Glivec, CML, Abl and targeted molecular therapy for cancer
This essay was written by Qiling Xu and was first published in the 2003 Mill Hill Essays.
Basel, Switzerland, 3 January 2000. The pharmaceutical company Novartis announced that the European Commission approved Glivec as a first-line treatment for adult and child patients with chronic myeloid leukemia (CML), enabling doctors to provide the drug to newly diagnosed patients. The approval was based on information collected over twelve months in a large study comparing Glivec with the best previous treatment for CML. Patients treated with Glivec were nine times more likely to achieve a response in which no cancer cells remained. In other patients Glivec significantly delayed the time to progression of the more advanced stages of CML.
What is Glivec? How did this drug come about? It is a long story and the protagonists include the white blood cell cancer, chronic myeloid leukemia (CML); the culprit, a gene called Bcr-Abl; a new drug that was given the name Glivec; numerous detectives, and important historical influences.
Normal cells produced in the bone marrow develop to maturity in a step-by-step process, until they leave the bone marrow and enter into the blood stream. Among immature bone marrow cells are stem cells that will become red blood cells and different kinds of white blood cells. One can think of a tree when looking at the process. The trunk represents the stem cells that produce many branches of intermediate cells, which each in turn form a distinct set of mature cell types at their tips. Leukemia is an uncontrolled multiplication of one branch, the white blood cells. As in many other forms of cancer, the leukemic cells are descended from a single cell that lost its ability to maintain normal control over cell division and growth. There are a number of types of leukemia, corresponding to the many types of white blood cells and the stages they go through as they mature. Chronic myeloid leukemia, or CML, arises in a bone marrow cell, which normally forms two types of cells called granulocytes and platelets. So, CML patients have abnormally high numbers of these cells in their blood. Clinically there are three phases of CML depending on the number of immature blood cells in the blood and bone marrow. During the initial chronic phase the number of immature stem cells is increased but they can still mature to form different white blood cells that function normally. At this stage the patients have mild symptoms and respond to treatment. After an average of four to five years, the CML typically progresses to the accelerated phase, in which there are more immature cells in the blood and the disease is not as responsive to treatment. The final, often fatal stage, is the “blast crisis”, where immature cells dominate and chronic leukemia has become an aggressive acute leukemia.
There are three major treatments for CML. Firstly, chemotherapy and radiotherapy can be used to kill cancerous cells but this has the disadvantage that normal cells in the blood are also killed. The treatment offers some relief rather than a cure since the methods are poorly targeted and highly toxic. A second treatment is aimed at improving the body’s immune system to fight the cancer. Interferon alpha is a protein naturally produced in our bodies to fight viruses. In the 1980s interferon alpha was introduced to treat CML in combination with chemotherapy. This treatment has achieved long-lasting remissions in 30% of patients compared with 5% of those using conventional chemotherapy. It does not, however, completely remove the cancer cells. The final treatment option is high-dose chemotherapy and total body irradiation followed by bone marrow transplantation. This is the only treatment capable of disease eradication, but is restricted to patients who can identify a suitable bone marrow donor and who are medically fit to undergo the procedure. The most effective therapy would be one that specifically eliminates the cancer cells. However, such a treatment was not available before scientists had a molecular understanding of the disease. This was achieved in the revolution triggered by the discovery of the DNA double helix half a century ago, which established that DNA contains the blueprint for inheritance (i.e. our genes) and that genes encode proteins. The discovery has brought us into the protean world of genes and more importantly the profound influence on our understanding of biological systems and medicine.
One copy of each of the long-chain DNA molecules which constitute the genetic material is contained within the nucleus of each cell, packaged into microscopic structures called chromosomes. Human cells have 46 chromosomes, 23 pairs in females, 22 pairs plus 2 unpaired in males. Each of these chromosome pairs has a characteristic length and pattern of light and dark bands when stained with dyes. In 1960 two researchers in Philadelphia noticed that an abnormally small chromosome was consistently present in the cells of CML patients. This was given the name of the Philadelphia (Ph) chromosome. This was the first time that a chromosomal abnormality had been associated with a malignant disease. In 1973 the Ph chromosome was found to be the result of swapping parts of chromosome 9 and 22, producing a longer chromosome 9 and a shorter chromosome 22. Two genes flanking the points in the DNA where this swap over occurs were identified in the early 1980s. One is the abl (ablation) gene from chromosome 9 that was already known as the human counterpart of a mouse virus gene. The other gene from chromosome 22 was given the name of bcr (short for breakpoint cluster region). At that time the function of Bcr was unknown, but the abl gene was widely studied. The Abl protein is an enzyme, known as a protein kinase, that attaches a phosphate group to specific proteins. When this happens it triggers a cascade of events in the cells that regulate the division of cells and cell death. These are both common mechanisms used to control the number of cells in the body. Cell death is a protection mechanism used to eliminate excess or abnormal cells. Disruption of either of these mechanisms can result in uncontrolled cell division manifested as cancer. Abl protein in normal cells is only enzymatically active when the cell is stimulated by a growth factor. The effect of the Ph chromosome translocation is to bring the bcr and abl genes together resulting in a fused bcr-abl gene. A major breakthrough in understanding the molecular nature of CML came when it was discovered that the fused Bcr-Abl protein produced from the fused gene is a continuously active protein kinase. This unrestrained activation of the kinase activity leads to an increase in the rate of cell division and the protection of the cells from cell death. Consequently a large number of Ph chromosome-containing cells is found in CML patients.
The realization that the bcr-abl gene is not just diagnostic of the Philadelphia chromosome, but more importantly is the cause of the disease, focussed efforts to find a targeted treatment for CML sufferers. Since mutations in the bcr-abl gene that disrupt the kinase activity of the Bcr-Abl protein also removes its effects, researchers began to search for a potent anti-leukemic protein kinase inhibitor among a wide variety of candidate chemical compounds. In 1995 and 1996 a few such compounds that specifically inhibited the kinase activity of Bcr-Abl were identified. Later on with various modifications and improvements, one compound emerged as the lead for clinical development. Normal protein kinases have highly flexible molecular structures that exist in equilibrium between shapes in which they are active or inactive. Changes to the structure, such as replacement or removal of key amino acids, can fix the shape in a form which is active at all times and Bcr-Abl is a classic example of this. The specificity of kinase inhibitors lies in their ability to bind to the unique shape of the kinase region and, in the case of STI-571, to trap the Abl in an inactive shape. Protein kinases play important roles in controlling various aspects of cell activities and have therefore become a common target for drug discovery and treatment.
Virtually all patients with the chronic phase of CML who are treated with STI-571, now known as Glivec, achieve complete remission. The drug is also now approved as a first-line treatment for gastrointestinal tumours. No wonder Glivec was hailed as a wonder drug that brings hope and cure to life threatening illnesses. However, CML comes in many ugly faces and Glivec cannot cure them all. When treated during the more aggressive stage of blast crisis, most CML patients ultimately develop a drug-resistant disease. The majority of relapsed patients have mutations within the Bcr-Abl kinase region that reduce or abolish drug binding. The key regions of the kinase required both for regulation of Abl under normal circumstances, and for inhibition of Bcr-Abl by Glivec, have now been identified. It is of great importance to understand the underlying mechanisms of drug resistance so that a new generation of targeted anti-cancer agents can be designed. The resistance to Glivec has also led to a renewed interest in other drugs that may be helpful in this situation. These are targeted at the cellular events following the activation of Bcr-Abl, the stability of the Bcr-Abl proteins and alternative inhibitors of Abl kinase.
We are undergoing a revolution in medicine in which a better understanding of mechanisms of disease, such as cancer, and of pathways at the molecular and genetic levels has provided new opportunities for rational therapies and novel approaches for diagnosis. Molecular targets include molecules and pathways that are involved in cell proliferation, programmed cell death, DNA repair and secondary tumour formation. Examples are p53, protein kinases and genes that directly cause cancer when mutated, such as Bcr-Abl in CML and the breast cancer susceptibility genes BRCA1 and BRCA2 in breast and ovarian cancer. Different strategies for targeted anti-cancer therapy are currently under investigation. In addition to chemical inhibitors, biological molecules such as DNA or antibodies and even viruses or cells can be used to eliminate tumours. In recent years we have seen the development of antibody directed drug therapy. This uses monoclonal antibodies to destroy some types of cancer cells while causing little harm to normal cells. Monoclonal antibodies are protein molecules designed to recognize and stick to the surface of cancer cells. When an antibody binds to the protein target it can trigger the body’s immune system to attack the cancer cells. Some monoclonal antibodies have had an anti-cancer drug or radioactive substance attached to them, in order to deliver treatment directly to the cancer cells. Therapeutic monoclonal antibodies are available for cancer treatment, including Alemtuzumab (brand name Campath) that recognizes some proteins found in certain white blood cells and Trastuzumab (brand name Herceptin), which recognizes a protein found in breast cancer cells. Alemtuzumab acts by attacking both leukemic and normal white blood cells. However, the body quickly replaces any normal white blood cells that are damaged. Trastuzumab works by preventing certain proteins from binding to other proteins and thus preventing cancer cell growth. Trastuzumab also works by triggering the body’s immune system to destroy the cancer cells.
In recognition of the importance of our genes in our health and our response to treatment the Department of Health has issued a Genetics White Paper in June 2003. This sets out a plan of action and investment in genetics for the National Health Service. Through genetic testing and gene-based treatment we can anticipate dramatic changes in disease prevention, diagnosis and treatment. As each individual has a unique composition in our genetic make-up, a person-based medical treatment will be a reality in the not too distant future. It is noteworthy that these recent and future advances in our understanding of human diseases are deeply rooted in knowledge of genetics, molecules and pathways that come from basic research.
In UK 600-900 people are diagnosed with CML each year, often by routine blood count checks. It occurs mostly in adults and very few children develop this type of leukemia. According to Cancer Research UK, there are 6,600 new cases of leukemia each year in the UK and three-quarters of these are in adults. Leukemia causes about 4 percent of all cancer-related deaths and is the number one cause of death from cancer in children and young adults under the age of 20. It is clear that leukemia is a genetic disorder, since more than 500 chromosome changes have been found in leukemia patients, and recurrent chromosomal changes occur in more than one half of all cases of leukemia. Progress in understanding the molecular biology of different types of leukemia is the key to the development of accurate diagnosis and targeted treatment. Glivec has set a precedent for the approach of molecularly targeted therapy and has demonstrated that it is pivotal to identify the right target for the right group of patients.