Tokai University takes on challenge of developing therapeutic drugs for leukemia

A research group consisting of Professor Takashi Yahata, Professor Kyoshi Ando and Associate Professor Makoto Onizuka of the School of Medicine, Tokai University and their colleagues is engaged in a study aiming to develop university-originated therapeutic drugs for leukemia based on the results of basic research.

An elegant experimental system

With regard to the appeal of this research, Professor Yahata noted, 'I believe that to clinical doctors who are in daily contact with patients, hematopoietic stem cell transplantation is a demanding and difficult medical practice, and they are constantly wishing for further improvements to treatment results. On the other hand, I am involved in basic research, and I use an extremely elegant experiment system to carry out research using experimental animals. The fact that transplanted donor cells derived from other individuals can be engrafted and function in the body of a recipient is a situation that lies outside of expectations in the natural world, and I feel a sense of romanticism in humans striving to apply these unique circumstances to medicine and treating incurable diseases.'

Previously, Professor Yahata and his colleagues have worked with Professor Toshio Miyata of Tohoku University and his colleagues, focusing on clarifying the physiological actions of the plasminogen activator inhibitor PAI-1, and clarified that PAI-1 inhibits hematopoietic regeneration by negatively regulating the fibrinolysis system (fibrinolytic system). When the group carried out experiments involving hematopoietic stem cell transplantation into PAI-1 gene knockout mice, they confirmed rapid hematopoietic regeneration and, interestingly, also found that this was more efficient than administering tissue plasminogen activator (tPA), a factor involved in fibrinolysis.

Thus, the research group worked with Visiting Professor Noriaki Hirayama (then a Specially Appointed Professor of the Institute of Advanced Biosciences) to explore and identify low-molecular-weight compounds that inhibit PAI-1 activity via simulations based on the 3D spatial structure of PAI-1. They confirmed that these compounds (PAI-1 inhibitors) function to encourage hematopoietic regeneration and focused on a clinical study using these PAI-1 inhibitors.

A hematologist's greatest wish

In the past, leukemia was thought to be an incurable and terminal illness, but highly effective drugs such as tyrosine kinase inhibitors have been developed one after another since the 1980s, and now many patients are able to control the illness and go into remission. On the other hand, in cases of a certain severity, such as highly malignant cases or cases with chemotherapy resistance, there are no other options except accepting hematopoietic stem cell transplantation, such as bone marrow or umbilical cord blood transplants.

Associate Professor Onizuka, who is responsible for the clinical study, noted, 'While I was engaged in clinical work over a long period, there were lots of patients who could not be cured and patients who lost their lives due to complications even after having a hematopoietic stem cell transplant. I want to help these patients somehow - that is the greatest wish of hematologists around the country, including myself.'

Even today, when treatment results have improved remarkably, hematopoietic stem cell transplantation still has aspects of experimental treatment.

Tokai University has a long history of operating an umbilical cord blood bank (this was transferred to the Japanese Red Cross Society in 2014 when the Act on Organ Transplantation was enacted). The use of umbilical cord blood in hematopoietic stem cell transplantation has the advantage of being non-invasive. On the other hand, fewer stem cells can be obtained from umbilical cord blood than from bone marrow, so raising the survival rate is a problem.

Thus, in 2016 the research group launched a doctor-led clinical trial aiming to develop a drug that improves hematopoietic function by also using PAI-1 inhibitors in hematopoietic stem cell transplantation with umbilical cord blood for chronic myelogenous leukemia. This was a very small-scale trial with few subjects, and in one patient an adverse event - hepatic impairment - occurred; later analysis clarified that the PAI-1 inhibitors did not cause this, and a new clinical study was planned with consideration for future adverse events.

Associate Professor Onizuka noted, 'In many blood disorders, patients have to keep taking medication for the rest of their lives, even if they don't have a transplant; this becomes a large psychological and financial burden. In the future, I want to continue clinical research that will enable patients to stop taking medication.'

As Professor Yahata and his colleagues continued their research on the physiological actions of PAI-1, they learned that PAI-1 is also associated with life processes other than the fibrinolytic system.

In a mouse experiment, the group observed that administering PAI-1 inhibitors during a hematopoietic stem cell transplant increased the hematopoietic stem cell homing function (arriving in bone marrow from peripheral blood). When the group were verifying whether this function is the primary cause of fibrinolytic system activation in mice, they clarified that PAI-1 regulates the motility of hematopoietic stem cells through a mechanism other than the fibrinolytic system. In the past, researchers had only focused on PAI-1's function of regulating the fibrinolytic system in extracellular body fluids, but the group discovered novel functions: that it also functions within cells, negatively regulates the motility of hematopoietic stem cells, and ensures their quiescence in the unique microenvironment (niche) of bone marrow.

Freedom from anticancer drugs

Hematopoietic stem cells are dormant in this niche and activate to maintain the homeostasis of the hematopoietic system by maintaining stem cell activity. Anticancer drugs have little effect on cancerous stem cells in a dormant state, when the cell cycle is paused, and it is thought that the dormant state of leukemia stem cells, from which leukemia originates, is induced by interaction through adherence to the bone marrow niche.

Thus, Professor Yahata and his colleagues used mice to investigate whether they could end the quiescent state of leukemia stem cells by inhibiting PAI-1 and thus weaken their anticancer drug treatment resistance.

In doing so, they learned that by using a PAI-1 inhibitor at the same time as anticancer drugs, the motility of leukemia stem cells increases and they become highly responsive to treatment, leading to a remarkable antitumor effect being displayed, including a reduced mortality rate.

Since 2019, when the group obtained these results, they have worked with Renascience, a venture company originating in Tohoku University, to carry out a clinical study to verify this effect after improving inhibitors.

Professor Yahata commented, "PAI-1 is a protein that has been known for a long time, and researchers didn't pay it a lot of attention. However, as our analysis progressed, we learned that this protein has multiple functions and frequently appears in situations involving various life processes. Reports showing that inhibitors affect diseases other than leukemia are being produced one after another. By promoting analysis of the functions of PAI-1, we anticipate that we will find out how PAI-1 is connected with the onset and advancement of different diseases, and will therefore be able to predict which additional diseases we can use inhibitors for."

The clinical study currently underway involves the joint administration of PAI-1 inhibitors and tyrosine kinase inhibitors for chronic myelogenous leukemia (no transplants). The aim is Associate Professor Onizuka's greatest wish: to stop the need for anticancer drugs that people must continue to take for the rest of their lives. There were very few of the adverse events that occur to a greater or lesser degree in clinical studies to verify the efficacy of general drugs, which, conversely, led to some unease, but phase two was completed smoothly, and preparations for phase three are now progressing.

Research for drug discovery is taking place in many universities, but there are few cases in which research is able to progress to clinical trials. Professor Ando and Professor Miyata's strong passion towards drug discovery in academia is pushing this forward. There are great hopes for future development.