A Two-Fold Rise of BCR-ABL Transcript Levels Advises BCR-ABL Mutation Analysis in Imatinib-Treated Chronic Myeloid Leukemia (CML) - an Analysis of the Randomized CML-Study IV

![Graphic][1] Introduction: The clonal selection of a mutant BCR-ABL positive clone can be observed in about one of two patients with imatinib-resistant chronic myeloid leukemia (CML). The early detection of BCR-ABL kinase domain mutations is crucial, since it allows to change the tyrosine kinase inhibitor (TKI) regimen in a timely manner and may therefore prevent disease progression and the accumulation of further genetic lesions. European LeukemiaNet (ELN) recommendations suggest a mutation analysis if optimal response criteria are not achieved at 3, 6, 12 or 18 months, or whenever a loss of optimal response occurs (Soverini et al., Blood 2011). Several attempts have been made to derive this indication from a specific increase of BCR-ABL levels. Here we report on the correlation of a rise in BCR-ABL transcript levels and the prevalence of BCR-ABL kinase domain mutations in imatinib-treated patients of the CML-Study IV. Methods: A total of 1,173 patients were enrolled until 2009 and randomized to one of four imatinib-based treatment arms. BCR-ABLIS of 988 patients was determined in 7,876 samples by quantitative RT-PCR in the central laboratory (median sample number per patient: 8.4, range 1-37; median follow up: 34 months, range 0-86), representing the eligible patients for the study. Thereby, the estimated intra-laboratory variance is assumed to be about 20%. A first rise of BCR-ABLIS to at least two-fold and >0.1% between two samples of a patient's molecular course defined a sample suspected of bearing a mutant BCR-ABL positive clone. A mutation analysis was performed on this critical sample by direct sequencing of ABL exons 4 to 10. Results: A critical rise in BCR-ABLIS was observed in 231 of 988 patients (23%) after a median of 15.2 months on treatment (range 2.8-59.4). In the corresponding sample 33 mutant clones could be detected in 31 patients (13%). Thereby a steeper rise of BCR-ABLIS was correlated with a higher incidence of BCR-ABL mutations in the respective group (table). A total of 18 different mutations could be detected, the most frequent were: M244V, n=7 (21%); E255K, n=4 (12%); T315I, n=3 (9%); L248V, G250E, L387M and F486S, n=2 (6%), respectively. Mutations occur in a substantial proportion (8%) of patients with an only 2 to 3-fold rise of BCR-ABLIS transcript levels (table). Therefore, the most sensitive cut-off should be applied and mutation analysis may be triggered by a doubling of BCR-ABL transcripts at levels >0.1% IS. Conclusion: BCR-ABL kinase domain mutations occur already in a substantial proportion of patients with a doubling of BCR-ABL transcript levels, which should determine mutation analysis. | Rise of BCR-ABL expression | Patients (n) | Patients with BCR-ABL mutations (n) | Patients with BCR-ABL mutations (%) | Inter-sample interval (median, days) | | -------------------------- | ------------ | ----------------------------------- | ----------------------------------- | ------------------------------------------- | | 2 to 3-fold | 72 | 6 | 8.3 | 98 | | 3 to 5-fold | 50 | 3 | 6.0 | 100 | | 5 to 10-fold | 39 | 4 | 10.3 | 99 | | 10 to 100-fold | 49 | 10 | 20.4 | 98 | | > 100-fold | 21 | 8 | 38.1 | 125 | | > 2-fold (total) | 231 | 31 | 13.4 | 101 | Table 1. Disclosures Hanfstein: Novartis: Research Funding; Bristol-Myers Squibb: Honoraria. Hehlmann: Novartis: Research Funding; Bristol-Myers Squibb: Research Funding. Saussele: Novartis: Honoraria, Research Funding, Travel Other; Bristol-Myers Squibb: Honoraria, Research Funding, Travel, Travel Other; Pfizer: Honoraria, Travel, Travel Other. Schnittger: MLL Munich Leukemia Laboratory: Equity Ownership. Neubauer: MedUpdate: Honoraria, Speakers Bureau. Kneba: Novartis: Consultancy, Equity Ownership, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Pfirrmann: Novartis: Consultancy; Bristol-Myers Squibb: Honoraria. Hochhaus: Pfizer: Consultancy, Research Funding; ARIAD: Honoraria, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding. Muller: Novartis: Honoraria, Research Funding; Bristol Myers Squibb: Honoraria, Research Funding; ARIAD: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding. [1]: /embed/inline-graphic-2.gif