Abstract Abstract 2790 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). Recent evidence suggests that CML patients presenting BCR-ABL/ABLIS levels >10% after 3 months of IM or >1% after 6 months of treatment have inferior outcomes in terms of both overall survival (OS) and progression-free survival. We wanted to establish if high BCR-ABL transcripts at diagnosis would also be associated with unfavorable responses to IM. To this end, we correlated quantitative determinations of BCR-ABL measured at diagnosis with the outcome of 230 newly diagnosed CML patients receiving IM 400 mg/die. BCR-ABL transcripts were measured from peripheral blood samples drawn at diagnosis 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. Median follow-up of the study population was 42 months. Estimated 5-year cumulative incidences of complete hematologic response, complete cytogenetic response (CCyR) and major molecular response were 97.9%, 89.5% and 64.7%. Five-year probabilities of OS, transformation-free survival (TFS: survival without disease transformation to the accelerated phase or blast crisis) and failure-free survival (FFS: survival without IM failure as defined by the 2009 European Leukemia Net recommendations) were 93.8%, 97.8% and 76%. Correlations between high BCR-ABL transcripts at diagnosis and unsatisfactory IM responses were much stronger using GUS in place of ABL as a reference gene. Indeed, while elevated BCR-ABL/GUSIS (p<0.0001) and elevated BCR-ABL/ABLIS (p<0.0001) both correlated with inferior probabilities of optimal response, high BCR-ABL/GUSIS (p<0.0001) but not high BCR-ABL/ABLIS (p=0.18) was associated with lower rates of CCyR after 12 months of IM. Neither BCR-ABL/GUSIS nor BCR-ABL/ABLIS at diagnosis were predictive of OS. However, high BCR-ABL/GUSIS was more accurately associated with lower probabilities of FFS (p<0.0001) and TFS (p=0.01) as compared to BCR-ABL/ABLIS (p=0.02 for FFS; p=0.36 for TFS). When we employed the 2009 European Leukemia Net criteria to subdivide our patient cohort in optimal responders, suboptimal responders and individuals failing IM, we found that elevated BCR-ABL/GUSIS (p<0.0001) was superior to elevated BCR-ABL/ABLIS (p<0.004) in distinguishing patient outcome. Furthermore, while BCR-ABL/ABLIS at diagnosis only discriminated optimal from resistant subjects (p=0.005), BCR-ABL/GUSIS transcripts were significantly different between the three patient groups (optimal vs suboptimal p=0.0002; optimal vs resistant p<0.0001; suboptimal vs resistant p<0.0001). Using receiver operating characteristic curves and the achievement of an optimal response as a specific endpoint, we found that 16.46% BCR-ABL/GUSISat diagnosis defined a threshold distinguishing low risk from high risk patients. 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 IM that should receive alternative forms of treatment. Disclosures: Turri: Novartis: Consultancy, Novartis Other; Bristol Myers Squibb: Bristol Myers Squibb, Bristol Myers Squibb Other, Consultancy. Morabito:Celgene: Honoraria.