Rational therapeutic decision-making using mutational analysis for tyrosine kinase inhibitor therapy in BCR-ABL positive leukemias

A13

Introduction: Although imatinib is highly efficacious for the treatment of BCR-ABL positive leukaemias, imatinib-resistance due to the selection of BCR-ABL kinase domain (KD) mutations is an emerging problem. Two newer tyrosine kinase inhibitors (TKI), dasatinib and nilotinib, are more potent than imatinib and have in vitro activity against most imatinib-resistant mutations. However some mutations retain varying degrees of resistance in vitro to dasatinib and/or nilotinib and can be recovered in mutagenesis assays in the presence of either one or both drugs. This in vitro data can be used to guide therapeutic-decision making, depending on which mutations are detected.
 Methods: RNA was extracted from peripheral blood and BCR-ABL KD amplified by a hemi-nested RT/PCR approach and the PCR products subjected to direct sequencing.
 Results: BCR-ABL KD mutations were detected in 43 out of 83 imatinib-resistant chronic myeloid leukemia or Philadelphia-positive acute lymphoblastic leukemia patients. These mutations were located in the ATP-binding site (n=18), imatinib-binding site (n=10), catalytic domain (n=2), activation loop (n=2) and other sites (n=13). The imatinib-binding site mutant, F317L is known to be nilotinib-sensitive but dasatinib-resistant in vitro. This mutant was detected in 2 patients and both were treated with nilotinib, rather than dasatinib and achieved a major cytogenetic remission. Patients which had dasatinib- and nilotinib-sensitive mutations achieved hematologic and cytogenetic remission with either drug. However new mutations were detected which led to nilotinib- and dasatinib-resistance in 7 out of 10 and 13 out of 23 patients respectively. The T315I mutant was detected in 11 of these patients. This was not surprising as this mutant had been recovered in mutagenesis assays with dasatinib and nilotinib and were highly resistant to both drugs. A clinical trial with a new TKI with activity against the T315I mutant is underway and will benefit patients who harbor this mutant. Mutational analysis has also allowed rational switching from nilotinib to dasatinib and vice versa with favorable clinical outcomes in 2 patients. The F317V mutant was detected in the first patient who developed progressive disease on dasatinib. This mutant was known to be selected in vitro in the presence of dasatinib but remains sensitive to nilotinib. The patient was switched to nilotinib and achieved a complete cytogenetic remission (CCR). The Y253H mutant is recovered in mutagenesis assays in the presence of nilotinib and was detected in the second patient who progressed while on nilotinib. This mutant is sensitive to dasatinib and the patient achieved a CCR after switching to dasatinib.
 Conclusions: The detection of BCR-ABL KD mutants is important in the understanding of TKI resistance and also allows a rational molecular-based approach for TKI treatment.