High BCR-ABL/GUSIS Levels at Diagnosis Are Associated with Unfavorable Responses to Standard Dose Imatinib

Background The approval of second-generation tyrosine kinase inhibitors (TKIs) for the first line treatment of Chronic Myeloid Leukemia (CML) has generated a need for early molecular parameters associated with inadequate responses to Imatinib Mesylate (IM). Objective We correlated quantitative determination of BCR-ABL transcripts at diagnosis with the outcome (defined according to the 2013 European Leukemia Net recommendations) of 272 newly diagnosed CML patients receiving IM 400 mg/die. Methods BCR-ABL transcripts were measured from peripheral blood samples drawn at diagnosis before patients received any pharmacological treatment using Real-Time Quantitative PCR (RQ-PCR). All molecular determinations were performed twice (in triplicates) on the same sample using either ABL or glucuronidase-beta ( GUS ) as reference genes. BCR-ABL values were then reported on the international scale (IS). Results With a median follow-up of 60 months, 65.4% of patients achieved an optimal response, 5.6% presented a response currently defined as "warning", 22.4% failed IM treatment and 6.6% switched to a different tyrosine kinase inhibitor because of intolerance to the drug. We recorded 19 deaths (6.9%), 7 (2.5%) attributable to disease progression. We applied Receiver Operating Characteristic (ROC) curves to define BCR-ABL/GUS IS expression levels that would separate patients likely (i.e. below the threshold) or unlikely (i.e. above the threshold) to achieve multiple endpoints, namely: optimal response (OR), failure-free survival (FFS), event-free survival (EFS), transformation-free survival (TFS) and overall survival (OS). Employing the specific threshold calculated for each endpoint we found that high BCR-ABL/GUS IS levels at diagnosis were associated with inferior probabilities of OR (p IS levels were also associated with higher rates of disease transformation to the accelerated phase or blast crisis (p=0.029) but not with OS (p=0.132). Conclusions High BCR-ABL transcripts at diagnosis measured by RQ-PCR employing GUS as a reference gene allow the identification of CML patients unlikely to benefit from standard dose IM that should be considered for alternative forms of treatment. Disclosures Hochhaus: Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; ARIAD: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding.