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Disease and treatment

A Trojan horse can sneak medicine through the gut wall

Even using the most advanced types of pills, only 1 in 100 biomedicine molecules passes through the body and arrives at the intended target. Of all the obstacles medicine meets on the way, the gut wall is the most insurmountable. Danish researchers now plan to disguise molecules so they can reach their target.

Most people swallowing a pill assume that the contents will arrive in the right place – but this does not always happen. During its journey, the pill is attacked by both enzymes in the gut and gastric acid before it reaches its greatest obstacle – the gut barrier. The active ingredient is often stopped here because of its size or chemistry. And if it gets through, it still has to reach its final target. In the future, medicine will be targeted better.

“Of all the challenges the medicine meets on the way, the gut wall is clearly the most insurmountable. By changing the surface of medicines, we can get them to function as a kind of Trojan horse that can cross the gut barrier and function on the other side. Using the world’s most advanced microscope, we can even monitor the process and learn which chemical modifications work best at getting the medicine through,” explains Thomas Lars Andresen, Professor and Head of Department, Department of Micro- and Nanotechnology, Technical University of Denmark (DTU Nanotech).

Sparing the gut

The greatest challenge in delivering the medicine to the intended target involves understanding how to get the active ingredients across the gut barrier in animals and humans. Enzymes in our digestive system normally break down the protein we eat, and specific amino acids broken down from the protein are absorbed through our gut barrier.

“The gut can be viewed as externally oriented, like our skin, which faces outward towards our environment. Our gut barrier is designed to block bacteria, viruses and various substances that pass through our gut; their molecules should not be absorbed by our body. So the whole system is set up such that we cannot transport a peptide-based drug across our gut barrier – unless we play some magic tricks.”

To be as specific as possible, the researchers decided to perform their tricks on several existing biopharmaceuticals, including insulin (for treating diabetes), calcitonin (for osteoporosis) and interleukin-12 (for autoimmune diseases and cancer).

“We chose these ingredients to include several relevant biopharmaceutical compounds but especially because they are chemically and structurally diverse. This will provide us with mechanistic understanding of how the compounds are able to pass through the membrane of cells and thereby provide methodological understanding that can be used more generally. The goal is to succeed in transporting at least 10 times as much of the therapeutic peptides across the gut wall during the project.”

Like alcohol for the cells

The researchers will use two strategies. One is to alter the surface of the ingredients so that their chemistry becomes more compatible with the gut membrane. This has previously been tried with some success by adding chemicals that increase penetrability. However, this has not been very effective, and no one knows the long-term consequences of constantly penetrating the gut in this way.

“We will try to modify the peptides so that they actually function as their own Trojan horse. When they encounter the gut barrier, they will have physical and chemical properties that enable them to transport themselves through. Once they have crossed the barrier, the Trojan horse will open up to allow the active ingredient of the peptide to be released into the bloodstream.”

The trick performed by the researchers can be compared with what happens when a person drinks alcohol. The structure of ethanol allows it to pass through our membranes unaided so that it can spread throughout the whole body relatively quickly.

“These peptides, which are pharmaceutical ingredients, can be theoretically altered so that they mimic ethanol. So we are trying to modify them such that, even though they are large molecules, they behave like ethanol and can pass through our systems in a simple way. The excipients that have helped the medicine pass through are then automatically cleaved off,” explains Thomas Lars Andresen.

An alternative way across

In parallel with the chemical modification of medicine, the researchers are also working on two groundbreaking strategies that may revolutionize the transport of medicine. One is based on designing small medicine capsules on a nanometer scale. The capsules, which are no larger than a small bacterium, can contain tens of thousands of biopharmaceutical molecules. In addition to size as a factor in promoting absorption, their surface also helps.

“We can coat these small capsules with a surface that is very similar to the surface of the gut membrane itself. This means that they will easily be able to pass through and continue their journey on the other side. In addition, we can put several small antibodies on their exterior – a sort of address identity – so that they end up in the correct mailbox, both in the gut membrane and at their final destination, such as the pancreas.”

The team is developing another potential landmark strategy together with their partners at Monash University in Australia. The have experimented with following an unorthodox path through the human body. Once a normal drug has passed through the gut, it continues through the liver into the bloodstream. Instead the researchers are experimenting with transport through the human lymph system.

“Using the lymphatic system as an alternative transport system has several obvious advantages. Lymphatic tissue allows pharmaceutical ingredients that are less soluble in water to pass through. Further, the lymph glands are full of immune cells, so this provides opportunities for immunotherapy and thus, in cancer, for example, being able to activate immune substances in the lymphatic system to battle cancer.”

Monitoring progress with the world’s best microscope

A major challenge in creating new types of pills that will ensure more effective delivery of medicine is how to monitor progress. Instead of, for example, giving various types of pills to mice and then taking samples of their blood, tissue and organs, the researchers are hoping instead to move the body into the laboratory itself.

“We will initially create a 2D model system, but we also aim to develop a 3D model system using the guts of rats, for example, so we obtain a more realistic model of what a gut barrier looks like and so that it is more like tissue. In this way, we can incorporate somewhat more mechanical properties into the model we are building.”

To monitor what is happening, the researchers at DTU Nanotech have teamed up with researchers from Harvard Medical School, which has one of the world’s most advanced lattice light-sheet microscopes. According to Thomas Lars Andresen, the combination of the Danish researchers’ experience with fluorescent labelling of molecules may be the key to achieving their target within 6 years.

“The microscope not only enables us to track whether the drug passes through the gut but also how it manages to pass through – right down to the molecular level. We can see whether the compounds on the surface of the drug slow the progress and can thereby change the composition of the surface and remove the obstacles. At the end of the project, we will therefore obtain knowledge that can ensure that people receive more effective and safer treatment,” concludes Thomas Lars Andresen.

“Acylation of salmon calcitonin modulates in vitro intestinal peptide flux through membrane permeability enhancement” has been published in the European Journal of Pharmaceutics and Biopharmaceutics. In 2017, the Novo Nordisk Foundation awarded a Challenge Programme grant to Thomas Lars Andresen for the project Oral Drug Delivery of Biopharmaceuticals.

Thomas Lars Andresen
Professor
The major challenge for obtaining high drug bioavailability of peptide biopharmaceuticals is to circumvent the intestinal barrier constituted road-block where the physiochemical properties of peptides, being relatively large macromolecules with medium to high hydrophilicity, constitutes an intrinsic challenge. Modifications that can be used to change the physiochemical properties of the peptides will often render them non-active except at a few selected positions that do not interfere with their binding to target receptors. Effective delivery of peptide biopharmaceuticals requires more advanced peptide engineering strategies and/or new drug delivery technology developments, which may need to be mutually optimized, hand-in-hand, to achieve radical improvement in oral delivery efficiency. We will innovate the next generation drug delivery systems that effectively transport peptide biopharmaceuticals across the small intestine, which can successively seek out target cells and tissues, and ultimately release the carried drugs with spatial and temporal control in a radically more efficient way than current state-of-the-art. The key challenge to be addressed is to mechanistically understand how a delivery system can be designed to transport biopharmaceuticals across the intestinal barrier with a design that provides high bioavailability of the drug to the target cells and tissues within animals and humans.