The Persistence of p190 BCR-ABL Transcripts Is Associated with Lower Probability of Molecular Response to Imatinib in Early and Late Chronic Phase CML Patients.

Background. It has been demonstrated that about 70% of patients with CML in chronic phase (CP) at diagnosis co-expressed p210 and p190 BCR/ABL transcripts, although at a much lower level (

Materials and Methods. BM samples were obtained from 83 pts with CP-CML treated with Imatinib at a daily oral dose ranging between 300–500mg. These included 192 samples from 43 pts with late CP-CML (post-IFN failure) and 140 samples from 40 pts with early CP-CML who received Imatinib as first line therapy. Median follow-up was 18 (3–58) and 39 (12–58) months for early and late CP-CML, respectively. As part of a diagnostic work-up, BM samples from each patient were assessed for expression of both p210 and p190 BCR/ABL levels by real-time quantitative reverse transcription PCR (QRT-PCR) using a TAQ-Man system (ABI Prism 7700 Perkin Elmer) for BCR-ABL and ABL genes. The median number of BM assessment was 3 (2–6) for early CP-CML and 4 (2–10) for late CP-CML. A major molecular response (MMR) was defined as BCR-ABL/ABL ratios less than 0.05%. A specific nested RT-PCR screening was assessed for detection of p210 (b2a2, b3a2) and p190 (e1a2) BCR-ABL transcripts to confirm the negative data of p210 and p190 in QRT-PCR.

Results. A MMR was obtained in 20 pts (50%) and 20 pts (46%) in early and late CP-CML respectively. However, early CP-CML pts showed a significantly greater reduction in p210 BCR-ABL levels compared to late CP-CML after 12 months of Imatinib therapy (p=0.006), indicating a different kinetic of molecular response. Co-expression of p210 and p190 BCR-ABL transcripts at diagnosis was 73% for early CP-CML, whereas it was not available for late CP-CML. To test whether the persistence of p190 BCR-ABL transcript was predictive of MMR, we divided CML pts in 2 groups, those with 0 or 1 p190 BCR-ABL positive samples (group 1) and those with 2 or more positive samples (group 2) during the follow-up. We found that CP-CML pts of the group 2 showed a significant lower probability to obtain MMR molecular response compared to pts of group 1 both for late and early CML patients respectively [17/24 (71%) vs 5/19 (26%) with p=0.0039)], [15/21 (71%) vs 6/18 (33%) with p=0.017)]. This correlation holds also for complete cytogenetic response (data not shown).

Conclusions. In this study, approximately 50% of pts reached a MMR; half of them had undetectable values of p210 BCR-ABL transcripts. However, in a proportion of pts with complete cytogenetic response and low level of p210 BCR-ABL transcript, the expression of p190 is still detectable. The persistence of p190 signal despite the 2–3log fall in p210 BCR-ABL levels, may be of prognostic significance and may disclose unfolded concepts of biological relevance.