Illustration realized in the framework of a collaboration between the Image/Recit option of the HEAD (Haute École d'Art et de Design) - Genève and the Faculty of Sciences of the University of Geneva.

Globally, 1 in 4 adults will experience a stroke, but treatment options are limited.¹ Thrombin an essential enzyme in blood clotting, plays a crucial role in strokes. Thrombin converts fibrinogen, a soluble molecule, into fibrin, an insoluble substance. Fibrin acts like a fishing net, trapping red blood cells to form clots. In certain cases, people are at risk of unprompted blood clotting which can lead to strokes. Patients are commonly treated with anti-thrombin agents to ensure that excessive blood clotting does not occur. One of the therapeutic challenges in addressing blood clotting is that under or over-inhibition is equally dangerous. It has been estimated that anticoagulant-related bleeding is responsible for 15% of all hospital visits for adverse drug events.² Inspired by nature and supramolecular chemistry, our aim within the collaborative project between the University of Geneva and the University of Sydney was to develop an effective anti-coagulant that can be quickly reversed with an antidote.
 

For many people, blood sucking insects (ticks, leeches, mosquitoes, etc.) are the source of phobia and disgust. For scientists working in the anti-coagulation field, they provide a playground for discovery. Numerous blood sucking insects inject peptide-based anti-coagulants when they bite, in order to enjoy their meal. These natural anticoagulants also keep the meal liquid, which is essential for successful digestion. Hirudin, a protein found in leeches, has inspired approved anti-coagulants. A research group at the University of Sydney, directed by Prof. Richard Payne, previously synthesized these molecules in their lab to study the anti-coagulation activity. These molecules bind to different binding pockets on thrombin and in doing so, they block the transformation of fibrinogen to fibrin.

We transformed insect-derived molecules into two separate, inactive fragments that, when combined, form a powerful anticoagulant. We assembled the two fragments using a dynamic link called Peptide Nucleic Acids (PNA), a type of supramolecular interaction. PNA is similar to DNA, in that the genetic code (nucleobases A, T, C and G) is identical, and the structure is helical, but the backbone is peptide based, resulting in a more stable structure. The two PNA strands come together in the same way as DNA does: like Velcro, but in a very specific manner depending on the PNA sequence. This link has the advantage of being dynamic and reversible. The developed molecules were extensively studied and showed that, individually, the molecules have no effect on blood coagulation but when combined, they strongly decrease thrombin’s activity resulting in decreased blood clotting. They were further tested in vivo (mice-model) and showed comparable effects to an approved anti-coagulant (Argatroban) at lower doses.

As mentioned previously, reversibility is a strongly desired characteristic for the development of novel anti-coagulants, in order to facilitate dosage and to reduce the side-effects and risks. By controlling the link between the two fragments, we can manage the anticoagulant activity. We showed that a fast-acting antidote could unlink the fragments. In practice this is done by adding a third PNA strand which outcompetes for the interaction between these two components. This strand basically ‘unzips’ the drug resulting in two fragments which have little to no effect. This results in switching off the drug and rapidly restoring thrombin’s blood clotting ability. This reversibility was also tested in the in vivo model and showed rapid recovery of blood clotting similar to that when no drug was administered.

This study demonstrated the development of an effective anticoagulant which showed similar effects to a marketed drug. Our new therapeutic could be rapidly reversed by the addition of a simple antidote. We demonstrated this in the case of anticoagulants, but the technology is a general strategy for drug reversibility and could be applied to many fields, such as immunotherapy (where rapid reversal would be desired in the case of infection). 

 

1-      Global, Regional, and Country-Specific Lifetime Risks of Stroke, 1990 and 2016. New England Journal of Medicine 2018, 379 (25), 2429-2437. DOI: 10.1056/NEJMoa1804492. 

2-      Geller, A. I. et al. Emergency Visits for Oral Anticoagulant Bleeding. Journal of General Internal Medicine 2020, 35, 371-373. DOI: 10.1007/s11606-019-05391-y