Measuring response to BCR-ABL inhibitors in chronic myeloid leukemia

In patients with chronic myeloid leukemia (CML), the hallmark Philadelphia chromosome is the marker of disease that can be detected by conventional metaphase cytogenetics, fluorescence in situ hybridization, or polymerase chain reaction. The current "gold standard" of treatment response is cytogenetic response. Cytogenetic response to imatinib is strongly associated with disease progression and survival. Various strategies aimed at improving cytogenetic response have been explored, such as escalation of imatinib and switching to the newer breakpoint cluster region/v-abl Abelson murine leukemia viral oncogene (BCR-ABL) inhibitors dasatinib and nilotinib. Data from recent randomized trials of dasatinib and nilotinib as first-line therapy of newly diagnosed chronic-phase CML suggest that these agents are more effective than imatinib in achieving 6-month and 12-month complete cytogenetic responses. However, it is still too early to know whether or not this early response will translate into a long-term survival advantage. In addition, more sensitive assays to detect residual disease also may be associated with improved long-term outcomes. The deepest measure of response-a complete molecular response-may help identify patients who can stop taking imatinib for the short term, although the long-term consequences of this strategy remain unknown.

BCR-ABL tyrosine kinase inhibitors in the treatment of Philadelphia chromosome positive chronic myeloid leukemia: a review

Chronic Myeloid Leukemia (CML) is a clonal disease characterized by the presence of the Philadelphia (Ph+) chromosome and its oncogenic product, BCR-ABL, a constitutively active tyrosine kinase, that is present in >90% of the patients. Epidemiologic data indicates that almost 5000 new cases are reported every year and 10% of these patients eventually succumb to the disease. The treatment of CML was revolutionized by the introduction of imatinib mesylate (IM, Gleevec), a BCR-ABL tyrosine kinase inhibitor (TKI). The clinical use of specific BCR-ABL inhibitors has resulted in a significantly improved prognosis, response rate, overall survival, and patient outcome in CML patients compared to previous therapeutic regimens. However, the complete eradication of CML in patients receiving imatinib was limited by the emergence of resistance mostly due to mutations in the ABL kinase domain and to a lesser extent by molecular residual disease after treatment. The second-generation BCR-ABL TKIs nilotinib (Tasigna) and dasatinib (Sprycel), showed significant activity in clinical trials in patients intolerant or resistant to imatinib therapy, except in those patients with the T315I BCR-ABL mutation. Identifying key components involved in the CML pathogenesis may lead to the exploration of new approaches that might eventually overcome resistance mediated to the BCR-ABL TKIs. Here, we present an overview about the current treatment of Ph+ CML patients with the TKIs and the obstacles to successful treatment with these drugs.

Dasatinib

Dasatinib, (former BMS 354825), is an orally available small-molecule multikinase inhibitor. It potently inhibits BCR-ABL and SRC-family kinases (SRC, LCK, YES, FYN), but also c-KIT, PDGFR-alpha and beta, and ephrin receptor kinase.Dasatinib is about 300 times more potent than imatinib in cells expressing unmutated BCR-ABL in vitro. The drug has demonstrated activity against clinically relevant mutations, including those associated with poor prognosis during ongoing imatinib therapy.Dasatinib is approved for the treatment of patients with BCR-ABL-positive chronic myeloid leukemia (CML), resistant or intolerant to imatinib in chronic, accelerated, and blast phase. It also is approved for the treatment of Philadelphia Chromosome positive (Ph+) acute lymphoblastic leukemia (ALL) resistant or intolerant to imatinib.A single daily dose of 100 mg in chronic phase CML results in high hematologic and molecular remission rates and prolongation of survival. In accelerated and blastic phase as well as in ALL, 70 mg twice daily is recommended. Complete hematologic and cytogenetic remissions (CR) frequently occur even in this patient group with poor prognosis. Remissions however are very short.Side effects of dasatinib are frequent but mostly moderate and manageable and include cytopenias and pleural effusions. The role of dasatinib in other diseases, including solid tumors, has to be identified.

Optimizing first-line therapy for patients with chronic myeloid leukemia

Imatinib is now established as the gold standard first-line therapy for patients with chronic myeloid leukemia (CML). Responses to imatinib are superior to those seen with interferon alfa and also occur earlier, demonstrating a stronger and deeper response to therapy. Imatinib therapy also provides long-term clinical benefit and outcomes, with improved progression-free survival (PFS) and overall survival (OS) compared with historical controls, at 6 years of follow-up. Recent data show that annual event rates decline over time with imatinib therapy, suggesting that long-term disease control is possible in continuously responding patients. Despite these treatment successes, new strategies are continually being evaluated to maximize responses to imatinib and ensure the best treatment outcomes for all patients. For example, high-dose imatinib therapy, with doses up to 800 mg/d, has been shown to improve response rates. Prospective, randomized trials are ongoing to assess the benefits of high-dose imatinib therapy and determine whether it extends PFS and OS compared with standard-dose imatinib.