Discovery and development of anti-plasmin drugs (ε-aminocaproic acid and Tranexamic acid)

Do you know “Tranexamic acid”?

“Have you heard of the name, Tranexamic Acid?” Many will answer “Yes, I have.” in Japan. In the past, there was a TV commercial for tooth paste that asked, “Do you bleed when biting an apple?” and more recently we hear in TV commercials that tranexamic acid is effective for skin whitening.
Tranexamic acid is a drug often used to reduce surgical and traumatic bleeding as well as to reduce abnormal bleeding. It is also used as an anti-inflammatory agent for the treatment of a viral sore throat or in urticaria.
Tranexamic acid has a history of over 50 years, it is used in over 120 countries worldwide, but even now clinical trials are ongoing. In recent years, the astonishing results of three large-scale clinical trials (in traumatic, postpartum and traumatic brain hemorrhage) have been big news stories around the world.

 

Tranexamic acid is a cheap lifesaver of the heamorrhage patients!

CRASH-2: Effects of tranexamic acid in traumatic haemorrhage
http://www.crash2.lshtm.ac.uk/

On the 3rd June 2010, the results of large-scale clinical trial (called CRASH-2) published in the Lancet that looked at the effectiveness of tranexamic acid in bleeding trauma patients.
The researchers estimated how many lives could be saved if all trauma patients world-wide were treated with tranexamic acid. They estimated that 128,000 lives could be saved if tranexamic acid was given within an hour of the injury or 112,000 lives if it was given at least within 3 hours of the injury. The countries where most lives could be saved were India, China, USA, Brazil and Russia where the population is dense, and the risk of road traffic crashes is high.

Link:Ker et al. BMC Emargency Medicine 2012, 12:3
https://bmcemergmed.biomedcentral.com/articles/10.1186/1471-227X-12-3

 

Woman trial: Effects of tranexamic acid in postpartum haemorrhage
https://www.lshtm.ac.uk/newsevents/news/2017/woman-trial-results

On the 26th April 2017, 7 years later, the result of large-scale clinical trial (the WOMAN trial) reported the effects of tranexamic acid in women with postpartum hemorrhage in the Lancet, one of the leading international medical journals. The astonishing results were featured on global news. The BBC reported: “Postpartum haemorrage: Cheap lifesaver’ cuts deaths by a third.” The New York Times reported: “Dangerous bleeding after childbirth could be treated with ＄1 injection.” However, the results were not only featured in Europe and USA, but also similar news stories were reported all around the world including Africa and Asia.

CRASH-3: Effects of tranexamic acid in traumatic brain haemorrhage
https://crash3.lshtm.ac.uk/blog/crash-3-trial-results/

On the 15th October 2019, the astonishing results were featured on global news again. The results of CRASH-3 published in the Lancet that lowered the risk of death by 20% on the patients with mild or moderate traumatic brain injury by the administration of tranexamic acid. It has suggested that hundreds of thousands of lives could be saved by giving patients with head injuries a £6 drug to stop killer bleeds in the brain.

Based on these results, tranexamic acid, invented over half a century ago is once again attracting attention.

 

Mechanism of action of Tranexamic acid

Tranexamic acid is an anti-plasmin drug. So, what does plasmin do? And why is it necessary to inhibit plasmin? Usually, when you get injured and start to bleed, provided the damage to the vessel is not too large, the bleeding will stop within a short period. First, platelets gather to cover the damaged site (Primary hemostasis.) However, this may not stop the bleeding completely and so a fibrous net called fibrin covers the platelets to fortify the damaged site and stop the bleeding (Secondary hemostasis.) Eventually, when the vessel is repaired, blood clot that is no longer necessary is removed. And it is plasmin’s job to remove the old blood clot. Plasmin gets rid of the blood clot by cutting the fibrin net. However, when plasmin’s action is too strong, blood clot may be dissolved before the damaged vessel is repaired, and that may cause bleeding. In such case, Tranexamic acid is used to inhibit plasmin.

 

History of Tranexamic acid

Tranexamic acid was invented by Japanese researchers led by husband and wife Shosuke and Utako Okamoto. The story starts in 1947, 2 years after the second world war. It started in a laboratory in a Tokyo suburb as a joint research project between the Hayashi laboratory and the Mitsui Chemical corporation (whose pharmaceutical branch is currently the Mitsubishi Tanabe Pharma Corporation.) Shosuke Okamoto (Keiko university) represented the Hayashi laboratory and Fujio Nagazawa (Vice director of Mitsui Chemical laboratory) represented the Mitsui side.
Nagasawa suggested the following 3 research objectives:

·First, to exceed the international standards
·Second, focus on an area that has been untouched by other researcher
·Third, work towards the development of a drug that reduces human suffering

In response to this, Shousuke Okamoto came up with the creative idea to invent an anti-plasmin. This was an entirely new concept since there were no anti-plasmin drugs.

 

Clues from the amino acid “Lysine” and the birth of ε-aminocaproic acid

Research on an anti-plasmin started with a series of Sulphur compounds, based on the view that Sulphur compounds have a small inhibitory effect on plasmin. However, the results were not encouraging. After this, we extended the search to readily available low molecular weight compounds and to the amino acids. After hundreds of failures, they started to think that the research was a failure.
But one sunny summer afternoon, there was a shout from a research assistant. “Oh my goodness, lysine inhibits plasmin even at a weak concentration.” Because lysine was not readily available it had not been looked at previously, but the results showed a fascinating ability to inhibit plasmin even at concentrations as weak as 0.01%. This was nothing like the effects of other compounds. Whilst it did not mean that that lysine could become a drug straightaway, this discovery was a major clue. And fortunately, Lysine was not only harmless to the human body, but its structure was easy to understand.
Various compounds with a similar structure to lysine were developed and tested to see if they could inhibit plasmin. In this way, we discovered epsilon which has a structure similar to lysine but with the alpha amino group removed. It was ten times more effective than lysine as regards plasmin inhibition.

 

Looking for a more effective anti-plasmin and the birth of Tranexamic acid

Epsilon became widely used both in and outside of Japan; however, the effect was not strong enough for very severe bleeding such as that seen in lung surgery. In France and Sweden, large doses (30-60g) of epsilon were being administered. “If only a stronger antiplasmin could be invented” was the sorrowful wish from treating doctors. In response to their demand, further research was started to find a drug that can inhibit plasmin action at ever weaker concentrations. The clue to a new drug was the molecular skeleton of epsilon. The distance between the positive charge and negative charge of epsilon was critical. Maintaining this distance and changing the shape of the molecule from a single strand to a ring structure, gave rise to aminomethylcyclohexanecarboxylic acid (AMCHA.) Animal experiments and clinical trials confirmed the anti-plasmin effect of AMCHA at 1/8 to 1/10 of the dose of epsilon and without drug toxicity.

What was remarkable here was that there were stereoisomers (cis-form and trans-form) of AMCHA. The Japanese team believed that trans-form was effective while the Swedish team believed the cis-form was effective. In those days it was not easy to separate or to identify the isomers. Eventually, researchers in Daiichi pharmaceutical co. were successful in separating the isomers and confirmed that the trans-form was effective. Thus, tranexamic acid that has antiplasmin effect 10 times stronger than epsilon was introduced to the world. (Daiichi pharmaceutical started selling it as Transamin in 1965.) Following this, epsilon was replaced by tranexamic acid which became one of the most influential drugs in the 20th century.

Stereoisomers of cyclohexane

 

A time capsule to reach people in 5000 years later

In 1970, an international exhibition was held in Osaka. One of the attractions and something that was popular at the time was the “Time capsule EXPO’70”. The plan was to create a time capsule with the objective of transmitting contemporary culture to people living in 5000 years’ time. The plan cost about two hundred million yen. Among an enormous number of items from the 1970’s, 2098 items (out of which, 742 items are from natural science field, 686 items were from social field, 592 items are art related and the rest 78 items) were selected by interviewing 632 leading experts from 36 countries.
One of the items selected was tranexamic acid (storage number: N-15-3-7). Two of these time capsules were buried in Osaka castle close to the inner fortifications. The first capsule will remain buried until 6970 without being disturbed at all. The second capsule was opened in 2000, 30 years after the international exhibition and will be opened every 100 years to check the state of preservation.

Time capsule EXPO '70 was buried adjacent to Osaka Castle.