Slipping friends through the barrier
(appeared in Dec 2015)

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The body defenses to keep invaders out also block medicines from making an entry, , says S.Ananthanarayanan.

A most convenient way of taking medicine is by swallowing a pill. But the body has created barriers to prevent pathogens, like bacteria, which could come in along with food, from entering the bloodstream. These same barriers could also prevent useful drugs which are taken orally from reaching place where they are needed. Oral medication thus needs to have special arrangements to ensure absorption.

Debora Walker, Benjamin T. Käsdorf, Hyeon-Ho Jeong, Oliver Lieleg and Peer Fischer at the Max Planck Institute for Intelligent Systems at Stuttgart, the University of Stuttgart and the Institute for Medical Engineering at the Technical University at Munich report a step in this direction in the journal, Science Advances. They have piggybacked, they report, on a stratagem used by an adapted bacterium to design a drug carrier that can drill its way through the mucus armor of the stomach wall.

One of the main defenses of the body is the layer of mucus, a viscous fluid that covers some tissue, either internal or which is exposed to the exterior, like the nostrils, eyelids, the airways in the lungs, the stomach and intestines, the genitals or the anus. The function is both to moisten and prevent the loss of moisture through exposed tissue as well as to prevent access by dirt, toxins or micro-organisms. The primary component of mucus is the protein, mucin, whose molecules form an interconnected network that serves to prevent larger particles from passing though, but does not come in the way of nutrients in the food being absorbed by the intestines. But this barrier, compounded by the acidic environment in the stomach, is also an impediment in oral the administration of therapeutic agents, which are often large molecules or encapsulated to be effective and have large particle size.

One way to get around this difficulty is with the use of chemical agents that break up the mucin network and clear the way. But this method cannot be followed to any great extent as it would compromise the protective role of mucus. Many pathogens have devised ways to get around the barrier, the Stuttgart/Munich paper says, including interaction with the mucus surface, affecting the production of mucin and then of liquefying the mucus. The only way available to drug carriers, so far, the paper says, has been to coat the carriers with agents that affect the mucus-surface interaction, if not to use enzymes, whose effect of degrading the mucus layer is not reversible. A particular bacterium, Heliobacter pylori, which has flagelles, or whip-like organs for propulsion, however, uses a variation, in the form of an enzyme, urease, which directly affects only the level of acidity of the medium, but manages to get through the mucus barrier anyway.

In the highly acid environment of the stomach H. pylori secretes a large quantity of urease, which has a first effect of neutralising the acidity. How it works is that urease acts on urea, which is a waste product that is present in circulation. Urea arises in the liver, out of digestion of protein and reduction of levels of ammonia, and gets taken out for excreting by the action of the kidneys. But at the spot where H. pylorihas secreted urease, the enzyme works as a catalyst to convert urea back to ammonia. The rising ammonia level is alkaline and this reduces the local level of acidity, which is good for the bacterium. But the fall in acidity is not so good for the mucin membrane, which undergoes changes that make it less viscous. The H. pylorithen has free passage and its flagelles propel the bacterium through the mucus to the intestine wall, where it does its work of promoting peptic ulcers and gastritis. The paper says that the bacterium is able to move through mucus so long as there is urea and the medium is not acidic. But even moving flagelles cannot help if there is acidity and there is no urea and the movement resumes when urea is added.

Once the bacterium has gone through and there is no more urease, acidity is restored and the mucin network thickens again, to close the door to other large particles. This ‘altruism’, of letting the fence get mended once its purpose is served may be the secret of H. Pylori’s success, as the body may have evolved differently if the damage had been long lived.

Imitating H. pylori

The Stuttgart/Munich group has created an artificial drug delivery microstructure with a magnetic propulsion arrangement and a urease dispenser to make use of the H. Pylorimethod of breaching the mucus layer. Magnetic propulsion, where a screw-like piece of magnetic material is subjected to a rotating magnetic field has been found to work in low viscosity fluids in a number of studies, the paper says. The micro-helices, or screw-shaped propellers were fashioned by a process of ‘vapour deposition’ and were made of glass, with nickel, as the magnetic material deposited towards the end of the fabrication. The surface was then coated with alumina, aluminum oxide, to protect the nickel from acid. The nickel material was magnetised in a sense transverse to the axis of the screw, so that a rotating magnetic field set the screw turning, which would move it forward.

Just before the micro-propellers were magnetised, they a charge of urease was deposited on their surface, using agents that would bind to the surface and then hold on to the urease. Testing with glass beads of the same dimensions and treated in the same way showed that they effectively acted on urea to reduce acidity below the level need for mucin to remain gelled and viscous.

In practice, however, it was found that even urease treated rotating micro-propellers were not able to actually get moving when placed in mucus. This appeared to be because mucin adheres to the propeller surface, which gets entangled with the web of mucin molecules. After trying different agents to reduce this adhesion, it was found that bile salts, which are present in any case in the intestines, were effective. Trials then showed that micro-propellers could effectively go through a layer of mucus that was largely like the mucus actually found in the stomach and intestines, except for the need to adjust some parameters.

The study would prove useful both for artificial devices, like remotely steerable ‘robot-type’ devices and even to help non-propelled drug or particle delivery across the mucus barrier in the gastro-intestinal tract, the paper says.

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