Absolute BCR-ABL Transcript Levels At 3 Months but Not At Diagnosis Predict Survival of Chronic Myeloid Leukemia (CML) Patients On Imatinib Therapy

Early assessment of molecular and cytogenetic response at 3 months of imatinib treatment has been shown to predict survival and might trigger treatment intensification in slow responders who are supposed to harbor a BCR-ABL positive clone with inferior susceptibility to tyrosine kinase inhibition (Hanfstein et al., Leukemia 2012). BCR-ABL transcript levels at 3 months depend on levels at diagnosis and the subsequent decline under treatment. Which of both parameters determines the clinical course and allows for prediction of survival is unclear. The BCR-ABL/ABL ratio is supposed to be skewed for high values, e.g. >10%, due to the fact that ABL transcripts are also amplified from the fusion gene and in fact BCR-ABL/(ABL + BCR-ABL) is determined. Therefore, Beta-glucuronidase (GUS) was used as reference gene to determine high transcript levels at diagnosis. In addition, the linearity of the BCR-ABL/GUS scale allowed for an optimization of prognostic cut-off levels. We compared the significance of 1) BCR-ABL/GUS at diagnosis, 2) BCR-ABL/GUS at 3 months, 3) the individual reduction of transcripts given by (BCR-ABL/GUS at 3 months)/(BCR-ABL/GUS at diagnosis), and 4) the established 10% BCR-ABL/ABL landmark expressed on the international scale (BCR-ABL^IS).

A total of 337 patients (pts) were investigated. According to the protocol of the German CML study IV pts could have been pre-treated with imatinib up to 6 weeks before randomization. 56 pts with imatinib onset before initial blood sampling within the study were excluded from the analysis. A total of 281 evaluable patients (median age 51 years, range 17–85, 42% female) were treated with an imatinib-based therapy consisting of imatinib 400 mg/d (n=76), imatinib 800 mg/d (n=110) and combinations of standard dose imatinib with interferon alpha (n=84) and low-dose cytarabine (n=11). Median follow-up was 4.8 years (range 1–10). Transcript levels of BCR-ABL, ABL, and GUS were determined by quantitative RT-PCR from samples taken before imatinib onset (“at diagnosis”) and 3 month samples. Only patients expressing typical BCR-ABL transcripts (b2a2 and/or b3a2) were considered. Disease progression was defined by the incidence of accelerated phase, blastic phase or death from any reason. A landmark analysis was performed for progression free survival (PFS) and overall survival (OS) after dichotomizing patients by a cut-off optimized by the cumulative martingale residuals method.

The median BCR-ABL/GUS ratio was 15.5% at diagnosis (0.07–271) and 0.62% at 3 months (0–34.7) reflecting a decline by 1.4 log. Disease progression was observed in 17 patients (6.0%), 14 of them died (5.0%). With regard to the above described parameters the following findings were observed: 1) at diagnosis no cut-off level could be identified for BCR-ABL/GUS ratios to separate two prognostic groups according to long-term PFS or OS. 2) At 3 months an optimized 2.8% BCR-ABL/GUS cut-off separated a high-risk group of 61 pts (22% of pts, 8-year PFS 78%, 8-year OS 81%) from a good-risk group of 220 pts (78% of pts, 8-year PFS 94%, 8-year OS 94%, p<0.001, respectively). 3) At 3 months an individual reduction of BCR-ABL transcripts to at least 40% (0.4 log) of the initial level separated best and divided a high-risk group of 33 pts (12% of pts, 8-year PFS 74%, 8-year OS 80%) from a good-risk group of 248 pts (88% of pts, 8-year PFS 93%, 8-year OS 93%, p<0.001, respectively). 4) When the established 10% BCR-ABL^IS at 3 months was investigated, 63 pts were high-risk (22% of pts, 8-year PFS 82%, 8-year OS 85%) and 218 good-risk (78% of pts, 8-year PFS 91%, 8-year OS 93%, p=0.002 for PFS, p=0.011 for OS).

Initial BCR-ABL transcript levels at diagnosis did not show prognostic significance. To predict survival at 3 months of treatment the absolute transcript level normalized by ABL or GUS can be used.

Schnittger:MLL Munich Leukemia Laboratory: Equity Ownership. Hochhaus:Novartis, BMS, MSD, Ariad, Pfizer: Consultancy Other, Honoraria, Research Funding. Müller:Novartis, BMS: Consultancy, Honoraria, Research Funding.