive to produce a band in the gel themselves, but a band corresponding to BSA did appear after the particles were treated with an agent that can cleave disuphide bonds, thus releasing the BSA. This provides good evidence that BSA is attached to the polymer particles via disulphide bonds between the pyridyl disulphide groups and the thiol groups.
Next, the chemists used dynamic light scattering (DLS) to determine what effect varying the protein concentration, the polymer concentration and the average number of thiol groups had on the size of the coated particles. As would be expected, higher concentrations of polymer produced larger particles. But higher concentrations of protein or numbers of thiol groups produced smaller particles, as both increased the speed with which the proteins bound to the polymer particles. This caused the particles to become covered in proteins at smaller sizes, after which no more polymers could join the particles to make them bigger.
Stable and active
Zhao and his team also used DLS to test whether the particles were degraded by exposure to urea and heat, because this would be reflected in a change in size. They didn’t detect any noticeable change, showing that the coated particles are fairly stable and thus should be able to withstand conditions within the body.
Finally, they analyzed the coated particles with circular dichroism spectroscopy, which is used to probe the secondary structure of proteins by measuring their absorption of circularly polarized light. This showed that the BSA retained its usually structure when bound to the polymer particles, implying it should also retain its usual activity. Zhao and his team then confirmed this by demonstrating that BSA bound to a particle could still hydrolyze a protein called p-nitrophenyl acteate.
All of which convincingly demonstrates that this novel method can produce very effective drug delivery vehicles, and Zhao and his team are now testing it out with other combinations of polymer particles and proteins.
Chemistry - A European Journal, 2017, 23, 3366 – 3374: "Covalently connected polymer–protein nanostructures fabricated by a reactive self-assembly approach"
Article by Jon Evans
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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