CML is a clonal hematopoietic stem cell disorder. It accounts for 15%-20% of all cases of leukemia with an annual incidence of 1 to 2 cases per 100,000 per year. The median age at diagnosis is between 50 and 60 years of age. Clinically, the disease progresses through distinct phases referred to as chronic or stable, accelerated, and blast. The chronic or stable phase of the disease is characterized by excess numbers of myeloid cells that differentiate normally. After an average of 4-6 years, the disease transforms through an "accelerated phase" to an invariably fatal acute leukemia, also known as blast crisis. Disease progression is likely due to the accumulation of molecu lar abnormalities that lead to a progressive loss of the capacity for terminal differentiation of the leukemic clone (Faderl et al. 1999; Sawyers 1999).
Treatment choices for patients with CML prior to the introduction of imatinib included stem cell transplantation, hydroxyurea, or interferon-a-based regimens, with allogeneic stem cell transplantation being the only proven curative therapy. As the average age of onset of CML is greater than 50 years of age, this factor, plus the inability to identify a suitably matched donor limits this option to a minority of patients. Thus, fewer than 20% of CML patients were cured with these treatment options (Faderl et al. 1999; Sawyers 1999).
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