22 Targeted therapies: Gleevec
We will now discuss Gleevec (Glivec, Imatinib) as a second example of targeted therapies.
22.1 Chronic Myelogenous Leukemia and its origin
Chronic Myelogenous Leukemia (CML) is a slowly progressing disease, which is more common in the elderly with a median age of diagnosis of 65 years. Its symptoms include fatigue, appetite loss and resulting weightloss, mild fever and night sweats (due to elevated basal levels of metabolism) and enlargement of the spleen. Blood stem cells, located in the bone marrow, give raise to both myeloidand lymphoid lineages of blood cells (Figure 22.1). The daughter cellsof the blood cells, myeloid and lymphoid progenitor cells, then lead to the production of more specific types of blood cells:
- Lymphocytes, primarily composed of T-cells and B-cells,
- Red blood cells, platelets,
- and granulocytes.
CML is a cancer of white blood cells and accounts for 14% of all leukemias. “It is a form of leukemia characterised by the increased and unregulated growth of predominantly myeloid cells in the bone marrow and the accumulation of these cells in the blood.”
Expert box: what are leukemias?
22.2 Diagnosis and stages of Chronic Myelogenous Leukemia
CML is often suspected on the basis of a complete blood count, which shows increased numbers of granulocytes. A complete blood count is requested by a doctor to provide information about the cells in a patient’s blood. This includes a cell count for each cell type, concentrations of various proteins and minerals and cells that circulate in the blood stream. These are generally divided into three types:
- White blood cells (leukocytes),
- Red blood cells (erythrocytes),
- and Platelets (thrombocytes).
Abnormally high or low counts may indicate the presence of various forms of disease.
Bone marrow samples can be obtained by bone marrow aspiration 22.2. A needle is inserted through the skin using manual pressure and forced until it abuts the bone. With a twisting motion the needle is advanced through the bone cortex into the marrow cavity. Once the needle is in the marrow cavity, a syringe is attached and used to aspirate liquid bone marrow. An aspiration usually yields about 300 µL of bonemarrow. The liquid bone marrow can then be examined by a pathologistunder the light microscope and analysed by flow cytometry, chromosomeanalysis or PCR.
The progress of CML is divided into three phases:
At time of diagnosis, about 85% of patients with CML are in the chronic phase. They have only mild symptoms and without treatment they progress over the course of several years to the accelerated phase.
Criteria for the accelerated phase may vary, but include for example 10% to 19% myeloblasts in the blood or bone marrow. This phase signals that the disease is progressing. Drug treatment is often less effective in the advanced stages.
Blast crisis is the final phase of CML with rapid progression and short survival. Blast crisis is diagnosed for example when there are more than 20% myeloblasts in the blood or bone marrow.
22.3 The molecular origin of Chronic Myelogenous Leukemia
What is the molecular basis of CML? The Philadelphia chromosome was first discovered in 1960 by Peter Nowell from the University of Pennsylvania School of Medicine and David Hungerford from the Fox Chase Cancer Center and was therefore named after the City in which both institutes are located. The chromosomal defect in the Philadelphia chromosome is a chromosomal translocation (Figure 19.3). Part of chromosome 9 can swap places with part of chromosome 22. This results in an elongated chromosome 9 and a shorter or truncated chromosome 22, named the Philadelphia Chromosome. This translocation results in an oncogenic Bcr-Abl gene fusion (Abl: Abelson proto-oncogene; Bcr: breakpoint cluster region) on the shorter chromosome 22. The resulting gene codes for a Bcr-Abl fusion protein. This fusion protein is always ON or continuously activated and this constitutively active fusion protein leads to increased cell division or proliferation.
Depending on the precise location of the fusion, three clinicallyimportant variants exist, namely the p190, p210 and p230 isoforms.
- P190 is generally associated with Acute Lymphoblastic Leukemia (ALL),
- P210 is generally associated with Chronic Myelogenous Leukemia (CML), but can also cause ALL,
- and p230 is usually linked to Chronic Neutrophilic Leukemia (CNL).
22.4 Main Effects of the fusion protein Bcr-Abl on cell signalling
The Bcr-Abl fusion protein influences many downstream pathways including the:
- Ras/MAPK pathway, leading to increased proliferation,
- Src/Pax/Fak/Raf pathway, affecting the cytoskeleton, resulting in increased cell motility,
- JAK/STAT pathway, responsible for proliferation,
- PI3K/AKT/Bcl2 pathway, leading to suppression of cell death and increased survival.
22.5 Mechanism of action of Gleevec
CML can be treated with targeted drugs called tyrosine kinase inhibitors (TkI). Imatinib (marketed as Gleevec, Figure 22.5)) is a Bcr-Abl tyrosine-kinase inhibitor and specifically targets Bcr-Abl. It was approved by the FDA in 2011. Because Bcr-Abl exclusively exists in cancer cells but not in healthy cells, Gleevec works as a form of targeted therapy since only cancer cells are killed through the drug’s action. It is able to inhibit the progression of CML in about 70% of patients, but the treatment has to be continued indefinitely, because some leukemic cells persist in nearly all patients.
As previously mentioned the tyrosine kinase domain of Bcr-Abltyrosine kinase is stuck in the ON position and it is constitutively active. Gleevec specifically blocks the tyrosine kinase domain in Abl. Gleevec works by binding close to the ATPase site locking it in a closed (or self-inhibited) conformation and therefore inhibiting the enzyme activity of the kinase domain. Gleevec therefore inhibits the different downstream pathways induced by the Bcr-Abl fusion protein. As a result these cells stop growing and eventually die by apoptotic cell death: the transmission of proliferative signals to the nucleus is blocked and leukemic cell apoptosis is induced.
22.6 Administration, clinical uses and side effects of Gleevec
Gleevec is taken orally. It is used for the treatment of CML, but also against gastrointestinalstromal tumours.
Side effects are generally mild or well-tolerated and include nausea, diarrhea, loss of appetite, neutropenia, thrombocytopenia and anemia. One rare complication is severe congestive cardiac failure.
22.7 Cytotoxic drugs versus targeted therapies
We have heard that Gleevec has generally mild side effects, compared tocytotoxic chemotherapy. Why is this the case? With cytotoxic drugs, we work at the maximum tolerated dose, whereas with targeted therapy, we work at the biologically effective dose. That leads to a reduction of side effects: the less drug is administered to the patient, the less side effects the patient may suffer. Whereas we need to treat patients in cycles with standard chemotherapy, we can use targeted therapy such as Gleevec using daily doses.
- Cytotoxicity = is thequality of being toxic to cells
- Cytostasis = is the inhibition of cell growth and mutliplication
References ot be added