Fluctuating Values of Molecular Residual Disease (MRD) without Molecular Progression After Imatinib Discontinuation in Patients (pts) with Chronic Myeloid Leukemia (CML) Who Have Maintained Complete Molecular Response: Implications for Re-Treatment Criteria and Role of Prior Interferon Therapy. A Pilot Study of the French CML Group (FILMC),

We have reported the results of imatinib discontinuation in CML pts in complete molecular response (CMR) for more than 2 years under imatinib therapy (STIM study, Mahon et al. Lancet Oncol. 2010). Among the group of pts without confirmed molecular relapse, a small proportion exhibited low levels of detectable residual disease during a prolonged period of time.

In order to better characterize this phenomenon, we decided to analyse pts who stopped IM following a maintained CMR or an undetectable molecular residual disease (UMRD) and resumed therapy upon loss of major molecular response (MMR). We also aimed to validate the loss of MMR as a robust criterion for the re-introduction of tyrosine kinase inhibitors (TKIs).

CP-CML pts were eligible if they were in CMR (CMR^4.5: BCR-ABL/ABL IS ratio <0.0032%) or UMRD (undetectable Bcr-Abl using standardized RTQ-PCR) under imatinib therapy for more than 2 years. Those pts were not enrolled in the STIM study because the study was closed or because they experienced at least one positive value of the BCR-ABL/ABL ratio during the 2 years follow-up. The proposed criterion for resuming imatinib was the loss of MMR (BCR-ABL/ABL IS ratio >0.1%). We calculated relapse free survival (RFS) using three different end-points: First loss CMR/UMRD defined by one occurrence MRD positivity; second loss of CMR/UMRD using the STIM definition (two consecutive increasing values of MRD); third loss of MMR. We also described pts with long lasting fluctuating PCR values.

34 CP-CML pts were included in the analysis. Median follow-up after imatinib discontinuation was 21.3 months (2.2–83.1). Sex ratio (M/F) was 50% with a median age of 54.1 years (27.4–78.2). Sokal score distribution was 34.5%, 37.9% and 27.6% for low, intermediate and high values respectively. 19 out of 34 (55.9%) of the pts received interferon therapy prior to imatinib. Median duration of imatinib therapy and median duration of CMR/UMRD prior to discontinuation was 63.8 months (30.1–120.8) and 33.7 months (7.3–72.8) respectively (only two pts had CMR/UMRD duration less than 2 years). Of note 18 out of 34 pts (52.9%) had a least one MRD positive value after the achievement of CMR/UMRD. After imatinib discontinuation, we identified 11 pts (32.4%) who experienced repeated low levels of detectable MRD without losing their MMR. Median follow-up for these pts with fluctuating values of MRD was 15.4 months (3.5–59.5) and none of them restarted imatinib. We next analysed relapse free survival (RFS) using the loss of MMR criteria (RFS-MMR). Median RFS-MMR was not reached, compared to median RFS using the loss of CMR/UMRD criteria (4.8 months) and median RFS using the STIM criteria (13.8 months) (p=0.003). As a consequence, 62.8% of the pts remain treatment free at 2 years using the loss of MMR criteria for resuming imatinib. Fluctuating values of MRD has already been described after interferon cessation in CML interferon treated pts. We thus asked if prior therapy with interferon before imatinib may influence treatment free survival. Duration of imatinib therapy and Sokal score risk distribution were comparable between pre-treated and non pre-treated pts (p=0.7). However, the median RFS was longer in interferon pre-treated pts as compared to pts who received imatinib first line (not reached versus 7 months, p=0.047). Furthermore, this difference was not significant using the loss of CMR/URMD (p=0.27) to define molecular relapse.

We were able to identify a significant number of pts with fluctuating values of MRD after imatinib discontinuation, a proportion underestimated in previous studies. We also validated the loss of MMR as the most accurate and robust criteria for restarting imatinib after imatinib discontinuation. Applying this criterion, we demonstrated that treatment free survival is significantly better in pts previously treated with interferon before imatinib compared to pts who received imatinib as first line therapy. An update of this pilot study on a larger number of patients will be presented.

Rousselot:BMS, Novartis: Research Funding. Tulliez:Novartis:. Mahon:Novartis Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Honoraria; Pfizzer: Honoraria.