Targeted therapies in the treatment of Philadelphia chromosome-positive acute lymphoblastic leukemia

Imatinib mesylate (Gleevec, Novartis Pharmaceuticals Corp, East Hanover, NJ; Glivec, Novartis Pharma AG, Basel, Switzerland), a signal transduction inhibitor with preferential effects against the tyrosine kinase activity of the protein product of the ABL proto-oncogene, induced hematologic responses in >or=90% of patients with chronic-phase chronic myeloid leukemia (CML). In Philadelphia chromosome-positive (Ph(+)) acute lymphoblastic leukemia (ALL), the BCR-ABL translocation is the main transforming event, making it another hematologic malignancy targeted by this ABL-tyrosine kinase inhibitor. In an international multicenter phase II trial, imatinib-induced hematologic responses (typically brief) were achieved in 60% of patients with relapsed or refractory Ph(+) ALL. Subsequently, the German Multicenter Study Group for Adult ALL (GMALL) analyzed 59 patients treated in two successive nonrandomized phase II trials of imatinib in patients with relapsed or refractory Ph(+) ALL. Peripheral blood blasts cell clearance occurred within 8 to 14 days in most patients. However, in a significant proportion, blast counts subsequently increased 16 to 50 days after treatment onset. Imatinib mesylate was particularly effective in patients with relapse after stem cell transplantation (SCT); 75% of patients achieved complete leukemic response. Rapid development of resistance during treatment with imatinib mesylate remains a major problem. Further research efforts should explore the mechanisms of resistance to imatinib mesylate; effectiveness of other targeted therapies (eg, farnesyl transferase inhibitors [FTIs]); combination therapies; and inclusion of strategies for immune response modification (eg, donor lymphocyte infusions, interferon-alpha) for Ph/BCR-ABL-positive leukemias.